Bonneau J, Ferland G, Karelis AD et al.
Unité de recherche en physiologie intégrative et moléculaire, Institut de Recherches Cliniques de Montréal, Montréal, Québec H2W 1R7, Canada; Programme de biologie moléculaire, Université de Montréal, Québec H3T 3J7, Canada.
Journal of diabetes and its complications. Feb 2017.
In mice, osteocalcin (OCN) acts as a bone-derived hormone promoting insulin sensitivity and glucose tolerance. In that species, OCN endocrine action is inhibited when its first glutamic acid residue (Glu13) is γ-carboxylated (Gla). The importance of this posttranslational modification for OCN function in human is still unclear. Our objectives were to identify an assay to assess γ-carboxylation of human OCN on its first Glu residue (Glu17) and to test its association with insulin resistance and inflammation profile in overweight women.Several ELISAs were tested to determine their specificity toward various forms of human OCN. Associations between OCN γ-carboxylation and determinants of glucose tolerance, insulin sensitivity, liver function and subclinical inflammation were then investigated in 129 non-diabetic overweight and obese postmenopausal women.We identified assays allowing the measurement of total OCN (tOCN) and the ratio of Gla17/tOCN. Circulating Gla17/tOCN levels correlated negatively with insulin sensitivity assessed by hyperinsulinemic-euglyceamic clamp (P=0.02) or insulin sensitivity index derived from oral glucose tolerance test (P=0.00003), and positively with insulin resistance assessed by HOMA-IR (P=0.0005) and with markers of subclinical inflammation and liver enzymes, including C-reactive protein (CRP; P=0.007) and aspartate aminotransferase (AST; P=0.009).γ-carboxylation of OCN on Glu17 residue correlates with insulin resistance and subclinical inflammation, suggesting that γ-carboxylation of OCN negatively regulates its endocrine action in humans.