Pbx1 represses osteoblastogenesis by blocking Hoxa10-mediated recruitment of chromatin remodeling factors. Mol Cell Biol. 2010;30:3531-3541. [PMID: 20439491 DOI: 10.1128/mcb.00889-09]

MicroRNA control of bone formation and homeostasis

MicroRNAs (miRNAs) repress cellular protein levels to provide a sophisticated parameter of gene regulation that coordinates a broad spectrum of biological processes. Bone organogenesis is a complex process involving the differentiation and crosstalk of multiple cell types for formation and remodeling of the skeleton. Inhibition of mRNA translation by miRNAs has emerged as an important regulator of developmental osteogenic signaling pathways, osteoblast growth and differentiation, osteoclast-mediated bone resorption activity and bone homeostasis in the adult skeleton. miRNAs control multiple layers of gene regulation for bone development and postnatal functions, from the initial response of stem/progenitor cells to the structural and metabolic activity of the mature tissue. This Review brings into focus an emerging concept of bone-regulating miRNAs, the evidence for which has been gathered largely from in vivo mouse models and in vitro studies in human and mouse skeletal cell populations. Characterization of miRNAs that operate through tissue-specific transcription factors in osteoblast and osteoclast lineage cells, as well as intricate feedforward and reverse loops, has provided novel insights into the supervision of signaling pathways and regulatory networks controlling normal bone formation and turnover. The current knowledge of miRNAs characteristic of human pathologic disorders of the skeleton is presented with a future goal towards translational studies.

The role of Pbx1 in T cells

Tissue and organ differentiation is tightly controlled to ensure proper development and function of the growing embryo as well as cells such as lymphocytes that differentiate throughout the adult stage. Therefore it is vital that the genes and the protein they encode that are involved in these processes function accurately. Hence, any mutation or error that occurs along the way can result in extensive damage, which is expressed in various ways in the embryo and can result in immune pathogenesis, including immunodeficiency and autoimmune diseases, when lymphocyte development is altered. A number of studies have been carried out to look at the genes regulating transcription in tissue differentiation, including the transcription factors Pbx1. This gene is of particular interest to us as we have identified that it is associated with systemic lupus erythematosus susceptibility (Cuda et al., in press). This perspective summarizes the known roles of Pbx1 in tissue differentiation as well as our recent findings associating genetic variations in Pbx1 to lupus susceptibility, and we will speculate on how this gene controls the maintenance of immune tolerance in T cells.

Inhibition of fetal bone development through epigenetic down-regulation of HoxA10 in obese rats fed high-fat diet

Epidemiological studies show that maternal obesity during intrauterine and early postnatal life increases the risk of low bone mass and fracture later in life. Here, we show that bone development is inhibited in gestational embryonic day 18.5 (E18.5) embryos from rat dams made obese by feeding a high-fat diet (HFD). Moreover, fetal rat osteogenic calvarial cells (FOCCs) from these obese dams have significantly less potential to develop into mature osteoblasts compared to cells from AIN-93G diet-fed controls. Profiling of transcriptional genes for osteogenesis revealed a profound decrease in the homeodomain-containing factor A10 (HoxA10) in FOCCs from fetuses of HFD-induced obese dams. Significant methylation of the HoxA10 promoter was found in those FOCCs, as well as in mouse ST2 cells treated with a mixture of free fatty acids similar to that found in serum from HFD-induced obese rats. This was accompanied by lower expression of osteogenic markers, but higher levels of PPARγ. Control FOCCs depleted of the HoxA10 gene (shRNA) ex vivo behave similarly to cells from fetuses of obese dams; conversely, overexpression of HoxA10 gene in FOCCs from HFD rats exhibit the same phenotype as controls. Treatment of FOCCs from control rats or of ST2 cells with an artificial mixture of free fatty acids significantly down-regulated HoxA10 protein expression, and cells exhibited adipocyte-like properties. These results suggest that maternal obesity impairs fetal skeletal development through down-regulation of the HoxA10 gene, which may lead to an increase in the prevalence of low bone mass in the offspring later in life.

Epigenetic regulation of early osteogenesis and mineralized tissue formation by a HOXA10-PBX1-associated complex

Homeodomain-containing (HOX) factors such as the abdominal class homeodomain protein HOXA10 and the TALE-family protein PBX1 form coregulatory complexes and are potent transcriptional and epigenetic regulators of tissue morphogenesis. We have identified that HOXA10 and PBX1 are expressed in osteoprogenitors; however, their role in osteogenesis has not been established. To determine the mechanism of HOXA10-PBX-mediated regulation of osteoblast commitment and the related gene expression, PBX1 or HOX10 were depleted (shRNA or genetic deletion, respectively) or exogenously expressed in C3H10T1/2, bone marrow stromal progenitors, and MC3T3-E1 (preosteoblast) cells. Overexpression of HOXA10 increased the expression of osteoblast-related genes, osteoblast differentiation and mineralization; expression of PBX1 impaired osteogenic commitment of pluripotent cells and the differentiation of osteoblasts. In contrast, the targeted depletion of PBX1 by shRNA increased the expression of bone marker genes (osterix, alkaline phosphatase, BSP, and osteocalcin). Chromatin-associated PBX1 and HOXA10 were present at osteoblast-related gene promoters preceding gene expression, but PBX1 was absent from promoters during the transcription of bone-related genes, including osterix (Osx). Further, PBX1 complexes were associated with histone deacetylases normally linked with chromatin inactivation. Loss of PBX1 but not of HOXA10 from the Osx promoter was associated with increases in the recruitment of histone acetylases (p300), as well as decreased H3K9 methylation, reflecting transcriptional activation. We propose PBX1 plays a central role in attenuating the activity of HOXA10 as an activator of osteoblast-related genes and functions to establish the proper timing of gene expression during osteogenesis, resulting in proper matrix maturation and mineral deposition in differentiated osteoblasts.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

Metastatic bone disease: role of transcription factors and future targets

Progression of cancer from the earliest event of cell transformation through stages of tumor growth and metastasis at a distal site involves many complex biological processes. Underlying the numerous responses of cancer cells to the tumor microenvironment which support their survival, migration and metastasis are transcription factors that regulate the expression of genes reflecting properties of the tumor cell. A number of transcription factors have been identified that play key roles in promoting oncogenesis, tumor growth, metastasis and tissue destruction. Relevant to solid tumors and leukemias, tissue-specific transcription factors that are deregulated resulting from mutations, being silenced or aberrantly expressed, have been well characterized. These are the master transcription factors of the Runx family of genes, the focus of this review, with emphasis placed on Runx2 that is abnormally expressed at very high levels in cancer cell lines that are metastatic to bone. Recent evidence has identified a correlation of Runx2 levels in advanced stages of prostate and breast cancer and demonstrated that effective depletion of Runx2 by RNA interference inhibits migration and invasive properties of the cells prevents metastatic bone disease. This striking effect is consistent with the broad spectrum of Runx2 properties in activating many genes in tumor cells that have already been established as indicators of bone metastasis in poor prognosis. Potential strategies to translate these findings for therapeutic applications are discussed.