Transcriptional activation of the fra-1 gene by AP-1 is mediated by regulatory sequences in the first intron

The Fra-1: Novel role in regulating extensive immune cell states and affecting inflammatory diseases

Fra-1(Fos-related antigen1), a member of transcription factor activator protein (AP-1), plays an important role in cell proliferation, apoptosis, differentiation, inflammation, oncogenesis and tumor metastasis. Accumulating evidence suggest that the malignancy and invasive ability of tumors can be significantly changed by directly targeting Fra-1. Besides, the effects of Fra-1 are gradually revealed in immune and inflammatory settings, such as arthritis, pneumonia, psoriasis and cardiovascular disease. These regulatory mechanisms that orchestrate immune and non-immune cells underlie Fra-1 as a potential therapeutic target for a variety of human diseases. In this review, we focus on the current knowledge of Fra-1 in immune system, highlighting its unique importance in regulating tissue homeostasis. In addition, we also discuss the possible critical intervention strategy in diseases, which also outline future research and development avenues.

Fos-Related Antigen 1 May Cause Wnt-Fzd Signaling Pathway-Related Nephroblastoma in Children

This study aimed to investigate the role of the primary Fos-related antigen 1 (Fosl-1) oncogene in nephroblastoma by studying 60 childhood nephroblastoma and 58 paraneoplastic carcinoma cases. The Fosl-1 expression was detected using immunohistochemistry. In vitro culture of nephroblastoma cells was performed by viral transfection to establish Fosl-1 overexpression and gene knockout models. Flow cytometry and nano-PCR were used to detect apoptosis and mRNA expression in related pathway genes. Immunohistochemical results showed that the positive expression of Fosl-1 in the nuclei of nephroblastoma tissue was 78%, among which metastasis rate was 61.7%; correspondingly, it was 8%, and 100% in adjacent tissues. The qPCR results indicated that MMP9, Wnt1, and Fzd1 were significantly upregulated after Fosl-1 overexpression compared with the normal embryonic tissue cells, control, and gene knockout groups (P <0.05). Fosl-1 could cause the occurrence, development, and metastasis of childhood nephroblastoma through wingless/int1/Frizzled-related signaling pathways.

Chondroprotective effects of CDK4/6 inhibition via enhanced ubiquitin-dependent degradation of JUN in synovial fibroblasts

OBJECTIVE

Targeting synovial fibroblasts (SF) using a cyclin-dependent kinase (CDK) 4/6 inhibitor (CDKI) could be a potent therapy for rheumatoid arthritis (RA) via inhibition of proliferation and MMP-3 production. This study was designed to elucidate the mechanism of chondroprotective effects on SFs by CDK 4/6 inhibition.

METHODS

CDK4/6 activity was inhibited using CDKI treatment or enhanced by adenoviral gene transduction. Chondroprotective effects were evaluated using a collagen induced arthritis model (CIA). Gene and protein expression were evaluated with quantitative PCR, ELISA, and Western blotting. The binding of nuclear extracts to DNA was assessed with an electrophoresis mobility shift assay. RNA-Seq was performed to identify gene sets affected by CDKI treatment.

RESULTS

CDKI attenuated cartilage destruction and MMP-3 production in CIA. In RASFs, CDKI impaired the binding of AP-1 components to DNA and inhibited the production of MMP-1 and MMP-3, which contain the AP-1 binding sequence in their promoter. CDK4/6 protected JUN from proteasome-dependent degradation by inhibiting ubiquitination. The RNA-Seq analysis identified CDKI-sensitive inflammatory genes, which were associated with the pathway of RA-associated genes, cytokine-cytokine receptor interaction, and IL-17 signalling. Notably, the AP-1 motif was enriched in these genes.

CONCLUSION

The mechanism of chondroprotective effects by CDK4/6 inhibition was achieved by the attenuation of AP-1 transcriptional activity via the impaired stability of JUN. Since the pharmacologic inhibition of CDK4/6 has been established as tolerable in cancer treatment, it could also be beneficial in patients with RA due to its chondroprotective and anti-inflammatory effects.

Equine penile squamous cell carcinoma: expression of biomarker proteins and EcPV2

Equine penile squamous cell carcinoma (EpSCC) is a relatively common cutaneous neoplasm with a poor prognosis. In this study, we aimed to determine the protein expression and colocalisation of FRA1, c-Myc, Cyclin D1, and MMP7 in normal (NT), tumour (T), hyperplastic epidermis and/or squamous papilloma (Hyp/Pap), poorly-differentiated (PDSCC), or well-differentiated (WDSCC) EpSCC using a tissue array approach. Further objectives were to correlate protein expression to (i) levels of inflammation, using a convolutional neural network (ii) equine papillomavirus 2 (EcPV2) infection, detected using PCR amplification. We found an increase in expression of FRA1 in EpSCC compared to NT samples. c-Myc expression was higher in Hyp/Pap and WDSCC but not PDSCC whereas MMP7 was reduced in WDSCC compared with NT. There was a significant increase in the global intersection coefficient (GIC) of FRA1 with MMP7, c-Myc, and Cyclin D1 in EpSCC. Conversely, GIC for MMP7 with c-Myc was reduced in EpSCC tissue. Inflammation was positively associated with EcPV2 infection in both NT and EpSCC but not Hyp/Pap. Changes in protein expression could be correlated with EcPV2 for Cyclin D1 and c-Myc. Our results evaluate novel biomarkers of EpSCC and a putative correlation between the expression of biomarkers, EcPV2 infection and inflammation.

Induction of AP-1 by YAP/TAZ contributes to cell proliferation and organ growth

Yes-associated protein (YAP) and its homolog transcriptional coactivator with PDZ-binding motif (TAZ) are key effectors of the Hippo pathway to control cell growth and organ size, of which dysregulation yields to tumorigenesis or hypertrophy. Upon activation, YAP/TAZ translocate into the nucleus and bind to TEAD transcription factors to promote transcriptional programs for proliferation or cell specification. Immediate early genes, represented by AP-1 complex, are rapidly induced and control later-phase transcriptional program to play key roles in tumorigenesis and organ maintenance. Here, we report that YAP/TAZ directly promote FOS transcription that in turn contributes to the biological function of YAP/TAZ. YAP/TAZ bind to the promoter region of FOS to stimulate its transcription. Deletion of YAP/TAZ blocks the induction of immediate early genes in response to mitogenic stimuli. FOS induction contributes to expression of YAP/TAZ downstream target genes. Genetic deletion or chemical inhibition of AP-1 suppresses growth of YAP-driven cancer cells, such as Lats1/2-deficient cancer cells as well as Gαq/11 mutated uveal melanoma. Furthermore, AP-1 inhibition almost completely abrogates the hepatomegaly induced by YAP overexpression. Our findings reveal a feed-forward interplay between immediate early transcription of AP-1 and Hippo pathway function.

Developmental expression patterns of fosl genes in Xenopus tropicalis

Fos-like antigens (Fosl) including Fosl1 and Fosl2 exclusively heterodimerize with Jun members to form AP-1 complex, thereby participating in various cellular progresses including cell cycle regulation. However, expression patterns of these two genes during embryonic development remains largely unknown. In the present study, both temporal and spatial expression patterns of fosl1 and fosl2 were examined during embryonic development of Xenopus tropicalis. Real-time quantitative PCR results showed that the expression of the two genes was increased from stage 2 to stage 42. However, expression level of fosl1 is much higher than that of fosl2 at stage 42. Whole-mount in situ hybridization showed that fosl1 was expressed in eyes, branchial arch, notochord, otic vesicle, and liver. However, fosl2 was expressed in lung primordium from stage 34 to stage 38, in addition to the moderate expression in eyes and branchial arch at stage 42. Thus, the developmental expression patterns of these two fosl genes is different in Xenopus embryos. These results provide a basis for further functional study of these two genes.

The overexpression of Fra1 disorders the inflammatory cytokine secretion by mTEC of myasthenia gravis thymus

The thymus of a myasthenia gravis (MG) patient is often accompanied by and effected with follicular hyperplasia. Inflammatory cytokines in thymus induce the formation of germinal centres (GC). MG thymic inflammatory cytokines are predominantly secreted by stromal cells. Our previous studies revealed that the expression level of the Fra1 protein, which is a Fos member of the activator protein 1 transcription factors (AP-1), was higher in the MG thymus compared with that of the normal thymus. Based on that, we demonstrated that Fra1 was mainly expressed in medulla thymic epithelial cells (mTECs) and that the rate of Fra1 positive mTECs in the MG thymus was higher than normal. In vitro, we found that the expression of CCL-5, CCL-19 and CCL-21 could be regulated by Fra1 in mTEC and that IL-1β, IL-6, IL-8 and ICAM1 were downregulated in the Fra1 overexpression group and upregulated in the Fra1 knock-down group. Meanwhile, we detected that the expression levels of suppressor of cytokine signalling 3 (SOCS3) were significantly upregulated along with the overexpression of Fra1. Hence, we considered that the overexpression of Fra1 disrupted inflammatory cytokine secretion by mTEC in the MG thymus and that STAT3 and SOCS3 were strongly involved in this process.

microRNA-130a suppresses breast cancer cell migration and invasion by targeting FOSL1 and upregulating ZO-1

FOSL1 is frequently overexpressed in multiple types of human cancers including invasive breast cancers and implicated in cancer invasion and metastasis. However, how FOSL1 is overexpressed in cancers remains to be elucidated. Several microRNAs (miRNAs) have been shown to target FOSL1 and are downregulated in human cancers. Here, we report that miR-130a is a novel FOSL1 targeting miRNA. Using gene expression microarray analysis, we found that FOSL1 is among the most up-regulated genes in cells transfected with miR-130a inhibitors. Transient transfection-immunoblot, RNA-immunoprecipitation, and luciferase reporter assays revealed that miR-130a directly targets FOSL1 mRNA at its 3'-UTR. Overexpression of miR-130a significantly reduced the levels of FOSL1 in invasive breast cancer MDA-MB-231 and Hs578T cell lines and suppresses their migration and invasion. This inhibition can be rescued by ectopic expression of miR-130a-resistant FOSL1. Interestingly, we show that overexpression of miR-130a increased the levels of tight-junction protein ZO-1 while inhibition of miR-130a reduced the levels of ZO-1. We further show that miR-130a expression is significantly reduced in cancer tissues from triple-negative breast cancer (TNBC) patients, correlating significantly with the upregulation of FOSL1 expression, compared to non-TNBC tissues. Together, our results reveal that miR-130a directly targets FOSL1 and suppresses the inhibition of ZO-1, thus inhibiting cancer cell migration and invasion, in TNBCs.

Integrin-uPAR signaling leads to FRA-1 phosphorylation and enhanced breast cancer invasion

Background

The Fos-related antigen 1 (FRA-1) transcription factor promotes tumor cell growth, invasion and metastasis. Phosphorylation of FRA-1 increases protein stability and function. We identify a novel signaling axis that leads to increased phosphorylation of FRA-1, increased extracellular matrix (ECM)-induced breast cancer cell invasion and is prognostic of poor outcome in patients with breast cancer.

Methods

While characterizing five breast cancer cell lines derived from primary human breast tumors, we identified BRC-31 as a novel basal-like cell model that expresses elevated FRA-1 levels. We interrogated the functional contribution of FRA-1 and an upstream signaling axis in breast cancer cell invasion. We extended this analysis to determine the prognostic significance of this signaling axis in samples derived from patients with breast cancer.

Results

BRC-31 cells display elevated focal adhesion kinase (FAK), SRC and extracellular signal-regulated (ERK2) phosphorylation relative to luminal breast cancer models. Inhibition of this signaling axis, with pharmacological inhibitors, reduces the phosphorylation and stabilization of FRA-1. Elevated integrin α_Vβ₃ and uPAR expression in these cells suggested that integrin receptors might activate this FAK-SRC-ERK2 signaling. Transient knockdown of urokinase/plasminogen activator urokinase receptor (uPAR) in basal-like breast cancer cells grown on vitronectin reduces FRA-1 phosphorylation and stabilization; and uPAR and FRA-1 are required for vitronectin-induced cell invasion. In clinical samples, a molecular component signature consisting of vitronectin-uPAR-uPA-FRA-1 predicts poor overall survival in patients with breast cancer and correlates with an FRA-1 transcriptional signature.

Conclusions

We have identified a novel signaling axis that leads to phosphorylation and enhanced activity of FRA-1, a transcription factor that is emerging as an important modulator of breast cancer progression and metastasis.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-0936-8) contains supplementary material, which is available to authorized users.

Loss of Igf2 Gene Imprinting in Murine Prostate Promotes Widespread Neoplastic Growth

Loss of imprinting (LOI) is an epigenetic event that relaxes an allele-specific restriction on gene expression. One gene that experiences LOI is the paracrine insulin-like growth factor IGF2, which occurs commonly in human prostate tissues during aging and tumorigenesis. However, the relationship between IGF2 LOI and prostate tumorigenesis has not been established functionally. In this study, we created a mouse model with CTCF-binding site mutations at the Igf2-H19 imprint control region that abolishes CTCF insulator activity, resulting in biallelic Igf2 expression that mimics increased levels seen with aging-induced LOI. We found that Igf2 LOI increased the prevalence and severity of prostatic intraepithelial neoplasia (PIN), a premalignant lesion. Engineering Nkx3.1 deficiency into our model increased the frequency of PIN lesions in an additive fashion. Prostates harboring LOI displayed increased MAPK signaling and epithelial proliferation. In human prostate tissue arrays, we documented a positive correlation in benign tissues of IGF2 levels with phospho-ERK and phospho-AKT levels. Overall, our results establish that Igf2 LOI is sufficient on its own to increase rates of neoplastic development in the prostate by upregulating critical cancer-associated signaling pathways. Cancer Res; 77(19); 5236-47. ©2017 AACR.

Expression of Fra-1 in human hepatocellular carcinoma and its prognostic significance

This study aimed to explore the clinical significance and prognostic value of Fra-1 in hepatocellular carcinoma patients after curative resection. Fra-1 expression was investigated using a combination of techniques: immunohistochemistry for 66 samples of hepatocellular carcinoma and quantitative real-time polymerase chain reaction and western blotting assays for 19 matched hepatocellular carcinoma specimens. Fra-1 was present in 38 of 66 (57.6%) tumor tissues, with intense staining in the nuclei. There was also positive staining in 14 of 66 (21.2%) adjacent peritumoral tissues, with weak staining in the cytoplasm. Quantitative real-time polymerase chain reaction and western blotting assays confirmed higher expression of Fra-1 messenger RNA and Fra-1 protein in tumor tissues than adjacent non-tumor tissues for 19 hepatocellular carcinoma samples (p < 0.001). Positive expression of Fra-1 was significantly related to vascular invasion and serum alpha-fetoprotein. Kaplan-Meier survival analysis found that overexpressed Fra-1 was correlated with poor overall survival and disease-free survival. Multivariate analysis identified Fra-1 as an independent prognostic factor. Fra-1 may be involved in the progress of hepatocellular carcinoma and could be a promising molecular candidate in the diagnosis and treatment of hepatocellular carcinoma.

MLK3 regulates FRA-1 and MMPs to drive invasion and transendothelial migration in triple-negative breast cancer cells

Mixed-lineage kinase 3 (MLK3), a mitogen-activated protein kinase kinase kinase (MAP3K), has critical roles in metastasis of triple-negative breast cancer (TNBC), in part by regulating paxillin phosphorylation and focal adhesion turnover. However the mechanisms and the distinct step(s) of the metastatic processes through which MLK3 exerts its influence are not fully understood. Here we report that in non-metastatic, estrogen receptor-positive breast cancer (ER+ BC) cells, induced MLK3 expression robustly upregulates the oncogenic transcription factor, FOS-related antigen-1 (FRA-1), which is accompanied by elevation of matrix metalloproteinases (MMPs), MMP-1 and MMP-9. MLK3-induced ER+ BC cell invasion is abrogated by FRA-1 silencing, demonstrating that MLK3 drives invasion through FRA-1. Conversely, in metastatic TNBC models, high FRA-1 levels are significantly reduced upon depletion of MLK3 by either gene silencing or by the CRISPR/Cas9n editing approach. Furthermore, ablation of MLK3 or MLK inhibitor treatment decreases expression of both MMP-1 and MMP-9. Consistent with the role of tumor cell-derived MMP-1 in endothelial permeability and transendothelial migration, both of these are reduced in MLK3-depleted TNBC cells. In addition, MLK inhibitor treatment or MLK3 depletion, which downregulates MMP-9 expression, renders TNBC cells defective in Matrigel invasion. Furthermore, circulating tumor cells derived from TNBC-bearing mice display increased levels of FRA-1 and MMP-1 compared with parental cells, supporting a role for the MLK3–FRA-1–MMP-1 signaling axis in vascular intravasation. Our results demonstrating the requirement for MLK3 in controlling the FRA-1/MMPs axis suggest that MLK3 is a promising therapeutic target for treatment of TNBC.

Activation of Tax protein by c-Jun-N-terminal kinase is not dependent on the presence or absence of the early growth response-1 gene product

The Tax protein of human T cell leukemia virus type 1 plays a major role in the pathogenesis of adult T cell leukemia (ATL), an aggressive neoplasia of CD4+ T cells. In the present study, we investigated whether the EGR-1 pathway is involved in the regulation of Tax-induced JNK expression in human Jurkat T cells transfected to express the Tax protein in the presence or absence of PMA or ionomycin. Overexpression of EGR-1 in Jurkat cells transfected to express Tax, promoted the activation of several genes, with the most potent being those that contained AP-1 (Jun/c-Fos), whereas knockdown of endogenous EGR-1 by small interfering RNA (siRNA) somewhat reduced Tax-mediated JNK-1 transcription. Additionally, luciferase-based AP-1 and NF-κB reporter gene assays demonstrated that inhibition of EGR-1 expression by an siRNA did not affect the transcriptional activity of a consensus sequence of either AP-1 or NF-κB. On the other hand, the apoptosis assay, using all-trans retinoic acid (ATRA) as an inducer of apoptosis, confirmed that siRNA against EGR-1 failed to suppress ATRA-induced apoptosis in Jurkat and Jurkat-Tax cells, as noted by the low levels of both DEVDase activity and DNA fragmentation, indicating that the induction of apoptosis by ATRA was Egr-1-independent. Finally, our data showed that activation of Tax by JNK-1 was not dependent on the EGR-1 cascade of events, suggesting that EGR-1 is important but not a determinant for the activity for Tax-induced proliferation of Jurkat cells.

FRA-1 as a driver of tumour heterogeneity: a nexus between oncogenes and embryonic signalling pathways in cancer

Tumour heterogeneity is a major factor undermining the success of therapies targeting metastatic cancer. Two major theories are thought to explain the phenomenon of heterogeneity in cancer--clonal evolution and cell plasticity. In this review, we examine a growing body of work implicating the transcription factor FOS-related antigen 1 (FRA-1) as a central node in tumour cell plasticity networks, and discuss mechanisms regulating its activity in cancer cells. We also discuss evidence from the FRA-1 perspective supporting the notion that clonal selection and cell plasticity represent two sides of the same coin. We propose that FRA-1-overexpressing clones featuring high plasticity undergo positive selection during consecutive stages of multistep tumour progression. This model underscores a potential mechanism through which tumour cells retaining elevated levels of plasticity acquire a selective advantage over other clonal populations within a tumour.

Promoter-level expression clustering identifies time development of transcriptional regulatory cascades initiated by ErbB receptors in breast cancer cells

The analysis of CAGE (Cap Analysis of Gene Expression) time-course has been proposed by the FANTOM5 Consortium to extend the understanding of the sequence of events facilitating cell state transition at the level of promoter regulation. To identify the most prominent transcriptional regulations induced by growth factors in human breast cancer, we apply here the Complexity Invariant Dynamic Time Warping motif EnRichment (CIDER) analysis approach to the CAGE time-course datasets of MCF-7 cells stimulated by epidermal growth factor (EGF) or heregulin (HRG). We identify a multi-level cascade of regulations rooted by the Serum Response Factor (SRF) transcription factor, connecting the MAPK-mediated transduction of the HRG stimulus to the negative regulation of the MAPK pathway by the members of the DUSP family phosphatases. The finding confirms the known primary role of FOS and FOSL1, members of AP-1 family, in shaping gene expression in response to HRG induction. Moreover, we identify a new potential regulation of DUSP5 and RARA (known to antagonize the transcriptional regulation induced by the estrogen receptors) by the activity of the AP-1 complex, specific to HRG response. The results indicate that a divergence in AP-1 regulation determines cellular changes of breast cancer cells stimulated by ErbB receptors.

Visualization of Fra-1/AP-1 activation during LPS-induced inflammatory lung injury using fluorescence optical imaging

Inappropriate lung inflammatory response following oxidant and toxicant exposure can lead to abnormal repair and disease pathogenesis, including fibrosis. Thus early detection of molecular and cellular processes and mediators promoting lung inflammation is necessary to develop better strategies for therapeutic intervention and disease management. Previously, we have shown that transcription factor Fra-1/AP-1 plays key roles in lung inflammatory response, as Fra-1-null mice are less susceptible than wild-type mice to LPS-induced lung injury and mortality. Herein, we developed a transgenic reporter mouse model expressing tdTomato under the control of FRA-1 (human) promoter (referred to as FRA-1(TdTg) mice) to monitor its activation during inflammatory lung injury using fluorescence protein-based optical imaging and molecular analysis in vivo and ex vivo. A higher red fluorescent signal was observed in the lungs of LPS-treated FRA-1(TdTg) mice compared with vehicle controls, and Western blot and qRT-PCR analyses revealed a significant correlation with the FRA-1-tdTomato reporter expression. Immunocolocalization demonstrated expression of FRA-1-tdTomato largely in lung alveolar macrophages and to some extent in epithelial cells. Moreover, we validated these results with a second reporter mouse model that expressed green fluorescent protein upon activation of endogenous Fra-1 promoter. Additionally, we demonstrated increased expression of FRA-1 in alveolar macrophages in human lung instilled with Escherichia coli ex vivo. Collectively, our data obtained from two independent reporter mouse models and from human samples underscore the significance of Fra-1 activation in alveolar macrophages during inflammatory lung injury and may aid in developing strategies to target this transcription factor in lung injury and repair.

FOS-like antigen 1 is highly expressed in human psoriasis tissues and promotes the growth of HaCaT cells in vitro

Psoriasis is a multifactorial disease and the mechanisms involved in its pathogenesis remain to be elucidated. FOS‑like antigen 1 (Fra‑1) is a proto‑oncogene. It is a negative inhibitor of activator protein‑1 activity and possesses transforming activity. The effect of and possible mechanisms underlying Fra‑1 in psoriasis remain to be elucidated. In the present study, western blot analysis and reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) techniques were used to identify differentially expressed Fra‑1 in psoriatic and in normal control tissues. Compared with the control samples, the expression of normalized Fra‑1 genes in psoriasis was 12.6 times higher. Western blot analysis was used to assess the protein levels of Fra‑1. The results demonstrated that the protein expression of Fra-1 was high in tissues affected by psoriasis. This also corresponded with the results of RT‑qPCR. Fra‑1‑stable expressing HaCaT/Fra‑1 or control HaCaT/vector cell lines were then generated to elucidate the function of Fra‑1 in the growth of HaCaT cells. The results demonstrated that Fra‑1 promoted the growth of HaCaT cells in vitro by arresting the cell cycle and inhibiting cell apoptosis. These results suggested that Fra‑1 may be important in psoriasis.

Retinoblastoma protein and MyoD function together to effect the repression of Fra-1 and in turn cyclin D1 during terminal cell cycle arrest associated with myogenesis

The acquisition of skeletal muscle-specific function and terminal cell cycle arrest represent two important features of the myogenic differentiation program. These cellular processes are distinct and can be separated genetically. The lineage-specific transcription factor MyoD and the retinoblastoma protein pRb participate in both of these cellular events. Whether and how MyoD and pRb work together to effect terminal cell cycle arrest is uncertain. To address this question, we focused on cyclin D1, whose stable repression is required for terminal cell cycle arrest and execution of myogenesis. MyoD and pRb are both required for the repression of cyclin D1; their actions, however, were found not to be direct. Rather, they operate to regulate the immediate early gene Fra-1, a critical player in mitogen-dependent induction of cyclin D1. Two conserved MyoD-binding sites were identified in an intronic enhancer of Fra-1 and shown to be required for the stable repression of Fra-1 and, in turn, cyclin D1. Localization of MyoD alone to the intronic enhancer of Fra-1 in the absence of pRb was not sufficient to elicit a block to Fra-1 induction; pRb was also recruited to the intronic enhancer in a MyoD-dependent manner. These observations suggest that MyoD and pRb work together cooperatively at the level of the intronic enhancer of Fra-1 during terminal cell cycle arrest. This work reveals a previously unappreciated link between a lineage-specific transcription factor, a tumor suppressor, and a proto-oncogene in the control of an important facet of myogenic differentiation.

Small molecule inhibitors targeting activator protein 1 (AP-1)

Activator protein 1 (AP-1) is a pivotal transcription factor that regulates a wide range of cellular processes including proliferation, apoptosis, differentiation, survival, cell migration, and transformation. Accumulating evidence supports that AP-1 plays an important role in several severe disorders including cancer, fibrosis, and organ injury, as well as inflammatory disorders such as asthma, psoriasis, and rheumatoid arthritis. AP-1 has emerged as an actively pursued drug discovery target over the past decade. Excitingly, a selective AP-1 inhibitor T-5224 (51) has been investigated in phase II human clinical trials. Nevertheless, no effective AP-1 inhibitors have yet been approved for clinical use. Despite significant advances achieved in understanding AP-1 biology and function, as well as the identification of small molecules modulating AP-1 associated signaling pathways, medicinal chemistry efforts remain an urgent need to yield selective and efficacious AP-1 inhibitors as a viable therapeutic strategy for human diseases.

FOSL1 controls the assembly of endothelial cells into capillary tubes by direct repression of αv and β3 integrin transcription

To form three-dimensional capillary tubes, endothelial cells must establish contacts with the extracellular matrix that provides signals for their proliferation, migration, and differentiation. The transcription factor Fosl1 plays a key role in the vasculogenic and angiogenic processes as Fosl1 knockout embryos die with vascular defects in extraembryonic tissues. Here, we show that Fosl1(-/-) embryonic stem cells differentiate into endothelial cells but fail to correctly assemble into primitive capillaries and to form tube-like structures. FOSL1 silencing affects in vitro angiogenesis, increases cell adhesion, and decreases cell mobility of primary human endothelial cells (HUVEC). We further show that FOSL1 is a repressor of αv and β3 integrin expression and that the down-modulation of αvβ3 rescues the angiogenic phenotype in FOSL1-silenced HUVEC, while the ectopic expression of αvβ3 alone reproduces the phenotypic alterations induced by FOSL1 knockdown. FOSL1 represses the transcription of both αv and β3 integrin genes by binding together with JunD to their proximal promoter via the transcription factor SP1. These data suggest that FOSL1-dependent negative regulation of αvβ3 expression on endothelial cells is required for endothelial assembly into vessel structures.

FOSL1 as a candidate target gene for 11q12 rearrangements in desmoplastic fibroblastoma

Desmoplastic fibroblastoma (DF) is a benign fibroblastic/myofibroblastic tumor. Cytogenetic analyses have revealed consistent rearrangement of chromosome band 11q12, strongly suggesting that this region harbors a gene of pathogenetic importance. To identify the target gene of the 11q12 rearrangements, we analyzed six cases diagnosed as DF using chromosome banding, fluorescence in situ hybridization (FISH), single-nucleotide polymorphism array and gene expression approaches. Different structural rearrangements involving 11q12 were found in five of the six cases. Metaphase FISH analyses in two of them mapped the 11q12 breakpoints to an ~20-kb region, harboring FOSL1. Global gene expression profiling followed by quantitative real-time PCR showed that FOSL1 was expressed at higher levels in DF with 11q12 rearrangements than in desmoid-type fibromatoses. Furthermore, FOSL1 was not upregulated in the single case of DF that did not show cytogenetic involvement of 11q12; instead this tumor was found to display a hemizygous loss on 5q, including the APC (adenomatous polyposis coli) locus, raising the possibility that it actually was a misdiagnosed Gardner fibroma. 5'RACE-PCR in two 11q12-positive DF did not identify any fusion transcripts. Thus, in agreement with the finding at chromosome banding analysis that varying translocation partners are involved in the 11q12 rearrangement, the molecular data suggest that the functional outcome of the 11q12 rearrangements is deregulated expression of FOSL1.

Global expression analysis identified a preferentially nerve growth factor-induced transcriptional program regulated by sustained mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) and AP-1 protein activation during PC12 cell differentiation

Neuronal differentiation of PC12 cells in response to NGF is a prototypical model in which signal duration determines a biological response. Sustained ERK activity induced by NGF, as compared with transient activity induced by EGF, is critical to the differentiation of these cells. To characterize the transcriptional program activated preferentially by NGF, we compared global gene expression profiles between cells treated with NGF and EGF for 2-4 h, when sustained ERK signaling in response to NGF is most distinct from the transient signal elicited by EGF. This analysis identified 69 genes that were preferentially up-regulated in response to NGF. As expected, up-regulation of these genes was mediated by sustained ERK signaling. In addition, they were up-regulated in response to other neuritogenic treatments (pituitary adenylate cyclase-activating polypeptide and 12-O-tetradecanoylphorbol-13-acetate plus dbcAMP) and were enriched for genes related to neuronal differentiation/function. Computational analysis and chromatin immunoprecipitation identified binding of CREB and AP-1 family members (Fos, FosB, Fra1, JunB, JunD) upstream of >30 and 50%, respectively, of the preferentially NGF-induced genes. Expression of several AP-1 family members was induced by both EGF and NGF, but their induction was more robust and sustained in response to NGF. The binding of Fos family members to their target genes was similarly sustained in response to NGF and was reduced upon MEK inhibition, suggesting that AP-1 contributes significantly to the NGF transcriptional program. Interestingly, Fra1 as well as two other NGF-induced AP-1 targets (HB-EGF and miR-21) function in positive feedback loops that may contribute to sustained AP-1 activity.

Fra-1 controls motility of bladder cancer cells via transcriptional upregulation of the receptor tyrosine kinase AXL

Fos-related antigen 1 (Fra-1) is a Fos family member overexpressed in several types of human cancers. Here, we report that Fra-1 is highly expressed in the muscle-invasive form of the carcinoma of the bladder (80%) and to a lesser extent in superficial bladder cancer (42%). We demonstrate that in this type of cancer Fra-1 is regulated via a C-terminal instability signal and C-terminal phosphorylation. We show that manipulation of Fra-1 expression levels in bladder cancer cell lines affects cell morphology, motility and proliferation. The gene coding for AXL tyrosine kinase is directly upregulated by Fra-1 in bladder cancer and in other cell lines. Importantly, our data demonstrate that AXL mediates the effect of Fra-1 on tumour cell motility but not on cell proliferation. We suggest that AXL may represent an attractive therapeutic target in cancers expressing high Fra-1 levels.

JunD-mediated repression of GADD45α and γ regulates escape from cell death in prostate cancer

The AP-1 transcription factor complex has been implicated in a variety of biological processes including cell differentiation, proliferation, apoptosis and oncogenic transformation. We previously established that activation of the AP-1 family member JunD contributes to deregulated expression of the anti-apoptotic IL-6 gene in prostate cancer cells. We now show that inhibition of JunD in prostate cancer cells results in GADD45α- and γ-dependent induction of cell death and inhibition of tumor growth that is mediated at least partially via c-Jun N-terminal kinase (JNK) and p38 kinase activation. Apoptosis induction by dominant negative JunD and JNK and p38 kinase activation are impeded upon knock down of GADD45α and γ expression by small interfering RNA, most vividly demonstrating the central role of GADD45α and γ in JunD-mediated escape of prostate cancer cells from programmed cell death.

PMA up-regulates the transcription of Axl by AP-1 transcription factor binding to TRE sequences via the MAPK cascade in leukaemia cells

BACKGROUND

Axl is a receptor tyrosine kinase promoting anti-apoptosis, invasion and mitogenesis, and is highly expressed in different solid cancers. Axl basal transcriptional activity is driven by Sp1/Sp3, and overexpression of MZF-1 (myeloid zinc-finger 1) induces Axl transcription and gene expression. Furthermore, Axl expression is epigenetically controlled by CpG hypermethylation; however, little is known about inducible Axl gene expression and Axl regulation in haematopoetic malignancies.

RESULTS

In the present study, we studied Axl transcriptional regulation under PMA-stimulated conditions in leukaemia cells. Luciferase analysis with sequential 5'-deletion constructs revealed that the -660/-580 region of the Axl promoter is indispensable for induced promoter activity under PMA stimulation. This region includes AP-1 (activator protein 1)/CREB [CRE (cAMP-response-element)-binding protein] motifs, five times partially overlapping TGCGTG repeats and multiple GT repeats. Mutational, supershift and ChIP (chromatin immunoprecipitation) analysis determined that AP-1 family members bind to AP-1 motifs and to the 5 × TGCGTG overlapping repeats, thus transactivating Axl promoter activity. Furthermore, specific inhibitors of PKC (protein kinase C), ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 reduced Axl expression. Additionally, mithramycin treatment abolished constitutive and PMA-induced Axl expression.

CONCLUSIONS

Taken together the results of the present study suggest that PMA-induced Axl gene expression in leukaemia cells is mediated by AP-1 motifs and 5 × TGCGTG repeats within the promoter region -660/-580, and through the PKC/ERK1/2/AP-1 or PKC/p-38/AP-1 signalling axis.

Role of MSK1 in the malignant phenotype of Ras-transformed mouse fibroblasts

Activated by the RAS-MAPK signaling pathway, MSK1 is recruited to immediate-early gene (IEG) regulatory regions, where it phosphorylates histone H3 at Ser-10 or Ser-28. Chromatin remodelers and modifiers are then recruited by 14-3-3 proteins, readers of phosphoserine marks, leading to the occupancy of IEG promoters by the initiation-engaged form of RNA polymerase II and the onset of transcription. In this study, we show that this mechanism of IEG induction, initially elucidated in parental 10T1/2 murine fibroblast cells, applies to metastatic Hras1-transformed Ciras-3 cells. As the RAS-MAPK pathway is constitutively activated in Ciras-3 cells, MSK1 activity and phosphorylated H3 steady-state levels are elevated. We found that steady-state levels of the IEG products AP-1 and COX-2 were also elevated in Ciras-3 cells. When MSK1 activity was inhibited or MSK1 expression was knocked down in Ciras-3 cells, the induction of IEG expression and the steady-state levels of COX-2, FRA-1, and JUN were greatly reduced. Furthermore, MSK1 knockdown Ciras-3 cells lost their malignant phenotype, as reflected by the absence of anchorage-independent growth.

MLK3 is critical for breast cancer cell migration and promotes a malignant phenotype in mammary epithelial cells

The malignant phenotype in breast cancer is driven by aberrant signal transduction pathways. Mixed-lineage kinase-3 (MLK3) is a mammalian mitogen-activated protein kinase kinase kinase (MAP3K) that activates multiple MAPK pathways. Depending on the cellular context, MLK3 has been implicated in apoptosis, proliferation, migration and differentiation. Here we investigated the effect of MLK3 and its signaling to MAPKs in the acquisition of malignancy in breast cancer. We show that MLK3 is highly expressed in breast cancer cells. We provide evidence that MLK3's catalytic activity and signaling to c-jun N-terminal kinase (JNK) is required for migration of highly invasive breast cancer cells and for MLK3-induced migration of mammary epithelial cells. Expression of active MLK3 is sufficient to induce the invasion of mammary epithelial cells, which requires AP-1 activity and is accompanied by the expression of several proteins corresponding to AP-1-regulated invasion genes. To assess MLK3's contribution to the breast cancer malignant phenotype in a more physiological setting, we implemented a strategy to inducibly express active MLK3 in the preformed acini of MCF10A cells grown in 3D Matrigel. Induction of MLK3 expression dramatically increases acinar size and modestly perturbs apicobasal polarity. Remarkably, MLK3 expression induces luminal repopulation and suppresses the expression of the pro-apoptotic protein BimEL, as has been observed in Her2/Neu-expressing acini. Taken together, our data show that MLK3-JNK-AP-1 signaling is critical for breast cancer cell migration and invasion. Our current study uncovers both a proliferative and novel antiapoptotic role for MLK3 in the acquisition of a malignant phenotype in mammary epithelial cells. Thus, MLK3 may be an important therapeutic target for the treatment of invasive breast cancer.

Heterodimerization with Fra-1 cooperates with the ERK pathway to stabilize c-Jun in response to the RAS oncoprotein

Multiple tumorigenic pathways converge on the activating protein-1 (AP-1) family of dimeric transcription complexes by affecting transcription, mRNA decay, posttranslational modifications, as well as stability of its JUN and FOS components. Several mechanisms have been implicated in the phosphorylation- and ubiquitylation-dependent control of c-Jun protein stability. Although its dimer composition has a major role in the regulation of AP-1, little is known about the influence of heterodimerization partners on the half-life of c-Jun. The FOS family member Fra-1 is overexpressed in various tumors and cancer cell lines wherein it controls motility, invasiveness, cell survival and cell division. Oncogene-induced accumulation of Fra-1 results from both increased transcription and phosphorylation-dependent stabilization of the protein. In this report, we describe a novel role of Fra-1 as a posttranslational regulator of c-Jun. By using both constitutively and inducible transformed rat thyroid cell lines, we found that c-Jun is stabilized in response to RAS oncoprotein expression. This stabilization requires the activity of the extracellular signal-related kinase (ERK) pathway, along with c-Jun heterodimerization with Fra-1. In particular, heterodimerization with Fra-1 inhibits c-Jun breakdown by a mechanism dependent on the phosphorylation of the Fra-1 C-terminal domain that positively controls the stability of the protein in response to ERK signaling. Therefore, Fra-1 modulates AP-1 dimer composition by promoting the accumulation of c-Jun in response to oncogenic RAS signaling.

Transactivation and expression patterns of Jun and Fos/AP-1 super-family proteins in human oral cancer

Transcription factor activator protein-1 (AP-1) super-family is known to modulate expression of array of genes during development of many cancers and considered as an important target for modern therapeutics. But the role of AP-1 during development of human oral cancers is still poorly understood. Because oral cancer is one of the most common cancers in India and south-east Asia, we studied the activation and expression pattern of AP-1 family of proteins and mRNA in different stages of oral carcinogenesis. Gel-shift assay, western blotting, immunohistochemistry and northern blotting have been used to assess the binding activity and expression pattern of AP-1 family (c-Jun, JunB, JunD, c-Fos, FosB, Fra-1 and Fra-2) proteins and mRNA transcripts in a total of 100 fresh oral tissue specimens comprising precancer (n = 40), cancer (n = 50) and healthy control (n = 10). Constitutive activation of AP-1 with concomitant upregulated expression of majority of AP-1 family of proteins and mRNA was observed in cancer cases. Interestingly, almost all precancerous cases showed JunD homodimers, whereas c-Fos/JunD was the most prevalent complex found in cancer tissues. The overexpression of EGFR mRNA, p50:p50/NF-kappaB homodimer formation, together with overexpression of pERK and c-Fos proteins in this study suggests an interesting cross talk between AP-1 and NF-kappaB pathways in oral cancers. Thus, this study demonstrates differential expression and activation of AP-1 super-family proteins in relation to severity of lesion and their crucial role in human oral carcinogenesis.

Emerging roles of ATF2 and the dynamic AP1 network in cancer

Cooperation among transcription factors is central for their ability to execute specific transcriptional programmes. The AP1 complex exemplifies a network of transcription factors that function in unison under normal circumstances and during the course of tumour development and progression. This Perspective summarizes our current understanding of the changes in members of the AP1 complex and the role of ATF2 as part of this complex in tumorigenesis.

Fos proteins suppress dextran sulfate sodium-induced colitis through inhibition of NF-kappaB

The Fos family proteins, c-Fos and Fra-1, are components of the dimeric transcription factor AP-1, which is typically composed of Fos and Jun family proteins. We have previously shown that mice lacking c-Fos (Fos(-/-) mice) respond more strongly to LPS injection than do wild-type (wt) controls. We then examined the sensitivity of Fos(-/-) mice to acute inflammatory stress in a dextran sulfate sodium (DSS)-induced colitis model. We found that Fos(-/-) mice exhibited more severe weight loss, bleeding, diarrhea, and colon shortening than did wt mice, in association with higher TNF-alpha production and NF-kappaB activity in colon segments of DSS-treated Fos(-/-) mice. Furthermore, NF-kappaB inhibition suppressed severe DSS-induced colitis in Fos(-/-) mice. In contrast, Fra-1 transgenic (Tg) mice responded poorly to LPS injection, and Fra-1-overexpressing macrophages and fibroblasts showed reduced production of proinflammatory cytokines, NO, and NF-kappaB activity. Remarkably, in the DSS-induced colitis model, Fra-1 Tg mice showed less severe clinical scores of colitis than did wt mice. Consistently, proinflammatory cytokine production and NF-kappaB activity in colon segments of DSS-treated Fra-1 Tg mice were lower than in wt controls. These findings reveal that the absence of c-Fos and overexpression of Fra-1 respectively enhance and suppress the activation of NF-kappaB in DSS-induced inflammatory stress. In this paper, we propose that AP-1 transcription factors containing c-Fos or Fra-1 are negative regulators of NF-kappaB-mediated stress responses.

Ligand-specific sequential regulation of transcription factors for differentiation of MCF-7 cells

Background

Sharing a common ErbB/HER receptor signaling pathway, heregulin (HRG) induces differentiation of MCF-7 human breast cancer cells while epidermal growth factor (EGF) elicits proliferation. Although cell fates resulting from action of the aforementioned ligands completely different, the respective gene expression profiles in early transcription are qualitatively similar, suggesting that gene expression during late transcription, but not early transcription, may reflect ligand specificity. In this study, based on both the data from time-course quantitative real-time PCR on over 2,000 human transcription factors and microarray of all human genes, we identified a series of transcription factors which may control HRG-specific late transcription in MCF-7 cells.

Results

We predicted that four transcription factors including EGR4, FRA-1, FHL2, and DIPA should have responsibility of regulation in MCF-7 cell differentiation. Validation analysis suggested that one member of the activator protein 1 (AP-1) family, FOSL-1 (FRA-1 gene), appeared immediately following c-FOS expression, might be responsible for expression of transcription factor FHL2 through activation of the AP-1 complex. Furthermore, RNAi gene silencing of FOSL-1 and FHL2 resulted in increase of extracellular signal-regulated kinase (ERK) phosphorylation of which duration was sustained by HRG stimulation.

Conclusion

Our analysis indicated that a time-dependent transcriptional regulatory network including c-FOS, FRA-1, and FHL2 is vital in controlling the ERK signaling pathway through a negative feedback loop for MCF-7 cell differentiation.

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

Fos family protein degradation by the proteasome

c-Fos proto-oncoprotein defines a family of closely related transcription factors (Fos proteins) also comprising Fra-1, Fra-2, FosB and ΔFosB, the latter two proteins being generated by alternative splicing. Through the regulation of many genes, most of them still unidentified, they regulate major functions from the cell level up to the whole organism. Thus they are involved in the control of proliferation, differentiation and apoptosis, as well as in the control of responses to stresses, and they play important roles in organogenesis, immune responses and control of cognitive functions, among others. Fos proteins are intrinsically unstable. We have studied how two of them, c-Fos and Fra-1, are degraded. Departing from the classical scenario where unstable key cell regulators are hydrolysed by the proteasome after polyubiquitination, we showed that the bulk of c-Fos and Fra-1 can be broken down independently of any prior ubiquitination. Certain conserved structural domains suggest that similar mechanisms may also apply to Fra-2 and FosB. Computer search indicates that certain motifs shared by the Fos proteins and putatively responsible for instability are found in no other protein, suggesting the existence of degradation mechanisms specific for this protein family. Under particular signalling conditions, others have shown that a part of cytoplasmic c-Fos requires ubiquitination for fast turnover. This poses the question of the multiplicity of degradation pathways that apply to proteins depending on their intracellular localization.

Synergistic activation of human involucrin gene expression by Fra-1 and p300--evidence for the presence of a multiprotein complex

Involucrin is expressed in the differentiated suprabasal epidermal layers, and an AP1 transcription factor-binding site present in the involucrin promoter distal regulatory region is required for this regulation. This site binds Fra-1, but cofactor interaction at this site has not been adequately characterized. We show that Fra-1 and p300 histone acetyltransferase are present at the AP1 site, as detected by chromatin immunoprecipitation. This interaction is functional, as treating p300 expressing keratinocytes with calcium or 12-O-tetradeconylphorbol-13-acetate, results in a synergistic increase in hINV expression, and this enhanced activation can be reproduced by coexpression of Fra-1 and p300. p300 also co-precipitates with Fra-1, but protein fractionation studies suggest that this interaction requires an additional protein. Fra-1 also interacts with other proteins that interact at the AP1-5 site, including JunD, JunB, Sp1, and P/CAF. Contrary to results in some other systems, Fra-1 functions as a positive transcriptional regulator in human keratinocytes. These studies suggest that a large multiprotein complex, which includes Fra-1, p300, P/CAF, junD, junB, and Sp1 acts at the AP1-5 site to produce a synergistic increase in hINV gene expression.

Ubiquitin-independent- versus ubiquitin-dependent proteasomal degradation of the c-Fos and Fra-1 transcription factors: is there a unique answer?

The Fos family of transcription factors comprises c-Fos, Fra-1, Fra-2 and FosB, which are all intrinsically unstable proteins. Fos proteins heterodimerize with a variety of other transcription factors to control genes encoding key cell regulators. Their best known partners are the Jun family proteins (c-Jun, JunB, and JunD). At the cellular level, Fos-involving dimers control proliferation, differentiation, apoptosis and responses to environmental cues. At the organism level, they play paramount parts in organogenesis, immune responses and cognitive functions, among others. fos family genes are subjected to exquisite, complex and intermingled transcriptional and post-transcriptional regulations, which are necessary to avoid pathological effects. In particular, the Fos proteins undergo to numerous post-translational modifications, such as phosphorylations and sumoylation, regulating their transcriptional activity, their subcellular localization and their turnover. The mechanisms whereby c-Fos and Fra-1 are degraded have been studied in detail. Contrasting with the classical scenario, according to which most unstable key cell regulators are hydrolyzed by the proteasome after conjugation of polyubiquitin chains, the bulk of c-Fos and Fra-1 can be hydrolyzed independently of any prior ubiquitylation in different situations. c-Fos and Fra-1 share a common destabilizing domain whose primary sequence is conserved in Fra-2 and FosB, suggesting that similar breakdown mechanisms might be at play in the latter two proteins. However, a database search indicates that this domain is not found in any other protein, suggesting that the mechanisms underlying Fos protein destruction may be specific to this family. Interestingly, under particular conditions, a fraction of cytoplasmic c-Fos is ubiquitylated, leading to faster turnover. This poses the question of the multiplicity of degradation pathways that can target the same substrate depending on its activation state, its protein partnership and/or its intracellular localization. This issue is discussed here together with the, thus far, overlooked roles of the various proteasomal complexes found in all cells.

FRA-1 proto-oncogene induces lung epithelial cell invasion and anchorage-independent growth in vitro, but is insufficient to promote tumor growth in vivo

FRA-1 forms activator protein-1 complexes in association with members of the JUN family and drives gene transcription. FRA-1 has been implicated in the development of airway squamous metaplasia and is frequently overexpressed in squamous cell carcinomas of the esophagus and stomach. We and others have shown a high level of persistent induction of FRA-1 by lung carcinogens, such as cigarette smoke and asbestos, in pulmonary epithelial cells. However, the exact roles of FRA-1 in regulating lung epithelial cell growth and invasion are poorly understood. To examine this aspect, we have stably overexpressed FRA-1 in human type-II-like alveolar malignant cell line (A549) and a nonmalignant bronchial epithelial cell line (BEAS-2B). FRA-1 greatly enhanced the rate of proliferation, motility, and invasion of A549 and BEAS-2B cells. In athymic nude mice, FRA-1, but not the control vector, rapidly enhanced tumor formation and metastasis by A549 cells. In contrast, FRA-1 failed to promote tumor formation by BEAS-2B. We suggest that FRA-1 can promote motility, invasion, and anchorage-independent growth of lung epithelial cells in vitro, but is insufficient for tumor formation.

PIM1-dependent phosphorylation of histone H3 at serine 10 is required for MYC-dependent transcriptional activation and oncogenic transformation

The serine/threonine kinase human Pim1 (hereafter PIM1) cooperates with human c-Myc (hereafter MYC) in cell cycle progression and tumorigenesis. However, the nature of this cooperation is still unknown. Here we show that, after stimulation with growth factor, PIM1 forms a complex with the dimer of MYC with MAX (Myc-associated factor X) via the MYC BoxII (MBII) domain. MYC recruits PIM1 to the E boxes of the MYC-target genes FOSL1 (FRA-1) and ID2, and PIM1 phosphorylates serine 10 of histone H3 (H3S10) on the nucleosome at the MYC-binding sites, contributing to their transcriptional activation. MYC and PIM1 colocalize at sites of active transcription, and expression profile analysis revealed that PIM1 contributes to the regulation of 20% of the MYC-regulated genes. Moreover, PIM1-dependent H3S10 phosphorylation contributes to MYC transforming capacity. These results establish a new function for PIM1 as a MYC cofactor that phosphorylates the chromatin at MYC-target loci and suggest that nucleosome phosphorylation, at E boxes, contributes to MYC-dependent transcriptional activation and cellular transformation.

Role of heterodimerization of c-Fos and Fra1 proteins in osteoclast differentiation

Bone resorbing osteoclasts are specialized macrophages that cannot differentiate in the absence of c-Fos, a member of the dimeric transcription factor AP-1 (activator protein-1). However, osteoclast differentiation in the absence of c-Fos can be rescued in vitro and in vivo by Fra1, a Fos-like protein and transcriptional target of c-Fos. To enable AP-1 proteins binding to DNA, c-Fos or Fra1 must heterodimerize with a partner such as c-Jun, JunB and JunD. In this study, we investigated the dimerization partners of c-Fos and Fra1 required for osteoclast differentiation using synthetic "single-chain" AP-1 dimers in which c-Fos or Fra1 is tethered via a linker to Jun proteins. When c-Fos was analyzed in combination with any Jun protein, including a c-Jun mutant lacking major phosphorylation sites for c-Jun amino-terminal kinase (JNK), osteoclasts were efficiently formed from c-Fos-deficient hematopoietic precursors. However, Fra1 in combination with any Jun protein could not rescue osteoclastogenesis. The ability to rescue was compared to transcriptional activity measured in transient transfection assays using promoters driven by consensus AP-1 sites or a composite AP-1/NFAT binding site. These data show that a single Jun/c-Fos dimer is sufficient for osteoclast differentiation, likely due to its transactivation ability for a broader range of promoters, in particular consensus AP-1 sites. We propose that Fra1 together with a dimerization partner different from Jun proteins can rescue osteoclast differentiation in c-Fos-deficient precursors.

The duration of ERK1/2 activity determines the activation of c-Fos and Fra-1 and the composition and quantitative transcriptional output of AP-1

The duration of ERK1/2 activation influences the nature of the biological response to agonist. Members of the AP-1 transcription factor family are well known targets of the ERK1/2 pathway and are expressed in a temporally coordinated fashion during cell cycle re-entry. In CCl39 fibroblasts, sustained ERK1/2 activation is required for the expression of Fra-1, Fra-2, c-Jun and JunB, whereas expression of c-Fos is still strongly induced even in response to transient ERK activation. However, the significance of this pattern of expression for AP-1 activity has not been addressed. Here we show that growth factor stimulated activation of the C-terminal c-Fos transactivation domain (TAD) serves as a sensor for ERK1/2 signal duration whereas the c-JunTAD is not responsive to growth factors. In addition, sustained ERK1/2 activation determines the duration of increases in AP-1 DNA binding complexes as well as their qualitative make up. Finally, this is reflected in both the duration and quantitative transcriptional output of stably integrated AP-1 reporter constructs, indicating that AP-1 activity is finely tuned to ERK1/2 signal duration. These results provide new insights into the importance of ERK1/2 signal duration in the regulation of AP-1 and provide an explanation for how differences in signal duration can lead to both quantitative and qualitative changes in gene expression.

Ubiquitin-independent proteasomal degradation of Fra-1 is antagonized by Erk1/2 pathway-mediated phosphorylation of a unique C-terminal destabilizer

Fra-1, a transcription factor that is phylogenetically and functionally related to the proto-oncoprotein c-Fos, controls many essential cell functions. It is expressed in many cell types, albeit with differing kinetics and abundances. In cells reentering the cell cycle, Fra-1 expression is transiently stimulated albeit later than that of c-Fos and for a longer time. Moreover, Fra-1 overexpression is found in cancer cells displaying high Erk1/2 activity and has been linked to tumorigenesis. One crucial point of regulation of Fra-1 levels is controlled protein degradation, the mechanism of which remains poorly characterized. Here, we have combined genetic, pharmacological, and signaling studies to investigate this process in nontransformed cells and to elucidate how it is altered in cancer cells. We report that the intrinsic instability of Fra-1 depends on a single destabilizer contained within the C-terminal 30 to 40 amino acids. Two serines therein, S252 and S265, are phosphorylated by kinases of the Erk1/2 pathway, which compromises protein destruction upon both normal physiological induction and tumorigenic constitutive activation of this cascade. Our data also indicate that Fra-1, like c-Fos, belongs to a small group of proteins that may, under certain circumstances, undergo ubiquitin-independent degradation by the proteasome. Our work reveals both similitudes and differences between Fra-1 and c-Fos degradation mechanisms. In particular, the presence of a single destabilizer within Fra-1, instead of two that are differentially regulated in c-Fos, explains the much faster turnover of the latter when cells traverse the G(0)/G(1)-to-S-phase transition. Finally, our study offers further insights into the signaling-regulated expression of the other Fos family proteins.

Characterization of E-cadherin-dependent and -independent events in a new model of c-Fos-mediated epithelial–mesenchymal transition

Fos proteins have been implicated in control of tumorigenesis-related genetic programs including invasion, angiogenesis, cell proliferation and apoptosis. In this study, we demonstrate that c-Fos is able to induce mesenchymal transition in murine tumorigenic epithelial cell lines. Expression of c-Fos in MT1TC1 cells led to prominent alterations in cell morphology, increased expression of mesenchymal markers, vimentin and S100A4, DNA methylation-dependent down-regulation of E-cadherin and abrogation of cell-cell adhesion. In addition, c-Fos induced a strong beta-catenin-independent proliferative response in MT1TC1 cells and stimulated cell motility, invasion and adhesion to different extracellular matrix proteins. To explore whether loss of E-cadherin plays a role in c-Fos-mediated mesenchymal transition, we expressed wild-type E-cadherin and two different E-cadherin mutants in MT1TC1/c-fos cells. Expression of wild-type E-cadherin restored epithelioid morphology and enhanced cellular levels of catenins. However, exogenous E-cadherin did not influence expression of c-Fos-dependent genes, only partly suppressed growth of MT1TC1/c-fos cells and produced no effect on c-Fos-stimulated cell motility and invasion in matrigel. On the other hand, re-expression of E-cadherin specifically negated c-Fos-induced adhesion to collagen type I, but not to laminin or fibronectin. Of interest, mutant E-cadherin which lacks the ability to form functional adhesive complexes had an opposite, potentiating effect on cell adhesion to collagen I. These data suggest that cell adhesion to collagen I is regulated by the functional state of E-cadherin. Overall, our data demonstrate that, with the exception of adhesion to collagen I, c-Fos is dominant over E-cadherin in relation to the aspects of mesenchymal transition assayed in this study.

An association of a simultaneous nuclear and cytoplasmic localization of Fra-1 with breast malignancy

Background

Overexpression of Fra-1 in fibroblasts causes anchorage-independent cell growth and oncogenic transformation. A high level of Fra-1 expression is found in various tumors and tumorigenic cell lines, suggesting that Fra-1 may be involved in malignant progression. This study aimed to investigate the significance of Fra-1 expression in breast carcinogenesis.

Methods

The expression of Fra-1 was investigated by immunohistochemistry in neoplastic breast diseases ranging from benign fibroadenoma to very aggressive undifferentiated carcinoma. The correlations of Fra-1 expression with other indicators of breast carcinoma prognosis (ER, PR and ErbB2 receptors) were analyzed.

Results

All neoplastic breast tissues, either benign or malignant breast tissues, were nuclear immunoreactive for Fra-1-recognizing antibody. The pattern of Fra-1 expression by benign neoplastic cells was predominantly nuclear. However, the nuclear/cytoplasmic concomitant immunoreactivity was observed in all types of breast carcinomas. A clear shift in Fra-1 immunoreactivity, from an exclusively nuclear to a simultaneous nuclear and cytoplasmic localization was noticed in ~90% of breast carcinomas.

Conclusion

The overall expression, pattern and intensity of Fra-1 proteins were correlated with breast oncogenesis. Overexpression of Fra-1, leading to a persistent high cytoplasmic accumulation, may play a role in the process of breast carcinogenesis.