Albumin induces cellular fibrosis by upregulating transforming growth factor-beta ligand and its receptors in renal distal tubule cells

Albuminuria is indicative of nephropathy. However, little literature has focused on the role of albumin in renal distal tubule fibrosis. We used a well‐defined distal tubule cell, Madin‐Darby Canine Kidney (MDCK). Proliferation and cytotoxicity were examined. The conditioned supernatant was collected and subjected to ELISA assay for detection of fibronectin and TGF‐β1. Reverse transcription‐PCR and Western blot assay were performed to evaluate the expression of mRNA and protein of two types of TGF‐β receptors (TbetaR). Flow cytometry assay and phosphotyrosine (pY)‐specific antibodies were used to assay the phosphorylation status of TbetaR. We showed that albumin dose dependently (0, 0.1, 1, or 10 mg/ml) inhibited cellular growth in MDCK cells without inducing cellular cytoxicity. In addition, albumin significantly upregulated the secretion of both fibronectin and TGF‐β1 at dose over 1 mg/ml. Moreover, 24 h pretreatment of albumin significantly enhanced exogenous TGF‐β1‐induced secretion of fibronectin. These observations were reminiscent of the implications of TbetaR since TbetaR appears to correlate with the susceptibility of cellular fibrosis. We found that albumin significantly increased protein levels of type I TbetaR (TbetaRI) instead of type II receptors (TbetaRII). In addition, phosphorylation level of TbetaRII of both pY259 and pY424 was significantly enhanced instead of pY336. The novel observation indicates that extreme dose of albumin upregulates TGF‐β autocrine loop by upregulating TGF‐β1, TbetaRI, and the receptor kinase activity of TbetaRII by inducing tyrosine phosphorylation on key amino residue of TbetaRII in renal distal tubule cells. These combinational effects might contribute to the pathogenesis of renal fibrosis. J. Cell. Biochem. 97: 956–968, 2006. © 2005 Wiley‐Liss, Inc.