['To determine the role of epithelial-mesenchymal transition (EMT) as a potential mechanism contributing to the characteristic tubulointerstitial renal fibrosis in multiple myeloma, we examined whether myeloma light chains (LCs) directly induce EMT in human renal proximal tubule epithelial cells (PTECs).', 'As positive controls we used TGF-beta1 and cyclosporine A (CsA), two agents known to induce EMT in PTECs. Human LCs were isolated and purified from the urine of myeloma patients with modest renal insufficiency without evidence of glomerular involvement. HK-2 cells were exposed to kappa LC (25 microM) for periods up to 72 h.', 'LCs induced marked cellular morphological alterations in PTECs, accompanied with increased expression levels of profibrotic TGF-beta1, FSP-1 and extracellular matrix components. Using semiquantitative immunoblotting and RT-PCR, we observed that the expression of E-cadherin decreased after 24 h, while the expression of alpha-SMA increased in PTEC after continuous exposure to kappa-LCs. Human serum albumin (HSA; 160 microM) had less potent effect on the expression of EMT-related molecules. Neutralizing TGF-beta1 antibody blocked CsA-induced EMT but had no effect on LC-exposed cells. LC-induced EMT and the secretions of IL-6 and MCP-1 were, however, markedly attenuated by p38 MAPK interference. The use of bone morphogenetic protein-7 or pituitary adenylate cyclase-activating polypeptide (PACAP) induced the formation of cell aggregates, and the reacquisition of E-cadherin expression and renal proximal tubule epithelial morphology within the confluent cell monolayer during and after LC exposure.'] Do myeloma light chains induce epithelial-mesenchymal transition in human renal proximal tubule epithelial cells?