Expression of Cytosolic Phospholipase A2 in Equine Endometrium during the Oestrous Cycle and Early Pregnancy

Abstract

Phospholipase A2 (PLA2) is a key enzyme for biosynthesis of PGF2α. Real-time RT-PCR and immunohistochemistry were used to determine transcription and cellular distribution of cytosolic PLA2 (cPLA2) in the equine endometrium during the oestrous cycle and early pregnancy. Endometrial biopsy and blood samples were collected from cycling mares on days (d) 8, 11, 15 and 18 (oestrus) (Day 0 = Day of ovulation; n = 5 for each day) and from pregnant mares (n = 4) on d15. Except for mares on d18 and some cyclic mares (n = 2) on d15 with low progesterone (P₄) concentrations (< 3.18 nm), P₄ concentrations were high. Cytosolic PLA2 was mainly localized in the luminal epithelium and stroma was negative. Cytosolic PLA2 expression was negatively correlated with P₄ concentration (r = -0.75, P < 0.001) and differed according to the stage of the oestrous cycle (P < 0.05). Cytosolic PLA2 expression was high during oestrus and declined to basal levels on d8 (P < 0.05). Thereafter, there was a trend for increased cPLA2 expression as luteal phase progressed. However, as P₄ dropped below 3.18 nm on d15, cPLA2 expression increased (P < 0.05). In pregnant mares, cPLA2 expression was not different from cyclic mares on d15 with high P₄ concentrations. However, it was lower than cyclic mares with low P₄ concentrations (p < 0.05). In conclusion, cPLA2 isoform, a member of the PLA2 family that controls PGF2α secretion, is highly expressed in the endometrium at the expected time of luteolysis. Progesterone may be an inhibitor for cPLA2 expression. During pregnancy, cPLA2 expression may be sufficient to initiate the cascade for PGF2α secretion and does not play a direct role in maternal recognition of pregnancy.