IMMUNOCYTOCHEMICAL LOCALIZATION OF THE PREHORMONE CONVERTASES PC1 ANDPC2 IN RAT PROLACTIN CELLS

The prohormone convertases PC1 and PC2 are subtilisin-related endopept idases that process prohormone and neuropeptide precursors. Using diff erent ultrastructural immunocytochemical approaches, we have investiga ted their intracellular distribution in a neuroendocrine cell type tha t has not been examined thus far, the rat anterior pituitary lactotrop e. These cells secrete mainly prolactin and also express the neuroendo crine-specific protein secretogranin ii, which is considered a peptide precursor. Our study provides evidence for the expression of PC1 and PC2 in rat lactotropes and provides new information on their subcellul ar localization. Apart from their presence in the secretory granules, PC1 and PC2 displayed different major localization along the secretory pathway. PC1 immunoreactivity was concentrated in the Golgi apparatus , whereas PCZ immunoreactivity was prominent in the rough endoplasmic reticulum (RER). These observations provide morphological support for previous biochemical analysis of proPC1 and proPC2 post-translational processing, which has demonstrated that PC1 exits very rapidly from th e RER, whereas PCZ is retained much longer in this compartment.