EXPRESSION OF TGF-ALPHA AUTOCRINE ACTIVITY IN HUMAN COLON-CARCINOMA CBS CELLS IS AUTOREGULATED AND INDEPENDENT OF EXOGENOUS EPIDERMAL GROWTH-FACTOR

Autocrine transforming growth factor alpha (TGF alpha) activity and co ntrol mechanisms for its expression were examined in a representative clonal isolate (CBS4) of a well-differentiated human colon carcinoma c ell line designated CBS. CBS4 cells expressed TGF alpha and its recept or, epidermal growth factor receptor (EGFr). Blockade of EGFr and TGF alpha by neutralizing antibodies inhibited clonal growth and the initi ation of DNA synthesis from quiescence in CBS4 cells. Therefore, TGF a lpha is an autocrine growth factor for CBS4 cells. Several studies hav e indicated that activation of the EGFr by exogenous EGF stimulates TG F alpha expression. However, in CBS4 cells EGF did not induce TGF alph a mRNA expression, indicating that EGF does not affect TGF alpha trans cription in these cells. Exogenous treatment of exponentially growing cells with either EGF or EGFr blocking antibody enhanced release of TG F alpha protein into the conditioned medium. This indicated that the r elease of TGF alpha into the conditioned medium by exogenous EGF was a t least partially due to the displacement of TGF alpha from the TGF al pha/EGFr complexes. Similarly to exponentially growing cells, the EGFr blocking antibody and EGF also enhanced TGF alpha release into the me dium of CBS4 cells after release from quiescence. These results indica ted that exogenous EGF had little if any effect on TGF alpha expressio n in these cells and suggested that TGF alpha expression might be unde r endogenous TGF alpha control. Blockade of the autocrine TGF alpha lo op by TGF alpha neutralizing antibody suppressed TGF alpha mRNA both i n exponentially growing and quiescent cells, demonstrating that autocr ine TGF alpha is autoregulatory in this system. (C) 1998 Wiley-Liss, I nc.