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Gene regulation and deregulation by MAL/MRTF coactivators

Proteins of the MRTF family play a key role in regulated gene expression downstream of Rho family GTPases. MAL/MKL1/MRTF-A is a transcriptional coactivator of serum response factor (SRF) and is bound and inhibited by G-actin in the non-induced state. In acute megakaryocytic leukaemia in infants, a recurrent translocation t(1;22) results in the OTT-MAL/RBM15-MKL1 fusion oncoprotein. How it contributes to the malignancy is unknown. We tested by biochemical analysis whether OTT-MAL is functionally deregulated. We showed that OTT-MAL is a constitutive activator of SRF and target gene expression. This requires the SRF binding motif and the MAL-derived transactivation domain. OTT-MAL localises to the nucleus and is not regulated by upstream signalling. OTT-MAL deregulation reflects its independence from control by G-actin, which fails to interact with OTT-MAL in co-immunoprecipitation experiments. OTT-MAL also caused a delayed induction of the MAL-independent, TCF-dependent target genes c-fos and egr-1, and the MAPK/Erk pathway. In addition, RBPJ/CBF-1 regulated gene expression was activated by OTT-MAL but not by MAL. Our data suggest that the deregulated activation of MAL-dependent and independent promoters results in tissue-specific functions of OTT-MAL.

When tested in heterologous tissue culture systems, however, we observed strong anti-proliferative effects of OTT-MAL. Similarly, overexpression of MAL exhibited antiproliferative and pro-apoptotic effects requiring transcription through SRF. To gain insight into the molecular mechanisms involved, we performed gene expression analysis. By using a combination of actin binding drugs, which specifically interfere with the actin-MAL complex, we identified on a genome wide basis 210 genes primarily regulated by G-actin. We found many known MAL-dependent SRF target genes, as well as novel directly regulated genes. Several putative antiproliferative target genes were newly identified. We showed that a group of MAL regulated genes negatively interferes with the EGFR-MAPK pathway, thereby reducing proliferative signalling. Our results show the existence of negatively acting transcriptional networks between pro- and antiproliferative signalling pathways towards subsets of SRF target genes.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Posern, G. Gene regulation and deregulation by MAL/MRTF coactivators. Cell Commun Signal 7 (Suppl 1), A66 (2009).

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