Fructose-induced AGEs-RAGE signaling in skeletal muscle contributes to impairment of glucose homeostasis

Abstract

Increased fructose intake has been linked to the development of dyslipidemia, obesity and impaired glucose tolerance. Due to its specific metabolic fate, fructose impairs normal lipid and carbohydrate metabolism and facilitates the non-enzymatic glycation reaction leading to enhanced accumulation of advanced glycation end products (AGEs). However, the formation of fructose-AGEs under in vivo setup and its tissue specific accumulation is less explored. Here, we investigated the impact of high fructose on AGEs accumulation in skeletal muscle and its causal role in impaired glucose homeostasis. In L6 rat skeletal muscle cells, chronic exposure to fructose induced AGEs accumulation and the cellular level of the receptor for AGEs (RAGE) and the effect was prevented by pharmacological inhibition of glycation. Under in vivo settings, Sprague Dawley rats exposed to 20% fructose in drinking water for 16 weeks, displayed increased fasting glycemia, impaired glucose tolerance, decreased skeletal muscle Akt (Ser-473) phosphorylation, and enhanced triglyceride levels in serum, liver and gastrocnemius muscle. We also observed a high level of AGEs in serum and gastrocnemius muscle of fructose-supplemented animals, associated with methylglyoxal accumulation and up regulated expression of RAGE in gastrocnemius muscle. Treatment with aminoguanidine inhibited fructose-induced AGEs accumulation and normalized the expression of RAGE and Dolichyl-Diphosphooligosaccharide-Protein Glycosyltransferase (DDOST) in gastrocnemius muscle. Inhibition of AGEs-RAGE axis counteracted fructose-mediated glucose intolerance without affecting energy metabolism. These data reveal diet-derived AGEs accumulation in skeletal muscle and the implication of tissue specific AGEs in metabolic derangement, that may open new perspectives in pathogenic mechanisms and management of metabolic diseases.

Introduction

The universal increase in consumption of calories and specifically of refined carbohydrates correlates positively with an alarming increase in metabolic disorders. In particular, the increased consumption of fructose has been associated with worldwide rise in obesity, insulin-resistance, impaired glucose tolerance, and other metabolic disorders [[1], [2], [3], [4]]. Fructose is a lipogenic sugar, acquainted to stimulate de novo lipogenesis to drive hepatic triglycerides synthesis contributing to insulin resistance and dyslipidemia [5]. Besides its well-recognized lipogenic effect, chronic fructose exposure has been reported to generate higher level of advanced glycation end products (AGEs) in rodent model [6]. AGEs are highly reactive molecules formed by the non-enzymatic glycoxidation reactions between the amino group of proteins and reducing sugars like glucose or fructose [7]. AGEs are well reported for their pathogenic role in the development of various patho-physiological conditions including, insulin resistance [8], diabetes and related complications [9], [10], cardiovascular diseases [11], and neurological disorders [12]. Persistent hyperglycemia in diabetes facilitates endogenous generation and accumulation of AGEs [13]. Further, increasing body of evidences has shown that AGEs can also be derived from exogenous sources. Consumption of processed food [14], [15] and smoking [16] are important sources of exogenous AGEs. Moreover, increased intake of processed sugar (e.g. fructose) has proposed to be a source for endogenous AGEs production [17].

Fructose is 10 times more active than glucose in glycation reaction due to its higher stability in open chain and keto group, leading to accelerated AGEs formation [18]. In addition to dietary intake, fructose level is elevated in various tissues under diabetic condition, where the polyol pathway is active, stipulating its possible contribution in intracellular AGEs accumulation in metabolic tissues [19]. The causal role of dietary AGEs in obesity and insulin resistance has been well reported [8]. On the contrary, restriction of dietary AGEs has been reported to reduce insulin resistance and inflammation [20], and the inhibition of AGEs has shown to improve obesity in high fat diet fed mice [21].

Nevertheless, the effect of fructose intake on AGEs level and metabolism in individual tissue and their contribution on whole body metabolic response are poorly understood. Skeletal muscle is the foremost site for postprandial glucose utilization and major determinant for whole body insulin response. Defective insulin response in skeletal muscle and consequent impairment of glucose tolerance is considered to be the primary defect in the establishment of type 2 diabetes mellitus [22]. The accumulation of AGEs has been reported to modulate skeletal muscle function during aging [23], and to drive lipogenic pathway leading to lipid production and accumulation in diet-induced obese mouse model [24]. We have previously reported that acute exposure to fructose attenuates glucose utilization in skeletal muscle cells [25], associated with induction of oxidative stress causing mitochondrial dysfunction and apoptosis [26]. Here, we sought to elucidate the implication of fructose in skeletal muscle AGEs accumulation and characterize the causal role of AGEs in fructose-mediated impairment of glucose homeostasis. We report that exposure to fructose caused AGEs accumulation and activation of AGE-RAGE signaling in skeletal muscle, implicated in impaired lipid metabolism and glucose intolerance.