Adenovirus-mediated overexpression of p15(INK4B) inhibits human glioma cell growth, induces replicative senescence, and inhibits telomerase activity similarly to p16(INK4A)

The genes encoding the cyclin-dependent kinase inhibitors p16(INK4A) (CDKN2 A) and p15(INK4B) (CDKN2B) are frequently homozygously deleted in a variety of tumor cell lines and primary tumors, including glioblastomas in which 4 0-50% of primary tumors display homozygous deletions of these two loci. Alt hough the role of pig as a tumor suppressor has been well documented, it ha s remained less well studied whether p15 plays a similar growth-suppressing role. Here, we have used replication-defective recombinant adenoviruses to compare the effects of expressing wild-type pie and p15 in glioma cell lin es. After infection, high levels of p16 and p15 were observed in two human glioma cell lines (U251 MG and U373 MG). Both inhibitors were found in comp lex with CDK4 and CDK6, Expression of pie and p15 had indistinguishable eff ects on U251 MG, which has homozygous deletion of CDKN2A and CDKN2B, but a wild-type retinoblastoma (RB) gene. Cells were growth-arrested, showed no i ncreased apoptosis, and displayed a markedly altered cellular morphology an d repression of telomerase activity. Transduced cells became enlarged and f lattened and expressed senescence-associated beta-galactosidase, thus fulfi lling criteria for replicative senescence. In contrast, the growth and morp hology of U373 MG, which expresses pie and p15 endogenously, but undetectab le levels of RE protein, were not affected by exogenous overexpression of e ither inhibitor. Thus, we conclude that overexpression of p15 has a similar ability to inhibit cell proliferation, to cause replicative senescence, an d to inhibit telomerase activity as p16 in glioma cells with an intact RE p rotein pathway.