PMA Inhibits the Growth of Human Fibroblasts after the Induction of Immediate-Early Genes

Abstract

WS-1 cells, human fibroblasts derived from embryonic skin, become quiescent in serum-free medium. Addition of serum or thrombin stimulates incorporation of 3 H-labeled thymidine 16-40 h later. The tumor promoter phorbol 12-myristate 13-acetate (PMA) inhibited this response in quiescent cells stimulated with serum or thrombin. PMA also inhibited thymidine incorporation in rapidly growing WS-1 cells. Since the inhibition caused by PMA could result from activation of protein kinase C (PKC) or the down-regulation of PKC, the effect of the PKC inhibitor, GF 109203X, was tested. GF 109203X did not affect thymidine incorporation induced by serum. GF 109203X prevented the inhibition caused by PMA, suggesting that the activation of PKC is required for the inhibition of DNA synthesis. A prolonged activation of PKC is probably required, since a single treatment of a synthetic diacylglycerol does not inhibit and addition of GF 109203X 2 to 8 h after PMA reverses the inhibition. One possible target for PMA activity is the induction of immediate-early response genes. Thrombin or serum treatment of quiescent WS-1 cells induced protooncogenes c- fos and c- jun. PMA also induced c- fos and c- jun with a time course similar to that of serum or thrombin. The induction of c- fos by PMA was sensitive to staurosporine or GF 109203X. A time course of PMA addition indicated that maximal inhibition of DNA synthesis occurred when PMA was added 4 h after stimulation with serum. These results suggest that the PMA-induced inhibition of DNA synthesis occurs through an activation of PKC that inhibits DNA synthesis at a point after the induction of c- fos and c- jun.