Expression of cyclooxygenase-II (COX-II) and 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD)/prostaglandin F-synthase (PGFS) in bovine placentomes: implications for the initiation of parturition in cattle.

The chain of events leading to prepartal luteolysis in cattle is not yet fully understood. Prostaglandin F(2alpha) (PGF(2alpha)) seems to be a major factor involved. However, only little information is available about the underlying regulatory mechanisms. Consequently, the expression of cyclooxygenase-II (COX-II) and 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), an enzyme recently shown to be most likely responsible for the production of luteolytic PGF(2alpha) in the endometrium of cyclic cows, was investigated in bovine placentomes. Immunohistochemical methods were applied to placentomes from 17 pregnant cows between days 100 and 284, from three cows during the prepartal progesterone decrease (days 273-282) and from five parturient cows. COX-II was found in uninucleated trophoblast cells (UTC) from day 100 until parturition. However, between days 100 and 235 expression was only weak to moderate, focal and mainly restricted to the chorionic plate and adjacent basal parts of chorionic stem villi. In placentomes from a 270 and a 284 day pregnant cow, in which the prepartal decline of progesterone levels had not started yet, staining had substantially increased and extended to secondary and tertiary chorionic villi. In prepartal and parturient cows strong to intense staining was present in UTC all over the villous tree. Real time RT-PCR confirmed an extensive pre- and intrapartal rise of COX-II expression in bovine placentomes with a 70-100-fold increase of COX-II-mRNA levels. 20alpha-HSD/PGFS was widely expressed in UTC of chorionic villi. Like COX-II it was down-regulated in UTC differentiating into trophoblast giant cells. Immunostaining pattern did not change remarkably during the period under investigation, and 20alpha-HSD/PGFS-mRNA levels increased only 2.6-fold in the prepartal phase. Thus, in UTC PGF(2alpha) may be produced via COX-II and 20alpha-HSD/PGFS, but only COX-II may be substantially involved in the control of a putative prepartal placentomal output of luteolytic PGs, whereas 20alpha-HSD/PGFS seems to be expressed in a more constitutive manner.