Tbx2 Is Overexpressed and Plays an Important Role in Maintaining Proliferation and Suppression of Senescence in Melanomas

Heart chamber size in zebrafish is regulated redundantly by duplicated tbx2 genes

The Tbx2 transcription factor is implicated in growth control based on its association with human cancers. In the heart, Tbx2 represses cardiac differentiation to mediate development of the atrioventricular canal (AVC). The zebrafish genome retains two tbx2 genes, and both are required for formation of the AVC. Here, we show that both genes are also expressed earlier in the primitive heart tube, and we describe a previously unrecognized role for Tbx2 in promoting proliferation of presumptive myocardium at the heart tube stage. In contrast to single knockdowns, depletion of both gene products causes chamber defects, resulting in an expanded atrium and a smaller ventricle, associated with decreased proliferation of ventricular cardiomyocytes. The phenotype correlates with changes in the expression for known cardiac growth factors. Therefore, in zebrafish, two tbx2 genes are functionally redundant for regulating chamber development, while each gene is required independently for development of the AVC.

PMA-induced up-regulation of TBX3 is mediated by AP-1 and contributes to breast cancer cell migration

The T-box transcription factor TBX3 provides an important link between embryonic development and cancer. TBX3 mediates limb, mammary gland and heart development and, in humans, mutations resulting in haplo-insufficiency of TBX3 lead to ulnar-mammary syndrome. Importantly, the de-regulation of TBX3 gene expression has been linked to several cancers, where it acts to suppress senescence and promotes proliferation and tumour invasion. Despite the negative impact of de-regulated TBX3 expression as seen by developmental defects and cancer, surprisingly little is known about the regulation of the TBX3 gene. In the present paper, we show that the phorbol ester PMA increases TBX3 protein and mRNA levels in a protein kinase C-dependent manner via the AP-1 (activator protein 1) transcription factors c-Jun and JunB. Furthermore, these AP-1 factors are shown to mediate the activation of the TBX3 gene by binding a non-consensus PMA-response element in the TBX3 promoter in vitro and in vivo. We also demonstrate that TBX3 contributes to the PMA-induced migration previously observed for the MCF-7 breast epithelium cancer cell line. Our present results reveal a previously unidentified pathway that up-regulates TBX3 expression and provides additional evidence that increased levels of TBX3 contribute to metastasis.

Commentary. A picture of Mitf in melanoma immortality

The Mitf gene has a key role in melanocytes and melanoma by regulating cell cycle progression, survival and differentiation. Two papers in this issue of Oncogene (Cheli et al., 2011; Strub et al., 2011) reveal that low-Mitf cells can initiate tumors with high efficiency, and that Mitf blocks senescence by regulating genes implicated in S-phase progression and mitosis.

Progressive genetic alterations of adenoid cystic carcinoma with high-grade transformation

CONTEXT

Although genome-wide imbalances have been characterized in conventional adenoid cystic carcinoma, other than p53 mutational status, the molecular profile of adenoid cystic carcinoma with high-grade transformation has not been explored.

OBJECTIVE

To evaluate progressive genetic alterations in adenoid cystic carcinoma with high-grade transformation using array comparative genomic hybridization.

DESIGN

Five adenoid cystic carcinomas with high-grade transformation (4 primary tumors and 1 paired metastasis) were selected and characterized at the DNA level by array comparative genomic hybridization on formalin-fixed paraffin-embedded tissue. Select alterations were validated by fluorescence in situ hybridization.

RESULTS

Chromosomal gains were mostly confined to the areas of high-grade transformation while losses were seen only in the conventional areas. Chromosomal regions with significant gains included 8q24, 17q11.2-q12, 17q23, and 15q11-13. Regions that showed the significant losses included 9q34, 4p16, 1p36.1, and 11q22. Fluorescence in situ hybridization analysis demonstrated increases in C-MYC (8q24.12-q24.13) and a low level increases in ERBB2 ( formerly HER2/neu ) (17q11.2-q12) in cases showing gains by array comparative genomic hybridization in these regions. However, no tumor showed HER2/ neu immunopositivity.

CONCLUSIONS

High-grade transformation in adenoid cystic carcinoma is a complex process that is reflected by several chromosomal alterations. Our findings implicate C-MYC amplification in this progression, although the role of HER2/neu is still unclear. Other candidate oncogenes, particularly on chromosome 17q23, warrant investigation in this rare tumor.

TBX2 and TBX3: the special value for anticancer drug targets

TBX2 and TBX3 are members of the T-box family of transcription factors, which are implicated in embryonic development. Unlike most members of the T-box family, TBX2 and TBX3 are the only mammalian T-box factors which function as transcriptional repressors, mediated by the repression domain in the C-terminal. In addition to a role in development, recent evidence suggests that TBX2 and TBX3 are overexpressed in a number of cancers, including melanoma, breast, liver, lung, pancreas, ovarian, and cervical cancers. However, there is little information about the mechanisms for how these T-box genes contribute to tumorigenesis. Upregulation of TBX2 and TBX3 suppresses the expression of p14(ARF) and p21(CIP1) and promotes bypass of senescence through inactivation of p53 pathway. TBX2 functionally interacts with pRb, and pRb modulates TBX2 functional specificity. In addition, TBX2 is a player of Wnt signaling while TBX3 is a downstream target of the Wnt/beta-catenin pathway, and overexpression of TBX2 and TBX3 represses the expression of E-cadherin, which is demonstrated to be a prerequisite for epithelial tumor cell invasion. Moreover, TBX2 is shown to interact with EGR1 to block multiple downstream tumor suppressors. Here, we review the current knowledge on TBX2 and TBX3 in tumorigenesis and prospect their special value for development of target-based anticancer drugs.

MITF-siRNA formulation is a safe and effective therapy for human melasma

It is unclear whether siRNA-based agents can be a safe and effective therapy for diseases. In this study, we demonstrate that microphthalmia-associated transcription factor-siRNA (MITF-siR)-silenced MITF gene expression effectively induced a significant reduction in tyrosinase (TYR), tyrosinase-related protein 1, and melanocortin 1 receptor (MC1R) levels. The siRNAs caused obvious inhibition of melanin synthesis and melanoma cell apoptosis. Using a novel type of transdermal peptide, we developed the formulation of an MITF-siR cream. Results demonstrated that hyperpigmented facial lesions of siRNA-treated subjects were significantly lighter after 12 weeks of therapy than before treatment (P < 0.001); overall improvement was first noted after 4 weeks of siRNA treatment. At the end of treatment, clinical and colorimetric evaluations demonstrated a 90.4% lightening of the siRNA-treated lesions toward normal skin color. The relative melanin contents in the lesions and adjacent normal skin were decreased by 26% and 7.4%, respectively, after treatment with the MITF-siR formulation. Topical application of siRNA formulation significantly lightens brown facial hypermelanosis and lightens normal skin in Asian individuals. This treatment represents a safe and effective therapy for melasma, suggesting that siRNA-based agents could be developed for treating other diseases such as melanoma.

Generation of mice with a conditional null allele for Tbx2

The T-box transcription factor Tbx2 plays important roles in patterning and development, and has been implicated in cell-cycle regulation and cancer. Conventional disruption of Tbx2 results in abnormalities of the heart, limbs, eye and other structures, and early fetal lethality. To gain insight into the role of Tbx2 in different tissues and at different stages of development, we have generated a conditional null allele of Tbx2 by flanking Exon 2 with loxP sites (Tbx2(fl2)). Homozygous Tbx2(fl2) mice are viable and fertile, indicating that the Tbx2(fl2) allele is a fully functional Tbx2 allele. Cre-mediated recombination, using a ubiquitously active CMV-Cre line, results in deletion of Exon 2 and loss of protein expression. Embryos homozygous for the recombined allele (Tbx2(Delta2)) show the same heart and limb defects as conventional Tbx2-deficient embryos. This Tbx2 conditional null allele will be a valuable tool to uncover tissue-specific roles of Tbx2 in development and disease.

Cancer stem cells versus phenotype-switching in melanoma

Tumours comprise multiple phenotypically distinct subpopulations of cells, some of which are proposed to possess stem cell-like properties, being able to self-renew, seed and maintain tumours, and provide a reservoir of therapeutically resistant cells. Here, we use melanoma as a model to explore the validity of the cancer stem cell hypothesis in the light of accumulating evidence that melanoma progression may instead be driven by phenotype-switching triggered by genetic lesions that impose an increased sensitivity to changes in the tumour microenvironment. Although at any given moment cells within a tumour may exhibit differentiated, proliferative or invasive phenotypes, an ability to switch phenotypes implies that most cells will have the potential to adopt a stem cell-like identity. Insights into the molecular events underpinning phenotype-switching in melanoma highlight the close relationship between signalling pathways that generate, maintain and activate melanocyte stem cells as well as the inverse correlation between proliferation and invasive potentials. An understanding of phenotype-switching in melanoma, and in particular the signalling events that regulate the expression of the microphthalmia-associated transcription factor Mitf, points to new therapeutic opportunities aimed at eradicating therapeutically resistant stem cell-like melanoma cells.

SWItching on the transcriptional circuitry in melanoma

Melanoma is an aggressive malignancy that is resistant to current therapy, and the most lethal of all human skin cancers. It is characterized by several genetic alterations that lead to changes in gene expression and tumorigenesis by triggering alterations in the normal transcriptional circuitry. Transformation and tumor progression are thought to be promoted by a complex interplay between the accumulation of genetic alterations and epigenetic changes. In this review, we discuss recent studies that have implicated SWI/SNF chromatin remodeling enzymes as epigenetic regulators of a transcriptional circuit that operates within the context the genetic alterations that frequently occur in melanoma.

Resistance to transforming growth factor β-mediated tumor suppression in melanoma: are multiple mechanisms in place?

Resistance to transforming growth factor (TGF) β-mediated tumor suppression in melanoma appears to be a crucial step in tumor aggressiveness since it is usually coupled with the ability of TGFβ to drive the oncogenic process via autocrine and paracrine effects. In this review, we will focus mainly on the mechanisms of escape from TGFβ-induced cell cycle arrest because the mechanisms of resistance to TGFβ-mediated apoptosis are still essentially speculative. As expected, some of these mechanisms can directly affect the function of the main downstream effectors of TGFβ, Smad2 and Smad3, resulting in compromised Smad-mediated antiproliferative activity. Other mechanisms can counteract or overcome TGFβ-mediated cell cycle arrest independently of the Smads. In melanoma, some models of resistance to TGFβ have been suggested and will be described. In addition, we propose additional models of resistance taking into consideration the information available on the dysregulation of fundamental cellular effectors and signaling pathways in melanoma.

The retinoblastoma protein modulates Tbx2 functional specificity

This study demonstrates that Tbx2 binds Rb1. The interaction with Rb1 increases Tbx2 DNA-binding activity and enhances the ability of Tbx2 to repress transcription. The results show that Tbx2 regulates the expression of genes involved in cell division and DNA replication and that Rb1 modulates Tbx2 target gene recognition and specificity.

Tbx2 is a member of a large family of transcription factors defined by homology to the T-box DNA-binding domain. Tbx2 plays a key role in embryonic development, and in cancer through its capacity to suppress senescence and promote invasiveness. Despite its importance, little is known of how Tbx2 is regulated or how it achieves target gene specificity. Here we show that Tbx2 specifically associates with active hypophosphorylated retinoblastoma protein (Rb1), a known regulator of many transcription factors involved in cell cycle progression and cellular differentiation, but not with the Rb1-related proteins p107 or p130. The interaction with Rb1 maps to a domain immediately carboxy-terminal to the T-box and enhances Tbx2 DNA binding and transcriptional repression. Microarray analysis of melanoma cells expressing inducible dominant-negative Tbx2, comprising the T-box and either an intact or mutated Rb1 interaction domain, shows that Tbx2 regulates the expression of many genes involved in cell cycle control and that a mutation which disrupts the Rb1-Tbx2 interaction also affects Tbx2 target gene selectivity. Taken together, the data show that Rb1 is an important determinant of Tbx2 functional specificity.

MicroRNA-340-mediated degradation of microphthalmia-associated transcription factor mRNA is inhibited by the coding region determinant-binding protein

Alternative cleavage and polyadenylation generate multiple transcript variants of mRNA isoforms with different length of 3'-untranslated region (UTR). Alternative cleavage and polyadenylation enable differential post-transcriptional regulation of transcripts via the availability of different cis-acting elements in 3'-UTRs. Microphthalmia-associated transcription factor (MITF) is a master regulator of melanocyte development and melanogenesis. It has also been implicated in melanoma development. Here we show that melanoma cells favor the expression of MITF mRNA with shorter 3'-UTR. This isoform of mRNA is regulated by microRNA, miR-340. miR-340 interacts with two of its target sites on the 3'-UTR of MITF mRNA, causing mRNA degradation and decreased expression and activity of MITF. On the other hand, the RNA-binding protein coding region determinant-binding protein, shown to be highly expressed in melanoma, directly binds to the 3'-UTR of MITF mRNA and prevents the binding of miR-340 to its target sites, resulting in stabilization of the MITF transcript and elevated expression and transcriptional activity of MITF. This interplay between RNA-binding protein and miRNA describes the important mechanism of regulation of MITF in melanocytes and malignant melanomas.

Classical and Novel Prognostic Markers for Breast Cancer and their Clinical Significance

The use of biomarkers ensures breast cancer patients receive optimal treatment. Established biomarkers such as estrogen receptor (ER) and progesterone receptor (PR) have been playing significant roles in the selection and management of patients for endocrine therapy. HER2 is a strong predictor of response to trastuzumab. Recently, the roles of ER as a negative and HER2 as a positive indicator for chemotherapy have been established. Ki67 has traditionally been recognized as a poor prognostic factor, but recent studies suggest that measurement of Ki67-positive cells during treatment will more effectively predict treatment efficacy for both anti-hormonal and chemotherapy. p53 mutations are found in 20–35% of human breast cancers and are associated with aggressive disease with poor clinical outcome when the DNA-binding domain is mutated. The utility of cyclin D1 as a predictor of breast cancer prognosis is controversial, but cyclin D1b overexpression is associated with poor prognosis. Likewise, overexpression of the low molecular weight form of cyclin E1 protein predicts poor prognosis. Breast cancers from BRCA1/2 carriers often show high nuclear grades, negativity to ER/PR/HER2, and p53 mutations, and thus, are associated with poor prognosis. The prognostic values of other molecular markers, such as p14^ARF, TBX2/3, VEGF in breast cancer are also discussed. Careful evaluation of these biomarkers with current treatment modality is required to determine whether their measurement or monitoring offer significant clinical benefits.

Methylthioadenosine phosphorylase and activated insulin-like growth factor-1 receptor/insulin receptor: potential therapeutic targets in chordoma

Currently there is no effective chemotherapy for chordoma. Recent studies report co-expression of insulin-like growth factor-1 receptor (IGF1R) and its cognate ligand in chordoma, but it is unknown whether this receptor tyrosine kinase is activated in these tumours. Additionally, genetic studies have confirmed frequent deletions of chromosome 9p in chordomas, which encompasses the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus. Another gene in this region, methylthioadenosine phosphorylase (MTAP), is an essential enzyme of the purine salvage pathway and has therapeutic relevance because MTAP-deficient cells are particularly sensitive to inhibitors of de novo purine synthesis. We investigated whether these pathways might be potential therapeutic targets for chordoma. Paraffin-embedded tissue samples from 30 chordomas were analysed by immunohistochemistry for expression of the phosphorylated isoforms of IGF1R or the insulin receptor (pIGF1R/pIR) and selected downstream signalling molecules, including BCL2-associated agonist of cell death protein (BAD). Expression of CDKN2A and MTAP proteins was also assessed. Skeletal chondrosarcomas, benign notochordal cell tumours, and fetal notochord were studied for comparison. Phosphorylated IGF1R/IR was detected in 41% of chordomas, together with activated downstream signalling molecules, and pIGF1R/pIR was absent in benign notochordal cell tumours and fetal notochord. Thirty-nine per cent of chordomas were negative for MTAP immunoreactivity. Patients with pIGF1R/pIR-positive tumours showed significantly decreased median disease-free survival in multivariate survival analysis (p = 0.036), whereas phosphorylation of BAD at serine-99 was found to be associated with a favourable prognosis (p = 0.002). Approximately 40% of chordomas demonstrate evidence of activation of the IGF1R/IR signalling pathway or loss of a key enzyme in the purine salvage pathway. Aberrant signalling cascades and disrupted metabolic pathways such as these may represent opportunities for novel targeted therapeutic approaches for the treatment of chordoma.

T-box 2 represses NDRG1 through an EGR1-dependent mechanism to drive the proliferation of breast cancer cells

T-box 2 (TBX2) is a transcription factor involved in mammary development and is known to be overexpressed in a subset of aggressive breast cancers. TBX2 has previously been shown to repress growth control genes such as p14(ARF) and p21(WAF1/cip1). In this study we show that TBX2 drives proliferation in breast cancer cells and this is abrogated after TBX2 small interfering RNA (siRNA) knockdown or after the expression of a dominant-negative TBX2 protein. Using microarray analysis we identified a large cohort of novel TBX2-repressed target genes including the breast tumour suppressor NDRG1 (N-myc downregulated gene 1). We show that TBX2 targets NDRG1 through a previously undescribed mechanism involving the recruitment of early growth response 1 (EGR1). We show EGR1 is required for the ability of TBX2 to repress NDRG1 and drive cell proliferation. We show that TBX2 interacts with EGR1 and that TBX2 requires EGR1 to target the NDRG1 proximal promoter. Abrogation of either TBX2 or EGR1 expression is accompanied by the upregulation of cell senescence and apoptotic markers. NDRG1 can recapitulate these effects when transfected into TBX2-expressing cells. Together, these data identify a novel mechanism for TBX2-driven oncogenesis and highlight the importance of NDRG1 as a growth control gene in breast tissue.

The T-box transcription factor Tbx2: its role in development and possible implication in cancer

Tbx2 is a member of the T-box family of transcription factors that are crucial in embryonic development. Recent studies suggest that T-box factors may also play a role in controlling cell cycle progression and in the genesis of cancer. Tbx2 has been implicated in several developmental processes such as coordinating cell fate, patterning and morphogenesis of a wide range of tissues and organs including limbs, kidneys, lungs, mammary glands, heart, and craniofacial structures. Importantly, Tbx2 is overexpressed in several cancers including melanoma, small cell lung carcinoma, breast, pancreatic, liver, and bladder cancers and can suppress senescence, a cellular process, which serves as a barrier to cancer development. This review presents a state of the art overview of the role and regulation of Tbx2 in early embryonic development and in cancer.

Skeletal muscle phenotypically converts and selectively inhibits metastatic cells in mice

Skeletal muscle is rarely a site of malignant metastasis; the molecular and cellular basis for this rarity is not understood. We report that myogenic cells exert pronounced effects upon co-culture with metastatic melanoma (B16-F10) or carcinoma (LLC1) cells including conversion to the myogenic lineage in vitro and in vivo, as well as inhibition of melanin production in melanoma cells coupled with cytotoxic and cytostatic effects. No effect is seen with non-tumorigenic cells. Tumor suppression assays reveal that the muscle-mediated tumor suppressor effects do not generate resistant clones but function through the down-regulation of the transcription factor MiTF, a master regulator of melanocyte development and a melanoma oncogene. Our findings point to skeletal muscle as a source of therapeutic agents in the treatment of metastatic cancers.

Mitf silencing cooperates with IL-12 gene transfer to inhibit melanoma in mice

Malignant melanoma is a malignant neoplasm originating from the melanocyte lineage. Microphthalmia-associated transcription factor (Mitf) is crucially involved in the melanin synthesis as well as proliferation and survival of melanocyte and melanoma. We previously showed that short interfering RNA (siRNA) that is specific for the Mitf gene (Mitf-siRNA) significantly inhibited growth of B16 melanoma after electro-transfected in vivo into preestablished tumor in mice. Here we assessed efficacy of electroporation-mediated co-transfection of Mitf-siRNA and IL-12 gene in the treatment of murine melanoma. As results, the tumor growth was more strongly inhibited by intratumor co-transfection with Mitf-siRNA and IL-12-encoding plasmid DNA than by transfection with either of the molecules alone. The co-transfection induced intratumor infiltration of CD4+ and CD8+ T cells, and hampered neoangiogenesis in the tumor. The findings suggest that the RNAi/cytokine gene combination therapy by means of electroporation may become a novel and efficacious therapeutic modality to treat neoplasms including melanoma.

Increased Oral Fibroblast Lifespan Is Telomerase-independent

Oral mucosal wound-healing is characterized by rapid re-epithelialization and remodeling, with minimal scar formation. This may be attributed to the distinct phenotypic characteristics of the resident fibroblasts. To test this hypothesis, we investigated patient-matched oral mucosal and skin fibroblasts. Compared with skin fibroblasts, oral mucosal fibroblasts had longer proliferative lifespans, underwent more population doublings, and experienced senescence later, which was directly related to longer telomere lengths within oral mucosal fibroblasts. The presence of these longer telomeres was independent of telomerase expression, since both oral oral mucosal fibroblasts and skin fibroblasts were negative for active telomerase, as assessed according to the Telomeric Repeat Amplification Protocol. This study has demonstrated that, compared with skin fibroblasts, oral mucosal fibroblasts are ‘younger’, with a more embryonic/fetal-like phenotype that may provide a notable advantage for their ability to repair wounds in a scarless fashion.

Transcriptional regulation in melanoma

Transcriptional regulation in melanoma is a complex process that tends to hijack the normal melanocyte signaling pathways involved in melanocyte development, pigmentation, and survival. At the center of these often overlapping networks of transcriptional activation and repression is microphthalmia-associated transcription factor (MITF), a melanocyte lineage marker that increases pigment production and exhibits diverse effects on cell survival, proliferation, and cell cycle arrest. The particular conditions that allow MITF to produce these potentially contradictory roles have not yet been fully elucidated, but analysis of the pathways involved provides opportunities to learn about new therapeutic strategies.

14-3-3sigma gene silencing during melanoma progression and its role in cell cycle control and cellular senescence

Background

The family of 14-3-3 proteins plays an important role in cancer biology by interfering with intracellular signalling pathways and cell cycle checkpoints. The 14-3-3σ isoform acts as a tumor suppressor and is often inactivated during tumor development.

Results

Here, we demonstrate enhanced CpG methylation of the 14-3-3σ gene in lymph node and cutaneous melanoma metastases compared with primary tumors, associated with dramatically reduced mRNA expression. In line with this, treatment of different metastatic melanoma cell lines with 5-aza-2'-deoxycytidine (5-Aza-CdR), a potent inhibitor of cytosine methylation, significantly induces 14-3-3σ protein expression. Additional treatment with histone deacetylase inhibitor 4-phenylbutyric acid (Pba) further enhances 14-3-3σ expression. Induction of 14-3-3σ expression by 5-Aza-CdR/Pba treatment leads to almost complete inhibition of cell proliferation, with cells predominantly arrested in G2-M. The antiproliferative effect of 5-Aza-CdR/Pba was reversed in 14-3-3σ knockdown cells. Similarly, melanoma cell lines stably overexpressing 14-3-3σ show dramatically reduced cell proliferation rates. Moreover, synchronous 14-3-3σ stably overexpressing cells do not progress through cell cycle, but display a permanent increase in the population of 4n DNA containing cells. Interestingly, overexpression of 14-3-3σ induces senescence of melanoma cells and is involved in melanoma cell senescence under genotoxic stress. Finally, 14-3-3σ knockdown supports migratory capacity of melanoma cells in vitro, while 14-3-3σ overexpression has opposing effects.

Conclusion

Taken together, the present report indicates that epigenetic silencing of 14-3-3σ might contribute to tumor progression in malignant melanoma via loss of cell cycle control, impaired cellular senescence program and support of migratory capacity.

Phosphorylation of histone H3 by protein kinase C signaling plays a critical role in the regulation of the developmentally important TBX2 gene

The mechanism(s) regulating the expression of the TBX2 gene, a key regulator of development, is poorly understood and thus limits an understanding of its function(s). Here we demonstrate that 12-O-tetradecanoylphorbol-13-acetate (TPA) induces TBX2 expression in normal human fibroblasts in a protein kinase C (PKC)-dependent and MAPK-independent manner. Our data further reveal that TPA activates transcription of TBX2 through activating MSK1, which leads to an increase in phosphorylated histone H3 and the recruitment of Sp1 to the TBX2 gene. In addition, TPA was shown to activate MSK1 in a PKC-dependent and MAPK-independent manner. This study is the first to provide evidence that phosphorylation of histone H3 leads to the transcriptional activation of the TBX2 gene and to link MSK1 to PKC.

Protein interactions at the heart of cardiac chamber formation

The vertebrate heart is a muscular pump that contracts in a rhythmic fashion to propel the blood through the body. During evolution, the morphologically complex four-chambered heart of birds and mammals has evolved from a single-layered tube with peristaltic contractility. The heart of Drosophila, referred to as the dorsal vessel, is a blind sac composed of myogenic cells that contract rhythmically. The fish heart is composed of a single atrial chamber connected to a single ventricular chamber. The evolutionary development of fast-contracting chambers allowed the heart to build up high blood pressures. In amphibians two atrial chambers exist, separated by a septum, connecting to a single ventricle. The division of a common atrium and ventricle into right and left-sided chambers represents an evolutionary milestone in the development of the four-chambered heart and is necessary for separation of oxygenated and deoxygenated blood. In amphibians and reptiles, pulmonary and systemic circulations are incompletely separated allowing adaptable blood flows to both circulations. In contrast, the hearts of birds and mammals, in which septa completely separate the pulmonary and systemic circulations, both circulations have similar flows, but blood pressures can be regulated separately. In this review we focus, in a morphologically integrated fashion, on the molecular interactions that govern the intricate cardiac design.

Loss of Tbx2 delays optic vesicle invagination leading to small optic cups

Tbx2 is a T-box transcription factor gene that is dynamically expressed in the presumptive retina during optic vesicle invagination. Several findings implicate Tbx2 in cell cycle regulation, including its overexpression in tumours and regulation of proliferation during heart development. We investigated the role of Tbx2 in optic cup formation by analysing mice with a targeted homozygous mutation in Tbx2. Loss of Tbx2 caused a reduced presumptive retinal volume due to increased apoptosis, and a delay in ventral optic vesicle invagination leading to the formation of small and abnormally shaped optic cups. Tbx2 is essential for maintenance, but not induction of expression of the dorsal retinal determinant, Tbx5, and acts downstream of Bmp4, a dorsally expressed gene implicated in human microphthalmia. The small retina showed a hypocellular ventral region, loss of Fgf15, normally expressed in proliferating central retinal cells, and increased numbers of mitotic cells in the dorsal region, indicating that Tbx2 is required for normal growth and development across the D-V axis. Dorsal expression of potential regulators of retinal growth, Cyp1b1 and Cx43, and the topographic guidance molecule ephrinB2, was increased, and intraretinal axons were disorganised resulting in a failure of optic nerve formation. Our data provide evidence that Tbx2 is required for proper optic cup formation and plays a critical early role in regulating regional retinal growth and the acquisition of shape during optic vesicle invagination.

Genetic alterations and oncogenic pathways associated with breast cancer subtypes

Breast cancers can be divided into subtypes with important implications for prognosis and treatment. We set out to characterize the genetic alterations observed in different breast cancer subtypes and to identify specific candidate genes and pathways associated with subtype biology. mRNA expression levels of estrogen receptor, progesterone receptor, and HER2 were shown to predict marker status determined by immunohistochemistry and to be effective at assigning samples to subtypes. HER2(+) cancers were shown to have the greatest frequency of high-level amplification (independent of the ERBB2 amplicon itself), but triple-negative cancers had the highest overall frequencies of copy gain. Triple-negative cancers also were shown to have more frequent loss of phosphatase and tensin homologue and mutation of RB1, which may contribute to genomic instability. We identified and validated seven regions of copy number alteration associated with different subtypes, and used integrative bioinformatics analysis to identify candidate oncogenes and tumor suppressors, including ERBB2, GRB7, MYST2, PPM1D, CCND1, HDAC2, FOXA1, and RASA1. We tested the candidate oncogene MYST2 and showed that it enhances the anchorage-independent growth of breast cancer cells. The genome-wide and region-specific differences between subtypes suggest the differential activation of oncogenic pathways.

BMP4-Smad signaling pathway mediates adriamycin-induced premature senescence in lung cancer cells

Cell senescence, an irreversible cell cycle arrest, reflects a safeguard program that limits the capacity of uncontrolled cell proliferation. Treatment of tumor cells with certain chemotherapeutic agents activates premature senescence to decrease the tumorigenecity. Here we show that sublethal concentrations of adriamycin could induce premature senescence in lung cancer cells. Adriamycin treatment resulted in the up-regulation of BMP4, which is underexpressed in NSCLC (non-small cell lung cancers). Moreover, the BMP4-Smad pathway played a key role in mediating adriamycin-induced senescence. Overexpression of BMP4 was able to induce premature senescence in lung cancer cells and this process required the participation of cyclin/cyclin-dependent kinase (cdk) inhibitors p16(INK4a) and p21(WAF1/cip1). We also show that increases of p16(INK4a) and p21(WAF1/cip1) expression in response to BMP4 were mediated by the Smad signaling pathway. Furthermore, our data revealed that p300 was recruited to P16(INK4a) and P21(WAF1/cip1) promoters by Smad1/5/8 to induce the hyperacetylation of histones H3 and H4 at the promoters. The present study provides useful clues to the evaluation of the potentiality of BMP4 as a responsive molecular target for cancer chemotherapy.

Expression of TBX2 promotes anchorage-independent growth and survival in the p53-negative SW13 adrenocortical carcinoma

The transcriptional regulator TBX2 is genetically amplified in several cancers and has, in addition, important roles in development. In carcinogenesis, TBX2 regulates the cell cycle by suppressing the expression of cyclin-dependent kinase (CDK) inhibitors and destabilizes p53 by suppressing expression of ARF. In embryogenesis, however, TBX2 appears to act independently of the cell cycle or p53 and is regulated by growth factors. Tumorigenic functions of TBX2 that are independent of p53 or cell cycle regulation remain poorly understood. Here we used SW13 carcinoma cells which express inactive p53 and have no detectable p16 or p21 CDK-inhibitors as a model to study these functions. Expression of TBX2 in SW13 cells had no effect on the cell cycle but promoted anchorage-independence and increased resistance to apoptotic stimuli including UV-irradiation, the cytotoxic drug doxorubicin and lethal endoplasmic-reticulum stress. This is a cell type-dependent effect as TBX2 overexpression in PANC1 pancreatic cancer cells which are p53-negative has no effect on colony formation or survival after irradiation. Mechanistically, in SW13 cells, TBX2 overexpression strongly reduced the activation of caspase 3, 8 and 9 following UV-irradiation but without altering the expression of the corresponding procaspases. There were, however, dramatic and specific decreases in the expression of procaspases 1 and 4. The expression of the inhibitor of apoptosis, cIAP2/BIRC3, increased in TBX2-overexpressing cells. TBX2 was upregulated in a PI3K-dependent manner by growth factors that are tumorigenic for SW13. Inhibition of Akt phosphorylation abrogates upregulation of TBX2 by FGF-4. Our findings identify TBX2 as a cell type-dependent survival factor under a p53-negative background, and are indicative of a potentially wider role for TBX2 in carcinogenesis than hitherto described.

Role of cyclin D1 as a mediator of c-Met- and beta-catenin-induced hepatocarcinogenesis

Activation of c-Met signaling and beta-catenin mutations are frequent genetic events observed in liver cancer development. Recently, we demonstrated that activated beta-catenin can cooperate with c-Met to induce liver cancer formation in a mouse model. Cyclin D1 (CCND1) is an important cell cycle regulator that is considered to be a downstream target of beta-catenin. To determine the importance of CCND1 as a mediator of c-Met- and beta-catenin-induced hepatocarcinogenesis, we investigated the genetic interactions between CCND1, beta-catenin, and c-Met in liver cancer development using mouse models. We coexpressed CCND1 with c-Met in mice and found CCND1 to cooperate with c-Met to promote liver cancer formation. Tumors induced by CCND1/c-Met had a longer latency period, formed at a lower frequency, and seemed to be more benign compared with those induced by beta-catenin/c-Met. In addition, when activated beta-catenin and c-Met were coinjected into CCND1-null mice, liver tumors developed despite the absence of CCND1. Intriguingly, we observed a moderate accelerated tumor growth and increased tumor malignancy in these CCND1-null mice. Molecular analysis showed an up-regulation of cyclin D2 (CCND2) expression in CCND1-null tumor samples, indicating that CCND2 may replace CCND1 in hepatic tumorigenesis. Together, our results suggest that CCND1 functions as a mediator of beta-catenin during HCC pathogenesis, although other molecules may be required to fully propagate beta-catenin signaling. Moreover, our data suggest that CCND1 expression is not essential for liver tumor development induced by c-Met and beta-catenin.

Tbx3 represses E-cadherin expression and enhances melanoma invasiveness

The T-box transcription factors Tbx2 and Tbx3 are overexpressed in many cancers and in melanoma promote proliferation by actively suppressing senescence. Whether they also contribute to tumor progression via other mechanisms is not known. Here, we identify a novel role for these factors, providing evidence that Tbx3, and potentially Tbx2, directly repress the expression of E-cadherin, a keratinocyte-melanoma adhesion molecule whose loss is required for the acquisition of an invasive phenotype. Overexpression of Tbx2 and Tbx3 in melanoma cells down-regulates endogenous E-cadherin expression, whereas depletion of Tbx3, but not Tbx2, increases E-cadherin mRNA and protein levels and decreases melanoma invasiveness in vitro. Consistent with these observations, in melanoma tissue, Tbx3 and E-cadherin expression are inversely correlated. Depletion of Tbx3 also leads to substantial up-regulation of Tbx2. The results suggest that Tbx2 and Tbx3 may play a dual role during the radial to vertical growth phase transition by both inhibiting senescence via repression of p21(CIP1) expression, and enhancing melanoma invasiveness by decreasing E-cadherin levels.

Anchorage-independent growth of pocket protein-deficient murine fibroblasts requires bypass of G2 arrest and can be accomplished by expression of TBX2

Mouse embryonic fibroblasts (MEFs) deficient for pocket proteins (i.e., pRB/p107-, pRB/p130-, or pRB/p107/p130-deficient MEFs) have lost proper G(1) control and are refractory to Ras(V12)-induced senescence. However, pocket protein-deficient MEFs expressing Ras(V12) were unable to exhibit anchorage-independent growth or to form tumors in nude mice. We show that depending on the level of pocket proteins, loss of adhesion induces G(1) and G(2) arrest, which could be alleviated by overexpression of the TBX2 oncogene. TBX2-induced transformation occurred only in the absence of pocket proteins and could be attributed to downregulation of the p53/p21(CIP1) pathway. Our results show that a balance between the pocket protein and p53 pathways determines the level of transformation of MEFs by regulating cyclin-dependent kinase activities. Since transformation of human fibroblasts also requires ablation of both pathways, our results imply that the mechanisms underlying transformation of human and mouse cells are not as different as previously claimed.

Expression of Tbx2 and Tbx3 in the developing hypothalamic-pituitary axis

TBX2 and TBX3 are transcription factors that belong to the T-box family, members of which play important roles during mammalian embryogenesis. Mutations in T-box genes have been linked to several human genetic disorders and increasing evidence suggests that Tbx2 and Tbx3 may play a key role in cancer. The primary functions of Tbx2 and Tbx3 remain poorly defined, mainly because of their widespread expression in several tissues and their multiple potential roles in morphogenesis, organogenesis and cell-fate commitment. Here, we describe in detail the expression of Tbx2 and Tbx3 in the developing hypothalamic-pituitary axis. Localized transcripts can be detected during the early stages of pituitary commitment. Expression of Tbx2 is restricted to the infundibular region of the ventral diencephalon (VD) at all ages examined, whereas Tbx3 can be detected in both the VD and Rathke's pouch, the precursor of the anterior pituitary. Outside the developing hypophyseal organ novel sites of Tbx3 and Tbx2 expression include migrating branchiomotor (BM) and visceromotor (VM) neurons in the hindbrain, neuroepithelial cells of the developing tongue (Tbx3) as well as the developing blood vessel network (Tbx2).

Expression and mutation analysis of Tpit in the canine pituitary gland and corticotroph adenomas

Pituitary-dependent hyperadrenocorticism (PDH) in dogs is caused by a pituitary corticotroph adenoma. Although PDH is a common disorder in dogs, little is known about the underlying pathogenesis. In the pituitary glands of humans and mice, the pro-opiomelanocortin (POMC)-expressing cell lineages, the corticotrophs and melanotrophs, have a specific marker in common, the T-box transcription factor Tpit (Tbx19), which is obligate for POMC expression. Tpit also regulates the late differentiation of the corticotrophs and melanotrophs, and therefore may contribute to the pathogenesis of the corticotroph adenomas. The aim of this study was to perform an expression and mutation analysis of Tpit in the normal canine pituitary and in corticotroph adenomas. The distribution of the Tpit protein in the pituitary gland was studied with immunohistochemistry and the expression of the gene with RT-PCR. The coding region of Tpit cDNA from 14 dogs with PDH was screened for mutations. Tpit was expressed in corticotroph and melanotroph cells of the normal and adenomatous canine pituitary, and remained present in non-adenomatous corticotrophs of pituitaries from PDH dogs. No tumor-specific mutation in the Tpit cDNA from the corticotroph adenomas was found. However, a missense polymorphism in the highly conserved DNA-binding domain, the T-box, was discovered in one dog. It is concluded that Tpit can be used as a reliable marker for the corticotroph and melanotroph cells in the canine pituitary tissue and that mutations in the Tpit gene are unlikely to play a major role in the pathogenesis of canine corticotroph adenomas.

A role for Tbx2 in the regulation of the alpha2(1) collagen gene in human fibroblasts

The T-box gene family encodes highly conserved transcription factors that play important roles in embryonic development and have been implicated in carcinogenesis. One member of the family, Tbx2, is generally regarded as a transcriptional repressor but appears to be capable of functioning as an activator depending on the cellular context. This study shows that Tbx2 is expressed in normal human fibroblasts but is drastically reduced in several transformed fibroblast cell lines. This pattern of Tbx2 expression correlated with that observed for the human alpha2(1) collagen gene (COL1A2). Interestingly, stable expression of transfected Tbx2 in transformed fibroblast cell lines further reduces expression of the human endogenous COL1A2 gene. This ability of Tbx2 to repress the human COL1A2 gene was confirmed in luciferase reporter assays and shown to be independent of the consensus T-box binding element.

UV-mediated regulation of the anti-senescence factor Tbx2

Several lines of evidence have implicated members of the developmentally important T-box gene family in cell cycle regulation and in cancer. Importantly, the highly related T-box factors Tbx2 and Tbx3 can suppress senescence through repressing the cyclin-dependent kinase inhibitors p19(ARF) and p21(WAF1/CIP1/SDII). Furthermore, Tbx2 is up-regulated in several cancers, including melanomas where it was shown to function as an anti-senescence factor, suggesting that this may be one of the mechanisms by which T-box proteins contribute to the oncogenic process. However, very little is known about whether Tbx2 is regulated by p21-mediated stress-induced senescence signaling pathways. In this study, using the MCF-7 breast cancer cell line known to overexpress Tbx2, we show that in response to stress induced by ultraviolet irradiation the Tbx2 protein is specifically phosphorylated by the p38 mitogen-activated protein kinase. Using site-directed mutagenesis and in vitro kinase assays, we have identified serine residues 336, 623, and 675 in the Tbx2 protein as the p38 target sites and show that these sites are phosphorylated in vivo. Importantly, we show by Western blotting, immunofluorescence, and reporter assays that this phosphorylation leads to increased Tbx2 protein levels, predominant nuclear localization of the protein, and an increase in the ability of Tbx2 to repress the p21(WAF1/CIP1/SDII) promoter. These results show for the first time that the ability of Tbx2 to repress the p21 gene is enhanced in response to a stress-induced senescence pathway, which leads to a better understanding of the regulation of the anti-senescence function of Tbx2.

Proteomic analysis of protein expression changes in a model of gliomagenesis

Loss of p53 function is a common event in a variety of human cancers including tumors of glial origin. Using an in vitro mouse model of malignant astrocyte transformation, three cleavable isotope coded affinity tag (cICAT) experiments were performed comparing cultured wild-type astrocytes and two p53(-/-) astrocyte cultures before and after malignant transformation. We identified and quantitated an average of 1366 proteins per experiment and demonstrated that the protein quantitation ratios in each individual cICAT experiment correlated well to ratios determined in the other two studies. These data were further supported by microarray analysis which also correlated to changes in protein expression. The results showed significant changes in protein expression in association with malignant transformation. Proteins overexpressed in malignant astrocytes were typically involved in ribosome biogenesis/protein synthesis and DNA replication, while underexpressed proteins were generally associated with the regulation of cell cycle checkpoint control, tumor suppression, and apoptosis. Among the significantly up-regulated proteins and transcripts in malignant mouse astrocytes were members of the minichromosome maintenance (MCM) family. Western blot analysis verified increased expression of MCM proteins in malignant human astrocytoma cell lines, which had not previously been described. These results demonstrate the usefulness of the cICAT approach for comparing differences in protein expression profiles between normal and malignant cells.

T-box factors determine cardiac design

The heart of higher vertebrates is a structurally complicated multi-chambered pump that contracts synchronously. For its proper function a number of distinct integrated components have to be generated, including force-generating compartments, unidirectional valves, septa and a system in charge of the initiation and coordinated propagation of the depolarizing impulse over the heart. Not surprisingly, a large number of regulating factors are involved in these processes that act in complex and intertwined pathways to regulate the activity of target genes responsible for morphogenesis and function. The finding that mutations in T-box transcription factor-encoding genes in humans lead to congenital heart defects has focused attention on the importance of this family of regulators in heart development. Functional and genetic analyses in a variety of divergent species has demonstrated the critical roles of multiple T-box factor gene family members, including Tbx11, −2, −3, −5, −18 and −20, in the patterning, recruitment, specification, differentiation and growth processes underlying formation and integration of the heart components. Insight into the roles of T-box factors in these processes will enhance our understanding of heart formation and the underlying molecular regulatory pathways.

Ectopic Tbx2 expression results in polyploidy and cisplatin resistance

T-box factors play critical roles in embryonic development and have been implicated in cell cycle regulation and cancer. For example, Tbx2 can suppress senescence through a mechanism involving the repression of the cyclin-dependent kinase inhibitors, p19(ARF) and p21(WAF1/CIP1/SDII), and the Tbx2 gene is deregulated in melanoma, breast and pancreatic cancers. In this study, several transformed human lung fibroblast cell lines were shown to downregulate Tbx2. To further investigate the role of Tbx2 in oncogenesis we therefore stably reexpressed Tbx2 in one such cell line. Compared to their parental cells, the resulting Tbx2-expressing cells are larger, with binucleate and lobular nuclei containing double the number of chromosomes. Moreover, these cells had an increase in frequency of several features of genomic instability such as chromosome missegregation, chromosomal rearrangements and polyploidy. While grossly abnormal, these cells still divide and give rise to cells that are resistant to the chemotherapeutic drug cisplatin. Furthermore, this is shown to be neither species nor cell type dependent, as ectopically expressing Tbx2 in a murine melanoma cell line also induce mitotic defects and polyploidy. These results have important implications for our understanding of the role of Tbx2 in tumorigenesis because polyploidy frequently precedes aneuploidy, which is associated with high malignancy and poor prognosis.

Transcriptional Repression by the T-box Proteins Tbx18 and Tbx15 Depends on Groucho Corepressors

Tbox18 (Tbx18) and Tbox15 (Tbx15) encode a closely related pair of vertebrate-specific T-box (Tbx) transcription factors. Functional analyses in the mouse have proven the requirement of Tbx15 in skin and skeletal development and of Tbx18 in the formation of the vertebral column, the ureter, and the posterior pole of the heart. Despite the accumulation of genetic data concerning the embryological roles of these genes, it is currently unclear how Tbx18 and Tbx15 exert their function on the molecular level. Here, we have initiated a molecular analysis of Tbx18 and Tbx15 proteins and have characterized functional domains for nuclear localization, DNA binding, and transcriptional modulation. We show that both proteins homo- and heterodimerize, bind to various combinations of T half-sites, and repress transcription in a Groucho-dependent manner. Competition with activating T-box proteins may constitute one mode of action as we show that Tbx18 interacts with Gata4 and Nkx2-5 and competes Tbx5-mediated activation of the cardiac Natriuretic peptide precursor type a-promoter and that ectopic expression of Tbx18 down-regulates Tbx6-activated Delta-like 1 expression in the somitic mesoderm in vivo.

T-box factors: targeting to chromatin and interaction with the histone H3 N-terminal tail

T-box transcription factors play a crucial role in development where they are implicated in patterning and cell fate decisions. Tbx2 and Tbx3 have also been implicated in several cancers including melanoma, and can act as antisenescence factors through their ability to repress p19(ARF) and p21(CIP1) expression. Although several target genes for T-box factors have been identified, it is unknown whether this family of proteins can bind chromatin, a property that would facilitate the epigenetic reprogramming that occurs in both development and cancer progression. Here, we show that Tbx2 has the potential to recognize mitotic chromatin in a DNA-dependent fashion, can interact specifically with the histone H3 N-terminal tail, a property shared with Tbx4, Tbx5 and Tbx6, and can also recognize nucleosomal DNA, with binding to nucleosomes being antagonized by the presence of the histone tails. Strikingly, in vivo Tbx2 co-localization with pericentric heterochromatin appears to be regulated and ectopic expression of Tbx2 leads to severe mitotic defects. Taken together our results suggest that Tbx2, and most likely other members of the T-box family, are able to target chromatin and may indicate a role for the T-box factors in epigenetic reprogramming events.

Confirmation of a BRAF mutation-associated gene expression signature in melanoma

Mutations in the BRAF oncogene occur in the majority of melanomas, leading to the activation of the mitogen-activated protein kinase pathway and the transcription of downstream effectors. As BRAF and its effectors could be good melanoma therapy targets, defining the repertoire of genes that are differentially regulated because of BRAF mutational activation is an important objective. Towards this goal, we and others have attempted to determine whether a BRAF mutation-associated gene expression profile exists. Results have been mixed, with some groups reporting a BRAF-signature and another group not. Here we resolve this issue and confirm that while gene-by-gene correlations fail to reveal a specific gene(s) whose expression correlates with BRAF status, a BRAF signature can be distinguished by analysis of global expression patterns. Specifically, we have here applied support vector machine (SVM) analysis to Affymetrix microarray data from a panel of 63 melanoma cell lines. SVMs found a BRAF signature in training samples and predicted BRAF mutation status with high accuracy (AUC=0.840) in the remaining samples. We verified this is a generalized BRAF signature by repeating the analysis in three published microarray datasets, and again found that SVMs predicted BRAF mutation well (Philadelphia: AUC=0.788; Zurich: AUC=0.688; Mannheim: AUC=0.686). An ensemble of 300 SVMs trained on our data also predicted BRAF mutation status in two of the three published datasets (Philadelphia AUC=0.778; Zurich AUC=0.719; Mannheim AUC=0.564). Taken together, these data support the existence of a BRAF mutation-specific expression signature.

Role for histone deacetylase 1 in human tumor cell proliferation

Posttranslational modifications of core histones are central to the regulation of gene expression. Histone deacetylases (HDACs) repress transcription by deacetylating histones, and class I HDACs have a crucial role in mouse, Xenopus laevis, zebra fish, and Caenorhabditis elegans development. The role of individual class I HDACs in tumor cell proliferation was investigated using RNA interference-mediated protein knockdown. We show here that in the absence of HDAC1 cells can arrest either at the G(1) phase of the cell cycle or at the G(2)/M transition, resulting in the loss of mitotic cells, cell growth inhibition, and an increase in the percentage of apoptotic cells. On the contrary, HDAC2 knockdown showed no effect on cell proliferation unless we concurrently knocked down HDAC1. Using gene expression profiling analysis, we found that inactivation of HDAC1 affected the transcription of specific target genes involved in proliferation and apoptosis. Furthermore, HDAC2 downregulation did not cause significant changes compared to control cells, while inactivation of HDAC1, HDAC1 plus HDAC2, or HDAC3 resulted in more distinct clusters. Loss of these HDACs might impair cell cycle progression by affecting not only the transcription of specific target genes but also other biological processes. Our data support the idea that a drug targeting specific HDACs could be highly beneficial in the treatment of cancer.