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Title: The role of two different isoforms of VEGF-A in human tumour angiogenesis, development, and prognosis
Authors: Giacca, Mauro
Zacchigna, Serena
Kazemi, Maryam
Carrer, Alessandro
Keywords: VEGF
alternative splicing
colorectal cancer
Issue Date: 24-Apr-2012
Publisher: Universit√† degli studi di Trieste
Abstract: To grow beyond 1-2 mm in size, a tumour needs to establish an independent blood supply to sustain an ever-increasing demand for oxygen and nutrients. This requirement is fulfilled through the production of several pro-angiogenic growth factors, eventually leading to accelerated neo-vessel formation. While numerous pro-angiogenic factors have been characterized so far, Vascular Endothelial Growth Factor A, (VEGF-A) has been identified as a predominant regulator of tumour angiogenesis by facilitating endothelial cell (EC) migration, proliferation, tube formation, and survival. VEGF-A gene structure and function have been extensively elucidated over last decades. Notwithstanding, due to conflicting results both in experimental and clinical settings, VEGF-A contribution to tumour progression is still debated. Some controversies arise due to the fact that most of the studies have underestimated the impact of VEGF-A gene alternative splicing, which eventually gives rise to several splicing isoforms. In keeping with this possibility, we previously demonstrated that the two most abundant VEGF-A splicing isoforms (VEGF165 and VEGF121) have different biological effects in vivo. In particular, we showed that both isoforms are equally able to activate the local endothelium, eventually leading to capillary sprouting, but only the longer isoform, VEGF165, is also able to promote full vessel maturation. This ability can be ascribed to the capacity of VEGF165, but not VEGF121, to recruit a peculiar population of accessory myeloid cells, which sustains vessel maturation. We termed these cells Nrp-1 Expressing Mononuclear cells (NEMs) as they express Neuropilin-1 (Nrp-1), a non-tyrosine kinase co-receptor crucially involved in their mobilization. Hence, our data strongly point towards a differential contribution of VEGF-A splicing isoforms to tumour angiogenesis. Based on these findings, we investigated the impact of VEGF165 and VEGF121 on tumour angiogenesis and progression in human tumour samples. To assess the relevance of VEGF-A splicing in the context of human malignancies, we first quantified the relative abundance of VEGF165 and VEGF121 in a set of colorectal cancer patients. The ratio of isoform expression (VEGF165/VEGF121) was analysed both in tumour mass and in a matched sample of healthy mucosa, harvested at least 10 cm far from the tumour site. The VEGF isoform ratios were not statistically different in mucosa and tumour. Similarly, no correlation with Grading (G), Stage (T), or Vascular Extra-Parietal Invasion (IVEP) was detected. However, the VEGF165/VEGF121 ratio was significantly higher in tumours derived from patients with metastatic disease, arguing for a facilitating role of VEGF165 in hematogenous tumour dissemination. Despite the absence of any obvious trend in VEGF-A splicing regulation, the VEGF165/VEGF121 ratio showed a significant degree of variability. We thus further analyzed a restricted group of patients who strongly up-regulated VEGF165 (VEGF165high patients). Interestingly, these patients showed reduced lymph node infiltration but improved vessel morphology. In accordance with the above information, tumours that strongly up-regulated Semaphorin3A (Sema3A - known Nrp-1 ligand and NEM recruiter) showed improved vessel structure and better outcome (no metastasis, no lymph node infiltration and longer survival). Taken together, these data suggest that two major Nrp-1 ligands, VEGF165 (but not VEGF121) and Sema3A, are able to enhance vessel maturation in tumours, possibly through the recruitment of NEMs. This evidence is consistent with the conclusion that improved tumour vascular function is detrimental for tumour growth, in concert with a number of recent reports highlighting the role of vessel normalization in tumour progression. According to this notion, high levels of either VEGF165 or Sema3A correlated with better disease outcome, as evaluated by lymph node infiltration and overall survival. Of interest, however, up-regulation of VEGF165 was not associated with a reduced metastatic spread (as Sema3A did), while, on the contrary, higher levels of VEGF165 were found in patients with metastatic disease. Taken together, these data appear fully consistent with recently published information supporting the concept that the vascular normalization induced by anti-angiogenic therapies indeed elicits tumour cell spreading and metastasis. Our additional studies on human specimens and cell culture identified hypoxia as a crucial regulator of VEGF splicing. Ongoing experiments to further elucidate the effect of other environmental factors on the regulation of VEGF splicing balance is in progress.
Description: 2010/2011
NBN: urn:nbn:it:units-4464
Appears in Collections:Scienze mediche

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