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The epithelial-mesenchymal transition (EMT) is an embryonic transdifferentiation process consisting of conversion of polarized epithelial cells to motile mesenchymal ones. EMT-inducing transcription factors are aberrantly expressed in multiple tumor types and are known to favor the metastatic dissemination process. Supporting oncogenic activity within primary lesions, the TWIST and ZEB proteins can prevent cells from undergoing oncogene-induced senescence and apoptosis by abolishing both p53- and RB-dependent pathways. Here we show that they also downregulate PP2A phosphatase activity and efficiently cooperate with an oncogenic version of H-RAS in malignant transformation of human mammary epithelial cells. Thus, by down-regulating crucial tumor suppressor functions, EMT inducers make cells particularly prone to malignant conversion. Importantly, by analyzing transformed cells generated in vitro and by characterizing novel transgenic mouse models, we further demonstrate that cooperation between an EMT inducer and an active form of RAS is sufficient to trigger transformation of mammary epithelial cells into malignant cells exhibiting all the characteristic features of claudin-low tumors, including low expression of tight and adherens junction genes, EMT traits, and stem cell-like characteristics. Claudin-low tumors are believed to be the most primitive breast malignancies, having arisen through transformation of an early epithelial precursor with inherent stemness properties and metaplastic features. Challenging this prevailing view, we propose that these aggressive tumors arise from cells committed to luminal differentiation, through a process driven by EMT inducers and combining malignant transformation and transdifferentiation., Competing Interests: The authors have declared that no competing interests exist.

The SNAIL and SLUG transcription factors play important roles in embryogenesis owing to their anti-apoptotic properties and their ability to promote morphogenetic changes by inducing epithelial-mesenchymal transitions (EMT). These characteristics provide many of the proteins in these families with oncogenic and pro-metastatic capabilities when reactivated in cancers. The SCRATCH subgroup of the SNAIL superfamily, including SCRATCH1 and SCRATCH2, display distinct embryonic functions and diverge early in evolution. Despite the described overexpression of SCRT1 (encoding for SCRATCH1) in a small subset of human lung cancers, there is little data supporting a role of SCRATCH proteins in tumorigenesis. To further explore this possibility, we assessed SNAI1 (SNAIL), SNAI2 (SLUG) and SCRT1 (SCRATCH1) expression in a wide panel of human and murine tumors encompassing 151 primary tumors and 6 different cancer types, including melanomas and multiple different carcinomas. Whereas SNAI1 and SNAI2 are widely expressed in human and murine tumors, our results reveal that SCRT1 transcripts are undetectable in nearly all of the examined tumors suggesting that SCRATCH1 plays a minor role, if any, in tumorigenesis. Our data therefore suggest that oncogenic properties are not shared by all SNAIL superfamily members but instead are specifically allotted to the SNAIL subgroup supporting the conclusions that SNAIL and SCRATCH subgroups are functionally divergent and strengthening the hypothesis that the oncogenic potential of SNAIL and SLUG proteins relies on the hijacking of their embryonic functions.

Epidemiological and biological studies indicate that solar UVB radiation is involved in cutaneous malignant melanoma etiology. Indeed, melanocytes are very frequently exposed to solar UV radiation, which induces cell damage and may promote cell transformation. We previously showed that melanocytes and melanoma cells exposed to UVB radiation activates a p53-independent pathway involving Gadd45a and, more recently, that Gadd45a plays a critical role in UVB-induced G2 cell cycle arrest of melanoma cells. In this study, we demonstrate that the inhibition of UV-induced Gadd45a overexpression by RNA interference results in a dramatic increase of cell death. We identify this cell death as apoptosis, with activation of Caspase-3 and a decrease in Bcl-x(L) expression. Furthermore, we show that inhibition of UV-induced Gadd45a overexpression also leads to increased sensitivity of melanoma cells to therapeutic agents such as DTIC and Cisplatin. We conclude that UVB-induced Gadd45a overexpression protects melanoma cells from apoptosis, both by causing a G2 cell cycle arrest and by inhibiting the mitochondrial apoptotic pathway. These observations suggest that Gadd45a inactivation could be a useful way to sensitize melanoma cells to chemotherapy. JID journal club article: For questions, answers, and open discussion about this article please go to http://network.nature.com/group/jidclub

Exposure to solar UVB radiation is involved in the development of cutaneous melanoma. We previously showed that human melanocytes and melanoma cells respond to UVB radiation via a p53-independent pathway involving GADD45A activation. Here, we determined that UVB-induction of Gadd45a is necessary for G(2) arrest and that Gadd45a and its partner p21(Waf1) colocalize in nuclear bodies called Nuclear Speckles. We further observed that UVB-induced G(2) arrest is associated with Cdc2 accumulation in these Nuclear Speckles. Knock-down of Gadd45a expression by RNA interference prevents both UVB-induced Cdc2 accumulation in Nuclear Speckles and G(2) arrest. Our results demonstrate that UVB-induced G(2) arrest of melanoma cells is Gadd45a-dependent. Furthermore, we show that Cdc2 sequestration by Gadd45a occurs in Nuclear Speckles, suggesting a new role for these nuclear bodies, so far only linked to RNA maturation.

Susceptibility to solar ultraviolet is an important melanoma risk factor. We investigated the relationship between individual susceptibility to ultraviolet and risk of melanoma by measuring the apoptosis triggered in peripheral lymphocytes by a low-dose ultraviolet B irradiation (50 J/m(2)) in young and older melanoma patients and controls. Melanoma patients below the age of 40 are more sensitive to UVB-induced apoptosis than older melanoma patients and healthy controls. Analysis of data (adjusted for age and phototype) shows that UVB-induced apoptosis is an important risk factor for melanoma (OR 9.1, 95% CI [3-28], P=0.0001). UVB-induced apoptosis is independent of phototype (P=0.11, Wald test) and tumour thickness (P=0.88, Spearman correlation, for all cases and 0.26 for patients younger than 40 years), and may be used as a functional laboratory test for studying the genetic-environment interactions involved in melanoma occurrence.