Stereospecific Inhibitory Effects of CCG-1423 on the Cellular Events Mediated by Myocardin-Related Transcription Factor A

Subretinal fibrosis secondary to neovascular age-related macular degeneration: mechanisms and potential therapeutic targets

Subretinal fibrosis is the end-stage sequelae of neovascular age-related macular degeneration. It causes local damage to photoreceptors, retinal pigment epithelium, and choroidal vessels, which leads to permanent central vision loss of patients with neovascular age-related macular degeneration. The pathogenesis of subretinal fibrosis is complex, and the underlying mechanisms are largely unknown. Therefore, there are no effective treatment options. A thorough understanding of the pathogenesis of subretinal fibrosis and its related mechanisms is important to elucidate its complications and explore potential treatments. The current article reviews several aspects of subretinal fibrosis, including the current understanding on the relationship between neovascular age-related macular degeneration and subretinal fibrosis; multimodal imaging techniques for subretinal fibrosis; animal models for studying subretinal fibrosis; cellular and non-cellular constituents of subretinal fibrosis; pathophysiological mechanisms involved in subretinal fibrosis, such as aging, infiltration of macrophages, different sources of mesenchymal transition to myofibroblast, and activation of complement system and immune cells; and several key molecules and signaling pathways participating in the pathogenesis of subretinal fibrosis, such as vascular endothelial growth factor, connective tissue growth factor, fibroblast growth factor 2, platelet-derived growth factor and platelet-derived growth factor receptor-β, transforming growth factor-β signaling pathway, Wnt signaling pathway, and the axis of heat shock protein 70-Toll-like receptors 2/4-interleukin-10. This review will improve the understanding of the pathogenesis of subretinal fibrosis, allow the discovery of molecular targets, and explore potential treatments for the management of subretinal fibrosis.

mDia2 is an important mediator of MRTF-A-dependent regulation of breast cancer cell migration

Dysregulated actin cytoskeleton gives rise to aberrant cell motility and metastatic spread of tumor cells. This study evaluates the effect of overexpression of wild-type versus functional mutants of MRTF-A on migration and invasion of breast cancer (BC) cells. Our studies indicate that SRF's interaction is critical for MRTF-A-induced promotion of both two-dimensional and three-dimensional cell migration, while the SAP-domain function is important selectively for three-dimensional cell migration. Increased MRTF-A activity is associated with more effective membrane protrusion, a phenotype that is attributed predominantly to SRF's interaction with MRTF. We demonstrate formin-family protein mDia2 as an important mediator of MRTF-stimulated actin polymerization at the leading edge and cell migration. Multiplexed quantitative immunohistochemistry and transcriptome analyses of clinical BC specimens further demonstrate a positive correlation between nuclear localization of MRTF with malignant traits of cancer cells and enrichment of MRTF-SRF gene signature in pair-matched distant metastases versus primary tumors. In conclusion, this study establishes a novel mechanism of MRTF-dependent regulation of cell migration and provides evidence for the association between MRTF activity and increased malignancy in human BC, justifying future development of specific small molecule inhibitors of the MRTF-SRF transcriptional complex as potential therapeutic agents in BC.

The benzoylphenylurea derivative BPU17 acts as an inhibitor of prohibitin and exhibits antifibrotic activity

Inflammation-induced choroidal neovascularization followed by the epithelial-mesenchymal transition (EMT) of retinal pigment epithelial cells (RPEs) is a cause of neovascular age-related macular degeneration (nAMD). RPE-derived myofibroblasts overproduce extracellular matrix, leading to subretinal fibrosis. We already have demonstrated that benzylphenylurea (BPU) derivatives inhibit the function of cancer-associated fibroblasts. Here, we investigated the anti-myofibroblast effects of BPU derivatives and examined such BPU activity on subretinal fibrosis. A BPU derivative, BPU17, exhibits the most potent anti-myofibroblast activity among dozens of BPU derivatives and inhibits subretinal fibrosis in a mouse model of retinal degeneration. Investigations with primary cultured RPEs reveal that BPU17 suppresses cell motility and collagen synthesis in RPE-derived myofibroblasts. These effects depend on repressing the serum response factor (SRF)/CArG-box-dependent transcription. BPU17 inhibits the expression of SRF cofactor, cysteine and glycine-rich protein 2 (CRP2), which activates the SRF function. Proteomics analysis reveals that BPU17 binds to prohibitin 1 (PHB1) and inhibits the PHB1-PHB2 interaction, resulting in mild defects in mitochondrial function. This impairment causes a decrease in the expression of CRP2 and suppresses collagen synthesis. Our findings suggest that BPU17 is a promising agent against nAMD and the close relationship between PHB function and EMT.

mDia2 is an important mediator of MRTF-A-dependent regulation of breast cancer cell migration

Dysregulated actin cytoskeleton gives rise to aberrant cell motility and metastatic spread of tumor cells. This study evaluates the effect of overexpression of wild-type vs functional mutants of MRTF-A on migration and invasion of breast cancer (BC) cells. Our studies indicate that SRF's interaction is critical for MRTF-A-induced promotion of both 2D and 3D cell migration, while the SAP-domain function is important selectively for 3D cell migration. Increased MRTF-A activity is associated with more effective membrane protrusion, a phenotype that is attributed predominantly to SRF's interaction of MRTF. We demonstrate formin-family protein mDia2 as an important mediator of MRTF-stimulated actin polymerization at the leading edge and cell migration. Multiplexed quantitative immunohistochemistry and transcriptome analyses of clinical BC specimens further demonstrate a positive correlation between nuclear localization of MRTF with malignant traits of cancer cells and enrichment of MRTF-SRF gene signature in pair-matched distant metastases vs primary tumors. In conclusion, this study establishes a novel mechanism of MRTF-dependent regulation of cell migration and provides evidence for the association between MRTF activity and increased malignancy in human breast cancer, justifying future development of a specific small molecule inhibitor of the MRTF-SRF transcriptional complex as a potential therapeutic agent in breast cancer.

SIGNIFICANCE

Actin cytoskeletal dysregulation gives rise to metastatic dissemination of cancer cells. This study mechanistically investigates the impact of specific functional disruption of MRTF (a transcriptional co-factor of SRF) on breast cancer cell migration.This study establishes a novel mechanism linking mDia2 to MRTF-dependent regulation of cell migration and provides clinical evidence for the association between MRTF activity and increased malignancy in human breast cancer.Findings from these studies justify future exploration of specific small molecule inhibitor of the MRTF-SRF transcriptional complex as a potential therapeutic agent in breast cancer.

Novel ability of diflubenzuron as an inhibitor of mitochondrial function

Compounds classified as benzoylphenylurea (BPU), such as diflubenzuron (DFB), are used as insecticides. Although BPU disrupts molting by inhibiting chitin biosynthesis and exhibits insecticidal activity, their exact mode of action remains unknown. Since epidermal cells proliferate and morphologically change from squamous to columnar cells during the early stages of insect molting, we speculate that a transition similar to that from epithelium to mesenchyme occurs and that BPU may inhibit this transition. Here, we addressed this possibility. We found that DFB decreases actin expression in insect cells (the tissue cultures of insect integument). Detailed analysis in Schneider S2 cells reveals that DFB inhibits the expression of actin isoforms (Act5C and Act42A) and the Drosophila ortholog of myocardin-related transcription factor (Mrtf), leading to cell growth suppression. Proteomics identified the Drosophila ortholog of prohibitin (Phb1D and Phb2E) as one of the DFB-binding proteins. DFB inhibits the interaction between Phb1D and Phb2E and induces mitochondrial dysfunction. The knock-down of Phb2E suppresses the expression of Act5C, Act42A, and Mrtf, leading to cell growth inhibition. Thus, the disruption of Phb function is a possible novel target of DFB.

SRF inhibitors reduce prostate cancer cell proliferation through cell cycle arrest in an isogenic model of castrate-resistant prostate cancer

Castrate-resistant prostate cancer (CRPC) is challenging to treat, despite improvements with next-generation anti-androgens such as enzalutamide, due to acquired resistance. One of the mechanisms of such resistance includes aberrant activation of co-factors of the androgen receptor (AR), such as the serum response factor (SRF), which was associated with prostate cancer progression and resistance to enzalutamide. Here, we show that inhibition of SRF with three small molecules (CCG-1423, CCG-257081 and lestaurtinib), singly and in combination with enzalutamide, reduces cell viability in an isogenic model of CRPC. The effects of these inhibitors on the cell cycle, singly and in combination with enzalutamide, were assessed with western blotting, flow cytometry and β-galactosidase staining. In the androgen deprivation-sensitive LNCaP parental cell line, a synergistic effect between enzalutamide and all three inhibitors was demonstrated, while the androgen deprivation-resistant LNCaP Abl cells showed synergy only with the lestaurtinib and enzalutamide combination, suggesting a different mechanism of action of the CCG series of compounds in the absence and presence of androgens. Through analysis of cell cycle checkpoint proteins, flow cytometry and β-galactosidase staining, we showed that all three SRF inhibitors, singly and in combination with enzalutamide, induced cell cycle arrest and decreased S phase. While CCG-1423 had a more pronounced effect on the expression of cell cycle checkpoint proteins, CCG-257081 and lestaurtinib decreased proliferation also through induction of cellular senescence. In conclusion, we show that inhibition of an AR co-factors, namely SRF, provides a promising approach to overcoming resistance to AR inhibitors currently used in the clinic.

Rho/SRF Inhibitor Modulates Mitochondrial Functions

CCG-1423 is a Rho A pathway inhibitor that has been reported to inhibit Rho/SRF-mediated transcriptional regulation. Serum response factor and its cofactors, which include ternary complex factors and myocardin-related transcription factors, regulate various cellular functions. In this study, we observed that CCG-1423 modulates the mitochondrial functions. The effect of this small molecule drug was determined by measuring mitochondrial function using an XFe96 Analyzer and an Oxygraph 2k (O2k) high-resolution respirometer. CCG-1423 treatment significantly reduced oxidative phosphorylation in a dose-dependent manner. However, CCG-1423 increased the glycolytic rate. We also observed that histone 4 at lysine-16 underwent hyperacetylation with the treatment of this drug. Immunolabeling with F-actin and MitoTracker revealed the alteration in the actin cytoskeleton and mitochondria. Taken together, our findings highlight a critical role of CCG-1423 in inhibiting the transcription of SRF/p49 and PGC-1α, β, resulting in the downregulation of mitochondrial genes, leading to the repression of mitochondrial oxidative phosphorylation and overall ATP reduction. This study provides a better understanding of the effects of CCG-1423 on mitochondria, which may be useful for the assessment of the potential clinical application of CCG-1423 and its derivatives.

Emerging role for the Serum Response Factor (SRF) as a potential therapeutic target in cancer

INTRODUCTION

The Serum Response Factor (SRF) is a transcription factor involved in three hallmarks of cancer: the promotion of cell proliferation, cell death resistance and invasion and metastasis induction. Many studies have demonstrated a leading role in the development and progression of multiple cancer types, thus highlighting the potential of SRF as a prognostic biomarker and therapeutic target, especially for cancers with poor prognosis.

AREAS COVERED

This review examines the role of SRF in several cancers in promoting cellular processes associated with cancer development and progression. SRF co-factors and signalling pathways are discussed as possible targets to inhibit SRF in a tissue and cancer-specific way. Small-molecule inhibitors of SRF, such as the CCGs series of compounds and lestaurtinib, which could be used as cancer therapeutics, are also discussed.

EXPERT OPINION

Targeting of SRF and its co-factors represents a promising therapeutic approach. Further understanding of the molecular mechanisms behind the action of SRF could provide a pipeline of novel molecular targets and therapeutic combinations for cancer. Basket clinical trials and the use of SRF immunohistochemistry as companion diagnostics will help testing of these new targets in patients.

Inhibition of the Myocardin-Related Transcription Factor Pathway Increases Efficacy of Trametinib in NRAS-Mutant Melanoma Cell Lines

Simple Summary

Malignant melanoma is the most aggressive skin cancer, and treatment is often ineffective due to the development of resistance to targeted therapeutic agents. The most prevalent form of melanoma with a mutated BRAF gene has an effective treatment, but the second most common mutation in melanoma (NRAS) leads to tumors that lack targeted therapies. In this study, we show that NRAS mutant human melanoma cells that are most resistant to inhibition of the oncogenic pathway have a second activated pathway (Rho). Inhibiting that pathway at one of several points can produce more effective cell killing than inhibition of the NRAS pathway alone. This raises the possibility that such a combination treatment could prove effective in those melanomas that fail to respond to existing targeted therapies such as vemurafenib and trametinib.

Abstract

The Ras/MEK/ERK pathway has been the primary focus of targeted therapies in melanoma; it is aberrantly activated in almost 80% of human cutaneous melanomas (≈50% BRAFV600 mutations and ≈30% NRAS mutations). While drugs targeting the MAPK pathway have yielded success in BRAFV600 mutant melanoma patients, such therapies have been ineffective in patients with NRAS mutant melanomas in part due to their cytostatic effects and primary resistance. Here, we demonstrate that increased Rho/MRTF-pathway activation correlates with high intrinsic resistance to the MEK inhibitor, trametinib, in a panel of NRAS mutant melanoma cell lines. A combination of trametinib with the Rho/MRTF-pathway inhibitor, CCG-222740, synergistically reduced cell viability in NRAS mutant melanoma cell lines in vitro. Furthermore, the combination of CCG-222740 with trametinib induced apoptosis and reduced clonogenicity in SK-Mel-147 cells, which are highly resistant to trametinib. These findings suggest a role of the Rho/MRTF-pathway in intrinsic trametinib resistance in a subset of NRAS mutant melanoma cell lines and highlight the therapeutic potential of concurrently targeting the Rho/MRTF-pathway and MEK in NRAS mutant melanomas.

Myocardin-Related Transcription Factor A (MRTF-A) Regulates the Balance between Adipogenesis and Osteogenesis of Human Adipose Stem Cells

Previous studies have demonstrated that myocardin-related transcription factor A (MRTF-A) generates a link between the dynamics of the actin cytoskeleton and gene expression with its coregulator, serum response factor (SRF). MRTF-A has also been suggested as a regulator of stem cell differentiation. However, the role of MRTF-A in human mesenchymal stem cell differentiation remains understudied. We aimed to elucidate whether MRTF-A is a potential regulator of human adipose stem cell (hASC) differentiation towards adipogenic and osteogenic lineages. To study the role of MRTF-A activity in the differentiation process, hASCs were cultured in adipogenic and osteogenic media supplemented with inhibitor molecules CCG-1423 or CCG-100602 that have been shown to block the expression of MRTF-A/SRF-activated genes. Our results of image-based quantification of Oil Red O stained lipid droplets and perilipin 1 staining denote that MRTF-A inhibition enhanced the adipogenic differentiation. On the contrary, MRTF-A inhibition led to diminished activity of an early osteogenic marker alkaline phosphatase, and export of extracellular matrix (ECM) proteins collagen type I and osteopontin. Also, quantitative Alizarin Red staining representing ECM mineralization was significantly decreased under MRTF-A inhibition. Image-based analysis of Phalloidin staining revealed that MRTF-A inhibition reduced the F-actin formation and parallel orientation of the actin filaments. Additionally, MRTF-A inhibition affected the protein amounts of α-smooth muscle actin (α-SMA), myosin light chain (MLC), and phosphorylated MLC suggesting that MRTF-A would regulate differentiation through SRF activity. Our results strongly indicate that MRTF-A is an important regulator of the balance between osteogenesis and adipogenesis of hASCs through its role in mediating the cytoskeletal dynamics. These results provide MRTF-A as a new interesting target for guiding the stem cell differentiation in tissue engineering applications for regenerative medicine.

Profibrotic epithelial phenotype: a central role for MRTF and TAZ

Epithelial injury is a key initiator of fibrosis but - in contrast to the previous paradigm - the epithelium in situ does not undergo wide-spread epithelial-mesenchymal/myofibroblast transition (EMT/EMyT). Instead, it assumes a Profibrotic Epithelial Phenotype (PEP) characterized by fibrogenic cytokine production. The transcriptional mechanisms underlying PEP are undefined. As we have shown that two RhoA/cytoskeleton-regulated transcriptional coactivators, Myocardin-related transcription factor (MRTF) and TAZ, are indispensable for EMyT, we asked if they might mediate PEP as well. Here we show that mechanical stress (cyclic stretch) increased the expression of transforming growth factor-β1 (TGFβ1), connective tissue growth factor (CTGF), platelet-derived growth factor and Indian Hedgehog mRNA in LLC-PK1 tubular cells. These responses were mitigated by siRNA-mediated silencing or pharmacological inhibition of MRTF (CCG-1423) or TAZ (verteporfin). RhoA inhibition exerted similar effects. Unilateral ureteral obstruction, a murine model of mechanically-triggered kidney fibrosis, induced tubular RhoA activation along with overexpression/nuclear accumulation of MRTF and TAZ, and increased transcription of the above-mentioned cytokines. Laser capture microdissection revealed TAZ, TGFβ1 and CTGF induction specifically in the tubular epithelium. CCG-1423 suppressed total renal and tubular expression of these proteins. Thus, MRTF regulates epithelial TAZ expression, and both MRTF and TAZ are critical mediators of PEP-related epithelial cytokine production.

SRF'ing and SAP'ing - the role of MRTF proteins in cell migration

Actin-based cell migration is a fundamental cellular activity that plays a crucial role in a wide range of physiological and pathological processes. An essential feature of the remodeling of actin cytoskeleton during cell motility is the de novo synthesis of factors involved in the regulation of the actin cytoskeleton and cell adhesion in response to growth-factor signaling, and this aspect of cell migration is critically regulated by serum-response factor (SRF)-mediated gene transcription. Myocardin-related transcription factors (MRTFs) are key coactivators of SRF that link actin dynamics to SRF-mediated gene transcription. In this Review, we provide a comprehensive overview of the role of MRTF in both normal and cancer cell migration by discussing its canonical SRF-dependent as well as its recently emerged SRF-independent functions, exerted through its SAP domain, in the context of cell migration. We conclude by highlighting outstanding questions for future research in this field.

Myocardin-related transcription factor A (MRTF-A) contributes to acute kidney injury by regulating macrophage ROS production

A host of pathogenic factors induce acute kidney injury (AKI) leading to insufficiencies of renal function. In the present study we evaluated the role of myocardin-related transcription factor A (MRTF-A) in the pathogenesis of AKI. We report that systemic deletion of MRTF-A or inhibition of MRTF-A activity with CCG-1423 significantly attenuated AKI in mice induced by either ischemia-reperfusion or LPS injection. Of note, MRTF-A deficiency or suppression resulted in diminished renal ROS production in AKI models with down-regulation of NAPDH oxdiase 1 (NOX1) and NOX4 expression. In cultured macrophages, MRTF-A promoted NOX1 transcription in response to either hypoxia-reoxygenation or LPS treatment. Interestingly, macrophage-specific MRTF-A deletion ameliorated AKI in mice. Mechanistic analyses revealed that MRTF-A played a role in regulating histone H4K16 acetylation surrounding the NOX gene promoters by interacting with the acetyltransferase MYST1. MYST1 depletion repressed NOX transcription in macrophages. Finally, administration of a MYST1 inhibitor MG149 alleviated AKI in mice. Therefore, we data illustrate a novel epigenetic pathway that controls ROS production in macrophages contributing to AKI. Targeting the MRTF-A-MYST1-NOX axis may yield novel therapeutic strategies to combat AKI.

Regulated methionine oxidation by monooxygenases

Protein function can be regulated via post-translational modifications by numerous enzymatic and non-enzymatic mechanisms, including oxidation of cysteine and methionine residues. Redox-dependent regulatory mechanisms have been identified for nearly every cellular process, but the major paradigm has been that cellular components are oxidized (damaged) by reactive oxygen species (ROS) in a relatively unspecific way, and then reduced (repaired) by designated reductases. While this scheme may work with cysteine, it cannot be ascribed to other residues, such as methionine, whose reaction with ROS is too slow to be biologically relevant. However, methionine is clearly oxidized in vivo and enzymes for its stereoselective reduction are present in all three domains of life. Here, we revisit the chemistry and biology of methionine oxidation, with emphasis on its generation by enzymes from the monooxygenase family. Particular attention is placed on MICALs, a recently discovered family of proteins that harbor an unusual flavin-monooxygenase domain with an NADPH-dependent methionine sulfoxidase activity. Based on structural and kinetic information we provide a rational framework to explain MICAL mechanism, inhibition, and regulation. Methionine residues that are targeted by MICALs are reduced back by methionine sulfoxide reductases, suggesting that reversible methionine oxidation may be a general mechanism analogous to the regulation by phosphorylation by kinases/phosphatases. The identification of new enzymes that catalyze the oxidation of methionine will open a new area of research at the forefront of redox signaling.

Inhibition of SRF/myocardin reduces aortic stiffness by targeting vascular smooth muscle cell stiffening in hypertension

AIMS

Increased aortic stiffness is a fundamental manifestation of hypertension. However, the molecular mechanisms involved remain largely unknown. We tested the hypothesis that abnormal intrinsic vascular smooth muscle cell (VSMC) mechanical properties in large arteries, but not in distal arteries, contribute to the pathogenesis of aortic stiffening in hypertension, mediated by the serum response factor (SRF)/myocardin signalling pathway.

METHODS AND RESULTS

Four month old male spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were studied. Using atomic force microscopy, significant VSMC stiffening was observed in the large conducting aorta compared with the distal arteries in SHR (P < 0.001), however, this regional variation was not observed in WKY rats (P > 0.4). The increase of VSMC stiffness was accompanied by a parallel increase in the expression of SRF by 9.8-fold and of myocardin by 10.5-fold in thoracic aortic VSMCs from SHR vs. WKY rats, resulting in a significant increase of downstream stiffness-associated genes (all, P < 0.01 vs. WKY). Inhibition of SRF/myocardin expression selectively attenuated aortic VSMC stiffening, and normalized downstream targets in VSMCs isolated from SHR but not from WKY rats. In vivo, 2 weeks of treatment with SRF/myocardin inhibitor delivered by subcutaneous osmotic minipump significantly reduced aortic stiffness and then blood pressure in SHR but not in WKY rats, although concomitant changes in aortic wall remodelling were not detected during this time frame.

CONCLUSIONS

SRF/myocardin pathway acts as a pivotal mediator of aortic VSMC mechanical properties and plays a central role in the pathological aortic stiffening in hypertension. Attenuation of aortic VSMC stiffening by pharmacological inhibition of SRF/myocardin signalling presents a novel therapeutic strategy for the treatment of hypertension by targeting the cellular contributors to aortic stiffness.