Abstract: BACKGROUND: Fluorosis is a common endemic disease, and its severity depends on the dose and duration of fluoride exposure. Bone morphogenetic protein 2 plays an important role in the pathogenesis of skeletal fluorosis.
OBJECTIVE: To explore the relationship between the expression of bone morphogenetic protein 2 and bone fluoride content in rabbit femur and the changes of bone formation markers, osteocalcin and procollagen l N-terminal propeptide, under different fluoride concentrations.  
METHODS: Sixteen New Zealand white rabbits were randomly divided into control group, low dose group, middle dose group, and high dose group, with four rabbits in each group. Animals drank tap water containing 0, 100, 200, and 400 mg/L sodium fluoride, respectively. The experimental period was 6 months. During the feeding period, all animals had free access to water and food. Occurrence and development of dental fluorosis was observed regularly. The fluorine content in rabbit femur was determined by ashing distillation-fluorine reagent colorimetry. The expression of bone morphogenetic protein 2 in femur was detected by western blot. The levels of osteocalcin and procollagen l N-terminal propeptide in serum were detected by enzyme-linked immunosorbent assay.  
RESULTS AND CONCLUSION: The rabbits in fluoride exposure groups showed different degrees of fluorosis. In the low dose group, mild fluorosis was found and mainly manifested with tarnished teeth and some fine lines. The middle dose group was dominated by moderate dental fluorosis, manifested as uneven tooth surface and color. Some models with dental defects presented with severe dental fluorosis. The high dose group was dominated by severe dental fluorosis, which was characterized by dull tooth surface and deformed tooth development. The bone fluoride content in the femur of rabbits increased with the increase of fluoride intake, which was higher in the high dose group than the other three groups (P < 0.01, P < 0.001). The serum osteocalcin level in the high dose was higher than that in the control group (P < 0.05) and the serum level of procollagen l N-terminal propeptide was lower than that in the control group and the low dose group (P < 0.05). The expression of bone morphogenetic protein 2 in the femur was higher in the low dose group than the control and high dose groups (P < 0.05). Pearson correlation analysis showed that the expression of bone morphogenetic protein 2 in the low dose group was positively correlated with the bone fluoride content (r=0.951, P < 0.05), while there was no significant correlation between the expression of bone morphogenetic protein 2 and the content of bone fluoride in the middle and high dose groups (r=0.889, r=-0.116, P > 0.05). To conclude, the expression of bone morphogenetic protein 2 in rabbit femur is bidirectionally related to bone fluoride concent, which is related to fluoride exposure concentration, that is, too high or too low fluoride content will alter the expression of bone morphogenetic protein 2.

Key words: fluorosis, bone fluoride, bone morphogenetic protein 2, osteocalcin, procollagen l N-terminal propeptide, rabbit