Degradation of cellular and viral Fos proteins

Progress in the Study of Fra-2 in Respiratory Diseases

Fos-related antigen-2 (Fra-2) is a member of the activating protein-1 (AP-1) family of transcription factors. It is involved in controlling cell growth and differentiation by regulating the production of the extracellular matrix (ECM) and coordinating the balance of signals within and outside the cell. Fra-2 is not only closely related to bone development, metabolism, and immune system and eye development but also in the progression of respiratory conditions like lung tumors, asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease (COPD). The increased expression and activation of Fra-2 in various lung diseases has been shown in several studies. However, the specific molecular mechanisms through which Fra-2 affects the development of respiratory diseases are not yet understood. The purpose of this research is to summarize and delineate advancements in the study of the involvement of transcription factor Fra-2 in disorders related to the respiratory system.

Transcription factor Fra-2 and its emerging role in matrix deposition, proliferation and inflammation in chronic lung diseases

Fos-related antigen-2 (Fra-2) belongs to the activator protein 1 (AP-1) family of transcription factors and is involved in a broad variety of cellular processes, such as proliferation or differentiation. Aberrant expression of Fra-2 or regulation can lead to severe growth defects or diverse pathologies. Elevated Fra-2 expression has been described in several chronic lung diseases, such as pulmonary fibrosis, chronic obstructive pulmonary disease and asthma. However, the pathomechanisms behind the Fra-2-induced pulmonary remodelling are still not fully elucidated. Fra-2 overexpressing mice were initially described as a model of systemic sclerosis associated organ fibrosis, with predominant alterations in the lung. High levels of Fra-2 expression give rise to profound inflammation with severe remodelling of the parenchyma and the vasculature, resulting in fibrosis and pulmonary hypertension, respectively, but also alters bronchial function. In this review we discuss the central role of Fra-2 connecting inflammation, cellular proliferation and extracellular matrix deposition underlying chronic lung diseases and what we can learn for future therapeutic options.

miR-101 inhibits the G1-to-S phase transition of cervical cancer cells by targeting Fos

OBJECTIVES

The chief objective of this study was to identify the miRNAs targeting Fos, a well-recognized proto-oncogene that is commonly overexpressed in cervical cancer, and its biological significance on the cellular behaviors of HeLa, a cervical cancer cell.

MATERIALS AND METHODS

We initially analyzed the 3'untranslated region (3'UTR) of Fos and screened the potential miRNAs targeting Fos using 3 bioinformatical Web sites. Luciferase reporter assay, real-time polymerase chain reaction, and Western blotting were used to validate the binding of chosen miRNA (miR-101) on the 3'UTR of Fos and the downstream regulation on its mRNA and protein levels. Furthermore, flow cytometry along with the Fos rescue strategy was applied to analyze the modulation of cell cycle of HeLa cells by miR-101.

RESULTS

Among these predicted candidate miRNAs, miR-101 was the miRNAs preferred by all the 3 used Web sites. The results of luciferase reporter assay, real-time polymerase chain reaction, and Western blotting demonstrated that miR-101 directly targeted on the 3'UTR of Fos and down-regulated the expression of Fos at mRNA and protein levels. Furthermore, cell cycle analysis showed that miR-101 arrests G1-to-S phase transition of HeLa cells, at least partially by targeting Fos.

CONCLUSIONS

We concluded that by targeting the proto-oncogene Fos, miR-101 is involved in G1-to-S phase transition in cervical cancer cells in vitro and might provide a new approach for the pharmacological interference node in cervical cancer treatment.

Antagonistic regulation of cell-matrix adhesion by FosB and DeltaFosB/Delta2DeltaFosB encoded by alternatively spliced forms of fosB transcripts

Among fos family genes encoding components of activator protein-1 complex, only the fosB gene produces two forms of mature transcripts, namely fosB and DeltafosB mRNAs, by alternative splicing of an exonic intron. The former encodes full-length FosB. The latter encodes DeltaFosB and Delta2DeltaFosB by alternative translation initiation, and both of these lack the C-terminal transactivation domain of FosB. We established two mutant mouse embryonic stem (ES) cell lines carrying homozygous fosB-null alleles and fosB(d) alleles, the latter exclusively encoding DeltaFosB/Delta2DeltaFosB. Comparison of their gene expression profiles with that of the wild type revealed that more than 200 genes were up-regulated, whereas 19 genes were down-regulated in a DeltaFosB/Delta2DeltaFosB-dependent manner. We furthermore found that mRNAs for basement membrane proteins were significantly up-regulated in fosB(d/d) but not fosB-null mutant cells, whereas genes involved in the TGF-beta1 signaling pathway were up-regulated in both mutants. Cell-matrix adhesion was remarkably augmented in fosB(d/d) ES cells and to some extent in fosB-null cells. By analyzing ES cell lines carrying homozygous fosB(FN) alleles, which exclusively encode FosB, we confirmed that FosB negatively regulates cell-matrix adhesion and the TGF-beta1 signaling pathway. We thus concluded that FosB and DeltaFosB/Delta2DeltaFosB use this pathway to antagonistically regulate cell matrix adhesion.

Role of Fra-2 in breast cancer: influence on tumor cell invasion and motility

Fra-2 (Fos-related antigen 2) is a member of the Fos family of AP-1 transcription factors which is often up-regulated in mammary carcinomas. Previous results suggested that it might be involved in the regulation of breast cancer invasion and metastasis. In order to analyze the role of Fra-2 in breast cancer cells, it was silenced in the highly invasive MDA-MB231 cells using RNA interference. On the other hand, stable transfectants of the weakly invasive MCF7 cell line were established in order to analyze the effects of Fra-2 overexpression. In both approaches, cell proliferation was not or only weakly influenced by Fra-2. In contrast, the invasive potential of the cells was increased, and a weaker effect on motility was observed. By cDNA microarray analysis of the MCF7 transfectants followed by validation on a protein level, we identified several Fra-2 target genes which might be involved in cell invasion and migration, i.e., ALCAM and connexin 43. Additionally, mRNA expression levels of various genes which are associated with a more malignant behavior of the tumors in vivo were up- or downregulated, i.e., members of the MAGE family, S100P, TIMP2, IL24 etc. These results show that Fra-2 overexpression is associated with a more aggressive tumor phenotype and is probably involved in breast cancer progression in vivo.

Cytoplasmic-to-nuclear volume ratio affects AP-1 complex formation as an indicator of cell cycle responsiveness

Cytoplasmic volume undergoes a series of changes during mitosis in eukaryotes; in turn, signaling events such as osmotic stress can regulate the cytoplasmic volume in cells. In some organisms, increase in cytoplasmic-to-nuclear volume ratio was seen to affect the growth potential in cells, however, the mechanistics of such a regulation, if at all present, was unclear. In a computational model, we have constructed a growth factor-induced signaling pathway leading to AP-1 heterodimer formation through transcriptional regulation, and analyzed the effects of increasing the cytoplasmic-to-nuclear ratio on c-jun transcription and AP-1 complex. We have observed that larger cytoplasmic volumes caused both an increase in the final AP-1 product and a delay in the time of AP-1 accumulation.

Loss of Net as Repressor Leads to Constitutive Increased c-fos Transcription in Cervical Cancer Cells

We have investigated the expression of c-fos in cervical carcinoma cells and in somatic cell hybrids derived therefrom. In malignant cells, c-fos was constitutively expressed even after serum starvation. Dissection of the c-fos promoter showed that expression was mainly controlled by the SRE motif, which was active in malignant cells, but repressed in their non-malignant counterparts. Constitutive SRE activity was not mediated by sustained mitogen-activated protein kinase activity but because of inefficient expression of the ternary complex factor Net, which was either very low or even barely discernible. Chromatin immunoprecipitation assays revealed that Net directly binds to the SRE nucleoprotein complex in non-tumorigenic cells, but not in malignant segregants. Small interfering RNA targeted against Net resulted in enhanced c-fos transcription, clearly illustrating its repressor function. Conversely, stable ectopic expression of Net in malignant cells negatively regulated endogenous c-fos, resulting in a disappearance of the c-Fos protein from the AP-1 transcription complex. These data indicate that loss of Net and constitutive c-fos expression appear to be a key event in the transformation of cervical cancer cells.

MEK1-dependent delayed expression of Fos-related antigen-1 counteracts c-Fos and p65 NF-kappaB-mediated interleukin-8 transcription in response to cytokines or growth factors

Binding sites for the dimeric transcription factor activator protein (AP)-1 are found in numerous immunoregulatory and inflammatory genes. The precise mechanisms by which AP-1 activates or represses immune response genes and in particular the roles of individual AP-1 subunits in inflammatory responses are largely unknown. We report here that c-Fos and Fos-related antigen-1 (Fra-1), two inducible components of AP-1, are recruited to the endogenous interleukin (IL)-8 promoter in an IL-1-dependent manner. c-Fos activates IL-8 transcription and synergizes in this effect with p65 NF-kappaB. In contrast, Fra-1 strongly inhibits inducible IL-8 transcription. Fra-1 activation involves its stabilization, ubiquitination, and interaction with histone deacetylase-1. Blockade of MEK1 by PD98059 suppresses c-Fos and Fra-1 expression and, thus, affects two counteractive signals for IL-8 mRNA synthesis simultaneously. This disturbs the inducible recruitment of TATA box-binding protein and RNA polymerase II to the IL-8 promoter. Additional experiments reveal that, in conjunction with p65 NF-kappaB, the MEK1-ERK-dependent synthesis of c-Fos and Fra-1 serves to adjust the overall expression level of IL-8 in response to two of its physiological inducers, IL-1 and epidermal growth factor. Relative to c-Fos, the delayed recruitment of Fra-1 to the IL-8 promoter provides an example how AP-1 subunits may dampen excessive chemokine synthesis.

MEKK1 regulates the AP-1 dimer repertoire via control of JunB transcription and Fra-2 protein stability

Activator protein 1 (AP-1) transcription factor dimers are composed of Jun, Fos, and ATF member proteins, but the mechanisms that determine AP-1 composition are not clearly defined and the function of specific dimers is not well understood. MEKK1 is a mitogen-activated protein kinase (MAPK) kinase kinase and an ubiquitin ligase that regulates both the extracellular signal-regulated kinase 1/2 and the c-Jun amino-terminal kinase. Herein, we demonstrate that MEKK1 regulates the AP-1 protein repertoire. Both FGF-2 and phorbol ester-inducible urokinase-type plasminogen activator (uPA) expression requires AP-1 binding to an enhancer element in the uPA promoter, and we have previously shown that FGF-2 or PMA induction of uPA expression is strongly dependent on MEKK1. JunB mRNA is significantly increased in MEKK1-/- cells, demonstrating that MEKK1 suppresses JunB mRNA expression. Upregulation of JunB expression in MEKK1-/- cells forms an inhibitory AP-1 complex that binds to the uPA promoter and inhibits uPA transcription. MEKK1 also regulates Fra-2 protein stability by inducing Fra-2 ubiquitination and degradation. MEKK1 regulates AP-1-dependent gene expression by regulating the expression, activity and degradation of component members of the AP-1 complex. Controlling the repertoire of a transcription factor complex is a newly defined function for an MAPK kinase kinase.

Biochemical identification of proteasome-associated endonuclease activity in sunflower

Proteasomes have been purified from sunflower hypocotyles. They elute with a molecular mass of 600 kDa from gel filtration columns and two-dimensional gel electrophoresis indicates that the complex contains at least 20 different protein subunits. Peptide microsequencing revealed the presence of four subunits homologous to subunits Beta2, Beta6, Alpha5 and Alpha6 of plant proteasomes. These proteasomes have chymotrypsin-like activity and the highly purified fraction of this complex is associated with an endonuclease activity hydrolyzing Tobacco mosaic virus RNA and Lettuce mosaic virus RNA with a cleavage pattern showing fragments of well-defined size. This is the first evidence of a RNA endonuclease activity associated with plant proteasomes.