Activation of the TGFα autocrine loop is downstream of IGF-I receptor activation during mitogenesis in growth factor dependent human colon carcinoma cells

The inappropriate expression of TGFα in growth arrest contributes to malignant progression in human colon carcinoma cells. Early stage, non-progressed colon tumor cells show a down-regulation of TGFα in growth arrest and require both nutrients and growth factors for re-entry into the cell cycle. In contrast, highly progressed cells up-regulate TGFα during growth arrest and require only nutrients for re-entry. Given the importance of TGFα in malignant progression, this work addressed the regulation of TGFα expression in the early stage colon carcinoma cell line, FET. Growth-arrested FET cells down-regulated the expression of TGFα, EGFr and, in turn, EGFr activation. These quiescent cells continued to express high levels of IGF-IR protein, but IGF-IR activation was undetectable. Cell cycle re-entry required exogenous growth factor activation of the IGF-IR by insulin or IGF-I. This IGF-IR activation resulted in S phase re-entry and was accompanied by an approximate threefold induction of TGFα expression along with EGFr activation at 1 h following release from growth arrest. Activation of IGF-IR occurred within 5 min of cell-cycle re-entry. Previously identified DNA binding proteins which bind to a unique TGFα/EGF response element within the TGFα promoter were similarly induced following IGF-IR activation. The addition of EGFr neutralizing antibodies abolished the activated IGF-IR stimulated S phase re-entry. Moreover, disruption of the growth arrest associated down-regulation of TGFα in FET cells by constitutive TGFα expression abrogated the requirement for IGF-IR activation for cell cycle re-entry. Consequently, this study indicates, for the first time, that IGF-IR activation up-regulates components of the TGFα autocrine loop resulting in TGFα-mediated EGFr activation which was critical for IGF-IR mediated re-entry into the cell cycle from the growth-arrested state.