Expression of the mitochondrial chaperone heat shock protein 60 (HSP60) is regulated by leptin in the brain and seems to be critical to both mitochondrial function and insulin response, according to new research.

Andre Kleinridders, PhD, of Harvard University's Joslin Diabetes Center in Boston, and colleagues showed that type 2 diabetic mice and humans exhibited reduced HSP60 mRNA in brain cells and that in obese, diabetic mice this was also associated with insulin resistance and mitochondrial dysfunction.

Their research in animal models and with human tissue -- dissected brain samples from diabetic patients had reduced HSP60 mRNA at the level of the hypothalamus -- provides new insight into the development of type 2 diabetes, suggesting that leptin regulation of HSP60 may be a potential therapeutic target for the disease, they wrote online in The Journal of Clinical Investigation.

"These data provide what we believe to be a novel pathway of leptin/insulin crosstalk in the central nervous system (CNS) that may impact on hypothalamic control of energy homeostasis in obesity and insulin-resistant states," the group wrote. "Central insulin resistance may be an important component of, and a potential target for future treatments for diabetes and metabolic syndrome."

HSP60 is critical for the maintenance of mitochondrial integrity and cell viability, and it has been shown to be crucial for cell survival. It has also been suggested to have a role in the regulation of mitochondrial function in type 2 diabetes and obesity.

In the study, leptin-deficient mice that received twice daily injections of leptin showed a threefold (P≤0.05) increase in HSP60 protein level. Injecting leptin into C57B1/6 mice that had fasted for 18 hours led to a twofold increase in HSP60 protein levels. Feeding the fasted mice produced a 3.6-fold increase in serum leptin levels and a 36% increase in hypothalamic HSP60 levels, the authors reported.

Quantitative PCR analysis of hypothalamic samples from the diabetic mice showed a 43% reduction of HSP60 mRNA, paralleled by a 64% decrease in HSP60 at the protein level (determined by Western blot analysis). Reductions were seen in more than one diabetic mouse model.

Even more significant, the downregulation was seen in human brain cells. When dissected brain samples from three human controls and four patients with type 2 diabetes were compared, the patients' cells showed a 50% decreased expression of HSP60.

"It was quite surprising to see that what we saw in diabetic mouse models was also happening in human patients," Kleinridders told MedPage Today.

The researchers were also able to show that downregulation of HSP60 in hypothalamic cells caused insulin resistance that was reversed with antioxidant treatment.

To address whether increased oxidative stress was responsible for the observed insulin resistance phenotype, they treated control and HSP60 KD cells with the antioxidant N-acetyl-L-cysteine (L-NAC) for 24 hours, followed by stimulation with insulin for 5 to 20 minutes.

Pretreatment with L-NAC reversed the insulin resistance phenotype by increasing phosphorylation of AKT and ERK in HSP60 KD cells, while having no effect on the control cells. Phosphorylation of IRS1 Ser307 was decreased by 41% in HSP60 KD cells pretreated with L-NAC and was indistinguishable from the control cells, indicating that reducing oxidative stress rescues the insulin resistance phenotype in HSP60 KD cells.

"Treatment of cells with a different antioxidant, ascorbic acid, confirmed the observed phenotype and demonstrated that pre-treatment with an antioxidant can reverse the insulin-resistant phenotype in HSP60 KD cells," the researchers wrote. "Thus, reduced expression of HSP60 leads to increased mitochondrial stress and oxidative stress, increased IRS1 Ser307 phosphorylation, and resistance to insulin and IGF1 signaling, and this can be blocked by antioxidant treatment."

Kleinridders acknowledged that applying these results to a human patient population will be a challenge.

"Not only would you have to penetrate the blood-brain barrier, you would have to get the antioxidant into the cell at the specific place where the mitochondrial dysfunction is occurring," he explained.

Nevertheless, the research shows for the first time that mitochondrial dysfunction can cause insulin resistance, he said. It also shows that leptin plays an important role in regulating mitochondrial function in the hypothalamus by regulating the HSP60 chaperone.

"HSP60 and its regulation by leptin are crucial for normal mitochondrial function in the hypothalamus, and HSP60 is a novel integrator that regulates insulin and leptin crosstalk in the brain," the researchers concluded.