FGF-1 SECRETION PATHWAY

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The heparin-binding protein, fibroblast growth factor (FGF)- 1, is a prototype member of a large family of related genes and it lacks the structural feature to direct its secretion through the endoplasmic reticulum-Golgi complex. Because FGF-1 is a potent angiogenesis factor and has been implicated as a mediator of vascular pathophysiology, the mechanism by which FGF-1 is secreted may be an important regulatory feature to control the biological activity of this protein. Thus the long term goal of this application is to define the mechanism responsible for FGF-1 secretion. We have previously shown that temperature results in the secretion of FGF-1 as a protein which is biologically inactive and fails to associate with heparin with high affinity. However, (NH4)2S04 and more recently reducing agents are able to activate the heparin-binding and biological activity of latent FGF-1. Indeed, recent mutagenesis studies with FGF-1 confirm the latter observation since a cysteine-free FGF-1 mutant is not secreted in response to temperature . In addition, we have characterized FGF-1 as a phosphatydylserine (pLS)binding protein and have implicated the function of synaptotagnin (stg), a pLS-binding protein involved in exocytotic traffic, as a potential candidate for mediating the secretion of FGF-1. Lastly, we have obtained preliminary evidence that oxidized low density lipoprotein and hypoxia are able to induce the release of FGF-1 as a heparin-binding growth factor. These data support our immediate goals and these include (i) the definition of the role of FGF-1 cysteine residues and the potential function of stg in the FGF-1 secretion pathway, (ii) the identification and characterization of intracellular factors that participate in the FGF-1 secretion pathway and (iii) determine whether alternative biological stresses such as hypoxia and profound hypoxia are able to utilize this non-conventional pathway for FGF-1 secretion. It is anticipated that these results will in ultimately contribute not only to our understanding of how FGF-1 is able to regulate angiogenesis in vivo but definition of the FGF-1 secretion pathway may also yield new insight into mechanisms by which the FGF-1 secretion pathway may be inhibited. Indeed, it has recently been possible to co- localize FGF-1 and stg using novel electrophoretic methods from the medium of FGF-1-transfected NIH 3T3 cells conditioned by heat shock and NIH 3T3 cells co-transfected with FGF-1 and antisense stg fail to release FGF-1 in response to temperature stress.