Greg H. Tesch, 1,2 Frank Y. Ma, 1,2 Yingjie Han, 1,2 John T. Liles, 3 David G. Breckenridge, 3 and David J. Nikolic-Paterson 1,2 ASK1 Inhibitor Halts Progression of Diabetic Nephropathy in Nos3-Deficient Mice Diabetes 2015;64:3903–3913 | DOI: 10.2337/db15-0384 p38 mitogen-activated protein kinase (MAPK) signaling promotes diabetic kidney injury. Apoptosis signal- regulating kinase (ASK)1 is one of the upstream kinases in the p38 MAPK-signaling pathway, which is activated by inflammation and oxidative stress, suggesting a possible role for ASK1 in diabetic nephropathy. In this study, we examined whether a selective ASK1 inhibitor can pre- vent the induction and progression of diabetic nephrop- athy in mice. Diabetes was induced in hypertensive endothelial nitric oxide synthase (Nos3)-deficient mice by five low-dose streptozotocin (STZ) injections. Groups of diabetic Nos3 2/2 mice received ASK1 inhibitor (GS- 444217 delivered in chow) as an early intervention (2–8 weeks after STZ) or late intervention (weeks 8–15 after STZ). Control diabetic and nondiabetic Nos3 2/2 mice received normal chow. Treatment with GS-444217 abro- gated p38 MAPK activation in diabetic kidneys but had no effect upon hypertension in Nos3 2/2 mice. Early intervention with GS-444217 significantly inhibited dia- betic glomerulosclerosis and reduced renal dysfunction but had no effect on the development of albuminuria. Late intervention with GS-444217 improved renal func- tion and halted the progression of glomerulosclerosis, renal inflammation, and tubular injury despite having no effect on established albuminuria. In conclusion, this study identifies ASK1 as a new therapeutic target in diabetic nephropathy to reduce renal inflammation and fibrosis independent of blood pressure control. Diabetic nephropathy is the most common single cause of end-stage renal failure in many countries. Current ther- apies of controlling blood pressure and blood glucose levels have only a limited benefit in slowing progression to end-stage disease (1), indicating a major gap in our treat- ment options. The diabetic kidney is stressed by multiple factors including hyperglycemia, reactive oxygen species, advanced glycation end products, proinflammatory cyto- kines, and angiotensin II—all of which can induce signaling via the p38 mitogen-activated protein kinase (MAPK) (2–5). Increased activation of p38 MAPK in glomeruli and the tubulointerstitial compartment has been described in animal models of diabetic nephropathy as well as in patients with diabetic nephropathy (6–9). Inhibition of p38 MAPK activation via pharmacologic and genetic approaches can suppress the induction of albuminuria, glomerular matrix expansion, and inflammation in models of type 1 and type 2 diabetic nephropathy (9,10), although intervention studies in established disease are lacking. Importantly, clinical trials of p38 inhibitor compounds in rheumatoid arthritis have failed to deliver the promise of animal stud- ies owing to toxicity issues, which have led to investigation of the upstream kinases involved in context-dependent p38 MAPK activation (11). Apoptosis signal-regulating kinase 1 (ASK1/MAP3K5) is a member of the large family of MAPK kinase kinase enzymes, many of which have the potential to activate the downstream p38 MAPK via phosphorylation of the MKK3 and MKK6 enzymes (12,13). p38 MAPK and, to a lesser extent, c-Jun terminal kinase are the only known down- stream targets of ASK1 signaling (14). ASK1 is activated in response to oxidative stress; specifically, ASK1 exists as an inactive dimer coupled to thioredoxin and undergoes autoactivation after the oxidation and dissociation of thi- oredoxin (13). ASK1 is widely expressed in diverse tissues and is most highly expressed in the kidney (15). Mice lacking the Ask1 gene (Ask1 2/2 ) have a normal phenotype, 1 Department of Nephrology, Monash Medical Centre, Clayton, Victoria, Australia 2 Monash University Department of Medicine, Clayton, Victoria, Australia 3 Gilead Sciences, Foster City, CA Corresponding author: Greg H. Tesch, greg.tesch@monash.edu. Received 19 March 2015 and accepted 6 July 2015. This article contains Supplementary Data online at http://diabetes .diabetesjournals.org/lookup/suppl/doi:10.2337/db15-0384/-/DC1. © 2015 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. Diabetes Volume 64, November 2015 3903 COMPLICATIONS