SENSITIVITY TO TRANSFORMING GROWTH-FACTOR BETA-1-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 beta 1 (TGF-beta 1) is a potent inhibitor o f keratinocyte proliferation and a potential tumor suppressor of squam ous cell carcinomas (SCCs), TGF-beta 1 exerts its antiproliferative ef fects by inhibiting key transitions required for progression from G(1) to the S phase of the cell cycle, exemplified by a rapid reduction of c-MYC and inhibition of the G(1) cyclin/cyclin-dependent kinases by i nduction of their inhibitors p21(waf1), p27(Kip1), and p15(INK4B). A s ignificant majority of a new series of human SCC cell lines were found to be as sensitive as primary human epidermal keratinocytes to TGF-be ta 1 growth inhibition, Only a minority of cell lines derived from lat e-stage tumors were resistant. An early and rapid increase in p21(Waf1 ) and reduction in c-MYC protein levels were important concomitants fo r TGF-beta 1 growth inhibition; these changes occurred exclusively in each of the sensitive cell lines. Expression of p15(INK4B) was found t o be neither necessary nor sufficient for TGF-beta 1 growth arrest in the sensitive and resistant cell lines, respectively. TGF-beta 1 induc ed alterations in other cell cycle regulatory molecules, cyclin-depend ent kinase 4, cyclin D1, pRB, and p27(Kip1), occurred late and were di spensable in some of the sensitive cell lines. Expression of exogenous mycER fusion protein in one of the sensitive cell lines did not rende r the cells resistant to TGF-beta 1-induced growth arrest nor prevent p21(waf1) induction or down-regulation of both c-MYC and mycER protein s, However, in TGF-beta 1-resistant subclones of sensitive mycER-expre ssing cells, p21(waf1) was not induced, whereas both c-MYC and mycER p rotein levels decreased following TGF-beta 1 treatment. We conclude th at TGF-beta 1 activates multiple cell cycle inhibitory pathways depend ent upon p21(waf1) induction and c-MYC degradation and that it does no t function as a tumor suppressor in the majority of SCCs.