Low circulating levels of undercarboxylated osteocalcin (ucOC) is associated with a higher risk of cardiovascular disease, yet whether ucOC has a direct effect on endothelium-dependent vasorelaxation, or in proximity to its postulated receptor, the class CG protein-coupled receptor (GPCR6A), in blood vessels remains unclear. Immunohistochemistry and proximity ligation assays were used to localize the presence of ucOC and GPRC6A and to determine the physical proximity (< 40 nm) in radial artery segments collected from patients undergoing coronary artery bypass surgery (n = 6) which exhibited calcification (determined by Von Kossa) and aorta from New Zealand white rabbits exhibiting atherosclerotic plaques. Endothelium-dependent vasorelaxation was assessed using cumulative doses of acetylcholine in vitro on abdominal aorta of rabbits fed a normal chow diet (n = 10) and a 4-week atherogenic diet (n = 9) pre-incubated with ucOC (10 ng/mL) or vehicle. Both ucOC and GPRC6A were localized in human and rabbit diseased-blood vessels. Proximity ligation assay staining demonstrated physical proximity of ucOC with GPRC6A only within plaques in rabbit arteries and the endothelium layer of rabbit arterioles. Endothelium-dependent vasorelaxation was impaired in atherogenic abdominal aorta compared to healthy aorta and ucOC attenuated this impairment. ucOC attenuated impaired endothelium-dependent vasorelaxation in rabbit abdominal aorta following an atherogenic diet, however, this effect may be independent of GPRC6A. It is important that future studies determine the underlying cellular mechanisms by which ucOC effects blood vessels as well as whether it can be used as a therapeutic agent against the progression of atherosclerosis.
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Associate Professor Itamar Levinger was supported by a Heart Foundation Future Leader Fellow (ID: 100040). This study was funded by The Rebecca L Cooper Medical Research Foundation.
Conflict of interest
Tawar Qaradakhi, Laura K. Gadanec, Alex B. Tacey, David L. Hare, Brian F. Buxton, Vasso Apostoloppoulos, Itamar Levinger and Anthony Zulli declare that they have no conflict of interest. These authors also declare no competing financial interests.
The study was approved by the Austin Hospital Medical Research Ethics Committee and followed institutional guidelines that conform with the Declaration of Helsinki.
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Patients gave informed consent to collect discarded arteries.
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Qaradakhi, T., Gadanec, L.K., Tacey, A.B. et al. The Effect of Recombinant Undercarboxylated Osteocalcin on Endothelial Dysfunction. Calcif Tissue Int 105, 546–556 (2019). https://doi.org/10.1007/s00223-019-00600-6
- Blood vessels
- Radial artery