Possible Anti-Apoptotic Roles Of Prostaglandins in Bovine Luteal Cells

Abstract

Regression of the corpus luteum (CL) is characterized by loss of the capacity to synthesize and secrete progesterone (P4) (functional luteolysis) and tissue degradation by apoptosis (structural luteolysis). Luteinizing hormone (LH) is one of the most potent stimulators in progesterone (P4) production by the corpus luteum (CL) of most mammals, including cow. The action of LH on luteal cells is mainly mediated by cAMP which is induced by adenylate cylclase activation. This pathway is considered to be a primary second messenger of LH action. Recently, we have demonstrated that P4 and prostaglandins produced by CL suppress apoptosis of bovine luteal cells. Furthermore, our previous study showed that 8-bromo-cAMP prevents apoptotic cell death in cultured bovine luteal cells. Based on the above findings, we hypothesized that LH suppresses apoptosis of bovine luteal steroidogenic cells (LSC). In order to confirm the hypothesis, we examined the effect of LH on cell death and expression of crucial apoptosis related proteins in LSC. In addition, to investigate whether the anti-apoptotic effect of LH is mediated via stimulating P4 or via intracellular cAMP and/or phospholipase C (PLC), we examined the effects of a P4 antagonist, an adenylate cyclase and PLC inhibitor on the cell viability of LH-treated LSC. Cultured bovine LSC obtained from the CL at the mid stage (Days 8–12 of the cycle) were treated for 24 h with recombinant human tumor necrosis factor alpha (TNF; 2.9 nM) and recombinant bovine interferon gamma (IFNG; 2.5 nM) in the presence or absence of LH (0.34 nM). LH reduced the levels of cell death in the LSC induced by TNF and IFNG (P<0.05) as demonstrated by the WST-1 colorimetric assay. Furthermore, DNA fragmentation induced by TNF and IFNG was observed to be suppressed by LH as demonstrated by the TUNEL assay. A real-time quantitative RT PCR analysis revealed that LH attenuated mRNA expression of apoptosis related proteins, i.e. FAS, BAX and CASP3, as well as FAS and cleaved CASP3 protein expressions (P<0.05) as quantified by Western immunoblot in TNF and IFNG treated cells. Moreover, LH increased mRNA expression of BCL2 and BCL2:BAX mRNA ratio (P<0.05), whereas LH did not affect the BAX and BCL2 protein expression and BCL2:BAX protein ratio. Exposure to a specific P4 receptor antagonist (onapristone; 100 μM) and PLC inhibitor (U73122; 10 μM) suppressed the anti-apoptotic effects of LH in the absence and presence of TNF and IFNG. Exposure to a specific adenylate cyclase inhibitor (2’,5’- dideoxyadenosine, DDA; 100 nM) reduced cell viability in LH-treated cells in the absence or presence of TNF and IFNG (P<0.05). In conclusion, LH affects viability of luteal cells as an anti-apoptotic factor in bovine CL, (1) by stimulating P4 production, i.e. LH-stimulated P4 via PLC suppresses apoptosis, and (2) by directly regulating the mRNA expression of FAS, BAX, BCL2 and CASP3, and FAS and cleaved CASP3 protein expressions via cAMP signaling pathway. The overall findings suggest that LH plays luteoprotective roles in bovine CL by suppressing apoptosis of LSC.