Involvement of protein kinase C and an elastase-like enzyme in the processing of transforming growth factor-alpha in human colon carcinoma cell lines.

Human colon carcinoma cell lines secrete transforming growth factor-alpha (TGF-alpha). Previous work indicated that the apparent m.w. of the TGF-alpha secreted by these cells ranged between 5 and 25 kDa. The more differentiated GEO cell line secreted a higher percentage of high m.w. TGF-alpha than did the poorly differentiated HCT 116 cell line. In addition, the HCT 116 cells secreted 5-fold more TGF-alpha. Treatment of HCT 116 and GEO cells with a phorbol ester (TPA) resulted in a 4-fold increase in TGF-alpha in the conditioned media of both cell types. The TPA-induced release of TGF-alpha was blocked by an inhibitor of elastase-like enzymes. This suggested a role for protein kinase C (PKC) in TGF-alpha processing in colon carcinoma cells. Direct measurement of PKC activity indicated that the HCT 116 cells (which secrete more fully processed TGF-alpha) had 10-fold more PKC activity than GEO cells. The presence of an elastase-like activity in detergent extracts and the ability of an elastase inhibitor to block the TPA-induced secretion of TGF-alpha suggests that PKC and an elastase-like enzyme are involved in the processing and secretion of TGF-alpha by human colon carcinoma cell lines.