Stimulation of osteoblast activity by homocysteine

Homocysteine (HCY) has recently been linked to fragility fractures. Moreover, HCY activates osteoclasts. Little is known about the effect of HCY on activity of human osteoblasts (OBs). We hypothesized that HCY decreases the activity of OBs. Osteoblasts obtained from tra-becular human bone specimens of eight donors were cultured with conditioned medium. Culture medium was adjusted to 0, 100, 500, 1000 and 2000 muM HCY. After 14 days alkaline phosphatase (AP) activity, pro-collagen type I N-terminal peptide (PINP) and osteocalcin (OC) secretion in the supernatant were measured. After 20 days the formation of mineralized matrix was analyzed. HCY-stimulated AP activity gradually (100 muM HCY: 118%, P= 0.006; 500 muM HCY: 125%, P < 0.001). At 1000 and 2000 muM HCY the increase of AP activity was reversible (1000 muM HCY: 106%, P= 0.317; 2000 muM HCY: 102%, P < 0.737). The PINP secretion was also stimulated by HCY reaching a maximum of 260 +/- 154 mug/l at 500 mumol/l versus 205 +/- 94 mu,g/l in controls. After 20 days of culture the formation of bone matrix was increased at 100 and 500 muM HCY. OC secretion was not significantly changed. The results of the present study consistently demonstrate a moderate stimulation of primary human OB activity by increasing concentrations of HCY. However, the magnitude of this effect seems to be less pronounced than recent observations on primary human osteoclasts, suggesting a dysbalance between OBs and osteoclasts in favour of osteoclasts.