title: Activation of the TGFalpha autocrine loop is downstream of IGF-I receptor activation during mitogenesis in growth factor dependent human colon carcinoma cells.
abstract: The inappropriate expression of TGFalpha in growth arrest contributes to malignant progression in human colon carcinoma cells. Early stage, non-progressed colon tumor cells show a down-regulation of TGFalpha in growth arrest and require both nutrients and growth factors for re-entry into the cell cycle. In contrast, highly progressed cells up-regulate TGFalpha during growth arrest and require only nutrients for re-entry. Given the importance of TGFalpha in malignant progression, this work addressed the regulation of TGFalpha expression in the early stage colon carcinoma cell line, FET. Growth-arrested FET cells down-regulated the expression of TGFalpha, 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 TGFalpha 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 TGFalpha/EGF response element within the TGFalpha 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 TGFalpha in FET cells by constitutive TGFalpha 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 TGFalpha autocrine loop resulting in TGFalpha-mediated EGFr activation which was critical for IGF-IR mediated re-entry into the cell cycle from the growth-arrested state.
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Ohio, PO Box 10008, Toledo, Ohio, OH 43699-0008, USA.
authors.forename: Degeng
authors.initials: D
authors.lastname: Wang
authors.forename: Sandip
authors.initials: S
authors.lastname: Patil
authors.forename: Wenhui
authors.initials: W
authors.lastname: Li
authors.forename: Lisa E
authors.initials: LE
authors.lastname: Humphrey
authors.forename: Michael G
authors.initials: MG
authors.lastname: Brattain
authors.forename: Gillian M
authors.initials: GM
authors.lastname: Howell
mesh_terms: D002455:Cell Division; D003110:Colonic Neoplasms; D004247:DNA / Q000096:biosynthesis; D066246:ErbB Receptors / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D016196:S Phase; D015398:Signal Transduction; D016211:Transforming Growth Factor alpha / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D007328:Insulin; D016211:Transforming Growth Factor alpha; D007334:Insulin-Like Growth Factor I; D004247:DNA; D066246:ErbB Receptors; D017526:Receptor, IGF Type 1