br GLP R expression in the vascular endothelium

GLP-1R expression in the vascular endothelium Immunohistochemistry studies utilizing GLP-1R antibodies that have now been demonstrated to be non-specific [46], initially identified GLP-1R protein expression in both mouse mesenteric artery SMCs and heart coronary SMCs [7]. However, evidence for VSMC GLP-1R expression was also demonstrated by Richards et al., who showed co-expression of Glp1r-promoter driven yellow fluorescent protein and smooth muscle Darunavir Ethanolate in cells within ventricular blood vessels [48]. Moreover, GLP-1R expression was detected in mouse thoracic artery SMCs via immunohistochemistry, utilizing a NOVUS Biologicals antibody that remains to be validated for its specificity towards the GLP-1R [32]. In contrast, studies in human heart samples acquired from the University of Pennsylvania Heart Biobank failed to detect full-length GLP1R mRNA transcripts in coronary artery SMCs [5]. With regard to endothelial cell (EC) GLP-1R expression, both human coronary artery ECs (HCAECs) and human umbilical vein ECs (HUVECs) demonstrate positive GLP-1R protein expression via western blotting [28,44], though validity of the GLP-1R antibodies used in these studies remains to be determined, or has been shown to be non-specific, respectively. Conversely, coronary artery ECs in human heart samples from the University of Pennsylvania Heart Biobank once again did not demonstrate full-length GLP1R mRNA transcript expression [5]. Taken together, while species-specific findings may explain some of the discrepancies regarding VSMC/EC GLP-1R expression, it remains to be conclusively determined whether the GLP-1R is expressed within these cell-types in the vascular endothelium of animals or humans. In addition, whether specific vascular beds express the GLP-1R (e.g. coronary vasculature versus the renal vasculature), and whether this may change in the presence of underlying disease (e.g. coronary artery disease, renal failure, etc.) are important questions that future studies will need to address.
Vascular/endothelial GLP-1/GLP-1R action in vitro Despite the ongoing debate regarding the expression of the GLP-1R in VSMCs and ECs, a number of studies have demonstrated that direct treatment of VSMCs/ECs with native GLP-1 or GLP-1R agonists produces biological action(s). For example, treatment of mouse aortic VSMCs with exendin-4 (10 nM) reduced platelet-derived growth factor (PDGF, 25 ng/mL for 24 h)-induced cell proliferation [22], whereas exendin-4 (5 nM) decreased PDGF (10 ng/mL for 48 h)-induced proliferation of rat aortic VSMCs, which could be prevented via inhibiting protein kinase A (PKA) [25]. In addition, liraglutide (0.01–1 μg/mL) reduced oxidized low-density lipoprotein (LDL)-induced mitochondrial reactive oxygen species (ROS) generation in human aortic SMCs [14]. In studies using cultured VSMCs isolated from mouse abdominal aorta, over-expression of the lectin-like oxidized LDL scavenger receptor-1 (LOX-1) abrogated the liraglutide-mediated reduction in mitochondrial ROS [14]. With regards to ECs, treatment of HUVECs with liraglutide (0.1–100 μg/mL) increased endothelial NO synthase (eNOS) phosphorylation and NO production in a 5′AMP-activated protein kinase (AMPK)-dependent manner [23]. Likewise, both GLP-1 (100 nM) and exendin-4 (10 nM) increased eNOS phosphorylation and NO production in HCAECs [18]. Endothelial GLP-1R action has also been linked to reductions in oxidative stress, as treatment with GLP-1 (0.03–0.3 nM) reduced ROS and vascular cell adhesion molecule-1 mRNA expression in HUVECs following exposure to advanced glycation end products [28]. Similarly, liraglutide (1 μg/mL) treatment increased NO production and reduced tumor necrosis factor α (TNFα, 10 ng/mL)-induced nuclear factor kappa B activation in HUVECs, while also decreasing inflammatory gene expression (e.g. VCAM-1 and MCP-1) [23]. These salutary actions on ECs extend to an in vitro model of vascular aging, as treatment of HUVECs with GLP-1 or exendin-4 reduced hydrogen peroxide (30 μM)-induced senescence, actions that could be prevented via inhibiting either the GLP-1R (exendin(9–39)) or PKA (H89) [45]. It should be noted that the majority of the abovementioned studies used very high concentrations versus the normal physiological levels of GLP-1 observed following nutrient ingestion (∼20–30 pM) [4], and the reported observations could potentially be artifacts arising from promiscuous actions on related GPCRs.