A. Positive feedback. In this example, EGFR-dependent activation of transcription of the genes for the EGFR ligands heparin-binding EGF-like growth factor (HB-EGF) and TGF-α (output) augments the signaling input. B. Negative feedback. Ligand activation of EGFR activates MAPK downstream signaling. Activated MAPK activates insulin receptor substrate 1 (IRS1), which provides negative feedback that inhibits signaling from the IGF-1 receptor. Blockade of EGFR cancels this negative feedback, and induces compensatory activation of IGF-1R. IGF-1R activates PI3K and v-akt murine thymoma viral oncogene homolog (Akt) leading to apoptosis resistance [220].

A. Known (yellow) and potential (green) signaling proteins that regulate resistance to EGFR inhibitors. Blue, canonical transporter proteins involved in drug resistance. Analysis of these proteins in Cytoscape [221] reveals numerous direct physical interactions (lines) involving many of these proteins. ADAM: a disintegrin and metalloprotease; AKT: v-akt murine thymoma viral oncogene homolog; BCRP: breast cancer resistance protein; ERB3: v-erb-b oncogene homolog 3; HB-EGF: heparin-binding EGF-like growth factor; KRAS: Kirsten rat sarcoma viral oncogene homolog; MRP: multidrug resistance protein; P-GP: P glycoprotein; PI3K: phosphinositol 3 kinase; PTEN: phosphatase and tensin homolog; RAF: v-raf murine sarcoma viral oncogene homolog; RALGDS: v-ral simian leukemia viral oncogene homolog guanine nucleotide dissociation stimulator; STAT :Signal transducer and activator of transcription. B. Cytoscape was used to retrieve the set of direct binding partners for the 19 EGFR resistance modulator proteins shown in A (a total of 549 proteins), then map physical interactions between these direct interactors (black lines). Yellow, green, and blue represent either the initial ‘seed’ groups shown in A, or their direct interactors; proteins indicated in red interact with two or more of the ‘seeds’. The network is highly connected, involving 8537 interactions.

A. Transported substrates of P-glycoprotein (P-GP), also known as multidrug resistance gene 1 (MDR1), and as ATP binding casette B1 (ABCB1), multidrug resistance protein 1 (MRP1)/ABCC1, breast cancer resistance protein (BCRP)/ABCG2 and Ral binding protein 1 (RalBP1)/Ral interacting protein, 76 kDa (RLIP76). ABC transporters efflux a wide range of xenobiotics from the cell. Among these, erlotinib (erl), gefitinib (gef), and imatinib (imb) target EGFR; colchicine (col), doxorubicin (dox), flavopyridol (flav), methotrexate (met), paclitaxel (pac) and vinorelbine (vrl) are cytotoxic agents commonly used in conjunction with EGFR-targeted therapies. B. EGFR signaling mechanisms that regulate efflux pumps. EGFR signaling pathways regulate the expression of the P-GP/MDR1, MRP1, BCRP/ABCG2 and RalBP1/RLIP76 transporters. At least three ABC transporters are regulated by EGFR via the phosphinositol 3 kinase (PI3K)–v-akt murine thymoma viral oncogene homolog (AKT) arm of the EGFR signaling pathway; phosphatase and tensin homolog (PTEN) and NF-κB contribute to this regulation. GRB2: growth factor receptor-bound protein 2; RALGDS: v-ral simian leukemia viral oncogene homolog guanine nucleotide dissociation stimulator; Ras: RAS viral oncogene homolog; SHC: v-src sarcoma viral oncogene homology 2 domain-containing protein; SOS-1: son of sevenless homolog 1.

A, EGFR structure. Defined protein domains are numbered I‐IV [113]. Binding of ligand (black circles) to domains I and III of EGFR induces unfolding from a tethered to an ‘activated’ conformation. The resulting exposure of dimerization domain II allows it to pair with another EGFR molecule, or heterodimerize with another ErbB family member. Intracellular juxtaposition of the EGFR kinase domains induces trans-phosphorylation of essential tyrosine residues, activating downstream signaling. Cetuximab targets domain III, preventing ligand binding and subsequent activation steps. B. The ErbB receptor family, collaborating receptors, and receptor ligands [10]. EGFR homodimerizes, and heterodimerizes with all other ErbB family members [219]. All family members except ErbB3 contain a kinase domain (small ovals); ErbB3 is active in downstream signaling because of transphosphorylation. The profile of ligand binding to specific ErbB family members is indicated. AREG: amphiregulin; BTC: betacellulin; EREG: epiregulin; HB-EGF: heparin-binding EGF-like growth factor; HGF: hepatocyte growth factor; NRG: neuregulin; PI3K: phosphinositol 3 kinase; STAT :Signal transducer and activator of transcription. Signals emanating from c-Met, IGF-1R and ErbB3 modulate similar downstream outputs as ErbB family members: see text for details. C. Mechanisms of resistance to anti-EGFR agents. Deletion of EGFR exons 2-7 leads to constitutively active conformation of the EGFR [101,104]. EGFR kinase domain mutations, such as the common T790M mutation [86], may result in increased ATP binding and resistance to erlotinib. Receptor non-intrinsic mechanisms of resistance to anti-EGFR agents include ligand excess [115], resistance to antibody-induced endocytosis [16], or transphosphorylation [129,130].