Aberrant expression of
cyclin D1, frequently observed in human malignant disorders, has been linked to the control of G(1)→S cell cycle phase transition and development and progression in
carcinogenesis.
Cyclin D1 level changes are partially controlled by GSK-3β-dependent phosphorylation at threonine-286 (Thr286), which targets
cyclin D1 for ubiquitination and proteolytic degradation. In our continuing studies on the mechanism of
prostate cancer prevention by
resveratrol, focusing on the role of its recently discovered target
protein,
quinone reductase 2 (
NQO2), we generated
NQO2 knockdown CWR22Rv1 using
short hairpin RNA (
shRNA)-mediated gene silencing approach. We found that, compared with cells expressing
NQO2 (shRNA08),
NQO2 knockdown cells (shRNA25) displayed slower proliferation and G(1) phase cell accumulation. Immunoblot analyses revealed a significant decrease in phosphorylation of
retinoblastoma Rb and
cyclin D1 in shRNA25 compared with shRNA08. Moreover, shRNA25 cells showed a 37% decrease in
chymotrypsin-like
proteasome activity. An increase in AKT activity was also observed in shRNA25, supported by a ∼1.5-fold elevation in phosphorylation and ∼50% reduction/deactivation of GSK-3α/β at Ser21/9, which were accompanied by a decrease in phosphorylation of
cyclin D1 at T286.
NQO2 knockdown cells also showed attenuation of
resveratrol-induced downregulation of
cyclin D1. Our results indicate a hitherto unreported role of
NQO2 in the control of AKT/GSK-3β/
cyclin D1 and highlight the involvement of
NQO2 in degradation of
cyclin D1, as part of mechanism of
chemoprevention by
resveratrol.