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J Biol Chem, Vol. 273, Issue 45, 29935-29941, November 6, 1998

Modulation of Osteopontin Post-translational State by 1,25-(OH)₂-Vitamin D₃
DEPENDENCE ON Ca²⁺ INFLUX

From the Department of Basic Sciences, University of Texas-Houston Dental Branch, Houston, Texas 77030

In osteoblastic ROS 17/2.8 cells, 1,25-(OH)₂-vitamin D₃ stimulates transcription of the extracellular matrix phosphoprotein osteopontin (OPN). We now show post-translational regulation of OPN production by 1,25-(OH)₂D₃. Prior to transcriptional up-regulation of OPN, 1,25-(OH)₂D₃ induces a shift in OPN isoelectric point (pI) from 4.6 to 5.1. Loading equal amounts of OPN recovered from ROS 17/2.8 cells exposed to 1,25-(OH)₂D₃ or carrier for 3 h reveals that the pI shift represents reduced phosphorylation. Trypsin cleavage patterns of OPN produced after 1,25-(OH)₂D₃ treatment indicates phosphorylation changes in the resulting peptides. Using structural analogs to 1,25-(OH)₂D₃, we found that analog AT (25-(OH)-16-ene-23-yne-D₃), which triggers Ca²⁺ influx but does not bind to the vitamin D receptor, mimicked the OPN pI shift, whereas analog BT (1,25-(OH)₂-22-ene-24-cyclopropyl-D₃), which binds to the vitamin D receptor without triggering Ca²⁺ influx, did not. Likewise, inclusion of the Ca²⁺ channel blocker nifedipine blocks the charge conversion of OPN. Isolation of OPN from rat femurs and tibiae demonstrates the existence of two OPN charge forms in vivo. We conclude that 1,25-(OH)₂D₃ regulates OPN not only at the transcriptional level, but also modulates OPN phosphorylation state. The latter involves a short term (<3 h) treatment and is associated with membrane-initiated Ca²⁺ influx.

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