Lead alters parathyroid hormone-related peptide and transforming growth factor-β1 effects and AP-1 and NF-κB signaling in chondrocytes

The skeletal system is an important target for lead toxicity. One of the impacts of lead in the skeleton, the inhibition of axial bone development, is likely due to its effect on the normal progression of chondrocyte maturation that is central to the process of endochondral ossification. Since little is known about the effect of lead on chondrocyte function/maturation, its impact on (1) growth factor-induced proliferation, (2) expression of maturation-specific markers type X collagen and BMP-6, and (3) the activity of AP-1 and NF-κB was examined in chick growth plate and sternal chondrocyte models. Exposure to lead alone (1-30 μM) resulted in a dose-dependent inhibition of thymidine incorporation in growth plate chondrocytes. Lead also blunted the stimulation of thymidine incorporation by parathyroid hormone-related peptide (PTHrP) and transforming growth factor-β1 (TGF-β1), two critical regulators of chondrocyte maturation. Lead (1 and 10 μM), TGF-β1 (3 ng/ml) and PTHrP (10^-7 M) all significantly inhibited the expression of type X collagen, a marker of chondrocyte terminal differentiation. However, when in combination, lead completely reversed the inhibition of type X collagen by PTHrP and TGF-β1. The effect of lead on BMP-6, an inducer of terminal differentiation, was also examined. Independently, lead and TGF-β1 were without effect on BMP-6 expression, but PTHrP significantly suppressed it. Comparatively, lead did not alter PTHrP-mediated suppression of BMP-6, but in combination with TGF-β1, BMP-6 expression was increased 3-fold. To determine if lead effects on signaling might play a role in facilitating these events, the impact of lead on NF-κB and AP-1 signaling was assessed using luciferase reporter constructs in sternal chondrocytes. Lead had no effect on the AP-1 reporter, but it dose-dependently inhibited the NF-κB reporter. PTHrP, which signals through AP-1, did not activate the NF-κB reporter and did not affect inhibition of this reporter by lead. In contrast, PTHrP activation of the AP-1 reporter was dose-dependently enhanced by lead. These findings, which establish that chondrocytes are important targets for lead toxicity, suggest that the effects of lead on bone growth are derived from its impact on the modulation of chondrocyte maturation by growth factors and second messenger signaling responses.