Loss of MMP-2 in osteoblasts upregulates osteopontin and bone sialoprotein expression in a circuit regulating bone homeostasis

Abstract

Summary: Multicentric osteolysis with arthropathy (MOA; MIM 605156) is an inherited osteolyses/arthritis syndrome resulting from loss of MMP-2. We recently demonstrated that Mmp2-/- mice represent a unique model for the study of the human disease, sharing many features of the human syndrome including skeletal dysplasia and defects in osteoblast behavior. We therefore sought to explore, secondary molecular effects of MMP-2 loss which coexist with the underlying skeletal and osteoblast phenotypes. We used quantitative real-time PCR to measure osteoblast-related gene expression through ex vivo osteoblast differentiation of bone marrow stromal cells (BMSC) from Mmp2-/- and Mmp2+/+ mice. We used western blot to measure OPN serum levels and immunohistochemical staining to examine bone expression. MMP-2 expression was inhibited in SaOS2 cells using siRNA and decreased MMP-2 RNA and protein expression was confirmed by qRT-PCR and western blot, respectively. Mmp2-/- BMSC induced to differentiate into osteoblasts were shown to significantly upregulate OPN and BSP expression levels when compared to controls. Transcriptional upregulation is maintained in vivo as demonstrated by increased levels of OPN in serum and bone in Mmp2-/- mice. Finally, these effects are generalizable since siRNA-mediated inhibition in cultured cells also upregulates OPN and BSP. OPN and BSP were known to affect MMP-2 expression and activity but had not previously been shown to be regulated by MMP-2. Identification of this novel circuitry provides insight into the potential molecular landscape underlying the MOA phenotype and highlights a pathway that may play a role in normal bone homeostasis.