The present study aimed to evaluate the effect of DPP-4 inhibitors on serum levels of leptin in T2DM. Leptin is associated with metabolism, insulin sensitivity, and diabetes. This meta-analysis demonstrated that DPP-4 inhibitors exerted no significant effect on changing circulating leptin levels in T2DM patients compared to placebo or active drugs.
Leptin is a critical metabolic hormone that plays a key role in regulating the physiologic switch between the fed and starved states. Since its discovery in 1994, leptin provides deep insights into the regulation of central nervous system energy balance circuits[15]. It is a protein containing 167 amino acid mainly secreted by white adipose tissue into the blood and can be transported across the blood-brain barrier[16]. This adipokine regulates metabolic homeostasis by inhibiting food intake and increasing energy expenditure. Leptin significantly reduced blood glucose in mouse models of insulin-deficient diabetes, suggesting that leptin modulated glucose homeostasis independently of insulin[17, 18]. Although its exact mechanism of lowering glucose levels remains unknown, data have shown that leptin decreases appetite, suppresses insulin secretion, and increases insulin sensitivity. Overall, the identification of leptin has provided a framework for studying the pathogenesis of obesity in the diabetic population. Decreases in the sensitivity to leptin might contribute to the development of T2DM[19].
Leptin therapy has been found to effectively reverse hyperglycemia and prevent mortality in mouse models of diabetes[20]. The pathogenesis of obesity is analogous to diabetes and can result from either leptin hyposecretion or leptin resistance. The former type of obesity is characterized as low endogenous plasma leptin levels who respond to leptin therapy, while the latter form describes most obese subjects, who are leptin resistant but might respond to leptin therapy in combination with other drugs such as leptin sensitizers[21]. In states of leptin resistance such as obesity and T2DM, leptin action is decreased in the brain parenchyma and vessels, despite its elevated concentrations in the plasma[18]. In patients with T2DM, elevated levels of leptin are often linked with increased cardiovascular risk, as well as with the presence of macro- and microvascular complications. Treatment of diabetes in human beings might benefit from correction of leptin resistance as well as insulin resistance[20]. In patients with severe coronary artery disease, abdominal obesity is commonly related to increased leptin levels and decreased adiponectin concentrations. Leptin/adiponectin imbalance might mediate the increased risk of developing T2DM and cardiovascular disease associated with abdominal obesity[22]. Our team previously found that DPP-4 inhibitors increased serum adiponectin levels in T2DM[23]. In the current study, we demonstrated that DPP-4 inhibitors did not significantly change serum leptin concentrations, suggesting that these drugs provided a neutral effect without aggravating leptin resistance in the diabetic state.
Although the cardiovascular safety of DPP-4 inhibitors has been proven in T2DM, the net effect of these drugs on leptin concentrations in obesity-related disease remains unclear[24]. In Kitamura’s study, leptin sensitivity was enhanced after anagliptin treatment in high-fat diet fed mice[25]. In the study evaluating the inhibitory effect of vildagliptin on fibrosis markers on white adipose tissue of high-fat diet-induced obese mice, vildagliptin prevents the increase of fibrosis markers and reduces leptin levels[26]. The effect of sitagliptin on reducing BMI and the occurrence of hypoglycemia in obese patients with insulin treatment-induced diabetes mellitus might be correlated with decreased leptin levels and increased adiponectin levels[27]. Add-on therapy with anagliptin in Japanese T2DM patients treated with metformin for 52 weeks also reduced leptin concentrations[28]. Our study included most studies with relatively shorter treatment durations lasting from 1 month to 6 months, with only 1 study lasting for 13 months. Further studies with a longer duration and a larger number of participants will be needed to illuminate the effect of DPP-4 inhibitors on leptin concentrations and leptin sensitivity.
T2DM is associated with metabolic dysregulation and chronic inflammation. Data emerged from the research of leptin in diabetes has been discussed as an inflammatory mediator sustaining multifactorial diseases[29]. Leptin induces tumor necrosis factor-α (TNF-α)-dependent inflammation in acquired generalized lipodystrophy disease[30]. Statin[31] and antidiabetic agents[32] including sitagliptin, metformin, pioglitazone, liraglutide, and empagliflozin exhibit certain effects on inflammation. Sitagliptin ameliorated diet-induced metabolic syndrome and fatty liver via regulation of adipose tissue inflammation and hepatic adiponectin/ leptin levels[33]. Another study also proved that the novel DPP-4 inhibitor teneligliptin prevents high-fat diet-induced obesity accompanied by increased energy expenditure in mice[34]. DPP-4 inhibitor anagliptin exerts anti-inflammatory effects on macrophages, adipocytes, and mouse livers by suppressing NF-kB activation[35]. We also found that the inflammatory marker of C-reactive protein was effectively reduced after DPP-4 inhibition[36]. Further data should be reviewed regarding the role of leptin in inflammation, and the role of inflammation on the development of leptin resistance and obesity.
Although there were some reports investigating the effect of antidiabetic agents including DPP-4 inhibitors, there were no confirming answers to draw on whether DPP4 inhibitors could modulate leptin. We could not get further information for other comparisons for modulating leptin levels in T2DM. In the absence of comparative evidence between DPP-4i and other anti-diabetic medications, this meta-analysis added detailed illustration on adipokine of leptin levels. In this pooled analysis, comparisons of DPP-4i therapy and other treatment for type 2 diabetes (with 10 included trials) were performed, providing evidence that DPP-4i treatment was not significantly associated with changing leptin levels in participants from different regions in comparison with placebo. This effect of DPP-4i on leptin levels had not been changed by potential variables of treatment duration, age, and baseline HbA1c.
This study is the first meta-analysis demonstrating the effect of DDP-4i on serum leptin concentrations in T2DM. It suggested that DPP-4i did not exert an effect on leptin resistance in T2DM in patients with diabetes and obesity-associated cardiovascular diseases. It also provides insights into the therapeutic implications of obesity-related atherosclerotic disease in humans for the potentially protective effects on leptin sensitivity. Secondly, the pooled results suggest that leptin potentially serves as an effective cardiovascular biomarker in T2DM. Thirdly, subgroup analysis has been performed to explore the effect of therapy duration, diabetes duration, dosage, and age.
This study also has some limitations needed to be disclosed. Firstly, only literatures published in English were searched, which could inevitably generate publication bias and unstable estimates of treatment effects. Secondly, the pooled analysis should be interpreted with consideration for the moderate heterogeneity across identified studies, although measures had been taken to alleviate it by performing the sensitivity analysis and subgroup analysis. Thirdly, the follow-up periods were relatively short, and evaluating the long-term effect of DPP-4i treatment is necessary.