Circulating microparticles and microRNAs as players in atherosclerosis

Abstract

Microparticles (MPs) are extracellular membrane vesicles released from normal, apoptotic and pathological cells following a process of detachment from cells of origin. MPs are typically defined by their size, exposure of phosphatidylserine, the expression of surface antigens, proteins and genetic material, originating from their donor cells, and as important vehicles of intercellular communication across numerous biological processes. MPs contain the major source of systemic RNA including microRNA (miRNA) of which aberrant expression appears to be associated with stage and progression of atherosclerosis. The involvement and influence of miRNA during the onset and progression of atherosclerotic disease have generated a lot of interest in assessing the feasibility of therapeutic regulation of miRNAs to manipulate them with a special focus on cardiovascular disease. We speculate on the future developments of MPs which contain miRNA as new therapeutic targets for proliferative vascular diseases such as atherosclerosis.

Key Words: Microparticles; MicroRNA; Atherosclerosis

Core tip: Circulating microparticles (MPs) and microRNAs (miRNA) play an important role in atherosclerotic disease. MPs contain the major source of systemic RNA including microRNA of which aberrant expression appears to be associated with stage and progression of atherosclerosis. We speculate on the future developments of MPs which contain miRNA as new therapeutic targets for proliferative vascular diseases such as atherosclerosis.

ATHEROSCLEROSIS

Atherosclerosis is one of the most important and common cause of death and disability in the world and remains an occult but important precursor of significant cardiovascular events. Yet atherosclerosis is a systemic disease with important consequences in many other regional circulations, including those supplying the brain, kidneys, mesentery, and limbs. Atherosclerosis as a chronic inflammatory disease of the arterial wall is closely related to subendothelial lipoprotein retention, endothelial activation and migration of the immune cells to the inflamed intima which result in formation of fatty streaks and subsequent atheromas[1,2]. Monocytes are prominently involved in initiation, progression and complication of the atherosclerotic lesions. They enter atherosclerotic plaque and transform to macrophage-like, lipid-loaded foam cells[3]. Also, it is now increasingly admitted that cell-derived microparticles (MPs) may contribute to the initiation, progression and clinical complications of cardiovascular diseases and they appear interesting biomarkers to predict this pathology[4]. In addition, the ratio of circulating MPs to endothelial progenitor cells may be a new and valuable cellular marker of vascular dysfunction in atherosclerosis[5].

MP

MPs were described as small (0.1-1 μm), pro-inflammatory vesicles released by various cell types (e.g., leucocytes, endothelial cells, platelets, monocytes) in a tightly regulated process. MP vesiculation occurs as a cellular response to various physiological conditions including: apoptosis, senescence, cellular activation, shearing stress and biochemical triggers (such as cytokines and chemotherapeutics)[6]. They contain cytoplasm and surface markers of their cells of origin[7,8]. Once released into the circulation, MPs bind and fuse with their target cells through receptor-ligand interactions, thereby acting as biological vectors mediating vascular inflammation and coagulation[9]. Therefore, MPs have been shown to play a fundamental role in several cardiovascular diseases[10,11]. Increasing evidence indicates that inflammatory and pro-coagulatory effects of MPs on their target cells are caused by a specific lipid composition (e.g., byphosphatidylserine) as well as by the transfer of inflammatory cell components from their cells of origin[7]. However, it was also shown that MPs transport messenger RNAs (mRNAs) thereby affecting protein expression of their target cells[12]. Recent progress in the understanding of microRNA (miRNAs) has prompted the questions of whether MPs also affect their target cells via transferring endogenous miRNAs and whether these MPs have different miRNA patterns than their maternal cells.

miRNA

miRNAs are a class of small (about 22 nucleotides long), non-coding RNA that bind to mRNAs thereby acting as endogenous post-translational gene regulators[13]. Since more then 1000 different human miRNAs have already been discovered, the interaction between miRNAs and mRNAs is highly complex and currently not completely understood. miRNAs are potent and crucial regulators of important cellular processes such as differentiation, growth, and survival. miRNAs regulate gene expression through binding to 3' UTRs of target mRNAs whereby inducing either messenger RNA degradation or inhibition of protein translation. However, approximately one-third of human protein-encoding genes are miRNAs regulated, underlining the extraordinary impact of miRNAs on protein expression[14,15].

Recent data indicate that miRNAs play a vital role in many cardiovascular diseases and can be found in cardiac tissue as well as in circulating blood, opening the possibility to use them as diagnostic surrogate markers[16-19]. They were reported alterations in the expression of specific miRNAs in human atherosclerotic plaques which suggest that miRNAs may have an important role in regulating the evolution of atherosclerotic plaque toward instability and rupture[20,21]. Bidzhekov et al[22] identified miRNAs co-expressed in plaque tissue and classical monocytes (miR-99b, miR-152), or non-classical monocytes (miR-422a) which serve as therapeutic targets for treating inflammatory vascular diseases. Also, miR-126 is highly expressed in the heart and vasculature of zebrafish[23] and miR-221 positively regulates smooth muscle proliferation and neointimal formation[24,25]. The effects of miR-221 are strengthened by the concurrent upregulation of miR-21 in neointimal lesions[26]. miR-21, which is a ubiquitously expressed prosurvival miRNA, was shown to inhibit PTEN expression under these conditions[26]. Other miRNAs that may also contribute to vessel formation include miR-130a, which is upregulated in endothelial cells during tubulogenesis and facilitates the process by targeting and inhibiting the expression of antiangiogenic transcription factors GAX and HOXA5[27]. In contrast, overexpression of miR-92a in endothelial cells blocked tubulogenesis and was associated with reduced cell migration and adhesion, but did not affect viability or proliferation[28]. On the other hand miRNA-145 has been studied for its therapeutic properties in atherosclerotic disease. It was found that miRNA is abundant in arteries[26] and in differentiated vascular smooth muscle cells[29].

CONCLUSION

Furthermore, it was promoted the idea that MPs represent transport vehicles for a large number of specific miRNAs in circulation[30,31]. Moreover, it was showed that embryonic stem cell-derived MPs contain abundant miRNAs and that they can transfer a subset of miRNAs to mouse embryonic fibroblast in vitro[32]. Additionally, MPs released from mesenchymal stem cells protect from acute kidney injury induced by ischaemia reperfusion injury and from subsequent chronic renal damage[33]. Other studies demonstrated that the expression of miRNAs and RNAs in secreted vesicles does not necessarily reflect the intracellular expression of RNAs[34,35]. Hergenreider et al[36] suggest that extracellular vesicles from Krüppel-like factor 2 (KLF2)-transduced cells contain a specific combination of miRNAs, including miR-143/145, which mediate the biological properties. Thus, vesicles from KLF2-transduced endothelial cells, but not from controls, reduce atherosclerotic lesion formation in vivo, in a miR-143/145-dependent manner and they may provide a promising strategy to combat atherosclerosis. Also, stem cell-derived MPs appear to be naturally equipped to mediate tissue regeneration under certain conditions[37]. Therefore, the extracellular vesicles are considered potent sources of genetic information transfer between mammalian cells and tissues resulting in both beneficial (cell communication, stem cell plasticity and repair of injured tissues) and potentially detrimental (spread of disease) outcomes[37].

These data open new research perspectives on the use of MPs to transfer miRNAs-based information from stem cells/precursors/cells to target differentiated cells. Further studies on MPs biology and function may help elucidate the exact role that MPs as gene therapy tools.