Type 2 diabetes mellitus (T2DM) has become a new global pandemic due to the prevalence of Westernized diets and unhealthy lifestyles (Unnikrishnan et al., 2017; Cefalu, 2016). Estimates of data from China suggested that 43.8 million cases of T2DM in 2011 were the result of being overweight or obese (Zimmet et al., 2018). From 2013 to 2018, the number of overweight and obese people and T2DM patients continued to increase(Wang et al., 2021a). Insulin resistance (IR) is a key link between obesity and T2DM, so hypoglycaemic drugs that can improve IR are attracting an increasing amount of attention(Li et al., 2022).

Glucagon-like peptide-1 (GLP-1) is an incretin hormone released by L-cells in the small intestine that augments insulin secretion and lowers glucose concentrations, but native GLP-1 is degraded within 2–3 min in the circulation, so various GLP-1 receptor agonists (GLP-1RAs) act to provide prolonged in vivo actions of GLP-1(6). Liraglutide (Lira), as a typical GLR-1RA, has became a representative of new hypoglycaemic drugs (Michael A Nauck, 2021), because it reduces body weight and improves IR among patients with T2DM and T2DM mice in addition to lowering glucose (Zhang et al., 2021a; Davies et al., 2015). In T2DM, Lira has also been reported to contribute to protecting mitochondria in the soleus muscle(Yamada et al., 2022).

Skeletal muscle is the key insulin target organ and the main site of lipid-induced IR in T2DM(11, 12). Moreover, it is an important endocrine organ that can secrete a variety of myokines to communicate with other organs, such as adipose tissue(Pedersen and Febbraio, 2012). Irisin, proteolytically released from the transmembrane fibronectin type III domain-containing protein 5 (FNDC5), is a recently identified myokine that possesses a range of bioactivities, such as bone remodeling, neuroprotection and promoting brown-fat-like development (Maak et al., 2021). Study has shown that small amounts of irisin can also be released from adipose tissue as adipokine (Roca-Rivada et al., 2013).

White adipose tissue (WAT) and brown adipose tissue (BAT) are two types of fat tissue in mammals.WAT is typically specialized as an energy reservoir mainly in the form of triglycerides (TG), which are hydrolyzed to free fatty acids (FFA) and glycerol released into the circulation in times of food shortage, while BAT is responsible for heat generation and maintenance of body temperature mediated by uncoupling protein 1 (UCP1). Recently, irisin has been reported to stimulate the expression of browning-specific genes by binding to an α-integrin receptor, which in turn promotes weight loss via the p38 mitogen-activated protein kinase (p38 MAPK) and extracellular signal-related kinase (ERK) signaling pathways (Yuan Zhang, 2014; Kim et al., 2018).

Recently it was reported that pentamethylquercetin regulates lipid metabolism by modulating skeletal muscle-adipose tissue crosstalk in obese mice (Wu et al., 2022). Circulating irisin, mainly secreted by the skeletal muscle in response to exercise, can up-regulate UCP1 expression in WAT and combat obesity(Boström et al., 2012). Thus, pharmacologically inducing irisin production and weight loss may be a subject of interest for patients who are obese and unable or unwilling to exercise for personal reasons (Polyzos et al., 2018).

Additive treatment with Lira significantly increases serum irisin levels in T2DM patients inadequately controlled with insulin or other hypoglycaemic drugs(Guarnotta et al., 2021). Lira induces the expression of FNDC5 in mouse pancreas and increases the serum level of secreted FNDC5, which provides new mechanistic insights into Lira's anti-obesity actions(Li et al., 2021). One recent clinical study involving 54 obese T2DM patients also revealed that exenatide significantly increases the serum irisin(Liu Jia, 2016). However, the secretion and mechanism of irisin induced by Lira has not been reported.

AMP-activated protein kinase (AMPK) is an energy sensor and is involved in diabetes and diabetic complications(Madhavi et al., 2019). Previously, we demonstrated that acetyl-CoA carboxylase 1 (ACC1) is the downstream molecule of AMPK in WAT and macrophages(Zhang et al., 2021a). Because irisin secretion from skeletal muscle was positively associated with UCP1 gene expression in WAT (Boström et al., 2012), we hypothesised that Lira modulates muscle-to-adipose signaling pathway via promoting irisin secretion from muscle cells. Lira works mainly through binding to its cognate membrane-associated GLP-1R, so the specific antagonist Exendin 9-39 was used to confirm the molecular target of Lira on differentiated C2C12 cells. AMPK inhibitor Compound C and siAMPKα were used to treat differentiated 3T3-L1 cells to confirm the role of AMPK. Finally, transcriptome analysis was used to confirm the changes of the related molecules and pathways in irisin-treated differentiated 3T3-L1 cells.