Abstract
It is well established that epidermal growth factor (EGF) is a potent mitogen in cells expressing EGF receptor (EGFR). However, a body of evidence indicated that the effects of mitogenic EGF signaling exhibit a non-monotonic, or biphasic dose response curve; EGF at low concentrations elicits a mitogenic signaling pathway to stimulate cell proliferation while at high concentrations, EGF inhibits cell growth. However, the molecular mechanism underlying this paradoxical effect of EGF on cell proliferation remains largely unknown. Here, we investigated the molecular mechanisms underlying the biphasic EGF signaling in ER-negative breast cancer MDA-MB-231 and MDA-MB-436 cells, both of which express endogenous EGFR. We found that EGF at low concentrations induced the phosphorylation of the Src-Y416 residue, an event to activate Src, while at high concentrations allowed Src-Y527 phosphorylation that inactivates Src. EGF at 10 ng/ml also induced phosphorylation of the MAPK/ERK and activated cyclin D1 promoter activity through the Src/EGFR/STAT5 pathways but not at a higher concentration (500 ng/ml). Our results thus demonstrated that Src functions as a switch of EGF signaling depending on concentrations of EGF.
Highlights
epidermal growth factor (EGF) is one of the most potent mitogens, which transmit signals for cell growth, survival and motility by binding to and activating the EGF receptor (EGFR) [1,2,3]
It has been reported that co-expression of EGFR and c-Src in breast cancer cell lines results in their association and c-Srcmediated phosphorylation of the EGFR at tyrosine 845 (Tyr845) within its catalytic domain, which contributes to enhanced cell proliferation and tumor formation in vivo [7,8,9]
To examine whether signal transducer and activator of transcription 5 (STAT5) is involved in the observed biphasic EGF signaling, we transfected MDA-MB-231 and MDA-MB-436 cells with a consensus STAT5 reporter construct, containing a six-repeat sequence of the lactogenic hormone response element (LHRE) [23] and treated with EGF at 10 ng/ml and 500 ng/ml We found that 10 ng/ml EGF potently activates the promoter activity of the reporter plasmid while 500 ng/ml EGF failed to do so (Figure 3A), suggesting that EGF at low concentrations was able to activate STAT5 protein-mediated transcription
Summary
EGF is one of the most potent mitogens, which transmit signals for cell growth, survival and motility by binding to and activating the EGF receptor (EGFR) [1,2,3]. It has been reported that co-expression of EGFR and c-Src in breast cancer cell lines results in their association and c-Srcmediated phosphorylation of the EGFR at tyrosine 845 (Tyr845) within its catalytic domain, which contributes to enhanced cell proliferation and tumor formation in vivo [7,8,9]. EGFR-Tyr845 phosphorylation recruits the signal transducer and activator of transcription 3/5 (STAT3/5) that transmits the EGF signals to the cell nuclear and induces expression of the growth promoting genes such as c-Myc and cyclin D1 [10]. The tyrosine kinase Etk/Bmx, a downstream signaling molecule of EGF pathway is involved in the EGF-induced activation of STAT1 and apoptosis [20]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
