Decorin deficiency in diabetic mice: aggravation of nephropathy due to overexpression of profibrotic factors, enhanced apoptosis and mononuclear cell infiltration

Although deficiency of the small leucine-rich proteoglycan decorin aggravates diabetic nephropathy in mice, the precise mechanisms of action are not fully understood. In the present study we used decorin-deficient mice (Dcn(-/-)) to further elucidate the molecular mechanisms involved in the protective action of decorin in diabetes. We discovered that streptozotocin-induced diabetes in Dcn(-/-) mice led to increased proteinuria associated with enhanced cyclin-dependent kinase inhibitor p27Kip1 in podocytes and tubular epithelial cells. Furthermore, lack of decorin increased the rate of apoptosis and caused overexpression of the IGF-IR in tubular epithelial cells of diabetic kidneys. In vitro experiments using human proximal renal epithelial cells showed that recombinant decorin was bound to the IGF-IR and protected against high glucose-mediated apoptosis. Furthermore, overexpression of TGFbeta1 and CTGF triggered by decorin deficiency resulted in enhanced accumulation of extracellular matrix in diabetic kidneys. Notably, diabetic Dcn(-/-) kidneys revealed marked upregulation of the proinflammatory proteoglycan biglycan and enhanced infiltration of mononuclear cells. Collectively, our results indicate that decorin is a protective agent during the development of diabetic nephropathy. Future therapeutic approaches that would either enhance the endogenous production of decorin or deliver recombinant decorin to the diseased kidney might improve the outcome of patients with diabetic nephropathy.

Biglycan, a danger signal that activates the NLRP3 inflammasome via toll-like and P2X receptors

The role of endogenous inducers of inflammation is poorly understood. To produce the proinflammatory master cytokine interleukin (IL)-1beta, macrophages need double stimulation with ligands to both Toll-like receptors (TLRs) for IL-1beta gene transcription and nucleotide-binding oligomerization domain-like receptors for activation of the inflammasome. It is particularly intriguing to define how this complex regulation is mediated in the absence of an infectious trigger. Biglycan, a ubiquitous leucine-rich repeat proteoglycan of the extracellular matrix, interacts with TLR2/4 on macrophages. The objective of this study was to define the role of biglycan in the synthesis and activation of IL-1beta. Here we show that in macrophages, soluble biglycan induces the NLRP3/ASC inflammasome, activating caspase-1 and releasing mature IL-1beta without the need for additional costimulatory factors. This is brought about by the interaction of biglycan with TLR2/4 and purinergic P2X(4)/P2X(7) receptors, which induces receptor cooperativity. Furthermore, reactive oxygen species formation is involved in biglycan-mediated activation of the inflammasome. By signaling through TLR2/4, biglycan stimulates the expression of NLRP3 and pro-IL-1beta mRNA. Both in a model of non-infectious inflammatory renal injury (unilateral ureteral obstruction) and in lipopolysaccharide-induced sepsis, biglycan-deficient mice displayed lower levels of active caspase-1 and mature IL-1beta in the kidney, lung, and circulation. Our results provide evidence for direct activation of the NLRP3 inflammasome by biglycan and describe a fundamental paradigm of how tissue stress or injury is monitored by innate immune receptors detecting the release of the extracellular matrix components and turning such a signal into a robust inflammatory response.

Nitric oxide inhibits glomerular TGF-beta signaling via SMOC-1

Cytokines and nitric oxide (NO) stimulate rat mesangial cells to synthesize and secrete inflammatory mediators. To understand better the signaling pathways that contribute to this response, we exposed rat mesangial cells to the prototypic inflammatory cytokine IL-1beta and analyzed the changes in the pattern of gene expression. IL-1beta downregulated the gene encoding the matricellular glycoprotein secreted modular calcium-binding protein 1 (SMOC-1) in mesangial cells. Inflammatory cytokines attenuated SMOC-1 mRNA and protein expression through endogenous production of NO, which activated the soluble guanylyl cyclase. Silencing SMOC-1 expression with small interfering RNA decreased the formation of TGF-beta, reduced SMAD binding to DNA, and decreased mRNA expression of genes regulated by TGF-beta. In a rat model of anti-Thy-1 glomerulonephritis, glomerular SMOC-1 mRNA and protein decreased and inducible NO synthase expression increased simultaneously. Treatment of nephritic rats with the inducible NO synthase-specific inhibitor l-N(6)-(1-iminoethyl)-lysine prevented SMOC-1 downregulation. In summary, these data suggest that NO attenuates SMOC-1 expression in acute glomerular inflammation, thereby limiting TGF-beta-mediated profibrotic signaling.