Preliminary Study Of RAD21 In Promoting Triple-negative Breast Cancer Cells Proliferation By Activating MTOR Signaling Pathway

Breast cancer is a malignant tumor that arises in the glandular epithelium of the breast.Due to its high invasiveness,easy recurrence and insensitivity to endocrine therapy,treatment for Triple-negative breast cancer(TNBC)has become a major clinical challenge.The rapid proliferation of cancer cells is highly dependent on the function of ribosomes and efficient protein translation.Ribosome biogenesis is regulated by a variety of factors(eg MyC)and signaling pathways(eg MAPK,PI3K/AKT,and mTOR signaling pathways).The upregulation of ribosome biogenesis and the abnormal expression of ribosomal proteins promote migration and invasion ability of cancer cells.Therefore,identification of tumorspecific ribosome regulators will help establish new strategies to inhibit cancer cell growth and develop new drug targets.Cohesin is a multi-subunit complex composed of SMC1,SMC3,RAD21,and STAG1/2.Cohesin plays an important role in mediating sister chromatid adhesion,maintaining genome stability,and participating in genome expression regulation.As the core subunit of the cohesin complex,RAD21 is highly expressed in various malignant tumors such as breast cancer,ovarian cancer,and gastric cancer,and is closely related to poor prognosis.Cohesin can inhibit the transcription of ribosomal DNA(rDNA)when rDNA double-strand breaks occur to promote DNA damage repair.However,the regulatory relationship between RAD21 and nucleolar structure and function maintenance and mTOR pathway in breast cancer has not been elucidated.In this study,we have identified that knockdown of RAD21 in triple-negative breast cancer MDA-MB-231 could lead to activation of cellular ATM signaling pathway and increase of pATM levels.RAD21-knockdown MDA-MB-231 cells underwent nucleolar stress.The nucleolar protein NPM1 translocated from the nucleolus to the nuclear matrix in RAD21 depleted cells.However,activation of the ATM signaling pathway did not inhibit rRNA synthesis in RAD21-knockdown cells compared with control cells.It suggested that there may be other signaling pathway involved in upregulation of rRNA synthesis induced by RAD21 knockdown.RAD21 knockdown resulted in increase of nuclear distribution of p-mTOR in MDA-MB-231 cells.It may account for up-regulation of rDNA transcription induced by knockdown of RAD21.RAD21 was also involved in promoting the activation of mTOR/p70S6K signaling pathway and regulation of protein synthesis in triple-negative breast cancer cells and ovarian cancer cells.RAD21 was required for MDA-MB-231 cells response to mTOR inhibitor Rapamycin.The regulation of mTOR/p70S6K signaling pathway by RAD21 is partially dependent on the phosphatase PP2A.Treatment with the inhibitor of PP2A,okadaic acid,partially rescued the reduced phosphorylation level of p70S6K caused by RAD21 knockdown.It was unexpected that in RAD21 konckdown MDA-MB-231 cells the reduction of cytoplasmic p21 protein was partially dependent on PP2A.Our preliminary findings suggest that RAD21 promotes the proliferation of triple-negative breast cancer cells by activating the mTOR/p70S6K signaling pathway,which is partially dependent on the inhibition of PP2A activity.