High fructose intake has been shown to induce insulin resistance, weight gain, and hyperlipidemia in animals and humans . In our study, insulin resistance developed in SD rats following 8 weeks of feeding on a high fructose diet. The animals experienced characteristic changes that include increased body weight, glucose intolerance, increased serum insulin levels, and dyslipidemia. Treatment with metformin significantly improved these metabolic abnormalities. The observed effects of metformin are supported by the evidence that metformin decreases hepatic production of glucose and increases hepatic insulin sensitivity and glucose clearance [25, 26].
Insulin resistance is strongly associated with the development of atherosclerosis and cardiovascular disease [1, 2, 27]. In our study, severe neointimal hyperplasia developed following balloon injury in insulin-resistant rats compared to normal chow controls. The neointimal hyperplasia observed was consistent with neointimal SMC proliferation, suggesting SMC proliferation contributed largely to the increased neointimal formation [28, 29]. The neointimal hyperplasia of the carotid arteries was dramatically inhibited following treatment with metformin as compared with the untreated high fructose-fed rats those were determined at 1 week and 4 weeks after balloon injury. Neointimal SMC proliferation indicated by BrdU-positive cells was also inhibited in the group receiving metformin treatment. Metformin is known to reduce the development of atherosclerotic lesions and attenuate carotid intima-media thickness , but this is the first time that a similar effect on balloon injury-induced neointimal hyperplasia in insulin resistant rats has been demonstrated. Moreover, our in vitro scratch assay suggests that both fructose and MG-induced SMC migration may contribute to the neointimal formation and the inhibitory effect of metformin on SMC migration could contribute beneficial effects by reducing stenosis. Our in vitro experiment demonstrates that MMP activities were significantly increased in fructose or MG-treated SMCs. Treatment with metformin inhibited MMP activities. Previous studies have shown that MMP activity plays a major role in matrix accumulation and neointimal formation [24, 30]. The observed inhibitory effect on MMP activity and the upregulation of AMPK expression by metformin in our study may suggest that the suppressive effect on MMP is AMPK-dependent . Taken together, the increased SMCs proliferation, migration, and MMP activity induced by fructose are likely to contribute to the severe neointimal formation induced by balloon injury in insulin-resistant rats. Thus, metformin attenuated neointimal formation is mediated through inhibition of SMCs proliferation, migration, and MMP activity.
Plasma MG levels were highly increased in fructose-fed rats, while treatment with metformin dramatically decreased the MG levels. This effect can be attributed to metformin’s action as a scavenger of MG or through normalization of metabolic abnormalities . In a previous study, we have shown that fructose is the precursor of MG . Early research has also demonstrated that MG induces insulin resistance and that treatment with MG scavengers attenuates this effect in SD rats [32, 33]. Studies have shown that MG induces an inhibitory effect on phosphorylation of IRS-1, PI3K activity, and insulin-stimulated phosphorylation of protein kinase B (PKB) and extracellular-regulated kinase ½ (ERK½) . Metformin sensitizes insulin signalling through an AMP-activated protein kinase (AMPK)-mediated phosphatase and tensin homolog (PTEN) down-regulation . Our findings show that the expression of AMPKa and IRS-1 was significantly down regulated in the liver in fructose-induced insulin-resistant rats while treatment with metformin reversed this inhibitory effect. These findings are consistent with metformin improving insulin signalling probably through lowering of MG levels. Increased expression of JNK and NFKB in SMCs suggests that inflammation may play a role in the pathogenesis of stenosis in fructose-induced insulin resistance and metformin plays a protective role through the inhibition of inflammatory pathway in insulin resistance and stenosis.
Vasorelaxation was severely impaired in balloon-catheter-injured carotid arteries in insulin-resistant rats. Nevertheless, vasorelaxation was significantly restored in rats that received treatment with metformin. We and others have demonstrated that impairment of endothelium-dependent vasorelaxation may be mediated by MG while metformin treatment significantly improves endothelial function and vasorelaxation [8, 18, 35–37]. These findings suggest that the improvement of endothelial function and vessel reactivity may be attributed to the effects of metformin on reduction of MG levels and improvement of insulin sensitivity . Another study demonstrated that metformin reduces catecholamine-induced vasoconstriction through endothelium dependent and independent mechanisms in rats . Demonstration of the vasoprotective effect of metformin in an insulin resistant model has not been previously reported.
In conclusions, this study has demonstrated that balloon-injury-induced neointimal formation of the carotid arteries is facilitated by fructose-induced insulin resistance. Treatment with metformin significantly attenuates balloon-injury induced neointimal hyperplasia through inhibition of SMC proliferation, migration, MMP activities, and inflammation, as well as by improvement of the insulin signaling pathway.