Interaction of the 1alpha,25-dihydroxyvitamin D3 receptor at the distal promoter region of the bone-specific osteocalcin gene requires nucleosomal remodelling

Epigenetic Mechanisms Regulating Mesenchymal Stem Cell Differentiation

Human Mesenchymal Stem Cells (hMSCs) have emerged in the last few years as one of the most promising therapeutic cell sources and, in particular, as an important tool for regenerative medicine of skeletal tissues. Although they present a more restricted potency than Embryonic Stem (ES) cells, the use of hMCS in regenerative medicine avoids many of the drawbacks characteristic of ES cells or induced pluripotent stem cells. The challenge in using these cells lies into developing precise protocols for directing cellular differentiation to generate a specific cell lineage. In order to achieve this goal, it is of the upmost importance to be able to control de process of fate decision and lineage commitment. This process requires the coordinate regulation of different molecular layers at transcriptional, posttranscriptional and translational levels. At the transcriptional level, switching on and off different sets of genes is achieved not only through transcriptional regulators, but also through their interplay with epigenetic modifiers. It is now well known that epigenetic changes take place in an orderly way through development and are critical in the determination of lineage-specific differentiation. More importantly, alteration of these epigenetic changes would, in many cases, lead to disease generation and even tumour formation. Therefore, it is crucial to elucidate how epigenetic factors, through their interplay with transcriptional regulators, control lineage commitment in hMSCs.

C/EBPβ binds the P1 promoter of the Runx2 gene and up-regulates Runx2 transcription in osteoblastic cells

The Runx2 factor is an essential component of the regulatory mechanisms that control transcription during skeletogenesis. Runx2/p57 expression in osteoblastic cells is controlled by the P1 promoter, which is recognized by key regulators of osteoblast differentiation including homeodomain factors and Wnt- and BMP-signaling mediators. Here, we report that the transcription factor C/EBPβ up-regulates Runx2/p57 expression by directly binding to the Runx2 P1 promoter in mesenchymal, pre-osteoblastic, and osteoblastic cells. This C/EBPβ-mediated up-regulation is principally dependent on C/EBP site II that is located within the first 180 bp of the proximal P1 promoter region and is highly conserved among mouse, rat, and human Runx2 genes. Our studies reveal how the C/EBPβ factor, known to have a key role during osteogenesis, contributes to regulating the expression of Runx2, the master regulator of osteoblast differentiation.

1alpha,25-dihydroxy vitamin D(3) induces nuclear matrix association of the 1alpha,25-dihydroxy vitamin D(3) receptor in osteoblasts independently of its ability to bind DNA

1alpha,25-dihydroxy vitamin D(3) (vitamin D(3)) has an important role during osteoblast differentiation as it directly modulates the expression of key bone-related genes. Vitamin D(3) binds to the vitamin D(3) receptor (VDR), a member of the superfamily of nuclear receptors, which in turn interacts with transcriptional activators to target this regulatory complex to specific sequence elements within gene promoters. Increasing evidence demonstrates that the architectural organization of the genome and regulatory proteins within the eukaryotic nucleus support gene expression in a physiological manner. Previous reports indicated that the VDR exhibits a punctate nuclear distribution that is significantly enhanced in cells grown in the presence of vitamin D(3). Here, we demonstrate that in osteoblastic cells, the VDR binds to the nuclear matrix in a vitamin D(3)-dependent manner. This interaction of VDR with the nuclear matrix occurs rapidly after vitamin D(3) addition and does not require a functional VDR DNA-binding domain. Importantly, nuclear matrix-bound VDR colocalizes with its transcriptional coactivator DRIP205/TRAP220/MED1 which is also matrix bound. Together these results indicate that after ligand stimulation the VDR rapidly enters the nucleus and associates with the nuclear matrix preceding vitamin D(3)-transcriptional upregulation.

Insights into the transcriptional and chromatin regulation of mesenchymal stem cells in musculo-skeletal tissues

Utilizing adult stem cells for regenerative medicine of skeletal tissues requires the development of molecular and biochemical tools that will allow isolation of these cells and direction of their differentiation towards a desired lineage and tissue formation. Stem cell commitment and fate decision into specialized functional cells involve coordinated activation and silencing of lineage-specific genes. Transcription factors and chromatin-remodeling proteins are key players in the control process of lineage commitment and differentiation during embryogenesis and adulthood. Transcription factors act in cooperation with co-regulator proteins to generate tissue-specific responses that elicits the tissue specific gene expression. Consequently, one of the main challenges of today's research is to characterize molecular pathways that coordinate the lineage-specific differentiation. Epigenetic regulation includes chromatin remodeling that control structural changes of DNA required for the binding of transcription factors to promoter regions. Revealing the mechanisms of action of such factors will provide understanding of how transcription and chromatin regulatory factors function together to regulate stem cell lineage fate decision.

Nucleosome organization and targeting of SWI/SNF chromatin-remodeling complexes: contributions of the DNA sequence

Chromatin organization within the nuclear compartment is a fundamental mechanism to regulate the expression of eukaryotic genes. During the last decade, a number of nuclear protein complexes with the ability to remodel chromatin and regulate gene transcription have been reported. Among these complexes is the SWI/SNF family, which alters chromatin structure in an ATP-dependent manner. A considerable effort has been made to understand the molecular mechanisms by which SWI/SNF catalyzes nucleosome remodeling. However, limited attention has been dedicated to studying the role of the DNA sequence in this remodeling process. Therefore, in this minireview, we discuss the contribution of nucleosome positioning and nucleosome excluding sequences to the targeting and activity of SWI/SNF complexes. This discussion includes results from our group using the rat osteocalcin gene promoter as a model. Based on these results, we postulate a model for chromatin remodeling and transcriptional activation of this gene in osteoblastic cells.

Phosphorylation at serine 208 of the 1alpha,25-dihydroxy Vitamin D3 receptor modulates the interaction with transcriptional coactivators

Upon ligand binding the 1alpha,25-dihydroxy Vitamin D3 receptor (VDR) undergoes a conformational change that allows interaction with coactivator proteins including p160/SRC family members and the multimeric DRIP complex through the DRIP205 subunit. Casein kinase II (CKII) phosphorylates VDR both in vitro and in vivo at serine 208 within the hinge domain. This phosphorylation does not affect the ability of VDR to bind DNA, but increases its ability to transactivate target promoters. Here, we have analyzed whether phosphorylation of VDR by CKII modulates the ability of VDR to interact with coactivators in vitro. We find that both mutation of serine 208 to aspartic acid (VDRS208D) or phosphorylation of VDR by CKII enhance the interaction of VDR with DRIP205 in the presence of 1alpha,25-dihydroxy Vitamin D3. We also find that the mutation VDRS208D neither affects the ability of this protein to bind DNA nor to interact with SRC-1 and RXRalpha. Together, our results indicate that phosphorylation of VDR at serine 208 contributes to modulate the affinity of VDR for the DRIP complex and therefore may have a role in vivo regulating VDR-mediated transcriptional enhancement.

An architectural perspective of vitamin D responsiveness

Vitamin D serves as a principal modulator of skeletal gene transcription, thus necessitating an understanding of interfaces between the activity of this steroid hormone and regulatory cascades that are functionally linked to the regulation of skeletal genes. Physiological responsiveness requires combinatorial control where coregulatory proteins determine the specificity of biological responsiveness to physiological cues. It is becoming increasingly evident that the regulatory complexes containing the vitamin D receptor are dynamic rather than static. Temporal and spatial modifications in the composition of these complexes provide a mechanism for integrating regulatory signals to support positive or negative control through synergism and antagonism. Compartmentalization of components of vitamin D control in nuclear microenvironments supports the integration of regulatory activities, perhaps by establishing thresholds for protein activity in time frames that are consistent with the execution of regulatory signaling.

Effect of a Nutritional Supplementation on Bone Health in Chilean Elderly Subjects with Femoral Osteoporosis

OBJECTIVE

To study the effects of a special nutritional supplement on bone mineral density and bone turnover markers in Chilean elderly subjects with femoral osteoporosis.

SETTING

Public primary health care clinics in Chile.

SUBJECTS

Free living elderly subjects with femoral osteoporosis.

INTERVENTIONS

Subjects were randomized to receive the usual nutritional supplement provided by the Chilean Ministry of Health or a special nutritional supplement providing, among other nutrients, 90 mg isoflavones, 800 mg calcium, 400 IU vitamin D, 60 ug vitamin K and 31 g proteins per day.

MEASURES OF OUTCOME

At baseline, and after six and twelve months of supplementation, body composition, bone mineral density, serum 25 OH vitamin D, intact parathyroid hormone (iPTH), osteocalcin, decarboxylated osteocalcin, urinary aminoterminal telopeptide of type I collagen (NTX), deoxypyridoline cross links (Dpd) and equol were measured. Every month, urinary daidzein was measured in a morning urine sample.

RESULTS

No differences between treatment groups were observed in body composition or bone mineral density changes. The group receiving the special supplement had a significant increase in serum 25 OH vitamin D and a significant decrease in serum iPTH and decarboxylated osteocalcin. No association between daidzein or equol excretion and changes in bone mineralization was observed.

CONCLUSIONS

A special supplement delivered to elderly subjects with osteoporosis improved serum vitamin D and reduced serum iPTH and undercarboxylated osteocalcin levels but did not affect BMD.

YY1 is involved in RANKL-induced transcription of the tartrate-resistant acid phosphatase gene in osteoclast differentiation

Receptor activator of nuclear factor kappa B (NF-kappaB) ligand (RANKL), a critical activator of osteoclast differentiation, plays a pivotal role in tartrate-resistant acid phosphatase (TRAP) gene expression. Previously, we showed that upstream stimulatory factors (USF) 1 and 2 are implicated in the RANKL-induced TRAP transcriptional activation via a 12-bp USF binding site in the TRAP promoter. In that study, we also demonstrated that a RANKL-induced nuclear protein binds to a 50-bp oligonucleotide (Oligo IV) corresponding to a distinct TRAP promoter region. Here we report the identification and functional characterization of the nuclear protein binding to Oligo IV. We identified a 21-bp sequence CTGTTTATGATGGCGAGGGGG in Oligo IV that specifically binds the RANKL-induced nuclear protein from RAW264.7 cells by performing a series of competition assays. Computer analysis of the 21-bp sequence revealed that the sequence contains a putative Yin Yang 1 (YY1) binding site overlapped with a putative activator protein-2 (AP-2) binding site. Competition and supershift assays indicated that the nuclear protein binding to the 21-bp sequence is YY1, not AP-2. Functionally, mutation of the YY1-binding site resulted in a reduction in the RANKL-induced TRAP transcription in RAW264.7 cells, demonstrating that YY1 positively regulates RANKL-induced TRAP transcriptional activation. In conclusion, our data demonstrated that YY1 plays a functional role in RANKL-mediated TRAP gene expression during osteoclast differentiation.

Skeletal hormones and the C/EBP and Runx transcription factors: interactions that integrate and redefine gene expression

Systemic hormones and local growth factors have significant and often complex roles in normal tissue development, growth, remodeling, and repair. Early efforts in skeletal tissue attempted to define active panels of these agents and their direct effects on cell proliferation, matrix production, and secretion of other soluble mediators of differentiated cell function. Initial results resolved many of these questions and began to unveil functional interactions between specific hormones and growth factors. More recent evidence suggests that interactions between individual hormone systems also occur in less anticipated but probably not less meaningful ways. In some cases, these interactions may help to define a spectrum of effects on gene expression by focusing, refocusing, or integrating the activity of previously recognized transcription regulators. Other studies in isolated osteoblasts predict that certain steroid hormones have distinctive effects on specific transcription factors with important roles in bone growth and repair. In this review, we focus on studies that define functional and physical interactions between molecular mediators of hormone activity that could directly effect skeletal growth factor biology.

Bone-specific transcription factor Runx2 interacts with the 1alpha,25-dihydroxyvitamin D3 receptor to up-regulate rat osteocalcin gene expression in osteoblastic cells

Bone-specific transcription of the osteocalcin (OC) gene is regulated principally by the Runx2 transcription factor and is further stimulated in response to 1alpha,25-dihydroxyvitamin D3 via its specific receptor (VDR). The rat OC gene promoter contains three recognition sites for Runx2 (sites A, B, and C). Mutation of sites A and B, which flank the 1alpha,25-dihydroxyvitamin D3-responsive element (VDRE), abolishes 1alpha,25-dihydroxyvitamin D3-dependent enhancement of OC transcription, indicating a tight functional relationship between the VDR and Runx2 factors. In contrast to most of the members of the nuclear receptor family, VDR possesses a very short N-terminal A/B domain, which has led to the suggestion that its N-terminal region does not contribute to transcriptional enhancement. Here, we have combined transient-overexpression, coimmunoprecipitation, in situ colocalization, chromatin immunoprecipitation, and glutathione S-transferase pull-down analyses to demonstrate that in osteoblastic cells expressing OC, VDR interacts directly with Runx2 bound to site B, which is located immediately adjacent to the VDRE. This interaction contributes significantly to 1alpha,25-dihydroxyvitamin D3-dependent enhancement of the OC promoter and requires a region located C terminal to the runt homology DNA binding domain of Runx2 and the N-terminal region of VDR. Together, our results indicate that Runx2 plays a key role in the 1alpha,25-dihydroxyvitamin D3-dependent stimulation of the OC promoter in osteoblastic cells by further stabilizing the interaction of the VDR with the VDRE. These studies demonstrate a novel mechanism for combinatorial control of bone tissue-specific gene expression. This mechanism involves the intersection of two major pathways: Runx2, a "master" transcriptional regulator of osteoblast differentiation, and 1alpha,25-dihydroxyvitamin D3, a hormone that promotes expression of genes associated with these terminally differentiated bone cells.

The Runx2 transcription factor plays a key role in the 1alpha,25-dihydroxy Vitamin D3-dependent upregulation of the rat osteocalcin (OC) gene expression in osteoblastic cells

Bone-specific transcription of the osteocalcin (OC) gene is principally regulated by the Runx2 transcription factor and further stimulated in response to 1alpha,25-dihydroxy Vitamin D3 via its specific receptor (VDR). The rat OC gene promoter contains three recognition sites for Runx2 (sites A-C). Mutation of sites A and B, which flank the 1alpha,25-dihydroxy Vitamin D3-responsive element (VDRE), abolishes 1alpha,25-dihydroxy Vitamin D3-dependent enhancement of OC transcription, indicating a tight functional relationship between VDR and Runx2 factors. Additionally, the transcriptional co-activator p300 is recruited to the OC promoter by Runx2 where it up-regulates both basal and 1alpha,25-dihydroxy Vitamin D3-enhanced OC expression. Here, we present an overview of how in osteoblastic cells expressing OC, Runx2 modulates the 1alpha,25-dihydroxy Vitamin D3-dependent stimulation of the OC promoter by first recruiting transcriptional co-activators and then by further stabilizing the interaction of the VDR with the VDRE.

Nuclear microenvironments support physiological control of gene expression

There is growing recognition that the organization of nucleic acids and regulatory proteins is functionally linked to the assembly, localization and activity of gene regulatory machinery. Cellular, molecular, biochemical and in-vivo genetic evidence support an obligatory relationship between nuclear microenvironments where regulatory complexes reside and fidelity of transcriptional control. Perturbations in mechanisms governing the intranuclear trafficking of transcription factors and the temporal/spatial organization of regulatory proteins within the nucleus occur with compromised gene expression that abrogates skeletal development and mediates leukemogenesis.

Regulation of the bone-specific osteocalcin gene by p300 requires Runx2/Cbfa1 and the vitamin D3 receptor but not p300 intrinsic histone acetyltransferase activity

p300 is a multifunctional transcriptional coactivator that serves as an adapter for several transcription factors including nuclear steroid hormone receptors. p300 possesses an intrinsic histone acetyltransferase (HAT) activity that may be critical for promoting steroid-dependent transcriptional activation. In osteoblastic cells, transcription of the bone-specific osteocalcin (OC) gene is principally regulated by the Runx2/Cbfa1 transcription factor and is stimulated in response to vitamin D(3) via the vitamin D(3) receptor complex. Therefore, we addressed p300 control of basal and vitamin D(3)-enhanced activity of the OC promoter. We find that transient overexpression of p300 results in a significant dose-dependent increase of both basal and vitamin D(3)-stimulated OC gene activity. This stimulatory effect requires intact Runx2/Cbfa1 binding sites and the vitamin D-responsive element. In addition, by coimmunoprecipitation, we show that the endogenous Runx2/Cbfa1 and p300 proteins are components of the same complexes within osteoblastic cells under physiological concentrations. We also demonstrate by chromatin immunoprecipitation assays that p300, Runx2/Cbfa1, and 1alpha,25-dihydroxyvitamin D(3) receptor interact with the OC promoter in intact osteoblastic cells expressing this gene. The effect of p300 on the OC promoter is independent of its intrinsic HAT activity, as a HAT-deficient p300 mutant protein up-regulates expression and cooperates with P/CAF to the same extent as the wild-type p300. On the basis of these results, we propose that p300 interacts with key transcriptional regulators of the OC gene and bridges distal and proximal OC promoter sequences to facilitate responsiveness to vitamin D(3).

Intranuclear organization of RUNX transcriptional regulatory machinery in biological control of skeletogenesis and cancer

RUNX (AML/CBFA/PEBP2) transcription factors serve as paradigms for obligatory relationships between nuclear structure and physiological control of phenotypic gene expression. The RUNX proteins contribute to tissue restricted transcription by sequence-specific binding to promoter elements of target genes and serving as scaffolds for the assembly of coregulatory complexes that mediate biochemical and architectural control of activity. We will present an overview of approaches we are pursuing to address: (1) the involvement of RUNX proteins in governing competency for protein/DNA and protein/protein interactions at promoter regulatory sequences; (2) the recruitment of RUNX factors to subnuclear sites where the machinery for expression or repression of target genes is organized; and (3) the trafficking and integration of regulatory signals that control RUNX-mediated transcription.