Liraglutide, an acylated analog of glucagon-like peptide 1 (GLP-1), could improve glycemic control in diabetes. Moreover, endogenous opioid peptides play a role in blood sugar regulation. Since GLP-1 receptors are also expressed in extra-pancreatic tissues, this study investigates the effect of liraglutide on endogenous opioid secretion in type 1-like diabetes. The endogenous opioid level was determined by enzyme-linked immunosorbent assay. The direct effect of liraglutide on endogenous opioid secretion was determined in the isolated adrenal medulla. Acute treatment with liraglutide dose-dependently attenuated hyperglycemia, and increased the plasma opioid neuropeptide, beta-endorphin (BER) levels in diabetic rats. These effects have been blocked by GLP-1 receptor antagonist, naloxone. Additionally, the effects of liraglutide were markedly reduced in adrenalectomized diabetic rats. In the isolated adrenal medulla, liraglutide induced BER secretion and increased the BER mRNA levels. Subcellular effects of liraglutide on the adrenal gland were further identified to mediate through the exchange proteins directly activated by cAMP, mainly using the pharmacological blockade. After repeatedly administering liraglutide, metabolic changes in diabetic rats were investigated, and genes associated with gluconeogenesis in the liver were downregulated. Naloxone pretreatment inhibited these effects of liraglutide, indicating the involvement of endogenous opioids. The present study indicated that liraglutide had an acute effect of reducing hyperglycemia by regulating endogenous opioid BER and modifying the glucose homeostasis.

Adipose-derived cytokines may contribute to the inflammation that occurs in metabolic syndrome (MetS). The Takeda G protein-coupled receptor (TGR5) regulates energy expenditure and affects the production of pro-inflammatory biomarkers in metabolic diseases. Etanercept, which acts as a tumor necrosis factor (TNF)-α antagonist, can also block the inflammatory response. Therefore, the interaction between TNF-α and TGR5 expression was investigated in rats with high-fat diet (HFD)-induced obesity. Heart tissues isolated from the HFD-induced MetS rats were analyzed. Changes in TGR5 expression were investigated with lithocholic acid (LCA) as the agonist. Betulinic acid (BA) was used to activate TGR5 in urinary bladders. LCA was more effective in the heart tissues of HFD-fed rats, although etanercept alleviated the function of LCA. STAT3 activation and higher TGR5 expression were observed in the heart tissues collected from HFD-fed rats. Thus, cardiac TGR5 expression is promoted by HFD through STAT3 activation in rats. Moreover, the urinary bladders of female rats fed a HFD showed a low response, which was reversed by etanercept. Relaxation by BA in the bladders was more marked in HFD-fed rats. The high TGR5 expression in HFD-fed rats was characterized using a mRNA assay, and the increased cAMP levels were found to be stimulated by BA in the isolated bladders. Therefore, TGR5 expression increases with a HFD in both the hearts and urinary bladders. Collectively, cytokine-medicated TGR5 activation was observed in the hearts and urinary bladders of rats.

Diet-induced obesity (DIO) is considered the main risk factor for cardiovascular diseases. Increases in the plasma levels of tumor necrosis factor alpha (TNF-α) is associated with DIO. Etanercept, a TNF-α inhibitor, has been shown to alleviate cardiac hypertrophy. To investigate the effect of etanercept on cardiac fibrosis in DIO model, rats on high fat diet (HFD) were subdivided into two groups: the etanercept group and vehicle group. Cardiac injury was identified by classic methods, while fibrosis was characterized by histological analysis of the hearts. Etanercept treatment at 0.8 mg/kg/week twice weekly by subcutaneous injection effectively alleviates the cardiac fibrosis in HFD-fed rats. STAT3 activation seems to be induced in parallel with fibrosis-related gene expression in the hearts of HFD-fed rats. Decreased STAT3 activation plays a role in the etanercept-treated animals. Moreover, fibrosis-related genes are activated by palmitate in parallel with STAT3 activation in H9c2 cells. Etanercept may inhibit the effects of palmitate, but it is less effective than a direct inhibitor of STAT3. Direct inhibition of STAT3 activation by etanercept seems unlikely. Etanercept has the ability to ameliorate cardiac fibrosis through reduction of STAT3 activation after the inhibition of TNF-α and/or its receptor.

Diabetes is commonly treated with glucagon-like peptide-1 receptor (GLP-1R) agonists including liraglutide and others. However, liraglutide was found to raise plasma glucose levels in normal rats. The current study aims to determine how liraglutide causes this contentious condition in rats, both normal and diabetic. An adrenalectomy was performed to investigate the relationship between steroid hormone and liraglutide. To investigate the effect of central liraglutide infusion on blood glucose in rats, rats were intracerebroventricularly administrated with liraglutide with or without HPA axis inhibitors such as berberine and dexamethasone. The results showed that a single injection of liraglutide caused a temporary increase in blood glucose in healthy rats. Another GLP-1R agonist, Exendin-4 (Ex-4), increased blood sugar in a manner similar to that of liraglutide. The effects of liraglutide were also blocked by guanethidine pretreatment and vanished in normal rats with adrenalectomy. Additionally, central infusion of liraglutide via intracerebroventricular (icv) injection into normal rats also causes a temporary increase in blood glucose that was blocked by GLP-1R antagonists or the inhibitors such as berberine and dexamethasone. Similarly, central liraglutide treatment causes temporary increases in plasma glucose, adrenocorticotropic hormone (ACTH), and cortisol levels, which were reversed by inhibitors for the hypothalamic-pituitary-adrenal (HPA) axis. In normal rats, the temporary glucose-increasing effect of liraglutide was gradually eliminated during consecutive daily treatments, indicating tolerance formation. Additionally, liraglutide and Ex-4 cross-tolerance was also discovered in normal rats. Liraglutide was more effective in diabetic rats than in normal rats in activating GLP-1R gene expression in the isolated adrenal gland. Interestingly, the effect of liraglutide on glycemic control varied depending on whether the rats were diabetic or not. In normal rats, bolus injection of liraglutide, such as Ex-4, may stimulate the HPA axis, resulting in hyperglycemia. The cross-tolerance of liraglutide and Ex-4 provided a novel perspective on GLP-1R activation.