Receptor Kinases & Phosphatases

EGFR

The epidermal growth factor receptor (EGFR) is a cell surface receptor that directs the initiation of processes such as growth, proliferation, apoptosis, adhesion, migration, and differentiation, when it is activated by specific growth factors. Overexpression, or increased abundance, of EGFR has been reported in numerous human cancers, and is generally associated with a poor clinical prognosis for the patient.

The ErbB Family

EGFR is a member of the ErbB receptor tyrosine kinases (RTK) family, which is comprised of EGFR (also known as ErbB-1), ErbB-2 (or HER-2), ErbB-3, and ErbB-4. Each of the EGFR, ErbB-3, and ErbB-4 receptors has a specific, but not exclusive, set of growth factor ligands that includes epidermal growth factor (EGF), transforming growth factor alpha (TGFα), epiregulin, β-cellulin, heparin-binding EGF (HB-EGF) and amphiregulin. ErbB-2/HER-2 does not have a ligand but is activated by heterodimerization with another of the ErbB members.

Receptor Conformation and Activation

Investigators from the LICR Melbourne Branch and their collaborators elucidated the three dimensional structure of EGFR when bound to two different ligands: transforming growth factor α (TGFα) and epidermal growth factor (EGF)(1). The crystal structure of ErbB-2/HER-2, which is overexpressed in breast cancers, in the active form adopted for binding to other ErbB members was also solved(2). A team formed by the LICR Melbourne Branch and the LICR Melbourne Centre for Clinical Sciences then used crystallographic data and mutation analyses to characterize EGFR homodimer conformation and activation following ligand binding. The team found that, upon binding of the ligand, EGFR transitions from an inactive, tethered state to an active, untethered state and that ligand binding was required absolutely for activation(3,4). These data provided a major advance in our understanding of growth factor receptor conformational epitopes for targeted antibodies. Knowing the three-dimensional structure of the receptor allows the rational design of potential therapeutic agents, small molecule inhibitors and targeted antibodies, that block EGFR signaling.

In another study, a team formed by the LICR Melbourne and New York Branches, the LICR Melbourne Centre for Clinical Sciences and LICR Affiliates from New Haven (USA) demonstrated that overexpression of EGFR leads to the accumulation of immature (under-glycosylated) EGFR in the cell’s endoplasmic reticulum (5). Unexpectedly, this immature form of the receptor was also detected at the cell surface. Since under-glycosylated EGFR is primed for activation, it may contribute to spontaneous receptor activity and cancer cell growth. This study also suggests a novel strategy for a therapy that does not target mature wild-type EGFR: the generation of antibodies that target the immature receptors abundant when EGFR is overexpressed.

EGFR Signal Transduction

After dimerization, the receptors’ intrinsic tyrosine kinase activity phosphorylates tyrosine amino acid residues in both receptors; an LICR finding that established a central principle for the study of other RTK. Various ‘adaptor’ proteins, such as Shc and Grb2, are then recruited to the phosphorylated tyrosines, and activate signal transduction pathways, including the phosphoinositide 3-kinase (PI3K) pathway, which was discovered and characterized primarily by investigators at the LICR University College London Branch(6). The sequential activation of these and other molecules results in the initiation of gene transcription. The final aspect of EGFR signaling occurs with the receptor undergoing a conformational change (concomitant with ligand binding) that results in the exposure of motifs for endocytosis (removal from the cell surface) and degradation. The endocytosis of the receptor is the mechanism for down-regulation of signaling from the activated EGFR.

EGFR and Cancer

Overexpression of the EGFR protein and/or mutation of the EGFR gene sequence has been found in glioblastomas (brain tumors), and in kidney, cervix, vulva, oesophagus, prostate, colon, breast, ovary, pancreatic, non-small cell lung carcinoma, head and neck, bladder, and renal cancers. Deletions that cause a truncation of the EGFR protein occur naturally in human cancers, and have been arranged into different classifications based on the size of the deletion and the region of the gene affected. One example studied extensively by investigators at the LICR San Diego Branch is a deletion commonly found in glioblastomas, which truncates the extracellular domain of EGFR and confers enhanced tumorigenicity(7). The mutant receptor is known as the ‘de2-7 EGFR’ (or Δ2-7EGFR, or EGFR variant III).

Anti-EGFR Therapies

The development of anti-EGFR therapies has been a high priority for many years given LICR’s extensive knowledge and experience with EGFR signaling. First-generation commercial anti-EGFR therapies became available in the early-2000’s, but while they have clinical efficacy, their effects are limited and they are not suitable for all patients with tumors that overexpress EGFR. Additionally, the specificity of these therapies is not restricted to mutant or overexpressed EGFR, thus side-effects include damage to normal liver and/or skin which have high levels of wild-type EGFR.

One potential therapeutic agent being investigated by LICR is the monoclonal antibody 806(8), which was derived from a collaboration between the San Diego and New York Branches to generate antibodies against the de2-7 EGFR. 806 is particularly promising as it targets de2-7 EGFR and also wild-type EGFR, but only when the EGFR is overexpressed (see Antibody Targeting Program for more information on 806). LICR is also investigating the use of small molecule inhibitors of EGFR, particularly the tyrosine kinase inhibitor AG1478, which may eventually be used as a stand-alone therapy, or in conjunction with antibody therapies such as 806.

Referencces

  1. Garrett T.P., McKern N.M., Lou M., Elleman T.C., Adams T.E., Lovrecz G.O., Zhu H.J., Walker F., Frenkel M.J., Hoyne P.A., Jorissen R.N., Nice E.C., Burgess A.W., and Ward C.W. Crystal structure of a truncated epidermal growth factor receptor extracellular domain bound to transforming growth factor alpha. Cell (2002) 110(6):763-773.
  2. Garrett T.P., McKern N.M., Lou M., Elleman T.C., Adams T.E., Lovrecz G.O., Kofler M., Jorissen R.N., Nice E.C., Burgess A.W., and Ward C.W. The crystal structure of a truncated ErbB2 ectodomain reveals an active conformation, poised to interact with other ErbB receptors. Mol.Cell (2003) 11(2):495-505.
  3. Johns T.G., Adams T.E., Cochran J.R., Hall N.E., Hoyne P.A., Olsen M.J., Kim Y.S., Rothacker J., Nice E.C., Walker F., Ritter G., Jungbluth A.A., Old L.J., Ward C.W., Burgess A.W., Wittrup K.D., and Scott A.M. Identification of the epitope for the epidermal growth factor receptor-specific monoclonal antibody 806 reveals that it preferentially recognizes an untethered form of the receptor. J.Biol.Chem. (2004) 279(29):30375-30384.
  4. Walker F., Orchard S.G., Jorissen R.N., Hall N.E., Zhang H.H., Hoyne P.A., Adams T.E., Johns T.G., Ward C., Garrett T.P., Zhu H.J., Nerrie M., Scott A.M., Nice E.C., and Burgess A.W. CR1/CR2 interactions modulate the functions of the cell surface epidermal growth factor receptor. J Biol.Chem. (2004) 279(21):22387-22398.
  5. Johns T.G., Mellman I., Cartwright G.A., Ritter G., Old L.J., Burgess A.W., and Scott A.M. The antitumor monoclonal antibody 806 recognizes a high-mannose form of the EGF receptor that reaches the cell surface when cells over-express the receptor. FASEB J. (2005).
  6. Vanhaesebroeck B. and Waterfield M.D. Signaling by distinct classes of phosphoinositide 3-kinases. Exp.Cell Res. (1999) 253(1):239-254.
  7. Nishikawa R., Ji X.D., Harmon R.C., Lazar C.S., Gill G.N., Cavenee W.K., and Huang H.J. A mutant epidermal growth factor receptor common in human glioma confers enhanced tumorigenicity. Proc.Natl.Acad.Sci.U.S.A (1994) 91(16):7727-7731.
  8. Jungbluth A.A., Stockert E., Huang H.J., Collins V.P., Coplan K., Iversen K., Kolb D., Johns T.J., Scott A.M., Gullick W.J., Ritter G., Cohen L., Scanlan M.J., Cavanee W.K., and Old L.J. A monoclonal antibody recognizing human cancers with amplification/overexpression of the human epidermal growth factor receptor. Proc.Natl.Acad.Sci.U.S.A (2003) 100(2):639-644.

Centers Involved in this Research