Mitogen activated protein kinase phosphatase-1 (MKP-1) regulation of bone homeostasis and the bone response to mechanical loading

Mechanical loading of bone is essential for maintaining bone mass and integrity. Prostaglandins (PGs) produced by cyclooxygenase-2 (COX-2) are important for mediating bone formation responses to mechanical loading. Fluid shear stress (FSS), which is used to model the stresses generated in vivo by loading of bone, induces COX-2 expression in osteoblasts via extracellular signal-regulated kinase (ERK). FSS also induces MKP-1, and MKP-1 can dephosphorylate ERK. The goal of this research was to understand the role of MKP-1 in mechanical loading of bone and in bone homeostasis.;First, we investigated the role of MKP-1 in the FSS induction of COX-2 in clonal MC3T3-E1 and primary calvarial osteoblasts (POBs). Cells were subjected to brief FSS (pulsatile, 10 dynes/cm2) and returned to static culture (post FSS) for varying times. FSS induced MKP-1 via ERK and protein kinase A (PKA) pathways. Inhibition of ERK phosphorylation decreased the induction of both MKP-1 and COX-2. At 4 h post FSS inhibition of PKA decreased MKP-1 and increased the FSS induction of both ERK phosphorylation and COX-2 mRNA. In POBs from MKP-1 knockout (KO) mice, the FSS induction of ERK phosphorylation and COX-2 mRNA was increased compared to POBs from wild type (WT) POBs. These data suggest that FSS-stimulated MKP-1 expression decreases FSS-stimulated ERK phosphorylation and subsequent COX-2 expression.;Next, we investigated the role of MKP-1 in the regulation of COX-2 expression in ulnar loading in vivo. We subjected 16 wk-old female MKP-1 WT and KO mice to 30 s of unlar loading and measured COX-2 mRNA expression 1 h after loading. COX-2 expression was stimulated by loading and tended to be higher in MKP-1 KO ulnae than in WT ulnae. We speculate that MKP-1 may play an important role in limiting loading-induced COX-2 expression in bone.;We also examined the skeletal phenotype of MKP-1 WT and KO mice. MKP-1 KO male mice had less bone and more fat in bone marrow than WT males in vivo. In vitro, absence of MKP-1 increased osteoblast proliferation, decreased osteoblast differentiation, increased adipogenesis and increased osteoclastogenesis. These results suggest that MKP-1 is a critical regulator of bone homeostasis.