Local Vitamin D Metabolism is Differentially Regulated by MKP‐1 in Osteoclasts Stimulated with RANKL vs. LPS

Abstract

Recent reports indicate that the substrate level activity of Vitamin D (VD) regulates local inflammatory cytokine production, including mediators that regulate osteoclast (OC) formation. MKP‐1, a phosphatase which controls p38 and JNK MAPK signaling has been shown to be dependent on VD. Conversely, VD‐dependent genes involved in VD metabolism (Cyp27b1, Cyp24a1) and bone turnover (RANKL) are regulated by MKP‐1, resulting in abnormal VD activity and reduced OC. Interestingly, we reported that MKP‐1 positively regulates RANKL‐induce OC formation, but also reported that it negatively regulates OC formation in pre‐OC stimulated with pathogenic Lipopolysaccaride (LPS). Given the recent reports of VD's regulation of the inflammatory microenvironment, the objective of these experiments was to investigate the role of MKP‐1 on VDR‐dependent gene expression in RANKL vs. LPS‐stimulated pre‐OC. WT and MKP‐1‐/‐ bone marrow cells were obtained and isolated for pre‐OC using magnetic sorting for CD11bloCD115+ monocytes, stimulated with MCSF and RANKL (50ng/ml) or LPS (100ng/ml) for 96hrs. mRNA was and measured for expression of VD metabolism using qPCR. By adding LPS, Vdr expression was significantly up‐regulated in the MKP‐1‐/‐ population (P<0.01), however, expression of Cyp27b1 was significantly blunted in MKP‐1‐/‐ OC (P<0.05). Cyp24a1 was not induced by LPS. In conclusion, MKP‐1 neg. regulates VDR and + regulates Cyp27b1 in OC in response to LPS, not RANKL, contrary to what has been observed in osteoblasts. In all, MKP‐1 does not inhibit VDR nuclear activity in OC cells stimulated with LPS, but may play a role in the regulation of enzymes involved in local VD activation.