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The transcription of neuron-specific genes must be repressed in nonneuronal cells. REST͞NRSF is a transcription factor that restricts the expression of many neuronal genes through interaction with the neuron-restrictive silencer element at the promoter level. PAHX-AP1 is a neuronal gene that is developmentally up-regulated in the adult mouse brain but that has no functional NRSE motif in its 5 upstream sequence. Here, we report that the transcription factor AP4 and the corepressor geminin form a functional complex in which SMRT and histone deacetylase 3 are recruited. The functional complex represses PAHX-AP1 expression in nonneuronal cells and participates in regulating the developmental expression of PAHX-AP1 in the brain. This complex also serves as a transcriptional repressor of DYRK1A, a candidate gene for Down's syndrome. Furthermore, compared with that in normal fetal brain, the expression of AP4 and geminin is reduced in Down's syndrome fetal brain at 20 weeks of gestation age, at which time premature overexpression of dual-specificity tyrosine-phosphorylated and regulated kinase 1A (DYRK1A) is observed. Our findings indicate that AP4 and geminin act as a previously undescribed repressor complex distinct from REST͞NRSF to negatively regulate the expression of target genes in nonneuronal cells and suggest that the AP4 -geminin complex may contribute to suppressing the precocious expression of target genes in fetal brain.

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Estrogen receptor-related receptor ␥ (ERR␥/ERR3/NR3B3) is a member of the orphan nuclear receptor with important functions in development and homeostasis. Recently it has been reported that ERR␣ is involved in osteoblast differentiation and bone formation. In the present study we examined the role of ERR␥ in osteoblast differentiation. Here, we showed that ERR␥ is expressed in osteoblast progenitors and primary osteoblasts, and its expression is increased temporarily by BMP2. Overexpression of ERR␥ reduced BMP2-induced alkaline phosphatase activity and osteocalcin production as well as calcified nodule formation, whereas inhibition of ERR␥ expression significantly enhanced BMP2-induced osteogenic differentiation and mineralization, suggesting that endogenous ERR␥ plays an important role in osteoblast differentiation. In addition, ERR␥ significantly repressed Runx2 transactivity on osteocalcin and bone sialoprotein promoters. We also observed that ERR␥ physically interacts with Runx2 in vitro and in vivo and competes with p300 to repress Runx2 transactivity. Notably, intramuscular injection of ERR␥ strongly inhibited BMP2-induced ectopic bone formation in a dose-dependent manner. Taken together, these results suggest that ERR␥ is a novel negative regulator of osteoblast differentiation and bone formation via its regulation of Runx2 transactivity.Bone formation is a series of well orchestrated lineage-specific differentiation events (1). Osteoblasts, which play key roles in bone formation, are derived from pluripotent mesenchymal stem cells that have the capacity to differentiate into myocytes, adipocytes, and chondrocytes (2). Osteoblasts possess the necessary components to form bone matrix, which allows subsequent mineralization. Several hormones, growth factors, cytokines, and nuclear receptor proteins regulate these sequential events to trigger a complex network of signaling pathways.Bone morphogenetic proteins (BMPs) 5 are members of the transforming growth factor ␤ family and were originally identified by their capacity to induce ectopic bone formation (3, 4). Among the BMP family members, the action of BMP2 has been studied extensively in embryonic skeletal development, postnatal bone remodeling, and bone repair (5, 6). BMP2 promotes the commitment of pluripotent mesenchymal cells to the osteoblast lineage by regulating the signals that stimulate the specific transcriptional programs required for bone formation (5, 7).A master regulator of osteoblasts, Runx2, is indispensable for a skeletal development and maturation. Targeted disruption of Runx2 results in a complete lack of functional osteoblasts (8, 9). Runx2 directly regulates osteoblast-specific genes such as osteocalcin (OC), bone sialoprotein (BSP), osteopontin, and type I collagen through binding to specific DNA enhancer elements of its target gene promoters (4). In addition, Runx2 interacts with a variety of transcription factors (10) and recruits both co-activators (11-13) and co-repressors (14, 15) to form a complex on its target promoter. Theref...

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) interacts with a diverse array of transcription factors and regulates a variety of cellular events such as cell proliferation, differentiation, and metabolism. However, the role of SHP in bone formation has not yet been elucidated. SHP expression is significantly increased during osteoblast differentiation, and its expression is partially regulated by bone morphogenetic protein 2 (BMP-2), which plays an important role in bone formation. In our study, inhibition of SHP expression significantly repressed BMP-2-induced osteoblast differentiation and ectopic bone formation. In accordance with these in vitro and in vivo results, osteoblast differentiation in SHP−/− mice primary osteoblasts was significantly repressed, and the mice showed decreased bone mass resulting from decreased numbers of osteoblasts. Finally, SHP physically interacts and forms a complex with runt-related transcription factor 2 (Runx2) on the osteocalcin gene promoter, and overexpression of SHP increased Runx2 transactivity via competition with histone deacetylase 4 (HDAC4), an enzyme that inhibits DNA binding of Runx2 to its target genes. Taken together, these results indicate that SHP acts as a novel positive regulator of bone formation by augmenting osteoblast differentiation through regulation of the transcriptional activity of Runx2. © 2010 American Society for Bone and Mineral Research

Previously, we reported that decreased brain-specific angiogenesis inhibitor 2 (BAI2) induced increased VEGF expression. The regulatory mechanisms for this process are not understood. Here we show that GA-binding protein gamma (GABPc) associates with the cytoplasmic domain of BAI2, and GABPa/c or GABPa/b works as a transcriptional repressor of VEGF in SHSY5Y cells. Transcriptional activity of wild-type VEGF promoter was significantly increased in anti-sense BAI2-transfected cells, but not that of VEGF promoter harboring mutated GABP sites. In in vivo focal cerebral ischemia model, the decrease in BAI2 accompanied by decreased GABPa and GABPc elicited increased VEGF expression before the onset of HIF-1a. Our results point out that BAI2 controls VEGF transcription through GABP under normal conditions and cerebral ischemia.

Background: Severe inflammatory reactions delay wound healing of bone. Results: Tumor necrosis factor ␣ (TNF␣) inhibition of osteoblast differentiation is associated with increased cAMP response element-binding protein H (CREBH) and Smurf1 expression. Conclusion: CREBH mediates the inhibitory actions of TNF␣ in bone regeneration. Significance: CREBH is identified as a new mediator of inflammation-dependent bone degradation and a potential therapeutic target.

Lovastatin and simvastatin may stimulate the osteoblastic differentiation of periodontal ligament cells via the ERK1/2 pathway. This suggests that the statins may be useful for regenerating periodontal hard tissue.