Astragaloside IV Attenuates Podocyte Apoptosis Mediated by Endoplasmic Reticulum Stress through Upregulating Sarco/Endoplasmic Reticulum Ca2+-ATPase 2 Expression in Diabetic Nephropathy.

Hengjiang Guo,Xiaodong Mao,Yingjun Zang,Shuang Chu,Wen Peng,Xuemei Zhang,Hao Wang,Yunman Wang,Aili Cao,Cheng Liu,Yi Wang

doi:10.3389/fphar.2016.00500

Abstract

Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) plays a central role in the pathogenesis of diabetes. This protein has been recognized as a potential target for diabetic therapy. In this study, we identified astragaloside IV (AS-IV) as a potent modulator of SERCA inhibiting renal injury in diabetic status. Increasing doses of AS-IV (2, 6, and 18 mg kg-1 day-1) were administered intragastrically to db/db mice for 8 weeks. Biochemical and histopathological approaches were conducted to evaluate the therapeutic effects of AS-IV. Cultured mouse podocytes were used to further explore the underlying mechanism in vitro. AS-IV dose-dependently increased SERCA activity and SERCA2 expression, and suppressed ER stress-mediated and mitochondria-mediated apoptosis in db/db mouse kidney. AS-IV also normalized glucose tolerance and insulin sensitivity, improved renal function, and ameliorated glomerulosclerosis and renal inflammation in db/db mice. In palmitate stimulated podocytes, AS-IV markedly improved inhibitions of SERCA activity and SERCA2 expression, restored intracellular Ca2+ homeostasis, and attenuated podocyte apoptosis in a dose-dependent manner with a concomitant abrogation of ER stress as evidenced by the downregulation of GRP78, cleaved ATF6, phospho-IRE1α and phospho-PERK, and the inactivation of both ER stress-mediated and mitochondria-mediated apoptotic pathways. Furthermore, SERCA2b knockdown eliminated the effect of AS-IV on ER stress and ER stress-mediated apoptotic pathway, whereas its overexpression exhibited an anti-apoptotic effect. Our data obtained from in vivo and in vitro studies demonstrate that AS-IV attenuates renal injury in diabetes subsequent to inhibiting ER stress-induced podocyte apoptosis through restoring SERCA activity and SERCA2 expression.

Highlights

Diabetic nephropathy remains the most common microvascular complication of diabetes and the leading cause of end-stage-renal disease (ESRD) worldwide
We found a significant reduction of SERCA2 expression and activity in db/db mouse kidney and cultured mouse podocytes exposed to palmitate, accompanied by an increase of cytosolic Ca2+ levels, an induction of endoplasmic reticulum (ER) stress and the activation of three unfolded protein response (UPR) pathways, ER stress-mediated apoptotic pathway and mitochondria-mediated apoptotic pathway, which lead to podocyte apoptosis in vivo and in vitro
We found that SERCA2 expression and Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity were significantly decreased in the kidney cortex of db/db mice (Figure 5) and mouse podocytes incubated with palmitate (Figure 8), paralleled by an elevation of intracellular Ca2+ levels in palmitate-incubated podocytes (Figure 9), an induction of ER stress and the activation of three UPR arms as well as ER stress-mediated apoptotic pathway as evidenced by

Summary

Introduction

Diabetic nephropathy remains the most common microvascular complication of diabetes and the leading cause of end-stage-renal disease (ESRD) worldwide. Some pathophysiological stress leads to the accumulation of aberrant unfolded proteins in the ER lumen, which in turn initiates a well-conserved signaling cascade called the UPR to mitigate ER stress through the mediation of three ER-resident transducers: activating transcription factor 6 (ATF6), PERK, and IRE1 (Inagi, 2010; Inagi et al, 2014). Under prolonged or excessive ER stress, the apoptotic signaling will be induced, leading to cell injury and death through the mediation of downstream molecules, such as CHOP, c-Jun N-terminal kinases (JNK), and caspase 12 (Inagi, 2010; Inagi et al, 2014). ER stress has emerged as one of the central mechanisms that lead to diabetic complications and inhibition of ER stress improves diabetic symptoms (Ozcan et al, 2006; Qi et al, 2011). Excessive ER stress results in podocyte apoptosis while suppression of ER stress attenuates podocyte apoptosis in vivo and in vitro (Chen et al, 2008; Cao et al, 2016)

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