Upregulation of parathyroid VDR expression by extracellular calcium is mediated by ERK1/2-MAPK signaling pathway

Abstract

We have previously demonstrated that the activation of rat parathyroid calcium-sensing receptor (CaSR) upregulates VDR expression in vivo (Garfia B, Cañadillas S, Luque F, Siendones E, Quesada M, Almadén Y, Aguilera-Tejero E, Rodríguez M. J Am Soc Nephrol 13: 2945–2952, 2002; Rodriguez ME, Almaden Y, Cañadillas S, Canalejo A, Siendones E, Lopez I, Aguilera-Tejero E, Martin D, Rodriguez M. Am J Physiol Renal Physiol 292: F1390–F1395, 2007). The present study was designed to characterize the signaling system that mediates the stimulation of parathyroid VDR gene expression by extracellular calcium. Experiments were performed in vitro by the incubation of rat parathyroid glands and in vivo with normal and uremic (Nx) rats receiving injections of CaCl₂or EDTA to obtain hypercalcemic or hypocalcemic clamps. A high calcium concentration increased VDR expression. The addition of arachidonic acid (AA) to the low-calcium medium produced an increase in VDR mRNA of the same magnitude as that observed with high calcium. The addition of ionophore to the low-calcium medium also increased VDR mRNA expression. High calcium or the addition of AA to the low-calcium medium induced the activation (phosphorylation) of ERK1/2-MAPK. The specific inhibition of the ERK1/2-MAPK activity prevented the stimulation of VDR expression by high calcium or AA. These results suggest that AA regulates parathyroid VDR gene expression through the activation of the ERK1/2-MAPK. CaSR activation induced the activation of transcription factor Sp1, but not of NF-κB p50 or p65 or activator protein-1. The addition of AA to the low-calcium medium increased specific DNA-binding activity of Sp1 to almost the same level as high calcium, which was prevented by the inhibition of ERK1/2. Furthermore, mithramycin A (a Sp1 inhibitor) prevented the upregulation of VDR mRNA by high calcium. Finally, both sham and Nx hypercalcemic rats showed similar increased levels of VDR mRNA compared with sham and Nx hypocalcemic rats. Our results demonstrate that extracellular calcium stimulates VDR expression in parathyroid glands through the elevation of the cytosolic calcium level and the stimulation of the PLA₂-AA-dependent ERK1/2-pathway. Furthermore, the transcription factor Sp1 mediates this effect.