Targeting macrophages GLP-1 receptor pathways as emerging therapies to ameliorate cardiac remodelling associated with diabetes

Indeed, we have recently reported that the GLP-1 analogue, exendin-4, promotes specific benefits on cardiac inflammation and ECM remodelling in a mouse model of experimental diabetes. Besides its direct action to maintain cardiovascular function and indirect action to control cardiovascular risk factors, it is becoming increasingly evident that GLP-1 confers cardioprotective effects through modulation of inflammatory signalling.

Therefore, the aim of this thesis was to study the effects of the GLP-1 analogue, liraglutide, in diabetes with a specific focus on cardiac inflammation. <br><br>Based on our previous findings, we initially designed modified GLP-1 peptides to specifically target macrophages. As macrophages express the mannose receptor, we hypothesised that addition of mannose to GLP-1 peptides would increase macrophage selectivity, promoting high affinity binding to the cell surface.

However, addition of mannose was found to reduce exendin-4-GLP-1R binding and activation. Therefore, we employed poly (lactic-co-glycolic acid) (PLGA) nanoparticles for preferential targeting of liraglutide and exendin-4 to inflammatory cells. Whilst liraglutide-PLGA nanoparticles displayed equivalent GLP-1R binding and activation to native liraglutide in U2OS cells and CHL cells, respectively, they showed lower efficacy with respect to macrophage GLP-1R binding in vitro and in vivo.

This observation may be explained by rapid phagocytosis of nanoparticles before the adsorbed liraglutide had opportunity to bind to the GLP-1R. Therefore, an alternate approach involving, for example, selection of mannosylated or polyethylene glycol (PEG) PLGA-nanoparticles, may minimise rapid macrophage phagocytosis thereby promoting liraglutide-GLP-1R binding. <br><br>The heart consists of different cell types, including cardiomyocytes, fibroblasts, endothelial cells, and infiltrating inflammatory cells, with intercellular communication playing a key role in cardiac remodelling.

Cardiac fibroblasts play a particularly important role in diabetes by promoting ECM remodelling and fibrosis, whilst coronary microvascular endothelial cells secrete inflammatory factors that negatively influence cardiac structure and function to induce adverse remodelling. Accumulating evidence suggests that both cardiac fibrosis and endothelial dysfunction in diabetes are driven by infiltrating inflammatory cells, all of which have been shown to be altered by GLP1 agonists, although cardiac fibroblasts do not express the GLP-1R, highlighting indirect actions which appear to be largely mediated by macrophages.

As such, macrophages were treated with high glucose which induced cytokine/chemokine expression, dependent upon NF-κB as a prototypical proinflammatory mediator associated with macrophages infiltration and adhesion, effects which were reversed by liraglutide. Furthermore, liraglutide attenuated fibroblast differentiation and endothelial cell dysfunction induced by incubation with conditioned media from high glucose-treated macrophages, highlighting modulation of paracrine signalling as a likely mechanism underlying its apparent cardioprotective actions. <br><br>In order to investigate potential translation of these experimental findings, initial clinical analyses were undertaken in type 2 diabetic patients receiving conventional glycaemic medication and those receiving additional liraglutide.

Interestingly, although preliminary in nature, this study highlighted potential modulation of circulating inflammatory cells showing some correlation with cardiac function, which could be subject to modulation by liraglutide. <br><br>Taken together, the findings of this thesis have established that macrophage-specific targeting by modified GLP-1 is more complex than anticipated. However, given our in vitro findings indicating that liraglutide modulates macrophage inflammatory autocrine and paracrine signalling, selective macrophage modulation holds clear potential to maximise cardioprotective effects of GLP-1 on ECM remodelling and endothelial dysfunction.

In order to establish translational relevance of such experimental data, it is critical to generate comprehensive clinical data in order to support the specific use of GLP-1 based therapies to prevent and reduce adverse cardiac remodelling and dysfunction associated with diabetes.