Albumin stimulates renal tubular inflammation through a HSP70-TLR4 axis in early diabetic nephropathy

Abstract

Increased urinary albumin excretion is not simply an aftermath of glomerular injury, and also involves in the progression of diabetic nephropathy (DN). While toll-like receptors (TLRs) are incriminated in renal inflammation of DN, whether and how albumin is involved in TLR-related renal inflammatory response remains to be clarified. Here we showed that both TLR2 and TLR4, one of their putative endogenous ligands HSP70, and NF-κB promoter activity were markedly elevated in the kidney of diabetic mice. A deficiency of TLR4, but not TLR2, alleviated albuminuria, tubulointerstitial fibrosis, and inflammation induced by diabetes. The protection against renal injury in diabetic Tlr4−/- mice was associated with reduced tubular injuries and preserved cubilin levels, rather than amelioration of glomerular lesions. In vitro studies revealed that albumin, a stronger inducer than high-glucose, induced the release of HSP70 from proximal tubular cells. HSP70 blockade ameliorated albumin-induced inflammatory mediators. HSP70 triggered the production of inflammatory mediators in a TLR4-dependent manner. Moreover, HSP70 inhibition in vivo ameliorates diabetes-induced albuminuria, inflammatory response, and tubular injury. Finally, we found that DN patients had higher levels of TLR4 and HSP70 in the dilated tubules than non-diabetic controls. Thus, activation of the HSP70-TLR4 axis, stimulated at least in part by albumin, in the tubular cell is a novel mechanism associated with inducing tubulointerstitial inflammation and aggravating pre-existing microalbuminuria in the progression of DN.