Tight regulation of growth factor signaling through receptor tyrosine kinases (RTK) is critical to cell differentiation, growth, and survival. Sprouty (Spry) proteins are generally considered inhibitors of RTK signaling whose effects are mediated by interference with the mitogen-activated protein kinase (MAPK) cascade. Studies by Wong et al. and Egan et al. challenge this paradigm by demonstrating that human Sprouty2 (hSpry2) can potentiate epidermal growth factor (EGF) signaling by inhibiting ubiquitylation of the epidermal growth factor receptor (EGFR) by the E3 ubiquitin ligase c-Cbl. Wong et al. used confocal microscopy to show that overexpressed hSpry2, but not truncated constructs or isoforms that don't bind c-Cbl, inhibited endocytosis of immunofluorescently labeled EGFR in COS-1 cells stimulated with EGF. Transfected c-Cbl accelerated EGF-stimulated receptor down-regulation, whereas hSpry2 enhanced cell surface retention of EGFRs as demonstrated by radiolabeled EGF to EGFR binding. Using Western analysis on immunoprecipitates of intact and mutant proteins from transfected COS-1 cells, as well as an in vitro ubiquitylation assay, Wong et al. demonstrated that hSpry2 inhibited c-Cbl-mediated ubiquitylation of EGFR. Analysis of immunoprecipitates of cells transfected with hSpry2 and UbcH7 (human E2 ubiquitin ligase) indicated that hSpry competed with UbcH7 for binding to the c-Cbl RING finger domain. Decreased EGFR ubiquitylation in response to hSpry correlated with sustained EGF-induced ERK phosphorylation, indicating potentiation of the MAPK pathway. In PC12 cells, transfected hSpry2 induced morphological differentiation--normally associated with nerve growth factor (NGF) or fibroblast growth factor (FGF) stimulation--presumably secondary to sustained EGF-induced extracellular signal-regulated kinase (ERK) phosphorylation. In contrast, hSpry2 inhibited the response to FGF and NGF. These results indicate that, rather than being general inhibitors of RTK signaling, Sprys may inhibit certain specific pathways and potentiate others to fine tune the responses to RTK activation.
E. S. M. Wong, M. Wong, C.W. Fong, J. Lin, P. Yusoff, B. C. Low, W. Y. Langdon, G. R. Guy, Sprouty2 attenuates epidermal growth factor receptor ubiquitylation and endocytosis, and consequently enhances Ras/ERK signalling. EMBO J. 21, 4796-4808 (2002). [Abstract] [Full Text]
J. E. Egan, A. B. Hall, B. A. Yatsula, D. Bar-Sagi, The bimodal regulation of epidermal growth factor signaling by human Sprouty proteins, Proc. Natl. Acad. Sci. U.S.A. 99, 6041-6046 (2002). [Abstract] [Full Text]