The Human Sef-a Isoform Utilizes Different Mechanisms to Regulate Receptor Tyrosine Kinase Signaling Pathways and Subsequent Cell Fate

Abstract

Negative feedback is among the key mechanisms for regulating receptor tyrosine kinase (RTK) signaling. Human Sef, a recently identified inhibitor of RTK signaling, encodes different isoforms, including a membrane spanning (hSef-a) and a cytosolic (hSef-b) isoform. Previously, we reported that hSef-b inhibited fibroblast proliferation and prevented the activation of mitogen-activated protein kinase (MAPK), without affecting protein kinase B/Akt or p38 MAPK. Conflicting results were reported concerning hSef-a inhibition of MAPK activation, and the effect of hSef-a on other RTK-induced signaling pathways is unknown. Here we show that, in fibroblasts, similar to hSef-b, ectopic expression of hSef-a inhibited fibroblast growth factorinduced cell proliferation. Unlike hSef-b, however, the growth arrest was mediated via a MAPK-independent mechanism, and was accompanied by elevated p38 MAPK phosphorylation and inhibition of protein kinase B/Akt. In addition, hSef-a, but not hSef-b, mediated apoptosis in fibroblast growth factor-stimulated cells. Chemical inhibitor of p38 MAPK abrogated the effect of hSef-a on apoptosis. In epithelial cells, ectopic expression of hSef-a inhibited the activation of MAPK, whereas downregulation of endogenous hSef-a significantly increased MAPK activation and accelerated growth factor-dependent cell proliferation. These results indicate that hSef-a is a multifunctional negative modulator of RTK signaling and clearly demonstrate that hSef-a can inhibit the activation of MAPK, although in a cell type-specific manner. Moreover, the differences between the activities of hSef-a and hSef-b suggest that hSef isoforms can control signal specificity and subsequent cell fate by utilizing different mechanisms to modulate RTK signaling.

Similar expression to FGF (Sef) inhibits fibroblast growth factor-induced tumourigenic behaviour in prostate cancer cells and is downregulated in aggressive clinical disease

“…The ability of hSef to act upstream or at Ras would imply that it should also inhibit alternate pathways activated by FGFR/Ras interaction, which include PI3 kinase (Gioeli, 2005). In keeping with this, both human and mouse Sef have been shown to block FGF induction of the PI3 kinase pathway (Kovalenko et al, 2003;Ziv et al, 2006). The exact mechanism of how hSef might inhibit FGFR/Ras activation of MAPK signalling in prostate cancer cells is currently unclear.…”

“…Current work in our group is analysing the exact mechanisms by which Sef might inhibit FGFR/Ras and MAPK signalling in prostate cancer cells. Only one human Sef (hSef) gene has been identified, although two isoforms are known to exist Ziv et al, 2006). Fulllength Sef is predicted to contain a signal peptide and transmembrane domain and is ubiquitously expressed in human tissues.…”

“…The short isoform is thought to be primarily cytosolic and has a more restricted pattern of expression (Preger et al, 2004). Both human isoforms have been shown to inhibit RTK signalling in cell line models Ziv et al, 2006). Differences in cell-specific isoform function have been previously reported.…”

“…In the other report, Sef was shown to immunoprecipitate and colocalise with the FGF receptor (FGFR) (Tsang et al, 2002). Work with human and mouse Sef has similarly shown differences with Sef, either acting at the level of Ras interface or downstream at the level of MEK (Kovalenko et al, 2003;Xiong et al, 2003;Yang et al, 2003;Ren et al, 2006;Ziv et al, 2006). An alternative mechanism has proposed that hSef does not block ERK phosphorylation but rather affects its substrate specificity by spatial regulation into the nucleus or cytoplasm (Torii et al, 2004).…”

Orphan receptor IL-17RD regulates Toll-like receptor signalling via SEFIR/TIR interactions

“…The Il17rd gene encodes a transmembrane protein of the IL-17 receptor family, but its ligand remains unidentified 33 . IL-17RD, originally termed similar expression to fibroblast growth factor ( fgf ) genes (Sef) as it belonged to the FGF synexpression group in zebrafish development 34,35 , was initially established as a negative regulator of FGF-induced Ras and MAPK signalling pathways in zebrafish, and subsequently mammalian IL-17RD was reported to inhibit ERK and PI3K pathways in response to a number of receptor tyrosine kinase ligands [36][37][38][39][40][41] .…”