Regulation of cellular α-MSH and β-endorphin during stimulated secretion from intermediate pituitary cells: involvement of aspartyl and cysteine proteases in the control of cellular levels of α-MSH and β-endorphin

Abstract

The regulation of cellular levels of α-melanocyte stimulating factor (α-MSH) and β-endorphin in response to stimulated secretion from intermediate pituitary cells in primary culture was investigated in this study. Regulation of the cell content of α-MSH and β-endorphin occurred in two phases consisting of (a) initial depletion of cellular levels of these peptide hormones during short-term secretion (3 h) induced by isoproterenol, forskolin, or phorbol myristate acetate (PMA) which was followed by (b) long-term (24 h) increases in cellular levels of α-MSH and β-endorphin in response to stimulated secretion induced by isoproterenol and PMA. In short-term experiments (3 h), cellular levels of α-MSH and β-endorphin were reduced by 30–50% during stimulated secretion of these peptide hormones by isoproterenol (agonist for the β-adrenergic receptor), forskolin that activates protein kinase A (PKA), and PMA that activates protein kinase C (PKC). Moreover, dopamine inhibited isoproterenol-induced depletion of cellular α-MSH and β-endorphin. During long-term incubation of cells (24 h) with isoproterenol, cellular α-MSH and β-endorphin were increased to twice that of controls (unstimulated cells). Treatment with PMA for 24 h also increased cellular levels of α-MSH and β-endorphin. Moreover, cellular levels of α-MSH and β-endorphin were decreased during long-term treatment of cells with an aspartyl protease inhibitor, pepstatin A, and with the cysteine protease inhibitor E64c. These results implicate aspartyl and cysteine proteases in the cellular production of α-MSH and β-endorphin that requires proteolytic processing of their common precursor proopiomelanocortin (POMC). These findings demonstrate the parallel regulation of cellular levels of α-MSH and β-endorphin during their cosecretion, which may involve aspartyl and cysteine proteases in the metabolism of these peptide hormones.