- Open Access
Cholesterol transport and steroidogenesis by the corpus luteum
Reproductive Biology and Endocrinology volume 1, Article number: 90 (2003)
The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.
The ephemeral nature of the corpus luteum (CL) makes it even more remarkable that this tissue is able to synthesize upwards of 40 mg of progesterone in the human on a daily basis . To accomplish this feat the steroidogenic machinery within the cells of the CL must be highly efficient. Because of progesterone's importance to reproductive success, the regulation of its synthesis by luteal tissue has been well studied in a variety of species [2–4]. However, while the synthesis and essentiality of luteal progesterone production is consistent among all eutherian mammals, luteal tissue can also produce androgens, estrogens, 20α-hydroxyprogesterone, and 5α-reduced progestins all of which vary dramatically across different species [5–7]. In addition, the uniqueness of the CL as an endocrine organ is also evident by the different mechanisms whereby luteal regression occurs and by the species specific mechanisms employed to maintain luteal progesterone secretion if a pregnancy ensues . This concept is clearly evident when the trophoblastic production of chorionic gonadotropin in primates is compared to the mechanisms employed in ungulates, which modulate uterine prostaglandin F2α production and/or secretion.
Regulation of steroid production by the CL varies remarkably for different species. In humans, monkeys and ruminants the CL is largely dependent on pituitary-derived luteinizing hormone (LH) acting through the cAMP/protein kinase A pathway . Conversely, in rodents and rabbits, it is well established that prolactin and estradiol are critical luteotrophic hormones . In addition to the direct effects of luteotrophic hormones on the luteal cells via interaction with their respective receptors, LH and the other luteotrophic hormones modulate luteal synthesis of growth factors, cytokines, and other factors that in turn influence luteal cell function [10, 11]. Understandably, the regulation of CL growth and regression is a unique process when compared to other steroidogenic tissues and this was best described by I. Rothchild in his treatise on " The regulation of the mammalian corpus luteum" . He concludes that luteal progesterone production occurs relatively autonomously; a classic-negative feedback system seen in the other endocrine tissues does not operate in the CL and at the end of the luteal phase, in spite of pituitary-support, the CL undergoes regression and progesterone secretion declines. In 1996, Dr. Rothchild updated his hypothesis and concluded that progesterone may not only stimulate but may also be directly involved in the process of luteolysis . Thus, the changing capacity for steroidogenesis by the CL is one of the more important aspects of luteal physiology.
Steroidogenic cells within the corpus luteum of most but not all species can be divided into two subpopulations of cells based on size and their putative follicular cell of origin (thecal or granulosa) . In addition to the gross morphologic differences, the biochemical and molecular phenotype of these two cell types varies throughout the luteal phase/pregnancy as does the proportion of these cells that make up the corpus luteum . Isolation of large and small cells in a variety of species has indicated that the large cells exhibit the greatest basal steroid production and are less or not responsive to addition of LH, while small luteal cells bind LH to a high degree and respond with pronounced increases in progesterone synthesis [4, 15]. Numerous reviews have described: 1) the differences between luteal cell types, 2) the role of LH and luteotrophic factors including those associated with pregnancy in regulation of luteal function, and 3) how luteal regression is postulated to proceed. In this minireview, we will focus on recent advances made in the understanding of luteal steroidogenic function, comparing primates to other species.
Cholesterol transport to and within luteal cells
The first challenge for any steroid producing cell including luteal cells is obtaining the precursor cholesterol. While, luteal cells can produce cholesterol de novo, this method of obtaining cholesterol typically plays a minor role in the normal functioning tissue as evidenced by the low levels of HMG-CoA reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway, and the relative lack of the other cholesterol biosynthetic enzymes . By default then, the major mechanisms for obtaining cholesterol are either the endocytosis of cholesterol rich low-density lipoprotein (LDL) or the selective uptake of cholesterol esters from high-density lipoprotein (HDL).
Whether LDL or HDL serves as the source of cholesterol for luteal steroidogenesis appears to be species dependent with mice, rats, and ruminants utilizing HDL and human, rhesus macaques, and porcine using primarily LDL  and references therein. In mice, the cloning of the scavenger receptor BI (SR-BI) clarified the mechanism of HDL sterol uptake and indicated that this receptor mediates the selective uptake of cholesterol esters from HDL . Targeted deletion of the SR-BI gene demonstrated that female mice were infertile and exhibited reduced lipid levels in the CL as measured by oil red O staining, suggesting a reduction in cholesterol ester storage . However, the decline in fertility could not be attributed to reduced steroid output, as endocrine profiles were normal, suggesting that de novo synthesis of cholesterol may have increased in these animals to rectify the absence of HDL delivery. A role for HDL and/or endogenous cholesterol stores in primate luteal steroidogenesis can also be envisioned in the hours immediately following the onset of the LH surge through the process of ovulation . First, preovulatory granulosa cells exhibit a build up of cholesterol ester, however, the source of this cholesterol is not known . Second, very low levels of LDL are found in human follicular fluid [22–24], while follicular fluid levels of HDL are similar to serum levels [23, 24]. Third, short-term cultures (2 hr) of luteal cells isolated from periovulatory macaque follicles at 12, 24, and 36 h after the LH surge demonstrated that these cells were not responsive to inclusion of LDL or cholesterol (control) in the medium . All of these observations suggest that HDL and/or an endogenous source of cholesterol plays a role in the early steroidogenesis by luteinizing granulosa cells. These data contrast those of long-term cultures of human granulosa-lutein and monkey luteal cells where LDL is known to be critical for progesterone production by isolated cells and HDL is ineffective [17, 26]. In addition to the selective uptake of cholesterol esters from HDL, selective uptake of cholesterol esters from LDL by gonadal tissue has also been demonstrated . The importance of selective cholesterol uptake from either HDL or LDL in primate luteal tissue remains to be determined, as does the mechanism that drives the initial increases in luteal cell cholesterol storage.
Processing of the LDL/LDL-receptor-clathrin coated pit complex has been well described, whereas the subsequent understanding of trafficking of LDL-derived cholesterol within the cell is not as far advanced . The study of the neurovisceral Niemann-Pick C (NPC) disorder has provided some insight in to this complex process. This disease is characterized by a mutation in the protein (NPC-1) that is responsible for intracellular trafficking of LDL-derived cholesterol and results in the accumulation of unesterified cholesterol in the lysosomes and Golgi complex . Treatment of fibroblast cells with pharmacological levels of progesterone is commonly used to induce the NPC-1 phenotype. Interestingly, the high progesterone level used to induce the phenotype in other cells is well within the levels that would be observed in luteal cells. Thus, luteal cells must have either adapted a mechanism to bypass this regulatory effect of progesterone on the NPC-1 protein, or luteal cells might utilize the high levels of progesterone to elevate free cholesterol in a positive feedback mechanism and thereby produce more progesterone. In human granulosa-lutein cells (collected 27 h post-LH surge), NPC-1 is localized to a subset of lysosomes and NPC-1 containing vesicles that are distributed in the cytoplasm in a random pattern distinctly different from those of free cholesterol and cytoplasmic neutral lipid droplets . In the only studies of NPC-1 in post-ovulatory luteal cells, Gervy et al. have demonstrated that expression of NPC-1 in pig luteal cells appears to be up-regulated by cAMP treatment and that when preovulatory porcine granulosa cells were luteinized in vitro there was a progressive increase in NPC-1 expression and protein [31, 32]. Furthermore, they observed that in early luteal tissue (24 hr post ovulation) the thecal cells stained with greater intensity than the granulosa cells. These results contrast those of Watari et al., who was unable to demonstrate an effect of 8-Br-cAMP on regulation of NPC-1 expression in human granulosa-lutein cells . This appears to be a species-specific difference and expression analyses in other species is warranted, as is the more thorough analysis of regulation of NPC-1 in human luteal tissues. Similar clinical and biochemical phenotypes for a second independent gene, called NPC-2 suggest that the two proteins may interact or function sequentially within a common pathway . NPC-2 expression and characterization in luteal tissue has not been reported.
Cholesterol exists in two forms in cells and plasma lipoproteins, namely free cholesterol and cholesterol esters. Free cholesterol is the precursor substrate for steroidogenesis. Cholesterol esters, on the other hand, consist of cholesterol esterified through the 3β-hydroxyl group to polyunsaturated fatty acids or to sulfate, which is catalyzed by the microsomal acyl coenzyme A:cholesterol acyltransferase (ACAT). Newly synthesized cholesterol esters accumulate within the rough endoplasmic reticulum and bud off as cytoplasmic lipid droplets; the abundance of the latter is a key feature of luteal cells. Present in cytoplasmic lipid droplets and lipoprotein particles, the fatty acid esters of cholesterol can neither replace free cholesterol as a structural ingredient of the plasma membrane nor serve as direct substrates for steroid production. Cholesterol esters found in cytoplasmic lipid droplets are hydrolyzed by an extralysosomal enzyme, neutral cholesterol ester hydrolase (NCEH), also known as hormone sensitive lipase because its activity is tightly regulated within steroidogenic tissues by tropic hormones including FSH, LH and hCG  and references therein. Both ACAT and NCEH are not dynamically regulated in luteal cells, and therefore do not limit steroidogenesis.
Luteal progesterone production
Immediately after the LH surge (or hCG administration) serum progesterone levels are known to rapidly (30 min) increase . The rapidity of this response suggests that most of the enzymes and proteins necessary for progesterone synthesis must be present in the cells or are rapidly induced. The lack of the full complement of the appropriate enzymatic machinery in the primate granulosa cells and granulosa cells of several other species [25, 33, 34] points to the luteinizing thecal cells as the possible source for this immediate increase in progesterone synthesis. Additionally, the limited vascularization of the granulosa cells prior to ovulation in most species would theoretically not only limit the secretory capability of the granulosa, but also the ability of these cells to obtain precursor cholesterol (HDL or LDL) via the vasculature. Luteal cells isolated from early human luteal tissues express and contain elevated levels of steroidogenic enzymes , including the steroidogenic acute regulatory protein (StAR), a critical protein involved in steroidogenesis . Examination of luteal tissue progesterone concentrations in primates demonstrated that early luteal phase tissues had similar or more steroid / mg of tissue when compared to mid-luteal phase tissues , even though luteal secretion (as indicated by circulating progesterone levels) is not maximal until several days later, coincident with the maturation of the vascular network [38–40]. The establishment of an inadequate vascular supply to the corpus luteum is postulated to have significant ramifications on steroid secretion later in the luteal phase also .
Progesterone biosynthesis requires only two enzymatic steps; the conversion of cholesterol to pregnenolone, catalyzed by P450 side chain cleavage (P450scc) located on the inner mitochondrial membrane, and its subsequent conversion to progesterone, catalyzed by 3β-hydroxysteroid dehydrogenase (3β-HSD) present in the smooth endoplasmic reticulum (SER). Interestingly, in luteinizing granulosa and theca cells, both the mitochondria and the SER undergo dramatic changes in organization and increases in quantity concurrent with dramatic increases in cellular progesterone synthesis. However, the mechanisms driving this organelle reorganization and formation in luteal cells and the impact these changes have on luteal function are not fully understood.
Examination of P450scc expression in primate luteal tissue indicates that the overall expression of this enzyme remains elevated and relatively constant throughout the luteal phase [35, 41]. Progesterone secretion by isolated small and large ovine luteal cells treated with excess cholesterol substrate in the form of hydroxylated cholesterol molecules that measure P450scc activity directly also indicate that P450scc was not limiting in this species [42, 43]. Furthermore, in the rat, expression of P450scc remains elevated in CL not receiving luteotrophic support and after serum progesterone levels begin to decline, suggesting that P450scc is not the limiting factor in progesterone secretion in the rat CL as well . Interestingly, monkey granulosa cells collected prior to the LH surge, contained P450scc mRNA but failed to exhibit P450scc activity (as measured by conversion of 25-hydroxycholesterol to progesterone) [25, 44]. These studies suggest that the granulosa cells either have not yet acquired the P450scc protein and/or the necessary electron transfer partners (i.e., adrenodoxin/adrenodoxin reductase). For example in human placental tissues there is evidence that the levels of adrenodoxin/andrenodoxin reductase limits P450scc activity . Conversely, cholesterol utilization may somehow be blocked in these cells. Following hCG stimulation the monkey granulosa-lutein cells (12 hr post-hCG) exhibited a decline in P450scc mRNA levels, coincident with greatest levels of P450scc activity [25, 44]. Thereafter, granulosa-lutein cell conversion of 25-hydroxycholesterol to progesterone fell off dramatically at 24 and 36 hr. Conversely, serum progesterone concentrations in these monkeys after exhibiting an initial ~25-fold increase 12 hr post-hCG treatment, remained at those levels at 24 hr before increasing again at 36 hr [25, 44]. Overall, the experiments cited suggest that P450scc is not limiting in luteal progesterone secretion, with the possible exception of the earliest stages of luteinization. Further studies are warranted to sort out whetherP450scc is a limiting factor during this early critical period.
The onset and regulation of 3β-HSD expression exhibits a wide variation among different species . In the human corpus luteum, the expression of 3β-HSD is observed to be greatest during the early luteal phase and then declines by the mid-luteal phase where it remains in the late luteal phase, unless stimulated with hCG . Pregnenolone conversion to progesterone by macaque granulosa cells collected before the LH surge indicated that these cells contain significant amounts of 3β-HSD activity, prior to in vivo progesterone biosynthesis . Increased 3β-HSD mRNA expression by macaque granulosa cells 12 h after in vivo hCG treatment were followed by a transient decline in 3β-HSD expression at 24 h followed by a rise to an intermediate level by 36 h after hCG treatment . Additionally, dramatic increases in progesterone secretion by both small and large ovine luteal cells were observed following incubation with pregnenolone, suggesting again that 3β-HSD is not limiting [42, 43]. Thus, similar to P450scc, 3β-HSD does not appear to be rate limiting in luteal progesterone biosynthesis. Indeed, the consensus of a large number of studies in primates and in other species indicate that the critical step in luteal progesterone secretion is the movement of cholesterol from the outer to inner mitochondrial membrane [3, 4, 15, 44].
Discovery of steroidogenic acute regulatory protein
Since the discovery of steroidogenic acute regulatory protein (StAR), the protein that governs the movement of cholesterol from the outer to inner mitochondrial membrane, the focus of many studies has been on its regulation and function in luteal and other tissues . Prior to the LH surge, StAR is virtually absent from the granulosa cells which are unable to metabolize and synthesize progesterone from cholesterol precursors [44, 47, 48]. Conversely, StAR is found in high levels in the periovulatory theca cells that are able to synthesize androgens from cholesterol. These points are illustrated nicely in a preovulatory human follicle collected during the initial rise of the LH surge and immunostained for StAR protein (Figure 1). Expression of StAR transcripts and protein was greatest in early and mid-luteal phase CL before declining in the late-luteal phase [49, 50]. In the human CL, the theca-lutein and granulosa-lutein cells exhibited marked heterogeneity in StAR protein concentrations, with theca-lutein cells expressing greater levels of StAR than granulosa-lutein cells, irrespective of the stage of the luteal phase . Theca-lutein cell StAR expression was greatest in the early luteal phase, moderate in the mid and least in the late luteal phase, while granulosa-lutein cells exhibited moderate StAR expression in early luteal tissue, increased levels in mid-luteal phase CL and declining expression in late luteal phase tissues . Immunodetection of StAR in granulosa-lutein cells was not homogenous, as cells adjacent to the central cavity contained greater amounts of StAR, than those near the capsule ; the cause and importance of this differential staining is unknown.
Administration of hCG to women preferentially increased theca-lutein cell StAR mRNA expression and protein levels in mid-luteal phase CL, while causing only a moderate increase in granulosa-lutein cell expression in mid-luteal phase CL . Human CG treatment during the late luteal phase caused a pronounced increase in both theca- and granulosa-lutein cell StAR gene expression. These in vivo results in women confirm previous observations in monkeys and in vitro results with isolated luteal cells, and demonstrate an age-dependent response of the CL to hCG [52, 53]. In species where luteolysis is well established as being driven by uterine prostaglandin-F2α, exogenous administration of this luteolytic compound has been shown to cause a pronounced decline in StAR gene expression [4, 54]. Thus, loss of StAR expression at the end of the luteal phase may play a key role in the decline in luteal progesterone biosynthesis.
Transcriptional regulation of StAR gene expression in luteal tissues
Early studies examining steroidogenesis demonstrated that protein synthesis was required for hormonal/cAMP stimulation of steroid secretion  and references therein. Indeed, the loss of StAR protein following cycloheximide treatment meshed well with this being the critical protein. Subsequent investigations, however, indicated that not only was StAR protein synthesis inhibited, so was StAR mRNA expression, suggesting that StAR and therefore steroidogenesis was primarily regulated at the transcriptional level  and references therein. The identification and elucidation of the transcription factors that bind to the StAR promoter has been an area of active research [56–58]. Since the human StAR promoter lacks the recently described cAMP-responsive element (CRE) observed in the mouse [59, 60] and because evidence supporting a role for CRE-binding protein activation of human StAR transcriptional activity is also lacking, other transcription factor families have been evaluated .
Steroidogenic factor-1 (SF-1/Ad4BP/NR5A-1), a member of the nuclear receptor superfamily, confers both basal and cAMP-dependent responsiveness to many of the genes encoding steroidogenic enzymes . Likewise, all StAR promoters examined contain SF-1 binding sites, and in all cases except the mouse these sites were shown to be essential for both basal and hormone-induced (cAMP) regulation [60, 62–66]. The method by which SF-1 binding sites confer cAMP-responsiveness and the regulatory factors involved in SF-1 function (i.e., ligands, phosphorylation, coactivators [67–69]) are not completely understood. One of the confounding issues with SF-1 was the observation that granulosa cells and luteal cells only minimally express this protein both before and after the LH surge. Recently, Lui et al.,  demonstrated that liver receptor homologue-1, (LRH-1, CPF/FTF/hB1F/NR5A-2), a closely related nuclear hormone receptor that shares identical DNA binding mechanisms and specificities with SF-1 , was present in granulosa cells before the LH surge. Moreover, LRH-1 exhibited a pronounced increase in expression after the LH surge and remained at high levels in rat luteal tissue as long as progesterone biosynthesis was elevated. Recently, Falender et al.,  confirmed the exclusive expression of follicular LRH-1 to the mouse granulosa cells and the upregulation of LRH-1 expression in rodent luteal cells. Because of the marked difference in the mouse and rat promoter with respect to SF-1 function, it will be interesting to determine whether StAR expression in rat/primate luteal tissue is regulated by LRH-1 and whether or not this occurs in the mouse model.
The human and rodent StAR gene promoters also contain cis elements that are responsive to CCAAT/enhancer-binding proteins (Cebp) that have been shown to regulate both basal and cAMP-dependent StAR gene expression [57, 64, 73, 74]. Promoter analysis has also indicated that over expression of Cebp and SF-1 had a synergistic effect on cAMP-dependent StAR promoter activity . LH and cAMP analogs increase nuclear Cebp β (Cebpb) levels in human granulosa-lutein cells  and in mouse granulosa cells luteinized in vivo (i.e., PMSG/hCG stimulation protocol) . The ability of Cebpb to regulate the StAR promoter suggests that this protein may be the potential StAR regulatory factor that is lost following cycloheximide treatment of cells.
The identification of a GATA site in the StAR promoter led investigators to test for its influence on StAR promoter activity [64, 74]. Electrophoretic mobility shift assays and StAR promoter analysis demonstrated a positive role for GATA-4 in basal and cAMP-dependent StAR promoter activity. Since GATA-4 is constitutively expressed in mouse granulosa cells, it would seem that this transcription factor would likely only function in a permissive versus an obligatory role in the hormonal induction of StAR gene transcription. However, recent studies have shown that GATA-4 is activated by phosphorylation following trophic hormonal stimulation and this is correlated with StAR activation [76, 77]. Additionally, these authors demonstrated that GATA-4 and Cebpb cooperate to mediate cAMP stimulation of the StAR promoter. While the experiments described above have narrowed the field of candidates mediating cAMP-regulated activity of the StAR promoter, the details of how these factors function individually or in concert remain to be elucidated and the role these factors play in luteal steroidogenesis, particularly in the two different luteal cell types remains to be determined.
Numerous studies have demonstrated that in addition to the major hormonal regulators of steroidogenic cell function, paracrine and autocrine factors influence StAR gene expression ,  and references therein. Some of these factors amplify StAR gene expression (e.g., IGFs) and others diminish expression (e.g. TGFβ, TNFα). The mechanisms by which these factors act to control StAR levels, by and large, have not been elucidated but may encompass transcriptional as well as post-transcriptional mechanisms. Interestingly, progesterone was recently shown to have a stimulatory effect on StAR gene expression in a mouse Leydig cell line (MA-10) through a yet to be determined mechanism . This mechanism does not require classical progesterone receptors as MA-10 cells are devoid of this receptor . This observation is of particular interest to those who study the corpus luteum, as Rothchild predicted that progesterone production by the CL was capable of stimulating its own synthesis as early as 1981. The subsequent identification of progesterone receptors in primate luteal tissue  provided a plausible mechanism for this to occur, and this recent finding suggests yet another mechanism via which progesterone is capable of modulating its own synthesis.
Posttranslational modification of StAR and interaction with other proteins
StAR was originally identified as a phosphoprotein and mutation of a conserved serine195 phosphorylation site resulted in approximately 50% reduction in steroid production . Phosphorylation of StAR is postulated to play a role in the movement or targeting of StAR to the outer mitochondrial membrane. This hypothesis is supported by the experimental observation that a N-terminal deletion of StAR, which lacks the mitochondrial targeting sequence when combined with a S195A mutation, exhibits no difference in pregnenolone synthesis when compared to the "wild-type" N-62 StAR protein . This contrasts the >60% reduction in pregnenolone synthesis detected with the full-length S195A mutant compared to the wild type protein. Presently, the role of StAR phosphorylation in luteal function remains to be determined.
The mechanism by which StAR is able to increase the transfer of cholesterol from the outer to the inner mitochondrial membrane has been the subject of many investigations [81, 82]. Differing viewpoints on whether StAR activity requires an interaction with other proteins, such as the peripheral type benzodiazepine receptor (PBR) exist [15, 83, 84]. Levels of PBR and StAR in isolated small and large ovine luteal cells did not differ when expressed on a per μg protein basis, however, the large luteal cells exhibited 3-fold more endozepine, the natural ligand for PBR . The mechanism by which endozepine influences PBR and/or PBR interaction with StAR is not known, however, recently it was shown that StAR and PBR can be closely associated in mitochondrial membranes . Understanding how these two proteins interact in luteal tissue, as well as determining the role endozepine plays in cholesterol movement remains to be elucidated.
Luteal production of other steroids
Thecal cell distribution and the proportion of these cells within the corpus luteum vary dramatically for different species. In the primate CL, many of the thecal/small luteal cells remain associated with the vascular tree and some of these cells maintain their androgenic phenotype as evident by intense P450-17α immunolocalization and androgen biosynthesis in culture [6, 86]. In primates, the CL also retains its ability to secrete estrogen throughout the luteal lifespan; some of the large/granulosa-lutein cells in the primate contain P450aromatase, thus indicating that the primate CL may retain the two cell-follicular model of estrogen biosynthesis during the luteal phase . Interestingly, estrogen secretion by the primate ovary does not appear essential for pregnancy as progesterone replacement in ovariectomized women and monkeys alone maintains pregnancy. Although the exact role for luteal estradiol secretion is unknown, it was originally postulated to be involved in luteolysis in the primate, where the luteolytic process is independent of uterine interaction. While the local luteolytic effect of estrogen has been largely discounted, the presence of both estrogen receptor α and β within primate luteal tissue supports a local role for this steroid in luteal function . Expression of aromatase by luteal cells declines during the late-luteal phase paralleling the decline in luteal estrogen biosynthesis . Recently, progesterone was shown to promote the survival of rat luteal cells by inhibiting apoptosis and stimulating luteal cell androstenedione synthesis . Interestingly, androstenedione synthesis in the rat also maintained luteal function and increased progesterone biosynthesis . The localization of the androgen receptors and estrogen receptors within the primate and rodent corpus luteum supports a role these steroids in local luteal function [37, 90]. The identification of genes downstream of these transcription factors will be important to clarify how these hormones regulate luteal function.
Our understanding of luteal steroidogenesis in primate species has made significant advances since the discovery of StAR and its subsequent characterization in luteinizing granulosa cells and luteal tissues. While it is clear that StAR is involved in luteal steroidogenesis, the factors that control it synthesis in luteal tissue are only now beginning to be investigated. One of the more interesting observations by Rothchild more than 20 years ago was that progesterone secretion by luteal tissue appeared to perpetuate its own synthesis. The subsequent discovery that the luteal tissue contained progesterone receptors that might mediate this progesterone response supported this hypothesis. However, the more recent observations that progesterone can itself stimulate StAR gene transcription, and that progesterone/androgens are capable of maintaining luteal function in the rat (which lacks a classical progesterone receptor) further supports this original hypothesis as well as providing a possible explanation for this phenomena. Recent technical advances combined with the large group of investigators interested in understanding how the corpus luteum of a diverse number of species develops and functions, promises to provide us with many additional exciting findings regarding luteal steroidogenesis in the near future.
Lipsett MB: In: Reproductive Endocrinology. Edited by: Yen SC, Jaffe RB. 1978, Philadelphia: WB Saunders, 80-92.
Niswender GD, Juengel JL, Silva PJ, Rollyson MK, McIntush EW: Mechanisms controlling the function and life span of the corpus luteum. Physiol Rev. 2000, 80: 1-29.
Devoto L, Kohen P, Vega M, Castro O, Gonzalez RR, Retamales I, Carvallo P, Christenson LK, Strauss JF: Control of human luteal steroidogenesis. Mol Cell Endocrinol. 2002, 186: 137-141. 10.1016/S0303-7207(01)00654-2.
Diaz FJ, Anderson LE, Wu YL, Rabot A, Tsai SJ, Wiltbank MC: Regulation of progesterone and prostaglandin F2alpha production in the CL. Mol Cell Endocrinol. 2002, 191: 65-80. 10.1016/S0303-7207(02)00056-4.
Albarracin CT, Parmer TG, Duan WR, Nelson SE, Gibori G: Identification of a major prolactin-regulated protein as 20 alpha-hydroxysteroid dehydrogenase: coordinate regulation of its activity, protein content, and messenger ribonucleic acid expression. Endocrinology. 1994, 134: 2453-2460. 10.1210/en.134.6.2453.
Sanders SL, Stouffer RL, Brannian JD: Androgen production by monkey luteal cell subpopulations at different stages of the menstrual cycle. J Clin Endocrinol Metab. 1996, 81: 591-596. 10.1210/jc.81.2.591.
Hodges JK, Heistermann M, Beard A, van Aarde RJ: Concentrations of progesterone and the 5 alpha-reduced progestins, 5 alpha-pregnane-3,20-dione and 3 alpha-hydroxy-5 alpha-pregnan-20-one, in luteal tissue and circulating blood and their relationship to luteal function in the African elephant, Loxodonta africana. Biol Reprod. 1997, 56: 640-646.
Roberts RM, Xie S, Mathialagan N: Maternal recognition of pregnancy. Biol Reprod. 1996, 54: 294-302.
Stormshak F, Zelinski-Wooten MB, Abdelgadir SE: Comparative aspects of the regulation of corpus luteum function in various species. Adv Exp Med Biol. 1987, 219: 327-360.
Devoto L, Vega M, Kohen P, Castro A, Castro O, Christenson LK, Carvallo P, Strauss JF: Endocrine and paracrine-autocrine regulation of the human corpus luteum during the mid-luteal phase. J Reprod Fertil Suppl. 2000, 55: 13-20.
Webb R, Woad KJ, Armstrong DG: Corpus luteum (CL) function: local control mechanisms. Domest Anim Endocrinol. 2002, 23: 277-285. 10.1016/S0739-7240(02)00163-7.
Rothchild I: The regulation of the mammalian corpus luteum. Recent Prog Horm Res. 1981, 37: 183-298.
Rothchild I: The corpus luteum revisited: are the paradoxical effects of RU486 a clue to how progesterone stimulates its own secretion?. Biol Reprod. 1996, 55: 1-4.
Murphy BD: Models of luteinization. Biol Reprod. 2000, 63: 2-11.
Niswender GD: Molecular control of luteal secretion of progesterone. Reproduction. 2002, 123: 333-339. 10.1530/reprod/123.3.333.
Gwynne JT, Strauss JFd: The role of lipoproteins in steroidogenesis and cholesterol metabolism in steroidogenic glands. Endocr Rev. 1982, 3: 299-329.
Brannian JD, Stouffer RL: Native and modified (acetylated) low density lipoprotein-supported steroidogenesis by macaque granulosa cells collected before and after the ovulatory stimulus: correlation with fluorescent lipoprotein uptake. Endocrinology. 1993, 132: 591-597. 10.1210/en.132.2.591.
Acton S, Rigotti A, Landschulz KT, Xu S, Hobbs HH, Krieger M: Identification of scavenger receptor SR-BI as a high density lipoprotein receptor [see comments]. Science. 1996, 271: 518-520.
Trigatti B, Rayburn H, Vinals M, Braun A, Miettinen H, Penman M, Hertz M, Schrenzel M, Amigo L, Rigotti A, Krieger M: Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology. Proc Natl Acad Sci U S A. 1999, 96: 9322-9327. 10.1073/pnas.96.16.9322.
Chaffin CL, Stouffer RL: Local role of progesterone in the ovary during the periovulatory interval. Rev Endocr Metab Disord. 2002, 3: 65-72. 10.1023/A:1012704903128.
Endresen MJ, Haug E, Abyholm T, Henriksen T: The source of cholesterol for progesterone synthesis in cultured preovulatory human granulosa cells. Acta Endocrinol (Copenh). 1990, 123: 359-364.
Simpson ER, Rochelle DB, Carr BR, MacDonald PC: Plasma lipoproteins in follicular fluid of human ovaries. J Clin Endocrinol Metab. 1980, 51: 1469-1471.
Carr BR, Sadler RK, Rochelle DB, Stalmach MA, MacDonald PC, Simpson ER: Plasma lipoprotein regulation of progesterone biosynthesis by human corpus luteum tissue in organ culture. J Clin Endocrinol Metab. 1981, 52: 875-881.
Enk L, Crona N, Olsson JH, Hillensjo T: Lipids, apolipoproteins and steroids in serum and in fluid from stimulated and non-stimulated human ovarian follicles. Acta Endocrinol (Copenh). 1986, 111: 558-562.
Chaffin CL, Hess DL, Stouffer RL: Dynamics of periovulatory steroidogenesis in the rhesus monkey follicle after ovarian stimulation. Hum Reprod. 1999, 14: 642-649. 10.1093/humrep/14.3.642.
Tureck RW, Wilburn AB, Gwynne JT, Paavola LG, Strauss JFd: The role of lipoproteins in steroidogenesis by human luteinized granulosa cells in culture. J Steroid Biochem. 1983, 19: 1033-1038. 10.1016/0022-4731(83)90050-X.
Azhar S, Luo Y, Medicherla S, Reaven E: Upregulation of selective cholesteryl ester uptake pathway in mice with deletion of low-density lipoprotein receptor function. J Cell Physiol. 1999, 180: 190-202. 10.1002/(SICI)1097-4652(199908)180:2<190::AID-JCP7>3.3.CO;2-Q.
Christenson LK, Strauss JF: Human genetic defects that influence cholesterol metabolism in steroidogenic tissues. In: Genetics of Steroid Biosynthesis and Function. Edited by: Mason JI. 2002, UK: Harwood Academic Publishers
Strauss JF, Liu P, Christenson LK, Watari H: Sterols and intracellular vesicular trafficking: lessons from the study of NPC1. Steroids. 2002, 67: 947-951. 10.1016/S0039-128X(02)00042-9.
Watari H, Blanchette-Mackie EJ, Dwyer NK, Sun G, Glick JM, Patel S, Neufeld EB, Pentchev PG, Strauss JF: NPC1-containing compartment of human granulosa-lutein cells: a role in the intracellular trafficking of cholesterol supporting steroidogenesis. Exp Cell Res. 2000, 255: 56-66. 10.1006/excr.1999.4774.
Gevry NY, Lalli E, Sassone-Corsi P, Murphy BD: Regulation of niemann-pick c1 gene expression by the 3'5'-cyclic adenosine monophosphate pathway in steroidogenic cells. Mol Endocrinol. 2003, 17: 704-715. 10.1210/me.2002-0093.
Gevry NY, Murphy BD: The role and regulation of the Niemann-Pick C1 gene in adrenal steroidogenesis. Endocr Res. 2002, 28: 403-412. 10.1081/ERC-120016815.
Oonk RB, Krasnow JS, Beattie WG, Richards JS: Cyclic AMP-dependent and -independent regulation of cholesterol side chain cleavage cytochrome P-450 (P-450scc) in rat ovarian granulosa cells and corpora lutea. cDNA and deduced amino acid sequence of rat P-450scc. J Biol Chem. 1989, 264: 21934-21942.
Conley AJ, Kaminski MA, Dubowsky SA, Jablonka-Shariff A, Redmer DA, Reynolds LP: Immunohistochemical localization of 3 beta-hydroxysteroid dehydrogenase and P450 17 alpha-hydroxylase during follicular and luteal development in pigs, sheep, and cows. Biol Reprod. 1995, 52: 1081-1094.
Duncan WC, Cowen GM, Illingworth PJ: Steroidogenic enzyme expression in human corpora lutea in the presence and absence of exogenous human chorionic gonadotrophin (HCG). Mol Hum Reprod. 1999, 5: 291-298. 10.1093/molehr/5.4.291.
Devoto L, Vega M, Kohen P, Castro O, Carvallo P, Palomino A: Molecular regulation of progesterone secretion by the human corpus luteum throughout the menstrual cycle. J Reprod Immunol. 2002, 55: 11-20. 10.1016/S0165-0378(01)00133-4.
Duffy DM, Abdelgadir SE, Stott KR, Resko JA, Stouffer RL, Zelinski-Wooten MB: Androgen receptor mRNA expression in the rhesus monkey ovary. Endocrine. 1999, 11: 23-30. 10.1385/ENDO:11:1:23.
Christenson LK, Stouffer RL: Proliferation of microvascular endothelial cells in the primate corpus luteum during the menstrual cycle and simulated early pregnancy. Endocrinology. 1996, 137: 367-374. 10.1210/en.137.1.367.
Reynolds LP, Redmer DA: Growth and development of the corpus luteum. J Reprod Fertil Suppl. 1999, 54: 181-191.
Fraser HM, Wulff C: Angiogenesis in the primate ovary. Reprod Fertil Dev. 2001, 13: 557-566. 10.1071/RD01055.
Ravindranath N, Little-Ihrig L, Benyo DF, Zeleznik AJ: Role of luteinizing hormone in the expression of cholesterol side-chain cleavage cytochrome P450 and 3 beta-hydroxysteroid dehydrogenase, delta 5–4 isomerase messenger ribonucleic acids in the primate corpus luteum. Endocrinology. 1992, 131: 2065-2070. 10.1210/en.131.5.2065.
Belfiore CJ, Hawkins DE, Wiltbank MC, Niswender GD: Regulation of cytochrome P450scc synthesis and activity in the ovine corpus luteum. J Steroid Biochem Mol Biol. 1994, 51: 283-290. 10.1016/0960-0760(94)90041-8.
Wiltbank MC, Belfiore CJ, Niswender GD: Steroidogenic enzyme activity after acute activation of protein kinase (PK) A and PKC in ovine small and large luteal cells. Mol Cell Endocrinol. 1993, 97: 1-7. 10.1016/0303-7207(93)90205-X.
Chaffin CL, Dissen GA, Stouffer RL: Hormonal regulation of steroidogenic enzyme expression in granulosa cells during the peri-ovulatory interval in monkeys. Mol Hum Reprod. 2000, 6: 11-18. 10.1093/molehr/6.1.11.
Tuckey RC, Sadleir J: The concentration of adrenodoxin reductase limits cytochrome p450scc activity in the human placenta. Eur J Biochem. 1999, 263: 319-325. 10.1046/j.1432-1327.1999.00483.x.
Stocco DM: The role of the StAR protein in steroidogenesis: challenges for the future. J Endocrinol. 2000, 164: 247-253.
Kiriakidou M, McAllister JM, Sugawara T, Strauss JF: Expression of steroidogenic acute regulatory protein (StAR) in the human ovary. J Clin Endocrinol Metab. 1996, 81: 4122-4128. 10.1210/jc.81.11.4122.
Pescador N, Soumano K, Stocco DM, Price CA, Murphy BD: Steroidogenic acute regulatory protein in bovine corpora lutea. Biol Reprod. 1996, 55: 485-491.
Devoto L, Kohen P, Gonzalez RR, Castro O, Retamales I, Vega M, Carvallo P, Christenson LK, Strauss JF: Expression of steroidogenic acute regulatory protein in the human corpus luteum throughout the luteal phase. J Clin Endocrinol Metab. 2001, 86: 5633-5639. 10.1210/jc.86.11.5633.
Juengel JL, Meberg BM, Turzillo AM, Nett TM, Niswender GD: Hormonal regulation of messenger ribonucleic acid encoding steroidogenic acute regulatory protein in ovine corpora lutea. Endocrinology. 1995, 136: 5423-5429. 10.1210/en.136.12.5423.
Kohen P, Castro O, Palomino A, Munoz A, Christenson LK, Strauss JF, Devoto L: The steroidogenic response and corpus luteum expression of the steroidogenic acute regulatory protein (StAR) following hCG administration at different times in the human luteal phase. J Clin Endocrinol Metab. 2003
Retamales I, Carrasco I, Troncoso JL, Las Heras J, Devoto L, Vega M: Morpho-functional study of human luteal cell subpopulations. Hum Reprod. 1994, 9: 591-596.
Hutchison JS, Zeleznik AJ: The corpus luteum of the primate menstrual cycle is capable of recovering from a transient withdrawal of pituitary gonadotropin support. Endocrinology. 1985, 117: 1043-1049.
Sandhoff TW, McLean MP: Repression of the rat steroidogenic acute regulatory (StAR) protein gene by PGF2alpha is modulated by the negative transcription factor DAX-1. Endocrine. 1999, 10: 83-91. 10.1385/ENDO:10:1:83.
Christenson LK, Strauss JF: Steroidogenic acute regulatory protein (StAR) and the intramitochondrial translocation of cholesterol. Biochim Biophys Acta. 2000, 1529: 175-187. 10.1016/S1388-1981(00)00147-5.
Stocco DM, Clark BJ, Reinhart AJ, Williams SC, Dyson M, Dassi B, Walsh LP, Manna PR, Wang X, Zeleznik AJ, Orly J: Elements involved in the regulation of the StAR gene. Mol Cell Endocrinol. 2001, 177: 55-59. 10.1016/S0303-7207(01)00423-3.
Reinhart AJ, Williams SC, Stocco DM: Transcriptional regulation of the StAR gene. Molecular and Cellular Endocrinology. 1999, 151: 161-169. 10.1016/S0303-7207(98)00257-3.
Christenson LK, Strauss JF: Steroidogenic acute regulatory protein: an update on its regulation and mechanism of action. Arch Med Res. 2001, 32: 576-586. 10.1016/S0188-4409(01)00338-1.
Sugawara T, Lin D, Holt JA, Martin KO, Javitt NB, Miller WL, Strauss JF: Structure of the human steroidogenic acute regulatory protein (StAR) gene: StAR stimulates mitochondrial cholesterol 27-hydroxylase activity. Biochemistry. 1995, 34: 12506-12512.
Manna PR, Dyson MT, Eubank DW, Clark BJ, Lalli E, Sassone-Corsi P, Zeleznik AJ, Stocco DM: Regulation of steroidogenesis and the steroidogenic acute regulatory protein by a member of the cAMP response-element binding protein family. Mol Endocrinol. 2002, 16: 184-199. 10.1210/me.16.1.184.
Parker KL, Schimmer BP: Steroidogenic factor 1: a key determinant of endocrine development and function. Endocr Rev. 1997, 18: 361-377. 10.1210/er.18.3.361.
Sugawara T, Kiriakidou M, McAllister JM, Kallen CB, Strauss JF: Multiple steroidogenic factor 1 binding elements in the human steroidogenic acute regulatory protein gene 5'-flanking region are required for maximal promoter activity and cyclic AMP responsiveness. Biochemistry. 1997, 36: 7249-7255. 10.1021/bi9628984.
Sandhoff TW, Hales DB, Hales KH, McLean MP: Transcriptional regulation of the rat steroidogenic acute regulatory protein gene by steroidogenic factor 1. Endocrinology. 1998, 139: 4820-4831. 10.1210/en.139.12.4820.
Wooton-Kee CR, Clark BJ: Steroidogenic factor-1 influences protein-deoxyribonucleic acid interactions within the cyclic adenosine 3,5-monophosphate-responsive regions of the murine steroidogenic acute regulatory protein gene. Endocrinology. 2000, 141: 1345-1355. 10.1210/en.141.4.1345.
Rust W, Stedronsky K, Tillmann G, Morley S, Walther N, Ivell R: The role of SF-1/Ad4BP in the control of the bovine gene for the steroidogenic acute regulatory (StAR) protein. J Mol Endocrinol. 1998, 21: 189-200.
LaVoie HA, Garmey JC, Veldhuis JD: Mechanisms of insulin-like growth factor I augmentation of follicle-stimulating hormone-induced porcine steroidogenic acute regulatory protein gene promoter activity in granulosa cells. Endocrinology. 1999, 140: 146-153. 10.1210/en.140.1.146.
Christenson LK, McAllister JM, Martin KO, Javitt NB, Osborne TF, Strauss JF: Oxysterol regulation of steroidogenic acute regulatory protein gene expression. Structural specificity and transcriptional and posttranscriptional actions. J Biol Chem. 1998, 273: 30729-30735. 10.1074/jbc.273.46.30729.
Hammer GD, Krylova I, Zhang Y, Darimont BD, Simpson K, Weigel NL, Ingraham HA: Phosphorylation of the nuclear receptor SF-1 modulates cofactor recruitment: integration of hormone signaling in reproduction and stress. Mol Cell. 1999, 3: 521-526.
Ito M, Yu RN, Jameson JL: Steroidogenic factor-1 contains a carboxy-terminal transcriptional activation domain that interacts with steroid receptor coactivator-1. Mol Endocrinol. 1998, 12: 290-301. 10.1210/me.12.2.290.
Liu DL, Liu WZ, Li QL, Wang HM, Qian D, Treuter E, Zhu C: Expression and functional analysis of liver receptor homologue-1 as a potential steroidogenic factor in rat ovary. Biol Reprod. 2003
Galarneau L, Pare JF, Allard D, Hamel D, Levesque L, Tugwood JD, Green S, Belanger L: The alpha1-fetoprotein locus is activated by a nuclear receptor of the Drosophila FTZ-F1 family. Mol Cell Biol. 1996, 16: 3853-3865.
Falender AE, Lanz RB, Malenfant D, Belanger L, Richards JS: Differential expression of SF-1 and FTF/LRH-1 in the rodent ovary. Endocrinology. 2003, doi:10.1210:
Christenson LK, Johnson PF, McAllister JM, Strauss JF: CCAAT/enhancer-binding proteins regulate expression of the human steroidogenic acute regulatory protein (StAR) gene. J Biol Chem. 1999, 274: 26591-26598. 10.1074/jbc.274.37.26591.
Silverman E, Eimerl S, Orly J: CCAAT enhancer-binding protein beta and GATA-4 binding regions within the promoter of the steroidogenic acute regulatory protein (StAR) gene are required for transcription in rat ovarian cells. Journal of Biological Chemistry. 1999, 274: 17987-17996. 10.1074/jbc.274.25.17987.
Sirois J, Richards JS: Transcriptional regulation of the rat prostaglandin endoperoxide synthase 2 gene in granulosa cells. Evidence for the role of a cis-acting C/EBP beta promoter element. J Biol Chem. 1993, 268: 21931-21938.
Tremblay JJ, Hamel F, Viger RS: Protein kinase A-dependent cooperation between GATA and CCAAT/enhancer-binding protein transcription factors regulates steroidogenic acute regulatory protein promoter activity. Endocrinology. 2002, 143: 3935-3945. 10.1210/en.2002-220413.
Tremblay JJ, Viger RS: Transcription factor GATA-4 is activated by phosphorylation of serine 261 via the cAMP/PKA signaling pathway in gonadal cells. J Biol Chem. 2003
Schwarzenbach H, Manna PR, Stocco DM, Chakrabarti G, Mukhopadhyay AK: Stimulatory Effect of Progesterone on the Expression of Steroidogenic Acute Regulatory Protein in MA-10 Leydig Cells. Biol Reprod. 2003, 68: 1054-1063.
Hild-Petito S, Stouffer RL, Brenner RM: Immunocytochemical localization of estradiol and progesterone receptors in the monkey ovary throughout the menstrual cycle. Endocrinology. 1988, 123: 2896-2905.
Arakane F, Kallen CB, Watari H, Foster JA, Sepuri NB, Pain D, Stayrook SE, Lewis M, Gerton GL, Strauss JF: The mechanism of action of steroidogenic acute regulatory protein (StAR). StAR acts on the outside of mitochondria to stimulate steroidogenesis. J Biol Chem. 1998, 273: 16339-16345. 10.1074/jbc.273.26.16339.
Strauss JFd, Kallen CB, Christenson LK, Watari H, Devoto L, Arakane F, Kiriakidou M, Sugawara T: The steroidogenic acute regulatory protein (StAR): A window into the complexities of intracellular cholesterol trafficking. Recent Progress in Hormone Research. 1999, 54: 369-395.
Bose H, Lingappa VR, Miller WL: Rapid regulation of steroidogenesis by mitochondrial protein import. Nature. 2002, 417: 87-91. 10.1038/417087a.
Hauet T, Liu J, Li H, Gazouli M, Culty M, Papadopoulos V: PBR, StAR, and PKA: partners in cholesterol transport in steroidogenic cells. Endocr Res. 2002, 28: 395-401. 10.1081/ERC-120016814.
Miller WL, Strauss JF: Molecular pathology and mechanism of action of the steroidogenic acute regulatory protein, StAR. J Steroid Biochem Mol Biol. 1999, 69: 131-141. 10.1016/S0960-0760(98)00153-8.
West LA, Horvat RD, Roess DA, Barisas BG, Juengel JL, Niswender GD: Steroidogenic acute regulatory protein and peripheral-type benzodiazepine receptor associate at the mitochondrial membrane. Endocrinology. 2001, 142: 502-505. 10.1210/en.142.1.502.
Sanders SL, Stouffer RL: Localization of steroidogenic enzymes in macaque luteal tissue during the menstrual cycle and simulated early pregnancy: immunohistochemical evidence supporting the two-cell model for estrogen production in the primate corpus luteum. Biol Reprod. 1997, 56: 1077-1087.
Duffy DM, Chaffin CL, Stouffer RL: Expression of estrogen receptor alpha and beta in the rhesus monkey corpus luteum during the menstrual cycle: regulation by luteinizing hormone and progesterone. Endocrinology. 2000, 141: 1711-1717. 10.1210/en.141.5.1711.
Goyeneche AA, Deis RP, Gibori G, Telleria CM: Progesterone promotes survival of the rat corpus luteum in the absence of cognate receptors. Biol Reprod. 2003, 68: 151-158.
Goyeneche AA, Calvo V, Gibori G, Telleria CM: Androstenedione interferes in luteal regression by inhibiting apoptosis and stimulating progesterone production. Biol Reprod. 2002, 66: 1540-1547.
Chaffin CL, Stouffer RL, Duffy DM: Gonadotropin and steroid regulation of steroid receptor and aryl hydrocarbon receptor messenger ribonucleic acid in macaque granulosa cells during the periovulatory interval. Endocrinology. 1999, 140: 4753-4760. 10.1210/en.140.10.4753.
This work was supported in part by a Mellon Foundation Grant (LKC) and by NIH grant TW001485 (LD) and FONDAP grant 150–10006 (LD). We would like to thank Drs. A. G. Brown, J.R. Wood, J.F. Strauss, III and C. L. Chaffin for reading and constructive comments regarding this manuscript.
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
Rights and permissions
About this article
Cite this article
Christenson, L.K., Devoto, L. Cholesterol transport and steroidogenesis by the corpus luteum. Reprod Biol Endocrinol 1, 90 (2003). https://doi.org/10.1186/1477-7827-1-90
- Granulosa Cell
- Corpus Luteum
- Luteal Cell
- Luteal Function
- Cytoplasmic Lipid Droplet