Alternative splicing of vascular endothelial growth factor (VEGF) mRNA results in three distinct molecular forms of 121 or 165 (V165) amino acids that are released in the conditioned medium of cultured cells and one longer isoform of 189 amino acids (V189) that remains cell-associated. V189 has been expressed in wild type CHO-K1 cells and in glycosaminoglycan-deficient pgsA-745 Chinese hamster ovary (CHO) mutant cells. It could be released from CHO-K1 cell membranes by heparin or a synthetic peptide designed on the sequence encoded by exon 6 but was freely released from CHO mutant cells. In both cases, the immunoreactive V189 was mainly released as a 40-kDa cleaved form, provided that the serine protease urokinase, but not plasmin, was active. Recombinant V189 was purified from insect cells infected with a recombinant baculovirus as a nonmitogenic 50-kDa precursor that binds to the receptor Flt-1 but not to Flk-1. It could be matured by urokinase as a 38-kDa fragment able to bind to Flk-1 and to trigger cell proliferation. V165 and V189, however, could be cleaved by plasmin as 34-kDa fragments that exhibit a decreased mitogenic activity. These findings indicate that the carboxyl-terminal domain of V189 masks its binding domain to Flk-1.
The 484-nucleotide (nt) alternatively translated region (ATR) of the human fibroblast growth factor 2 (FGF-2) mRNA contains four CUG and one AUG translation initiation codons. Although the 5-end proximal CUG codon is initiated by a cap-dependent translation process, the other four initiation codons are initiated by a mechanism of internal entry of ribosomes. We undertook here a detailed analysis of the cis-acting elements defining the FGF-2 internal ribosome entry site (IRES). A thorough deletion analysis study within the 5-ATR led us to define a 176-nt region as being necessary and sufficient for IRES function at four codons present in a downstream 308-nt RNA segment. Unexpectedly, a single IRES module is therefore responsible for translation initiation at four distantly localized codons. The determination of the FGF-2 5-ATR RNA secondary structure by enzymatic and chemical probing experiments showed that the FGF-2 IRES contained two stem-loop regions and a G quartet motif that constitute novel structural determinants of IRES function. The human FGF-21 gene belongs to a gene family of 23 fibroblast growth factors and is involved in various fundamental cellular processes, such as cell proliferation, differentiation, and angiogenesis (1). The contribution of translational control in FGF-2 gene expression has been particularly well documented. First, a process of alternative initiation of translation occurs on the unique FGF-2 mRNA and leads to the production of five FGF-2 protein isoforms with extended NH 2 -terminal ends (2-4). Second, translation of four of these isoforms is initiated at non-canonical CUG codons (2-4). Third, the use of alternative initiation codons is controlled in transformed and stressed cells (5). Fourth, protein synthesis on the FGF-2 mRNA can occur by internal ribosome entry (6).Whereas the usual mechanism of translation in eukaryotes involves the recruitment of the 40 S ribosomal subunit to the 5Ј-cap structure of the mRNA, a restricted but growing number of viral and cellular mRNA initiate their translation through the recruitment of the 40 S ribosomal subunit to internal sequences of the mRNA called IRES (7-9). As regards the FGF-2 mRNA, this process is active in vivo in transgenic mice (10). Interestingly, the FGF-2 IRES is able to contribute to the choice of initiation codons because translation at the cap-proximal CUG codon occurs by a cap-dependent process, whereas translation at the other four codons occurs by an IRES-based mechanism (4). To understand translation initiation codon selection in the FGF-2 mRNA, it is therefore important to define the cis-elements required for FGF-2 IRES function.The RNA sequences and structural features of cellular IRESs remain largely unknown. Various attempts to define the cis-acting elements required for the function of cellular IRES have failed to find a common RNA sequence. Cellular IRES seem to be very diverse in nature, without stringent sequence similarity. Because IRES elements in viral RNA genomes contain higher order structures whose inte...
Estradiol is known to exert a protective effect against the development of atherosclerosis, but the mechanism by which this protection is mediated is unclear. Since animal studies strongly suggest that production of endothelium-derived relaxing factor is enhanced by estradiol, we The incidence of cardiovascular disease, the leading cause of mortality in western societies, is higher in men than in premenopausal women but increases in postmenopausal women. An abundance of epidemiological data supports a role for estrogens in this atheroprotective effect, prompting recommendations for their widespread use in postmenopausal replacement therapy (1, 2). However, the mechanism whereby this protection is mediated remains obscure. It is traditionally thought to be due to potentially favorable changes in blood lipids and lipoproteins (1), but a number of human (3) as well as animal studies strongly suggest a direct effect on the vascular system (4-6) and more specifically that basal endotheliumderived relaxing factor is enhanced in estradiol-treated females compared with oophorectomized controls (7-11).The endothelium-derived relaxing factor has been identified as nitric oxide (NO) or a closely related compound derived from the amino acid L-arginine, able to induce stimulation of the soluble guanylate cyclase enzyme contained in vascular smooth muscle cells (12)(13)(14) Because the molecular mechanisms of the endotheliumderived relaxing factor-enhanced activity observed in estradiol-treated animals has not yet been precisely elucidated, we sought to determine the effects of estradiol on NO production and simultaneously on NOS mRNA, protein, and activity in a well-characterized culture system of endothelial cells. MATERIALS AND METHODSCell Culture and Materials. Bovine aortic endothelial cells (BAEC) were obtained and grown as described (25,26)
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