Expression of G1 phase regulators in MG-63 osteosarcoma cell line

Cyclins and cyclin-dependent kinases (cdks) form complexes that govern tran sitions during cell cycle phases. In this study we characterized a human os teosarcoma cell line, MG-63, for the expression level of cyclin D1, cyclin E, cdk4, cdk2, and cell cycle inhibitors pRb and p21. To investigate the ro le of these proteins we treated MG-63 cells with tumor necrosis factor-alph a (TNF-alpha) and interleukin-6 (IL-6). Cell proliferation analysis demonst rated an increased proliferation of MG-63 cells with IL-6, while TNF-alpha acted as an antiproliferative agent. Immunoblotting revealed an increased e xpression of p21 with TNF-alpha and its complex with cdk2. TNF-alpha reduce d the expression of the cyclin E-cdk2 complex. TNF-alpha did not affect the amount of cyclin D1, cyclin E, cdk4, cdk2, and of cyclin D1-cdk4 complex. IL-6 decreased p21 expression and its complex with cdk2, while it increased the cyclin E-cdk2 complex. Cyclin D1 and cdk4 expression and their complex did not change after IL-6 treatment, nor did cyclin E and cdk2 protein exp ression. Hyperphosphorylated/ dephosphorylated Rb protein ratio was reduced with TNF-a whereas it increased with IL-G. These results may suggest an im portant role of p21 and of cyclin E-cdk2 complex in the. G1 phase regulatio n through pRb phosphorylation in MG-63 cells.