In the present study, we found that GLP-1R protein levels decreased significantly in a knee OA rat model induced by monoiodoacetate, which was accompanied by the downregulation of the PKA/CREB pathway and upregulation of the inflammation-related proteins TNF-α, IL-6 and IL-1β. We also found that GLP-1R interacted with the PKA/CREB pathway and that liraglutide could activate PKA/CREB signals and inhibit the expression of inflammation-related proteins. Together these data indicate that liraglutide exhibits anti-inflammatory activity through the activation of the PKA/CREB pathway in an OA rat model. In addition, we also found that liraglutide could cause weight loss during treatment, which may be closely related to its mechanism in weight regulation. However, whether this weight loss had a bad effect on OA rats remained to be further studied.
OA was once defined as a non-inflammatory arthropathy, but it is now well-recognized that there is a major inflammatory component to this disease. Previous studies indicated that inflammation plays an important role in the development of OA, which is often closely related to low-grade synovitis [
17]. Moreover, cytokines produced in innate immune responses, such as TNFα, IL-6 and IL-1β are found at comparable levels in rheumatoid arthritis joint tissues, early OA and end-stage disease synovium [
18]. Analogously, in our study, we also found increased levels of those cytokines in the cartilage tissues of our knee OA rat model, suggesting that chondritis might also be implicated in the occurrence and development of OA. Thus, blocking inflammatory pathways during the early stages of OA could, at least in part, delay or impede the progress of the disease, which offers a reasonable basis for the exploitation of anti-arthritis drugs. For example Ran et al., suggested that schisandrin B might be a potential therapeutic agent in the treatment of OA due to its anti-inflammatory effects via suppression of the nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signal pathways [
19].
Inflammation is mediated by many upstream and downstream molecules. Among them, GLP-1/GLP-1R signals have been investigated as treatment targets for inflammation-related diseases, given their anti-inflammatory functions. GLP-1 is an incretin hormone, produced by L cells to regulate glucose and energy homeostasis via GLP-1R binding [
9]. GLP-1R is a G-protein coupled receptor and is widely localized in astrocytes, neurons and endothelial cells [
20‐
22]. In the musculoskeletal system, Chen et al. found that GLP-1R was also expressed in chondrocytes and was associated with the degeneration of cartilage. Moreover, the authors also found decreased expression of GLP-1R in an OA rat model, where OA was induced by ACL resection in a transected ACL and medial menisci in the right knee joint [
10]. Similarly, we also found that the expression of GLP-1R was lower in knee cartilage tissues with OA induced by the administration of monoiodoacetate compared with the control group, which suggests a potential and crucial role of GLP-1R in OA development. Although, the molecular mechanisms responsible for the anti-inflammatory effects of GLP-1/GLP-1R signals are still under investigation, the PKA/CREB pathway axis and associated active components may be major signals responsible for the anti-inflammatory effects of GLP-1/GLP-1R [
23]. In cartilage tissues of the OA knee, upregulation of inflammation-related proteins is accompanied by the downregulation of GLP-1R and PKA/CREB pathways. In addition, we used protein immunoprecipitation to verify the interaction between GLP-1R and the PKA/CREB pathway. Thus, we hypothesized that GLP-1R might participate in the regulation of PKA/CREB signals, which are likely to be involved in inflammatory processes. To test this hypothesis, liraglutide, an agonist of GLP-1/GLP-1R, was used to upregulate GLP-1R signals in our OA rat model. Surprisingly, we found that liraglutide could activate the PKA/CREB pathway and inhibit inflammation. After GLP-1 binding to GLP-1R, this complex could upregulate cAMP thereby activating the PKA/CREB pathway [
24]. CREB binds to CREB-binding protein (CBP), a member of the family of histone acetyltransferases (HATs), and catalyzes histone acetylation, thereby switching chromatin from a closed to an open state [
12]. This change promotes the binding of RNA polymerase II and basal transcription factors to the open DNA to initiate transcription, involving anti-inflammatory cytokines, such as IL-10 and dual specificity phosphatase 1 (DUSP1) [
25]. One key action of IL-10 is to transform macrophages of an M1 “inflammatory” phenotype that produce high levels of pro-inflammatory cytokines and low levels of anti-inflammatory cytokines, to an M2b “regulatory” phenotype that produce low levels of pro-inflammatory cytokines and high levels of anti-inflammatory molecules [
1,
26]. DUSP1 restricts activation of the MyD88 signaling pathway by dephosphorylation and inactivation of p38 MAPKs and c-Jun N-terminal kinases (JNKs), which can be induced by glucocorticoids [
26,
27]. These data suggest that the activation of CREB signals plays an important role in the anti-inflammatory response, with GLP-1R being implicated in this process.