Ets-1 p51 and p42 isoforms differentially modulate Stromelysin-1 promoter according to induced DNA bend orientation

Ets family proteins regulate the EMT transcription factors Snail and ZEB in cancer cells

The epithelial–mesenchymal transition (EMT) is a crucial morphological event that occurs during epithelial tumor progression. Snail and ZEB1/2 (ZEB1 and ZEB2), known as EMT transcription factors, are key regulators of this transition. ZEB1/2 are positively correlated with EMT phenotypes and the aggressiveness of cancers. On the contrary, Snail is also correlated with the aggressiveness of cancers, but is not correlated with the expression of EMT marker proteins. Snail is induced by transforming growth factor‐β (TGF‐β), a well‐known inducer of EMT, in various cancer cells. Interestingly, Snail induction by TGF‐β is markedly enhanced by active Ras signals. Thus, cancer cells harboring an active Ras mutation exhibit a drastic induction of Snail by TGF‐β alone. Here, we found that members of the E26 transformation‐specific (Ets) transcription factor family, Ets1 and Ets2, contribute to the upregulation of both Snail and ZEB1/2. Snail induction by TGF‐β and active Ras is dramatically inhibited using siRNAs against both Ets1 and Ets2 together, but not on their own; in addition, siRNAs against both Ets1 and Ets2 also downregulate the constitutive expression of Snail and ZEB1/2 in cancer cells. Examination of several alternatively spliced variants of Ets1 revealed that p54‐Ets1, which includes exon VII, but not p42‐Ets1, which excludes exon VII, regulates the expression of the EMT transcription factors, suggesting that Ets1 is a crucial molecule for regulating Snail and ZEB1/2, and thus cancer progression is mediated through post‐translational modification of the exon VII domain.

ETS1 induces transforming growth factor β signaling and promotes epithelial-to-mesenchymal transition in prostate cancer cells

Expression of the transcriptional regulator, E26 transformation-specific 1 (ETS1), is elevated in human prostate cancers, and this is associated with more aggressive tumor behavior and a rapid progression to castrate-resistant disease. Multiple ETS1 isoforms with distinct biological activities have been characterized and in 44 matched nonmalignant and malignant human prostate specimens, messenger RNAs for two ETS1 isoforms, ETS1p51 and ETS1p42, were detected, with ETS1p51 levels significantly lower in prostate tumor compared to matched nonmalignant prostate tissues. In contrast, ETS1p51 protein, the only ETS1 isoform detected, was expressed at significantly higher levels in malignant prostate. Analysis of epithelial-to-mesenchymal transition (EMT)-associated genes regulated following overexpression of ETS1p51 in the LNCaP prostate cancer cell line predicted promotion of transforming growth factor β (TGFβ) signaling and of EMT. ETS1p51 overexpression upregulated cellular levels of the EMT transcriptional regulators, ZEB1 and SNAIL1, resulted in reduced expression of the mesenchymal marker vimentin with concomitantly elevated levels of claudin 1, an epithelial tight junction protein, and increased prostate cancer cell migration and invasion. ETS1p51-induced activation of the pro-EMT TGFβ signaling pathway that was predicted in polymerase chain reaction arrays was verified by demonstration of elevated SMAD2 phosphorylation following ETS1p51 overexpression. Attenuation of ETS1p51 effects on prostate cancer cell migration and invasion by inhibition of TGFβ pathway signaling indicated that ETS1p51 effects were in part mediated by induction of TGFβ signaling. Thus, overexpression of ETS1p51, the predominant ETS1 isoform expressed in prostate tumors, promotes an EMT program in prostate cancer cells in part via activation of TGFβ signaling, potentially accounting for the poor prognosis of ETS1-overexpressing prostate tumors.

The role of the transcription factor Ets1 in carcinoma

Ets1 belongs to the large family of the ETS domain family of transcription factors and is involved in cancer progression. In most carcinomas, Ets1 expression is linked to poor survival. In breast cancer, Ets1 is primarily expressed in the triple-negative subtype, which is associated with unfavorable prognosis. Ets1 contributes to the acquisition of cancer cell invasiveness, to EMT (epithelial-to-mesenchymal transition), to the development of drug resistance and neo-angiogenesis. The aim of this review is to summarize the current knowledge on the functions of Ets1 in carcinoma progression and on the mechanisms that regulate Ets1 activity in cancer.

A Role for Autoinhibition in Preventing Dimerization of the Transcription Factor ETS1

ETS1 is the archetype of the ETS transcription factor (TF) family. ETS TFs share a DNA-binding domain, the ETS domain. All ETS TFs recognize a core GGA(A/T) binding site, and thus ETS TFs are found to redundantly regulate the same genes. However, each ETS TF has unique targets as well. One prevailing hypotheses explaining this duality is that protein-protein interactions, including homodimerization, allow each ETS TF to display distinct behavior. The behavior of ETS1 is further regulated by autoinhibition. Autoinhibition apparently modulates ETS1 DNA binding affinity, but the mechanism of this inhibition is not completely understood. We sought to characterize the relationship between DNA binding and ETS1 homodimer formation. We find that ETS1 interrogates DNA and forms dimers even when the DNA does not contain an ETS recognition sequence. Mutational studies also link nonspecific DNA backbone contacts with dimer formation, in addition to providing a new role for the recognition helix of ETS1 in maintaining the autoinhibited state. Finally, in showing that residues in the DNA recognition helix affect autoinhibition, we define a new function of ETS1 autoinhibition: maintenance of a monomeric state in the absence of DNA. The conservation of relevant amino acid residues across all ETS TFs indicates that the mechanisms of nonspecific DNA interrogation and protein oligomer formation elucidated here may be common to all ETS proteins that autoinhibit.

The level of Ets-1 protein is regulated by poly(ADP-ribose) polymerase-1 (PARP-1) in cancer cells to prevent DNA damage

Ets-1 is a transcription factor that regulates many genes involved in cancer progression and in tumour invasion. It is a poor prognostic marker for breast, lung, colorectal and ovary carcinomas. Here, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a novel interaction partner of Ets-1. We show that Ets-1 activates, by direct interaction, the catalytic activity of PARP-1 and is then poly(ADP-ribosyl)ated in a DNA-independent manner. The catalytic inhibition of PARP-1 enhanced Ets-1 transcriptional activity and caused its massive accumulation in cell nuclei. Ets-1 expression was correlated with an increase in DNA damage when PARP-1 was inhibited, leading to cancer cell death. Moreover, PARP-1 inhibitors caused only Ets-1-expressing cells to accumulate DNA damage. These results provide new insight into Ets-1 regulation in cancer cells and its link with DNA repair proteins. Furthermore, our findings suggest that PARP-1 inhibitors would be useful in a new therapeutic strategy that specifically targets Ets-1-expressing tumours.

Genomic and biochemical insights into the specificity of ETS transcription factors

ETS proteins are a group of evolutionarily related, DNA-binding transcriptional factors. These proteins direct gene expression in diverse normal and disease states by binding to specific promoters and enhancers and facilitating assembly of other components of the transcriptional machinery. The highly conserved DNA-binding ETS domain defines the family and is responsible for specific recognition of a common sequence motif, 5'-GGA(A/T)-3'. Attaining specificity for biological regulation in such a family is thus a conundrum. We present the current knowledge of routes to functional diversity and DNA binding specificity, including divergent properties of the conserved ETS and PNT domains, the involvement of flanking structured and unstructured regions appended to these dynamic domains, posttranslational modifications, and protein partnerships with other DNA-binding proteins and coregulators. The review emphasizes recent advances from biochemical and biophysical approaches, as well as insights from genomic studies that detect ETS-factor occupancy in living cells.

Caspase cleavage of Ets-1 p51 generates fragments with transcriptional dominant-negative function

Ets-1 is a transcription factor that plays an important role in various physiological and pathological processes, such as development, angiogenesis, apoptosis and tumour invasion. In the present study, we have demonstrated that Ets-1 p51, but not the spliced variant Ets-1 p42, is processed in a caspase-dependent manner in Jurkat T-leukaemia cells undergoing apoptosis, resulting in three C-terminal fragments Cp20, Cp17 and Cp14 and a N-terminal fragment, Np36. In vitro cleavage of Ets-1 p51 by caspase 3 produces fragments consistent with those observed in cells undergoing apoptosis. These fragments are generated by cleavage at three sites located in the exon VII-encoded region of Ets-1 p51. This region is absent from the Ets-1 p42 isoform, which therefore cannot be cleaved by caspases. In Ets-1 p51, cleavage generates C-terminal fragments containing the DNA-binding domain, but lacking the transactivation domain. The Cp17 fragment, the major cleavage product generated during apoptosis, is devoid of transcriptional activity and inhibits Ets-1 p51-mediated transactivation of target genes by competing with Ets-1 p51 for binding to Ets-binding sites present in the target promoters. In the present study, we have demonstrated that caspase cleavage of Ets-1 within the exon VII-encoded region leads to specific down-regulation of the Ets-1 p51 isoform during apoptosis. Furthermore, our results establish that caspase cleavage generates a stable C-terminal fragment that acts as a natural dominant-negative form of the full-length Ets-1 p51 protein.

DNA-dependent protein kinase is a novel interaction partner for Ets-1 isoforms

The Ets-1 transcription factor plays an important role in various physiological and pathological processes. These diverse roles of Ets-1 are likely to depend on its interaction partner proteins. We used our previously developed, recombinant biotinylated Ets-1 that conserves native Ets-1 properties to identify new interaction partners. Here, based on results from streptavidin pull-down assays, mass spectrometry and co-immunoprecipitation, we report a novel interaction partner for Ets-1 isoforms: a heterotrimeric complex of DNA-dependent protein kinase (DNA-PK), made up of Ku70, Ku86, and DNA-PK catalytic subunit (DNA-PKcs). Kinase assays performed in vitro showed that DNA-PK phosphorylates the Ets-1 protein. Furthermore, we demonstrated that Ku86, but not Ku70 or DNA-PKcs, down-regulated the transcriptional activity of Ets-1 when analysed using a reporter gene assay. These results illustrate how detecting novel molecular interactions may provide new clues for understanding the diverse functions of Ets-1.