P16 INHIBITION OF TRANSFORMED AND PRIMARY SQUAMOUS EPITHELIAL-CELLS

We and others have recently shown that p16 can potently and specifical ly inhibit progression through the G1 phase of the replicative cycle i n cells that express the retinoblastoma protein (pRB). However, none o f these studies examined cell types in which p16 has been firmly impli cated in tumorigenesis. We predicted that such cells would show sensit ivity to p16 inhibition, perhaps conferred by proteins in addition to or other than pRB. Intragenic, inactivating mutations of p16 have been found at significant frequency in primary tumors derived from squamou s epithelial cells of the esophagus (ESCC). We therefore examined p16 function in ESCC lines and in primary squamous epithelial cells cultur ed from mouse skin. We find that seven of eight ESCC lines tested are inhibited by p16 and fail to express the protein endogenously. The lon e p16-resistant line expresses endogenous p16 but lacks functional pRB . Primary squamous epithelial cells are also inhibited by p16. These d ata suggest that squamous epithelial cells are generally sensitive to inhibition by a regulatory pathway that involves p16 and pRB, and that , by the time of establishment in culture, there is nearly universal i nactivation of this pathway in ESCCs.