Loss of mouse fibroblast cell response to phorbol esters restored by microinjected protein kinase C

The phorbol esters in addition to being among the most potent mouse skin tumour promoters profoundly affect many different biological systems ^1,2. It is postulated that they act through activation of protein kinase C (ref. 3-5), but substantial heterogeneity in their pharmacological and binding behaviour in some systems⁶ has caused concern about whether this is their only target. Evidence linking protein kinase C activation with biological responses to the phorbol esters includes similarity in structure-activity relations for binding and response⁷; in vitro phosphorylation of specific proteins by protein kinase C at the same sites at which phorbol ester treatment induces phosphorylation in intact cells ³; and correlation in certain cell types between down regulation of protein kinase C on chronic phorbol ester treatment and loss of cellular responsiveness to the phorbol ester^8-10. Here we report that micro-injection of purified protein kinase C into Swiss 3T3 fibroblasts pretreated with the phorbol ester phorbol 12,13-diburyrate (PDBu) restores the mitogenic response of the cells to PDBu, directly establishing the involvement of protein kinase C in this response.