Proxidant Injection Causes the Onset of Type 2 Diabetes in the Spontaneously Hypertensive Obese (SHROB/Kol) Rat

Discussion

The success of proxidant treatment in triggering the onset of diabetes in the Spontaneously Hypertensive Obese (SHROB/Kol) rat confirms the association between oxidative stress and the onset of diabetes in a model of metabolic syndrome. Hydroquinone acted to promote oxidative stress through up-regulation of the production of oxidative damage-causing free radicals. L-buthionine sulfoximine, a glutathione synthase inhibitor, served to prevent the break down of harmful free radicals to water and oxygen. The combination of upregulation of free radical production and inhibition of free radical breakdown maximized oxidative stress in the metabolic syndromeexhibiting SHROB, thus leading to the onset of diabetes. These findings, in conjunction with the fact that salineinjected control SHROB did not develop diabetes, demonstrate that subjection to oxidative stress can cause type 2 diabetes in an at-risk, pre-diabetic model.

The transient elevation of morning blood glucose levels to above 125 mg/dl indicate that the regulation of oxidative damage in SHROB may have a mechanism for short-term recovery from oxidative stress. There are many potential points of regulation of oxidative damage in the SHROB, including the liver, pancreas, and peripheral muscle or fat tissue. Possible mechanisms may include increased glucose production from the liver, impaired insulin secretion from the pancreas, and receptor or postreceptor defects leading to insulin resistance in muscle and fat. Further studies will explore the effect of oxidative stress on each of these possible points of regulation, and seek to determine the possibility of adaptive measures to combat oxidative stress.

The impaired glucose tolerance observed in proxidant treated SHROB as compared to control SHROB receiving saline injection further demonstrates the effect of oxidative stress in this model. Impaired glucose tolerance is often associated with insulin resistance, a symptom seen in the pre-diabetic SHROB model. However, the further impairment of insulin resistance in the proxidant treated SHROB indicates the association between oxidative stress and the onset of diabetes.

Obesity is an underlying factor in prediabetes and diabetes. In this study, SHROB body weight remained unchanged despite the increased oxidative stress. This finding rules out the possibility that the onset of diabetes resulted indirectly from increased body weight. The lack of weight loss also suggests that the proxidant treatment is not simply toxic.

Elevated plasma peroxide levels within 1h of proxidant injection, with a return to baseline within 24h indicate the activity of internal mechanisms within SHROB for the control of oxidative stress. However, the fact that proxidant injections administered following a recovery period of three days lead to impaired glucose tolerance, and subsequently higher maximum glucose levels and longer recovery times in an oral glucose tolerance test, may indicate an underlying impairment of SHROB's ability to combat oxidative stress. Subsequent oxidative damage may contribute to insulin resistance in the liver as well as renal and hepatic dysfunction. These are characteristics of chronic type 2 diabetes.

Given the fact that oxidative stress can promote the onset of diabetes in a model of metabolic syndrome, further studies may be performed to explore the mechanism of this association and to investigate preventative treatments for those at risk for developing diabetes. These studies may include the exploration of the effects of diets containing varying levels of anti-oxidants in conjunction with proxidant treatment, as well as investigation of medications with the potential to inhibit oxidative stress. Anti-oxidant therapies alone or in combination with treatment for characteristics of metabolic syndrome, such as hypertension, may be beneficial in the prevention of the onset of type 2 diabetes in at-risk individuals. This has a widespread potential for benefit due to the increasing frequency of diabetes, obesity, and other characteristics of metabolic syndrome worldwide. Prevention and treatment of these aspects of disease is an important step in the improvement of the overall health of the international population, and future studies of the mechanism of the onset of diabetes through oxidative stress will work towards the achievement of this goal.

Conclusions

Induction of oxidative stress in prediabetic SHROB rats causes the onset of diabetes within two days of daily treatment, and further impairs glucose tolerance as measured by oral glucose tolerance testing. Weight gain is not a contributing factor to elevated the blood glucose levels or impaired glucose tolerance seen in proxidant treated SHROB. Plasma peroxide levels were found to be increased within 1h of proxidant injection with a return to baseline within 24h. The SHROB rat subjected to oxidative stress is a potential model to test the reduction of oxidative stress and subsequent diabetes prevention by antioxidants in individuals exhibiting metabolic syndrome.

Acknowledgments

I would like to thank my advisor Dr. Paul Ernsberger and our co-investigator Dr. Richard Koletsky for their guidance and support of my research endeavors. Thank you to Rachel Koletsky, Neema Patel, Alex Moore, Matt Koletsky, Liza Escobedo, Sabrina Jackson, and Simone Edwards for their help in conducting this research. I would like to thank the Dietrich Diabeties Research Institute of the Diabetes Association of Greater Cleveland and the SOURCE Office of Case Western Reserve University for their funding, and CWRU's Department of Nutrition for serving as home base for my research over the past three years.

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