Inhibition Of The Transcriptional Kinase CDK7 Overcomes Therapeutic Resistance In HER2-positive Breast Cancers

Background: Human epidermal growth factor receptor 2(HER2)plays an important role in cell growth and proliferation in breast cancer,and the invention of HER2 inhibitors gives rise to a new era of targeting HER2+ BC.Recent clinical studies showed that many HER2+ BC patients were found resistant to therapy of HER2 inhibitors.Resistance mechanismsmainly involve the activation and increased protein level of multiple receptor tyrosine kinases(RTKs).The heterogeneity of HER2 inhibitors-induced RTKs prevents kinase targeting by a single kinase inhibitor.Thus,there is a major challenge to the treatment of HER2+ BC patients and a novel method is critically needed.CDK7 is a member of cyclin-dependent kinase family and THZ1 is a covalent inhibitor of CDK7.In this study,we investigated the effects of inhibition of the transcriptional kinase CDK7 on therapeutically recalcitrant HER2+ BC,and explored its mechanism,which would provide a potential target of clinical treatment for drug-resistant HER2+BC.Methods: In this study,in vitro and in vivo assays were performed to evaluate the effect of THZ1 in HER2 inhibitor-resistant(HER2iR) and HER2 inhibitor-sensitive(HER2iS)cell lines.Moreover,we tested whether THZ1 in combination with HER2 inhibitor Lapatinib would resensitize HER2iR cancer cells to lapatinib in vitro.Cell viability assay,FACS,western blotting,RNA-seq,microarray,q RT-PCR,mouse xenotransplantation experiments,immunohistochemistry and data statistical analyses were employed to determine CDK7 mediated multi-kinase signaling.Results: Both HER2iR and HER2 i S cell lines exhibited high sensitivity to THZ1.CDK7 promoted cell cycle progression through inhibition of transcription,rather than via direct phosphorylation of classical CDK targets.THZ1 displayed potent synergy with the HER2 inhibitor Lapatinib in HER2iR BC cells in vitro and restored HER2iR sensitivity to Lapatinib.The combination of THZ1 and Lapatinib induced tumor regression and apoptosis in two HER2iR xenograft models in vivo.The transcriptional kinase activity of CDK7 is regulated not only by HER2,but also by the receptor tyrosine kinases which activated in response to HER2 inhibition,as well as by the downstream SHP2 and PI3K/AKT pathways.Conclusion: This study reveals how CDK7 acts as a transcriptional kinase in HER2+ BC and utilizes CDK7 inhibition as an additional therapeutic avenue that blocks the activation of genes engaged by multiple HER2iR kinases.