Date Published: February 14, 2019
Publisher: Public Library of Science
Author(s): Brad P. Dieter, Rick L. Meek, Robert J. Anderberg, Sheryl K. Cooney, Jen L. Bergin, Hongyu Zhang, Viji Nair, Matthias Kretzler, Frank C. Brosius, Katherine R. Tuttle, Stuart E Dryer.
Serum amyloid A (SAA), a potent inflammatory mediator, and Janus kinase 2 (JAK2), an intracellular signaling kinase, are increased by diabetes. The aims were to elucidate: 1) a JAK2-mediated pathway for increased SAA in the kidneys of diabetic mice; 2) a JAK2-SAA pathway for inflammation in podocytes.
Akita diabetic mice (129S6) with podocyte JAK2 overexpression and angiotensin II infusion (4 weeks) were given a JAK1,2 inhibitor (LY03103801, 3 mg/kg/day orally for the last two weeks). Kidneys were immunostained for SAA isoform 3 (SAA3). SAA3 knockout and control mouse podocytes were exposed to advanced glycation end products (AGE) or exogenous SAA with JAK2 inhibition (Tyrphostin AG 490, 50μM). JAK2 activity (phosphorylation, Western blot, 1 hour) and mRNA for SAA3 and associated inflammatory genes (Cxcl5, Ccl2, and Ccl5) were measured by RT-PCR (20 hours).
SAA3 protein was present throughout the diabetic kidney, and podocyte JAK2 overexpression increased tubulointerstitial SAA3 compared to wild type diabetic controls, 43% versus 14% (p = 0.007); JAK1,2 inhibition attenuated the increase in SAA3 to 15% (p = 0.003). Urine albumin-to-creatinine ratio (r = 0.49, p = 0.03), mesangial index (r = 0.64, p = 0.001), and glomerulosclerosis score (r = 0.51, p = 0.02) were associated with SAA3 immunostaining scores across mouse groups. Exposing podocytes to AGE or exogenous SAA increased JAK2 activity within one hour and mRNA for associated inflammatory genes after 20 hours. JAK2 inhibition reduced SAA3 mRNA expression in podocytes exposed to AGE or SAA. SAA3 knockout podocytes had >85% lower AGE-induced inflammatory genes.
JAK1,2 inhibition reduced SAA and histological features of DKD in podocyte JAK2-overexpressing mice. In podocytes exposed to a diabetes-like condition, JAK2 inhibition reduced expression of SAA, while SAA knockout blocked expression of associated pro-inflammatory mediators. SAA may promote JAK2-dependent inflammation in the diabetic kidney.
Pro-inflammatory mediators in the diabetic kidney are induced by activation of various signaling cascades [1–3]. Comparison of transcriptional networks between humans with diabetes and corresponding mouse models identified shared signals in the diabetic kidney [4, 5]. These signals, and their downstream pro-inflammatory mediators, provide candidate targets for new therapeutics for diabetic kidney disease (DKD). Janus kinase 2 (JAK2) is an intracellular tyrosine kinase that transduces cytokine-mediated signals. Compared to non-diabetic individuals, JAK2 is expressed at higher levels in the glomeruli and tubulointerstitium of patients with DKD . In diabetic mice, podocyte-specific JAK2 overexpression exacerbated histological features of DKD and produced a phenotype similar to human DKD .
Podocyte JAK2 overexpression in diabetes independently and synergistically increased SAA in the mouse kidney, which directly correlated with glomerular damage. Exposure to AGE or SAA activated JAK2 signaling to produce a pro-inflammatory response in podocytes, while knockout of SAA3 had a profound inhibitory effect on expression of JAK2-associated inflammatory mediators. Taken together, the present data show SAA to be a downstream mediator of JAK2 that may mechanistically contribute to podocyte-derived inflammation and consequent kidney damage in diabetes.