The non-canonical ubiquitin activating enzyme UBA6 suppresses epithelial-mesenchymal transition of mammary epithelial cells

Ubiquitination plays critical roles in the regulation of oncoproteins and tumor suppressors during carcinogenesis. The two ubiquitin activating enzymes (E1) in human genome, UBA1 and UBA6, initiate ubiquitination by ATP-dependent activation of ubiquitin. Recent evidence suggests that UBA1 and UBA6 play partially overlapped yet distinct roles in controlling the proteome. Here we demonstrate that ubiquitination pathways initiated specifically by UBA6 set a suppressive barrier against critical steps of mammary carcinogenesis such as loss of polarity, anoikis resistance and epithelial-mesenchymal transition (EMT). Mammary epithelial MCF-10A cells expressing shRNA against UBA6 fail in establishing cell cycle arrest in response to detachment from extracellular matrix, confluency with fully engaged cell-cell contact or growth factor deprivation. Moreover, UBA6-deficient MCF-10A cells undergo spontaneous EMT under growth factor deprivation and exhibit accelerated kinetics of TGF-β-induced EMT. The Rho-GTPase CDC42 is one of the specific targets of UBA6-initiated ubiquitination and plays a key role in the function of UBA6 in controlling epithelial homeostasis, since a CDC42 inhibitor, ML141, rescues UBA6-deficient cells from the EMT phenotype. Immunohistochemical analysis of human breast cancer tissues demonstrates that 38% of invasive carcinomas express low or undetectable expression of UBA6, suggesting that downregulation of this non-canonical E1 plays a role in breast cancer development.