Chromium (d-Phenylalanine)₃ alleviates high fat-induced insulin resistance and lipid abnormalities

Abstract

High-fat diet has been implicated as a major cause of insulin resistance and dyslipidemia. The objective of this study was to evaluate the impact of dietary-supplementation of chromium (d-phenylalanine)₃ [Cr(d-Phe)₃] on glucose and insulin tolerance in high-fat diet fed mice. C57BL/6-mice were randomly assigned to orally receive vehicle or Cr(d-Phe)₃ (45 μg of elemental chromium/kg/day) for 8-weeks. High-fat-fed mice exhibited impaired whole-body-glucose and -insulin tolerance and elevated serum triglyceride levels compared to normal chow-fed mice. Insulin-stimulated glucose up-take in the gastrocnemius muscles, assessed as 2-[³H-deoxyglucose] incorporation was markedly diminished in high-fat fed mice compared to control mice. Treatment with chromium reconciled the high-fat diet-induced alterations in carbohydrate and lipid metabolism. Treatment of cultured, differentiated myotubes with palmitic acid evoked insulin resistance as evidenced by lower levels of insulin-stimulated Akt-phosphorylation, elevated JNK-phosphorylation, (assessed by Western blotting), attenuation of phosphoinositol-3-kinase activity (determined in the insulin-receptor substrate-1-immunoprecipitates by measuring the extent of phosphorylation of phosphatidylinositol by γ-³²P-ATP), and impairment in cellular glucose up-take, all of which were inhibited by Cr(d-Phe)₃. These results suggest a beneficial effect of chromium-supplementation in insulin resistant conditions. It is likely that these effects of chromium may be mediated by augmenting downstream insulin signaling.

Introduction

Type 2 diabetes is increasing in epidemic proportions and it is predicted that > 366 million people will be afflicted globally by the year 2030 [1]. In most cases, metabolic syndrome, characterized by insulin resistance, dyslipidemia and hypertension is often observed years before the development of frank observable diabetes. Although the etiology of type 2 diabetes remain poorly understood, lifestyle evolution and excessive intake of fat-enriched diet have been attributed as major determinants of this fast-growing epidemic [2]. Several human and animal studies have demonstrated an association between high-fat feeding and insulin resistance, suggesting a causal role of high-fat diet in the pathophysiology of type 2 diabetes [3], [4], [5].

Diet and exercise have beneficial effects in slowing the progression of the disease, although, alone they provided little reprieve from the current type-2 epidemic. Drugs such as sulfonylurea's and glitazones can be used to treat insulin resistance but are known to cause weight gain and adverse cardiovascular effects. Identification and development of novel molecules and molecular targets to treat or prevent insulin resistance and thus to pre-empt the development of type-2 diabetes is therefore a strategy that merits attention. In this context, attention has been drawn towards a variety of mineral and dietary supplements that may alleviate insulin resistance. Chromium is an 'essential’ nutrient for the maintenance of normal glucose tolerance [6]. Consequently, chromium deficiency is thought to precipitate symptoms of type-2 diabetes and supplementation with chromium corrects some of these symptoms. Accordingly, several human and animal studies have demonstrated that chromium administration improves glucose tolerance and insulin sensitivity in type-2 diabetes and obesity [6], [7]. Despite these documented evidence of the benefits of chromium, there have been studies contradicting such findings and questioning the wisdom of using chromium in treating diabetic patients [8]. Recent studies indicating that the popular commercially available chromium supplement may cause developmental toxicities [9]. Although the exact mechanisms of toxicity of chromium complexes are unknown, studies by Shi and co-workers indicate that chromium undergoes redox activation to generate reactive oxygen species in the presence of hydrogen peroxide [10]. The fact that the picolinate ligand was attributed to these toxicities [11] prompted us to design and synthesize a novel chromium complex with the non-essential amino acid, d-phenylalanine, as ligand. Unlike chromium picolinate, Cr(d-Phe)₃ did not cause DNA-damage [12] and was found to be more potent than picolinate in augmenting glucose-uptake in animal models of obesity [13]. However, the effect of Cr(d-Phe)₃ on fatty acid-induced insulin resistance has not been evaluated thus far, although dietary fat is the major contributor of obesity and cardiovascular disease. Based on this background, the objective of this study was to examine the impact of oral supplementation with Cr(d-Phe)₃ on glucose and insulin tolerance in a high-fat-fed mouse model of insulin resistance. In addition, in an effort to elucidate the potential molecular mechanisms, the effect of Cr(d-Phe)₃ on free fatty acid-induced insulin resistance in cultured myotubes was also investigated.