Inflammation contributes to the tubulointerstitial lesions of diabetic nephropathy. Toll-like receptors (TLRs) modulate immune responses and inflammatory diseases, but their role in diabetic nephropathy is not well understood. In this study, we found increased expression of TLR4 but not of TLR2 in the renal tubules of human kidneys with diabetic nephropathy compared with expression of TLR4 and TLR2 in normal kidney and in kidney disease from other causes. The intensity of tubular TLR4 expression correlated directly with interstitial macrophage infiltration and hemoglobin A1c level and inversely with estimated glomerular filtration rate. The tubules also upregulated the endogenous TLR4 ligand high-mobility group box 1 in diabetic nephropathy. In vitro, high glucose induced TLR4 expression via protein kinase C activation in a time-and dose-dependent manner, resulting in upregulation of IL-6 and chemokine (C-C motif) ligand 2 (CCL-2) expression via IkB/NF-kB activation in human proximal tubular epithelial cells. Silencing of TLR4 with small interfering RNA attenuated high glucose-induced IkB/NF-kB activation, inhibited the downstream synthesis of IL-6 and CCL-2, and impaired the ability of conditioned media from high glucosetreated proximal tubule cells to induce transmigration of mononuclear cells. We observed similar effects using a TLR4-neutralizing antibody. Finally, streptozotocin-induced diabetic and uninephrectomized TLR4-deficient mice had significantly less albuminuria, renal dysfunction, renal cortical NF-kB activation, tubular CCL-2 expression, and interstitial macrophage infiltration than wild-type animals. Taken together, these data suggest that a TLR4-mediated pathway may promote tubulointerstitial inflammation in diabetic nephropathy.
Since the efficacy of mycophenolate mofetil (MMF) to treat immunoglobulin A (IgA) nephropathy is controversial, we extended our original study by following 40 Chinese patients with established IgA nephropathy for 6 years. All patients were maintained on their angiotensin blockade medication and half were randomized to receive MMF for 6 months. After 6 years, 11 patients required dialysis (2 from the MMF and 9 from the control group). Significantly, only 3 treated (as compared to 10 control) patients reached the composite end point of serum creatinine doubling or end-stage renal disease. Linear regression showed the annualized decline in the estimated glomerular filtration rate was significantly less in the MMF-treated group. Urinary protein excretion and the albumin-to-creatinine ratio were lower with MMF treatment during the first 24 months, beyond which there was no difference between groups. Multivariable Cox regression analysis showed that the baseline estimated glomerular filtration rate and proteinuria, and change in the urine albumin-to-creatinine ratio at 1 year to be important predictors of progression to end-stage renal disease. We found that among Chinese patients with IgA nephropathy who had mild histologic lesions and persistent proteinuria despite maximal angiotensin blockade, MMF treatment may result in transient and partial remission of proteinuria in the short-term and renoprotection in the long-term.
In chronic glomerulopathic disease, renal function correlates more with the degree oftubulointerstitial injury than that of the glomerular lesions. Proteinuria may be one of the pathologic links between these two intrarenal compartments. It is apparent that the proximal tubular epithelial cell (PTEC) assumes a proinflammatory and profibrotic role during proteinuria in which the PTEC expresses a variety of chemokines and injury signals that culminate in progressive interstitial inflammation and fibrosis. During diabetes, other substrates including advanced glycation end products (AGEs), AGE intermediates, and high glucose (HG) may provoke the PTEC even further. Glycated albumin, but not the equivalent dose of bovine serum albumin (BSA), stimulates tubular IL-8 and ICAM-1 expression via NF-κB-, MAPK- and STAT-1-dependent pathways. Human biopsies of diabetic nephropathy (DN) reveal colocalization of AGE and ICAM-1 in proximal tubules. The biologically active carbonyl intermediates methylglyoxal-BSA-AGE and AGE-BSA upregulate tubular expression of CTGF, TGF-β, and VEGF, whereas carboxymethyllysine-BSA stimulates tubular expression of IL-6, CCL-2, CTGF, TGF-β, and VEGF via RAGE activation and NF-κB signal transduction. Hyperglycemia (30 mM), but not the equivalent dose of mannitol, promotes proinflammatory (IL-6 and CCL-2), profibrotic (TGF-β) and angiogenic (VEGF) responses in tubular cells via MAPK and PKC signaling and induces epithelial mesenchymal transition, which is TGF-β1 mediated. It has recently been shown that toll-like receptor (TLR) is implicated in the diabetic kidney. In human DN biopsies and PTEC, TLR4is upregulated and plays a permissive role in HG-induced IL-6 and CCL-2 overexpression and monocyte transmigration. In streptozotocin-induced rat DN and PTEC, TLR2 appears to be upregulated. Other novel mediators that become activated in PTEC exposed to HG include macrophage inflammatory protein-3-α, Krüppel-like factor 6 and thioredoxin-interacting protein, which may be attenuated by peroxisome proliferator-activate dreceptor-γ activation. Collectively, these phenomena suggest that the renal tubules are heavily involved in the pathogenesis of DN. These pathophysiologic responses may be collectively described as diabetic tubulopathy.
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