TGFα is required for full expression of the transformed growth phenotype of NIH 3T3 cells overexpressing ornithine decarboxylase

Ornithine decarboxylase (ODC) overexpressed from a heterologous promoter drives the tumorigenic transformation of NIH 3T3 cells and provides a model to investigate the underlying molecular mechanisms. These transformed cells, designated NODC cells, exhibit elevated levels of epidermal growth factor receptor (EGFR) tyrosine kinase (Tyr-k) activity relative to control transfected cells and inhibition of EGFR Tyr-k activation suppresses the transformed growth phenotype of these cells. Thus, ODC-induced transformation of NIH 3T3 cells appears to be mediated, at least in part, by enhanced signaling through the EGFR pathway. Here we extend these studies by evaluating: (i) the effects on growth regulation of overexpressing ODC in EGFR-deficient NIH 3T3 cells; (ii) the potential role of TGFa in mediating the EGFR-dependent transformation of NIH 3T3 cells by ODC. Disruption of EGFR-TGFa interactions either by deleting EGFR, by treatment with anti-TGFa neutralizing antibody or by transfection with a TGFa antisense expression vector suppressed acquisition of the full transformed growth phenotype. Specifically, the loss of contact inhibition and the capacity for clonogenic growth appear more dependent on EGFR-TGFa interactions than anchorage-independent growth in ODC-overexpressing cells. ODC overexpression does not alter the amount, localization or secretion of TGFa. Thus, TGFa is not the ODC-responsive component of the EGFR signaling pathway but appears to be critically involved in development of the transformed phenotype of NODC cells.