|M.Sc Student||Ziv-Uziel Inbal|
|Subject||Characterization of the Biological Activity of a Novel|
Fibroblast Growth Factors Inhibitor -SEF
|Department||Department of Biology||Supervisor||Professor Dina Ron|
Receptor tyrosine kinases (RTKs) control a multitude of biological processes and are therefore subjected to multiple levels of regulation. Sef has recently been identified as a specific antagonist of fibroblast growth factor (FGF) signaling in zebrafish and subsequently in mouse and human. The human Sef gene encodes several isoforms that are generated via an alternative splicing mechanism (designated hSef-a to hSef-d). In this work, we explored several aspects regarding the biochemical properties and mechanism of action of hSef-a, an integral membrane protein, as compared to the cytosolic isoform hSef-b. We confirmed that hSef-a is a glycoprotein and found that both hSef-a and hSef-b are associated with FGF-receptor1 (FGFR1). Both isoforms are phosphorylated on tyrosine upon FGFR1 activation. An active receptor is essential for hSef phosphorylation but not for the association with hSef. hSef-a strongly inhibits Erk phosphorylation in human embryonic kidney cells HEK 293. In NIH/3T3 fibroblasts, hSef-a does not inhibit Erk phosphorylation or nuclear translocation; nevertheless, it induces apoptosis following stimulation with FGF but not serum. This activity is accompanied by inhibition of PKB/Akt and enhancement of p38 MAPK. Chemical inhibitor of p38 overrides hSef-a induced apoptosis. The b-isoform does not posses proapoptotic activity even though it efficiently inhibits cell proliferation. In contrast to hSef-a, hSef-b inhibits cellular response to PDGF. Our results demonstrate that alternative splicing expands the hSef feedback inhibition repertoire of RTK signaling. In addition, our results demonstrate, for the first time, that feedback antagonists can differentially modulate cell fate determination.