Gene profile analysis of osteoblast genes differentially regulated by histone deacetylase inhibitors.

BACKGROUND: Osteoblast differentiation requires the coordinated stepwise expression of multiple genes. Histone deacetylase inhibitors (HDIs) accelerate the osteoblast differentiation process by blocking the activity of histone deacetylases (HDACs), which alter gene expression by modifying chromatin structure. We previously demonstrated that HDIs and HDAC3 shRNAs accelerate matrix mineralization and the expression of osteoblast maturation genes (e.g. alkaline phosphatase, osteocalcin). Identifying other genes that are differentially regulated by HDIs might identify new pathways that contribute to osteoblast differentiation. RESULTS: To identify other osteoblast genes that are altered early by HDIs, we incubated MC3T3-E1 preosteoblasts with HDIs (trichostatin A, MS-275, or valproic acid) for 18 hours in osteogenic conditions. The promotion of osteoblast differentiation by HDIs in this experiment was confirmed by osteogenic assays. Gene expression profiles relative to vehicle-treated cells were assessed by microarray analysis with Affymetrix GeneChip 430 2.0 arrays. The regulation of several genes by HDIs in MC3T3-E1 cells and primary osteoblasts was verified by quantitative real-time PCR. Nine genes were differentially regulated by at least two-fold after exposure to each of the three HDIs and six were verified by PCR in osteoblasts. Four of the verified genes (solute carrier family 9 isoform 3 regulator 1 (Slc9a3r1), sorbitol dehydrogenase 1, a kinase anchor protein, and glutathione S-transferase alpha 4) were induced. Two genes (proteasome subunit, beta type 10 and adaptor-related protein complex AP-4 sigma 1) were suppressed. We also identified eight growth factors and growth factor receptor genes that are significantly altered by each of the HDIs, including Frizzled related proteins 1 and 4, which modulate the Wnt signaling pathway. CONCLUSION: This study identifies osteoblast genes that are regulated early by HDIs and indicates pathways that might promote osteoblast maturation following HDI exposure. One gene whose upregulation following HDI treatment is consistent with this notion is Slc9a3r1. Also known as NHERF1, Slc9a3r1 is required for optimal bone density. Similarly, the regulation of Wnt receptor genes indicates that this crucial pathway in osteoblast development is also affected by HDIs. These data support the hypothesis that HDIs regulate the expression of genes that promote osteoblast differentiation and maturation.

Histone deacetylase co-repressor complex control of Runx2 and bone formation.

A decade has passed since the transcription factor, Runx2, was found to be essential for osteoblast development and chondrocyte hypertrophy. During the last 10 years, our understanding of Runx2's physiological roles and the molecular mechanisms whereby it regulates gene expression to control cell-cycle progression and cellular differentiation has increased exponentially. Runx2 is expressed in osteoblasts, prehypertrophic chondrocytes, mesenchymal cells of the perichondrium, T lymphocytes, endothelial cells, and breast and prostate epithelial cells, with increased expression observed in breast and prostate carcinomas. Although Runx2 and other mammalian Runt domain proteins were originally described as transcriptional activators, they are also transcriptional repressors and thus maintain functional similarities with their Drosophila homolog, Runt. Runx2 binds a consensus DNA sequence but does not possess any enzymatic activities that directly affect chromatin structure. It alters gene expression by recruiting cofactors to gene regulatory elements. Histone deacetylases (HDACs) are among the co-repressors that interact with Runx2. In this review, we summarize data demonstrating that several HDACs and their associated proteins interact with Runx2, regulate its activity, and affect bone formation. HDACs are components of multiprotein complexes that interact with many transcription factors and are subject to regulation by extracellular signals. The elucidation of HDAC complex components that influence Runx2 activity in specific cell types and in response to various extracellular stimuli will increase our understanding of how this crucial transcription factor functions, and how we might be able to control its activity to influence bone formation or reduce bone disease associated with cancer metastasis.