Itai-itai disease is thought to be the result of chronic
cadmium (Cd) intoxication. Renal proximal tubules are a major target of Cd toxicity. The whole mechanism of the adverse effects of Cd remains unresolved, especially how renal damage is related to the development of bone lesions.
Fibroblast growth factor 23 (FGF23) is a bone-derived phosphaturic factor that regulates
vitamin D and
inorganic phosphate metabolism in the kidney. To clarify the role of FGF23 on Cd toxicity, we investigated the mechanisms of Cd-induced FGF23 production in the bone. Cd injection into mice significantly increased plasma FGF23 concentrations, but did not change FGF23
mRNA expression in bone.
GalNAc-T3 is involved in secreting intact FGF23. To determine potential roles of
GalNAc-T3 in Cd-induced FGF23 production, we examined the effect of Cd on
GalNAc-T3 mRNA expression in vivo and in vitro.
GalNAc-T3 gene expression was significantly increased in the bones of Cd-injected mice. Cd also enhanced the expression of
GalNAc-T3 in cultured
osteosarcoma UMR106 cells and primary osteocytes. Cd activated
aryl hydrocarbon receptors (AhR) and AhR were required for
GalNAc-T3 gene expression induced by Cd. In addition, Cd-dependent FGF23 production was completely inhibited by an AhR antagonist. AhR
siRNA markedly suppressed the stimulation of transcriptional activity by Cd. Furthermore, Cd induced AhR activation via phosphorylation of Ser-68 by p38
kinase in the
nuclear export signal of AhR. Thus, Cd stimulated
GalNAc-T3 gene transcription via enhanced AhR binding to the
GalNAc-T3 promoter. These findings suggest that the Cd-induced increase in
GalNAc-T3 suppresses proteolytic processing of FGF23 and increases serum FGF23 concentrations.