High activity of serum response factor in the mesenchymal transition of epithelial tumor cells is regulated by RhoA signaling

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.

MICAL2 is a novel nucleocytoplasmic shuttling protein promoting cancer invasion and growth of lung adenocarcinoma

MICAL2 is a tumor-promoting factor involved in cell migration, invasion, deformation, and proliferation not yet fully explored in lung adenocarcinoma (LUAD). This study demonstrated that MICAL2 was overexpressed and cytoplasm-enriched in LUAD tissues. Moreover, high cytoplasmic MICAL2 and/or total MICAL2 expression levels were positively correlated with lymphatic metastasis and shorter overall survival in LUAD patients. MICAL2 promoted LUAD cell proliferation, migration, invasion, and epithelial to mesenchymal transition-all of which involved the AKT and myosin-9 pathways. Furthermore, MICAL2 was identified as a nucleoplasm shuttling protein dependent on myosin-9 and its C-terminal fragment. MICAL2-ΔC-enriched in the nucleus-had less impact on tumor malignancy in LUAD cells in vitro and in vivo. Tumor promotion by MICAL2 was reduced by nuclear-export inhibitor, myosin-9 inhibitor, or si-myosin-9-all of which effectively inhibited MICAL2's nuclear export. Finally, the expression and subcellular location as well as clinical significance of MICAL2 and myosin-9 were analyzed across TCGA data and LUAD tissue arrays. Our data revealed that MICAL2 overexpression and nuclear export were associated with cancer progression; inhibiting its expression and/or nuclear export may provide a new target for LUAD therapy.

SRF promotes gastric cancer metastasis through stromal fibroblasts in an SDF1-CXCR4-dependent manner

It has been suggested that the overexpression of serum response factor (SRF) in cancer cells may promote cancer metastasis. However, the exact pathway by which SRF promotes metastasis has not been clarified. Here we showed that SRF promotes gastric cancer (GC) metastasis through stromal fibroblasts in an SDF1-CXCR4-dependent manner. SRF expression was significantly increased in metastatic GCs compared with the non-metastatic GCs (n=50, p=0.013). Immuno-staining indicated that SRF was primarily expressed in a-smooth muscle actin (αSMA)-expressing periglandular fibroblasts in GCs. The conditioned medium (CM) from CCD18Co fibroblasts stably transfected with the SRF vector (CCD18Co-SRF) significantly enhanced migration of MKN45 gastric cancer cells. In contrast, the CM from CCD18Co fibroblasts, in which SRF was downregulated, inhibited mobility of MKN45 cells. Similar results were observed in cultured BGC823 cells even when they were treated with the NIH3T3-SRF CM. When MKN45 cells and SRF-upregulated or downregulated CCD18Co cells were simultaneously co-injected into the tail veins of NOD-SCID mice, a significant increase or decrease was, respectively, observed in the experimental pulmonary metastasis of cancer cells. Furthermore, SRF overexpression significantly upregulated `SMA and stromal cell derived factor1 (SDF1) expression in these fibroblasts, and an anti-SDF1 antibody or the SDF1 receptor CXCR4-specific inhibitor AMD3100 treatment completely reversed the SRF-enhanced migration of cancer cells. Quantitative RT-PCR demonstrated that the expression level of SRF was positively correlated with that of SDF1 in 92 GC samples (r=0.63, p<0.001). In conclusion, SRF promote GC metastasis by facilitating myofibroblast-cancer cell crosstalk in an SDF1-CXCR4 dependent manner, and maybe a therapeutic target.

MicroRNA-647 Targets SRF-MYH9 Axis to Suppress Invasion and Metastasis of Gastric Cancer

MicroRNAs (miRNAs) play important roles in regulating tumour development and progression. Here we show that miR-647 is repressed in gastric cancer (GC), and associated with GC metastasis. Moreover, we identify that miR-647 can suppress GC cell migration and invasion in vitro. Mechanistically, we confirm miR-647 directly binds to the 3' untranslated regions of SRF mRNA, and SRF binds to the CArG box located at the MYH9 promoter. CCG-1423, an inhibitor of RhoA/SRF-mediated gene transcription, inhibits the expression of MYH9, especially in SRF downregulated cells. Overexpression of miR-647 inhibits MGC 80-3 cells' metastasis in orthotropic GC models, but increasing SRF expression in these cells reverses this change. Importantly, we found the synergistic inhibition effect of CCG-1423 and agomir-647, an engineered miRNA mimic, on cancer metastasis in orthotropic GC models. Our study demonstrates that miR-647 functions as a tumor metastasis suppressor in GC by targeting SRF/MYH9 axis.

Epithelial-to-Pericyte Transition in Cancer

During epithelial-to-mesenchymal transition (EMT), cells lose epithelial characteristics and acquire mesenchymal properties. These two processes are genetically separable and governed by distinct transcriptional programs, rendering the EMT outputs highly heterogeneous. Our recent study shows that the mesenchymal products generated by EMT often express multiple pericyte markers, associate with and stabilize blood vessels to fuel tumor growth, thus phenotypically and functionally resembling pericytes. Therefore, some EMT events represent epithelial-to-pericyte transition (EPT). The serum response factor (SRF) plays key roles in both EMT and differentiation of pericytes, and may inherently confer the pericyte attributes on EMT cancer cells. By impacting their intratumoral location and cell surface receptor expression, EPT may enable cancer cells to receive and respond to angiocrine factors produced by the vascular niche, and develop therapy resistance.

A basal-like breast cancer-specific role for SRF-IL6 in YAP-induced cancer stemness

The switch between stem/progenitor cell expansion and differentiation is critical for organ homeostasis. The mammalian Hippo pathway effector and oncoprotein YAP expands undifferentiated stem/progenitor cells in various tissues. However, the YAP-associated transcription factors and downstream targets underlying this stemness-promoting activity are poorly understood. Here we show that the SRF–IL6 axis is the critical mediator of YAP-induced stemness in mammary epithelial cells and breast cancer. Specifically, serum response factor (SRF)-mediated binding and recruitment of YAP to mammary stem cell (MaSC) signature-gene promoters induce numerous MaSC signature genes, among which the target interleukin (IL)-6 is critical for YAP-induced stemness. High SRF–YAP/TAZ expression is correlated with IL6-enriched MaSC/basal-like breast cancer (BLBC). Finally, we show that this high SRF expression enables YAP to more efficiently induce IL6 and stemness in BLBC compared with luminal-type breast cancer. Collectively, our results establish the importance of SRF–YAP–IL6 signalling in promoting MaSC-like properties in a BLBC-specific manner.

The downstream effector of the Hippo pathway, YAP, is a critical regulator of both normal and cancer stem cell properties. In this study, the authors show that in basal-like breast tumors YAP forms a complex with SRF and together they induce a mammary stem cell gene signature through the transcriptional activation of IL-6.

JNK-dependent gene regulatory circuitry governs mesenchymal fate

The epithelial to mesenchymal transition (EMT) is a biological process in which cells lose cell–cell contacts and become motile. EMT is used during development, for example, in triggering neural crest migration, and in cancer metastasis. Despite progress, the dynamics of JNK signaling, its role in genomewide transcriptional reprogramming, and involved downstream effectors during EMT remain largely unknown. Here, we show that JNK is not required for initiation, but progression of phenotypic changes associated with EMT. Such dependency resulted from JNK-driven transcriptional reprogramming of critical EMT genes and involved changes in their chromatin state. Furthermore, we identified eight novel JNK-induced transcription factors that were required for proper EMT. Three of these factors were also highly expressed in invasive cancer cells where they function in gene regulation to maintain mesenchymal identity. These factors were also induced during neuronal development and function in neuronal migration in vivo. These comprehensive findings uncovered a kinetically distinct role for the JNK pathway in defining the transcriptome that underlies mesenchymal identity and revealed novel transcription factors that mediate these responses during development and disease.

Large-scale characterization of DNA methylation changes in human gastric carcinomas with and without metastasis

PURPOSE

Metastasis is the leading cause of death for gastric carcinoma. An epigenetic biomarker panel for predicting gastric carcinoma metastasis could have significant clinical impact on the care of patients with gastric carcinoma. The main purpose of this study is to characterize the methylation differences between gastric carcinomas with and without metastasis.

EXPERIMENTAL DESIGN

Genome-wide DNA methylation profiles between 4 metastatic and 4 nonmetastatic gastric carcinomas and their surgical margins (SM) were analyzed using methylated-CpG island amplification with microarray. The methylation states of 73 candidate genes were further analyzed in patients with gastric carcinoma in a discovery cohort (n=108) using denatured high performance liquid chromatography, bisulfite-sequencing, and MethyLight. The predictive values of potential metastasis-methylation biomarkers were validated in cohorts of patients with gastric carcinoma in China (n=330), Japan (n=129), and Korea (n=153).

RESULTS

The gastric carcinoma genome showed significantly higher proportions of hypomethylation in the promoter and exon-1 regions, as well as increased hypermethylation of intragenic fragments when compared with SMs. Significant differential methylation was validated in the CpG islands of 15 genes (P<0.05) and confirmed using bisulfite sequencing. These genes included BMP3, BNIP3, CDKN2A, ECEL1, ELK1, GFRA1, HOXD10, KCNH1, PSMD10, PTPRT, SIGIRR, SRF, TBX5, TFPI2, and ZNF382. Methylation changes of GFRA1, SRF, and ZNF382 resulted in up- or downregulation of their transcription. Most importantly, the prevalence of GFRA1, SRF, and ZNF382 methylation alterations was consistently and coordinately associated with gastric carcinoma metastasis and the patients' overall survival throughout discovery and validation cohorts in China, Japan, and Korea.

CONCLUSION

Methylation changes of GFRA1, SRF, and ZNF382 may be a potential biomarker set for prediction of gastric carcinoma metastasis.

Serum response factor induces epithelial to mesenchymal transition with resistance to sorafenib in hepatocellular carcinoma

The epithelial to mesenchymal transition (EMT) is a crucial process in tumor progression. EMT of tumor cells not only causes increased metastasis, but also contributes to drug resistance. Serum response factor (SRF) is a transcription factor that plays a central role in carcinogenesis and tumor progression in several types of cancers. We investigated the effect of EMT-related SRF, focusing on its promotion of chemoresistance against sorafenib in hepatocellular carcinoma (HCC). We examined SRF and Snail expression in 146 cases of HCCs by immunohistochemistry. We also examined the chemoresistance effect of SRF in HCC cells by transfecting HLE cells with SRF cDNA and SH-J1 cells with SRF antisense cDNA. Expression of SRF and Snail were detected in 37.6% (55 of 146 cases) and in 12.3% (18 of 146 cases) of the HCCs, respectively. None of the tumor-free liver tissues showed SRF or Snail expression. SRF expression was closely correlated with the expression of Snail (p<0.001) and expression of both SRF and Snail showed significant correlation with the high histological grade (p=0.015 and 0.003, respectively). Overexpression of SRF in HLE cells led to increased expression of mesenchymal markers, as well as increased cell growth and colony formation. Overexpression of SRF also led to a significant reduction in the cytotoxic effect of sorafenib in HLE cells. Conversely, inhibition of SRF expression in the SH-J1 cells significantly enhanced the apoptotic effects of sorafenib, along with the reduced expression of mesenchymal markers and restored the expression of E-cadherin. These results suggest that SRF is critical for HCC to acquire a mesenchymal phenotype, which leads to resistance against a sorafenib-mediated apoptotic effect.

An activating beta1 integrin mutation increases the conversion of benign to malignant skin tumors

Identifying the physiologic relevance of cancer-associated genetic polymorphisms is a major challenge. Several changes in the coding sequence of beta integrin subunits have now been described in human tumors. One of these, T188Ibeta1, was identified as a heterozygous mutation in a poorly differentiated squamous cell carcinoma (SCC) and shown to activate extracellular matrix adhesion and inhibit keratinocyte differentiation in vitro. To study its contribution to tumor development, we overexpressed the mutant or wild-type (WT) human beta1 subunit in the basal layer of mouse epidermis using the keratin 14 promoter. The transgenic integrins were expressed at the cell surface and were functional, with the T188Ibeta1 subunit promoting cell spreading to a greater extent than WTbeta1. Epidermal proliferation and differentiation were unaffected and no expansion of the stem cell compartment was detected. During chemical carcinogenesis, both transgenes increased papilloma formation, but only the T188Ibeta1 transgene stimulated the conversion of papillomas to SCCs. Papillomas bearing the mutation showed increased Erk activity and reduced differentiation. SCCs expressing T188Ibeta1 were less well-differentiated than those expressing WTbeta1. These observations establish that the expression of a genetic variant in the I-like domain of beta1 integrins does not affect normal epidermal homeostasis, but increases tumor susceptibility and influences tumor type.

Myofibroblasts in pulmonary and brain metastases of alveolar soft-part sarcoma: a novel target for treatment?

Alveolar soft-part sarcoma (ASPS) is a rare neoplasm with chromosomal translocation that results in ASPL-TFE3 fusion. It is a slow-growing lesion associated with a high incidence of pulmonary and brain metastases indicating poor survival. We demonstrated that the ASPS metastases include also stromal myofibroblasts. These cells proliferate, express smooth-muscle genes, and synthesize extracellular matrix proteins, all of which are characteristics of activated myofibroblasts. The tumor cells also exhibited stromal components such as transforming growth factor beta (TGFbeta)-dependent, hypoxia-regulated cytoglobin (stellate cell activation association protein, cytg/STAP) and prolyl 4-hydroxylase, a collagen cross-linking enzyme. The pulmonary ASPS myofibroblasts synthesize serum response factor (SRF), a repressor of Smad3-mediated TGFbeta signaling essential for myofibroblast differentiation and Smad3. The phosphorylated active Smad3 was found mostly in the tumor cells. The brain tumor cells express cytg/STAP, but in contrast to the lung metastases, they also express SRF, Smad3, and phospho-Smad3. Halofuginone, an inhibitor of myofibroblasts' activation and Smad3 phosphorylation, inhibited tumor development in xenografts derived from renal carcinoma cells harboring a reciprocal ASPL-TFE3 fusion transcript. This inhibition was associated with the inhibition of TGFbeta/SRF signaling, with the inhibition of myofibroblasts' activation, and with the complete loss in TFE3 synthesis by the tumor cells. These results suggest that the myofibroblasts may serve as a novel target for treatment of ASPS metastases.

Egr-1 and serum response factor are involved in growth factors- and serum-mediated induction of E2-EPF UCP expression that regulates the VHL-HIF pathway

E2-EPF ubiquitin carrier protein (UCP) has been shown to be highly expressed in common human cancers and target von Hippel-Lindau (VHL) for proteosomal degradation in cells, thereby stabilizing hypoxia-inducible factor (HIF)-1alpha. Here, we investigated cellular factors that regulate the expression of UCP gene. Promoter deletion assay identified binding sites for early growth response-1 (Egr-1) and serum response factor (SRF) in the UCP promoter. Hepatocyte or epidermal growth factor (EGF), or phorbol 12-myristate 13-acetate induced UCP expression following early induction of Egr-1 expression in HeLa cells. Serum increased mRNA and protein levels of SRF and UCP in the cell. By electrophoretic mobility shift and chromatin immunoprecipitation assays, sequence-specific DNA-binding of Egr-1 and SRF to the UCP promoter was detected in nuclear extracts from HeLa cells treated with EGF and serum, respectively. Overexpression of Egr-1 or SRF increased UCP expression. RNA interference-mediated depletion of endogenous Egr-1 or SRF impaired EGF- or serum-mediated induction of UCP expression, which was required for cancer cell proliferation. Systemic delivery of EGF into mice also increased UCP expression following early induction of Egr-1 expression in mouse liver. The induced UCP expression by the growth factors or serum increased HIF-1alpha protein level under non-hypoxic conditions, suggesting that the Egr-1/SRF-UCP-VHL pathway is in part responsible for the increased HIF-1alpha protein level in vitro and in vivo. Thus, growth factors and serum induce expression of Egr-1 and SRF, respectively, which in turn induces UCP expression that positively regulates cancer cell growth.

Comparison of mRNA expression of transcriptional factors and intercalated disk constituent proteins between in vivo and cultured cardiomyocytes

The weak contractile force exerted by engineered cardiac muscle is a big problem in cardiac muscle tissue engineering, even though the field has made great progress over the past decade. We believe that one major reason for the weak contractile force is that the expression of genes regulating cardiomyocyte differentiation and cardiac tissue syncytium may be different for in vivo and cultured cells. In the present study, we investigated the difference of mRNA expression under in vivo and culture conditions in order to seek a target for further gene transfer treatment in the process of cardiac tissue construction. To this end, mRNA expression of four major transcriptional factors (SRF, p300, Nkx2.5, and myocardin) and two intercalated disk constituent proteins (N-cadherin and connexin43) in rat cardiomyocytes was measured by means of ratiometric reverse-transcription polymerase chain reaction. Cardiomyocytes were harvested from the hearts of 18-day (about 3 days before birth) Wistar-rat embryos (embryonic cells), 12-day neonatal rat hearts (neonatal cells), or 14-day successive dish culture of the embryonic cells harvested from 18-day embryos (cultured cells). The results indicated that, except for SRF, the mRNAs had a lower expression tendency in cultured cells than in embryonic and in neonatal cells; in particular, the mRNA expression of myocardin, N-cadherin, and connexin43 of cultured cardiomyocytes was significantly lower than that of neonatal cells. Therefore, myocardin is a candidate for forced gene up-expression during the construction of engineered cardiac tissue; in addition, a plausible reason for the weak contractile force of engineered cardiac tissue is the weak constitution of intercalated disk, because it was elucidated that mRNA expression of proteins related to intercalated disk were lower in culture.

Constitutive serum response factor activation by the viral chemokine receptor homologue pUS28 is differentially regulated by Galpha(q/11) and Galpha(16)

Expression of the human cytomegalovirus (HCMV)-encoded chemokine receptor homologue pUS28 in mammalian cells results in ligand-dependent and -independent changes in the activity of multiple cellular signal transduction pathways. The ligand-dependent signalling activity of pUS28 has been shown to be predominantly mediated by heterotrimeric G proteins of the G(i/o) and G(12/13) subfamilies. Ligand-independent constitutive activity of pUS28 causing stimulation of inositol phosphate formation has been correlated with the coupling of pUS28 to G proteins of the G(q) family. It is well known that activation of G(q) proteins by cell surface receptors is coupled to activation of the Rho GTPase RhoA. Activated RhoA regulates numerous cellular functions, including the activity of the transcription factor serum response factor (SRF). The marked activation of G(q) proteins by pUS28 in transfected and HCMV-infected cells prompted us to investigate its effect on SRF activity. The results presented herein demonstrate that expression of pUS28 in COS-7 cells caused a vigorous induction of SRF activity. This effect was observed in the absence of chemokines known to interact with pUS28, and was specifically mediated by endogenous G(q) and/or G(11) as well as RhoA and/or a closely related Rho GTPase. The stimulatory effect of pUS28 and Galpha(q/11) was independent of phospholipase C-beta (PLCbeta) activation and was markedly sensitive to inhibition by wild-type, but not by constitutively active Galpha(16), thus identifying Galpha(16) as a modulator of Galpha(q/11) function likely to act by competing with Galpha(q/11) for and thus uncoupling Galpha(q/11) from activation by pUS28.

Protein expression profiling of breast cancer cells by dissociable antibody microarray (DAMA) staining

Dissociable antibody microarray (DAMA) staining is a technology that combines protein microarrays with traditional immunostaining techniques. It can simultaneously determine the expression and subcellular location of hundreds of proteins in cultured cells and tissue samples. We developed this technology and demonstrated its application in identifying potential biomarkers for breast cancer. We compared the expression profiles of 312 proteins among three normal breast cell lines and seven breast cancer cell lines and identified 10 differentially expressed proteins by the data analysis program DAMAPEP (DAMA protein expression profiling). Among those proteins, RAIDD, Rb p107, Rb p130, SRF, and Tyk2 were confirmed by Western blot and statistical analysis to have higher expression levels in breast cancer cells than in normal breast cells. These proteins could be potential biomarkers for the diagnosis of breast cancer.

Hic-5 contributes to epithelial-mesenchymal transformation through a RhoA/ROCK-dependent pathway

Epithelial-mesenchymal transformation (EMT) in response to TGFbeta1 is a coordinated process of tissue morphogenesis that occurs during embryonic development as well as during certain pathologic events including kidney tubulointerstitial fibrosis. It is characterized by the disassembly of cell-cell junctions and dramatic alterations in the actin cytoskeleton that facilitates cell-matrix adhesion and stimulates migration. The focal adhesion adapter protein, Hic-5, has previously been reported to be upregulated during TGFbeta1-induced EMT in mouse mammary epithelial cells and the current study recapitulates this result in both mouse kidney proximal tubule epithelial, MCT, cells and human mammary epithelial, MCF10A, cells. To evaluate a causative role for Hic-5 in EMT, Hic-5 RNA interference (siRNA) was used to prevent Hic-5 expression in response to TGFbeta1 stimulation and was shown to suppress cell migration and actin stress fiber formation. It also resulted in the retention of a robust epithelial cell morphology characterized by elevated E-cadherin protein expression and well-organized adherens junctions. In addition, Hic-5 siRNA treatment led to the suppression of TGFbeta1 induction of RhoA activation. In contrast, forced expression of Hic-5 led to the formation of ROCK-dependent actin stress fibers. Furthermore, the induction of Hic-5 expression in response to TGFbeta1 was shown to be a RhoA/ROCK I-dependent process. Together, these data implicate Hic-5 as a key regulator of EMT and suggest that RhoA stimulated Hic-5 expression in response to TGFbeta1 may be functioning in a feed forward mechanism whereby Hic-5 maintains the mesenchymal phenotype through sustained RhoA activation and signaling.

SRF is a nuclear repressor of Smad3-mediated TGF-beta signaling

Serum response factor (SRF) is a widely expressed transcription factor involved in immediate-early and tissue-specific gene expression, cell proliferation and differentiation. We defined a new role of SRF as a nuclear repressor of the tumor growth factor beta1 (TGF-beta1) growth-inhibitory signal during cell proliferation. We show that SRF significantly inhibits the TGF-beta1/Smad-dependent transcription by associating with Smad3. SRF causes resistance to the TGF-beta1 cytostatic response by directly repressing the Smad transcriptional activity and Smad binding to DNA. Furthermore, we demonstrated that overexpression of SRF markedly decreases the level of Smad3 complex binding to the promoters of Smad3 target genes, p15(INK4b) and p21(Cip1). This leads to the inhibition of expression of TGF-beta1-responsive genes. SRF therefore acts as a nuclear repressor of Smad3-mediated TGF-beta1 signaling.

Binding of serum response factor to cystic fibrosis transmembrane conductance regulator CArG-like elements, as a new potential CFTR transcriptional regulation pathway

CFTR expression is tightly controlled by a complex network of ubiquitous and tissue-specific cis-elements and trans-factors. To better understand mechanisms that regulate transcription of CFTR, we examined transcription factors that specifically bind a CFTR CArG-like motif we have previously shown to modulate CFTR expression. Gel mobility shift assays and chromatin immunoprecipitation analyses demonstrated the CFTR CArG-like motif binds serum response factor both in vitro and in vivo. Transient co-transfections with various SRF expression vector, including dominant-negative forms and small interfering RNA, demonstrated that SRF significantly increases CFTR transcriptional activity in bronchial epithelial cells. Mutagenesis studies suggested that in addition to SRF other co-factors, such as Yin Yang 1 (YY1) previously shown to bind the CFTR promoter, are potentially involved in the CFTR regulation. Here, we show that functional interplay between SRF and YY1 might provide interesting perspectives to further characterize the underlying molecular mechanism of the basal CFTR transcriptional activity. Furthermore, the identification of multiple CArG binding sites in highly conserved CFTR untranslated regions, which form specific SRF complexes, provides direct evidence for a considerable role of SRF in the CFTR transcriptional regulation into specialized epithelial lung cells.

Depletion of serum response factor by RNA interference mimics the mitogenic effects of platelet derived growth factor-BB in vascular smooth muscle cells

Promoters of many smooth muscle-specific genes (SM-genes) contain multiple CArG boxes, which represent a binding site for serum response factor (SRF). Transcriptional control through these regions involves interactions with SRF and specific coactivators such as myocardin. We have previously reported that suppression of SM-gene expression by platelet derived growth factor (PDGF) is associated with redistribution of SRF, leading to lower intra-nuclear levels, and a reduction in SRF transactivation. To further assess the role of SRF depletion on VSMC phenotype, the current study used RNA interference (RNAi). Two SRF-specific sequences constructed as hairpins were stably expressed in rat VSMC. Clones expressing SRF RNAi had no detectable SRF expression by immunoblotting, and showed diminished levels of SM alpha-actin protein and promoter activity. Unexpectedly, depletion of VSMC resulted in increased rates of proliferation and migration. Several genes whose expression is increased by PDGF stimulation, including c-Jun, were similarly induced in cells lacking SRF. Effects of SRF depletion were not attributable to altered PDGF receptor activity or alterations in activation of Akt. These data indicate that loss of SRF transactivation in VSMC, in this case through suppression via RNAi, induces biological responses similar to that seen with PDGF.

Patterns of gene expression differentially regulated by platelet-derived growth factor and hypertrophic stimuli in vascular smooth muscle cells: markers for phenotypic modulation and response to injury

In vascular smooth muscle cells (VSMC), platelet-derived growth factor (PDGF) suppresses expression of multiple smooth muscle contractile proteins, useful markers of differentiation. Conversely, hypertrophic agents induce expression of these genes. The goal of this study was to employ genomic approaches to identify classes of genes differentially regulated by PDGF and hypertrophic stimuli. Changes in gene expression were determined using Affymetrix RAE-230 GeneChips in rat aortic VSMC stimulated with PDGF. For comparison with a model hypertrophic stimulus, a microarray was performed with VSMC stably expressing constitutively active Galpha(16), which strongly induces smooth muscle marker expression. We identified 75 genes whose expression was increased by exposure to PDGF and decreased by expression of Galpha(16) and 97 genes whose expression was decreased by PDGF and increased by Galpha(16). These genes included many smooth muscle-specific proteins; several extracellular matrix, cytoskeletal, and chemotaxis-related proteins; cell signaling molecules; and transcription factors. Changes in gene expression for many of these were confirmed by PCR or immunoblotting. The contribution of signaling pathways activated by PDGF to the gene expression profile was examined in VSMC stably expressing gain-of-function H-Ras or myristoylated Akt. Among the genes that were confirmed to be differentially regulated were CCAAT/enhancer-binding protein delta, versican, and nexilin. All of these genes also had altered expression in injured aortas, consistent with a role for PDGF in the response of injured VSMC. These data indicate that genes that are differentially regulated by PDGF and hypertrophic stimuli may represent families of genes and potentially be biomarkers for vascular injury.

Serum response factor is alternatively spliced in human colon cancer

BACKGROUND

Serum response factor (SRF) is a transcription factor that plays an important role in cellular differentiation and cell cycle regulation. SRF function is regulated in part by alternative splicing. Little is known about the expression or role of these alternatively spliced isoforms during tumorigenesis. We hypothesized that there is a change in the splice variants during intestinal tumorigenesis and that this change promotes the tumor phenotype.

MATERIALS AND METHODS

SRF expression was determined by Western blotting of benign intestinal cells and human colon cancer cell lines. To determine the effect of alternative splicing of SRF on intestinal growth and proliferation, the predominant alternatively spliced isoform of SRF that we identified in colon cancer cells, SRFDelta5, was transfected into IEC-6 cells. IEC-6 and IEC-6SRFDelta5 cells were plated and cell numbers were determined at four time points.

RESULTS

Western blotting demonstrates that full-length SRF is the predominant form of SRF in rat IEC-6 cells, normal human colonic mucosa, and HT-29 cells, derived from a well-differentiated human colonic adenocarcinoma. In the colon cancer cell lines derived from poorly differentiated tumors (WiDr, HCT 116, LoVo, and SW480), SRFDelta5 is the predominant isoform expressed. There was a significant increase in cell survival in IEC-6 cells transfected with SRFDelta5 compared to parental cells.

CONCLUSION

Our data demonstrate that an alternatively spliced isoform of SRF, SRFDelta5, is expressed in human colon cancer cell lines. Additionally, these data demonstrate that expression of SRFDelta5 may contribute to the tumor phenotype by affecting cell survival. This is the first study to document a change in expression of the alternatively spliced isoform of SRF in human malignancy.

Rho GTPases: potential candidates for anticancer therapy

Low molecular weight Rho GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell proliferation, apoptosis, lipid metabolism, cytoarchitecture, adhesion, migration, cell polarity, and transcriptional regulation. The high incidence of overexpression of some members of the Rho family of GTPases in human tumors suggests that these proteins are important in the carcinogenic process, and therefore potential candidates for a therapeutic intervention. In recent years, the characterization of downstream effectors to Rho GTPases has increased our understanding of the general cellular effects that permit aberrant proliferation and motility of tumor cells. In addition, several transcription factors have been identified to play important roles at various levels of Rho-induced tumorigenesis. Accordingly, drugs that specifically alter Rho signaling display antineoplastic properties both at the level of tumor growth and tumor metastasis. In this review, a brief summary of the progress made in understanding the biological functions elicited by Rho GTPases that contribute to tumor biology will be made. In addition, a description of new drugs available targeted to specific elements of Rho signaling with antineoplastic or antimetastatic activity is included.

Epithelial–mesenchymal transitions in development and pathologies

The epithelial-mesenchymal transition (EMT) is a fundamental process governing morphogenesis in multicellular organisms. This process is also reactivated in a variety of diseases including fibrosis and in the progression of carcinoma. The molecular mechanisms of EMT were primarily studied in epithelial cell lines, leading to the discovery of transduction pathways involved in the loss of epithelial cell polarity and the acquisition of a variety of mesenchymal phenotypic traits. Similar mechanisms have also been uncovered in vivo in different species, showing that EMT is controlled by remarkably well-conserved mechanisms. Current studies further emphasise the critical importance of EMT and provide a better molecular and functional definition of mesenchymal cells and how they emerged >500 million years ago as a key event in evolution.

Platelet-derived growth factor-BB-mediated activation of Akt suppresses smooth muscle-specific gene expression through inhibition of mitogen-activated protein kinase and redistribution of serum response factor

Platelet-derived growth factor (PDGF) inhibits expression of smooth muscle (SM) genes in vascular smooth muscle cells and blocks induction by arginine vasopressin (AVP). We have previously demonstrated that suppression of SM-alpha-actin by PDGF-BB is mediated in part through a Ras-dependent pathway. This study examined the role of phosphatidylinositol 3-kinase (PI3K)y and its downstream effector, Akt, in regulating SM gene expression. PDGF caused a rapid sustained activation of Akt, whereas AVP caused only a small transient increase. PDGF selectively caused a sustained stimulation of p85/p110 alpha PI3K. In contrast, p85/110 beta PI3K activity was not altered by either PDGF or AVP, whereas both agents caused a delayed activation of Class IB p101/110 gamma PI3K. Expression of a gain-of-function PI3K or myristoylated Akt (myr-Akt) mimicked the inhibitory effect of PDGF on SM-alpha-actin and SM22 alpha expression. Pretreatment with LY 294002 reversed the inhibitory effect of PDGF. Expression of myr-Akt selectively inhibited AVP-induced activation of c-Jun N-terminal kinase and p38 mitogen-activated protein kinases, which we have shown are critical for induction of these genes. Nuclear extracts from PDGF-stimulated or myr-Akt expressing cells showed reduced serum response factor binding to SM-specific CArG elements. This was associated with appearance of serum response factor in the cytoplasm. These data indicate that activation of p85/p110 alpha/Akt mediates suppression of SM gene expression by PDGF.

Transcriptional regulators and myelopoiesis: the role of serum response factor and CREB as targets of cytokine signaling

Hematopoiesis is a complex process in which mature myeloid and lymphoid cells are produced from a small population of pluripotent stem cells within the bone marrow. Blood cell formation occurs, in part, by progenitor cell exposure to humoral growth regulators, known as hematopoietic cytokines, as well as by the regulated expression of genes by transcription factors. In this paper, we review two important nuclear proteins, the serum response factor and the cyclic adenosine monophosphate response element-binding protein, as downstream targets of mitogens, with a specific focus on hematopoietic cytokine signaling and the role these proteins play in gene regulation.

Serum response factor function and dysfunction in smooth muscle

Tight control of smooth muscle cell (SM) proliferation, differentiation, and apoptosis requires a balance between signaling and transcriptional events. Recent developments in vascular research revealed that serum response factor (SRF) function is important for the regulation of each of these processes. The cloning and characterization of several SM specific genes and the discovery that SRF is central for their expression fueled studies aimed at understanding the role of molecular partners including co-activators and co-repressors. Perturbations of pathways involving SRF are associated with abnormalities in the myogenic program and aberrant phenotypic consequences. Surprisingly, studies on airway SM have remained an underrepresented area of investigation. Our laboratory described a novel regulatory mechanism of SRF function in airway myocytes by modulation of its subcellular localization. This review summarizes current knowledge on the structure and function of this essential transcription factor as well different modes of regulating SRF expression and activity that are becoming key players in directing SM function in health and disease.

ROCK and nuclear factor-kappaB-dependent activation of cyclooxygenase-2 by Rho GTPases: effects on tumor growth and therapeutic consequences

Rho GTPases are overexpressed in a variety of human tumors contributing to both tumor proliferation and metastasis. Recently, several studies demonstrate an essential role of transcriptional regulation in Rho GTPases-induced oncogenesis. Herein, we demonstrate that RhoA, Rac1, and Cdc42 promote the expression of cyclooxygenase-2 (COX-2) at the transcriptional level by a mechanism that is dependent on the transcription factor nuclear factor-kappaB (NF-kappaB), but not Stat3, a transcription factor required for RhoA-induced tumorigenesis. With respect to RhoA, this effect is dependent on ROCK, but not PKN. Treatment of RhoA-, Rac1-, and Cdc42-transformed epithelial cells with Sulindac and NS-398, two well-characterized nonsteroid antiinflammatory drugs (NSAIDs), results in growth inhibition as determined by cell proliferation assays. Accordingly, tumor growth of RhoA-expressing epithelial cells in syngeneic mice is strongly inhibited by NS-398 treatment. The effect of NSAIDs over RhoA-induced tumor growth is not exclusively dependent on COX-2 because DNA-binding of NF-kappaB is also abolished upon NSAIDs treatment, resulting in complete loss of COX-2 expression. Finally, treatment of RhoA-transformed cells with Bay11-7083, a specific NF-kappaB inhibitor, leads to inhibition of cell proliferation. We suggest that treatment of human tumors that overexpress Rho GTPases with NSAIDs and drugs that target NF-kappaB could constitute a valid antitumoral strategy.

Biphasic role of TGF-beta1 in signal transduction and crosstalk

TGF-beta1 induces cell cycle activation in mouse embryonic fibroblasts by down regulation of p27(Kip1) but it can also induce delay of EGF-induced cell cycle activation in these cells under similar conditions. In an attempt to determine the basis for these responses, the study of early TGF-beta1-induced signal transduction pathways in the presence and absence of EGF was undertaken. It is proposed that a likely target for the inhibition by TGF-beta1 of the early EGF-induced p42/p44 MAPK is at the c-Raf locus. The finding that the catalytic subunits of PKA are associated with Raf-1 within minutes following application of TGF-beta1 but not EGF in fibroblasts arrested in early G1 is suggestive of a role of PKA-Raf-1 interaction in TGF-beta1 induced delay of EGF-induced cell cycle kinetics. A model for TGF-beta1 induced translocation to the plasma membrane-associated Raf-1 is proposed. Reports that Rho-like GTPase activity is critical for the activation of TGF-beta1 downstream pathways raises the question as to whether Rho proteins are involved in these observed TGF-beta1-induced responses. Post-receptor signaling mechanisms for TGF-beta1 and cross-talk with PKA-mediated pathways are examined in an effort to explain the modulation by TGF-beta1 of mitogen-induced cell proliferation in mesenchymal cells.

Alterations in signal transduction pathways implicated in tumour progression during multistage mouse skin carcinogenesis

The multistage mouse skin carcinogenesis model, although an artificial one, is an ideal system to study the timing of qualitative and quantitative alterations which take place during the different stages of chemical carcinogenesis, allowing analysis of the events that lead to the transition from the stage of initiation to the stage of promotion and finally to the progression of carcinogenesis. In this review we focus on the role of the H-ras gene and its target molecules during mouse skin carcinogenesis. Besides H-ras, which is a critical target of chemical carcinogens, we report alterations in oncosuppressor genes. Finally, we examine the potential suppression of metastatic dynamics of spindle cells after biological or chemical inhibition of the signalling cascades involved in mouse skin carcinogenesis.

Actin dynamics control SRF activity by regulation of its coactivator MAL

Rho GTPases regulate the transcription factor SRF via their ability to induce actin polymerization. SRF activity responds to G actin, but the mechanism of this has remained unclear. We show that Rho-actin signaling regulates the subcellular localization of the myocardin-related SRF coactivator MAL, rearranged in t(1;22)(p13;q13) AML. The MAL-SRF interaction displays the predicted properties of a Rho-regulated SRF cofactor. MAL is predominantly cytoplasmic in serum-starved cells, but accumulates in the nucleus following serum stimulation. Activation of the Rho-actin signaling pathway is necessary and sufficient to promote MAL nuclear accumulation. MAL N-terminal sequences, including two RPEL motifs, are required for the response to signaling, while other regions mediate its nuclear export (or cytoplasmic retention) and nuclear import. MAL associates with unpolymerized actin through its RPEL motifs. Constitutively cytoplasmic MAL derivatives interfere with MAL redistribution and Rho-actin signaling to SRF. MAL associates with several SRF target promoters regulated via the Rho-actin pathway.

STAT5a activation mediates the epithelial to mesenchymal transition induced by oncogenic RhoA

The involvement of Rho GTPases in signal transduction pathways leading to transcription activation is one of the major roles of this family of GTPases. Thus, the identification of transcription factors regulated by Rho GTPases and the understanding of the mechanisms of their activation and its biological outcome are of great interest. Here, we provide evidence that Rho GTPases modulate Stat5a, a transcription factor of the family of signal transducers and activators of transcription. RhoA triggers tyrosine phosphorylation (Y696) of Stat5a via a JAK2-dependent mechanism and promotes DNA-binding activity of Stat5a. Tyrosine phosphorylation of Stat5a is also stimulated physiologically by lysophosphatidic acid (LPA) in a Rho-dependent manner. Simultaneously, RhoA reduces serine phosphorylation of Stat5a at both serine residues S726 and S780, resulting in a further increase of activity as defined by mutagenesis experiments. Furthermore, serine dephosphorylation of Stat5a by RhoA does not take place by down-modulation of either JNK1, MEK1, or p38 MAP kinases, as determined by transfection experiments or chemical inhibition of both MEK1, p38, and JNK serine kinases. Thus, RhoA regulates Stat5a via tyrosine phosphorylation and via a yet to be determined novel down-modulating pathway that involves serine dephosphorylation. Finally, we provide evidence for a role of Stat5a in RhoA-induced epithelial-to-mesenchymal transition with concomitant increase in vimentin expression, E-cadherin down-regulation, and cell motility.