Editors' ChoiceCell Biology

Thrombin Targets Notch Signaling

Science Signaling  04 Nov 2008:
Vol. 1, Issue 44, pp. ec375
DOI: 10.1126/scisignal.144ec375

Fibroblast growth factor 1 (FGF1) is a proangiogenic factor, lacks a signal peptide, and is secreted by a nonclassical mechanism. Thrombin is a serine protease that is involved in blood coagulation, as well as serving as the stimulating agent for protease-activated receptor 1 (PAR1), through which thrombin has been reported to stimulate FGF1 secretion and to have proangiogenic activity. FGF1 also has a thrombin cleavage site and is known to be cleaved by this protease. Now, Duarte et al. provide evidence supporting a model by which thrombin cleaves the Notch receptor ligand Jagged1 to inhibit Notch signaling, which leads to PAR1-independent promotion of FGF1 expression and secretion. In vitro, thrombin cleaved Jagged1, producing a 39-kD fragment. Treatment of cells overexpressing a tagged form of Jagged1 with thrombin stimulated the release of a 39-kD fragment of Jagged1 that the authors called sJ1 for soluble Jagged1. Forced expression of this sJ1 inhibited Notch-mediated transcription of a reporter gene, which was overcome by overexpression of full-length Jagged1. NIH 3T3 cells transfected with sJ1 exhibited increased mRNA for FGF1, but secretion was not detectable. Thus, the authors engineered cells to express a mutant of FGF1 that could not be cleaved by thrombin (FGF1R136K) and that was under the control of a heat shock–regulated promoter and found that FGF1 was secreted from cells coexpressing sJ1 under both normal and high-temperature conditions. To demonstrate that thrombin was acting directly on the Notch pathway and not through PAR1 activation, the authors showed that thrombin, but not a nonprotease ligand of PAR1, inhibited Notch signaling in cells transfected with full-length Jagged1. Furthermore, expression of constitutively active Notch (the intracellular domain) prevented thrombin from stimulating the release of FGF1R136K. Thrombin treatment of PAR1-null cells for 48 hours produced conditioned medium that stimulated FGF1 release from PAR1-null cells. Thus, there appears to be a PAR1-independent, Notch-dependent mechanism by which thrombin can regulate FGF1 secretion, which may contribute to thrombin's proangiogenic activity.

M. Duarte, V. Kolev, D. Kacer, C. Mouta-Bellum, R. Soldi, I. Graziani, A. Kirov, R. Friesel, L. Liaw, D. Small, J. Verdi, T. Maciag, I. Prudovsky, Novel cross-talk between three cardiovascular regulators: Thrombin cleavage fragment of Jagged1 induces fibroblast growth factor 1 expression and release. Mol. Biol. Cell 19, 4863-4874 (2008). [Abstract] [Full Text]