Angiogenesis, the process through which new blood vessels arise from preexisting ones, is regulated by several "classic" factors, among which the most studied are vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2). In recent years, investigations showed that, in addition to the classic factors, numerous endogenous peptides play a relevant regulatory role in angiogenesis. Such regulatory peptides, each of which exerts well-known specific biological activities, are present, along with their receptors, in the blood vessels and may take part in the control of the "angiogenic switch." An in vivo and in vitro proangiogenic effect has been demonstrated for erythropoietin, angiotensin II (ANG-II), endothelins (ETs), adrenomedullin (AM), proadrenomedullin N-terminal 20 peptide (PAMP), urotensin-II, leptin, adiponectin, resistin, neuropeptide-Y, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating polypeptide (PACAP), and substance P. There is evidence that the angiogenic action of some of these peptides is at least partly mediated by their stimulating effect on VEGF (ANG-II, ETs, PAMP, resistin, VIP and PACAP) and/or FGF-2 systems (PAMP and leptin). AM raises the expression of VEGF in endothelial cells, but VEGF blockade does not affect the proangiogenic action of AM. Other endogenous peptides have been reported to exert an in vivo and in vitro antiangiogenic action. These include somatostatin and natriuretic peptides, which suppress the VEGF system, and ghrelin, that antagonizes FGF-2 effects. Investigations on "nonclassic" regulators of angiogenesis could open new perspectives in the therapy of diseases coupled to dysregulation of angiogenesis.
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