Low bone mineral density (BMD) is a risk factor of
osteoporosis and has strong genetic determination. Genes influencing BMD and fundamental mechanisms leading to
osteoporosis have yet to be fully determined. Peripheral blood monocytes (
PBM) are potential osteoclast precursors, which could access to
bone resorption surfaces and differentiate into osteoclasts to resorb bone. Herein, we attempted to identify
osteoporosis susceptibility gene(s) and characterize their function(s), through an initial proteomics discovery study on
PBM in vivo, and multiscale validation studies in vivo and in vitro. Utilizing the quantitative proteomics methodology LC-nano-ESI-MS(E), we discovered that a novel
protein, i.e. ANXA2, was up-regulated twofold in
PBM in vivo in Caucasians with extremely low BMD (cases) versus those with extremely high BMD (controls) (n = 28, p < 0.05). ANXA2 gene up-regulation in low BMD subjects was replicated at the
mRNA level in
PBM in vivo in a second and independent case-control sample (n = 80, p < 0.05). At the
DNA level, we found that SNPs in the ANXA2 gene were associated with BMD variation in a 3(rd) and independent case-control sample (n = 44, p < 0.05), as well as in a random population sample (n = 997, p < 0.05). The above integrative evidence strongly supports the concept that ANXA2 is involved in the pathogenesis of
osteoporosis in humans. Through a follow-up cellular functional study, we found that ANXA2
protein significantly promoted monocyte migration across an endothelial barrier in vitro (p < 0.001). Thus, elevated ANXA2
protein expression level, as detected in low BMD subjects, probably stimulates more
PBM migration through the blood vessel walls to
bone resorption surfaces in vivo, where they differentiate into higher number of osteoclasts and resorb bone at higher rates, thereby decreasing BMD. In conclusion, this study identified a novel
osteoporosis susceptibility gene ANXA2, and suggested a novel pathophysiological mechanism, mediated by ANXA2, for
osteoporosis in humans.