Ligand-of-Numb protein X (LNX) was initially characterized as a RING finger type E3 ubiquitin ligase that targeted the intrinsic cell fate determinant Numb for ubiquitination dependent degradation. However, the physiological function of LNX remains largely unknown. In the present study, we demonstrate that ectopic expression of LNX in human proximal tubular epithelial cells (HK-2 cells) significantly enhanced TGF-beta1 induced epithelial to mesenchymal transition (EMT). The EMT-promoting effect of LNX manifested as strong inhibition of E-cadherin expression, enhanced expression of vimentin, fibronectin or PAI-1, and increased cell migration. This function of LNX was shown to be independent of its ligase activity because ectopic expression of a mutant form of LNX (C48ALNX) that lacks E3 ligase activity had the similar effect as the wild-type LNX. Overexpression of E-cadherin could inhibit LNX augmented EMT. This study suggests a potential role for LNX in promoting EMT in human proximal tubular epithelial cells.