Pro‐transforming growth factor‐alpha processing in human colon carcinoma cells: Role of protein kinase C

The human colon cancer cell lines HCT 116 (poorly differentiated) and CEO (well differentiated) express the mitogenic peptide transforming growth factor alpha (TGF‐α). The secretion of TGF‐α was enhanced by phorbol 12‐myristate 13‐acetate (PMA), indicating the possible role of protein kinase C (PKC) in the formation of mature TGF‐α. Cells were metabolically labeled with 3SS‐cysteine and the formation of the mature 6 kDa TGF‐α polypeptide from the 17 kDa pro‐TGF‐α precursor was determined. The conversion of pro‐TGF‐α was complete in 2–4 hr with the HCT 116 cells showing faster kinetics of TGF‐α formation than GEO cells. HCT 116 cells secreted more TGF‐α than GEO cells and the rate and extent of formation of TGF‐α was enhanced by PMA in both cell lines. The expression of several PKC isozymes by HCT 116 and GEO cells was examined by immunoblotting. The expression of all isozymes examined was higher in HCT 116 cells compared with GEO cells. Calphostin C, an inhibitor of PKC, reduced the enzyme activity and significantly inhibited the PMA‐induced secretion of TGF‐α by both cell lines. Two agonists of PKC that act on specific PKC isozymes, thymeleatoxin and 12‐deoxyphorbol 13‐phenylac‐etate 20‐acetate (dPPA), stimulated the release of TGF‐α into the medium to the same extent as PMA. Since dPPA has been reported to stimulate PKC‐4bT1 specifically, our results suggest a potential role for PKC‐β in the processing of pro‐TGF‐α by these 2 human colon carcinoma cell lines. © 1995 Wiley‐Liss, Inc.