The T-box transcription factors TBX2 and TBX3 are overexpressed in several cancers and are able to bypass senescence by repressing ARF and p21 WAF1/CIP1/SDII . Although these studies suggest that they may both contribute to the oncogenic process by repressing common targets, whether they have redundant or distinct roles in cancers where they are both overexpressed remains to be elucidated. Importantly, when Tbx2 function is inhibited in melanoma cells lacking Tbx3, the cells senesce, but whether this is possible in melanoma cells overexpressing both proteins is not known. An understanding of this issue may have important implications for the design of an effective pro-senescence therapy. In this study, the authors used a sh-RNA approach to knock down TBX2 and TBX3 individually in 2 human melanoma cell lines that overexpress both these factors and then examined their specific involvement in the oncogenic process. They demonstrate, using in vitro and in vivo cell proliferation, as well as colony-and tumor-forming ability and cell motility assays, that TBX2 and TBX3 have distinct roles in melanoma progression. In the tested lines, although TBX2 could promote proliferation and transformation and was required by primary melanoma cells for immortality, TBX3 was required for tumor formation and cell migration. These findings were reproducible in a human breast cancer cell line, which confirms that TBX2 and TBX3, although highly homologous, do not have redundant roles in the transformation process of cancers where they are both overexpressed. These results have important implications for the development of new cancer treatments and in particular for melanoma, which is a highly aggressive and intractable cancer.

TBX3, a member of the ancient and evolutionary conserved T-box transcription factor family, is a critical developmental regulator of several structures including the heart, mammary glands, limbs and lungs. Indeed, mutations in the human TBX3 lead to ulnar mammary syndrome which is characterized by several clinical malformations including hypoplasia of the mammary and apocrine glands, defects of the upper limb, areola, dental structures, heart and genitalia. In contrast, TBX3 has no known function in adult tissues but is frequently overexpressed in a wide range of epithelial and mesenchymal derived cancers. This overexpression greatly impacts several hallmarks of cancer including bypass of senescence, apoptosis and anoikis, promotion of proliferation, tumour formation, angiogenesis, invasion and metastatic capabilities as well as cancer stem cell expansion. The debilitating consequences of having too little or too much TBX3 suggest that its expression levels need to be tightly regulated. While we have a reasonable understanding of the mutations that result in low levels of functional TBX3 during development, very little is known about the factors responsible for the overexpression of TBX3 in cancer. Furthermore, given the plethora of oncogenic processes that TBX3 impacts, it must be regulating several target genes but to date only a few have been identified and characterised. Interestingly, while there is compelling evidence to support oncogenic roles for TBX3, a few studies have indicated that it may also have tumour suppressor functions in certain contexts. Together, the diverse functional elasticity of TBX3 in development and cancer is thought to involve, in part, the protein partners that it interacts with and this area of research has recently received some attention. This review provides an insight into the significance of TBX3 in development and cancer and identifies research gaps that need to be explored to shed more light on this transcription factor.

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The T-box transcription factor, TBX3, is overexpressed in several cancers and has been proposed as a chemotherapeutic target. Several lines of evidence suggest that TBX3 may be a key contributor to malignant melanoma, a highly aggressive and intractable disease. Using in vitro and in vivo assays we demonstrate here for the first time that overexpressing TBX3 in non-tumourigenic early stage melanoma cells is sufficient to promote tumour formation and invasion. Furthermore, we show that TBX3 may play an important role as a reciprocal switch between substrate dependent cell proliferation and tumour invasion.

The AKT3 signalling pathway plays a critical role in melanoma formation and invasion and components of this signalling cascade are therefore attractive targets for the treatment of malignant melanoma. Recent evidence show that the embryonically important TBX3 transcription factor is upregulated in a subset of melanomas and plays a key role in promoting melanoma formation and invasion, in part by repressing the cell adhesion molecule E-cadherin. We have identified TBX3 as a key substrate of AKT3 in melanomagenesis. Briefly, using site-directed mutagenesis and in vitro kinase assays, we have identified the AKT3 target site at serine residue 720 in the TBX3 protein and show that this site is phosphorylated in vivo. Importantly, we show by western blotting, immunofluorescence, reporter, migration and invasion assays that the phosphorylation at S720 promotes TBX3 protein stability, nuclear localization, transcriptional repression of E-cadherin, and its role in cell migration and invasion. Our results identify a novel signalling and transcriptional network linking AKT3, TBX3 and E-cadherin during melanoma migration and invasion and reveals TBX3 as a potential target for anti-metastatic therapeutics.

The transcription factor, TBX3, is critical for the formation of, among other structures, the heart, limbs and mammary glands and haploinsufficiency of the human TBX3 gene result in ulnar-mammary syndrome which is characterized by hypoplasia of these structures. On the other hand, the overexpression of TBX3 is a feature of a wide range of cancers and it has been implicated in several aspects of the oncogenic process. This includes its ability to function as an immortalizing gene and to promote proliferation through actively repressing negative cell cycle regulators. Together this suggests that TBX3 levels may need to be tightly regulated during the cell cycle. Here we demonstrate that this is indeed the case and that TBX3 mRNA and protein levels peak at S-phase and that the TBX3 protein is predominantly localized to the nucleus of S-phase cells. The increased levels of TBX3 in S-phase are shown to occur transcriptionally through activation by c-Myc at E-box motifs located at ¡1210 and ¡701 bps and post-translationally by cyclin A-CDK2 phosphorylation. Importantly, when TBX3 is depleted by shRNA the cells accumulate in S-phase. These results suggest that TBX3 is required for cells to transit through S-phase and that this function may be linked to its role as a proproliferative factor.

The emergence of drug resistant tumours that are able to escape cell death pose a major problem in the treatment of cancers. Tumours develop resistance to DNA-damaging chemotherapeutic agents by acquiring the ability to repair their DNA. Combination therapies that induce DNA damage and disrupt the DNA damage repair process may therefore prove to be more effective against such tumours. The developmentally important transcription factor TBX2 has been suggested as a novel anticancer drug target, as it is overexpressed in several cancers and possesses strong anti-senescence and pro-proliferative functions. Importantly, we recently showed that when TBX2 is silenced, we are able to reverse several features of transformation in both breast cancer and melanoma cells. Overexpression of TBX2 has also been linked to drug resistance and we have shown that its ectopic expression results in genetically unstable polyploidy cells with resistance to cisplatin. Whether the overexpression of endogenous TBX2 levels is associated with cisplatin resistance in TBX2-driven cancers has, however, not been shown. To address this we have silenced TBX2 in a cisplatin-resistant breast cancer cell line and we show that knocking down TBX2 sensitises the cells to cisplatin by disrupting the ATM-CHK2-p53 signalling pathway. Cell cycle analyses demonstrate that when TBX2 is knocked down there is an abrogation of an S-phase arrest but a robust G2/M arrest that correlates with a reduction in phosphorylated CHK2 and p53 levels. This prevents DNA repair resulting in TBX2-deficient cells entering mitosis with damaged DNA and consequently undergoing mitotic catastrophe. These results suggest that targeting TBX2 in combination with chemotherapeutic drugs such as cisplatin could improve the efficacy of current anticancer treatments.

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