Momilactone B, a
terpenoid phytoalexin present in rice bran, has been shown to exhibit several
biological activities. The present study was conducted using cultured human
leukemia U937 cells to elucidate the possible mechanisms by which
momilactone B exerts its anticancer activity, which to date has remained poorly understood.
Momilactone B treatment of U937 cells resulted in a dose-dependent inhibition of cell growth and induced apoptotic cell death as detected by
chromatin condensation, DNA fragmentation, the cleavage of
poly(ADP-ribose) polymerase and
Annexin V-FITC staining. Flow cytometric analysis revealed that
momilactone B resulted in G1 arrest in cell cycle progression, which was associated with the dephosphorylation of
retinoblastoma protein (pRB) and enhanced binding of pRB with the
E2F transcription factor family
proteins. Treatment with
momilactone B also increased the expression of
cyclin-dependent kinase (Cdk) inhibitor p21Waf1/Cip1 in a p53-independent manner, without any noticeable changes in G1
cyclins and
cyclin-dependent kinases (Cdks), except a slight decrease in
cyclin E. Moreover, in vitro
kinase assay indicated that
momilactone B significantly decreased Cdk4- and Cdk6-associated
kinase activities through a notably increased binding of p21 to Cdk4 and Cdk6. Our results demonstrated that
momilactone B caused G1 cell cycle arrest and apoptosis in U937 cells through the induction of p21 expression, inhibition of Cdk/
cyclin-associated
kinase activities, and reduced phosphorylation of pRB, which may be related to anticancer activity.