Selective Increased Luteal Prostaglandin E2 Compared to F2 Alpha Biosynthesis and Signaling Are Critical for Luteal Protection and Resistance During Establishment of Pregnancy in Sheep.

Abstract

In ruminants, pulsatile secretion of prostaglandin F2 alpha (PGF2a) by the endometrium is locally transported to corpus luteum (CL) and stimulates intraluteal PGF2a biosynthesis and induces luteal regression/luteolysis. Interestinlgy, luteolytic PGF2a fails to accomplish regression of CL of early pregnancy. This increased resistance of the CL to PGF2a may suggest existence of luteal protective mechanisms. Early physiological studies proposed prostaglandin (E2) PGE2 as luteal protective mediator because PGE2 counteracted the effects of PGF2a and protected the CL from regression. However, the molecular and cellular aspects of resistance of the CL to luteolytic PGF2a are largely unknown. The objective of this study is to determine biosynthesis, transport, and signaling of PGF2a and PGE2 in CL during the estrous cycle and early pregnancy in sheep. Suffolk cross-bred ewes were ovariohysterectomized on days 12, 14 and 16 of the estrous cycle (ECY) or pregnancy (PNY), the CLs were collected, and temporal expression of PGF2a and PGE2 biosynthetic, transport and signaling proteins were determiend by western blot. Blood samples were collected from the ovarian artery and ovarian vein and transport of PGF2a, PGFM, PGE2, and PGEM were determiend by ELISA. Results indicated that: (i) COX-2 protein was abundantly expressed on days 12-16 of ECY and PNY and not affected by the ECY or PNY status, and COX-1 protein was expressed at very low level; (ii) PGES-1 protein was abundantly expressed on days 12-14 of ECY and PNY, and then decreased (4 fold) on day 16 of the ECY compared to that of PNY, expression of PGES-2 protein was not modulated by ECY and PNY, and expression of PGES-3 protein was decreased (5 fold) on days 12-16 of ECY compared to that of PNY; (iii) expression of PGFS-AKR1C1 protein was not modulated by ECY and PNY, and PGFS-AKR1C3 protein was expressed at steady sate level on day 10-16 of ECY but decreased (several fold) on days 12-16 of PNY; (iv) expression of FP, EP1 and EP3 proteins were not modulated by ECY and PNY, but interestingly, expression of EP2 and EP4 proteins were decreased (4 fold) on day 16 of ECY compared to that of PNY; (v) PGT protein was abundantly expressed on days 12-16 of ECY and PNY and its expression was decreased on day 16 of ECY compared to that of PNY; and (vi) PGDH protein expression was increased on days 14-16 of PNY compared to that of ECY. (vi) Ovarian venous plasma concentrations of PGF2a, PGFM, PGE2 and PGEM were 1500-2000 pg/ml, 400-500 pg/ml, 1000-1250 pg/ml, and 2-5 pg/ml on days 14-16 of the estrous cycle, respectively, and 400-500 pg/ml, 2000-2500 pg/ml, 3000-3500 pg/ml, and 3-5 pg/ml on days 14-16 of pregnancy, respectively. These results together suggest that the luteal PG biosynthetic and signaling machinery is selectively directed towards PGF2a at the time of luteolysis, by contrast; towards PGE2 at the time of establishment of pregnancy. In addition, CL of pregnancy has innate capacity to catabolize PGF2 to inactive PGFM but not PGE2 to PGEM and favors intraluteal PGE2 production. Thus, increased intraluteal PGE2 production and action are the critical mechanisms that resist/protect CL of early pregnancy to luteolytic PGF2a. Selective inhibition of PGES-1 or EP2 and EP4 along with PGF2a could be the potential future therapeutic strategy to improve reproductive efficiency in ruminants. Supported by USDA award 2008-35203-19101 to JAA. (poster)