CRYβB2 is a biomarker for poor prognosis and response to CDK4/6 inhibitors in breast cancer

Background Hazard rates of breast cancer death are significantly higher for women of African American (AA) origin compared with European American (EA) and the molecular mechanisms underlying this difference in outcome are understudied. Previously, we showed that β-crystallin B2 (CRYβB2) expression is up-regulated in breast cancer of AA women, activates nucleolin (NCL), and mediates oncogenesis in triple negative breast cancer (TNBC). Presently no biomarkers, other than estrogen receptor (ER)/ progesterone receptor (PR) expression, are used to prescribe cyclin-dependent kinases 4/6 inhibitors (CDK4/6i) for women with hormone receptor positive (HR⁺) metastatic breast cancer. Methods Western blot and flow cytometry was used to determine the activation of CDK4/6 pathway and cell cycle of cells isolated from CRYβB2 overexpressing tumors, respectively. Response to CDK4/6i was determined following treatment of mice containing control and CRYβB2- overexpressing tumors and TNBC and ER⁺ cells. The correlation of CRYβB2 expression with CDK4/6 activation and survival was determined by Western blot, immunohistochemistry, and Kaplan Meier curves using TNBC and ER⁺ tumors from AA and EA women. Results Here, we report that tumors overexpressing CRYβB2 showed an increase in cell cycle progression and activation of the CDK4/6 pathway in models of premalignant- and ductal carcinoma in situ (DCIS) lesions, and TNBC and ER⁺ breast cancer cells. Targeting CRYβB2 and NCL resulted in lower levels of CDK4, cell division cycle 25 A (Cdc25a), and phosphorylated retinoblastoma (ppRb). Only tumors expressing CRYβB2 showed growth inhibition by the CDK4/6i, palbociclib. The expression of CRYβB2 protein inversely correlated with the IC50 of palbociclib in both TNBC and ER⁺ cells. In accord with this, CRYβB2 knockdown in TNBC and ER⁺ cells conferred greater resistance to inhibition with palbociclib. Further, the NCL aptamer AS-1411 sensitized TNBC and ER⁺ cells to palbociclib. Higher levels of CRYβB2 expression in TNBC and ER⁺ tumors of AA patients correlated with higher CDK4/pRb activation. Expression of both CRYβB2 and ppRb correlated with decreased survival in AA women with TNBC. Conclusions CRYβB2 expression correlated with CDK4/6 activation and response to CDK4/6i, and may be useful as a biomarker of prognosis and response to palbociclib therapy.