Abstract
The adaptor protein CrkII is involved in several biological activities, including mitogenesis, phagocytosis, and cytoskeleton reorganization. Previously, we demonstrated that CrkII plays an important role in osteoclast differentiation and function through Rac1 activation both in vitro and in vivo. In this study, we investigated whether CrkII also regulates the differentiation and function of another type of bone cells, osteoblasts. Overexpression of CrkII in primary osteoblasts inhibited bone morphogenetic protein (BMP) 2-induced osteoblast differentiation and function, whereas knockdown of CrkII expression exerted the opposite effect. Importantly, CrkII strongly enhanced c-Jun-N-terminal kinase (JNK) phosphorylation, and the CrkII overexpression-mediated attenuation of osteoblast differentiation and function was recovered by JNK inhibitor treatment. Furthermore, transgenic mice overexpressing CrkII under control of the alpha-1 type I collagen promoter exhibited a reduced bone mass phenotype. Together, these results indicate that CrkII negatively regulates osteoblast differentiation and function through JNK phosphorylation. Given that CrkII acts as a negative and positive regulator of osteoblast and osteoclast differentiation, respectively, the regulation of CrkII expression in bone cells may help to develop new strategies to enhance bone formation and inhibit bone resorption.
Highlights
Osteoblasts are mononuclear cells that are responsible for bone formation
To determine the role of CrkII in osteoblast differentiation and function, control- or CrkII-transduced osteoblasts were cultured in osteogenic medium (OGM), and the alkaline phosphatase (ALP) activity and alizarin red staining intensities were quantified
ALP activity was strongly induced by OGM stimulation, and this induction was greatly reduced in CrkII-overexpressing cells (Fig. 1a)
Summary
The differentiation of mesenchymal cells into osteoblasts is regulated by various anabolic factors, such as bone morphogenetic proteins (BMPs), insulin, and insulin-like growth factor-11–3. Binding of BMPs to their type I and II serine/threonine kinase receptors activates the Smad signaling pathway, inducing the expression of various target genes, such as runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), and osteocalcin[4,5,6]. Various transcription factors involved in osteoblast differentiation and function, such as the Runx[2], osterix, and homeodomain-containing Msx proteins, have been well studied[1,7,8,9,10]. Runx[2] is a master transcription factor that regulates the expression of many osteoblastic marker genes, including ALP, bone sialoprotein (BSP), and osteocalcin to induce osteoblast differentiation and bone formation[11,12]. Mice with a heterozygous mutation in Runx[2] exhibit cleidocranial dysplasia disorders associated with defective
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