Overexpressed Cyclin D1 and CDK4 proteins are responsible for the resistance to CDK4/6 inhibitor in breast cancer that can be reversed by PI3K/mTOR inhibitors

CDK4/6 inhibitors are the standard treatment in advanced HR+/HER2-breast cancer patients.Nevertheless,the resistance to CDK4/6 inhibitors is inevitable and the strategies to overcome resistance are of great interest.Here,we show that the palbociclibresistant breast cancer cells expressed significantly higher levels of Cyclin D1 and CDK4 proteins because of upregulated protein synthesis.Silencing Cyclin D1 or CDK4 led to cell cycle arrest while silencing Cyclin E1 or CDK2 restored the sensitivity to palbociclib.Furthermore,PI3K/mTOR pathway was hyper-activated in palbociclib-resistant cells,leading to more phosphorylated 4E-BP1 and higher levels of Cyclin D1 and CDK4 translation.Targeting PI3K/mTOR pathway with a specific PI3Kαinhibitor(BYL719)or an mTOR inhibitor(everolimus)reduced the protein levels of Cyclin D1 and CDK4,and restored the sensitivity to palbociclib.The tumor samples expressed significantly higher levels of Cyclin D1,CDK4,p-AKT and p-4E-BP1 after progression on palbociclib treatment.In conclusion,our findings suggest that overexpressed Cyclin D1 and CDK4 proteins lead to the resistance to CDK4/6 inhibitor and PI3K/mTOR inhibitors are able to restore the sensitivity to CDK4/6 inhibitors,which provides the biomarker and rationale for the combinational use of CDK4/6 inhibitors and PI3K/mTOR inhibitors after CDK4/6 inhibitor resistance in breast cancer.

LIN9 confers paclitaxel resistance in triple negative breast cancer cells by upregulating CCSAP

LIN9 functions to regulate cell mitotic process.Dysregulation of LIN9 expression is associated with development of human cancers.In this study we assessed the association of LIN9 expression with paclitaxel resistance and clarified the underlying mechanisms for the first time.LIN9 expression in breast cancer tissues was retrieved from publicly available online databases and statistically analyzed.Human TNBC cell lines MDA-MB-231 and MDA-MB-468 and their corresponding paclitaxelresistant sublines 231PTX and 468PTX were used to assess the expression of LIN9 by qRT-PCR and Western blot,cell growth by cell counting,cell viability by MTS assay,and cell apoptosis by flow cytometry.The data showed that high LIN9 expression in breast cancer patients receiving chemotherapy was related to poor overall survival(OS).LIN9 expression was upregulated in paclitaxel-resistant TNBC cells compared to their parental cells.Knockdown of LIN9 or treatment of paclitaxel-resistant TNBC cells with a bromo-and extra-terminal domain inhibitor(BETi)JQ1 which also decreased LIN9 expression enhanced the sensitivity of paclitaxel-resistant TNBC cells to paclitaxel.Mechanistically,decreased LIN9 in resistant cell lines reduced tumor cell viability,promoted multinucleated cells formation and induced tumor cell apoptosis,potentially by directly regulating microtubule-binding protein CCSAP.In conclusion,high LIN9 expression contributed to poor clinical outcomes and paclitaxel resistance in TNBC and BETi,targeting LIN9 expression,could be a reversible drug for PTX-resistant TNBC patients.