The anti-proliferative function of the TGF-ß1 signaling pathway involves the repression of the oncogenic TBX2 by its homologue TBX3.

A growing body of work has shown that the highly homologous T-box transcription factors TBX2 and TBX3 play critical but distinct roles in embryonic development and cancer progression. For example, TBX2 and TBX3 are up-regulated in several cancers and recent evidence suggests that whereas TBX2 functions as a pro-proliferative factor, TBX3 inhibits cell proliferation but promotes cancer cell migration and invasion. While the molecular mechanisms regulating these functions of TBX2 and TBX3 are poorly understood we recently reported that the TGF-ß1 signaling pathway up-regulates TBX3 expression to mediate, in part, its well described anti-proliferative and pro-migratory roles. The TBX3 targets responsible for these functions were however not identified. Here we reveal for the first time that the TGF-ß1 signaling pathway represses TBX2 transcriptionally and we provide a detailed mechanism to show that this is mediated by TBX3. Furthermore, we implicate the down-regulation of TBX2 in the anti-proliferative function of the TGF-ß1-TBX3 axis. These findings have important implications for our understanding of the regulation of TBX2 and TBX3 and shed light on the mechanisms involved in the anti-proliferative and pro-migratory roles of TGF-ß1.

The oncogenic TBX3 is a downstream target and mediator of the TGF-ß1 signaling pathway.

The T-box transcription factor, TBX3, plays an important role in embryonic development, and haploinsufficiency of TBX3 causes ulnar-mammary syndrome. Overexpression of TBX3, on the other hand, is associated with several cancers, and preliminary evidence suggests that increased levels of TBX3 may inhibit cell proliferation but promote tumor migration and invasion. Although this suggests that deregulated levels of TBX3 are deleterious in development and promotes disease, very little is known about the signaling pathways that regulate TBX3 expression. Here we show that overexpressing TBX3 inhibits proliferative ability while promoting the migration of breast epithelial cells. We demonstrate that the transforming growth factor ß1 (TGF-ß1) pathway up-regulates TBX3 protein and mRNA levels and show a detailed transcriptional mechanism by which this occurs. Using in vitro and in vivo assays, we show that Smad3/4 and JunB bind and cooperatively regulate TBX3 promoter activity through a Smad-binding element at -67 base pairs. Further, we show that TBX3 plays a pivotal role in mediating the antiproliferative and promigratory role of TGF-ß1 in breast epithelial and skin keratinocytes. This study identifies the TGF-ß1 signaling pathway as a potentially important player in the regulation of TBX3 in development and cancer.

The Highly Homologous T-Box Transcription Factors, TBX2 and TBX3, Have Distinct Roles in the Oncogenic Process.

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.