Interstitial fibrosis in mice with overload proteinuria: deficiency of TIMP-1 is not protective

Progressive renal interstitial fibrosis is characterized by up-regulated expression of the gene that encodes the tissue inhibitor of metalloproteinases-1 (TIMP-1), a regulator of extracellular matrix remodeling, suggesting that impaired matrix turnover contributes to the fibrogenic process. The present study was designed to develop a murine model of renal interstitial fibrosis, and to determine the functional significance of up-regulated Timp-1 expression by comparing the severity of this renal disease in wild-type mice and mice genetically deficient in Timp-1.

Initial pilot studies developed and characterized a murine model of bovine serum albumin (BSA)-induced protein-overload proteinuria with respect to the degree of proteinuria, severity of interstitial fibrosis, and renal mRNA levels for genes encoding matrix proteins, transforming growth factor-beta1 (TGF-beta1), and TIMP-1, -2, -3, and -4. In the final study, the severity of interstitial fibrosis was compared in wild-type and Timp-1-deficient mice after six weeks of proteinuria.

Mice injected with large daily intraperitoneal doses of BSA developed proteinuria, interstitial inflammation, and progressive interstitial fibrosis. A time course study based on measurements after one, two, and six weeks of BSA injections showed increased renal mRNA levels for the matrix genes procollagens alpha1(I), alpha1(III), and alpha2(IV) and TGF-beta1 and Timp-1. Timp-2 and Timp-3 genes were constitutively expressed at high levels in the normal kidneys and showed little change in the proteinuric kidneys. Timp-4 transcripts were not detected in any of the kidneys. After six weeks of BSA overload-proteinuria, the groups (N = 8 per group) of wild-type and Timp-1-deficient mice developed significant interstitial fibrosis compared with the control saline-injected groups. The severity of the interstitial fibrosis was similar in both proteinuric groups based on an assessment of the final kidney weight, total kidney collagen content, and the number of interstitial fields with increased fibronectin staining.

Results of the present study indicate that TIMP-1 deficiency does not alter the degree of interstitial fibrosis in the murine overload proteinuria model. Potential explanations include Timp-1 genetic redundancy, as suggested by the observation that, despite significant intrarenal induction of the Timp-1 gene expression, net renal metalloproteinase-9 (MMP-9) activity was not significantly altered. TIMP-1 is a multifunctional protein that may play a metalloproteinase-independent role in response to renal injury.