Kidney Proximal Tubular Epithelial-Specific Overexpression of Netrin-1 Suppresses Inflammation and Albuminuria through Suppression of COX-2-Mediated PGE2 Production in Streptozotocin-Induced Diabetic Mice

Inflammation plays a key role in the development and progression of diabetic kidney disease; however, the role of the anti-inflammatory molecule netrin-1 in diabetic kidney disease is unknown. We examined the role of netrin-1 in diabetes-induced kidney inflammation and injury using tubule-specific netrin-1 transgenic mice. Diabetes was induced using streptozotocin in wild-type and netrin-1 transgenic animals. Kidney function, fibrosis, glucose excretion, albuminuria, and inflammation were evaluated. The mechanism of netrin-1-induced suppression of inflammation was studied in vitro using a proximal tubular epithelial cell line. Diabetes was associated with increased infiltration of neutrophils and macrophages, chemokine expression, and tubular epithelial cell apoptosis in kidney. These changes were minimal in kidney of netrin-1 transgenic mice. In addition, diabetes induced a large increase in the excretion of prostaglandin E2 (PGE2) in urine, which was suppressed in netrin-1 transgenic mice. Netrin-1-induced suppression of PGE2 production was mediated through suppression of NFκB-mediated cyclooxygenase-2 (COX-2) in renal tubular epithelial cells. Furthermore, netrin-1 also increased albumin uptake by proximal tubular epithelial cells through the PI3K and ERK pathways without increasing glucose uptake. These findings suggest that netrin-1 is a major regulator of inflammation and apoptosis in diabetic nephropathy and may be a useful therapeutic molecule for treating chronic kidney diseases such as diabetic nephropathy.Diabetic nephropathy is the largest single cause of end-stage renal failure worldwide.¹ Despite the available modern therapies of glycemic and blood pressure control for diabetes, many patients continue to experience progressive renal damage.^2,3 It is extremely important, therefore, to identify novel interventions for halting the progression of diabetic nephropathy. Diabetic nephropathy has traditionally been considered a nonimmune disease; however, an increased presence of glomerular and interstitial immune cell infiltrates and increased expression of inflammatory cytokines in diabetic kidney have been reported in both human biopsies and animal models.^4–7 Moreover, recent studies suggest that diabetic nephropathy is also a tubular disease, and that early changes in tubular epithelial cells may be a critical factor in development of progressive kidney diseases.^8–12 Inflammation from tubular epithelial cells can damage other areas of the kidney, including the vasculature and glomerular mesangial cells, via inflammatory mediators such as prostanoid metabolites, cytokines, and chemokines. These mediators will induce hyperfiltration, matrix expansion, apoptosis, and vasodilation, and further increase the production of their own and other mediators of cell injury. Suppression of local inflammation in the tubular epithelium may therefore provide a more effective prevention strategy against the development of diabetic nephropathy, compared with treatments such as glycemic and blood pressure control. Recent studies from our laboratory have shown that netrin-1 effectively suppresses inflammation in an acute model of kidney disease. However, the role of netrin-1 in chronic kidney diseases is unknown. Moreover, the mechanisms as to how netrin-1 suppresses inflammation are unknown.Netrin-1 is a laminin-related secreted molecule that has been identified as a neuronal guidance cue, directing neurons and their axons to targets during development of the nervous system. However, guidance is unlikely to be the only function of netrin-1, netrins are widely expressed outside the nervous system, including in vascular endothelial^13,14 and kidney tubular epithelial cells. Vascular endothelial cells form a critical barrier for leukocyte migration into organs by producing repellent factors to leukocytes, such as netrin-1. Down-regulation of netrin-1 during organ injury is reported to exacerbate inflammation.^13,14 We have reported that administration or overexpression of netrin-1 protects the kidney against ischemia-reperfusion injury.¹³ However, nothing was known about the involvement of netrin-1 in diabetic nephropathy, warranting further investigation. The purpose of the present study was to determine the effect of tubular-specific overexpression of netrin-1 on diabetes-induced inflammation and nephropathy in mice.