The Protective Roles of GLP-1R Signaling in Diabetic Nephropathy

Possible Mechanism and Therapeutic Potential

Hiroki Fujita; Tsukasa Morii; Hiromi Fujishima; Takehiro Sato; Tatsunori Shimizu; Mihoko Hosoba; Katsushi Tsukiyama; Takuma Narita; Takamune Takahashi; Daniel J Drucker; Yutaka Seino; Yuichiro Yamada

Disclosures

Kidney Int. 2014;85(3) 

In This Article

Abstract and Introduction

Abstract

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone that has an antioxidative protective effect on various tissues. Here, we determined whether GLP-1 has a role in the pathogenesis of diabetic nephropathy using nephropathy-resistant C57BL/6-Akita and nephropathy-prone KK/Ta-Akita mice. By in situ hybridization, we found the GLP-1 receptor (GLP-1R) expressed in glomerular capillary and vascular walls, but not in tubuli, in the mouse kidney. Next, we generated C57BL/6-Akita Glp1r knockout mice. These mice exhibited higher urinary albumin levels and more advanced mesangial expansion than wild-type C57BL/6-Akita mice, despite comparable levels of hyperglycemia. Increased glomerular superoxide, upregulated renal NAD(P)H oxidase, and reduced renal cAMP and protein kinase A (PKA) activity were noted in the Glp1r knockout C57BL/6-Akita mice. Treatment with the GLP-1R agonist liraglutide suppressed the progression of nephropathy in KK/Ta-Akita mice, as demonstrated by reduced albuminuria and mesangial expansion, decreased levels of glomerular superoxide and renal NAD(P)H oxidase, and elevated renal cAMP and PKA activity. These effects were abolished by an adenylate cyclase inhibitor SQ22536 and a selective PKA inhibitor H-89. Thus, GLP-1 has a crucial role in protection against increased renal oxidative stress under chronic hyperglycemia, by inhibition of NAD(P)H oxidase, a major source of superoxide, and by cAMP-PKA pathway activation.

Introduction

Diabetic nephropathy (DN) is a serious complication of diabetes and the leading cause of end-stage renal disease in developed countries. Recent evidence indicates that oxidative stress has a central role in the development and progression of DN.[1,2] The increase in systemic oxidative stress becomes prominent from the incipient stage of DN.[3] In the kidney, reactive oxygen species (ROS) including superoxide anion (O2•–) are excessively produced by chronic hyperglycemia, leading to increased levels of renal oxidative stress. NAD(P)H oxidase is the most important source of superoxide anion,[4–7] and this enzyme is shown to be upregulated in the diabetic kidney.[5,8–10] A recent in vitro study using human HEK293 cells demonstrated that NAD(P)H oxidase NOX1-dependent ROS production is reduced by the elevation of cAMP and subsequent activation of protein kinase A (PKA).[11] Furthermore, treatment with cAMP-elevating agents such as isoproterenol and forskolin normalized the levels of NAD(P)H oxidase activity and superoxide in aortic vascular smooth muscle cells of spontaneously hypertensive rats.[12] Thus, the cAMP-PKA pathway appears to work as an important inhibitory factor for NAD(P)H oxidase–dependent production of ROS or superoxide.

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone that stimulates insulin secretion from pancreatic β-cells in a glucose-dependent manner.[13] Activation of the GLP-1 receptor (GLP-1R) stimulates adenylate cyclase and enhances the production of cAMP, the primary effector of GLP-1-induced insulin secretion.[13,14] Furthermore, increased levels of cAMP activate PKA or cAMP-regulated guanine nucleotide exchange factor II (Epac2), and contribute to mediating various physiological actions including insulin secretion.[14,15] The GLP-1R is expressed in pancreatic β-cells and in multiple extrapancreatic tissues including the gut, brain, heart, lung, and kidney.[16,17] Given the evidence indicating that cAMP and PKA pathways link to antioxidative effects, it is likely that GLP-1 protects various tissues from oxidative injury. However, the roles of GLP-1 in the kidney and DN have not been fully elucidated. First, the precise localization of GLP-1R in the kidney remains unclear. Second, it is unknown whether gain or loss of GLP-1R signaling modulates renal function and the progression of renal injury under conditions of chronic hyperglycemia.

In the present study, we investigated the role of endogenous GLP-1R signaling in DN. First, we examined the localization of GLP-1R in the mouse kidney by in situ hybridization and reverse transcriptase polymerase chain reaction (RT-PCR) analysis. Next, we studied two Ins2 Akita diabetic mouse models showing different susceptibility to the development and progression of DN, DN-resistant C57BL/6-Ins2 Akita (C57BL/6-Akita), and DN-prone KK/Ta-Ins2 Akita (KK/Ta-Akita).[18,19] We examined the renal phenotypes of C57BL/6-Akita mice with GLP-1R deficiency, and in complementary experiments, we tested whether a GLP-1R agonist, liraglutide, ameliorates nephropathic changes in KK/Ta-Akita mice that develop progressive DN.[18]

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