Abstract: Objective To observe the effects of Casitas B lymphoma (CBL) protein on proliferation, migration and invasion of breast cancer cells and explore its mechanism of action. Methods Cultured breast cancer cell lines MDA-MB-231 and MCF7A were transfected with a CBL-overexpressing plasmid and a specific siRNA targeting CBL (siRNA-CBL), respectively, and the changes in cell proliferation, migration and invasion were examined using colony-forming assay, cell counting kit-8 (CCK-8), scratch test and Transwell assay. Flow cytometry and Western blotting were performed to examine the effects of CBL overexpression on cell cycle and epithelial-mesenchymal transition (EMT) of MDA-MB-231 cells, and the changes in the number of filamentous pseudopodia were observed by rhodamine- labeled phalloidin staining of the cytoskeleton. IP-mass spectrometry identified NCK2 as the interacting proteins of CBL, and their interaction was verified by immunoprecipitation and immunofluorescence co-localization experiments in HEK-293T cells transfected with the plasmids for overexpression of CBL, NCK2, or both. Cycloheximide tracking and ubiquitination assays were used for assessing the effects of CBL on stability and ubiquitination of NCK2 protein in MDA-MB-231 cells; CCK-8 and Transwell assays were used to determine the effect of NCK2 overexpression on CBL-mediated proliferation and migration of the cells. Results The proliferation, migration and invasion were significantly suppressed in MDA-MB-231 cells overexpressing CBL (P<0.05) and significantly enhanced in MCF7 cells with CBL silencing (P<0.01). Silencing of CBL promoted G1/S transition in MCF7 cells (P<0.05). Overexpression of CBL significantly decreased the expressions of CDK2/4 (P<0.01), cyclin A2/B1/D1/D3/E2 (P<0.05), Snail, N-cadherin, claudin-1 (P<0.05), and upregulated the expression of E-cadherin (P<0.05). CBL silencing upregulated the expressions of CDK2/4/6 (P<0.05), cyclin A2/B1/D1/D3/E2 (P<0.05), Snail, vimentin, and claudin-1 (P<0.05) and down-regulated E-cadherin expression (P<0.05). CBL overexpression obviously reduced the number of filamentous pseudopodia in MDA-MB-231 cells, and the reverse changes were observed in MCF7 cells with CBL silencing. In MDA-MB-231 cells, CBL overexpression lowered NCK2 protein stability (P<0.05) and promoted its ubiquitin-mediated degradation (P<0.01). Overexpression of NCK2 obviously reversed CBL-mediated inhibition of cell proliferation and migration (P<0.01). Conclusion CBL can inhibit the proliferation, migration and invasion of breast cancer cells through ubiquitination-mediated degradation of NCK2.

Key words: CBL; tyrosine kinase non-catalytic region protein 2; E3 ligase; ubiquitylation; cell proliferation; cell invasion