Release of FGF1 and the Pathology of Angiogenesis

Maine Medical Center, Portland ME

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Abstract

The long-term goal of this laboratory has been to elucidate the mechanism by which fibroblast growth factor (FGF) 1 regulates the migration and proliferation of human endothelial cells in vitro and to apply this information to augment angiogenic responses during tissue and organ repair in vivo. While the recent application of the FGF prototypes for the generation of human collateral vessel growth in response to ischemic cardiovascular damage argues for the validity of this premise, structure-function studies with the members of the FGF gene family suggest that our understanding of FGF function is still primitive. Indeed, the function of two novel structural features within the FGF prototypes are not well understood and these include the presence of a functional nuclear/nucleolar localization signal(s) and the absence of a classical signal peptide sequence to direct its secretion through the conventional ER-Golgi apparatus-mediated pathway. Interestingly, these features are also found within members of the interleukin (IL)1 gene family which shares structural and crystallographic similarities with members of the FGF gene family. Since FGFs must gain access to the extracellular compartment to signal through their high affinity receptor tyrosine kinases in order to promote a biological response and undergo receptor-dependent nuclear/nucleolar translocation, we have utilized the resources from this award to define the mechanism by which FGF1 is released. Indeed, we have demonstrated during the current funding period that FGF1 but not FGF2 is released in response to biological stresses including heat shock, hypoxia, and serum deprivation in an apoptotic-independent manner. Structure- function analysis of the stress-induced FGF1 release pathway has demonstrated that FGF1 is exported into the extracellular compartment as a latent Cys30 FGF1 homodimer. The FGF1 homodimer is a component of a non-covalent high molecular weight complex which includes the extravesicular domain (p40) of the exocytotic trafficking-protein, p65 synaptotagmin (Syt1) and the annexin (Anx)2-binding protein, S100A13. Interestingly, like FGF1 and IL1alpha, these proteins also lack a classical signal peptide sequence and each is able to associate with acidic phospholipids including phosphatidylserine. In addition, we present preliminary data suggesting that the pathway responsible for the release of the mature but not the precursor form of the signal peptide-less pro-inflammatory cytokine, IL1alpha, is similar to the pathway utilized by FGF1 and that the precursor domain of IL1alpha acts as a dominant negative for the stress-induced release of FGF1. As a result, we request support to confirm and expand these results and propose to further define the mechanism of FGF1 release by (i) defining the structural prerequisites for the release of FGF1 and (ii) characterizing the cellular mechanisms by which these polypeptides facilitate FGF export. We suggest that these studies may not only provide new insight into the regulation of angiogenesis but may also enable access to novel pharmacologic targets for potential therapeutic management in man.