Renal fibrosis is defined by the exaggerated accumulation of extracellular matrix proteins. Tissue transglutaminase (TG2) modifies the stability of extracellular matrix proteins and renders the extracellular matrix resistant to degradation. In addition, TG2 also activates transforming growth factor-beta (TGF-beta). We investigated the involvement of TG2 in the development of renal fibrosis using mice with a knockout of the TG2 gene (KO). These mice were studied at baseline and 12 days after unilateral ureteral obstruction, which induced a significant increase in interstitial TG2 expression in wild-type mice (P < 0.001). Interstitial fibrosis was evident in both groups, but total and fibrillar collagen was considerably lower in KO mice as compared with wild-type (P < 0.001). Similarly, mRNA and protein expression of collagen I were significantly lower in KO animals (P < 0.05). A statistically significant reduction in renal inflammation and fewer myofibroblasts were observed in KO mice (P < 0.01). Free active TGF-beta was decreased in KO mice (P < 0.05), although total (active + latent) TFG-beta concentration did not differ between groups. These results show that mice deficient in TG2 are protected against the development of fibrotic lesions in obstructive nephropathy. This protection results from reduced macrophage and myofibroblast infiltration, as well as from a decreased rate of collagen I synthesis because of decreased TGF-beta activation. Our results suggest that inhibition of TG2 may provide a new and important therapeutic target against the progression of renal fibrosis.

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase (NOS) inhibitor, is accumulated in plasma during chronic kidney disease (CKD). It is considered an independent mortality and cardiovascular risk factor in CKD patients. To test the involvement of ADMA in CKD progression, we investigated the effects of chronic ADMA administration on renal structure and compared these effects with NG-nitro-L-arginine methyl ester (L-NAME) treatment, a widely used exogenous inhibitor of NOS that induces CKD. Three groups of uninephrectomized mice were studied: ADMA (60 mg/kg per day), L-NAME (60 mg/kg per day), and isotonic saline (control) were infused through osmotic mini-pumps for 8 weeks. ADMA and L-NAME induced hypertension (PAS 167 ± 16 and 168 ± 10 versus 100 ± 4 mmHg, p < 0.01, respectively). High level of ADMA was associated with increased renal oxidative stress. ADMA treatment induced glomerular and vascular fibrosis as evidenced by the elevated deposits of collagen I, III, and fibronectin (p < 0.01). A similar profile was observed in the L-NAME group. Mice treated with ADMA had reduced peritubular capillaries versus controls (p < 0.01). Collagen I mRNA expression and renal TGF-β1 concentrations were higher in the ADMA and L-NAME groups. Increased level of TGF-β1 was associated with a significant rise of HIF-1α and endothelin-1 expression. These results demonstrate for the first time that elevated concentrations of ADMA are associated with the development of renal fibrosis. These data suggest that in pathophysiological conditions of endothelial dysfunction, the exaggerated endogenous synthesis of ADMA could contribute to CKD progression by favouring hypertension, extracellular matrix synthesis, and rarefaction of peritubular capillaries.

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