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Osteoprotegerin Expression and Secretion Are Regulated by Calcium Influx through the L-Type Voltage-Sensitive Calcium Channel

Department of Biological Sciences, University of Delaware, Newark, Delaware 19716

Address all correspondence and requests for reprints to: Dr. Mary C. Farach-Carson, Department of Biological Sciences, University of Delaware, 326 Wolf Hall, Newark, Delaware 19716. E-mail: farachca{at}udel.edu.

Our previous studies showed that 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] modulates the activity of the Ca_V1.2 -subunit of the L-type voltage-sensitive calcium channel (VSCC) by two temporally distinct mechanisms. First, 1,25(OH)₂D₃ rapidly modulates local Ca²⁺ permeability in the plasma membrane of the proliferating osteoblast. Second, treatment with 1,25(OH)₂D₃ reduces biosynthesis of Ca_V1.2 such that transcript levels are half of original levels after 24 h. Osteoprotegerin (OPG) and receptor activator of nuclear factor kappa B ligand (RANKL) provide important regulatory mechanisms for controlling osteoclastogenesis and Ca²⁺ homeostasis. Because they often control Ca²⁺-activated secretion, we investigated the possibility that L-type VSCCs might regulate basal OPG and RANKL secretion in osteoblasts. We also studied 1,25(OH)₂D₃ effects on OPG and RANKL expression. To address this, we measured changes in expression and secretion of OPG and RANKL in MC3T3-E1 cells and calvarial organ cultures after treatment with 1,25(OH)₂D₃, VSCC inhibitors, and inhibitors of Ca²⁺-regulated signaling. RANKL production was increased in calvarial cultures by 1,25(OH)₂D₃ but was essentially undetectable in the medium of MC3T3-E1 cells. In contrast, OPG secretion in both systems was significantly reduced after 24 h treatment with 1,25(OH)₂D₃, by inhibitors of L-type VSCCs and calmodulin-sensitive protein kinases but not by inhibitors of protein kinase A, MAPKs, or other families of VSCCs. OPG secretion was abrogated by transfection with decoy cAMP response element binding sites. Our results suggest that OPG secretion is regulated through calmodulin-sensitive protein kinase signaling that depends on the activity of the L-type VSCC and is mediated through the cAMP response element-binding protein.

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