Identification and Functional Characterisation of Genes regulated by Monomeric Actin

Monomeric actin controls the activity of the transcription factor Serum Response Factor (SRF) via its coactivator MAL/MRTF-A. Upon signal induction, MAL is released from actin, binds SRF and activates target gene expression. In order to characterise the physiological role of this signalling pathway, I screened on a genome wide basis for target genes by transcriptome analysis. A combination of actin binding drugs (Cytochalasin D and Latrunculin B), targeting monomeric actin, was used to specifically and differentially interfere with the complex between MAL and actin. 210 genes primarily controlled by monomeric actin were identified in mouse fibroblasts. Among them more than 30% have been already found in screens for SRF target genes, supporting the validity of the screening approach. As expected, a lot of genes were involved in cytoskeleton organization. However, genes having anti-proliferative or pro-apoptotic features were identified surprisingly to the same extent. Consistently, I could demonstrate an antiproliferative function of MAL. More specifically, several genes interfering with the MAPK pathway were identified. One of them was Mig6/Errfi1, a negative regulator of EGF receptors. Mig6 induction by LPA or FCS revealed to be dependent on MAL, monomeric actin and the small GTPases Rho. Activated forms of MAL or SRF were sufficient to induce Mig6 expression. Subsequently, a Mig6 promoter element was found to be necessary to mediate MAL/SRF induction. Moreover, induction of Mig6 through the Actin-MAL pathway led to the downregulation of the mitogenic EGFR-MAP kinase cascade. For the first time a transcriptional link between G-actin levels sensed by MAL and the regulation of EGFR signalling was established. Furthermore, after having demonstrated that MAL induces apoptosis, I focused on the characterisation of two proapototic targets identified in the screen: Bok and Noxa. Bok and Noxa were induced by activators of the Rho-Actin-Mal-Srf pathway on a MAL dependent manner. The study of the Bok promoter revealed the existence of a response element that was necessary for the induction by MAL-SRF. Interestingly, apoptotic inducers like staurosporine, TNFα, or the DNA damaging agent Doxorubicin triggered MAL-SRF mediated transcription. As SRF controls the expression of the anti-apoptotic genes Bcl2 and Mcl1, the results from this work places thus SRF as a key transcription factor controlling the balance between pro and anti apoptotic genes in response to external cues

Epithelial Protein Lost in Neoplasm α (Eplin-α) is transcriptionally regulated by G-actin and MAL/MRTF coactivators

Epithelial Protein Lost in Neoplasm α is a novel cytoskeleton-associated tumor suppressor whose expression inversely correlates with cell growth, motility, invasion and cancer mortality. Here we show that Eplin-α transcription is regulated by actin-MAL-SRF signalling. Upon signal induction, the coactivator MAL/MRTF is released from a repressive complex with monomeric actin, binds the transcription factor SRF and activates target gene expression. In a transcriptome analysis with a combination of actin binding drugs which specifically and differentially interfere with the actin-MAL complex (Descot et al., 2009), we identified Eplin to be primarily controlled by monomeric actin. Further analysis revealed that induction of the Eplin-α mRNA and its promoter was sensitive to drugs and mutant actins which stabilise the repressive actin-MAL complex. In contrast, the Eplin-β isoform remained unaffected. Knockdown of MRTFs or dominant negative MAL which inhibits SRF-mediated transcription impaired Eplin-α expression. Conversely, constitutively active mutant actins and MAL induced Eplin-α. MAL and SRF were bound to a consensus SRF binding site of the Eplin-α promoter; the recruitment of MAL to this region was enhanced severalfold upon induction. The tumor suppressor Eplin-α is thus a novel cytoskeletal target gene transcriptionally regulated by the actin-MAL-SRF pathway, which supports a role in cancer biology

MAL/MRTF-A controls migration of non-invasive cells by upregulation of cytoskeleton-associated proteins

Monomeric actin regulates gene expression through serum response factor (SRF) by inhibiting its transcriptional coactivator myocardin-related transcription factor (MAL/MRTF). Many affected genes encode cytoskeletal components. We have analysed the migratory effects of actin–MAL signalling and of new target genes in non-invasive highly adherent cells. Expression of active MAL impaired migration of both fibroblasts and epithelial cells, whereas dominant-negative constructs and partial knockdown of MAL/MRTF enhanced motility. Knockdown of three newly characterised G-actin-regulated MAL targets, integrin α5, plakophilin 2 (Pkp2) and FHL1, enhanced cell migration. All three were upregulated by external stimulation through actin–MAL–SRF signalling, and MAL and SRF were inducibly recruited to cis-regulatory elements of the integrin α5 and Pkp2 genes. Finally, the reduced migration of epithelial cells stably expressing MAL was partially reversed by knockdown of Pkp2 and FHL1. We conclude that the actin–MAL pathway promotes adhesive gene expression, including integrin α5, Pkp2 and FHL1, and that this is anti-motile for non-invasive cells harbouring high basal activity.</jats:p

Stress signaling in breast cancer cells induces matrix components that promote chemoresistant metastasis

Abstract Metastatic progression remains a major burden for cancer patients and is associated with eventual resistance to prevailing therapies such as chemotherapy. Here, we reveal how chemotherapy induces an extracellular matrix (ECM), wound healing, and stem cell network in cancer cells via the c‐Jun N‐terminal kinase (JNK) pathway, leading to reduced therapeutic efficacy. We find that elevated JNK activity in cancer cells is linked to poor clinical outcome in breast cancer patients and is critical for tumor initiation and metastasis in xenograft mouse models of breast cancer. We show that JNK signaling enhances expression of the ECM and stem cell niche components osteopontin, also called secreted phosphoprotein 1 (SPP1), and tenascin C (TNC), that promote lung metastasis. We demonstrate that both SPP1 and TNC are direct targets of the c‐Jun transcription factor. Exposure to multiple chemotherapies further exploits this JNK‐mediated axis to confer treatment resistance. Importantly, JNK inhibition or disruption of SPP1 or TNC expression sensitizes experimental mammary tumors and metastases to chemotherapy, thus providing insights to consider for future treatment strategies against metastatic breast cancer