Studies on methionyl-tRNA synthetase -dependent stabilization of cyclin-dependent kinase 4 in lung cancer

Abstract

Cyclin-dependent kinase 4 (CDK4) is an important regulator of cell cycle progression which is related to cell proliferation. Interestingly, the level of CDK4 protein is remarkably elevated compared to the gene amplification rates in lung cancer. It suggests the existence of underlying mechanism that can further increase the CDK4 level after gene expression as well as the potential dependence on the CDK4 level in lung cancer. Our previous study reveals that methionyl-tRNA synthetase (MRS) specifically stabilizes CDK4 protein, while other CDKs are not affected in lung cancer. However, the mechanism of MRS-mediated stabilization of CDK4 is not well understood yet. Here, this study demonstrates that MRS enhances the complex formation among CDK4, heat shock protein 90 (HSP90) and cell division cycle 37 (CDC37), leading to increase in the CDK4 level. The increased CDK4 level by MRS overexpression was reduced by HSP90 inhibitor. MRS directly interacted with HSP90 and formed the ternary complex with CDK4 and HSP90. It implies that the function of MRS on stabilizing CDK4 requires the chaperone function of HSP90. To find out which functions of HSP90 MRS modulates for the stabilization of CDK4, the effects of MRS on the ATPase activity of HSP90 and the interaction between HSP90 and CDK4 were investigated. Whereas the ATPase activity of HSP90 was not affected by MRS addition, the association between CDK4 and HSP90 was clearly increased in a dose-dependent manner. It suggests that MRS plays a significant role in the recruitment of CDK4 into HSP90. Depletion of MRS using small interference RNA dissociated CDK4 from HSP90. The association was synergistically reduced by addition of a compound, the MRS-CDK4 interaction inhibitor. In particular, other HSP90 clients, such as CDK7 and p70S6K, known to be stabilized by HSP90 were not affected in this condition. It suggests that MRS exclusively works on the CDK4 stability, but not other HSP90 clients. Furthermore, MRS level was positively critical for the association between CDC37 and HSP90, as well. Taken together, this research reveals that MRS specifically stabilizes CDK4 by recruiting CDK4 and CDC37 into HSP90.