Title

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

Date of Completion

January 2010

Keywords

Biology, Molecular

Degree

Ph.D.

Abstract

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. ^