In domestic ruminants (sheep, cattle, goats) estrous cyclicity is regulated by uterine PGF2α-induced demise of the corpus luteum [1, 2]. PGF2α is released from endometrial luminal and superficial glandular epithelium in an episodic fashion toward the end of the estrous cycle. Oxytocin (OT), of neurohypophyseal and luteal origin, is believed to bind endometrial OT receptors and initiate pulsatile PGF2α secretion, which in turn, stimulates release of luteal OT and creates a positive feedback loop that results in several series of pulses of short duration which are effective in causing luteolysis [3, 4]. During early pregnancy, the conceptus protein interferon tau (IFNT) binds to endometrial receptors and attenuates episodic PGF2α secretion, thereby rescuing the CL from regression and maintaining progesterone production [5–7].
Release of arachidonic acid (AA) from membrane glycerophospholipids is catalyzed by phospholipase A2 enzymes and is considered the rate-limiting step in PG biosynthesis . Released AA is then metabolized to PGG2 by a cyclooxygenase reaction and then to PGH2 by a peroxidase reaction, both mediated by cyclooxygenase (COX) -1 and/or -2. Terminal PG synthases, which exhibit tissue-specific distribution, then convert PGH2 to bioactive PGs including PGF2α, PGE2, PGD2 and PGI2.
Although PLA2 activation catalyzes the rate-limiting step in PG biosynthesis, most investigations of the mechanisms and regulation of uterine PG production have focused on the activation and expression of down-stream enzymes, such as the cyclooxygenases and PG synthases. The few studies [9–12] that have examined the role of PLA2 in uterine PG production in domestic ruminants have focused on a single enzyme, cPLA2α (Group IVA PLA2 or PLA2G4A). Surprisingly, none of these studies identified consistent changes in expression of protein or mRNA for PLA2G4A in association with alterations in PG production. In contrast, non-specific PLA2 inhibitors significantly diminished both basal and agonist-induced PG production [10–12].
Mammalian cells contain structurally diverse PLA2 enzymes that differ in their location and regulation [reviewed in  and ]. At least 14 groups, and numerous subgroups, of PLA2s have been classified. These groups include four main families of PLA2s, the low molecular weight secretory PLA2s, the cytosolic PLA2G4s, the calcium-independent PLA2G6s and the platelet activating factor (PAF) acid hydrolases (PLA2G7 and PLA2G8). The intracellular PLA2G4 and PLA2G6 enzymes are specifically important in the regulation of PG biosynthesis because their sites of action are the perinuclear membranes where down-stream AA metabolizing enzymes (cyclooxygenases and PG synthases) reside.
The first cytosolic PLA2 discovered was PLA2G4A and it has received, by far, the most attention. It is a 749 amino acid, 85 kDa protein that migrates at 100–112 kDa in polyacrylamide gels. It is present in the cytosol of resting cells and upon activation by a variety of agonists, undergoes Ca++-directed translocation to perinuclear membranes. The N-terminal Ca++-dependent lipid binding (CaLB) or C2 domain is essential for membrane association and responsible for translocation in response to stimuli that increase intracellular calcium [14+]. Gene disruption studies have indicated an important role for PLA2G4A in normal fertility, eicosanoid production from inflammatory cells, brain injury and allergic response in mice [15–17]. Mice with a null mutation for the gene have fewer implantation sites, smaller litters and fail to undergo labor at term.
Several additional Group IV PLA2 enzymes have been cloned recently, including IVB, IVC, IVD, IVE and IVF [18–20]. Of these enzymes, IVC (PLA2γ) is of interest because it is constitutively associated with membranes, the major site of prostaglandin biosynthesis, lacks the C2 domain and does not require Ca++ for activation. Although first associated with membrane remodeling, recent studies demonstrate that PLA2G4C functions as a signaling PLA2 linked to PGE2 production .
In the literature, the term calcium-independent PLA2, has generally referred to the Group 6 family of PLA2 enzymes , despite the fact that Group 4C is Ca++-independent as well. Two distinct subclasses of Group 6 have been identified and characterized, 6A (also referred to as iPLA2 and iPLA2B) and 6B (iPLA2γ). More recently several novel iPLA2s have been identified, including iPLA2δ, iPLA2ε, iPLA2ζ and iPLA2η. These novel iPLA2 family members display primarily lysophospholipase, triacylglycerol lipase and/or transacylase activities rather than PLA2 activity. Group 6A has been cloned and characterized in several species and cell types [23–25]. It has a mass of 85–88 kDa, contains 7–8 terminal ankyrin repeats, and a consensus lipase motif. Several splice variants of the enzyme are expressed yielding multiple molecular weight isoforms . Recently, expression of a 50 kDa Group 6 enzyme in rat myometrium was associated with agonist-induced uterine contractions .
All PLA2 enzymes catalyze the hydrolysis of the sn-2 ester bond of phospholipids to produce free fatty acids and lsophospholipid [13, 14]. A distinguishing characteristic of PLA2G4A is that it exhibits greater selectivity for lipids containing arachidonic acid than other cytosolic PLA2s. For this reason, PLA2G4A has been the enzyme primarily associated with direct generation of arachidonic acid and initiation of the eicosanoid cascade. However, PLA2G4C  and PLA2G6 [32, 33] have been associated with agonist-stimulated arachidonic acid release and eicosanoid biosynthesis. In addition, it has been shown that PLA2G6 may mediate release of arachidonic acid directly by hydrolysis of arachidonate-containing plasmalogens or through an indirect, multi-step process involving hydrolysis of diacyl lipids to arachidonyl lysophosphatidylcholine .
The objectives of the present study were to investigate the expression and activities of Group 4A, 4C and 6A PLA2 enzymes in association with PGF2α and PGE2 production in bovine endometrial epithelial cell cultures. Results indicate that in addition to PLA2G4A, PLA2G6 and PLA2G4C contribute to regulation of uterine cell PG production and these latter two enzymes are activated in response to oxytocin and INFT, respectively.