Adenovirus-mediated gene transfer of MMAC1/PTEN to glioblastoma cells inhibits S phase entry by the recruitment of p27Kip1 into cyclin E/CDK2 complexes.

Genetic alterations in the MMAC1 tumor suppressor gene (also referred to as PTEN or TEP1) occur in several types of human cancers including glioblastoma. Growth suppression induced by overexpression of MMAC1 in cells with mutant MMAC1 alleles is thought to be mediated by the inhibition of signaling through the phosphatidylinositol 3-kinase pathway. However, the exact biochemical mechanisms by which MMAC1 exerts its growth-inhibitory effects are still unknown. Here we report that recombinant adenovirus-mediated overexpression of MMAC1 in three different MMAC1-mutant glioblastoma cell lines blocked progression from G0/G1 to S phase of the cell cycle. Cell cycle arrest correlated with the recruitment of the cyclin-dependent kinase (CDK) inhibitor, p27Kip1, to cyclin E immunocomplexes, which resulted in a reduction in CDK2 kinase activities and a decrease in levels of endogenous phosphorylated retinoblastoma protein. CDK4 kinase activities were unaffected, as were the levels of the CDK inhibitor p21Cip1 present in cyclin E immunocomplexes. Therefore, overexpression of MMAC1 via adenovirus-mediated gene transfer suppresses tumor cell growth through cell cycle inhibitory mechanisms, and as such, represents a potential therapeutic approach to treating glioblastomas.