Growth control in a human colon carcinoma cell line mediated by cell-associated transforming growth factor-alpha (TGF alpha).

Transforming growth factor-alpha (TGF alpha) is able to elicit growth in many target cells expressing a functional epidermal growth factor (EGF) receptor. Other laboratories have reported that the TGF alpha precursor polypeptide (proTGF alpha) is inefficiently cleaved from many target cells, resulting in accumulation of proTGF alpha on the cell surface. Since it has been shown that noncleavable, mutated cell-associated TGF alpha can stimulate cell growth on receptor-bearing adjacent cells, we have tried to determine whether cell-associated TGF alpha populations might be involved in supporting autonomous cell growth regulatory mechanisms in a human colon carcinoma cell line, HCT116. To address this question, the levels of secreted and nonsecreted TGF alpha produced were determined. Cells grown to medium cell density (40-60% confluent) expressed the greatest percentage of cell-associated TGF alpha (50%). Incubation of HCT116 cells with 0.1 U/ml porcine pancreatic elastase resulted in the release of 67% of the cell-associated TGF alpha into their medium and caused the treated cells to acquire a newly established growth sensitivity to exogenous TGF alpha at a ligand concentration of 1.0 nM. Western blot analysis of EGF receptor phosphotyrosine levels showed a decrease in phosphotyrosine content after elastase treatment. Phosphotyrosine content was restored to basal levels if elastase treatment was followed by addition of exogenous TGF alpha or EGF. These results suggest that HCT116 cells use a "closed" autocrine loop between cell-associated TGF alpha species and their EGF receptor to stimulate their cell growth.