Inactivation of both Rb and p53 pathways in mouse lung epithelial cell lines

Aberrant expression of key cell cycle regulatory genes is essential for the immortalization and transformation of cells in vitro. We examined 20 mouse lung epithelial cell lines (2 nontumorigenic, 5 nonmetastatic, and 13 meta static)for mutations or alterations in the expression of key components Of the Rb pathway (pRb and p(16INK4a) ) and the p53 Pathway (p53 and p19(ARF) ). Seven cell lines had a mutation in exons 5 to 8 of p53. P19(ARF) was ina ctivated in the remaining 13 cell lines, primarily by homozygous deletion. Rb expression was present and unaltered in all cell lines, with both phosph orylated and unphosphorylated protein forms detectable. p16(INK4a) transcri pts were undetectable in all cell lines tested except LM1. Loss of p16(INK4 a) expression was a result of homozygous deletion in II out of 20 lung cell lines and promoter-exon 1 hypermethylation in 6 out of the remaining 8 cel l lines. Other related components that were examined in this study Included p21 WAF1 and cyclin D1. Compared to normal lung tissue, P21WAF1 expression levels were reduced or undetectable in all cell lines, which did not corre late with loss of p53 function, but did correlate with inactivation of eith er p53 or p19(ARF). Although cyclin D1 expression was variable between cell lines, transcript levels were decreased by at least 50% in the nontumorige nic lines C10 and E10 compared to the tumorigenic cell lines. These results demonstrate mutually exclusive 1 relationships between p53 and p19(ARF) an d between Rt, and p16(INK4a), but perhaps not between cyclin DI and p16(INK 4a), and further describe the nature of involvement of both pathways in mou se lung tumorigenesis.