The induction of G(1) cell cycle arrest and apoptosis by second-generation
selenium compounds (e.g.,
methylselenol precursors such as
methylseleninic acid, MSeA) may contribute to their anti-
cancer activities. We have documented previously induction of G(1) arrest and apoptosis by MSeA in association with upregulation of
cyclin-dependent kinase inhibitor (
CDKI) proteins p21Cip1 and/or p27Kip1 in DU145
prostate cancer cells. However, whether these CDKIs play a critical mediator role in G(1) arrest and apoptosis by MSeA has not been addressed. In the present work, we show exposure of p53-mutant DU145 cells to sub-apoptotic concentrations of MSeA induced p21cip1
mRNA (3 h) and
protein (6 h) much faster than p27kip1
mRNA (12 h) and
protein (12 h). Knocking down of p21 by
siRNA completely abolished G(1) arrest induction by MSeA in DU145 cells, yet si-p27
RNA had no attenuation effect on the G(1) arrest. Depletion of p21Cip1 alone or both p21Cip1 and p27Kip1 increased MSeA-induced
caspase-mediated apoptosis. Immunoprecipitation detected increased binding of p21Cip1 to CDK2 and CDK6 in MSeA-exposed DU145 cells. In DU145 xenografts from mice acutely treated with MSeA p.o., the induction of p21Cip1 was observed at 72 h of daily exposure. In p53-wild type LNCaP PCa cells and p53-null PC-3 PCa cells, MSeA modestly and transiently upregulated p21Cip1
protein level, subsiding to basal level by 24 h, without affecting P27Kip1 abundance in the same duration. Si-p21
RNA knockdown in these cells have only a partial effect to reverse G(1) arrest induction by MSeA. Together, our data support persistent, p53-independent, p21Cip1 induction as a critical mediator of MSeA-induced G(1) arrest in DU145 PCa cells, however, p21Cip1 induction and G(1) arrest were not necessary for, and may antagonize,
caspase-mediated apoptosis.