Several cell types, including tumor cells of different origin (Moscatelli et al., 1986; Ohtani et al., 1993; Presta et al., 1986; Schulze-Osthoff et al., 1990; Takahashi et al., 1990; Yamanaka et al., 1993), macrophages (Baird et al., 1985) and T lymphocytes (Blotnik et al., 1994), express FGF2 in vitro and in vivo. FGF2 lacks a classic signal peptide for secretion (Abraham et al., 1986). However, cell damage may cause the release of FGF2 from producing cells (Gajdusek and Carbon, 1989; McNeil et al., 1989; Witte et al., 1989). Also, an alternative mechanism of exocytosis of FGF2, independent of the endoplasmic reticulum/Golgi pathway, has been proposed (Mignatti et al., 1991b; Mignatti et al., 1992). Accordingly, FGF2 has been found associated with the extracellular matrix (ECM) of cell cultures in vitro (Rogelj et al., 1989; Vlodavski et al., 1987a) and located in the basement membranes of blood vessels in vivo (DiMario et al., 1989; Folkman et al., 1988). On this basis, FGF2 is thought to exert its effects on endothelial cells via a paracrine mode consequent to its release by other cells and/or mobilization from ECM.
 

Paracrine activity of FGF2 on endothelium. FGF2 is released by tumor cells and inflammatory cells or is mobilized from ECM. Free FGF2 will act on endothelial cells in a paracrine mode of action.

Besides experimental evidence for paracrine mode of action for FGF2, some observations raise the hypothesis that FGF2 may also play an autocrine role in endothelial cells. In vitro, it has been shown that different endothelial cells produce FGF2 (Presta et al., 1989; Schweigerer et al., 1987; Vlodavski et al., 1987b) and that endogenous FGF2 modulates cell proliferation and migration, as well as the production of proteinases and their receptors (Itoh et al., 1992; Pepper et al., 1993; Sato and Rifkin, 1988). In vivo, it has been shown that FGF2 expression occurs in the endothelium adjacent to neoplastic cells in several human tumor types. These neoplasms include neuroblastoma, astrocytoma, glioblastoma and meningioma (Schulze-Osthoff et al., 1990; Takahashi et al., 1990; Zagzag et al., 1990), pheochromocytoma (Statuto et al., 1993), melanoma (Schulze-Osthoff et al., 1990), carcinomas of the stomach and colon (Ohtani et al., 1993; Schulze-Osthoff et al., 1990), and adenocarcinomas of the larynx, endometrium, and cervix. Thus, FGF2 expression is a common feature of vascular endothelium during tumor angiogenesis.

These observations strongly support the hypothesis that neovascularization may be triggered by molecule(s) released by tumor cells and/or infiltrating inflammatory cells that induce FGF2 upregulation in the quiescent endothelium. In keeping with this hypothesis is the observation that tumor cells of different origin release molecule(s) able to interact with endothelium and to upregulate the expression of FGF2 that, in turn, stimulates the fibrinolytic potential of the endothelial cell in an autocrine manner (Peverali et al., 1994). In addition, FGF2 itself, thrombin, and interleukin-2 stimulate FGF2 production in endothelial cells (Cozzolino et al., 1993; Weich et al., 1991).

FGF2 has been detected in cardiac myocytes (Speir et al., 1992) and cells of the coronary vasculature (Hawker and Granger, 1993). Also, cultured coronary endothelium exhibits the FGF receptor on its surface and expresses FGF2 mRNA (Hawker and Granger, 1993), suggesting that FGF2 might induce coronary angiogenesis by an autocrine/paracrine mechanism. Indeed, we have shown (Ziche et al., 1997) that nitric oxide (NO) induces an angiogenic phenotype (including cell proliferation and urokinase-type plasminogen activator upregulation) in coronary venular endothelial cells by inducing endogenous FGF2 and that this pathway mediates the angiogenetic response to the vasoactive neuropeptide substance P.
 
 

Endogenous FGF2 mediates NO-induced angiogenic phenotype in coronary venular endothelial cells. Substance P and the NO donor nitroprusside (NaNP) induce FGF2 upregulation that stimulates endothelial cell (EC) proliferation and urokinase-type plasminogen activator (uPA) upregulation. Neutralizing anti-FGF2 antibody prevents EC response to NO.