Activation of RB tumor suppressor protein and growth suppression of small cell lung carcinoma cells by reintroduction of p16INK4A gene.

The p16INK4A tumor suppressor gene is frequently inactivated in non-small cell lung carcinoma (NSCLC) and is less frequently inactivated in small cell lung carcinoma (SCLC) by mutation, deletion or DNA methylation. There are several reports that the reintroduction of the p16INK4A gene into p16(-) NSCLC cells results in significant growth suppression. However, there have been no reports of reintroduction of the p16INK4A gene into SCLC cells. To assess the biological significance of p16INK4A inactivation in the development of SCLC, full-length p16INK4A cDNA was introduced into an SCLC cell line, Ms-13, in which the p16INK4A protein was not detected. SCLC cells stably transfected with the p16INK4A expression vector formed only 2-16% of the number of neomycin-resistant colonies formed by cells transfected with a control vector, and no expression of exogenous p16INK4A protein was detected in any of 16 expanded clones. Transient transfection of the p16INK4A gene into SCLC cells resulted in exogeneous p16INK4A protein expression and dephosphorylation of endogenous retinoblastoma (RB) protein. These results suggest that the restoration of the p16INK4A function suppresses the growth of SCLC cells by dephosphorylation of the RB protein. Therefore, inactivation of p16INK4A may play an important role in the enhancement of growth of p16INK4A(-) RB(+) SCLC tumors in vivo.