Pharmacological Activation of the Melanocortin System Limits Plaque Inflammation and Ameliorates Vascular Dysfunction in Atherosclerotic Mice

Abstract

A therosclerosis is a chronic inflammatory disease causing progressive lesion formation and luminal narrowing of arteries. The underlying pathology is initiated by endothelial dysfunction and structural alterations after an imbalanced lipid metabolism and trapping of low-density lipoprotein (LDL) particles in the intima of arteries.1,2 Subsequently, intimal lipid accumulation triggers the release of various chemokines by activated endothelial cells, inducing a multiphase cascade of leukocyte infiltration and inflammatory responses in the arterial walls. As the knowledge of these inflammatory pathways has been growing during the past years, it has also uncovered new druggable targets that could complement the current treatment options, which do not directly interfere with the central inflammatory mechanisms driving atheroprogression. One endogenous pathway that has gained increasing attention for its wide-ranging and potent actions in suppressing maladaptive inflammatory responses is the melanocortin system consisting of melanocortin peptides and their cellular receptors. 3,4 However, the role of melanocortin signaling and its promise as a therapeutic target in atherosclerosis remain fully unexplored.Melanocortins are a family of peptides that are proteolytically cleaved from the common precursor molecule pro-opiomelanocortin. These peptides include melanocyte-stimulating hormones (α-, β-, and γ-MSH) and adrenocorticotropic hormone. They regulate important physiological functions by acting via G-protein-coupled melanocortin receptors (MC-Rs), named from MC1-R to MC5-R. 5,6 Our recent findings indicate that α-MSH serves as a protective regulator in the vasculature by enhancing endothelium-dependent control of blood vessel tone. 7 Mechanistically, α-MSH was shown to augment vascular nitric oxide (NO) availability by activating MC1-R in the endothelium. Besides promoting endothelial function, mounting evidence indicates that α-MSH, through the activation of © 2014 American Heart Association, Inc. Objective-Melanocortin peptides have been shown to elicit anti-inflammatory actions and to promote vascular endothelial function by activating type 1 and 3 melanocortin receptors. Here, we addressed whether these favorable properties of melanocortins could reduce atherosclerotic plaque inflammation and improve vasoreactivity in atherosclerotic mice. Approach and Results-Low-density lipoprotein receptor-deficient mice expressing only apolipoprotein B100 were fed a high-fat diet for 8 or 16 weeks and treated with either vehicle or a stable melanocortin analog, melanotan II (MT-II, 0.3 mg/kg per day, 4 weeks). We determined plaque uptake of fluorine-18-labeled fluorodeoxyglucose as a surrogate marker for atherosclerotic plaque inflammation and vascular function of the aorta by ex vivo analyses. MT-II had no effect on body weight or composition, or plasma cholesterol levels in atherosclerotic mice. Without attenuating atherosclerotic lesion size or lesional macrophage accumulation, MT-II treatment reduced fluorine-18-labeled...

Endothelium

“…Alterations in MRC-1 signaling impair endothelial and NO-dependent vasodilation in mice and in humans, leading to increased arterial stiffness. 18 These findings highlight the role of endothelial MRC-1 in the regulation of vascular tone. Expression of endothelial nitric oxide synthase (eNOS) is regulated by several exogenous stimuli, including tumor necrosis factor-a, which destabilizes eNOS mRNA leading to decreased eNOS expression.…”

“…17 Melanocortin-1 receptor (MRC-1), which regulates melanin pigmentation in the skin, has been also identified in endothelial cells where it increases NO availability. Rinne et al 18 report that alterations in MRC-1 signaling in mice and in humans leads to impaired endothelial-dependent vasodilation and NO availability and to increased arterial stiffness. These findings highlight the role of MRC-1 in the regulation of vascular tone.…”

“…Presence of MRC-1 in endothelial cells increases NO availability. 18 This receptor is abundantly expressed in the skin where it regulates melanin pigmentation. Alterations in MRC-1 signaling impair endothelial and NO-dependent vasodilation in mice and in humans, leading to increased arterial stiffness.…”

Comparison of Somatostatin Receptor 2-Targeting PET Tracers in the Detection of Mouse Atherosclerotic Plaques

“…The radioactivity uptake was also evaluated as plaque-to-wall and plaque-to-adventitia ratios. Cryosections (8 μm) adjacent to ARG-analyzed sections were immunohistochemically stained with a macrophage specific anti-mouse antibody (Mac-3, clone m3/84, BD Biosciences Pharmingen, Franklin Lakes, NJ, USA) to detect inflamed atherosclerotic plaques as previously described [26,27]. Dynamic acquisition, consisting of 11 time frames (2×30 s, 4×60 s and 5×300 s), was started immediately after intravenous tracer injection.…”

Systems Biology and Noninvasive Imaging of Atherosclerosis

“…PET, combined with the anatomical information from CT 101 or, more recently, MRI 88 , has been extensively validated to quantify vascular inflammation itself and its changes upon therapeutic intervention, both in humans and animals using the tracer 18F fluorodeoxyglucose (FDG) 102, 103 . Recently, 18F-FDG PET was used to demonstrate a decrease in vascular inflammation in Ldlr-/- atherosclerotic mice after treatment with melanocortin peptides 104 . Other PET tracers are now being investigated, such as sodium fluoride (NaF) 105 targeting micro-calcification, or tracers targeted to specific molecules, such as αvβ3, VCAM-1 102 , 68Ga-Fucoidan for P-selectin 106 (abundantly expressed in vulnerable, but not stable plaques), 64 Cu-FBP8 for thrombus detection and fibrin quantification 107, and 64 Cu-DOTATATE to selectively quantify plaque macrophages via the somatostatin receptor subtype-2 108 .…”