Induction of monocyte chemoattractant protein-1 in proximal tubule cells by urinary protein

Cytokines play a pivotal role in synthesis and deposition of extracellular matrix in chronic renal failure (CRF). The proinflammatory properties of monocyte chemoattractant protein (MCP)-1 make it an ideal candidate cytokine for the production of interstitial inflammation in CRF. To investigate the possible role of proteinuria in inducing proximal tubular (PT) MCP-1, MCP-1 mRNA levels were measured by Northern blot and reverse transcription PCR in confluent monolayers of PT cells in primary culture in media containing a variety of proteins. PT cells produced MCP-1 mRNA in response to bovine serum albumin (BSA), delipidated BSA (dBSA; 0.5 to 30 mg/ml), holotransferrin, and apotransferrin (1 to 8 mg/ml). Unstimulated PT cells expressed very low levels of MCP-1 mRNA, detectable by reverse transcription PCR but not by Northern blot. The expression of MCP-1 mRNA reached a peak (sixfold greater than control) within 4 h of exposure to dBSA and was maintained for at least 24 h with continued exposure. Removal of dBSA from the media led to a rapid decline in MCP-1 mRNA expression. dBSA-induced MCP-1 expression was inhibited by lysine, an inhibitor of protein uptake, and reproduced by dBSA purified by gel and size-selective filtration. dBSA influenced MCP-1 expression at the level of transcription and probably translation, as evidenced by abrogation of MCP-1 by actinomycin D and superinduction with the protein synthesis inhibitor cycloheximide. The concentration of MCP-1 protein in response to dBSA added to the apical surface of PT cells was 2.4-fold greater in basolateral than in apical media, indicating basolateral secretion of MCP-1 protein. In summary, PT cell MCP-1 mRNA and protein expression are upregulated by albumin and transferrin, in concentrations similar to those of proteinuric urine. This effect could explain the link between proteinuria and interstitial inflammation in CRF.