The geranylgeranyltransferase I inhibitor GGTI-298 induces hypophosphorylation of retinoblastoma and partner switching of cyclin-dependent kinase inhibitors - A potential mechanism for GGTI-298 antitumor activity

The geranylgeranyltransferase I inhibitor GGTI-298 has recently been shown to arrest human tumor cells in the G(1) phase of the cell cycle, induce apo ptosis, and inhibit tumor growth in nude mice. In the present manuscript, w e provide a possible mechanism by which GGTI-298 mediates its tumor growth arrest. Treatment of the human lung carcinoma cell line Calu-1 with GGTI-29 8 results in inhibition of the phosphorylation of retinoblastoma protein, a critical step for G(1)/S transition. The kinase activities of two G(1)/S c yclin-dependent kinases, CDK2 and CDK4, are inhibited in Calu-1 cells treat ed with GGTI-298, Furthermore, GGTI-298 has little effect on the expression levels of CDK2, CDK4, CDK6, cyclins D1 and E, but decreases the levels of cyclin A. GGTI-298 increases the levels of the cyclin-dependent kinase inhi bitors p21 and p15 and had little effect on those of p27 and p16, Most inte resting is the ability of GGTI-298 to induce partner switching for several CDK inhibitors. GGTI-298 promotes binding of p21 and p27 to CDK2 while decr easing their binding to CDK6, Reversal of partner switching and G(1) block was observed after removal of GGTI-298, Furthermore, GGTI-298 treatment res ults in an increased binding of p15 to CDK4, which is paralleled with decre ased binding to p27, The results demonstrate that the GGTI-298-mediated G(1 ) block in Calu-1 cells involves increased expression and partner switching of CDK inhibitors resulting in inhibition of CDK2 and CDK4, and retinoblas toma protein phosphorylation.