Sensitivity to transforming growth factor β1-induced growth arrest is common in human squamous cell carcinoma cell lines: c-MYC down-regulation and p21(waf1) induction are important early events

Transforming growth factor β1 (TGF-β1) is a potent inhibitor of keratinocyte proliferation and a potential tumor suppressor of squamous cell carcinomas (SCCs). TGF-β1 exerts its antiproliferative effects by inhibiting key transitions required for progression from G₁ to the S phase of the cell cycle, exemplified by a rapid reduction of c-MYC and inhibition of the G₁ cyclin/cyclin-dependent kinases by induction of their inhibitors p21(waf1), p27(Kip1), and p15(INK4B). A significant majority of a new series of human SCC cell lines were found to be as sensitive as primary human epidermal keratinocytes to TGF-β1 growth inhibition. Only a minority of cell lines derived from late-stage tumors were resistant. An early and rapid increase in p21(waf1) and reduction in c-MYC protein levels were important concomitants for TGF-β1 growth inhibition; these changes occurred exclusively in each of the sensitive cell lines. Expression of p15(INK4B) was found to be neither necessary nor sufficient for TGF-β1 growth arrest in the sensitive and resistant cell lines, respectively. TGF-β1 induced alterations in other cell cycle regulatory molecules, cyclin-dependent kinase 4, cyclin D1, pRB, and p27(Kip1), occurred late and were dispensable in some of the sensitive cell lines. Expression of exogenous mycER fusion protein in one of the sensitive cell lines did not render the cells resistant to TGF-β1-induced growth arrest nor prevent p21(waf1) induction or down-regulation of both c-MYC and mycER proteins. However, in TGF-β1-resistant subclones of sensitive mycER- expressing cells, p21(waf1) was not induced, whereas both c-MYC and mycER protein levels decreased following TGF-β1 treatment. We conclude that TGF- β1 activates multiple cell cycle inhibitory pathways dependent upon p21(waf1) induction and c-MYC degradation and that it does not function as a tumor suppressor in the majority of SCCs.