Melanocortin overexpression limits diet-induced inflammation and atherosclerosis in LDLR-/- mice

Neutrophils at the Crossroads of Inflammatory Bowel Disease and Atherosclerosis: A State-of-the-Art Review

Inflammatory bowel disease (IBD) is a growing global problem, particularly in regions with low sociodemographic indices and growing populations. IBD incidence is increasing among children and adolescents, leading to a growing economic burden. The prevalence of atherosclerotic cardiovascular diseases among patients with IBD is also higher than in the general population. While mortality rates have decreased, cardiovascular disease (CVD) remains a significant contributor to mortality and disability in IBD patients. According to the current understanding, neutrophils play an important role in both the atherogenesis and pathogenesis of IBD. This review addresses the state of the art of neutrophil involvement in the development of atherosclerosis and IBD. In the present review, we summarize the currently available evidence regarding neutrophils as a possible key driver of extraintestinal manifestations of IBD and cardiovascular complications. We provide a discussion on the potential role of neutrophil-derived markers in the development of new approaches for the precise diagnosis of atherosclerosis in patients with IBD, as well as new therapeutic targets.

Hepatocyte-specific loss of melanocortin 1 receptor disturbs fatty acid metabolism and promotes adipocyte hypertrophy

BACKGROUND/OBJECTIVES

Melanocortins mediate their biological functions via five different melanocortin receptors (MC1R - MC5R). MC1R is expressed in the skin and leukocytes, where it regulates skin pigmentation and inflammatory responses. MC1R is also present in the liver and white adipose tissue, but its functional role in these tissues is unclear. This study aimed at determining the regulatory role of MC1R in fatty acid metabolism.

METHODS

Male recessive yellow (Mc1re/e) mice, a model of global MC1R deficiency, and male hepatocyte-specific MC1R deficient mice (Mc1r LKO) were fed a chow or Western diet for 12 weeks. The mouse models were characterized for body weight and composition, liver adiposity, adipose tissue mass and morphology, glucose metabolism and lipid metabolism. Furthermore, qPCR and RNA sequencing analyses were used to investigate gene expression profiles in the liver and adipose tissue. HepG2 cells and primary mouse hepatocytes were used to study the effects of pharmacological MC1R activation.

RESULTS

Chow- and Western diet-fed Mc1re/e showed increased liver weight, white adipose tissue mass and plasma triglyceride (TG) concentration compared to wild type mice. This phenotype occurred without significant changes in food intake, body weight, physical activity or glucose metabolism. Mc1r LKO mice displayed a similar phenotype characterized by larger fat depots, increased adipocyte hypertrophy and enhanced accumulation of TG in the liver and plasma. In terms of gene expression, markers of de novo lipogenesis, inflammation and apoptosis were upregulated in the liver of Mc1r LKO mice, while enzymes regulating lipolysis were downregulated in white adipose tissue of these mice. In cultured hepatocytes, selective activation of MC1R reduced ChREBP expression, which is a central transcription factor for lipogenesis.

CONCLUSIONS

Hepatocyte-specific loss of MC1R disturbs fatty acid metabolism in the liver and leads to an obesity phenotype characterized by enhanced adipocyte hypertrophy and TG accumulation in the liver and circulation.

Melanocortin 1 Receptor Deficiency in Hematopoietic Cells Promotes the Expansion of Inflammatory Leukocytes in Atherosclerotic Mice

Melanocortin receptor 1 (MC1-R) is expressed in leukocytes, where it mediates anti-inflammatory actions. We have previously observed that global deficiency of MC1-R signaling perturbs cholesterol homeostasis, increases arterial leukocyte accumulation and accelerates atherosclerosis in apolipoprotein E knockout (Apoe^-/-) mice. Since various cell types besides leukocytes express MC1-R, we aimed at investigating the specific contribution of leukocyte MC1-R to the development of atherosclerosis. For this purpose, male Apoe^-/- mice were irradiated, received bone marrow from either female Apoe^-/- mice or MC1-R deficient Apoe^-/- mice (Apoe^-/- Mc1r^e/e) and were analyzed for tissue leukocyte profiles and atherosclerotic plaque phenotype. Hematopoietic MC1-R deficiency significantly elevated total leukocyte counts in the blood, bone marrow and spleen, an effect that was amplified by feeding mice a cholesterol-rich diet. The increased leukocyte counts were largely attributable to expanded lymphocyte populations, particularly CD4⁺ T cells. Furthermore, the number of monocytes was elevated in Apoe^-/- Mc1r^e/e chimeric mice and it paralleled an increase in hematopoietic stem cell count in the bone marrow. Despite robust leukocytosis, atherosclerotic plaque size and composition as well as arterial leukocyte counts were unaffected by MC1-R deficiency. To address this discrepancy, we performed an in vivo homing assay and found that MC1-R deficient CD4⁺ T cells and monocytes were preferentially entering the spleen rather than homing in peri-aortic lymph nodes. This was mechanistically associated with compromised chemokine receptor 5 (CCR5)-dependent migration of CD4⁺ T cells and a defect in the recycling capacity of CCR5. Finally, our data demonstrate for the first time that CD4⁺ T cells also express MC1-R. In conclusion, MC1-R regulates hematopoietic stem cell proliferation and tissue leukocyte counts but its deficiency in leukocytes impairs cell migration via a CCR5-dependent mechanism.

Antifibrotic and Anti-Inflammatory Actions of α-Melanocytic Hormone: New Roles for an Old Player

The melanocortin system encompasses melanocortin peptides, five receptors, and two endogenous antagonists. Besides pigmentary effects generated by α-Melanocytic Hormone (α-MSH), new physiologic roles in sexual activity, exocrine secretion, energy homeostasis, as well as immunomodulatory actions, exerted by melanocortins, have been described recently. Among the most common and burdensome consequences of chronic inflammation is the development of fibrosis. Depending on the regenerative capacity of the affected tissue and the quality of the inflammatory response, the outcome is not always perfect, with the development of some fibrosis. Despite the heterogeneous etiology and clinical presentations, fibrosis in many pathological states follows the same path of activation or migration of fibroblasts, and the differentiation of fibroblasts to myofibroblasts, which produce collagen and α-SMA in fibrosing tissue. The melanocortin agonists might have favorable effects on the trajectories leading from tissue injury to inflammation, from inflammation to fibrosis, and from fibrosis to organ dysfunction. In this review we briefly summarized the data on structure, receptor signaling, and anti-inflammatory and anti-fibrotic properties of α-MSH and proposed that α-MSH analogues might be promising future therapeutic candidates for inflammatory and fibrotic diseases, regarding their favorable safety profile.

Melanocortin 3 receptor activation with [D-Trp8]-γ-MSH suppresses inflammation in apolipoprotein E deficient mice

The melanocortin MC₁ and MC₃ receptors elicit anti-inflammatory actions in leukocytes and activation of these receptors has been shown to alleviate arterial inflammation in experimental atherosclerosis. Thus, we aimed to investigate whether selective targeting of melanocortin MC₃ receptor protects against atherosclerosis. Apolipoprotein E deficient (ApoE^-/-) mice were fed high-fat diet for 12 weeks and randomly assigned to receive either vehicle (n = 11) or the selective melanocortin MC₃ receptor agonist [D-Trp(8)]-gamma-melanocyte-stimulating hormone ([D-Trp8]-γ-MSH; 15 μg/day, n = 10) for the last 4 weeks. Lesion size as well as macrophage and collagen content in the aortic root plaques were determined. Furthermore, leukocyte counts in the blood and aorta and cytokine mRNA expression levels in the spleen, liver and aorta were quantified. No effect was observed in the body weight development or plasma cholesterol level between the two treatment groups. However, [D-Trp8]-γ-MSH treatment significantly reduced plasma levels of chemokine (C-C motif) ligands 2, 4 and 5. Likewise, cytokine and adhesion molecule expression levels were reduced in the spleen and liver of γ-MSH-treated mice, but not substantially in the aorta. In line with these findings, [D-Trp8]-γ-MSH treatment reduced leukocyte counts in the blood and aorta. Despite reduced inflammation, [D-Trp8]-γ-MSH did not change lesion size, macrophage content or collagen deposition of aortic root plaques. In conclusion, the findings indicate that selective activation of melanocortin MC₃ receptor by [D-Trp8]-γ-MSH suppresses systemic and local inflammation and thereby also limits leukocyte accumulation in the aorta. However, the treatment was ineffective in reducing atherosclerotic plaque size.

Charge Characteristics of Agouti-Related Protein Implicate Potent Involvement of Heparan Sulfate Proteoglycans in Metabolic Function

The endogenous melanocortin peptide agouti-related protein (AgRP) plays a well-known role in foraging, but its contribution to metabolic regulation is less understood. Mature AgRP_(83-132) has distinct residues for melanocortin receptor binding and heparan sulfate interactions. Here, we show that AgRP increases ad libitum feeding and operant responding for food in mice, decreases oxygen consumption, and lowers body temperature and activity, indicating lower energy expenditure. AgRP increased the respiratory exchange ratio, indicating a reduction of fat oxidation and a shift toward carbohydrates as the primary fuel source. The duration and intensity of AgRP's effects depended on the density of its positively charged amino acids, suggesting that its orexigenic and metabolic effects depend on its affinity for heparan sulfate. These findings may have major clinical implications by unveiling the critical involvement of interactions between AgRP and heparan sulfate to the central regulation of energy expenditure, fat utilization, and possibly their contribution to metabolic disease.

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AgRP increases both ad libitum and operant food intake and reduces energy expenditure

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AgRP reduces fat utilization as a fuel source, which promotes body fat accumulation

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These actions of AgRP depend on the positive charges, outside its ICK motif, that bind heparan sulfate

Pro-opiomelanocortin and its Processing Enzymes Associate with Plaque Stability in Human Atherosclerosis - Tampere Vascular Study

α-melanocyte-stimulating hormone (α-MSH) is processed from pro-opiomelanocortin (POMC) and mediates anti-inflammatory actions in leukocytes. α-MSH also promotes macrophage reverse cholesterol transport by inducing ATP-binding cassette transporters ABCA1 and ABCG1. Here we investigated the regulation of POMC and α-MSH expression in atherosclerosis. First, transcript levels of POMC and its processing enzymes were analyzed in human arterial plaques (n = 68) and non-atherosclerotic controls (n = 24) as well as in whole blood samples from coronary artery disease patients (n = 55) and controls (n = 45) by microarray. POMC expression was increased in femoral plaques compared to control samples as well as in unstable advanced plaques. α-MSH-producing enzyme, carboxypeptidase E, was down-regulated, whereas prolylcarboxypeptidase, an enzyme inactivating α-MSH, was up-regulated in unstable plaques compared to stable plaques, suggesting a possible reduction in intraplaque α-MSH levels. Second, immunohistochemical analyses revealed the presence of α-MSH in atherosclerotic plaques and its localization in macrophages and other cell types. Lastly, supporting the role of α-MSH in reverse cholesterol transport, POMC expression correlated with ABCA1 and ABCG1 in human plaque and whole blood samples. In conclusion, α-MSH is expressed in atherosclerotic plaques and its processing enzymes associate with plaque stability, suggesting that measures to enhance the local bioavailability of α-MSH might protect against atherosclerosis.