Cyclooxygenases (COXs) are key enzymes in the conversion of arachidonic acid to prostaglandins (PGs) and other eicosanoids. Nitric oxide synthase (NOS) is the enzyme that catalyzes the formation of nitric oxide (NO), a regulator of vascular permeability, from the guanidino nitrogen atom of L-arginine. Two isoforms of both enzymes occur: a constitutive one, Cox-1 and the inducible counterpart Cox-2; also NOS has a constitutive counterparts (cNOS) and an inducible form, called iNOS. The inducible isoforms of both enzymes are of maximum interest. It has been recently shown that cyclooxygenase-2 (Cox-2) is inducible by a variety of stimuli and that eicosanoids, mainly of the PGE2 species, are inducers of basic regulator of angiogenesis, including VEGF/VPF, bFGF, TGF-beta, PDGF, and endothelin-1. In addition, iNOS is inducible by Cox-2. p53 down-regulates the angiogenic process at various levels: it induces thrombospondin-1, a powerful antiangiogenic factor, down-regulates VEGF and NOS and, in addition, down-regulates hypoxia-induced angiogenesis, either inducing apoptosis or enhancing antiangiogenetic factors. It is noteworthy how important the p53 oncosuppressor is in the angiogenesis of solid tumor growth. Cox-2, iNOS and p53 are thus fundamental play-makers of the angiogenic process: they are discussed in detail and a tentative hierarchical cascade is proposed.

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