Polyomavirus large T antigen induces alterations in cytoplasmic signallingpathways involving Shc activation

It has been extensively demonstrated that growth factors play a key role in the regulation of proliferation. Several lines of evidence support the hyp othesis that for the induction of cell cycle progression in the absence of exogenous growth factors, oncogenes must either induce autocrine growth fac tor secretion or, alternatively, activate their receptors or their receptor substrates. Cells expressing polyomavirus large T antigen (PyLT) display r educed growth factor requirements, but the mechanisms underlying this pheno menon have yet to be explored. We conducted tests to see whether the reduct ion in growth factor requirements induced by PyLT was related to alteration s of growth factor-dependent signals. To this end, we analyzed the phosphor ylation status of a universal tyrosine kinase substrate, the transforming S hc adapter protein, in fibroblasts expressing the viral oncogene. We report that the level of Shc phosphorylation does not decrease in PyLT-expressing fibroblasts after growth factor withdrawal and that this PyLT-mediated eff ect does not require interaction with protein encoded by the retinoblastoma susceptibility gene. We also found that the chronic activation of the adap ter protein is correlated with the binding of Shc to Grb-2 and with defects in the downregulation of mitogen-activated protein kinases. In fibroblasts expressing the nuclear oncoprotein, we also observed the formation of a Py LT-Shc complex that might be involved in constitutive phosphorylation of th e adapter protein. Viewed comprehensively, these results suggest that the c ell cycle progression induced by PyLT may depend not only on the direct ina ctivation of nuclear antioncogene products but also on the indirect inducti on, through the alteration of cytoplasmic pathways, of growth factor-depend ent nuclear signals.