Here, we assessed and compared the anticancer efficacy and associated mechanisms of
silymarin and
silibinin in human
prostate cancer (PCA) PC3 cells;
silymarin is comprised of
silibinin and its other stereoisomers, including
isosilybin A,
isosilybin B,
silydianin,
silychristin and isosilychristin.
Silymarin and
silibinin (50-100 microg/ml) inhibited cell proliferation, induced cell death, and caused G1 and G2-M cell cycle arrest in a dose/time-dependent manner. Molecular studies showed that G1 arrest was associated with a decrease in
cyclin D1,
cyclin D3,
cyclin E,
cyclin-dependent kinase (CDK)4, CDK6 and
CDK2 protein levels, and CDK2 and CDK4
kinase activity, together with an increase in CDK inhibitors (CDKIs) Kip1/p27 and Cip1/p21. Further, both agents caused cytoplasmic sequestration of
cyclin D1 and CDK2, contributing to G1 arrest. The G2-M arrest by
silibinin and
silymarin was associated with decreased levels of
cyclin B1,
cyclin A, pCdc2 (Tyr15), Cdc2, and an inhibition of Cdc2
kinase activity. Both agents also decreased the levels of Cdc25B and cell division cycle 25C (Cdc25C)
phosphatases with an increased phosphorylation of Cdc25C at Ser216 and its translocation from nucleus to the cytoplasm, which was accompanied by an increased binding with 14-3-3beta. Both agents also increased checkpoint
kinase (Chk)2 phosphorylation at Thr68 and Ser19 sites, which is known to phosphorylate Cdc25C at Ser216 site. Chk2-specific
small interfering RNA largely attenuated the
silymarin and
silibinin-induced G2-M arrest. An increase in the phosphorylation of
histone 2AX and
ataxia telangiectasia mutated was also observed. These findings indicate that
silymarin and
silibinin modulate G1 phase
cyclins-CDKs-CDKIs for G1 arrest, and the Chk2-Cdc25C-Cdc2/
cyclin B1 pathway for G2-M arrest, together with an altered subcellular localization of critical cell cycle regulators. Overall, we observed comparable effects for both
silymarin and
silibinin at equal concentrations by weight, suggesting that
silibinin could be a major cell cycle-inhibitory component in
silymarin. However, other
silibinin stereoisomers present in
silymarin also contribute to its efficacy, and could be of interest for future investigation.