Methylseleninic acid inhibits microvascular endothelial G₁ cell cycle progression and decreases tumor microvessel density

Our previous work has shown that the cancer chemopreventive effect of selenium may in part be mediated by its antiangiogenic activities and that methylseleninic acid (MSeA) can induce G₁ arrest of human umbilical vein endothelial (macrovascular) cells. The objectives of the current study are to verify MSeA-induced G₁ arrest effect in microvascular endothelial cells and to elucidate the molecular mediators and targets involved. Flow cytometric analysis after MSeA exposure (2-10 μM) of telomerase-immortalized microvascular endothelial (TIME) cells for 24 hr showed a concentration- dependent increase of G₁-arrested cells. MSeA (3 μM) treatment delayed the mitogen-stimulated progression of TIME cells from G₁ to S phase. These effects of MSeA were accompanied by an early transient (6 hr) upregulation of P21/CIP1 and P27/KIP1 and a delayed modest increase of P16/INK4a (12 hr). MSeA increased P27/KIP1 mRNA transcript level and slowed the turnover of P21/CIP1 protein. MSeA-treated cells contained elevated levels of bound P16/INK4a within the CDK4/6/cyclin D1 complexes as well as bound P21/CIP1 and P27/KIP1 within the CDK2/cyclin E complex and decreased their kinase activities. MSeA suppressed the mitogen/CDK-driven phosphorylative inactivation of retinoblastoma (Rb) protein, diminishing E2F1 release from Rb. In vivo, daily oral MSeA treatment of nude mice bearing subcutaneously inoculated human prostate cancer DU145 xenografts inhibited tumor growth in a dose-dependent manner. The microvessel density of the tumors in the high MSeA group was decreased by more than half from the control. An inhibition of mitogen-stimulated proliferation of endothelial cells by MSeA may therefore contribute to the inhibition of tumor angiogenesis.