CDK5is Essential For TGF-β1-induced Epithelial-mesenchymal Transition And Breast Cancer Progression

Epithelial-mesenchymal transition (EMT) is a change of cellular plasticity and has beenidentified as a crucial process in embryonic development. In pathological process, thepotential role of EMT has been extensively studied over the years, including its role in theprogression of carcinoma and fibrosis of tissues and organs. Oncogenic EMT refers to theprocess in which epithelial malignant cells acquire mesenchymal cell phenotype, includingenhanced migratory capacity and invasiveness, and is generally accepted as a mechanismunderlying metastasis in many types of cancer. TGF-β1is a potent inducer of EMT; enhancedTGF-β1signaling in epithelial cells is thought to drive EMT. CDK5is a proline-directedserine/threonine kinase, and plays important roles in cancer progression.We found that CDK5and its co-activator p35were commonly overexpressed in breastcancers and were closely related with the breast cancer malignancy. Meanwhile, TGF-β1upregulated the expression of CDK5and p35in human mammary epithelial cell lineMCF10A. Moreover, knockdown of CDK5inhibited TGF-β1-induced EMT in MCF10A cells,as manifested by the changes in cell morphology and in expression levels of essential EMTmarkers. Likewise, knockdown of CDK5or inhibition of CDK5activity suppressed themigration and invasion of breast cancer cell lines MDA-MB-231and BT549. We thenshowed that nude mice orthotopically injected with MDA-MB-231-shCDK5andBT549-shCDK5cells in which CDK5expression was silenced formed less and smallertumors than that MDA-MB-231-shCtrl and BT549-shCtrl cells, indicating that CDK5functioned as a tumor-promoting factor in vivo.At the molecular level, shRNA-mediated CDK5silence or Roscovitine （Rv）-mediatedCDK5activity inhibition in breast cancer cells resulted in downregulation of mesenchymalmarker α-SMA, and in reduction of the phosphorylation of FAK protein at Ser-732.Furthermore, knockdown of CDK5altered the cellular distribution of F-actin. Significantly,we found that CDK5was abnormally overexpressed in clinical human breast cancer samplesand was significantly correlated with several poor prognostic parameters of breast cancer.Overall, our results suggest that, as a downstream step of the TGF-β1signaling, theCDK5-FAK pathway is essential for EMT and migration/invasion in breast cancer cells. Thisstudy implicates the potential perspective of the use of CDK5as a responsive target forinhibition of EMT and for prevention of cancer cell invasion and metastasis.