Effects of structurally related flavonoids on cell cycle progression of human melanoma cells: regulation of cyclin-dependent kinases CDK2 and CDK111Abbreviations: CDK, cyclin-dependent kinase; CKI, CDK inhibitor; PI 3-kinase, phosphatidylinositol 3-kinase; PKC, protein kinase C; DTT, dithiothreitol; RIPA, radioimmunoprecipitation assay buffer.