1. Introduction—Osteocalcin and Undercarboxylated Osteocalcin
The objectives of the present review are to highlight the role of osteocalcin, in particular its undercarboxylated form, as a hormone that regulates glucose homeostasis and cardiometabolic risk, by summarizing the available experimental and human evidence. Additionally, the emerging evidence implicating undercarboxylated osteocalcin in muscle function and glucose uptake, and its therapeutic potential for addressing cardiometabolic diseases are discussed.
Osteocalcin is a 49 amino acid polypeptide protein with a 5.7 KDa molecular weight. It is also known as gamma-carboxyglutamic acid-containing protein or bone gla protein [1
]. Osteocalcin is the most abundant non-collagenous protein within the bone matrix, primarily produced by osteoblasts during the late stage of their differentiation [2
]. However, the exact role of osteocalcin in the control of bone matrix formation, mineralization or maintenance is not fully understood [3
]. In osteoblasts, after protein translation at the endoplasmic reticulum, osteocalcin undergoes carboxylation at the 17, 21 and 24 glutamic acid (Glu) residues by γ-glutamyl carboxylase. This process is facilitated by vitamin K. This post-translational modification changes the conformation of the osteocalcin protein, resulting in an increased affinity for the calcium ions exposed at the surface of the hydroxyapatite crystal in the bone matrix [2
]. However, not all osteocalcin in bone is fully carboxylated, with a small amount of osteocalcin still containing one (mostly Glu17) or more empty Glu residue(s), collectively denoted as undercarboxylated osteocalcin (ucOC) [5
Osteocalcin in the circulation is a marker of bone turnover [4
]. Of the total amount of osteocalcin that is released into the circulation, a substantial proportion (40–60%) is ucOC, the amount of which is sensitive to vitamin K intake [3
]. As such, a higher percentage of ucOC is a marker for vitamin K status, generally indicative of lower vitamin K availability [8
]. Furthermore, a higher percentage of ucOC has been associated with increased risk of bone fracture in older adults, particularly women [11
]. Therefore, the detection of circulating ucOC has long been recognized as having clinical predictive value as a nutritional biomarker and indicator of fracture risk. In addition, recent studies have illuminated a role for ucOC to regulate insulin secretion by beta cells and adiponectin by fat cells [14
5. Discussion and Conclusions
Our understanding of the interaction between bone and energy metabolism has been improved by the identification of an endocrine role for ucOC in the modulation of insulin secretion and sensitivity, and therefore risk of Type 2 diabetes. While the initial discoveries were made in experimental mouse and cellular (in vitro) models, these were rapidly translated via observational clinical and epidemiological studies that supported the relevance of osteocalcin, and particularly ucOC, to glucose metabolism and diabetes risk in vivo. These studies have extended our understanding of ucOC, or the ratio of ucOC to total osteocalcin, as predictors not only of indices of insulin resistance and risk of Type 2 diabetes, but also of cardiovascular risk and incidence of myocardial infarction. Furthermore, the role of ucOC in muscle function and the effects of exercise to increase circulating ucOC in vivo illuminate new mechanisms by which exercise, bone and muscle are able to interact to regulate energy metabolism. While GPRC6A has been postulated as the putative receptor for ucOC, including in muscle, much more work is needed to clarify whether this is the primary route by which ucOC exerts its actions in different tissues.
At this stage, interventional studies that prove a causal role for ucOC in the reduction of diabetes and cardiovascular risk in humans are lacking. Anti-resorptive therapies for osteoporosis generally reduce bone turnover markers and randomised controlled trials of these agents have not reported differences in the incidence of diabetes in treatment vs control arms [128
]. However, those trials were never designed with incidence of diabetes as an outcome. By contrast, a retrospective analysis of a large cohort of primary care patients prescribed anti-resorptive therapy with bisphosphonates and practice-matched unexposed persons associated exposure to bisphosphonates with reduced risk of Type 2 diabetes [129
]. Neither of these studies reported ucOC results [128
In studies involving men and women, dietary intake of vitamin K1 (phylloquinone) was associated with insulin sensitivity [130
], while dietary intakes of either vitamin K1 or K2 (menaqionones) were associated with lower risk of Type 2 diabetes [131
]. In another study, vitamin K2 intake was associated with lower occurrence of metabolic syndrome [133
]. However, a higher intake of vitamin K might reflect better nutritional quality of the diet and greater engagement in healthy lifestyle behaviors overall, for example, consumption of leafy green vegetables which is the main source of vitamin K1 [134
]. Clinical trials of vitamin K supplementation have yielded mixed results. In a four-week trial in premenopausal women at risk of diabetes, vitamin K1 supplementation resulted in lower 2-h glucose concentrations and improved insulin sensitivity [135
]. By contrast, a 12-month study of vitamin K1 supplementation in postmenopausal women showed no change in indices of glucose homeostasis [136
]. A large randomized placebo-controlled trial in 355 older men and women of 36 months supplementation with vitamin K1 found less progression of insulin resistance in men, but not in women [137
]. A four-week study of vitamin K2 supplementation in healthy young men reported increased insulin sensitivity [138
]. However, studies of vitamin K2 supplementation in young men and women, and in postmenopausal women, have found no effect on adiponectin [139
]. A systematic review of eight trials involving 1077 participants reported no effect of vitamin K supplementation on insulin sensitivity, concluding that further well-designed randomized controlled trials with large sample sizes are needed [141
]. Vitamin K supplementation consistently reduced ucOC, increasing the proportion of carboxylated osteocalcin [135
]. Thus, the overall effect of vitamin K supplementation on indices of glucose metabolism remains to be fully clarified, reflecting the broader actions of vitamin K1 and K2 on pathways distinct from osteocalcin [141
Dedicated interventional studies are essential to determine whether increasing circulating ucOC or the ratio of ucOC relative to total osteocalcin would improve insulin sensitivity and thus reduce the risk of Type 2 diabetes in humans. Administering recombinant ucOC has been informative in mice [16
], but this approach is logistically more challenging in humans and as yet there have been no clinical trials of ucOC to determine its effects on diabetes and cardiovascular risk. Increasing circulating ucOC by exercise offers an alternative interventional approach, although care would be required in the interpretation of such studies when distinguishing the effects of exercise versus the effects of ucOC in the pancreas, fat and muscle tissues. Thus, the endocrine role of ucOC, linking as it may muscle, bone and metabolism to influence diabetes and cardiovascular risk, remains a key arena for future research to improve human health.
In terms of future perspectives and research directions, additional mechanistic studies to clarify the mechanisms by which ucOC exerts its actions in various tissues, including β-cells and muscle would be highly valuable. Further work is needed to ascertain whether GPRC6A is a major receptor for ucOC in these and other tissues, bearing in mind that another G-protein coupled receptor Gpr158 has been found to mediate the effect of osteocalcin on cognition in mice [145
]. Epidemiological studies utilizing robust assays for circulating ucOC will continue to provide important information regarding the associations of ucOC in humans. The interaction of ucOC, muscle function and glucose homeostasis is a key area further research, with exercise interventions representing a possible pathway to manipulate ucOC with the ultimate goal of improving cardiometabolic health.