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Corresponding author: Dr Rhian M. Touyz, Institute of Cardiovascular and Medical Sciences, BHF Glasgow Cardiovascular Research Centre, University of Glasgow, 126 University Place, Glasgow, United Kingdom G12 8TA. Tel.: 44(0)141 330 7775; fax: 44(0)141 330 3360.
Pathologically, arterial calcification contributes to advanced atherosclerosis (calcification of plaques) and increased vascular stiffness, leading to target organ injury in the heart, kidney, and brain through secondary microvascular damage.
Vascular calcification is associated with accumulation of calcium deposits in the vessel wall and mineralization of the internal elastic lamina and elastic fibres in the vascular media. This is a highly controlled process similar to events that regulate osteogenesis in bone cells. Factors that promote calcification include abnormalities in mineral metabolism, particularly hyperphosphatemia and hypercalcemia, which stimulate vascular smooth muscle cell (VSMC) differentiation to an osteoblastic phenotype.
Vascular calcification is an active cell-mediated response involving VSMC apoptosis and vesicle release, a shift in the balance of inhibitors and promoters of vascular calcification, and VSMC differentiation from a contractile to osteochondrogenic phenotype. This phenotypic shift requires phosphate, and the uptake of phosphate by the sodium-dependent phosphate cotransporters PiT-1 and PiT-2, which are upregulated by proinflammatory cytokines.
Molecular mechanisms regulating these events involve upregulation of transcription factors such as core-binding factor 1α (cbfa1)/runt-related transcription factor 2 (Runx2), msh homeobox 2 (MSX-2), and bone morphogenetic protein 2 (BMP-2), which are critically important in normal bone development and control the expression of osteogenic proteins, including osteocalcin, osteonectin, alkaline phosphatase, collagen-1, and bone sialoprotein.
Factors that trigger these events remain elusive, although multiple kinases including Src, phospholipase C (PLC), ERK1/2, p38 mitogen-activated protein kinase (MAPK), and protein kinase C (PKC) may be important.
mTOR is an intriguing candidate as a regulator of osteogenesis, because it is characteristically associated with cellular metabolism and longevity through its effects on cell growth, proliferation, and survival rather than on bone metabolism.
The finding that it is also instrumental in molecular processes associated with calcification of vessels is novel and may provide an interesting link between cardiovascular disease and metabolic diseases, because vascular calcification occurs in both pathologic processes.
Signalling through mTOR is complex. It is a serine/threonine kinase that belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase family and functions as an intracellular sensor for energy metabolism, nutrient availability, and cellular stresses and regulates cell growth and metabolism to adapt to environmental changes.
These adaptor proteins act as scaffold proteins to recruit mTOR substrates and regulators. mTOR is sensitive to the antifungal macrolide rapamycin, which forms a complex with FK506-binding protein that specifically inhibits mTORC1. Multiple upstream factors regulate mTOR, including growth factors, DNA damage, nutrients, and hypoxia. mTOR is a major activator of cell growth and proliferation. Once activated, mTORC1 phosphorylates p70 S6 kinase 1 (S6K1) and inhibits eIF-4E-binding protein 1 (4E-BP1), leading to activation of ribosomal protein S6 and eIF2E, protein synthesis, and cell proliferation. mTORC1 also activates transcription factors SREP1, peroxisome proliferator-activated receptor (PPAR) gamma, and hypoxia-inducible factor 1 (HIF-1), thereby regulating lipid biogenesis and glycolysis. Because of the essential role of mTOR in anabolic metabolism, energy storage, and cell growth/survival it is not surprising that this pathway is important in adipogenesis and glucose and lipid metabolism.
In obesity and metabolic disorders, mTORC1 is hyperactivated and may be associated with insulin resistance. In contrast, immunosuppressive drugs that inhibit mTOR, including cyclosporine A, tacrolimus, and rapamycin, enhance lipolysis and inhibit lipid storage and expression of lipogenic genes in adipose tissue, which may contribute to the development of dyslipidemia and insulin resistance associated with immunosuppressive therapy.
Although mTOR is now considered a major player in lipid and glucose metabolism, its role in the vasculature is still largely unknown. There is some evidence that it regulates vascular Ca2+ channels and intracellular Ca2+ mobilization, thereby influencing vasoconstriction.
Exact mechanisms whereby mTOR induces mineralization are unclear, but increased synthesis of osteogenic factors, such as bone morphogenetic proteins, may be important. Studies investigating the PI3K/AKT-mTOR axis in bone biological processes and mineralization are rare, although there is some evidence that this system influences chondrocyte maturation, endochondral ossification, and osteoblast differentiation through effects on activation of p70S6K and BMP expression.
Adiponectin is the most abundant adipokine produced by adipocytes and has been implicated as a key player in adiposity, insulin resistance, and inflammation through its effects on lipid and glucose metabolism.
First, studies were performed in isolated VSMCs in which only a few markers of osteoblastic differentiation were identified. This may not necessarily translate into vascular calcification in vivo. Second, although the study identifies a novel putative signalling pathway through mTOR, the exact molecular processes whereby mTOR-S6K1 influences activation of ion channels important in cellular Ca2+, Na+, and phosphate regulation, such as PiT-1 and PiT-2, are not addressed. However, despite these limitations, the study presents an interesting paradigm in which mTOR may be a link between metabolic disorders and vascular calcification. Zhan et al. provide new insights into the molecular biology of vascular calcification, and more research is certainly warranted.
It will be particularly interesting to know in whole animals whether inhibition of mTOR with adiponectin or rapamycin, which have been shown to influence adiposity and insulin resistance, would also ameliorate vascular calcification.
Work from the authors' laboratory was supported by grants 44018 and 57886, from the Canadian Institutes of Health Research, and grants from the British Heart Foundation. R.M.T. is supported through a British Heart Foundation Chair, and A.C.M. is supported through a Leadership Fellowship from the University of Glasgow.
The authors have no conflicts of interest to disclose.
Role of coronary artery calcium in cardiovascular risk assessment.