MKP-1 Is Essential for Canonical Vitamin D-Induced Signaling through Nuclear Import and Regulates RANKL Expression and Function

Vitamin D-3, and its most active form, 1,25(OH)(2)D-3, are well known to stimulate osteoclastogenesis through stromal cell induction of the receptor activator of nuclear factor-kappa 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)(2)D-3 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)(2)D-3-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)(2)D-3 revealed retinoid X receptor (RXR)alpha nuclear import was impaired in MKP-1(-/-) BMSC, whereas VDR import was not. Proximity ligation assays revealed that baseline VDR-RXR alpha heterodimer translocation was unchanged, yet 1,25(OH)(2)D-3-induced nuclear translocation of VDR-RXR alpha heterodimers was reduced in MKP-1(-/-) BMSC. A functional consequence was observed as BMSC from MKP-1(-/-) mice treated with 1,25(OH)(2)D-3 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)(2)D-3 signaling via VDR-RXR alpha heterodimer nuclear import and downstream osteoclastogenesis through stromal cell RANKL expression. (Molecular Endocrinology 26: 1682-1693, 2012)