Melatonin Treatment Improves Insulin Resistance and Pigmentation in Obese Patients with Acanthosis Nigricans

Objective This study aimed to determine the effects of melatonin on insulin resistance in obese patients with acanthosis nigricans (AN). Methods A total of 17 obese patients with acanthosis nigricans were recruited in a 12-week pilot open trial. Insulin sensitivity, glucose metabolism, inflammatory factors, and other biochemical parameters before and after the administration of melatonin were measured. Results After 12 weeks of treatment with melatonin (3 mg/day), homeostasis model assessment insulin resistance index (HOMA-IR) (8.99 ± 5.10 versus 7.77 ± 5.21, p < 0.05) and fasting insulin (37.09 5 ± 20.26 μU/ml versus 32.10 ± 20.29 μU/ml, p < 0.05) were significantly decreased. Matsuda index (2.82 ± 1.54 versus 3.74 ± 2.02, p < 0.05) was significantly increased. There were also statistically significant declines in the AN scores of the neck and axilla, body weight, body mass index, body fat, visceral index, neck circumference, waist circumference, and inflammatory markers. Conclusions It was concluded that melatonin could improve cutaneous symptoms in obese patients with acanthosis nigricans by improving insulin sensitivity and inflammatory status. This trial is registered with ClinicalTrials.gov NCT02604095.

Acanthosis nigricans (AN) is a disease characterized by skin pigmentation, hyperkeratosis, and velvet hyperplasia. Increased pigmentation commonly occurs in the posterior neck, axilla, and groin, and it can also be seen on the cubital fossa, labia, face, and other locations throughout the body [13]. AN can be divided into two categories: benign and malignant [14]. Studies have found that benign AN is closely related to hyperinsulinemia, insulin resistance (IR), and obesity [15,16]. Many different topical and oral treatments have been attempted for AN. Studies reported that treatment with both metformin and rosiglitazone are useful in AN characterized by IR [17,18]. Low-calorie diet, increasing physical activity, and weight reduction also can relieve symptoms of AN by improving the IR [19]. However, no safe and effective therapy exists especially when considering young patients with euglycemia. Patients with AN also are at increased risk of developing metabolic disorders such as diabetes and dyslipidemia.
Melatonin can regulate internal biological clocks and energy metabolism [20]. Melatonin influences insulin secretion mediated by Gi-protein-coupled melatonin receptors MT1 and MT2 [21]. It was found that melatonin treatment for one year could reduce fat mass and increase lean mass in postmenopausal women [22]. So this study was designed to determine whether treatment with melatonin is an effective treatment for AN as well as insulin resistance and metabolic disturbances.  Japan), with the same controlling of water, food, diuretics, alcohol and coffee intake, and urination. Neck circumference (NC), waist circumference (WC), hip circumference (HC), blood pressure (BP), and heart rate (HR) were measured twice, and the average was used for analysis. Scores of AN were calculated as described previously [23]. There are four degrees from 0 to 4 on the AN scales, which are strongly associated with fasting insulin and BMI.
Each patient received 3 mg/day melatonin (Schiff Nutrition Group: each tablet contains melatonin 3 mg, vitamin B6 5 mg, and calcium 52 mg) given orally at bedtime for 12 weeks.

Discussion
In 2000, AN was established as a formal risk factor for the development of diabetes in children by the American Diabetes Association [16]. Insulin resistance commonly occurs in association with AN. The cutaneous manifestations of AN are caused by hyperinsulinemia and the stimulation of keratinocytes and dermal fibroblasts by growth factors [26,27]. Patients with hyperinsulinemia have increased IGF-1 levels [28]. Hyperinsulinemia is able to reduce IGF binding protein-(IGFBP-) 1 and IGFBP-2, which regulate levels of IGFs. Reduced IGFBP-1 and IGFBP-2 levels could increase free IGF-1, thereby promoting the development of papillomatosis and hyperkeratosis observed in AN [29]. There are two membrane receptors of melatonin called MT1 and MT2, which are Gprotein-coupled receptors, in the periphery [30]. Both receptors are expressed in the islets of Langerhans and are involved in the regulation of glucagon secretion from α-cells and insulin secretion from β-cells [10]. Numerous studies support that activation of MT1 or MT2 leads to a reduction in second messenger 3′,5′-cyclic adenosine monophosphate (cAMP) or 3′,5′-cyclic guano-sine monophosphate (cGMP) accompanied by reduced insulin secretion [31,32]. Picinato et al. [33] found the effects of melatonin on the phosphorylation of IGF and insulin receptors triggering their signalization cascade. Therefore, the effect of melatonin on insulin secretion and IGF may explain the mechanism of action behind the utility of treating AN with melatonin.
Most patients with AN also have severe obesity. Accumulating evidence shows that obesity is a clinical indicator for the risk of cardiovascular diseases, diabetes mellitus, and the associated metabolic syndrome [34,35]. Somewhat surprisingly, we found that treatment with melatonin for only 12 weeks significantly decreased body weight, BMI, body fat, and visceral index of obese participants. Previous studies have shown that NC is associated with the metabolic disorders related to insulin resistance, cardiovascular diseases, metabolic syndrome, and obstructive sleep apnea syndrome [36,37]. Melatonin was also found to decrease NC and WC, which are commonly considered as risk factors for metabolic syndrome.
Numerous studies have demonstrated that melatonin has antioxidant [2,3] and anti-inflammatory [4] functions. In our study, serum CRP and TNF-α were decreased. Some investigators have suggested that the inflammation induced by metabolic surplus is different from classical inflammation [38]. TNF-α was the first inflammatory marker discovered in adipose tissue of obese mice [39]. As another inflammatory marker, CRP has been shown to be associated with an increased risk for development of type 2 diabetes mellitus (T2DM) [40]. We consider the anti-inflammatory function of melatonin as one of the possible mechanisms by which it improves AN.
This study had several limitations. Firstly, recruitment to diabetes trials has long been associated with placebomediated improvements in glycemic control and associated metabolic parameters [41]. But because of the lack resources to perform additional placebo arm studies, we do not have enough ability to recruit placebo-controlled subjects. Secondly, the sample size of the present study was relatively small. Because of the individual difference, we could not see significant changes in lipid metabolism and antioxidation functions. Secondly, we could not measure the overnight urinary 6-sulfatoxymelatonin excretion (UME), which can reflect the level of internal melatonin, of the outpatients. Further studies are necessary to prove these conclusions.

Conclusion
In summary, treatment with melatonin can decrease weight, adipose tissues, and cutaneous symptoms in obese patients with AN. It may act through improvement of insulin resistance and decreasing inflammation. However, further work must be done to elucidate its complete mechanism of action.

Conflicts of Interest
The authors declare that they have no conflicts of interest.