Chemical inhibition of methionyl-tRNA synthetase and cyclin-dependent kinase 4 interaction and its effect on p16INK4a-negative cancer cells

Abstract

PD0332991, also known as Palbociclib, is a specific inhibitor of cyclin-dependent kinase 4 (CDK4) and CDK6, which got accelerated approval by Food and Drug Administration in 2015 for estrogen receptor (ER)-positive and human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer patients. Unlike pan CDK inhibitors, PD0332991 is less toxic due to its specificity to CDK4/6, and doubled progression-free survival in metastatic breast cancer when combined with letrozole. However, PD0332991 treatment unexpectedly stabilized Cyclin D3 and CDK4/6 complexes and emergence of CDK4/6 mutation is highly expected based on the experiences of other kinase inhibitors. In the previous study, we proved that methionyl-tRNA synthetase (MRS) controls the stability of CDK4 by directly interacting with CDK4. MRS is an essential enzyme, which links methionine (Met) to tRNAMet doing a critical role in global translational regulation, but meanwhile, it facilitates cell proliferation via stabilizing CDK4, especially in p16INK4a-negative cancer. Inhibition of MRS by small interfering RNA or Met analogue treatments reduced CDK4 level resulting in cell cycle arrest at G0/G1. Based on this, I screened about 200 compounds to identify specific MRS-CDK4 interaction inhibitors, which can control CDK4 with different mode of action from that of PD0332991. I investigated the effects of the compounds on the cell proliferation, MRS activity and CDK4 level and finally found BC-MCI-CG-88 compound. BC-MCI-CG-88 directly hindered the interaction between MRS and CDK4 in immunoprecipitation assay and induced cell cycle arrest at G0/G1 suppressing cell proliferation. Moreover, it did not inhibit the catalytic activity of MRS implying that it would deliver its cytotoxic effects to only p16INK4a-negative cancer, but not to normal cells or tissues. This study is meaningful in that it suggests a novel possible therapeutics to target CDK4 via inhibiting the interaction between MRS and CDK4, which can be applicable to p16INK4a-negative cancer and hopefully, to PD0332991-resistant p16INK4a-negative cancer.