Targeting the Epidermal Growth Factor Receptor
Targeting the Epidermal Growth Factor Receptor
The epidermal growth factor receptor (EGFR) is a 170-kDa membrane-anchored protein tyrosine kinase that has been implicated in tumorigenesis. Protein kinases are targets for the treatment of numerous diseases including cancer, inflammatory disorders, and diabetes. There are 518 protein kinases that have a shared catalytic domain as far as structure, yet the regulation of their catalysis is variable.
The receptors in this family (ErbB/HER) consist of four tyrosine kinases that are structurally similar and include (EGFR; HER1 or ErbB-1), HER2/neu or ErbB-2, HER3 or ErbB-3, and HER4 or ErbB-4. All share an extracellular domain, an intracellular tyrosine kinase, and a transmembrane domain. Ligands such as transforming growth factor (TGF)-alpha or epidermal growth factor (EGF) activate the EGFR, resulting in its dimerization or heterodimerization with other receptors that are closely related, such as HER2/neu. Phosphorylation of these receptors through their tyrosine kinase domains leads to the recruitment of downstream effectors and activation of proliferation and cell-survival signals (Figure 1). This process appears to be overactive in malignancy. An important signaling route of the ErbB family is the Ras-Raf-MAP-kinase pathway. Through activation of Ras, a multistep cascade of phosphorylation is initiated; this leads to activation of the MAPKs ERK1 and ERK2,[4,5] both of which are linked to cell proliferation, transformation, and survival in laboratory studies.
We now have a better understanding of the structure of EGFR as well as mutations encountered in human cells because of data from the human genome project. It is of note that EGFR signaling may be affected by mechanisms other than EGFR expression. Mutation in the EGFR has been observed in a variety of tumors. The EGFRvIII mutant lacks the external ligand-binding domain but has a tyrosine kinase that is constitutively activated.
The ErbB network is key to important signaling pathways and has been preserved throughout evolution. Research in knockout and transgenic mice has clarified some of its important functions. Inactivation of the ErbB1 subtype seems to impair epithelial development such as tooth growth and eye opening.[9,10] ErbB1 seems to play an important role in differentiation of epithelial components of skin, lung, pancreas, and gastrointestinal tract. Mice that lack expression of TGF-alpha (an EGF ligand), have abnormalities in eye, skin, and hair development.
Although expressed in nonmalignant cells, the EGFR is highly expressed in a variety of tumors, and its expression is correlated with poor response to treatment and poor survival. Its activation has been shown to promote tumor proliferation, angiogenesis, as well as metastases and invasion.[8,13] It was earlier suggested that abnormal EGFR signaling is correlated with advanced disease, poor response to chemotherapy, and poor prognosis.[14,15] Human malignancies use several mechanisms that activate the EGF pathway, including overproduction of ligands, overproduction of receptors, or constitutive activation of receptors.[16,17] More recent data, however, indicate that overexpression of EGFR in non-small-cell lung cancer (NSCLC) may not be correlated with poor outcome. EGFR is overexpressed in 40% to 80% of NSCLC cases and numerous other epithelial cancers.
Similar chemical characteristics of EGFR in humans and mice indicate its preservation throughout a long evolutionary process. Radioimmunoassays have shown that EGFR is present in various body fluids including urine, saliva, amniotic fluid, and plasma. EGF has been shown to promote the growth of different cell types in tissue cultures including fibroblasts, mammary epithelium, and vascular endothelium.
The receptor is composed of a single chain with an outer portion that forms an EGF-binding domain (Figure 1). When the multipart portion of the receptor binds to EGF, it changes shape, allowing the receptor to dimerize with other receptors. The inner part of the receptor, which is a tyrosine kinase enzyme, is activated upon dimerization and adds phosphate groups to the tyrosine residues. This initiates a signaling cascade intracellularly that ultimately promotes DNA synthesis and cell growth. The growing information about the structure and function of this receptor has promoted the development and discovery of new anticancer drugs blocking the action of EGFR from both ends.
The potency of intracellular signaling seems to be determined by the ligand and which intracellular sites are autophosphorylated. The PI3K-activated AKT pathway and P70S6/P85S6K have different activation potencies that probably depend on the type of receptor (ErbB1/2/3/4). Several pathways including the MAPK pathway, the stress-activated protein kinase pathway, protein kinase C, and Akt are simultaneously activated. These lead to activation of different transcription programs in the nucleus. The result is activation of cell division and migration, both of which are characteristics of tumor cells. There appear to be other signaling pathways that are also integrated into the ErbB network including hormonal pathways, neurotransmitters, lymphokines, and stress inducers (Figure 1).
Tyrosine kinase inhibitors (TKIs) have been studied extensively in tissue culture of transformed cells and in animal models. They have been shown to inhibit receptor phosphorylation as well as tumor growth, invasion, and adhesion. They have also been shown to reverse cancer cells to phenotypically differentiated cells and hence reverse the process of transformation. Some quinazoline derivatives have been shown not only to compete with adenosine triphosphate but also to promote the formation of EGFR dimers that are inactive, and in the absence of ligands. Whether this property in these particular agents has any clinical implications is not yet known.