The role of kallistatin in renal fibrosis

Abstract

Chronic kidney disease (CKD) is a progressive loss of renal function over time. Among different kinds of CKD, diabetic nephropathy (DN) from type 2 diabetes, IgA nephropathy (IgAN), and hypertensive renal disease are the major causes of end-stage renal disease (ESRD) in many parts of the world including Hong Kong. Pharmacological treatment including angiotensin converting enzyme inhibitor (ACEi) or angiotensin II receptor blocker (ARB) are commonly used to ameliorate the decline of kidney function. However, their efficacy is limited. Renal fibrosis is an important component of the pathological process underpinning the progression of CKD. Kallistatin, a tissue kallikrein-binding protein, exerted an anti-fibrotic effect via attenuation of TGF-β signaling in the diabetic kidney and was reported to regulate Wnt/β-catenin signaling in cancers. Recent studies demonstrated that the Wnt/β-catenin signaling pathway also promoted renal interstitial fibrosis and podocyte dysfunction in CKD. Here, I present data showing the mechanisms where kallistatin ameliorates renal fibrosis via regulation of β-catenin signaling in human proximal tubular epithelial cells. My in vitro study revealed the protective function of kallistatin on TGF-β-induced fibrotic responses in human proximal tubular epithelial cells (HK-2 cells). Kallistatin overexpression by plasmid transfection not only reduced the expression of fibrotic markers such as collagen I and fibronectin in TGF-β-stimulated HK-2 cells, but also suppressed the nuclear translocation of β-catenin and expression of Snail, a transcription factor for the induction of epithelial-mesenchymal transition (EMT). In the animal model of unilateral ureteral obstruction (UUO), I further confirmed the role of kallistatin on renal fibrosis and Wnt/β-catenin signaling. Kallistatin overexpression using the ultrasound-microbubble-mediated gene transfer technique protected UUO mice from macrophage infiltration and tubulointerstitial fibrosis via downregulation of TGF-β expression. On the other hand, depletion of endogenous kallistatin by exogenous administration of an anti-kallistatin antibody aggravated the fibrotic events in UUO rats via upregulation of β-catenin expression. These data suggested that kallistatin suppressed TGF-β expression in human proximal tubular epithelial cells and subsequently inhibited the downstream Wnt/β-catenin activation to mitigate renal fibrosis. Finally, an elevation of urinary kallistatin protein level was observed in both UUO mice and patients with DN and IgAN. This increase in urine kallistatin levels was highly correlated with urine albumin-to-creatinine ratio (UACR) levels in DN patients, suggesting a compensatory mechanism of kallistatin in renoprotection and a potential use of urinary kallistatin as a diagnostic biomarker for DN, one of the main causes of CKD. In summary, an anti-fibrotic role of kallistatin, via modulating Wnt/β-catenin signaling, was found in the progression of CKD. Upregulation of kallistatin may be further exploited as a novel tool for kidney protection in CKD.