MKP-1 is essential for canonical vitamin D-induced signaling through nuclear import and regulates RANKL expression and function

Abstract

Vitamin D3, and its most active form, 1,25(OH)2D3, are well known to stimulate osteoclastogenesis through stromal cell induction of the receptor activator of nuclear factor-B ligand (RANKL). MAPK phosphatase-1 (MKP-1) is a phosphatase classically known to negatively regulate the innate immune response through dephosphorylation of p38, ERK, and c-Jun N-terminal kinase activity. This paper describes a new function of MKP-1 in permitting genomic 1,25(OH)2D3 signaling and downstream osteoclastogenesis through RANKL. Initially, quantitative RT-PCR (qRT-PCR) and immunoblot analysis comparing bone marrow stromal cells (BMSC) revealed that 1,25(OH)2D3-induced vitamin D receptor (VDR), cytochrome P 45024a1, and RANKL mRNA expression and protein were significantly attenuated or absent in MKP-1/ BMSC. Immunoblot analysis from cellular fractions of wild type and MKP-1 / BMSC stimulated with 10 7 M 1,25(OH)2D3 revealed retinoid X receptor (RXR)÷ nuclear import was impaired in MKP-1/ BMSC, whereas VDR import was not. Proximity ligation assays revealed that baseline VDR-RXR÷ heterodimer translocation was unchanged, yet 1,25(OH)2D3-induced nuclear translocation of VDR-RXR÷ heterodimers was reduced in MKP-1/ BMSC. A functional consequence was observed as BMSC from MKP-1/ mice treated with 1,25(OH)2D3 and cocultured with RAW 264.7 cells had a 91% decrease in osteoclastogenesis and a 94.5% decrease in mineralized matrix resorption compared with wild-type cocultures (P 0.01). These results reveal an unexpected, permissive role for MKP-1 in canonical 1,25(OH)2D3 signaling via VDR-RXR÷ heterodimer nuclear import and downstream osteoclastogenesis through stromal cell RANKL expression. © 2012 by The Endocrine Society.