The role of Toll-like receptor proteins (TLR) 2 and 4 in mediating inflammation in proximal tubules

• Published in: American journal of physiology. Renal physiology 2013-07, Vol.305 (2), p.F143-F154
• Main Authors: Mudaliar, Harshini, Pollock, Carol, Komala, Muralikrishna Gangadharan, Chadban, Steven, Wu, Huiling, Panchapakesan, Usha
• Format: Article
• Language: English
• Subjects: Animals; Cell Line; Cells; Cytokines; Diabetes; Diabetic Nephropathies - metabolism; Fibronectins - metabolism; Gene expression; Gene Silencing; Glucose - metabolism; HMGB1 Protein - metabolism; HMGB1 Protein - secretion; Humans; Hyperglycemia - metabolism; Inflammation - metabolism; Kidney Tubules, Proximal - metabolism; Kidneys; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; NF-kappa B - metabolism; Pathogenesis; PPAR gamma - metabolism; Rodents; T cell receptors; Toll-Like Receptor 2 - metabolism; Toll-Like Receptor 4 - metabolism; Up-Regulation
• ISSN: 1931-857X; 1522-1466
• DOI: 10.1152/ajprenal.00398.2012

Inflammatory responses are central to the pathogenesis of diabetic nephropathy. Toll-like receptors (TLRs) are ligand-activated membrane-bound receptors which induce inflammatory responses predominantly through the activation of NF-κB. TLR2 and 4 are present in proximal tubular cells and are activated by endogenous ligands upregulated in diabetic nephropathy, including high-mobility group box-1 (HMGB1) and fibronectin. Human proximal tubules were exposed to 5 mM (control), 11.2 mM (approximating the clinical diagnostic threshold for diabetes mellitus), and 30 mM (high) glucose for 72 h or 7 days. Cells were harvested for protein, mRNA, and nuclear extract to assess for TLR2, 4, and inflammatory markers. Glucose (11.2 mM) maximally increased TLR2 and 4 expression, HMGB1 release, and NF-κB activation with increased expression of cytokines. However, only TLR2 expression and subsequent NF-κB binding were sustained at 7 days. Recombinant HMGB1 induced NF-κB activation, which was prevented by both TLR2 silencing [small interfering (si)RNA] and TLR4 inhibition. Peroxisome proliferator-activated receptor-γ (PPAR-γ) transcription was reduced by exposure to 11.2 mM glucose with an increase observed at 30 mM glucose at 24 h. This may reflect a compensatory increase in PPAR-γ induced by exposure to 30 mM glucose, limiting the inflammatory response. Therefore, short-term moderate increases in glucose in vitro increase HMGB1, which mediates NF-κB activation through both TLR2 and 4. Furthermore, in vivo, streptozotocin-induced diabetic mice exhibited an increase in tubular TLR2 and HMGB1 expression. These results collectively suggest that TLR2 is likely to be the predominant long-term mediator of NF-κB activation in transducing inflammation in diabetic nephropathy.