Lead induces chondrogenesis and alters transforming growth factor-[beta] and bone morphogenetic protein signaling in mesenchymal cell populations

BACKGROUND: It has been established that skeletal growth is stunted in lead-exposed children. Because chondrogenesis is a seminal step during skeletal development, elucidating the impact of Pb on this process is the first step toward understanding the mechanism of Pb toxicity in the skeleton. OBJECTIVES: The aim of this study was to test the hypothesis that Pb alters chondrogenic commitment of mesenchymal cells and to assess the effects of Pb on various signaling pathways. METHODS: We assessed the influence of Pb on chondrogenesis in murine limb bud mesenchymal cells (MSCs) using nodule formation assays and gene analyses. The effects of Pb on transforming growth factor-[beta] (TGF-[beta]) and bone morphogenetic protein (BMP) signaling was studied using luciferase-based reporters and Western analyses, and luciferase-based assays were used to study cyclic adenosine monophosphate response element binding protein (CREB), [beta]-catenin, AP-1, and nuclear factor-kappa B (NF-[kappa]B) signaling. We also used an ectopic bone formation assay to determine how Pb affects chondrogenesis in vivo. RESULTS: Pb-exposed MSCs showed enhanced basal and TGF-[beta]/BMP induction of chondrogenesis, evidenced by enhanced nodule formation and up-regulation of Sox-9, type 2 collagen, and aggrecan, all key markers of chondrogenesis. We observed enhanced chondrogenesis during ectopic bone formation in mice preexposed to Pb via drinking water. In MSCs, Pb enhanced TGF-[beta] but inhibited BMP-2 signaling, as measured by luciferase reporter assays and Western analyses of Smad phosphorylation. Although Pb had no effect on basal CREB or Wnt/[beta]-catenin pathway activity, it induced NF[kappa]B signaling and inhibited AP-1 signaling. CONCLUSIONS: The in vitro and in vivo induction of chondrogenesis by Pb likely involves modulation and integration of multiple signaling pathways including TGF-[beta], BMP, AP-1, and NF[kappa]B. KEY WORDS: BMP signaling, chondrogenesis, lead, mesenchymal stem cells, TGF-[beta] signaling. Environ Health Perspect 115:1276-1282 (2007). doi:10.1289/ehp.10028 available via http://dx.doi.org/ [Online 3 July 2007]
In spite of effort to reduce human exposure, lead toxicity continues to be a major environmental health concern in the United States and in other industrial countries. Although recent research [...]