Macrophage deficiency of Akt2 reduces atherosclerosis in Ldlr null mice

Regulation of innate immune cell function by mTOR

The innate immune system is central for the maintenance of tissue homeostasis and quickly responds to local or systemic perturbations by pathogenic or sterile insults. This rapid response must be metabolically supported to allow cell migration and proliferation and to enable efficient production of cytokines and lipid mediators. This Review focuses on the role of mammalian target of rapamycin (mTOR) in controlling and shaping the effector responses of innate immune cells. mTOR reconfigures cellular metabolism and regulates translation, cytokine responses, antigen presentation, macrophage polarization and cell migration. The mTOR network emerges as an integrative rheostat that couples cellular activation to the environmental and intracellular nutritional status to dictate and optimize the inflammatory response. A detailed understanding of how mTOR metabolically coordinates effector responses by myeloid cells will provide important insights into immunity in health and disease.

Disruption of p21-activated kinase 1 gene diminishes atherosclerosis in apolipoprotein E-deficient mice

Pak1 plays an important role in various cellular processes, including cell motility, polarity, survival and proliferation. To date, its role in atherogenesis has not been explored. Here we report the effect of Pak1 on atherogenesis using atherosclerosis-prone apolipoprotein E-deficient (ApoE^−/−) mice as a model. Disruption of Pak1 in ApoE^−/− mice results in reduced plaque burden, significantly attenuates circulating IL-6 and MCP-1 levels, limits the expression of adhesion molecules and diminishes the macrophage content in the aortic root of ApoE^−/− mice. We also observed reduced oxidized LDL uptake and increased cholesterol efflux by macrophages and smooth muscle cells of ApoE^−/−:Pak1^−/− mice as compared with ApoE^−/− mice. In addition, we detect increased Pak1 phosphorylation in human atherosclerotic arteries, suggesting its role in human atherogenesis. Altogether, these results identify Pak1 as an important factor in the initiation and progression of atherogenesis.

Atherogenesis involves coordinated action of different cell types and factors. Here the authors show that the kinase Pak1 represents a key pro-atherogenic factor affecting the function of macrophages and vascular smooth muscle cells, including their production of proinflammatory cytokine IL-6 and chemokine MCP-1, and retention of cholesterol.

Akt isoforms in vascular disease

The mammalian serine/threonine Akt kinases comprise three closely related isoforms: Akt1, Akt2 and Akt3. Akt activation has been implicated in both normal and disease processes, including in development and metabolism, as well as cancer and cardiovascular disease. Although Akt signalling has been identified as a promising therapeutic target in cancer, its role in cardiovascular disease is less clear. Importantly, accumulating evidence suggests that the three Akt isoforms exhibit distinct tissue expression profiles, localise to different subcellular compartments, and have unique modes of activation. Consistent with in vitro findings, genetic studies in mice show distinct effects of individual Akt isoforms on the pathophysiology of cardiovascular disease. This review summarises recent studies of individual Akt isoforms in atherosclerosis, vascular remodelling and aneurysm formation, to provide a comprehensive overview of Akt function in vascular disease.

The influence of dysfunctional signaling and lipid homeostasis in mediating the inflammatory responses during atherosclerosis

Atherosclerosis, the underlying cause of myocardial infarction and thrombotic cerebrovascular events, is responsible for the majority of deaths in westernized societies. Mortality from this disease is also increasing at a marked rate in developing countries due to the acquisition of a westernized lifestyle accompanied with elevated rates of obesity and diabetes. Atherosclerosis is recognized as a chronic inflammatory disorder associated with lipid accumulation and the development of fibrotic plaques within the walls of medium and large arteries. A range of immune cells, such as macrophages and T-lymphocytes, through the action of various cytokines, such as interleukins-1 and -33, transforming growth factor-β and interferon-γ, orchestrates the inflammatory response in this disease. The disease is also characterized by marked dysfunction in lipid homeostasis and signaling pathways that control the inflammatory response. This review will discuss the molecular basis of atherosclerosis with particular emphasis on the roles of the immune cells and cytokines along with the dysfunctional lipid homeostasis and cell signaling associated with this disease.

Genetic Evidence Supports a Major Role for Akt1 in VSMCs During Atherogenesis

RATIONALE

Coronary artery disease, the direct result of atherosclerosis, is the most common cause of death in Western societies. Vascular smooth muscle cell (VSMC) apoptosis occurs during the progression of atherosclerosis and in advanced lesions and promotes plaque necrosis, a common feature of high-risk/vulnerable atherosclerotic plaques. Akt1, a serine/threonine protein kinase, regulates several key endothelial cell and VSMC functions including cell growth, migration, survival, and vascular tone. Although global deficiency of Akt1 results in impaired angiogenesis and massive atherosclerosis, the specific contribution of VSMC Akt1 remains poorly characterized.

OBJECTIVE

To investigate the contribution of VSMC Akt1 during atherogenesis and in established atherosclerotic plaques.

METHODS AND RESULTS

We generated 2 mouse models in which Akt1 expression can be suppressed specifically in VSCMs before (Apoe(-/-)Akt1(fl/fl)Sm22α(CRE)) and after (Apoe(-/-)Akt1(fl/fl)SM-MHC-CreER(T2E)) the formation of atherosclerotic plaques. This approach allows us to interrogate the role of Akt1 during the initial and late steps of atherogenesis. The absence of Akt1 in VSMCs during the progression of atherosclerosis results in larger atherosclerotic plaques characterized by bigger necrotic core areas, enhanced VSMC apoptosis, and reduced fibrous cap and collagen content. In contrast, VSMC Akt1 inhibition in established atherosclerotic plaques does not influence lesion size but markedly reduces the relative fibrous cap area in plaques and increases VSMC apoptosis.

CONCLUSIONS

Akt1 expression in VSMCs influences early and late stages of atherosclerosis. The absence of Akt1 in VSMCs induces features of plaque vulnerability including fibrous cap thinning and extensive necrotic core areas. These observations suggest that interventions enhancing Akt1 expression specifically in VSMCs may lessen plaque progression.

Deletion of Myeloid GSK3α Attenuates Atherosclerosis and Promotes an M2 Macrophage Phenotype

OBJECTIVE

Glycogen synthase kinase (GSK)-3α/β has been implicated in the pathogenesis of diabetes mellitus, cancer, Alzheimer, and atherosclerosis. The tissue- and homolog-specific functions of GSK3α and β in atherosclerosis are unknown. This study examines the effect of hepatocyte or myeloid cell deletion of GSK3α or GSK3β on atherosclerosis in low-density lipoprotein receptor (LDLR)(-/-) mice.

APPROACH AND RESULTS

We ablated GSK3α or GSK3β expression in hepatic or myeloid cells of LDLR(-/-) mice, and mice were fed a high-fat diet for 10 weeks. GSK3α or GSK3β deficiency in hepatic or myeloid cells did not affect metabolic parameters, including plasma lipid levels. Hepatic deletion of GSK3α or GSK3β did not affect the development of atherosclerosis or hepatic lipid content. Myeloid deletion of GSK3α, but not of GSK3β, reduced atherosclerotic lesion volume and lesion complexity. Mice lacking GSK3α in myeloid cells had a less inflammatory and more anti-inflammatory plasma cytokine profile. Macrophages within atherosclerotic lesions of myeloid GSK3α-deficient mice, but not of GSK3β-deficient mice, displayed reduced expression of markers associated with M1 macrophage polarization and enhanced expression of the M2 markers. Finally, bone marrow-derived macrophages were isolated and differentiated into classical M1 macrophages or alternative M2 macrophages in vitro. GSK3α deletion, but not GSK3β deletion, attenuated the expression of genes associated with M1 polarization while promoting the expression of genes associated with M2 polarization by modulating STAT3 and STAT6 activation.

CONCLUSIONS

Our findings suggest that deletion of myeloid GSK3α attenuates the progression of atherosclerosis by promoting an M2 macrophage phenotype.