Retinoblastoma gene-independent G1 phase arrest by flavone, phosphatidylinositol 3-kinase inhibitor, and histone deacetylase inhibitor

In most human malignant tumors, retinoblastoma tumor-suppressor gene (RB) product is inactivated by phosphorylation. Therefore, cancer preventive agents or molecular-targeting agents can inhibit the tumor growth at G1 phase through RB reactivation. However, little is known about the effectiveness of RB reactivating agents against malignancies with mutated RB. We report here that chemopreventive agent flavone, phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, and histone deacetylase (HDAC) inhibitor trichostatin A (TSA) also induce G1 phase arrest in malignant tumor cells with mutated RB. In human prostate cancer DU145 cells with mutated RB, flavone increased cyclin-dependent kinase (CDK) inhibitors p21 and p27, and reduced cdk4 and cdk6, resulting in decrement of phosphorylated RB family proteins p130 and p107. LY294002 also dephosphorylated p107 and p130 proteins, whereas TSA dephosphorylated p130, but not p107. Furthermore, flavone induced G1 phase arrest in both mouse embryo fibroblast (MEF) wild-type and MEF RB-/- cells, but did not do so in RB, p107, and p130 triple-knockout MEF cells. These results suggested that p130 and p107 contributed to G1 phase arrest by flavone in RB-mutated cells. However, flavone induced tumor suppressor microRNA miR-34a with reduction of E2F1 and E2F3, known to be downregulated by miR-34a, raising the possibility that miR-34a might partially contribute to G1 arrest by flavone. These results raise the possibility that RB reactivating chemopreventive agents or molecular targeting agents might also be effective against a variety of malignant tumor cells with mutant RB. (Cancer Sci 2012; 103: 2139-2143)