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LETTER TO THE EDITOR

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5-Methoxyindole-2-Carboxylic Acid (MICA) Fails to Retard Development and Progression of Type II Diabetes in ZSF1 Diabetic Rats

Chun-Yan Li1, 2, Wei-Xing Ma1, 3, 4, and Liang-Jun Yan1 

1Department of Pharmaceutical Sciences, UNT System College of Pharmacy, University of North Texas Health Science Center, Fort Worth, Texas 76107, USA; 2Shantou University Medical College, Shantou515041, Guangdong, China; 3Qingdao University of Science and Technology, Qingdao266042, Shandong, China; 4Technical Center of Qingdao Customs, Qingdao266002, Shandong, China 

Correspondence: liang-jun.yan@unthsc.edu (L-J.Y.) 

Li C-Y et al. Reactive Oxygen Species 9(27):144–147, 2020; ©2020 Cell Med Press

http://dx.doi.org/10.20455/ros.2020.821

(Received: December 21, 2019; Revised: January 4, 2020; Accepted: January 6, 2020) 

ABSTRACT | 5-Methoxyindole-2-carboxylic acid (MICA) is a well-established reversible inhibitor of mitochondrial dihydrolipoamide dehydrogenase (DLDH). This chemical, as an indole derivative, has been shown to be neuroprotective against ischemic stroke injury when administered either before or after ischemic stroke in animal models. MICA has also been studied as a potential antidiabetic agent by numerous investigators, though the underlying mechanisms remain sketchy. To attempt to elucidate the mechanisms of its antidiabetic action, we tested the effect of MICA on ZSF1 rat, a widely used rodent model of type 2 diabetes. ZSF1 rats as well as its healthy controls were fed with control diet or MICA-containing diet (200 mg/kg/day) for 9 weeks. Unexpectedly, comparison of body weight changes and blood glucose levels at the end of the 9-week’s feeding period indicated that MICA failed to show any anti-diabetic effect in the ZSF1 diabetic rats. The reasons for this failure were discussed. 

KEYWORDS | Diabetes; Dihydrolipoamide dehydrogenase; 2-Methoxyindole-2-carboxylic acid; Mitochondria 

ABBREVIATIONS | DLDH, dihydrolipoamide dehydrogenase; MICA, 5-methoxyindole-2-carboxylic acid 

5-Methoxyindole-2-carboylic acid (MICA) is a reversible inhibitor of mitochondrial dihydrolipoamide dehydrogenase (DLDH) [1–4]. This chemical was studied extensively in 1960s and 1970s; and many studies then indicated that MICA possesses glucose lowering ability [5, 6], and the underlying mechanism was thought to be due to its inhibitory effect on pyruvate carboxylase in the gluconeogenesis pathway in the liver [3, 7–10]. In fact, the exact mechanism of this action is unknown as MICA in vitro does not inhibit purified pyruvate carboxylase [3]. We recently also suggested that MICA inhibition of DLDH could lead to decreased blood glucose levels and attenuates diabetic oxidative stress induced by reactive oxygen species [11, 12]. 

In 1976, a group in Western Germany tested MICA’s glucose lowering ability in a diabetic model ofChinesehamsters[13].Unfortunately,theirstudy found that MICA, when administered via gavage at a dosage of 100 mg/kg body weight for a week, led to death of many animals in the experiments. Although the authors indeed observed MICA’s glucose lowering ability in this diabetes model, they concluded that MICA was not a good agent for managing type 2 diabetes due to its toxicity. Nevertheless, the idea that MICA can lower blood glucose in diabetes still seems to be attractive [11, 12]. 

We became interested in MICA in 2006 as we were then looking for a chemical or pharmacological drug that could inhibit DLDH, thereby creating a preconditioning or postconditioning effect for neuroprotection in ischemic stroke. In our neuroprotection and ischemic stroke studies, we indeed found that DLDH activity could be modulated by MICA and this modulation can produce both pre-conditioning and post-conditioning effect in stroke injury [14–16]. As we are also interested in exploring MICA’s potential anti-diabetic effect [11, 12], we thus set out to test our hypothesis that MICA retards development and progression of type 2 diabetes. 

To test our hypothesis, we used a well-characterized and widely used rodent diabetic model which is ZSF1 [17–19]. Male ZSF1 lean (control) and obese (diabetic) rats were purchased from Charles River (Wilmington, MA, USA) and all procedures involving this animal model of diabetes were reviewed and approved by our Intuitional Animal Care and Use Committee at University of North Texas Health Science Center. A total of 24 rats were used, comprising 12 lean control and 12 obese diabetics. For each group of healthy control and diabetes, 6 rats were on control diet (Purina 5008) and another 6 were on MICA-containing diet that was based on Purina 5008 diet, which was made by TestDiet (Richmond, IN, USA). Specifically, rats at the age of 8 weeks’ old were fed with either control diet or diet that contained MICA for 9 weeks and the dosage of MICA was 200 mg/kg based on our stroke studies whereby no toxic effect of MICA was observed [14]. At the end of the 9-week feeding period, no animals died and both body weight and blood glucose were measured. Data were presented in Figure 1. As can be seen in the figure, MICA did not exhibit any preventive effect on the increase of body weight in the diabetic group (Figure 1A). MICA also failed to bring down blood glucose in the diabetic group (Figure 1B). Taken together, these data indicate that MICA failed to attenuate development and progression of type 2 diabetes in the ZSF1 model.

 

FIGURE 1. Effect of MICA (200 mg/kg body weight per day) on body weight (A) and blood glucose (B). MICA diet feeding lasted for 9 weeks and end-point comparison was made between control diet and MICA diet within each group. Lean represents the healthy control group and Obese represents the diabetic group. Blood glucose was measured by FreeStyle Lite strips made by Abbott (Chicago, IL, USA). 

Our data, in contrast to what we have expected [11, 12], demonstrate that MICA has no beneficial effect in terms of managing type 2 diabetes, at least in the ZSF1 diabetic animal model at the tested dosage of 200 mg/kg. Our studies are also in disagreement with what had been reported in 1960s and 1970s that MICA possessed glucose lowering ability [5, 6, 13]. This disagreement may be due to the different animal models that were used. Nonetheless, whether MICA would show any beneficial effect in other rodent models of type 2 diabetes remains to be evaluated. Additionally, whether MICA has in vivo targets other than DLDH also remains to be investigated. Finally, given that induction of mild bioenergetics stress can produce cytoprotective effects [20–23], it should be pointed out that MICA, as a mild stressor of mitochondrial bioenergetics [14, 15, 24], may have beneficial effects in other age-related diseases or chronic metabolic disorders, which also remains to be explored. 

ACKNOWLEDGMENTS 

This work was supported by UNTHSC seed grants RI10015 and RI10039 (to LJY). LJY was also supported in part by the National Institutes of Neurological Disorders and Stroke (R01NS079792).The authors declare no conflicts of interest. 

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