Modulation of ambient temperature promotes inflammation and initiates atherosclerosis in wild type C57BL/6 mice.

OBJECTIVES: Obesity and obesity-associated inflammation is central to a variety of end-organ sequelae including atherosclerosis, a leading cause of death worldwide. Although mouse models have provided important insights into the immunopathogenesis of various diseases, modeling atherosclerosis in mice has proven difficult. Specifically, wild-type (WT) mice are resistant to developing atherosclerosis, while commonly used genetically modified mouse models of atherosclerosis are poor mimics of human disease. The lack of a physiologically relevant experimental model of atherosclerosis has hindered the understanding of mechanisms regulating disease development and progression as well as the development of translational therapies. Recent evidence suggests that housing mice within their thermoneutral zone profoundly alters murine physiology, including both metabolic and immune processes. We hypothesized that thermoneutral housing would allow for augmentation of atherosclerosis induction and progression in mice. METHODS: ApoE-/- and WT mice were housed at either standard (TS) or thermoneutral (TN) temperatures and fed either a chow or obesogenic "Western" diet. Analysis included quantification of (i) obesity and obesity-associated downstream sequelae, (ii) the development and progression of atherosclerosis, and (iii) inflammatory gene expression pathways related to atherosclerosis. RESULTS: Housing mice at TN, in combination with an obesogenic "Western" diet, profoundly augmented obesity development, exacerbated atherosclerosis in ApoE-/- mice, and initiated atherosclerosis development in WT mice. This increased disease burden was associated with altered lipid profiles, including cholesterol levels and fractions, and increased aortic plaque size. In addition to the mild induction of atherosclerosis, we similarly observed increased levels of aortic and white adipose tissue inflammation and increased circulating immune cell expression of pathways related to adverse cardiovascular outcome. CONCLUSIONS: In sum, our novel data in WT C57Bl/6 mice suggest that modulation of a single environmental variable, temperature, dramatically alters mouse physiology, metabolism, and inflammation, allowing for an improved mouse model of atherosclerosis. Thus, thermoneutral housing of mice shows promise in yielding a better understanding of the cellular and molecular pathways underlying the pathogenesis of diverse diseases.

MicroRNA-33-dependent regulation of macrophage metabolism directs immune cell polarization in atherosclerosis.

Cellular metabolism is increasingly recognized as a controller of immune cell fate and function. MicroRNA-33 (miR-33) regulates cellular lipid metabolism and represses genes involved in cholesterol efflux, HDL biogenesis, and fatty acid oxidation. Here, we determined that miR-33-mediated disruption of the balance of aerobic glycolysis and mitochondrial oxidative phosphorylation instructs macrophage inflammatory polarization and shapes innate and adaptive immune responses. Macrophage-specific Mir33 deletion increased oxidative respiration, enhanced spare respiratory capacity, and induced an M2 macrophage polarization-associated gene profile. Furthermore, miR-33-mediated M2 polarization required miR-33 targeting of the energy sensor AMP-activated protein kinase (AMPK), but not cholesterol efflux. Notably, miR-33 inhibition increased macrophage expression of the retinoic acid-producing enzyme aldehyde dehydrogenase family 1, subfamily A2 (ALDH1A2) and retinal dehydrogenase activity both in vitro and in a mouse model. Consistent with the ability of retinoic acid to foster inducible Tregs, miR-33-depleted macrophages had an enhanced capacity to induce forkhead box P3 (FOXP3) expression in naive CD4(+) T cells. Finally, treatment of hypercholesterolemic mice with miR-33 inhibitors for 8 weeks resulted in accumulation of inflammation-suppressing M2 macrophages and FOXP3(+) Tregs in plaques and reduced atherosclerosis progression. Collectively, these results reveal that miR-33 regulates macrophage inflammation and demonstrate that miR-33 antagonism is atheroprotective, in part, by reducing plaque inflammation by promoting M2 macrophage polarization and Treg induction.

Niacin inhibits skin dendritic cell mobilization in a GPR109A independent manner but has no impact on monocyte trafficking in atherosclerosis.

High-dose niacin therapy in humans reduces mortality from cardiovascular disease and may also protect against death from other causes, with benefits apparent more than a decade beyond the therapeutic period. Niacin therapy modulates circulating lipids, raising HDL and lowering LDL, but has the unwanted side effect of inducing skin flushing in response to treatment. Skin flushing results from niacin-induced activation of GPR109A and subsequent release of prostaglandins that promote vasodilation. GPR109A may also mediate HDL elevation. Recent data suggest that high-dose niacin may have benefits beyond improved lipid profiles, such as quelling inflammation, suggesting a potential role in immune cell trafficking. To explore effects of niacin on immune cell trafficking independently of its effects on lipid profiles, we took advantage of the fact that niacin therapy does not raise HDL in wild-type or apoE⁻/⁻ mouse strains. Wild-type and apoE⁻/⁻ C57BL/6 mice were fed standard chow or high-fat diets supplemented or not with 1% niacin. Against our predictions, this treatment did not modulate monocyte recruitment to or retention within atherosclerotic plaques. By contrast, stimulating the skin of niacin-treated mice with a contact sensitizer revealed impaired dendritic cell accumulation in draining lymph nodes and associated impaired adaptive immunity. Surprisingly, niacin-mediated impaired dendritic cell mobilization could not be reversed by cyclooxygenase inhibitor treatment nor deletion of the niacin receptor GPR109A, suggesting that the effects of niacin on modulating the migration of dendritic cells are not directly linked to skin flushing. Overall, these data suggest the existence of novel pathways triggered by niacin that, through suppression of dendritic cell migration, might impact adaptive immune responses that participate in sustained therapeutic benefits independent of niacin's cardioprotective capabilities.

Suppressed monocyte recruitment drives macrophage removal from atherosclerotic plaques of Apoe-/- mice during disease regression.

Experimental models of atherosclerosis suggest that recruitment of monocytes into plaques drives the progression of this chronic inflammatory condition. Cholesterol-lowering therapy leads to plaque stabilization or regression in human atherosclerosis, characterized by reduced macrophage content, but the mechanisms that underlie this reduction are incompletely understood. Mice lacking the gene Apoe (Apoe-/- mice) have high levels of cholesterol and spontaneously develop atherosclerotic lesions. Here, we treated Apoe-/- mice with apoE-encoding adenoviral vectors that induce plaque regression, and investigated whether macrophage removal from plaques during this regression resulted from quantitative alterations in the ability of monocytes to either enter or exit plaques. Within 2 days after apoE complementation, plasma cholesterol was normalized to wild-type levels, and HDL levels were increased 4-fold. Oil red O staining and quantitative mass spectroscopy revealed that esterified cholesterol content was markedly reduced. Plaque macrophage content decreased gradually and was 72% lower than baseline 4 weeks after apoE complementation. Importantly, this reduction in macrophages did not involve migratory egress from plaques or CCR7, a mediator of leukocyte emigration. Instead, marked suppression of monocyte recruitment coupled with a stable rate of apoptosis accounted for loss of plaque macrophages. These data suggest that therapies to inhibit monocyte recruitment to plaques may constitute a more viable strategy to reduce plaque macrophage burden than attempts to promote migratory egress.