Osteocalcin

Abstract

O steocalcin, or bone Gla protein, is a small protein secreted by osteoblasts that can undergo ␥-carboxylation. The ␥-carboxylated form binds hydroxyapatite and is abundant in bone extracellular matrix. In contrast, the undercarboxylated circulating form has been implicated as a novel hormone and positive regulator of glucose homeostasis. Importantly, osteocalcin expression has been described in calcifying vascular smooth muscle cells (VSMCs), although the physiological significance of this observation has remained unclear. In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Idelevich et al 1 show that osteocalcin is potentially a novel regulator of osteochondrogenic differentiation of pathologically mineralizing VSMCs. They reveal that osteocalcin, via hypoxia-inducible factor 1␣ signaling, stimulates expression of osteochondrogenic transcription factors in VSMCs, as well as a shift in cellular metabolism toward glycolysis. This study provides the first evidence that osteocalcin may be an active player in vascular calcification, with its presence in the calcified vasculature, and potentially the circulation, activating novel signaling pathways that promote mineralization. Pathological mineralization of the vasculature has a detrimental effect on cardiovascular function and is associated with increased mortality in patients with aging, atherosclerosis, type 2 diabetes, and chronic kidney disease. 2 Vascular smooth muscle cells (VSMCs) orchestrate the mineralization process, which is mediated in part by their osteochondrocytic differentiation in the vessel wall. This phenotypic transition is characterized by expression of Runx2 and Sox9, master transcription factors that regulate bone and cartilage differentiation during developmental osteochondrogenesis, as well as other bone-and cartilage-specific proteins, many with undefined functions. 3 One of these proteins, osteocalcin, is a small ␥-carboxylated protein that is expressed by both osteoblasts and VSMCs and abundantly deposited in the extracellular matrix of bone and in the calcified vasculature. 4 -6 Osteocalcin avidly binds hydroxyapatite via 3 ␥-carboxylated glutamic acid residues and was originally thought to play a role as a regulator of mineral nucleation. 3,7,8 However, osteocalcin is also found in the circulation, and a breakthrough in our understanding of the function of its circulating form came in 2007 when Gerard Karsenty's laboratory established that the undercarboxylated form of osteocalcin has a positive effect on glucose metabolism. 9 This effect on energy metabolism is mediated by upregulation of insulin secretion by pancreatic ␤-cells and adiponectin production by adipose tissues, which stimulates whole-body glucose uptake and utilization. Activation of insulin signaling pathways induces cross-talk between osteoblasts, osteoclasts, and nonskeletal tissues, leading to activation of bone resorption further stimulating the release of undercarboxylated osteocalcin. 9,10 Importantly, in the present issue of Arteriosclerosis, Th...

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“…In addition, OCN has been shown to regulate the mineralization process by inhibiting spontaneous mineral precipitation thereby limiting the creation of pseudomineral deposits . The carboxylated glutamic acid residues in osteocalcin have a high affinity for calcium‐containing hydroxyapatite; interestingly, the enzyme‐linked immunosorbent assay (ELISA) kit utilized in this study contained monoclonal antibodies specific for the carboxylated form of osteocalcin. Lastly, the enzymatic action of ALP serves to generate free inorganic phosphate and is well correlated with mineralization in osteoid tissues .…”

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“…However, recent data show that VC is a regulated process similar to bone formation in which calcification inducers and inhibitors have an active role. (4) On the one hand, an increase of inducers of VC such as RANKL, (5) RUNX2, (6) osteonectin, (7) osteocalcin, (8) or BMP2 promote the osteoblastic phenotype of vascular smooth muscle cells (VSMCs). (9) These osteoblastic markers are found to be elevated in the vasculature of both uremic rats and CKD patients.…”