**3.6 Annexin A2**

Annexin A2 (ANXA2) is a calcium-dependent phospholipid-binding protein expressed on the surface of peripheral blood monocytes, which can stimulate monocytes migration across endothelial cells and osteoclasts formation. However, ANXA2's role in bone remodeling is not limited to osteoclast formation, but can also promote the proliferation and differentiation of bone precursor cells, thereby affecting bone formation [95]. Increased expression of ANXA2 was found in postmenopausal Caucasian women patients with low bone mass and osteoporosis. A recent study found that compared with patients without fractures, the expression of ANXA2 protein in the PBMs of patients with osteoporotic fractures was significantly increased and plasma ANXA2 were inversely related to hip BMD in older population, which are significantly higher in the patients with very low BMD than those in very high BMD [96]. These studies suggest that ANXA2 may be a potential biochemical marker for osteoporosis, but there are few clinical studies on ANXA2.Thus, further longitudinal studies are needed to determine whether plasma ANXA2 levels can predict osteoporosis.
