Characterization of a delayed early serum response region

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.

Angiogenesis induced by signal transducer and activator of transcription 5A (STAT5A) is dependent on autocrine activity of proliferin

Multiple secreted factors induce the formation of new blood vessels (angiogenesis). The signal transduction events that orchestrate the numerous cellular activities required for angiogenesis remain incompletely understood. We have shown previously that STAT5 plays a pivotal role in angiogenesis induced by FGF2 and FGF8b. To delineate the signaling pathway downstream of STAT5, we expressed constitutively active (CA) or dominant-negative (DN) mutant STAT5A in mouse brain endothelial cells (EC). We found that the conditioned medium from CA-STAT5A but not from dominant-negative STAT5A overexpressing EC is sufficient to induce EC invasion and tube formation, indicating that STAT5A regulates the secretion of autocrine proangiogenic factors. Conversely, CA-STAT5A-induced conditioned medium had no effect on EC proliferation. Using a comparative genome-wide transcription array screen, we identified the prolactin family member proliferin (PLF1 and PLF4) as a candidate autocrine factor. The CA-STAT5A-dependent transcription and secretion of PLF by EC was confirmed by quantitative RT-PCR and Western blotting, respectively. CA-STAT5A binds to the PLF1 promoter region, suggesting a direct transcriptional regulation. Knockdown of PLF expression by shRNA or by blocking of PLF activity with neutralizing antibodies removed the CA-STAT5A-dependent proangiogenic activity from the conditioned medium of EC. Similarly, the ability of concentrated conditioned medium from CA-STAT5A transfected EC to induce angiogenesis in Matrigel plugs in vivo was abolished when PLF was depleted from the medium. These observations demonstrate a FGF/STAT5/PLF signaling cascade in EC and implicate PLF as autocrine regulator of EC invasion and tube formation.

Reactivation of proliferin gene expression is associated with increased angiogenesis in a cell culture model of fibrosarcoma tumor progression

Proliferin (PLF) is an angiogenic placental hormone. We now report that PLF gene expression can also occur in a progressive fibrosarcoma mouse tumor cell model. PLF mRNA and protein are detectable at very low levels in cell lines derived from the mild noninvasive stage of tumor development. Expression is greatly augmented in cell lines from the aggressively invasive stage of development, a stage at which the tumor becomes highly angiogenic, and PLF expression remains high in cell lines from the end stage of fibrosarcoma. Activator protein 1 factors present at high levels in the more invasive stages of the tumor may in part allow for increased PLF expression, as cells from the mild stage in which c-jun and junB are stably expressed secrete levels of PLF comparable to that of the advanced stages. Secreted PLF protein is functionally important in tumor cell angiogenic activity, as demonstrated by the reduction of angiogenic activity in fibrosarcoma cell culture medium by immunodepletion of PLF. These results suggest that an extraembryonic genetic program, which has evolved to support fetal growth, may be reactivated in certain tumors and contribute to tumor growth.

JunB is essential for mammalian placentation

Lack of JunB, an immediate early gene product and member of the AP-1 transcription factor family causes embryonic lethality between E8.5 and E10.0. Although mutant embryos are severely retarded in growth and development, cellular proliferation is apparently not impaired. Retardation and embryonic death are caused by the inability of JunB-deficient embryos to establish proper vascular interactions with the maternal circulation due to multiple defects in extra-embryonic tissues. The onset of the phenotypic defects correlates well with high expression of junB in wild-type extra-embryonic tissues. In trophoblasts, the lack of JunB causes a deregulation of proliferin, matrix metalloproteinase-9 (MMP-9) and urokinase plasminogen activator (uPA) gene expression, resulting in a defective neovascularization of the decidua. As a result of downregulation of the VEGF-receptor 1 (flt-1), blood vessels in the yolk sac mesoderm appeared dilated. Mutant embryos which escape these initial defects finally die from a non-vascularized placental labyrinth. Injection of junB-/- embryonic stem (ES) cells into tetraploid wild-type blastocysts resulted in a partial rescue, in which the ES cell-derived fetuses were no longer growth retarded and displayed a normal placental labyrinth. Therefore, JunB appears to be involved in multiple signaling pathways regulating genes involved in the establishment of a proper feto-maternal circulatory system.

A compilation of composite regulatory elements affecting gene transcription in vertebrates

Over the past years, evidence has been accumulating for a fundamental role of protein-protein interactions between transcription factors in gene-specific transcription regulation. Many of these interactions run within composite elements containing binding sites for several factors. We have selected 101 composite regulatory elements identified experimentally in the regulatory regions of 64 genes of vertebrates and of their viruses and briefly described them in a compilation. Of these, 82 composite elements are of the synergistic type and 19 of the antagonistic type. Within the synergistic type composite elements, transcription factors bind to the corresponding sites simultaneously, thus cooperatively activating transcription. The factors, binding to their target sites within antagonistic type composite elements, produce opposing effects on transcription. The nucleotide sequence and localization in the genes, the names and brief description of transcription factors, are provided for each composite element, including a representation of experimental data on its functioning. Most of the composite elements (3/4) fall between -250 bp and the transcription start site. The distance between the binding sites within the composite elements described varies from complete overlapping to 80 bp. The compilation of composite elements is presented in the database COMPEL which is electronically accessible by anonymous ftp via internet.