The retinoblastoma protein is an essential mediator of osteogenesis that links the p204 protein to the Cbfa1 transcription factor thereby increasing its activity

Monoallelic loss of RB1 enhances osteogenic differentiation and delays DNA repair without inducing tumorigenicity

The Retinoblastoma (RB1) gene plays a pivotal role in osteogenic differentiation. Our previous study, employing temporal gene expression analysis using quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), revealed the deregulation of osteogenic differentiation in patient-derived heterozygous RB1 mutant orbital adipose-derived mesenchymal stem cells (OAMSCs). The study revealed increased Alizarin Red staining, suggesting heightened mineralization without a corresponding increase in osteogenic lineage-specific gene expression. In this study, we performed high-throughput RNA sequencing on RB1+/+ and RB1+/- patient-derived OAMSCs differentiated towards the osteogenic lineage to investigate the pathways and molecular mechanisms. The pathway analysis revealed significant differences in cell proliferation, DNA repair, osteoblast differentiation, and cancer-related pathways in RB1+/- OAMSC-derived osteocytes. These findings were subsequently validated through functional assays. The study revealed that osteogenic differentiation is increased in RB1+/- cells, along with enhanced proliferation of the osteocytes. There were delayed but persistent DNA repair mechanisms in RB1+/- osteocytes, which were sufficient to maintain genomic integrity, thereby preventing or delaying the onset of tumors. This contrasts with our earlier observation of increased mineralization without corresponding gene expression changes, emphasizing the importance of high-throughput analysis over preselected gene set analysis in comprehending functional assay results.

Loss of Function of the Retinoblastoma Gene Affects Gap Junctional Intercellular Communication and Cell Fate in Osteoblasts

Loss of function of the Retinoblastoma gene (RB1) due to mutations is commonly seen in human osteosarcomas. One of the Rb1 gene functions is to facilitate cell fate from mesenchymal stem cells to osteoblasts and prevent adipocyte differentiations. In this study, we demonstrate that a stable reduction of Rb1 expression (RbKD) in murine osteoblasts causes them to express higher levels of PPAR-ɣ and other adipocyte-specific transcription factors while retaining high expression of osteoblast-specific transcription factors, Runx2/Cbfa1 and SP7/Osterix. Inhibition of gap junctional intercellular communication (GJIC) in osteoblasts is another mechanism that causes osteoblasts to transdifferentiate to adipocytes. We found that preosteoblasts exposed to osteoblast differentiating media (DP media) increased GJIC. RbKD cells showed reduced GJIC along with a reduction in expression of Cx43, the protein that mediates GJIC. Other membrane associated adhesion protein Cadherin 11 (Cad11) was also decreased. Since PPAR-ɣ is increased with Rb1 loss, we wondered if the reduction of this transcription factor would reverse the changes observed. Reduction of PPAR-ɣ in control osteoblasts slightly increased bone-specific expression and reduced adipocytic expression as expected along with an increase in Cad11 and Cx43 expression. GJIC remained high and was unaffected by a reduction in PPAR-ɣ in control cells. Knockdown of PPAR-ɣ in RbKD cells reduced adipocyte gene expression, while osteoblast-specific expression showed improvement. Cx43, Cad11 and GJIC remained unaffected by PPAR-ɣ reduction. Our observations suggest that increased PPAR-ɣ that happens with Rb1 loss only affects osteoblast-adipocyte-specific gene expression but does not completely reverse Cx43 gene expression or GJIC. Therefore, these effects may represent independent events triggered by Rb1loss and/or the differentiation process.

RIOK3 potentially regulates osteogenesis-related pathways in ankylosing spondylitis and the differentiation of bone marrow mesenchymal stem cells

RNA-binding proteins (RBPs), which are key effectors of gene expression, play critical roles in inflammation and immune regulation. However, the potential biological function of RBPs in ankylosing spondylitis (AS) remains unclear. We identified differentially expressed genes (DEGs) in peripheral blood mononuclear cells (PBMCs) of five patients with AS and three healthy persons by RNA-seq, obtained differentially expressed RBPs by overlapping DEGs and RBPs summary table. RIOK3 was selected as a target RBP and knocked down in mouse bone marrow mesenchymal stem cells (mBMSCs), and transcriptomic studies of siRIOK3 mBMSCs were performed again using RNA-seq. Results showed that RIOK3 knockdown inhibited the expression of genes related to osteogenic differentiation, ribosome function, and β-interferon pathways in mBMSCs. In vitro experiments have shown that RIOK3 knockdown reduced the osteogenic differentiation ability of mBMSCs. Collectively, RIOK3 may affect the differentiation of mBMSCs and participate in the pathogenesis of AS, especially pathological bone formation.

Activation and Immune Regulation Mechanisms of PYHIN Family During Microbial Infection

The innate immune system defenses against pathogen infections via patten-recognition receptors (PRRs). PRRs initiate immune responses by recognizing pathogen-associated molecular patterns (PAMPs), including peptidoglycan, lipopolysaccharide, and nucleic acids. Several nucleic acid sensors or families have been identified, such as RIG-I-like receptors (RLRs), Toll-like receptors (TLRs), cyclic GMP-AMP synthase (cGAS), and PYHIN family receptors. In recent years, the PYHIN family cytosolic DNA receptors have increased attention because of their important roles in initiating innate immune responses. The family members in humans include Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), interferon-inducible protein X (IFIX), and myeloid cell nuclear differentiation antigen (MNDA). The PYHIN family members are also identified in mice, including AIM2, p202, p203, p204, and p205. Herein, we summarize recent advances in understanding the activation and immune regulation mechanisms of the PYHIN family during microbial infection. Furthermore, structural characterizations of AIM2, IFI16, p202, and p204 provide more accurate insights into the signaling mechanisms of PYHIN family receptors. Overall, the molecular details will facilitate the development of reagents to defense against viral infections.

Heterozygous retinoblastoma gene mutation compromises in vitro osteogenesis of adipose mesenchymal stem cells - a temporal gene expression study

Osteosarcoma (OS) is a bone malignancy affecting children and adolescents. Retinoblastoma (RB) patients with germline RB1 mutations are susceptible to osteosarcoma in the second decade of their life. Several studies, particularly in mice, have revealed a role for RB1 in osteogenesis. Since, there is species specific difference attributed in retinoblastoma tumorigenesis between mice and human, we assumed, it is worthwhile exploring the role of RB1 in osteogenesis and thus onset of osteosarcoma. In this study, we analyzed the temporal gene expression of the osteogenic markers, tumor suppressor genes and hormone receptors associated with growth spurt during in vitro osteogenesis of mesenchymal stem cells derived from orbital adipose tissue of germline RB patients and compared it with those with wild type RB1 gene. Mesenchymal stem cells with the heterozygous RB1 mutation showed reduced expression of RB1 and other tumor suppressor genes and showed deregulation of osteogenic markers which could be an initial step for the onset of osteosarcoma.

RNA-Seq analysis of interferon inducible p204-mediated network in anti-tumor immunity

p204, a murine member of the interferon-inducible p200 protein family, and its human analogue, IFI16, have been shown to function as tumor suppressors in vitro, but the molecular events involved, in particular in vivo, remain unclear. Herein we induced the Lewis Lung carcinoma (LLC) murine model of human lung cancer in p204 null mice (KO) and their control littermates (WT). We compared the transcriptome in spleen from WT and p204 KO mice using a high-throughput RNA-sequencing array. A total 30.02 Gb of clean data were obtained, and overall Q30% was greater than 90.54%. More than 75% of clean data from 12 transcriptome samples were mapped to exons. The results showed that only 11 genes exhibited altered expression in untreated p204 KO mice relative to untreated WT mice, while 393 altered genes were identified in tumor-bearing p204 KO mice when compared with tumor-bearing WT mice. Further differentially expressed gene cluster and gene ontology consortium classification revealed that numerous cytokines and their receptors, chemoattractant molecules, and adhesion molecules were significantly induced in p204 KO mice. This study provides novel insights to the p204 network in anti-tumor immune response and also presents a foundation for future work concerning p204-mediated gene expressions and pathways.

p204 Is Required for Canonical Lipopolysaccharide-induced TLR4 Signaling in Mice

p204, a murine member of an interferon-inducible p200 family, was reported to recognize intracellular viral and bacterial DNAs, however, its role in the innate immunity in vivo remains unknown due to the lack of p204-deficient animal models. In this study we first generated the p204^−/− mice. Unexpectedly, p204 deficiency led to significant defect in extracellular LPS signaling in macrophages, as demonstrated by dramatic reductions of LPS-mediated IFN-β and pro-inflammatory cytokines. The serum levels of IFN-β and pro-inflammatory cytokines were also significantly reduced in p204^−/− mice following LPS challenge. In addition, p204^−/− mice were resistant to LPS-induced shock. LPS-activated NF-ĸB and IRF-3 pathways were all defective in p204-deficient macrophages. p204 binds to TLR4 through its Pyrin domain, and it is required for the dimerization of TLR4 following LPS-challenge. Collectively, p204 is a critical component of canonical LPS-TLR4 signaling pathway, and these studies also suggest that p204 could be a potential target to prevent and treat inflammatory and infectious diseases.

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p204 deficiency leads to significant defect in extracellular LPS signaling in macrophages.

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Serum levels of IFN-β and pro-inflammatory cytokines were also significantly reduced in p204^-/- mice following LPS challenge.

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p204^-/- mice were resistant to LPS-induced shock.

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p204 binds to TLR4 through its Pyrin domain, and it is required for the dimerization of TLR4 following LPS-challenge.

Effective anti-pathogenic responses, including production of type I IFNs and inflammatory response, are critical for host defense. p200 family members, including IFI16 and AIM2, have been reported to function as the sensors of pathogen components. However, investigation of their roles has largely focused on intracellular pathogen components, independent of extracellular pathogen receptors, such as TLRs. Here, we provide unexpected evidences demonstrating that p204, a murine counterpart of human IFI16, is required for extracellular but not intracellular LPS signaling. These results provide not only evidence of functional crosstalk and cooperation between intracellular p204 and extracellular LPS through TLR4 pathways in macrophage-mediated innate immunity, but also new insights into the mechanisms underlying p200 family proteins mediated antiviral and antibacterial infections.

Fluid shear stress induces osteoblast differentiation and arrests the cell cycle at the G0 phase via the ERK1/2 pathway

Numerous studies have demonstrated that fluid shear stress (FSS) may promote the proliferation and differentiation of osteoblast cells. However, proliferation and differentiation are mutually exclusive processes and are unlikely to be promoted by FSS simultaneously. Cell proliferation and differentiation induced by FSS has rarely been reported. In order to provide an insight into this process, the present study investigated the effects of FSS on osteoblast‑like MC3T3 cells in the G0/G1 phase, the period during which the fate of a cell is determined. The results of the present study demonstrated that FSS promoted alkaline phosphatase (ALP) activity, and the mRNA expression and protein expression of osteocalcin, collagen type I and runt‑related transcription factor 2 (Runx2), while inhibiting DNA synthesis and arresting the cell cycle at the G0/G1 phase. The increase in Runx2 and ALP activity was accompanied by the activation of calcium/calmodulin‑dependent protein kinase type II (CaMK II) and extracellular signal‑regulated kinases 1/2 (ERK1/2), which was completely abolished by treatment with KN93 and U0126, respectively. In addition, the inhibition of ERK1/2, although not CaMK II, decreased p21Cip/Kip activity, resulting in an increase in cell number and S phase re‑entry. The results of the present study indicated that in the G0/G1 phase, FSS promoted osteoblast differentiation via the CaMK II and ERK1/2 signaling pathways, and blocked the cell cycle at the G0/G1 phase via the ERK1/2 pathway only. The present findings provided an increased understanding of osteoblastic mechanobiology.

STING Contributes to Abnormal Bone Formation Induced by Deficiency of DNase II in Mice

OBJECTIVE

Cytosolic DNA sensors detect microbial DNA and promote type I interferon (IFN) and proinflammatory cytokine production through the adaptor stimulator of IFN genes (STING) to resolve infection. Endogenous DNA also engages the STING pathway, contributing to autoimmune disease. This study sought to identify the role of STING in regulating bone formation and to define the bone phenotype and its pathophysiologic mechanisms in arthritic mice double deficient in DNase II and IFN-α/β/ω receptor (IFNAR) (DNase II^-/- /IFNAR^-/- double-knockout [DKO] mice) compared with controls.

METHODS

Bone parameters were evaluated by micro-computed tomography and histomorphometry in DKO mice in comparison with mice triple deficient in STING, DNase II, and IFNAR and control mice. Cell culture techniques were employed to determine the parameters of osteoclast and osteoblast differentiation and function. NanoString and Affymetrix array analyses were performed to identify factors promoting ectopic bone formation.

RESULTS

Despite the expression of proinflammatory cytokines that would be expected to induce bone loss in the skeleton of DKO mice, the results, paradoxically, demonstrated an accumulation of bone in the long bones and spleens, sites of erythropoiesis and robust DNA accrual. In addition, factors promoting osteoblast recruitment and function were induced. Deficiency of STING significantly inhibited bone accrual.

CONCLUSION

These data reveal a novel role for cytosolic DNA sensor pathways in bone in the setting of autoimmune disease. The results demonstrate the requirement of an intact STING pathway for bone formation in this model, a finding that may have relevance to autoimmune diseases in which DNA plays a pathogenic role. Identification of pathways linking innate immunity and bone could reveal novel targets for the treatment of bone abnormalities in human autoimmune diseases.

Regulatory roles of interferon-inducible protein 204 on differentiation and vasculogenic activity of endothelial progenitor cells

BACKGROUND

Endothelial progenitor cells (EPCs) have shown great potential in angiogenesis either by their differentiation into endothelial cells or by secretion of angiogenic factors. Interferon-inducible protein 204 (Ifi204) has been reported to participate in the regulation of cell growth and differentiation. However, its role in differentiation of EPCs remains unknown. We proposed that Ifi204 could modulate the differentiation and regenerative abilities of EPCs.

METHODS

Ifi204-expressing lentivirus and Ifi204 siRNA were introduced into EPCs to overexpress and suppress the expression of Ifi204. Using fluorescence-activated cell sorting, immunocytochemistry, and quantitative PCR, endothelial markers including CD31, VE-cadherin, and vWF were detected in the modified EPCs. An in-vitro incorporation assay and a colony-forming assay were also performed.

RESULTS

Evidence showed that Ifi204 inhibition decreased the endothelial differentiation and vasculogenic activities of EPCs in vitro. In mice with hindlimb ischemia, downregulation of Ifi204 in EPCs, which was tracked by our newly synthesized nanofluorogen, impaired neovascularization, with a corresponding reduction in hindlimb blood reperfusion by postoperative day 14.

CONCLUSIONS

Ifi204 is required for EPC differentiation and neovascularization in vitro and in vivo. The regulatory roles of Ifi204 in EPC differentiation may benefit the clinical therapy of ischemic vascular diseases.

The roles of interferon-inducible p200 family members IFI16 and p204 in innate immune responses, cell differentiation and proliferation

p204 is a member of the interferon-inducible p200 family proteins in mice. The p200 family has been reported to be multifunctional regulators of cell proliferation, differentiation, apoptosis and senescence. Interferon-inducible protein 16 (IFI16) is regarded as the human ortholog of p204 in several studies. This is possibly due to the similarity of their structures. However the consistency of their functions is still elusive. Currently, an emerging focus has been placed upon the role of the p200 proteins as sensors for microbial DNA in innate immune responses and provides new insights into infections as well as autoimmune diseases. This review specially focuses on IFI16 and p204, the member of p200 family in human and murine respectively, and their pathophysiological roles in innate immune responses, cell differentiation and proliferation.

p107-Dependent recruitment of SWI/SNF to the alkaline phosphatase promoter during osteoblast differentiation

The retinoblastoma protein family is intimately involved in the regulation of tissue specific gene expression during mesenchymal stem cell differentiation. The role of the following proteins, pRB, p107 and p130, is particularly significant in differentiation to the osteoblast lineage, as human germ-line mutations of RB1 greatly increase susceptibility to osteosarcoma. During differentiation, pRB directly targets certain osteogenic genes for activation, including the alkaline phosphatase-encoding gene Alpl. Chromatin immunoprecipitation (ChIP) assays indicate that Alpl is targeted by p107 in differentiating osteoblasts selectively during activation with the same dynamics as pRB, which suggests that p107 helps promote Alpl activation. Mouse models indicate overlapping roles for pRB and p107 in bone and cartilage formation, but very little is known about direct tissue-specific gene targets of p107, or the consequences of targeting by p107. Here, the roles of p107 and pRB were compared using shRNA-mediated knockdown genetics in an osteoblast progenitor model, MC3T3-E1 cells. The results show that p107 has a distinct role along with pRB in induction of Alpl. Deficiency of p107 does not impede recruitment of transcription factors recognized as pRB co-activation partners at the promoter; however, p107 is required for the efficient recruitment of an activating SWI/SNF chromatin-remodeling complex, an essential event in Alpl induction.

IFI16 restricts HSV-1 replication by accumulating on the hsv-1 genome, repressing HSV-1 gene expression, and directly or indirectly modulating histone modifications

Interferon-γ inducible factor 16 (IFI16) is a multifunctional nuclear protein involved in transcriptional regulation, induction of interferon-β (IFN-β), and activation of the inflammasome response. It interacts with the sugar-phosphate backbone of dsDNA and modulates viral and cellular transcription through largely undetermined mechanisms. IFI16 is a restriction factor for human cytomegalovirus (HCMV) and herpes simplex virus (HSV-1), though the mechanisms of HSV-1 restriction are not yet understood. Here, we show that IFI16 has a profound effect on HSV-1 replication in human foreskin fibroblasts, osteosarcoma cells, and breast epithelial cancer cells. IFI16 knockdown increased HSV-1 yield 6-fold and IFI16 overexpression reduced viral yield by over 5-fold. Importantly, HSV-1 gene expression, including the immediate early proteins, ICP0 and ICP4, the early proteins, ICP8 and TK, and the late proteins gB and Us11, was reduced in the presence of IFI16. Depletion of the inflammasome adaptor protein, ASC, or the IFN-inducing transcription factor, IRF-3, did not affect viral yield. ChIP studies demonstrated the presence of IFI16 bound to HSV-1 promoters in osteosarcoma (U2OS) cells and fibroblasts. Using CRISPR gene editing technology, we generated U2OS cells with permanent deletion of IFI16 protein expression. ChIP analysis of these cells and wild-type (wt) U2OS demonstrated increased association of RNA polymerase II, TATA binding protein (TBP) and Oct1 transcription factors with viral promoters in the absence of IFI16 at different times post infection. Although IFI16 did not alter the total histone occupancy at viral or cellular promoters, its absence promoted markers of active chromatin and decreased those of repressive chromatin with viral and cellular gene promoters. Collectively, these studies for the first time demonstrate that IFI16 prevents association of important transcriptional activators with wt HSV-1 promoters and suggest potential mechanisms of IFI16 restriction of wt HSV-1 replication and a direct or indirect role for IFI16 in histone modification.

Wanderings in biochemistry

My Ph.D. thesis in the laboratory of Severo Ochoa at New York University School of Medicine in 1962 included the determination of the nucleotide compositions of codons specifying amino acids. The experiments were based on the use of random copolyribonucleotides (synthesized by polynucleotide phosphorylase) as messenger RNA in a cell-free protein-synthesizing system. At Yale University, where I joined the faculty, my co-workers and I first studied the mechanisms of protein synthesis. Thereafter, we explored the interferons (IFNs), which were discovered as antiviral defense agents but were revealed to be components of a highly complex multifunctional system. We isolated pure IFNs and characterized IFN-activated genes, the proteins they encode, and their functions. We concentrated on a cluster of IFN-activated genes, the p200 cluster, which arose by repeated gene duplications and which encodes a large family of highly multifunctional proteins. For example, the murine protein p204 can be activated in numerous tissues by distinct transcription factors. It modulates cell proliferation and the differentiation of a variety of tissues by binding to many proteins. p204 also inhibits the activities of wild-type Ras proteins and Ras oncoproteins.

Explore on the effect of ATF6 on cell growth and apoptosis in cartilage development

We previously report that BMP2 mediates mild ER stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. The mammalian unfolded protein response (UPR) protects the cell against the stress of misfolded proteins in the endoplasmic reticulum (ER). Failure to adapt to ER stress causes the UPR to trigger apoptosis. The transcription factor activating transcription factor 6 (ATF6), a key regulator of the UPR, is known to be important for ER stress-mediated apoptosis and cell growth, but the molecular mechanism underlying these processes remains unexplored. In this study, we demonstrate that ATF6 is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. ATF6 can enhance the level of IRE1a-spliced XBP1S protein in chondrogenesis. IRE1a and ATF6 can synergistically regulate endogenous XBP1S gene expression in chondrogenesis. Furthermore, overexpression ATF6 inhibited, while ATF6-knockdown enhanced, the cell proliferation in chondrocyte development with G1 phase arresting, S phase reducing and G2-M phase delaying. Besides, Ad-ATF6 can activate, whereas knockdown ATF6 by an siRNA-silencing approach inhibited, ER stress-mediated apoptosis in chondrogenesis induced by BMP2, as assayed by cleaved caspase3, CHOP, p-JNK expression in the course of chondrocyte differentiation. On the other hand, FCM, TUNEL assay and immunohistochemistry analysis also proved this result in vitro and in vivo. It was demonstrated that Ad-ATF6 activation of the ER stress-specific caspase cascade in developing chondrocyte tissue. Collectively, these findings reveal a novel critical role of ATF6 in regulating ER stress-mediated apoptosis in chondrocyte differentiation and the molecular mechanisms involved.

XBP1S, a BMP2-inducible transcription factor, accelerates endochondral bone growth by activating GEP growth factor

We previously reported that transcription factor XBP1S binds to RUNX2 and enhances chondrocyte hypertrophy through acting as a cofactor of RUNX2. Herein, we report that XBP1S is a key downstream molecule of BMP2 and is required for BMP2-mediated chondrocyte differentiation. XBP1S is up-regulated during chondrocyte differentiation and demonstrates the temporal and spatial expression pattern during skeletal development. XBP1S stimulates chondrocyte differentiation from mesenchymal stem cells in vitro and endochondral ossification ex vivo. In addition, XBP1S activates granulin-epithelin precursor (GEP), a growth factor known to stimulate chondrogenesis, and endogenous GEP is required, at least in part, for XBP1S-stimulated chondrocyte hypertrophy, mineralization and endochondral bone formation. Furthermore, XBP1S enhances GEP-stimulated chondrogenesis and endochondral bone formation. Collectively, these findings demonstrate that XBP1S, a BMP2-inducible transcription factor, positively regulates endochondral bone formation by activating GEP chondrogenic growth factor.

ATF6 upregulates XBP1S and inhibits ER stress-mediated apoptosis in osteoarthritis cartilage

As we previously reported, transcription factor XBP1S enhances BMP2-induced chondrocyte differentiation and acts as a positive mediator of chondrocyte hypertrophy. The purpose of this study was to determine (1) whether XBP1S influences ER stress-mediated apoptosis in osteoarthritis (OA); (2) whether ATF6 regulates IRE1/XBP1 signal pathway in OA cartilage; (3) what are the associated molecules affecting apoptosis in osteoarthritis and the molecular events underlying this process. Herein, we examined and found that ER stress-associated molecules were activated in OA patients, specifically XBP1S splice and expression were increased markedly by TNF-α and IL-1β treatments. Transcription factor ATF6 can specifically bind to the promoter of XBP1 gene and enhance the expression of XBP1S spliced by IRE1α in osteoarthritis cartilage. Furthermore, siXBP1S can enhance ER stress-mediated apoptosis and main matrix degradation in osteoarthritis. Whereas AdXBP1S can inhibit ER stress-mediated apoptosis and TNFα induced nitrite production in OA cartilage. In a word, our observations demonstrate the importance of XBP1S in osteoarthritis. ATF6 and IRE1α can regulate endogenous XBP1S gene expression synergistically in OA cartilage. More significantly, XBP1S was a negative regulator of apoptosis in osteoarthritis by affecting caspase 3, caspase 9, caspase 12, p-JNK1, and CHOP.

RETINOBLASTOMA-RELATED protein stimulates cell differentiation in the Arabidopsis root meristem by interacting with cytokinin signaling

Maintenance of mitotic cell clusters such as meristematic cells depends on their capacity to maintain the balance between cell division and cell differentiation necessary to control organ growth. In the Arabidopsis thaliana root meristem, the antagonistic interaction of two hormones, auxin and cytokinin, regulates this balance by positioning the transition zone, where mitotically active cells lose their capacity to divide and initiate their differentiation programs. In animals, a major regulator of both cell division and cell differentiation is the tumor suppressor protein RETINOBLASTOMA. Here, we show that similarly to its homolog in animal systems, the plant RETINOBLASTOMA-RELATED (RBR) protein regulates the differentiation of meristematic cells at the transition zone by allowing mRNA accumulation of AUXIN RESPONSE FACTOR19 (ARF19), a transcription factor involved in cell differentiation. We show that both RBR and the cytokinin-dependent transcription factor ARABIDOPSIS RESPONSE REGULATOR12 are required to activate the transcription of ARF19, which is involved in promoting cell differentiation and thus root growth.

Quantitative trait loci for bone mineral density and femoral morphology in an advanced intercross population of mice

Osteoporosis, characterized by low levels of bone mineral density (BMD), is a prevalent medical condition in humans. We investigated its genetic and environmental basis by searching for quantitative trait loci (QTLs) affecting six skeletal (including three BMD) traits in a G10 advanced intercross population produced from crosses of mice from the inbred strain C57BL/6J with mice from a strain selected for high voluntary wheel running. The mice in this population were fed either a high-fat or a matched control diet throughout the study, allowing us to test for QTL by diet interactions for the skeletal traits. Our genome scan uncovered a number of QTLs, the great majority of which were different from QTLs previously found for these same traits in an earlier (G4) generation of the same intercross. Further, the confidence intervals for the skeletal trait QTLs were reduced from an average of 18.5 Mb in the G4 population to an equivalent of about 9 Mb in the G10 population. We uncovered a total of 50 QTLs representing 32 separate genomic sites affecting these traits, with a distal region on chromosome 1 harboring several QTLs with large effects on the BMD traits. One QTL was located on chromosome 5 at 4.0 Mb with a confidence interval spanning from 4.0 to 4.6 Mb. Only three protein coding genes reside in this interval, and one of these, Cyp51, is an attractive candidate as others have shown that developing Cyp51 knockout embryos exhibit shortened and bowed limbs and synotosis of the femur and tibia. Several QTLs showed significant interactions with sex, although only two QTLs interacted with diet, both affecting only mice fed the high-fat diet.

Cooperative activation of tissue-specific genes by pRB and E2F1

The retinoblastoma tumor suppressor protein pRB is conventionally regarded as an inhibitor of the E2F family of transcription factors. Conversely, pRB is also recognized as an activator of tissue-specific gene expression along various lineages including osteoblastogenesis. During osteoblast differentiation, pRB directly targets Alpl and Bglap, which encode the major markers of osteogenesis alkaline phosphatase and osteocalcin. Surprisingly, p130 and repressor E2Fs were recently found to cooccupy and repress Alpl and Bglap in proliferating osteoblast precursors before differentiation. This raises the further question of whether these genes convert to E2F activation targets when differentiation begins, which would constitute a remarkable situation wherein pRB and E2F would be cotargeting genes for activation. Chromatin immunoprecipitation analysis in an osteoblast differentiation model shows that Alpl and Bglap are indeed targeted by an activator E2F, i.e., is E2F1. Promoter occupation of Alpl and Bglap by E2F1 occurs specifically during activation, and depletion of E2F1 severely impairs their induction. Mechanistically, promoter occupation by E2F1 and pRB is mutually dependent, and without this cooperative effect, activation steps previously shown to be dependent on pRB, including recruitment of RNA polymerase II, are impaired. Myocyte- and adipocyte-specific genes are also cotargeted by E2F1 and pRB during differentiation along their respective lineages. The finding that pRB and E2F1 cooperate to activate expression of tissue-specific genes is a paradigm distinct from the classical concept of pRB as an inhibitor of E2F1, but is consistent with the observed roles of these proteins in physiological models.

Transcriptional regulatory cascades in Runx2-dependent bone development

The development of the musculoskeletal system is a complex process that involves very precise control of bone formation and growth as well as remodeling during postnatal life. Although the understanding of the transcriptional mechanisms of osteogenesis has increased considerably, the molecular regulatory basis, especially the gene regulatory network of osteogenic differentiation, is still poorly understood. This review provides the reader with an overview of the key transcription factors that govern bone formation, highlighting their function and regulation linked to Runt-related transcription factor 2 (Runx2). Runx2 as the master transcription factor of osteoblast differentiation, Twist, Msh homeobox 2 (Msx2), and promyelocytic leukemia zinc-finger protein (PLZF) acting upstream of Runx2, Osterix (Osx) acting downstream of Runx2, and activating transcription factor 4 (ATF4) and zinc-finger protein 521 (ZFP521) acting as cofactors of Runx2 are discussed, and their relevance for tissue engineering is presented. References are provided for more in-depth personal study.

XBP1S associates with RUNX2 and regulates chondrocyte hypertrophy

BMP2 (bone morphogenetic protein 2) is known to activate unfolded protein response signaling molecules, including XBP1S and ATF6. However, the influence on XBP1S and ATF6 in BMP2-induced chondrocyte differentiation has not yet been elucidated. In this study, we demonstrate that BMP2 mediates mild endoplasmic reticulum stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. XBP1S is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. This expression is probably due to the activation of the XBP1 gene by ATF6 and splicing by IRE1a. ATF6 directly binds to the 5'-flanking regulatory region of the XBP1 gene at its consensus binding elements. Overexpression of XBP1S accelerates chondrocyte hypertrophy, as revealed by enhanced expression of type II collagen, type X collagen, and RUNX2; however, knockdown of XBP1S via the RNAi approach abolishes hypertrophic chondrocyte differentiation. In addition, XBP1S associates with RUNX2 and enhances RUNX2-induced chondrocyte hypertrophy. Altered expression of XBP1S in chondrocyte hypertrophy was accompanied by altered levels of IHH (Indian hedgehog) and PTHrP (parathyroid hormone-related peptide). Collectively, XBP1S may be a novel regulator of hypertrophic chondrocyte differentiation by 1) acting as a cofactor of RUNX2 and 2) affecting IHH/PTHrP signaling.

Tissue-specific gene targeting by the multiprotein mammalian DREAM complex

The mammalian DP, RB-like, E2F, and MuvB-like proteins (DREAM) complex, whose key components include p130 and E2F4, plays a fundamental role in repression of cell cycle-specific genes during growth arrest. Mammalian DREAM is well conserved with Drosophila and Caenorhabditis elegans complexes that repress pivotal developmental genes, but the mammalian complex has been thought to exist only in quiescent cells and not to be linked with development. However, new findings here identify tissue-specific promoters repressed by DREAM in proliferating precursors, revealing a new connection between control of growth arrest and terminal differentiation. Mechanistically, tissue-specific promoter occupation by DREAM is dependent on the integrity of a repressor form of the SWI/SNF chromatin-remodeling complex.

Disruption of calvarial ossification in E2f4 mutant embryos correlates with increased proliferation and progenitor cell populations

The E2F family of transcription factors, in association with pocket protein family members, are important for regulating genes required for cellular proliferation. The most abundant E2F, E2F4, is implicated in maintaining the G(0)/G(1) cell cycle state via transcriptional repression of genes that encode proteins required for S-phase progression. Here, we investigate E2F4's role in bone development using E2f4 germline mutant mice. We find that mutation of E2f4 impairs the formation of several bones that arise through intramembranous or endochondral ossification. The most severe defect occurred in the calvarial bones of the skull where we observed a striking delay in their ossification. In vivo and in vitro analyses established that E2F4 loss did not block the intrinsic differentiation potential of calvarial osteoblast progenitors. However, our data showed that E2f4 mutation elevated proliferation in the developing calvaria in vivo and it increased the endogenous pool of undifferentiated progenitor cells. These data suggest that E2F4 plays an important role in enabling osteoblast progenitors to exit the cell cycle and subsequently differentiate thereby contributing to the commitment of these cells to the bone lineage.

TIEG1/KLF10 modulates Runx2 expression and activity in osteoblasts

Deletion of TIEG1/KLF10 in mice results in a gender specific osteopenic skeletal phenotype with significant defects in both cortical and trabecular bone, which are observed only in female animals. Calvarial osteoblasts isolated from TIEG1 knockout (KO) mice display reduced expression levels of multiple bone related genes, including Runx2, and exhibit significant delays in their mineralization rates relative to wildtype controls. These data suggest that TIEG1 plays an important role in regulating Runx2 expression in bone and that decreased Runx2 expression in TIEG1 KO mice is in part responsible for the observed osteopenic phenotype. In this manuscript, data is presented demonstrating that over-expression of TIEG1 results in increased expression of Runx2 while repression of TIEG1 results in suppression of Runx2. Transient transfection and chromatin immunoprecipitation assays reveal that TIEG1 directly binds to and activates the Runx2 promoter. The zinc finger containing domain of TIEG1 is necessary for this regulation supporting that activation occurs through direct DNA binding. A role for the ubiquitin/proteasome pathway in fine tuning the regulation of Runx2 expression by TIEG1 is also implicated in this study. Additionally, the regulation of Runx2 expression by cytokines such as TGFβ1 and BMP2 is shown to be inhibited in the absence of TIEG1. Co-immunoprecipitation and co-localization assays indicate that TIEG1 protein associates with Runx2 protein resulting in co-activation of Runx2 transcriptional activity. Lastly, Runx2 adenoviral infection of TIEG1 KO calvarial osteoblasts leads to increased expression of Runx2 and enhancement of their ability to differentiate and mineralize in culture. Taken together, these data implicate an important role for TIEG1 in regulating the expression and activity of Runx2 in osteoblasts and suggest that decreased expression of Runx2 in TIEG1 KO mice contributes to the observed osteopenic bone phenotype.

BMD regulation on mouse distal chromosome 1, candidate genes, and response to ovariectomy or dietary fat

The distal end of mouse chromosome 1 (Chr 1) harbors quantitative trait loci (QTLs) that regulate bone mineral density (BMD) and share conserved synteny with human chromosome 1q. The objective of this article was to map this mouse distal Chr 1 region and identify gene(s) responsible for BMD regulation in females. We used X-ray densitometry [ie, dual-energy X-ray Absorptiometry (DXA), micro-computed tomography (µCT), and peripheral quantitative computed tomography (pQCT)] to phenotype a set of nested congenic strains constructed from C57BL/6BmJ (B6/Bm) and C3H/HeJ (C3H) mice to map the region associated with the BMD QTL. The critical region has been reduced to an interval of 0.152 Mb that contributes to increased BMD when C3H alleles are present. Histomorphometry and osteoblast cultures indicated that increased osteoblast activity was associated with increased BMD in mouse strains with C3H alleles in this critical region. This region contains two genes, Aim2, which binds with cytoplasmic dsDNA and results in apoptosis, and AC084073.22, a predicted gene of unknown function. Ovariectomy induced bone loss in the B6/Bm progenitor and the three congenic strains regardless of the alleles present in the critical BMD region. High dietary fat treatment (thought to suppress distal Chr 1 QTL for BMD in mice) did not induce bone loss in the congenics carrying C3H alleles in the critical 0.152 Mb carrying the AIM2 and AC084073.22 genes. Gene expression studies in whole bone of key congenics showed differential expression of AC084073.22 for strains carrying B6/Bm versus C3H alleles but not for Aim2. In conclusion, our data suggest that osteoblasts are the cellular target of gene action and that AC084073.22 is the best candidate for female BMD regulation in the distal region of mouse Chr 1.

Transcriptional activation by pRB and its coordination with SWI/SNF recruitment

A central question in cancer biology is why most tumor susceptibility genes are linked with only limited types of cancer. Human germ-line mutation of the retinoblastoma susceptibility gene Rb1 is closely linked with just retinoblastoma and osteosarcoma, although the gene is universally expressed. Functional analysis of pRB and its close relatives, p107 and p130, has largely focused on their roles in repression of proliferation across all tissue types, but genetic evidence indicates an active requirement for pRB in osteoblast differentiation that correlates more directly with osteosarcoma susceptibility. Still, potential promoter targets of pRB and its role in normally differentiating osteoblasts remain insufficiently characterized. Here, an early marker of osteoblast differentiation, alkaline phosphatase, is identified as a direct promoter activation target of pRB. One role of pRB on this promoter is to displace the histone lysine demethylase KDM5A, thereby favoring trimethylation of H3K4, a promoter activation mark. A major new aspect of pRB-mediated transcriptional activation revealed in this promoter analysis is its role in recruitment of an activating SWI/SNF chromatin-remodeling complex. SWI/SNF is a critical coordinator of tissue-specific gene expression. In osteoblasts, SWI/SNF complexes containing the BRM ATPase repress osteoblast-specific genes to maintain the precursor state, whereas the alternative ATPase BRG1 distinguishes an activating SWI/SNF complex necessary for RNA polymerase-II recruitment. A switch from BRM to BRG1 on the alkaline phosphatase promoter marks the onset of differentiation and is accomplished in a precise two-step mechanism. Dissociation of BRM-containing SWI/SNF depends on p300, and association of BRG1-containing SWI/SNF depends on pRB.

Transient expression of interferon-inducible p204 in the early stage is required for adipogenesis in 3T3-L1 cells

A member of the interferon-inducible p200 family of proteins, p204, has recently been reported to function in the development of many mesoderm-derived tissues, such as bone, muscle, and cartilage. However, no published study has yet investigated the role of p204 in adipogenesis. Our preliminary experiments showed that p204 can be found in 3T3-L1 preadipocytes, and its expression was up-regulated in a differentiation-dependent manner. As such, we hypothesized that p204 is associated with adipogenesis and focused on the influence of p204 on adipogenesis. In the present study, we investigated the transient elevated expression and cytoplasm-to-nucleus translocation of p204 in the early stage of adipogenesis. To determine the effect of p204 on adipogenesis, p204-siRNA and expression vector were produced for p204 suppression and overexpression, respectively. The knockdown of p204 resulted in a significantly depressed adipocyte differentiation, whereas p204 overexpression promoted adipocyte differentiation. The mRNA expression of adipogenic markers, such as peroxisome-proliferator-activated receptor (PPAR)gamma, CCAAT/enhancer-binding-protein (C/EBP)alpha, lipoprotein lipase, and adipsin, was decreased by p204 suppression and increased by p204 overexpression. A coimmunoprecipitation assay coupled with an indirect immunofluorescence assay also indicated that p204 interacted and colocalized with C/EBPdelta in the nucleus. Furthermore, the knockdown of p204 disrupted the interaction between p204 and C/EBPdelta and partially suppressed the PPARgamma transcriptional activity by dissociating C/EBPdelta with the PPARgamma promoter element. Collectively, our data indicate that the transient expression of p204 in the early stage is indispensable for adipocyte differentiation. Disruption of p204 expression patterns at this stage leads to irreversible damage in fat formation.

Restoration of regenerative osteoblastogenesis in aged mice: modulation of TNF

Skeletal changes accompanying aging are associated with both increased risk of fractures and impaired fracture healing, which, in turn, is due to compromised bone regeneration potential. These changes are associated with increased serum levels of selected proinflammatory cytokines, e.g., tumor necrosis factor alpha (TNF-alpha). We have used a unique model of bone regeneration to demonstrate (1) that aged-related deficits in direct bone formation can be restored to young mice by treatment with TNF blockers and (2) that the cyclin-dependent kinase inhibitor p21 is a candidate for mediation of the osteoinhibitory effects of TNF. It has been hypothesized recently that TNF antagonists may represent novel anabolic agents, and we believe that the data presented here represent a successful test of this hypothesis.

The p200 family protein p204 as a modulator of cell proliferation and differentiation: a brief survey

The expression of the murine p200 family protein p204 in numerous tissues can be activated by a variety of distinct, tissue-specific transcription factors. p204 modulates cell proliferation, cell cycling, and the differentiation of various tissues, including skeletal muscle myotubes, beating cardiac myocytes, osteoblasts, chondrocytes, and macrophages. This protein modulates these processes in various ways, such as by (1) blocking ribosomal RNA synthesis in the nucleolus, (2) inhibiting Ras signaling in the cytoplasm, (3) promoting the activity of particular transcription factors in the nucleus by forming complexes with them, and (4) overcoming the block of the activity of other transcription factors by inhibitor of differentiation (Id) proteins. Much remains to be learned about p204, particularly with respect to its expected involvement in the differentiation of several as yet unexplored tissues.

Concerted stimuli regulating osteo-chondral differentiation from stem cells: phenotype acquisition regulated by microRNAs

Bone and cartilage are being generated de novo through concerted actions of a plethora of signals. These act on stem cells (SCs) recruited for lineage-specific differentiation, with cellular phenotypes representing various functions throughout their life span. The signals are rendered by hormones and growth factors (GFs) and mechanical forces ensuring proper modelling and remodelling of bone and cartilage, due to indigenous and programmed metabolism in SCs, osteoblasts, chondrocytes, as well as osteoclasts and other cell types (eg T helper cells).This review focuses on the concerted action of such signals, as well as the regulatory and/or stabilizing control circuits rendered by a class of small RNAs, designated microRNAs. The impact on cell functions evoked by transcription factors (TFs) via various signalling molecules, also encompassing mechanical stimulation, will be discussed featuring microRNAs as important members of an integrative system. The present approach to cell differentiation in vitro may vastly influence cell engineering for in vivo tissue repair.

miR-199a, a bone morphogenic protein 2-responsive MicroRNA, regulates chondrogenesis via direct targeting to Smad1

MicroRNAs (miRNA) are short non-coding RNA molecules that regulate a variety of biological processes. The role of miRNAs in BMP2-mediated biological processes is of considerable interest. A comparative miRNA array led to the isolation of several BMP2-responsive miRNAs. Among them, miR-199a(*) is of particular interest, because it was reported to be specifically expressed in the skeletal system. Here we demonstrate that miR-199a(*) is an early responsive target of BMP2: its level was dramatically reduced at 5 h, quickly increased at 24 h and remained higher thereafter in the course of BMP2-triggered chondrogenesis of a micromass culture of pluripotent C3H10T1/2 stem cells. miR-199a(*) significantly inhibited early chondrogenesis, as revealed by the reduced expression of early marker genes for chondrogenesis such as cartilage oligomeric matrix protein (COMP), type II collagen, and Sox9, whereas anti-miR-199a(*) increased the expression of these chondrogenic marker genes. A computer-based prediction algorithm led to the identification of Smad1, a well established downstream molecule of BMP-2 signaling, as a putative target of miR-199a(*). The pattern of Smad1 mRNA expression exhibited the mirror opposite of miR-199a(*) expression following BMP-2 induction. Furthermore, miR-199a(*) demonstrated remarkable inhibition of both endogenous Smad1 as well as a reporter construct bearing the 3-untranslated region of Smad1 mRNA. In addition, mutation of miR-199a(*) binding sites in the 3'-untranslated region of Smad1 mRNA abolished miR-199a(*)-mediated repression of reporter gene activity. Mechanism studies revealed that miR-199a(*) inhibits Smad1/Smad4-mediated transactivation of target genes, and that overexpression of Smad1 completely corrects miR-199a(*)-mediated repression of early chondrogenesis. Taken together, miR-199a(*) is the first BMP2 responsive microRNA found to adversely regulate early chondrocyte differentiation via direct targeting of the Smad1 transcription factor.

p204, a p200 family protein, as a multifunctional regulator of cell proliferation and differentiation

The interferon-inducible p200 family comprises a group of homologous mouse and human proteins. Most of these have an N-terminal DAPIN domain and one or two partially conserved, 200 amino acid long C-terminal domains (designated as 200X domain). These proteins play important roles in the regulation of cell proliferation, tissue differentiation, apoptosis and senescence. p200 family proteins are involved also in autoimmunity and the control of tumor growth. These proteins function by binding to various target proteins (e.g. transcription factors, signaling proteins, oncoproteins and tumor suppressor proteins) and modulating target activity. This review concentrates on p204, a murine member of the family and its roles in regulating cell proliferation, cell and tissue differentiation (e.g. of skeletal muscle myotubes, beating cardiac myocytes, osteoblasts, chondrocytes and macrophages) and signaling by Ras proteins. The expression of p204 in various tissues as promoted by tissue-specific transcription factors, its distribution among subcellular compartments, and the controls of these features are also discussed.

The retinoblastoma protein tumor suppressor is important for appropriate osteoblast differentiation and bone development

Mutation of the retinoblastoma (RB) tumor suppressor gene is strongly linked to osteosarcoma formation. This observation and the documented interaction between the retinoblastoma protein (pRb) and Runx2 suggests that pRb is important in bone development. To assess this hypothesis, we used a conditional knockout strategy to generate pRb-deficient embryos that survive to birth. Analysis of these embryos shows that Rb inactivation causes the abnormal development and impaired ossification of several bones, correlating with an impairment in osteoblast differentiation. We further show that Rb inactivation acts to promote osteoblast differentiation in vitro and, through conditional analysis, establish that this occurs in a cell-intrinsic manner. Although these in vivo and in vitro differentiation phenotypes seem paradoxical, we find that Rb-deficient osteoblasts have an impaired ability to exit the cell cycle both in vivo and in vitro that can explain the observed differentiation defects. Consistent with this observation, we show that the cell cycle and the bone defects in Rb-deficient embryos can be suppressed by deletion of E2f1, a known proliferation inducer that acts downstream of Rb. Thus, we conclude that pRb plays a key role in regulating osteoblast differentiation by mediating the inhibition of E2F and consequently promoting cell cycle exit.

Rb depletion results in deregulation of E-cadherin and induction of cellular phenotypic changes that are characteristic of the epithelial-to-mesenchymal transition

The retinoblastoma tumor suppressor protein (Rb) is mutated or expressed at very low levels in several tumor types, including retinoblastoma and osteosarcoma, as well as small cell lung, colon, prostate, bladder, and breast carcinomas. Loss or reduction of Rb expression is seen most commonly in high-grade breast adenocarcinomas, suggesting that a relationship may exist between loss of Rb function and a less-differentiated state, increased proliferation, and high metastatic potential. In this study, we found that knockdown of Rb by small interfering RNA in MCF7 breast cancer cells disrupts cell-cell adhesion and induces a mesenchymal-like phenotype. The epithelial-to-mesenchymal transition (EMT), a key event in embryonic morphogenesis, is implicated in the metastasis of primary tumors. Additionally, Rb is decreased during growth factor- and cytokine-induced EMT and overexpression of Rb inhibits the EMT in MCF10A human mammary epithelial cells. Ectopic expression and knockdown of Rb resulted in increased or reduced expression of E-cadherin, which is specifically involved in epithelial cell-cell adhesion. Other EMT-related transcriptional factors, including Slug and Zeb-1, are also induced by Rb depletion. Furthermore, we confirmed that Rb binds to an E-cadherin promoter sequence in association with the transcription factor activator protein-2alpha. Finally, in breast cancer specimens, we observed a concurrent down-regulation of Rb and E-cadherin expression in mesenchymal-like invasive cancers. These findings suggest that Rb inactivation contributes to tumor progression due to not only loss of cell proliferation control but also conversion to an invasive phenotype and that the inhibition of EMT is a novel tumor suppressor function of Rb.

Transcription factors controlling osteoblastogenesis

The recent development of molecular biology and mouse genetics and the analysis of the skeletal phenotype induced by genetic mutations in humans led to a better understanding of the role of transcription factors that govern bone formation. This review summarizes the role of transcription factors in osteoblastogenesis and provides an integrated perspective on how the activities of multiple classes of factors are coordinated for the complex process of developing the osteoblast phenotype. The roles of Runx2, the principal transcriptional regulator of osteoblast differentiation, Osterix, beta-Catenin and ATF which act downstream of Runx2, and other transcription factors that contribute to the control of osteoblastogenesis including the AP1, C/EBPs, PPARgamma and homeodomain, helix-loop-helix proteins are discussed. This review also updates the regulation of transcription factor expression by signaling factors and hormones that control osteoblastogenesis.

p204 protein overcomes the inhibition of core binding factor alpha-1-mediated osteogenic differentiation by Id helix-loop-helix proteins

Id proteins play important roles in osteogenic differentiation; however, the molecular mechanism remains unknown. In this study, we established that inhibitor of differentiation (Id) proteins, including Id1, Id2, and Id3, associate with core binding factor alpha-1 (Cbfa1) to cause diminished transcription of the alkaline phosphatase (ALP) and osteocalcin (OCL) gene, leading to less ALP activity and osteocalcin (OCL) production. Id acts by inhibiting the sequence-specific binding of Cbfa1 to DNA and by decreasing the expression of Cbfa1 in cells undergoing osteogenic differentiation. p204, an interferon-inducible protein that interacts with both Cbfa1 and Id2, overcame the Id2-mediated inhibition of Cbfa1-induced ALP activity and OCL production. We show that 1) p204 disturbed the binding of Id2 to Cbfa1 and enabled Cbfa1 to bind to the promoters of its target genes and 2) that p204 promoted the translocation from nucleus to the cytoplasm and accelerated the degradation of Id2 by ubiquitin-proteasome pathway during osteogenesis. Nucleus export signal (NES) of p204 is required for the p204-enhanced cytoplasmic translocation and degradation of Id2, because a p204 mutant lacking NES lost these activities. Together, Cbfa1, p204, and Id proteins form a regulatory circuit and act in concert to regulate osteoblast differentiation.

p204 protein is a novel modulator of ras activity

The murine p200 family protein, p204, modulates cell proliferation and tissue differentiation. Many of its activities are exerted in the nucleus. However, in cardiac myocytes, p204 accumulated in the cytoplasm. A yeast two-hybrid assay revealed a p204-cytoplasmic Ras protein interaction. This was confirmed (i) by coimmunoprecipitation of p204 with Ras in mouse heart extract and with endogenous or ectopic H-Ras and K-Ras in cell lysates as well as (ii) by binding of purified H-Ras-GTP to purified p204 in vitro. p204 inhibited (i) the cleavage of RasGTP to RasGDP by RasGAP; (ii) the binding to RasGTP of Raf-1, phosphatidylinositol 3-kinase, and Ral-GDS, effectors of Ras signaling; and (iii) activation by the Ras pathway of the phosphorylation and thus activation of downstream targets (e.g. MEK, Akt, and p38 MAPK). Oncogenic Ras expression triggered the phosphorylation and translocation of p204 from the nucleus to the cytoplasm. This is expected to increase the interaction between the two proteins. Translocation triggered by Ras oncoprotein was blocked by the LY294002 inhibitor of phosphatidylinositol 3-kinase. Ras did not promote phosphorylation or translocation to the cytoplasm of mutated p204 in which serine 179 was replaced by alanine. p204 overexpression inhibited the anchorage-independent proliferation of cells expressing Ras(Q61L) oncoprotein. Ras oncoprotein triggered in MEF3T3 cells the rearrangement of the actin cytoskeleton and the enhancement of cell migration through a membrane. Overexpression of p204 inhibited both. Ras oncoprotein or activated, wild-type Ras was described to increase Egr-1 transcription factor expression. We report that a sequence in the gene encoding p204 bound Egr-1, and Egr-1 activated p204 expression. Ras oncoprotein or activated wild-type Ras increased the expression in 3T3 cells of p204 together with that of Egr-1. Furthermore, the activation of expression of a single copy of K-ras oncogene in cultured murine embryonic cells induced the expression of a high level of p204 as well as its distribution between the nuclei and the cytoplasm. Thus, p204 may serve as a negative feedback inhibitor of Ras activity.

Hedgehog and adipogenesis: fat and fiction

Morphogenes, abundantly described during embryogenesis have recently emerged as crucial modulators of cell differentiation processes. Hedgehog signaling, the dysregulation of which causing several pathologies such as congenital defects and cancer, is involved in several cell differentiation processes including adipogenesis. This review presents an overview of the relations between Hedgehog signaling, adipocyte differentiation and fat mass. While the anti-adipogenic role of Hedgehog signaling seems to be established, the effect of Hedgehog inhibition on adipocyte differentiation in vitro remains debated. Finally, Hedgehog potential as a pharmacological target to treat fat mass disorders is discussed.