Expression of proopiomelanocortin and prohormone convertase-1 and -2 in the late gestation fetal sheep pituitary

The biological activity of fetal plasma immunoreactive ACTH has been reported to increase during the final weeks of gestation in fetal sheep, indicative of enhanced processing of POMC to ACTH. The present study was aimed at examining the expression and localization of the prohormone convertases, PC1 and PC2, in the pituitary of fetal sheep during the final weeks of gestation. Pituitaries were obtained from fetal sheep during the final 50 days gestation (dGA) at 100-107 dGA (n = 6), 117-121 dGA (n = 6), 126-130 dGA (n = 7), and 144-147 dGA (n = 8; term = similar to 148 dGA). Pituitaries were cryosectioned and subjected to dual labeling in situ hybridization using S-35-labeled PC1 and/or PC2 complementary RNA probes with a digoxigenin-labeled POMC complementary RNA to localize and quantify PC1 and PC2 messenger RNA (mRNA) in POMC-hybridizing cells. Immunocytochemistry was also performed to assess coexpression of PC1 and PC2 with ACTH in the fetal pituitary. PC1 mRNA was heterogeneously distributed in the anterior pituitary (AP) at all gestational ages examined, with hybridization signals observed over POMC-expressing cells (corticotropes) as well as over noncorticotrope phenotypes. The inferior region of the AP contained an approximately 3-fold greater (P < 0.01) percentage of POMC cells containing PC1 transcripts compared with the superior region of the AP. The proportion of POMC cells containing PC1 was significantly higher (P < 0.01) in the 100-107 dGA and 144-147 dGA groups than in the 117-121 dGA and 126-130 dGA groups in both inferior and superior AP. The intensity of the PC1 hybridization signal over POMC-expressing cells was also about 2- to 4-fold greater (P < 0.01) in the inferior compared with the superior region of the fetal AP; the intensity of the PC1 hybridization signal associated with POMC cells remained constant within the AP region and did not change over the gestational ages examined. Hybridization for PC1 was highly variable over regions of AP not hybridizing for POMC, probably due to differences in the level of mRNA for PC1 between phenotypes. Similar to POMC cells, the average hybridization signal for PC1 over non-POMC-hybridizing regions was about P-fold greater in the inferior vs. superior AP. A weak PC2 hybridization signal was observed over a small number of unidentified phenotypes in the fetal AP at all ages examined; no POMC cells were found to contain PC2 hybridization signal. In the neurointermediate lobe, POMC, PC1, and PC2 were ubiquitously expressed at all ages. Levels of PC1 and PC2 mRNA in the fetal neurointermediate lobe did not change over the period of gestation examined. Immunocytochemical analysis of PC1 and PC2 with ACTH confirmed the pattern of expression and the extent of coexpression observed with in situ hybridization methods. We conclude that both PC1 and PC2 are likely to contribute to POMC processing in the fetal pituitary during the final weeks of gestation.