OBJECTIVE-Heat treatment and overexpression of heat shock protein 72 (HSP72) have been shown to protect against high-fat diet-induced insulin resistance, but little is known about the underlying mechanism or the target tissue of HSP action. The purpose of this study is to determine whether in vivo heat treatment can prevent skeletal muscle insulin resistance. RESEARCH DESIGN AND METHODS-MaleWistar rats were fed a high-fat diet (60% calories from fat) for 12 weeks and received a lower-body heat treatment (41°C for 20 min) once per week.RESULTS-Our results show that heat treatment shifts the metabolic characteristics of rats on a high-fat diet toward those on a standard diet. Heat treatment improved glucose tolerance, restored insulin-stimulated glucose transport, and increased insulin signaling in soleus and extensor digitorum longus (EDL) muscles from rats fed a high-fat diet. Heat treatment resulted in decreased activation of Jun NH 2 -terminal kinase (JNK) and inhibitor of B kinase (IKK-␤), stress kinases implicated in insulin resistance, and upregulation of HSP72 and HSP25, proteins previously shown to inhibit JNK and IKK-␤ activation, respectively. Mitochondrial citrate synthase and cytochrome oxidase activity decreased slightly with the high-fat diet, but heat treatment restored these activities. Data from L6 cells suggest that one bout of heat treatment increases mitochondrial oxygen consumption and fatty acid oxidation.CONCLUSIONS-Our results indicate that heat treatment protects skeletal muscle from high-fat diet-induced insulin resistance and provide strong evidence that HSP induction in skeletal muscle could be a potential therapeutic treatment for obesityinduced insulin resistance. Diabetes 58:567-578, 2009

Clinical studies have indicated a link between Parkinson's disease (PD) and Type 2 Diabetes. Although preclinical studies have examined the effect of high-fat feeding on dopamine function in brain reward pathways, the effect of diet on neurotransmission in the nigrostriatal pathway, which is affected in PD and parkinsonism, is less clear. We hypothesized that a high-fat diet, which models early-stage Type 2 Diabetes, would disrupt nigrostriatal dopamine function in young adult Fischer 344 rats. Rats were fed a high fat diet (60% calories from fat) or a normal chow diet for 12 weeks. High fat-fed animals were insulin resistant compared to chow-fed controls. Potassium-evoked dopamine release and dopamine clearance were measured in the striatum using in vivo electrochemistry. Dopamine release was attenuated and dopamine clearance was diminished in the high-fat diet group compared to chow-fed rats. Magnetic resonance imaging indicated increased iron deposition in the substantia nigra of the high fat group. This finding was supported by alterations in the expression of several proteins involved in iron metabolism in the substantia nigra in this group compared to chow-fed animals. The diet-induced systemic and basal ganglia-specific changes may play a role in the observed impairment of nigrostriatal dopamine function.

(PD), the clinical literature remains equivocal. We, therefore, sought to address the relationship between insulin resistance and nigrostriatal dopamine (DA) in a preclinical animal model. High-fat feeding in rodents is an established model of insulin resistance, characterized by increased adiposity, systemic oxidative stress, and hyperglycemia. We subjected rats to a normal chow or high-fat diet for 5 wk before infusing 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. Our goal was to determine whether a high-fat diet and the resulting peripheral insulin resistance would exacerbate 6-OHDA-induced nigrostriatal DA depletion. Prior to 6-OHDA infusion, animals on the high-fat diet exhibited greater body weight, increased adiposity, and impaired glucose tolerance. Two weeks after 6-OHDA, locomotor activity was tested, and brain and muscle tissue was harvested. Locomotor activity did not differ between the groups nor did cholesterol levels or measures of muscle atrophy. High-fat-fed animals exhibited higher homeostatic model assessment of insulin resistance (HOMA-IR) values and attenuated insulin-stimulated glucose uptake in fast-twitch muscle, indicating decreased insulin sensitivity. Animals in the high-fat group also exhibited greater DA depletion in the substantia nigra and the striatum, which correlated with HOMA-IR and adiposity. Decreased phosphorylation of HSP27 and degradation of IB␣ in the substantia nigra indicate increased tissue oxidative stress. These findings support the hypothesis that a diet high in fat and the resulting insulin resistance may lower the threshold for developing PD, at least following DA-specific toxin exposure. dopamine; diabetes; substantia nigra; striatum; insulin resistance CLINICAL STUDIES SUGGEST A LINK between type 2 diabetes (T2D) and Parkinson's disease (PD) (30,46), and between fat intake or adiposity and PD (1, 31, 34). Moreover, it was reported over 40 years ago that greater than 50% of PD patients exhibit abnormal glucose tolerance (4, 10) or diabetes (36). Despite this information, very little is known regarding the relationship of these diseases and the impact of comorbidity on their pathogenesis. By 2025, T2D is estimated to impact 300 million individuals (47), with the elderly at greatest risk (54), the population also at greatest risk for neurodegenerative diseases like PD. For these reasons, understanding the potential for T2D, obesity, high dietary fat intake, and insulin resistance to contribute to PD is critical. Although the exact cause of PD is unknown, various environmental factors such as aging, diet, and environmental toxin exposure have been implicated in contributing to its development (29, 34, 51). The idea that "multiple hits" play a role in PD degeneration is supported by the fact that 80% of dopamine (DA)-producing neurons must be lost for symptoms to appear (50). While diabetes and PD do not invariably coincide, several studies suggest that obesity may potentiate neuronal dysfunction or even neurodegeneration (reviewed in Ref. 11). High-f...

The antioxidant alpha-lipoic acid (LA) has been shown to improve insulin action in high-fat (HF)-fed animal models, yet little is known about its underlying mechanisms of action. We hypothesize that LA acts by inducing heat shock proteins (HSPs), which then inhibit stress kinases known to interfere with insulin signaling intermediates. Male Wistar rats were fed a HF diet (60% calories from fat) for 6 wk, while controls received a chow diet (10% calories from fat). One-half of the rats in each group received daily LA injections (30 mg/kg body wt). In rats fed a HF diet, LA increased expression of HSP72 and activation of HSP25 in soleus muscle, but it had no effect on HSPs in muscle from chow-fed rats. LA treatment reduced phosphorylation of c-Jun NH(2)-terminal kinase (JNK) and inhibitor of kappaB kinase-beta (IKKbeta) activity (IkappaBalpha protein levels) in rats fed a HF diet and effectively restored insulin responsiveness, as seen by insulin-stimulated phosphorylated Akt/Akt and 2-deoxyglucose uptake in soleus muscle. LA also induced activation of p38 MAPK and AMP-activated protein kinase, proteins previously implicated in insulin-independent glucose uptake. In addition, acute LA treatment induced HSPs in vitro in L6 muscle cells and prevented the activation of JNK and IKKbeta with stimulants such as anisomycin and TNF-alpha, respectively. In conclusion, our results suggest chronic LA treatment results in stress kinase inhibition and improved insulin signaling through a HSP-mediated mechanism.

Non-technical summary Previous studies show that oestrogen is beneficial for maintaining blood glucose levels and helping the body respond to insulin. Despite these previous findings, the mechanism by which oestrogen acts is unknown. We show that specific activation of oestrogen receptor α (ERα) increases glucose uptake into skeletal muscle when insulin is present. Activation of oestrogen receptor β (ERβ) alone or activation of both ERα and ERβ together did not increase glucose uptake into skeletal muscle. This suggests that oestrogen's beneficial effect occurs by activating ERα. These results have important implications for understanding the mechanisms of glucose homeostasis, particularly in postmenopausal women with low oestrogen levels.Abstract Previous studies suggest oestrogen receptor α (ERα) is involved in oestrogen-mediated regulation of glucose metabolism and is critical for maintenance of whole body insulin action. Despite this, the effect of direct ERα modulation in insulin-responsive tissues is unknown. The purpose of the current study was to determine the impact of ERα activation, using the ER subtype-selective ligand propylpyrazoletriyl (PPT), on skeletal muscle glucose uptake. Two-month-old female Sprague-Dawley rats, ovariectomized for 1 week, were given subcutaneous injections of PPT (10 mg kg −1 ), oestradiol benzoate (EB; 20 μg kg −1 ), the ERβ agonist diarylpropionitrile (DPN, 10 mg kg −1 ) or vehicle every 24 h for 3 days. On the fourth day, insulin-stimulated skeletal muscle glucose uptake was measured in vitro and insulin signalling intermediates were assessed via Western blotting. Activation of ERα with PPT resulted in increased insulin-stimulated glucose uptake into the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles, activation of insulin signalling intermediates (as measured by phospho-Akt (pAkt) and pAkt substrate (PAS)) and phosphorylation of AMP-activated protein kinase (AMPK). GLUT4 protein was increased only in the EDL muscle. Rats treated with EB or DPN for 3 days did not show an increase in insulin-stimulated skeletal muscle glucose uptake compared to vehicle-treated animals. These new findings reveal that direct activation of ERα positively mediates glucose uptake and insulin action in skeletal muscle. Evidence that oestrogens and ERα stimulate glucose uptake has important implications for understanding mechanisms of glucose homeostasis, particularly in postmenopausal women.