Proteolytic processing of a plasma membrane-bound precursor to human macrophage colony-stimulating factor (CSF-1) is accelerated by phorbol ester.

A transmembrane precursor to human macrophage colony-stimulating factor (M-CSF, CSF-1) is stably expressed at the cell surface where it is slowly and inefficiently cleaved to yield a soluble form of the growth factor. Incubation in the presence of phorbol ester resulted in rapid cleavage of the plasma membrane-bound precursor and release of soluble CSF-1. Within 60 min after phorbol treatment the quantity of growth factor recovered in the medium was more than 30-fold greater than that observed in the absence of the agent. The growth factor released in the presence of phorbol was biologically active and exhibited the same electrophoretic mobility as that obtained in the absence of the drug. Phorbol ester-accelerated processing of the cell surface CSF-1 precursor was abrogated by long-term exposure to phorbol, but was not inhibited by pretreatment with cycloheximide or incubation in serum-free medium. These results suggest that the enhanced post-translational processing of the CSF-1 precursor resulted from activation of a pre-existing cellular protease via a mechanism involving phorbol ester-mediated stimulation of protein kinase C.