VEGF-A mRNA processing, stability and translation: a paradigm for intricate regulation of gene expression at the post-transcriptional level

Abstract

Vascular Endothelial Growth Factor A (VEGF-A) is a potent secreted mitogen crucial for physiological and pathological angiogenesis. Post-transcriptional regulation of VEGF-A occurs at multiple levels. Firstly, alternative splicing gives rise to different transcript variants encoding diverse isoforms that exhibit distinct biological properties with regard to receptor binding and extra-cellular localization. Secondly, VEGF-A mRNA stability is regulated by effectors such as hypoxia or growth factors through the binding of stabilizing and destabilizing proteins at AU-rich elements located in the 3'-untranslated region. Thirdly, translation of VEGF-A mRNA is a controlled process involving alternative initiation codons, internal ribosome entry sites (IRESs), an upstream open reading frame (uORF), miRNA targeting and a riboswitch in the 3' untranslated region. These different levels of regulation cooperate for the crucial fine-tuning of the expression of VEGF-A variants. This review will be focused on our current knowledge of the complex post-transcriptional regulatory switches that modulate the cellular VEGF-A level, a paradigmatic model of post-transcriptional regulation.

Figure 1.

Human VEGFA gene structure and exon composition of the isoforms generated by alternative splicing. VEGFA gene spans 16 272 bp of chromosome 6p12 and consists of eight exons and seven introns. The two major transcription start sites, the two translation start sites (AUG and CUG) in the first exon and the two alternative stop codons in exon 8 are indicated. All currently described isoforms (named according to the amino acid number of the human synthesized protein) contain exons 1–5 and two different exons 8. The selection of the terminal exon splice site results in two isoform families, the pro-angiogenic VEGF-Axxx family and the antiangiogenic VEGF-Axxxb family. Exons 6 and 7 encode heparin-binding domains, responsible for the diffusibility and extra cellular matrix affinity of the alternative spliced isoforms.

Figure 2.

Human VEGF-A mRNAs and their major regulatory elements. mRNAs transcribed from two alternative promoters are schematized. Regulatory elements in the 5′ or 3′ untranslated regions and the coding region are indicated: the position of each regulatory element is numbered according to the VEGF-A189 5′ end mRNA sequence. - Two major alternative initiation codons (AUG and the non-canonic CUG) - IRES-B: Internal Ribosome Entry Site-B driving the expression of the CUG-initiated isoforms - IRES-A: Internal Ribosome Entry Site-A driving the expression of the AUG-initiated forms - uORF: upstream open reading frame, located in the IRES-A - Alternative spliced exons (see also Figure 1 for details) - PolyA signal: two alternative polyadenylation sites - AREs: AU-Rich Elements (consensus sequence AUUUA) - HSR: Hypoxia Stability Region - Riboswitch element: sequence involved in the stress responsive riboswitch driven by exclusive interaction with either hnRNPL or the IFN-γ-activated inhibitor of the GAIT translation complex. Beneath the mRNA, the two protein isoforms, L-VEGF-As and VEGF-As, are represented with their maturation products and localization.

Figure 3.

miRNA-targeting sites in the human VEGF-A mRNA 3′UTR. Position of each miRNA target site is represented and numbered according to the VEGF-A189 5′ end mRNA sequence.