Leukemia/lymphoma-related factor, a POZ domain-containing transcriptional repressor, interacts with histone deacetylase-1 and inhibits cartilage oligomeric matrix protein gene expression and chondrogenesis

Regulation of the chondrogenic phenotype in culture

In recent years, there has been a great deal of interest in the development of regenerative approaches to produce hyaline cartilage ex vivo that can be utilized for the repair or replacement of damaged or diseased tissue. It is clinically imperative that cartilage engineered in vitro mimics the molecular composition and organization of and exhibits biomechanical properties similar to persistent hyaline cartilage in vivo. Experimentally, much of our current knowledge pertaining to the regulation of cartilage formation, or chondrogenesis, has been acquired in vitro utilizing high-density cultures of undifferentiated chondroprogenitor cells stimulated to differentiate into chondrocytes. In this review, we describe the extracellular matrix molecules, nuclear transcription factors, cytoplasmic protein kinases, cytoskeletal components, and plasma membrane receptors that characterize cells undergoing chondrogenesis in vitro and regulate the progression of these cells through the chondrogenic differentiation program. We also provide an extensive list of growth factors and other extracellular signaling molecules, as well as chromatin remodeling proteins such as histone deacetylases, known to regulate chondrogenic differentiation in culture. In addition, we selectively highlight experiments that demonstrate how an understanding of normal hyaline cartilage formation can lead to the development of novel cartilage tissue engineering strategies. Finally, we present directions for future studies that may yield information applicable to the in vitro generation of hyaline cartilage that more closely resembles native tissue.

Granulin epithelin precursor: a bone morphogenic protein 2-inducible growth factor that activates Erk1/2 signaling and JunB transcription factor in chondrogenesis

Granulin epithelin precursor (GEP) has been implicated in development, tissue regeneration, tumorigenesis, and inflammation. Herein we report that GEP stimulates chondrocyte differentiation from mesenchymal stem cells in vitro and endochondral ossification ex vivo, and GEP-knockdown mice display skeleton defects. Similar to bone morphogenic protein (BMP) 2, application of the recombinant GEP accelerates rabbit cartilage repair in vivo. GEP is a key downstream molecule of BMP2, and it is required for BMP2-mediated chondrocyte differentiation. We also show that GEP activates chondrocyte differentiation through Erk1/2 signaling and that JunB transcription factor is one of key downstream molecules of GEP in chondrocyte differentiation. Collectively, these findings reveal a novel critical role of GEP growth factor in chondrocyte differentiation and the molecular events both in vivo and in vitro.

Functional identification of LRF as an oncogene that bypasses RASV12-induced senescence via upregulation of CYCLIN E

Mutant RAS (RAS(V12)) is known to transform most immortal cells but to induce premature senescence in primary cells. RAS(V12)-induced senescence in murine cells depends on the induction of the ARF/p53 and the retinoblastoma (Rb) family tumor suppressor pathways. We and others have shown previously that oncogene-induced senescence in vitro can be used as a tool to identify new cancer-related genes. In addition, we have shown that oncogene-induced senescence corresponds to an in vivo tumor suppressive mechanism. Therefore, we extended our search for novel genes that bypass of RAS(V12)-induced senescence, with the help of a previously designed unbiased functional screen with cDNA expression libraries. In this screen, we expected to find new mediators feeding into the p53 or Rb pathways or novel signaling factors. We report here the identification of leukemia/lymphoma related factor (Lrf) encoding a transcription factor with a BTB/POZ domain and Krüppel-like zinc fingers. This gene was previously identified as a potential oncogene that is overexpressed in human cancer. We find that LRF enhances E2F-dependent transcription and that it synergizes with RAS(V12) in activating E2F. Indeed, LRF-mediated bypass of RAS(V12)-induced senescence is accompanied by the induction of several E2F-target genes, including Cyclin E, Cyclin A and p107. Unexpectedly, LRF exerted this activity independent of several critical senescence inducers, such as p19(ARF), p21(CIP) and p16(INK4A). We show that CYCLIN E is necessary for LRF-mediated bypass, suggesting that it corresponds to a critical mediator of LRF-driven oncogenic transformation. Thus, LRF bypasses RAS(V12)-induced senescence in a CYCLIN E-dependent manner, which conceivably contributes to its role in cancer.

Curcumin decreases the expression of Pokemon by suppressing the binding activity of the Sp1 protein in human lung cancer cells

Pokemon, which stands for POK erythroid myeloid ontogenic factor, can regulate expression of many genes and plays an important role in tumorigenesis. Curcumin, a natural and non-toxic yellow compound, has capacity for antioxidant, free radical scavenger, anti-inflammatory properties. Recent studies shows it is a potential inhibitor of cell proliferation in a variety of tumour cells. To investigate whether curcumin can regulate the expression of Pokemon, a series of experiments were carried out. Transient transfection experiments demonstrated that curcumin could decrease the activity of the Pokemon promoter. Western blot analysis suggested that curcumin could significantly decrease the expression of the Pokemon. Overexpression of Sp1 could enhance the activity of the Pokemon promoter, whereas knockdown of Sp1 could decrease its activity. More important, we also found that curcumin could decrease the expression of the Pokemon by suppressing the stimulation of the Sp1 protein. Therefore, curcumin is a potential reagent for tumour therapy which may target Pokemon.

Proto-oncogene FBI-1 represses transcription of p21CIP1 by inhibition of transcription activation by p53 and Sp1

Aberrant transcriptional repression through chromatin remodeling and histone deacetylation has been postulated as the driving force for tumorigenesis. FBI-1 (formerly called Pokemon) is a member of the POK family of transcriptional repressors. Recently, FBI-1 was characterized as a critical oncogenic factor that specifically represses transcription of the tumor suppressor gene ARF, potentially leading indirectly to p53 inactivation. Our investigations on transcriptional repression of the p53 pathway revealed that FBI-1 represses transcription of ARF, Hdm2 (human analogue of mouse double minute oncogene), and p21CIP1 (hereafter indicated as p21) but not of p53. FBI-1 showed a more potent repressive effect on p21 than on p53. Our data suggested that FBI-1 is a master controller of the ARF-Hdm2-p53-p21 pathway, ultimately impinging on cell cycle arrest factor p21, by inhibiting upstream regulators at the transcriptional and protein levels. FBI-1 acted as a competitive transcriptional repressor of p53 and Sp1 and was shown to bind the proximal Sp1-3 GC-box and the distal p53-responsive elements of p21. Repression involved direct binding competition of FBI-1 with Sp1 and p53. FBI-1 also interacted with corepressors, such as mSin3A, NCoR, and SMRT, thereby deacetylating Ac-H3 and Ac-H4 histones at the promoter. FBI-1 caused cellular transformation, promoted cell cycle proliferation, and significantly increased the number of cells in S phase. FBI-1 is aberrantly overexpressed in many human solid tumors, particularly in adenocarcinomas and squamous carcinomas. The role of FBI-1 as a master controller of the p53 pathway therefore makes it an attractive therapeutic target.

Suppression of chondrogenesis by Id helix-loop-helix proteins in murine embryonic orofacial tissue

Inhibitors of differentiation (Id) proteins are helix-loop-helix (HLH) transcription factors lacking a DNA-binding domain. Id proteins modulate cell proliferation, apoptosis and differentiation in embryonic/fetal tissue. Perturbation of any of these processes in cells of the developing orofacial region results in orofacial anomalies. Chondrogenesis, a process integral to normal orofacial ontogenesis, is known to be modulated, in part, by Id proteins. In the present study, the mRNA and protein expression patterns of Id1, Id2, Id3 and Id4 were examined in developing murine orofacial tissue in vivo, as well as in murine embryonic maxillary mesenchymal cells in vitro. The functional role of Ids during chondrogenesis was also explored in vitro. Results reveal that cells derived from developing murine orofacial tissue (1) express Id1, Id2, Id3 and Id4 mRNAs and proteins on each of gestational days 12-14, (2) express all four Id proteins in a developmentally regulated manner, (3) undergo chondrogenesis and express genes encoding various chondrogenic marker proteins (e.g. Runx2, Type X collagen, Sox9) when cultured under micromass conditions and (4) can have their chondrogenic potential regulated via alteration of Id protein function through overexpression of a basic HLH factor. In summary, results from the current report reveal for the first time the expression of all four Id proteins in cells derived from developing murine orofacial tissue, and demonstrate a functional role for the Ids in regulating the ability of these cells to undergo chondrogenesis.

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.

Proto-oncogene FBI-1 (Pokemon/ZBTB7A) represses transcription of the tumor suppressor Rb gene via binding competition with Sp1 and recruitment of co-repressors

FBI-1 (also called Pokemon/ZBTB7A) is a BTB/POZ-domain Krüppel-like zinc-finger transcription factor. Recently, FBI-1 was characterized as a proto-oncogenic protein, which represses tumor suppressor ARF gene transcription. The expression of FBI-1 is increased in many cancer tissues. We found that FBI-1 potently represses transcription of the Rb gene, a tumor suppressor gene important in cell cycle arrest. FBI-1 binds to four GC-rich promoter elements (FREs) located at bp -308 to -188 of the Rb promoter region. The Rb promoter also contains two Sp1 binding sites: GC-box 1 (bp -65 to -56) and GC-box 2 (bp -18 to -9), the latter of which is also bound by FBI-1. We found that FRE3 (bp -244 to -236) is also a Sp1 binding element. FBI-1 represses transcription of the Rb gene not only by binding to the FREs, but also by competing with Sp1 at the GC-box 2 and the FRE3. By binding to the FREs and/or the GC-box, FBI-1 represses transcription of the Rb gene through its POZ-domain, which recruits a co-repressor-histone deacetylase complex and deacetylates histones H3 and H4 at the Rb gene promoter. FBI-1 inhibits C2C12 myoblast cell differentiation by repressing Rb gene expression.

The proto-oncogene LRF is under post-transcriptional control of MiR-20a: implications for senescence

MicroRNAs (miRNAs) are short 20–22 nucleotide RNA molecules that act as negative regulators of gene expression via translational repression: they have been shown to play a role in development, proliferation, stress response, and apoptosis. The transcriptional regulator LRF (Leukemia/lymphoma Related Factor) has been shown to prevent p19ARF transcription and consequently to inhibit senescence in mouse embryonic fibroblasts (MEF). Here we report, for the first time, that LRF is post-transcriptionally regulated by miR-20a. Using a gene reporter assay, direct interaction of miR-20a with the LRF 3′UTR is demonstrated. To validate the interaction miR-20a/3′UTR LRF miR-20a was over-expressed, either by transient transfection or retroviral infection, in wild type mouse embryo fibroblasts and in LRF-null MEF derived from LRF knock-out mice. We observed LRF decrease, p19ARF increase, inhibition of cell proliferation and induction of senescence. The comparison of miR-20a activity in wt and LRF-null MEF indicates that LRF is the main mediator of the miR-20a-induced senescence and that other targets are cooperating. As LRF down-regulation/p19ARF induction is always accompanied by E2F1 down-regulation and increase of p16, we propose that all these events act in synergy to accomplish miR-20a-induced senescence in MEF. Senescence has been recently revaluated as a tumor suppressor mechanism, alternative to apoptosis; from this point of view the discovery of new physiological “senescence inducer” appears to be promising as this molecule could be used as anticancer drug.

Development of an improved protocol to analyse gene expression in temporomandibular joint condylar cartilage of rats using DNA microarrays

PURPOSE

During recent years, gene expression analyses based on DNA chip technologies have allowed for the genome-wide identification of genes potentially associated with growth processes in a variety of organs. The present study aims to identify genes differentially expressed in the growing temporomandibular joint cartilage by means of transcriptome analyses.

MATERIAL AND METHODS

In total, the condylar cartilage of 32 rats comprising 4 age groups (newborn, 10 days, 21 days, 8 weeks) were used for analysis. Transcriptome analyses were carried out using Affymetrix Expression Arrays (Rat Genome 230 2.0 Arrays). The availability of high-quality RNA preparations from homogeneous tissue samples is a fundamental precondition of successful transcriptome analyses using DNA arrays. An optimised preparation protocol allowed RNA isolation of sufficient quality which was validated using capillary electrophoresis. RNA collected from 8 test animals of the 4 age groups respectively was mixed in equimolar RNA pools which served for the transcriptome analyses using Affymetrix arrays.

RESULTS

Statistical analysis of the gene expression data indicated the existence of genes differentially regulated in the growing temporomandibular cartilage. This evidence, however, requires validation by RT-PCR using individual animals' RNA. Preliminary candidate genes belong, among others, to the groups of matrix-degrading proteases, protease inhibitors and genes involved in cell growth, apoptosis and bone remodelling.

CONCLUSION

These differentially expressed genes in TMJ growth identified using DNA array technology may possibly contribute to a better understanding of growth biology and provide an approach to necessary therapy.

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

Bone formation requires the coordinated activity of numerous proteins including the transcription factor core-binding factor alpha1 (Cbfa1). Deregulation of Cbfa1 results in metabolic bone diseases including osteoporosis and osteopetrosis. The retinoblastoma protein (pRb) that is required for osteogenesis binds Cbfa1. We reported earlier that the p200 family protein p204, which is known to be involved in the differentiation of skeletal muscle myotubes, cardiac myocytes, and macrophages, also serves as a cofactor of Cbfa1 and promotes osteogenesis. In this study we established that suppression of p204 expression by an adenovirus construct encoding p204 antisense RNA inhibited osteoblast-specific gene activation by Cbfa1 in an osteogenesis assay involving the pluripotent C2C12 mesenchymal cell line. Using protein-protein interaction assays we established that Cbfa1, pRb, and p204 form a ternary complex in which pRb serves as a linker connecting p204 and Cbfa1. Chromatin immunoprecipitation assays revealed the binding of such a p204-pRb-Cbfa1 transcription factor complex to the promoter of the osteocalcin gene. The pRb requirement of the stimulation of Cbfa1 activity by p204 was established in experiments involving p204 mutants lacking one or two pRb binding (LXCXE) motifs. Such mutants failed to enhance the Cbfa1-dependent transactivation of gene expression as well as osteogenesis. Furthermore, as revealed in reporter gene and in vitro osteogenesis assays p204 synergized with pRb in the stimulation of Cbfa1-dependent gene activation and osteoblast differentiation.

Regulation of type II collagen expression by histone deacetylase in articular chondrocytes

Histone deacetylase (HDAC) regulates various cellular processes by modulating gene expression. Here, we investigated the role of HDAC in the expression of type II collagen, a marker of differentiated chondrocytes. We found that HDAC activity in primary articular chondrocytes decreases during dedifferentiation induced by serial monolayer culture and that the activity recovered during redifferentiation induced by three-dimensional culture in a cell pellet. Inhibition of HDAC with trichostatin A or PXD101 was sufficient to block type II collagen expression in primary culture chondrocytes. HDAC inhibition also blocked the redifferentiation of dedifferentiated chondrocytes by suppressing the synthesis and accumulation of type II collagen. HDAC inhibition promoted the expression of Wnt-5a, which is known to inhibit type II collagen expression, and knockdown of Wnt-5a blocked the ability of HDAC inhibitors to suppress type II collagen expression. In addition, the induction of Wnt-5a expression by HDAC inhibitors was associated with acetylation of the Wnt-5a promoter. Taken together, our results suggest that HDAC promotes type II collagen expression by suppressing the transcription of Wnt-5a.

The Effects of Histone Deacetylase Inhibitors on the Induction of Differentiation in Chondrosarcoma Cells

PURPOSE

Histologically, chondrosarcomas represent the degree of chondrogenic differentiation, which is associated with the prognosis of the disease. Histone acetylation and deacetylation play key roles in the regulation of chondrocytic differentiation. Here, we describe the antitumor effects of histone deacetylase (HDAC) inhibitors as differentiating reagents on chondrosarcomas.

EXPERIMENTAL DESIGN

We examined the effects of a HDAC inhibitor, depsipeptide, on the growth of chondrosarcoma cell lines. We also investigated the modulation of the expression levels of extracellular matrix genes and the induction of phenotypic change in chondrosarcoma cells treated with depsipeptide. Finally, we examined the antitumor effect of depsipeptide on chondrosarcoma in vivo.

RESULTS

Depsipeptide inhibited the growth of chondrosarcoma cells by inducing cell cycle arrest and/or apoptosis. HDAC inhibitors increased the expression of the alpha1 chain of type II collagen (COL2A1) gene due to the enhanced histone acetylation in the promoter and enhancer. Depsipeptide also up-regulated the expressions of aggrecan and the alpha2 chain of type XI collagen (COL11A2) mRNA in a dose-dependent manner. Moreover, long-term treatment with a low dose of depsipeptide resulted in the induction of differentiation into hypertrophic phenotype, as shown by the increment of the alpha1 chain of type X collagen (COL10A1) expression in chondrosarcoma cells. In vivo studies and histologic analyses confirmed that depsipeptide significantly inhibited tumor growth and induced differentiation into the hypertrophic and mineralized state in chondrosarcoma cells.

CONCLUSIONS

These results strongly suggest that HDAC inhibitors may be promising reagents for use as a differentiating chemotherapy against chondrosarcomas.

Crystal structure of the BTB domain from the LRF/ZBTB7 transcriptional regulator

BTB-zinc finger (BTB-ZF) proteins are transcription regulators with roles in development, differentiation, and oncogenesis. In these proteins, the BTB domain (also known as the POZ domain) is a protein-protein interaction motif that contains a dimerization interface, a possible oligomerization surface, and surfaces for interactions with other factors, including nuclear co-repressors and histone deacetylases. The BTB-ZF protein LRF (also known as ZBTB7, FBI-1, OCZF, and Pokemon) is a master regulator of oncogenesis, and represses the transcription of a variety of important genes, including the ARF, c-fos, and c-myc oncogenes and extracellular matrix genes. We determined the crystal structure of the BTB domain from human LRF to 2.1 A and observed the canonical BTB homodimer fold. However, novel features are apparent on the surface of the homodimer, including differences in the lateral groove and charged pocket regions. The residues that line the lateral groove have little similarity with the equivalent residues from the BCL6 BTB domain, and we show that the 17-residue BCL6 Binding Domain (BBD) from the SMRT co-repressor does not bind to the LRF BTB domain.

Cartilage oligomeric matrix protein is involved in human limb development and in the pathogenesis of osteoarthritis

As a member of the thrombospondin gene family, cartilage oligomeric protein (COMP) is found mainly in the extracellular matrix often associated with cartilage tissue. COMP exhibits a wide binding repertoire and has been shown to be involved in the regulation of chondrogenesis in vitro. Not much is known about the role of COMP in human cartilage tissue in vivo. With the help of immunohistochemistry, Western blot, in situ hybridization, and real-time reverse transcription-polymerase chain reaction, we aimed to elucidate the role of COMP in human embryonic, adult healthy, and osteoarthritis (OA) cartilage tissue. COMP is present during the earliest stages of human limb maturation and is later found in regions where the joints develop. In healthy and diseased cartilage tissue, COMP is secreted by the chondrocytes and is often associated with the collagen fibers. In late stages of OA, five times the COMP mRNA is produced by chondrocytes found in an area adjacent to the main defect than in an area with macroscopically normal appearance. The results indicate that COMP might be involved in human limb development, is upregulated in OA, and due to its wide binding repertoire, could play a role in the pathogenesis of OA as a factor secreted by chondrocytes to ameliorate the matrix breakdown.

Sox9 and p300 cooperatively regulate chromatin-mediated transcription

Chromatin structure is a fundamental component of gene regulation, expression, and cellular differentiation. We have previously reported that the multifunctional coactivator p300 is a member of the Sox9 (Sry-type high mobility group box 9)-related transcriptional apparatus and activates Sox9-dependent transcription during chondrogenesis. However, the mechanism of synergy between Sox9 and p300 in chromatin-mediated transcription has not been elucidated. In the present study we investigated the activity of Sox9 and p300 on chromatinized templates in vitro. Recombinant Sox9 was shown to be associated with several transcriptional cofactors including p300. In vitro transcription assays revealed that p300 potentiated Sox9-dependent transcription on chromatinized DNA and, importantly, was associated with hyperacetylated histones. Consistent with these results, the histone deacetylase inhibitor trichostatin A stimulated the expression of Sox9-regulated cartilage matrix genes and induced histone acetylation around the enhancer region of the collagen alpha1 (II) gene in chondrocytes. These findings suggest that Sox9 interacts with chromatin and activates transcription via regulation of chromatin modification.