Calcium Input Potentiates the Transforming Growth Factor (TGF)-β1-dependent Signaling to Promote the Export of Inorganic Pyrophosphate by Articular Chondrocyte

Transforming growth factor (TGF)-beta 1 stimulates extracellular PPi (ePP(i)) generation and promotes chondrocalcinosis, which also occurs secondary to hyperparathyroidism-induced hypercalcemia. We previously demonstrated that ANK was upregulated by TGF-beta 1 activation of ERK1/2 and Ca2+-dependent protein kinase C (PKC alpha). Thus, we investigated mechanisms by which calcium could affect ePP(i) metabolism, especially its main regulating proteins ANK and PC-1 (plasma cell membrane glycoprotein-1). We stimulated articular chondrocytes with TGF-beta 1 under extracellular (eCa(2+)) or cytosolic Ca2+ (cCa(2+)) modulations. We studied ANK, PC-1 expression (quantitative RT-PCR, Western blotting), ePP(i) levels (radiometric assay), and cCa(2+) input (fluorescent probe). Voltage-operated Ca2+-channels (VOC) and signaling pathways involved were investigated with selective inhibitors. Finally, Ank promoter activity was evaluated (gene reporter). TGF-beta 1 elevated cCa(2+) and ePP(i) levels (by up-regulating Ank and PC-1 mRNA/proteins) in an eCa(2+) dose-dependent manner. TGF-beta 1 effects were suppressed by cCa(2+) chelation or L- and T-VOC blockade while being mostly reproduced by ionomycin. In the same experimental conditions, the activation of Ras, the phosphorylation of ERK1/2 and PKC alpha, and the stimulation of Ank promoter activity were affected similarly. Activation of SP1 (specific protein 1) and ELK-1 (Ets-like protein-1) transcription factors supported the regulatory role of Ca2+. SP1 or ELK-1 overexpression or blockade experiments demonstrated a major contribution of ELK-1, which acted synergistically with SP1 to activate Ank promoter in response to TGF-beta 1. TGF-beta 1 promotes input of eCa(2+) through opening of L- and T-VOCs, to potentiate ERK1/2 and PKC alpha signaling cascades, resulting in an enhanced activation of Ank promoter and ePP(i) production in chondrocyte.