EGFR signaling patterns are regulated by its different ligands.

EGF receptor (EGFR) and its signaling have been investigated for many years, but how its different ligands regulate signaling has not been thoroughly explored. When investigating EGFR activation and downstream signaling in HeLa cells using a panel of ligands, we found a ligand-dependent differential activation of EGFR and the signaling pathways Akt, PLCγ and STAT with HB-EGF and BTC being the most potent ligands. All the tested ligands induced full activation of Erk signaling at 1 nM, whereas only HB-EGF and partly BTC and EGF induced strong activation of Akt, STAT3 and PLCγ at this concentration. Interestingly, we also found that the high activation potencies of HB-EGF and BTC could only partially be explained by their binding affinities, and are therefore likely to be regulated by other mechanisms. We thus suggest that the signaling pathways initiated from the EGFR vary depending on the ligands bound in a cell specific manner.

Internalization mechanisms of the epidermal growth factor receptor after activation with different ligands.

The epidermal growth factor receptor (EGFR) regulates normal growth and differentiation, but dysregulation of the receptor or one of the EGFR ligands is involved in the pathogenesis of many cancers. There are eight ligands for EGFR, however most of the research into trafficking of the receptor after ligand activation focuses on the effect of epidermal growth factor (EGF) and transforming growth factor-α (TGF-α). For a long time it was believed that clathrin-mediated endocytosis was the major pathway for internalization of the receptor, but recent work suggests that different pathways exist. Here we show that clathrin ablation completely inhibits internalization of EGF- and TGF-α-stimulated receptor, however the inhibition of receptor internalization in cells treated with heparin-binding EGF-like growth factor (HB-EGF) or betacellulin (BTC) was only partial. In contrast, clathrin knockdown fully inhibits EGFR degradation after all ligands tested. Furthermore, inhibition of dynamin function blocked EGFR internalization after stimulation with all ligands. Knocking out a number of clathrin-independent dynamin-dependent pathways of internalization had no effect on the ligand-induced endocytosis of the EGFR. We suggest that EGF and TGF-α lead to EGFR endocytosis mainly via the clathrin-mediated pathway. Furthermore, we suggest that HB-EGF and BTC also lead to EGFR endocytosis via a clathrin-mediated pathway, but can additionally use an unidentified internalization pathway or better recruit the small amount of clathrin remaining after clathrin knockdown.

EGF receptor inhibitors increase ErbB3 mRNA and protein levels in breast cancer cells.

The potential benefits of drugs directly targeting the ErbB receptors for cancer therapy have led to an extensive development within this field. However, the clinical effects of ErbB receptor-targeting drugs in cancer treatment are limited due to a high frequency of resistance. It has been reported that, when inhibiting the epidermal growth factor receptor (EGFR) with the tyrosine kinase inhibitor gefitinib, increased activation of ErbB3 via MET, or by re-localization of ErbB3 mediates cell survival. Here we show further evidence that members of the ErbB receptor family facilitate resistance to EGFR inhibitor treatment in ErbB2 overexpressing breast cancer cells. We found that gefitinib treatment increased ErbB3 expression, both at protein and mRNA levels. ErbB3 expression was upregulated not only by gefitinib but also by a panel of different EGFR inhibitors, suggesting that inhibition of EGFR in general affects ErbB3 expression. In addition, we found that gefitinib treatment increased ErbB2 expression levels while EGFR inhibitors decreased the activity of ErbB2. Concentrations of gefitinib that decreased phospho-ErbB2 reversely increased ErbB3 levels. We further examined changes induced by gefitinib treatment on mRNA levels of the most common genes known to be involved in breast cancer. As expected, we found that gefitinib downregulated genes whose functions were linked to cellular proliferation, such as Ki-67, topoisomerase II alpha and cyclins, and surprisingly downregulated gene expression of FAS which is involved in apoptotic signaling. Together, our data strongly suggest that resistance to EGFR inhibitors may result from the compensation of other family members and that combinations of anti-cancer drugs are required to increase the sensitivity of these treatments.

Differential effects of EGFR ligands on endocytic sorting of the receptor.

Endocytic downregulation is a pivotal mechanism turning off signalling from the EGF receptor (EGFR). It is well established that whereas EGF binding leads to lysosomal degradation of EGFR, transforming growth factor (TGF)-alpha causes receptor recycling. TGF-alpha therefore leads to continuous signalling and is a more potent mitogen than EGF. In addition to EGF and TGF-alpha, five EGFR ligands have been identified. Although many of these ligands are upregulated in cancers, very little is known about their effect on EGFR trafficking. We have compared the effect of six different ligands on endocytic trafficking of EGFR. We find that, whereas they all stimulate receptor internalization, they have very diverse effects on endocytic sorting. Heparin-binding EGF-like growth factor and Betacellulin target all EGFRs for lysosomal degradation. In contrast, TGF-alpha and epiregulin lead to complete receptor recycling. EGF leads to lysosomal degradation of the majority but not all EGFRs. Amphiregulin does not target EGFR for lysosomal degradation but causes fast as well as slow EGFR recycling. The Cbl ubiquitin ligases, especially c-Cbl, are responsible for EGFR ubiquitination after stimulation with all ligands, and persistent EGFR phosphorylation and ubiquitination largely correlate with receptor degradation.