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

Abstract

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.

Innate immunity in diabetes and diabetic nephropathy

“…In STZ-induced and HFD-induced diabetic mice, TLR2 localized on glomerular endothelial cells and repeated Porphyromonas gingivalis LPS administration enhanced urinary albumin excretion and production of IL6, TNF and LTA (TNFβ) in glomeruli 141 . In human proximal tubular cells, HMGB1-induced NFκB activation was prevented by TLR2 silencing, and the researchers of this study emphasized the importance of TLR2 as a mediator of NFκB activation in diabetic nephropathy 142 .…”

Beyond Tissue Injury—Damage-Associated Molecular Patterns, Toll-Like Receptors, and Inflammasomes Also Drive Regeneration and Fibrosis

“…160 The production of another proinflammatory molecule (i.e., HA) is also induced by hyperglycemia through a protein kinase C/TGF-b pathway. 163 Besides HA, several DAMPs, including the glucose-inducible HMGB1, 164 biglycan, and decorin, are overexpressed in diabetic kidneys and may trigger inflammation by activating TLR2/TLR4 receptors and NLRP3 inflammasomes. 47,51, 165 Increased renal biglycan levels correlate with enhanced infiltration of macrophages and renal LDL accumulation, and appear to promote kidney injury.…”

Links between coagulation, inflammation, regeneration, and fibrosis in kidney pathology

“…For example, tubular cell necrosis releases high-mobility group box 1 protein, an agonist to TLR2 and TLR4 and driver of inflammation during AKI. [67][68][69] Tamm-Horsfall protein/ uromodulin is exclusively expressed within the distal tubule and activates interstitial dendritic cells via TLR4 and NLRP3 whenever tubular injury promotes its leakage into the renal interstitium. 70 Toll-like receptors and inflammasomemediated immune recognition of necrotic cell death is a central element of rapidly progressive glomerulonephritis (RPGN), thrombotic microangiopathies, and acute tubular necrosis (ATN) as the release of TNF-alpha induces further cell necrosis, eg, necroptosis.…”