Cholesterol loading reprograms the microRNA-143/145-myocardin axis to convert aortic smooth muscle cells to a dysfunctional macrophage-like phenotype

So Much Cholesterol: the unrecognized importance of smooth muscle cells in atherosclerotic foam cell formation

PURPOSE OF REVIEW

Smooth muscle cells (SMCs) form the thickened intimal layer in atherosclerosis-prone arteries in early life, and provide the initial site for retention and uptake of atherogenic lipoproteins. Here we review current knowledge regarding the importance of SMCs in the deposition of cholesterol in atherosclerotic plaque.

RECENT FINDINGS

SMCs were found to comprise at least 50% of total foam cells in human coronary artery atherosclerosis, and exhibit a selective loss of expression of the cholesterol efflux promoter ATP-binding cassette transporter A1. Cholesterol loading induced a loss of SMC gene expression and an increase in macrophage and proinflammatory marker expression by cultured mouse and human arterial SMCs, with reversal of these effects upon removal of the excess cholesterol. Mice engineered to track all cells of SMC lineage indicated that, at most, SMCs make up about one-third of total cells in atherosclerotic plaque in these animals.

SUMMARY

SMCs appear to be the origin of the majority of foam cells in human atherosclerotic plaque. Recent studies suggest a renaissance of research on the role of SMCs in atherosclerosis is needed to make the next leap forward in the prevention and treatment of this disease.

Emerging roles for vascular smooth muscle cell exosomes in calcification and coagulation

Vascular smooth muscle cell (VSMC) phenotypic conversion from a contractile to 'synthetic' state contributes to vascular pathologies including restenosis, atherosclerosis and vascular calcification. We have recently found that the secretion of exosomes is a feature of 'synthetic' VSMCs and that exosomes are novel players in vascular repair processes as well as pathological vascular thrombosis and calcification. Pro-inflammatory cytokines and growth factors as well as mineral imbalance stimulate exosome secretion by VSMCs, most likely by the activation of sphingomyelin phosphodiesterase 3 (SMPD3) and cytoskeletal remodelling. Calcium stress induces dramatic changes in VSMC exosome composition and accumulation of phosphatidylserine (PS), annexin A6 and matrix metalloproteinase-2, which converts exosomes into a nidus for calcification. In addition, by presenting PS, VSMC exosomes can also provide the catalytic surface for the activation of coagulation factors. Recent data showing that VSMC exosomes are loaded with proteins and miRNA regulating cell adhesion and migration highlight VSMC exosomes as potentially important communication messengers in vascular repair. Thus, the identification of signalling pathways regulating VSMC exosome secretion, including activation of SMPD3 and cytoskeletal rearrangements, opens up novel avenues for a deeper understanding of vascular remodelling processes.

MicroRNA Regulation of Atherosclerosis

Atherosclerosis and its attendant clinical complications, such as myocardial infarction, stroke, and peripheral artery disease, are the leading cause of morbidity and mortality in Western societies. In response to biochemical and biomechanical stimuli, atherosclerotic lesion formation occurs from the participation of a range of cell types, inflammatory mediators, and shear stress. Over the past decade, microRNAs (miRNAs) have emerged as evolutionarily conserved, noncoding small RNAs that serve as important regulators and fine-tuners of a range of pathophysiological cellular effects and molecular signaling pathways involved in atherosclerosis. Accumulating studies reveal the importance of miRNAs in regulating key signaling and lipid homeostasis pathways that alter the balance of atherosclerotic plaque progression and regression. In this review, we highlight current paradigms of miRNA-mediated effects in atherosclerosis progression and regression. We provide an update on the potential use of miRNAs diagnostically for detecting increasing severity of coronary disease and clinical events. Finally, we provide a perspective on therapeutic opportunities and challenges for miRNA delivery in the field.

Beyond vascular inflammation--recent advances in understanding atherosclerosis

Atherosclerosis is the most life-threatening pathology worldwide. Its major clinical complications, stroke, myocardial infarction, and heart failure, are on the rise in many regions of the world--despite considerable progress in understanding cause, progression, and consequences of atherosclerosis. Originally perceived as a lipid-storage disease of the arterial wall (Die cellularpathologie in ihrer begründung auf physiologische und pathologische gewebelehre. August Hirschwald Verlag Berlin, [1871]), atherosclerosis was recognized as a chronic inflammatory disease in 1986 (New Engl J Med 314:488-500, 1986). The presence of lymphocytes in atherosclerotic lesions suggested autoimmune processes in the vessel wall (Clin Exp Immunol 64:261-268, 1986). Since the advent of suitable mouse models of atherosclerosis (Science 258:468-471, 1992; Cell 71:343-353, 1992; J Clin Invest 92:883-893, 1993) and the development of flow cytometry to define the cellular infiltrate in atherosclerotic lesions (J Exp Med 203:1273-1282, 2006), the origin, lineage, phenotype, and function of distinct inflammatory cells that trigger or inhibit the inflammatory response in the atherosclerotic plaque have been studied. Multiphoton microscopy recently enabled direct visualization of antigen-specific interactions between T cells and antigen-presenting cells in the vessel wall (J Clin Invest 122:3114-3126, 2012). Vascular immunology is now emerging as a new field, providing evidence for protective as well as damaging autoimmune responses (Int Immunol 25:615-622, 2013). Manipulating inflammation and autoimmunity both hold promise for new therapeutic strategies in cardiovascular disease. Ongoing work (J Clin Invest 123:27-36, 2013; Front Immunol 2013; Semin Immunol 31:95-101, 2009) suggests that it may be possible to develop antigen-specific immunomodulatory prevention and therapy-a vaccine against atherosclerosis.

Endothelial and smooth muscle cell transformation in atherosclerosis

PURPOSE OF REVIEW

Physiologically, endothelial integrity and smooth muscle homeostasis play key roles in the maintenance of vascular structure and functions. Under pathological conditions, endothelial and smooth muscle cells display great plasticity by transdifferentiating into other cell phenotypes. This review aims to update the progress in endothelial and smooth muscle cell transformation and to discuss their underlying mechanisms.

RECENT FINDINGS

At the early stage of atherosclerosis, it was traditionally believed that smooth muscle cells from the media migrate into the intima in which they proliferate to form neointimal lesions. Recently, endothelial cells were shown to undergo transformation to form smooth muscle-like cells that contribute to neointimal formation. Furthermore, not only can medial smooth muscle cells migrate and proliferate, they also have the ability to differentiate into macrophages in the intima in which they form foam cells by uptaking lipids. Finally, the discovery of stem/progenitor cells in the vessel wall that can differentiate into all types of vascular cells has complicated the research field even further.

SUMMARY

Based on the current progress in the research field, it is worthy to explore the contributions of cell transformation to the pathogenesis of atherosclerosis to understand the mechanisms on how they are regulated in order to develop novel therapeutic application targeting these processes to reverse the disease progression.

Recent insights into the cellular biology of atherosclerosis

Atherosclerosis occurs in the subendothelial space (intima) of medium-sized arteries at regions of disturbed blood flow and is triggered by an interplay between endothelial dysfunction and subendothelial lipoprotein retention. Over time, this process stimulates a nonresolving inflammatory response that can cause intimal destruction, arterial thrombosis, and end-organ ischemia. Recent advances highlight important cell biological atherogenic processes, including mechanotransduction and inflammatory processes in endothelial cells, origins and contributions of lesional macrophages, and origins and phenotypic switching of lesional smooth muscle cells. These advances illustrate how in-depth mechanistic knowledge of the cellular pathobiology of atherosclerosis can lead to new ideas for therapy.

KLF4-dependent phenotypic modulation of smooth muscle cells has a key role in atherosclerotic plaque pathogenesis

Previous studies investigating the role of smooth muscle cells (SMCs) and macrophages in the pathogenesis of atherosclerosis have provided controversial results owing to the use of unreliable methods for clearly identifying each of these cell types. Here, using Myh11-CreER(T2) ROSA floxed STOP eYFP Apoe(-/-) mice to perform SMC lineage tracing, we find that traditional methods for detecting SMCs based on immunostaining for SMC markers fail to detect >80% of SMC-derived cells within advanced atherosclerotic lesions. These unidentified SMC-derived cells exhibit phenotypes of other cell lineages, including macrophages and mesenchymal stem cells (MSCs). SMC-specific conditional knockout of Krüppel-like factor 4 (Klf4) resulted in reduced numbers of SMC-derived MSC- and macrophage-like cells, a marked reduction in lesion size, and increases in multiple indices of plaque stability, including an increase in fibrous cap thickness as compared to wild-type controls. On the basis of in vivo KLF4 chromatin immunoprecipitation-sequencing (ChIP-seq) analyses and studies of cholesterol-treated cultured SMCs, we identified >800 KLF4 target genes, including many that regulate pro-inflammatory responses of SMCs. Our findings indicate that the contribution of SMCs to atherosclerotic plaques has been greatly underestimated, and that KLF4-dependent transitions in SMC phenotype are critical in lesion pathogenesis.

MicroRNA-mediated mechanisms of the cellular stress response in atherosclerosis

Atherosclerosis is characterised by the accumulation of lipid-laden macrophages in atherosclerotic lesions and occurs preferentially at arterial branching points, which are prone to inflammation during hyperlipidaemic stress. The increased susceptibility at branching sites of arteries is attributable to poor adaptation of arterial endothelial cells to disturbed blood flow. In the past 5 years, several studies have provided mechanistic insights into the regulatory roles of microRNAs (miRNAs) in inflammatory activation, proliferation, and regeneration of endothelial cells during this maladaptive process. The intercellular transfer of vesicle-bound miRNAs contributes to arterial homeostasis, and the combinatorial effect of multiple miRNAs controls the unresolved inflammation orchestrated by macrophages in atherosclerotic lesions. In this Review, we highlight the miRNA-dependent regulation of the endothelial phenotype and the proliferative reserve that occurs in response to altered haemodynamic conditions as a prerequisite for atherogenic inflammation. In particular, we discuss the regulation of transcriptional modules by miRNAs and the protective role of complementary strand pairs, which encompasses remote miRNA signalling. In addition, we review the roles of miRNA tandems and describe the relevance of RNA target selection and competition to the behaviour of lesional macrophages. Elucidating miRNA-mediated regulatory mechanisms can aid the development of novel diagnostic and therapeutic strategies for atherosclerosis.

microRNAs Distinctively Regulate Vascular Smooth Muscle and Endothelial Cells: Functional Implications in Angiogenesis, Atherosclerosis, and In-Stent Restenosis

Endothelial cells (EC) and vascular smooth muscle cells (VSMC) are the main cell types within the vasculature. We describe here how microRNAs (miRs)--noncoding RNAs that can regulate gene expression via translational repression and/or post-transcriptional degradation--distinctively modulate EC and VSMC function in physiology and disease. In particular, the specific roles of miR-126 and miR-143/145, master regulators of EC and VSMC function, respectively, are deeply explored. We also describe the mechanistic role of miRs in the regulation of the pathophysiology of key cardiovascular processes including angiogenesis, atherosclerosis, and in-stent restenosis post-angioplasty. Drawbacks of currently available therapeutic options are discussed, pointing at the challenges and potential clinical opportunities provided by miR-based treatments.