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

The level of BMP4 signaling is critical for the regulation of distinct T-box gene expression domains and growth along the dorso-ventral axis of the optic cup

BACKGROUND

Polarised gene expression is thought to lead to the graded distribution of signaling molecules providing a patterning mechanism across the embryonic eye. Bone morphogenetic protein 4 (Bmp4) is expressed in the dorsal optic vesicle as it transforms into the optic cup. Bmp4 deletions in human and mouse result in failure of eye development, but little attempt has been made to investigate mammalian targets of BMP4 signaling. In chick, retroviral gene overexpression studies indicate that Bmp4 activates the dorsally expressed Tbx5 gene, which represses ventrally expressed cVax. It is not known whether the Tbx5 related genes, Tbx2 and Tbx3, are BMP4 targets in the mammalian retina and whether BMP4 acts at a distance from its site of expression. Although it is established that Drosophila Dpp (homologue of vertebrate Bmp4) acts as a morphogen, there is little evidence that BMP4 gradients are interpreted to create domains of BMP4 target gene expression in the mouse.

RESULTS

Our data show that the level of BMP4 signaling is critical for the regulation of distinct Tbx2, Tbx3, Tbx5 and Vax2 gene expression domains along the dorso-ventral axis of the mouse optic cup. BMP4 signaling gradients were manipulated in whole mouse embryo cultures during optic cup development, by implantation of beads soaked in BMP4, or the BMP antagonist Noggin, to provide a local signaling source. Tbx2, Tbx3 and Tbx5, showed a differential response to alterations in the level of BMP4 along the entire dorso-ventral axis of the optic cup, suggesting that BMP4 acts across a distance. Increased levels of BMP4 caused expansion of Tbx2 and Tbx3, but not Tbx5, into the ventral retina and repression of the ventral marker Vax2. Conversely, Noggin abolished Tbx5 expression but only shifted Tbx2 expression dorsally. Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.

CONCLUSION

Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up. Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

The other pigment cell: specification and development of the pigmented epithelium of the vertebrate eye

Vertebrate retinal pigment epithelium (RPE) cells are derived from the multipotent optic neuroepithelium, develop in close proximity to the retina, and are indispensible for eye organogenesis and vision. Recent advances in our understanding of RPE development provide evidence for how critical signaling factors operating in dorso-ventral and distal-proximal gradients interact with key transcription factors to specify three distinct domains in the budding optic neuroepithelium: the distal future retina, the proximal future optic stalk/optic nerve, and the dorsal future RPE. Concomitantly with domain specification, the eye primordium progresses from a vesicle to a cup, RPE pigmentation extends towards the ventral side, and the future ciliary body and iris form from the margin zone between RPE and retina. While much has been learned about the molecular networks controlling RPE cell specification, key questions concerning the cell proliferative parameters in RPE and the subsequent morphogenetic events still need to be addressed in greater detail.

Tbx20 regulation of endocardial cushion cell proliferation and extracellular matrix gene expression

While recent work has implicated Tbx20 in myocardial maturation and proliferation, the role of Tbx20 in heart valve development remains relatively unknown. Tbx20 expression was manipulated in primary avian endocardial cells in order to elucidate its function in developing endocardial cushions. Tbx20 gain of function was achieved with a Tbx20-adenovirus, and endogenous Tbx20 expression was inhibited with Tbx20-specific siRNA in cultured endocardial cushion cells. With Tbx20 gain of function, the expression of chondroitin sulfate proteoglycans (CSPG), including aggrecan and versican, was decreased, while the expression of the matrix metalloproteinases (MMP) mmp9 and mmp13 was increased. Consistent results were observed with Tbx20 loss of function, where the expression of CSPG genes increased and MMP genes decreased. In addition, cushion mesenchyme proliferation increased with infection of a Tbx20-adenovirus and decreased with transfection of Tbx20-specfic siRNA. Furthermore, BMP2 treatment resulted in increased Tbx20 expression in endocardial cushion cells, and loss of Tbx20 led to increased Tbx2 and decreased N-myc gene expression. Taken together, these data support a role for Tbx20 in repressing extracellular matrix remodeling and promoting cell proliferation in mesenchymal valve precursor populations in endocardial cushions during embryonic development.

The microphthalmia-associated transcription factor Mitf interacts with beta-catenin to determine target gene expression

Commitment to the melanocyte lineage is characterized by the onset of expression of the microphthalmia-associated transcription factor (Mitf). This transcription factor plays a fundamental role in melanocyte development and maintenance and seems to be crucial for the survival of malignant melanocytes. Furthermore, Mitf has been shown to be involved in cell cycle regulation and to play important functions in self-renewal and maintenance of melanocyte stem cells. Although little is known about how Mitf regulates these various processes, one possibility is that Mitf interacts with other regulators. Here we show that Mitf can interact directly with beta-catenin, the key mediator of the canonical Wnt signaling pathway. The Wnt signaling pathway plays a critical role in melanocyte development and is intimately involved in triggering melanocyte stem cell proliferation. Significantly, constitutive activation of this pathway is a feature of a number of cancers including malignant melanoma. Here we show that Mitf can redirect beta-catenin transcriptional activity away from canonical Wnt signaling-regulated genes toward Mitf-specific target promoters to activate transcription. Thus, by a feedback mechanism, Mitf can diversify the output of canonical Wnt signaling to enhance the repertoire of genes regulated by beta-catenin. Our results reveal a novel mechanism by which Wnt signaling and beta-catenin activate gene expression, with significant implications for our understanding of both melanocyte development and melanoma.

Cellular senescence and cancer treatment

Cellular senescence, an irreversible cell-cycle arrest, reflects a safeguard program that limits the proliferative capacity of the cell exposed to endogenous or exogenous stress signals. A number of recent studies have clarified that an acutely inducible form of cellular senescence may act in response to oncogenic activation as a natural barrier to interrupt tumorigenesis at a premalignant level. Paralleling the increasing insights into premature senescence as a tumor suppressor mechanism, a growing line of evidence identifies cellular senescence as a critical effector program in response to DNA damaging chemotherapeutic agents. This review discusses molecular pathways to stress-induced senescence, the interference of a terminal arrest condition with clinical outcome, and the critical overlap between premature senescence and apoptosis as both tumor suppressive and drug-responsive cellular programs.

Disruption of Scaffold Attachment Factor B1 Leads to TBX2 Up-regulation, Lack of p19ARFInduction, Lack of Senescence, and Cell Immortalization

Scaffold attachment factor B1 (SAFB1) is a multifunctional protein, which has previously been implicated in breast cancer. Here, we show that genetic deletion of SAFB1 in mouse embryonic fibroblasts (MEF) leads to spontaneous immortalization and altered expression of two proteins involved in immortalization and escape from senescence: low levels of p19(ARF) and high levels of TBX2. Inactivation of TBX2 using a dominant-negative TBX2 resulted in up-regulation of p19(ARF) in SAFB1 knockout MEFs. SAFB1 loss also caused lack of contact inhibition, increased foci formation, and increased oncogene-induced anchorage-independent growth. These findings suggest that SAFB1 is a novel player in cellular immortalization and transformation.

MITF: master regulator of melanocyte development and melanoma oncogene

Microphthalmia-associated transcription factor (MITF) acts as a master regulator of melanocyte development, function and survival by modulating various differentiation and cell-cycle progression genes. It has been demonstrated that MITF is an amplified oncogene in a fraction of human melanomas and that it also has an oncogenic role in human clear cell sarcoma. However, MITF also modulates the state of melanocyte differentiation. Several closely related transcription factors also function as translocated oncogenes in various human malignancies. These data place MITF between instructing melanocytes towards terminal differentiation and/or pigmentation and, alternatively, promoting malignant behavior. In this review, we survey the roles of MITF as a master lineage regulator in melanocyte development and its emerging activities in malignancy. Understanding the molecular function of MITF and its associated pathways will hopefully shed light on strategies for improving therapeutic approaches for these diseases.

Coexpression of NRASQ61Rand BRAFV600Ein Human Melanoma Cells Activates Senescence and Increases Susceptibility to Cell-Mediated Cytotoxicity

Activating mutations in BRAF and NRAS oncogenes in human melanomas are mutually exclusive. This finding has suggested an epistatic relationship but is consistent even with synthetic lethality. To evaluate the latter possibility, a mutated NRAS(Q61R) oncogene was expressed, under a constitutive or a doxycycline-regulated promoter, in a metastatic melanoma clone (clone 21) harboring an activated BRAF(V600E) oncogene. After the first 10 to 12 in vitro passages, the constitutive NRAS(Q61R) transfectant displayed progressive accumulation in G(0)-G(1) phase of the cell cycle and stained for the senescence-associated beta-galactosidase activity (SA-beta-Gal). Inducible expression of NRAS(Q61R), by the Tet-Off system, in clone 21 cells (21NRAS(61ON)) led to overactivation of the RAS/RAF/mitogen-activated protein kinase signaling pathway and, after the 10th in vitro passage, led to promotion of senescence. This was documented by reduced proliferation, flattened cell morphology, reduced growth in Matrigel, positive staining for SA-beta-Gal, and expression of AMP-activated protein kinase and of the cell cycle inhibitor p21(waf1/Cip1). These effects were detected neither in 21 cells with silenced NRAS(Q61R) (21NRAS(61OFF)) nor in cells transfected with an inducible wild-type NRAS gene (21NRAS(WTON)). In addition, when compared with parental 21 cells, or with 21NRAS(61OFF), 21NRAS(61ON) and constitutive NRAS(Q61R) transfectants cells showed increased susceptibility to cytotoxicity by both HLA class I antigen-restricted and nonspecific T cells and up-regulation of several MHC class I antigen processing machinery components. These results suggest a relationship of synthetic lethality between NRAS and BRAF oncogenes, leading to selection against "double-mutant" cells.

Cell cycle regulation of the T-box transcription factor tbx2

T-box transcription factors play key roles in development and in particular the determination or maintenance of cell fate. Tbx2 is a transcriptional repressor implicated in several developmental processes and which has also been implicated in cancer through its ability to suppress senescence via repression of the p19(ARF) and p21(Cip1) (CDKN1A) promoters. However, despite its importance, little is known about how Tbx2 may be regulated. Here, we show that Tbx2 protein expression is tightly regulated during cell cycle progression, with levels being low in G1, increasing in mid-S-phase and persisting at high levels though G2 until finally undergoing a dramatic reduction at the onset of mitosis. Moreover, in S-phase, Tbx2 is present at a subset of late, but not early, replication foci and a significant fraction of Tbx2 is tightly associated with the nucleus in small DNA-associated foci that do not correspond with telomeres, PML or cajal bodies. The results are consistent with Tbx2 playing a role in cell cycle progression and organization of subnuclear compartments.

Oncogene-induced senescence: putting the brakes on tumor development

Cellular senescence, a permanent cell cycle arrest, is considered a safeguard mechanism that may prevent aged or abnormal cells from further expansion. Although the term "replicative senescence" stands for the widely accepted model of a terminal growth arrest due to telomere attrition, the significance of "oncogene-inducible senescence" remained an issue of debate over the years. A number of recent studies now show the effect of this acute and telomere-independent form of senescence as a tumor-protective, fail-safe mechanism in vivo that shares conceptual and possibly therapeutic similarities with the genetically encoded apoptosis machinery.

T-box genes in vertebrate development

The myriad developmental roles served by the T-box family of transcription factor genes defy easy categorization. Present in all metazoans, the T-box genes are involved in early embryonic cell fate decisions, regulation of the development of extraembryonic structures, embryonic patterning, and many aspects of organogenesis. They are unusual in displaying dosage sensitivity in most instances. In humans, mutations in T-box genes are responsible for developmental dysmorphic syndromes, and several T-box genes have been implicated in neoplastic processes. T-box transcription factors function in many different signaling pathways, notably bone morphogenetic protein and fibroblast growth factor pathways. The few downstream target genes that have been identified indicate a wide range of downstream effectors.

T-box transcription factors and their roles in regulatory hierarchies in the developing heart

T-box transcription factors are important players in the molecular circuitry that generates lineage diversity and form in the developing embryo. At least seven family members are expressed in the developing mammalian heart, and the human T-box genes TBX1 and TBX5 are mutated in cardiac congenital anomaly syndromes. Here, we review T-box gene function during mammalian heart development in the light of new insights into heart morphogenesis. We see for the first time how hierarchies of transcriptional activation and repression involving multiple T-box factors play out in three-dimensional space to establish the cardiac progenitors fields, to define their subservient lineages, and to generate heart form and function.

Current strategies in overcoming resistance of cancer cells to apoptosis melanoma as a model

Most anticancer agents mediate their effects through common pathways which induce apoptosis or in some cases necrosis of cancer cells. The apoptotic pathways are regulated by Bcl-2 family proteins, which include both pro- and anti-apoptotic members. Much is known about the interactions of these proteins involved in apoptosis and this information is being utilized in the development of new reagents that may be used to treat patients with cancers. The inhibitor of apoptosis family of proteins constitute a second group of proteins which inhibit the effector caspases. Reagents that inhibit their activity are also under development. Resistance of cancer cells to treatment can in many instances be attributed to activation of intracellular signal pathways involved in survival, such as the Ras-Raf-MEK-ERK1/2 or the P13K-Akt pathway. Again, much has been learned about the control of these pathways and their activation of resistance mechanisms. Inhibitors of such pathways are being evaluated in preclinical and clinical studies and are showing promise as a new class of anticancer agents. Much of the progress in future studies will likely depend on the ability to target these new treatments to particular subgroups of patients with tumor characteristics that make them responsive to the agents in question.

MITF and cell proliferation: the role of alternative splice forms

Recent studies show that the melanocyte transcription factor MITF not only activates differentiation genes but also genes involved in the regulation of the cell cycle, suggesting that it provides a link between cell proliferation and differentiation. MITF, however, comes in a variety of splice isoforms with potentially distinct biological activities. In particular, there are two isoforms, (-) and (+) MITF, that differ in six residues located upstream of the DNA binding basic domain and show slight differences in the efficiency with which they bind to target DNA. Using in vitro BrdU incorporation assays and FACS analysis in transiently transfected cells, we show that (+) MITF has a strong inhibitory effect on DNA synthesis while (-) MITF has none or only a mild one. The strong inhibitory activity of (+) MITF is not influenced by a number of mutations that modulate MITF's transcriptional activities and is independent of the protein's carboxyl terminus but dependent on its aminoterminus. A further dissection of the molecule points to the importance of an aminoterminal serine, serine-73, which in both isoforms is phosphorylated to comparable degrees. The results suggest that one or several aminoterminal domains cooperate with the alternatively spliced hexapeptide to render MITF anti-proliferative in a way that does not depend on direct E box binding.

Histone deacetylase inhibitors and malignant melanoma

The search for antimelanoma agents acting by terminal differentiation via the pigmentation pathway has so far been unsuccessful, in part because of tumor heterogeneity and loss of function of pigmentation genes. Some differentiation agents, however, have emerged as inhibitors of histone deacetylases (HDAC), with consequences for chromosome remodeling, cell cycle arrest and selective toxicity in cultured melanoma cells compared with normal melanocytes. Few effects have been found on pigmentation, except paradoxically the down-regulation of TRP-1. Of the many genes regulated by HDAC inhibitors, induction of p21(WAF1/Cip1) is the most consistent finding and is associated with G(1) or G(2) phase blocks. Some melanoma cell lines appear to lack an HDAC inhibitor-specific G(2) checkpoint and viability is thus compromised by dividing with inappropriately-modified chromatin. Most cultured melanoma cells undergo apoptosis following treatment with HDAC inhibitors, via a mitochondrial and caspase-dependent pathway. However, the molecular mechanism may vary with cell line and HDAC inhibitor class. Tumor selectivity cannot yet be attributed to specific types or levels of HDACs, nor has the possibility of acetylation of non-histone targets been excluded. Elucidation of these complexities may be rewarding, in terms of directing the multiple consequences of inhibiting histone deacetylation towards overcoming the therapeutic problems of melanoma heterogeneity and emergence of resistance. Success in the clinic may require combination with agents that synergize with the cell cycle blocking and pro-apoptotic action of HDAC inhibitors.