The Cancer Preventive Flavonoid Silibinin Causes Hypophosphorylation of Rb/p107 and Rb2/p130 Via Modulation of Cell Cycle Regulators in Human Prostate Carcinoma DU145 Cells

Abstract

© 2 0 0 2 L a n d e s B i o s c i e n c e . N o t f o r d i s t r i b u t i o n . ABSTRACTPhosphorylation status of retinoblastoma (Rb) and related proteins is important to drive cell cycle progression. In hyperphosphorylated state, they are growth stimulatory, but their hypophosphorylation is growth inhibitory. Here we assessed whether silibinin causes hypophosphorylation of Rb-related proteins as its growth inhibitory response in human prostate cancer (PCA) DU145 cells. Silibinin treatment of cells resulted in a strong increase (up to 2.3-and 5.4-fold) in the levels of hypophosphorylated Rb/p107 and Rb2/p130, respectively, but a strong decrease (91, 78 and 45%) in protein levels of transcription factors E2F3, E2F4 and E2F5, respectively. In the studies analyzing whether this effect of silibinin is via modulation of cell cycle regulators, silibinin-treated cells showed a strong increase (up to 13-and 6-fold) in Cip1/p21 and Kip1/p27 levels, respectively. Silibinin treatment also resulted in 90 and 70% decrease in CDK4 and CDK2 levels, respectively, but did not alter the protein levels of cyclin D1 and cyclin E. Consistent with its effect on G1 cell cycle regulators, silibinin treated cells exhibited a strong G1 arrest, almost complete growth inhibition, and morphological changes suggestive of differentiation. Together, these results suggest that silibinin caused hypophosphorylation of Rb-related proteins may in part be responsible for its cancer preventive and anti-carcinogenic efficacy in different cancer models including PCA.

p21 and p27 induction by silibinin is essential for its cell cycle arrest effect in prostate carcinoma cells

“…Silibinin treatments induced strong G 1 arrest (P < 0.001) in DU145 cells in a dose-dependent manner (Fig. 1C), which was consistent with earlier observations (34). Analyses of the results showed that DU145 cells started accumulating in G 1 phase between 12 and 24 h at the expense of a dose-dependent decrease (P < 0.001) in S-phase population.…”

Unique natural antioxidants (NAOs) and derived purified components inhibit cell cycle progression by downregulation of ppRb and E2F in human PC3 prostate cancer cells

“…Recently, several studies with the antioxidant vitamin E and the polyphenolic flavonoid silibinin showed similar results concerning their anti-proliferation effects on PCA cells. The signaling pathway includes elevation in the hypophosphorylated form of pRb at the expense of the phosphorylated form of Rb, reduction at the protein level of CDKs, and cyclin typical to G1 phase of the cell cycle together with enhanced expression of the CDK inhibitor p21 cip1 [55][56][57].…”

PC-SPES Inhibits Cell Proliferation by Modulating p21, Cyclins D, E and B and Multiple Cell Cycle-Related Genes in Prostate Cancer Cells

“…Interestingly, these effects were similar with recently reported effects of cancer-preventive flavionoids, suggesting the link between cancer-preventing and therapeutic phytochemicals. 25,26 In sequence, Rb binds to transcription factor E2F which is critically involved in the control of several S phase requiring proteins including the expression of mini-chromosome maintenance (MCM) family of proteins that regulate DNA replication. Such a mechanistic scenario was also demonstrated by the array data showing that PC-SPES inhibited E2F, MCM2, 5 and 7 expressions (Table 1).…”