Ontogeny of the prohormone convertases PC1 and PC2 in the mouse hypophysis and their colocalization with corticotropin and alpha-melanotropin.

In the adult pituitary, anterior lobe corticotrophs and intermediate lobe melanotrophs differentially process proopiomelanocortin (POMC). Within the corticotrophs, POMC is processed mainly to corticotropin (ACTH) and beta-lipotropin, while alpha-melanotropin (alpha MSH) and beta-endorphin are the major end products in the melanotrophs. The observed transient presence of alpha MSH-like immunoreactivity during ontogeny suggested an age-dependent variation in POMC processing in the adenohypophysis. In this tissue, cell-specific POMC products are likely the result of differential expression of the two known prohormone convertases PC1 and PC2. In the present ontogeny study done in the mouse intermediate and anterior pituitary, we examined how the expression pattern of PC1 and PC2 mRNA transcripts correlates with that of ACTH and alpha MSH-like immunoreactivities. Our data demonstrated that both PC1 and PC2 transcripts can be detected in the presumptive adenohypophysis starting on embryonic day 15 (E15). In the intermediate lobe, PC1 and PC2 mRNAs appear on E18 and E16, respectively, and their levels increased during ontogeny, reaching maximal expression in the adult. Similarly, PC1 expression in the anterior pituitary increased from E15 to adulthood. However, PC2 mRNA expression peaked between postnatal days 1 (P1) and 14 (P14) and then decreased to adult levels. The distribution of PC1 and PC2 immunoreactivity is nicely correlated with the in situ hybridization data. In the anterior lobe, during the P1-P14 postnatal period, PC2 immunoreactivity was detected within cells synthesizing an alpha MSH-like peptide(s). This observation substantiates our earlier biochemical data suggesting that PC2 is the important convertase in the processing of POMC into alpha MSH. Furthermore, the demonstrated variation in the relative ratio of PC1/PC2 expression during ontogeny rationalizes the observed plasticity of POMC processing in the adenohypophysis. It is expected that beta-endorphin processing will follow that of alpha MSH.