Elevated Lifetime Lead Exposure Impedes Osteoclast Activity and Produces an Increase in Bone Mass in Adolescent Mice

The heavy metal lead (Pb) has a deleterious effect on skeletal health. Because bone mass is maintained through a balance of bone formation and resorption, it is important to understand the effect of Pb levels on osteoblastic and osteoclastic activity. Pb exposure is associated with low bone mass in animal models and human populations; however, the correlation between Pb dosing and corresponding bone mass has been poorly explored. Thus, mice were exposed to increasing Pb and at higher levels (500 ppm), there was unexpectedly an increase in femur-tibial bone mass by 3 months of age. This is contrary to several studies alluded to earlier. Increased bone volume (BV) was accompanied by a significant increase in cortical thickness of the femur and trabecular bone that extended beyond the epiphyseal area into the marrow cavity. Subsequent evaluations revealed an increase in osteoclast numbers with high Pb exposure, but a deficiency in osteoclastic activity. These findings were substantiated by observed increases in levels of the resorption-altering hormones calcitonin and estrogen. In addition we found that pro-osteoclastic nuclear factor-kappa beta (NF-kappa B) pathway activity was dose dependently elevated with Pb, both in vivo and in vitro. However, the ability of osteoclasts to resorb bone was depressed in the presence of Pb in media and within test bone wafers. These findings indicate that exposure to high Pb levels disrupts early life bone accrual that may involve a disruption of osteoclast activity. This study accentuates the dose dependent variation in Pb exposure and consequent effects on skeletal health.