Decorin, a small dermatan-sulfate proteoglycan, participates in extracellular matrix assembly and influences directly and indirectly cell behavior via interactions with signaling membrane receptors and transforming growth factor (TGF)-beta. We have therefore compared the development of tubulointerstitial kidney fibrosis in wild-type (WT) and decorin-/- mice in the model of unilateral ureteral obstruction. Without obstruction, kidneys from decorin-/- mice did not differ in any aspect from their WT counterparts. However, already 12 hours after obstruction decorin-/- animals showed lower levels of p27(KIP1) and soon thereafter a more pronounced up-regulation and activation of initiator and effector caspases followed by enhanced apoptosis of tubular epithelial cells. Later, a higher increase of TGF-beta1 became apparent. After 7 days, there was an up to 15-fold transient up-regulation of the related proteoglycan biglycan, which was mainly caused by the appearance of biglycan-expressing mononuclear cells. Other small proteoglycans showed no similar response. Because of enhanced degradation of type I collagen, end-stage kidneys from decorin-/- animals were more atrophic than WT kidneys. These data suggest that decorin exerts beneficial effects on tubulointerstitial fibrosis, primarily by influencing the expression of a key cyclin-dependent kinase inhibitor and by limiting the degree of apoptosis, mononuclear cell infiltration, tubular atrophy, and expression of TGF-beta1.

Small leucine-rich proteoglycans (SLRPs), for example, decorin, biglycan, fibromodulin, and lumican, are extracellular matrix organizers and binding partners of TGF-b. Decorin is also involved in growth control and angiogenesis. Hence, these proteoglycans are likely of importance in the pathogenesis of diabetic glomerulosclerosis. In normal kidney, SLRPs were preferentially expressed in the tubulointerstitium. Weak expression occurred in the mesangial matrix. Biglycan was expressed by glomerular endothelial cells and, together with fibromodulin, by distal tubular cells and in collecting ducts. In all stages of diabetic nephropathy, there was a marked up-regulation of the proteoglycans in tubulointerstitium and glomeruli. Decorin and lumican became expressed in tubuli. However, in glomeruli, overexpression was not mirrored by local proteoglycan accumulation except in advanced nephropathy. In severe glomerulosclerosis, increased decorin concentrations were found in plasma and urine, and urinary TGF-b/decorin complexes could be demonstrated indirectly. The failure to detect an increased glomerular proteoglycan quantity during the development of nephropathy could be explained by assuming that they are secreted into the mesangial matrix, but cleared via the vasculature or the urinary tract, in part as complexes with TGF-b. They could thereby counteract the vicious circle being characterized by increased TGF-b production and increased matrix deposition in diabetic nephropathy.

The data indicate that the different cells of the adult human kidney are characterized by a distinct expression pattern of the four small proteoglycans. It is suggested that these proteoglycans may have distinct pathophysiological roles depending upon whether they are expressed by mesangial cells, endothelial cells, epithelial cells, or cells of the tubulointerstitium.

During glomerular inflammation mesangial cells are the major source and target of nitric oxide that profoundly influences proliferation, adhesion, and death of mesangial cells. The effect of nitric oxide on the mRNA expression pattern of cultured rat mesangial cells was therefore investigated by RNA-arbitrarily-primed polymerase chain reaction. Employing this approach, biglycan expression turned out to be down-regulated timeand dose-dependently either by interleukin-1␤-stimulated endogenous nitric oxide production or by direct application of the exogenous nitric oxide donor, diethylenetriamine nitric oxide. There was a corresponding decline in the rate of biglycan biosynthesis and in the steady state level of this proteoglycan. In vivo, in a model of mesangioproliferative glomerulonephritis upregulation of inducible nitric-oxide synthase mRNA was associated with reduced expression of biglycan in isolated glomeruli. Biglycan expression could be normalized, both in vitro and in vivo, by using a specific inhibitor of the inducible nitric-oxide synthase, l-N 6 -(l-iminoethyl)-l-lysine dihydrochloride. Further studies showed that biglycan inhibited cell adhesion on type I collagen and fibronectin because of its binding to these substrates. More importantly, biglycan protected mesangial cells from apoptosis by decreasing caspase-3 activity, and it counteracted the proliferative effects of platelet-derived growth factor-BB. These findings indicate a signaling role of biglycan and describe a novel pathomechanism by which nitric oxide modulates the course of renal glomerular disease through regulation of biglycan expression.