Herceptin - Mechanism of Action
 BasicsIntroductionBreast cancer is the most common type of cancer found among women except for skin cancers[1]. Although it is rarely seen in men, one in eight women will be diagnosed with breast cancer within their lifetime. Patients exhibiting an over-expression of Human Epidermal Growth Factor Receptor 2 (HER2) account for 25% of all breast cancer [2][3]. HER2+ patients often experience a more aggressive cancer resulting in more metastasized tumors [3]. The statistics show a poor prognosis for HER2+ patients with a 5-year survival rate of 68%. Herceptin (also known as trastuzumab) was approved by the FDA in September of 1998 for HER2+ patients and has been shown to be an effective tool in the battle against breast cancer [4]. HER2HER2 is one of four human epidermal growth factor receptors (EGFR , HER2, HER3, and HER4)[5][6]. These receptors are part of a family of receptor tyrosine kinases responsible for cell proliferation and differentiation[2][7].  This family is known as the ErbB family, because they are encoded by the ERBB genes, and is also known as the HER family. The HER family (refer to figure 2 and figure 5) are plasma membrane-bound receptors that contain an extracellular ligand-binding domain, a transmembrane domain, and an intracellular domain [3]. These human epidermal growth factor receptors exist on the cell surface and, with the exception of HER2, bind to specific ligands (epidermal growth factors) [2]. Over 11 different ligands for the epidermal growth factor receptors have been identified [5]. After the ligand binds to the receptor the receptor is able to homodimerize or heterodimerize with one another [8]. Dimerization is a chemical interaction between two "like" molecules in which they form a union. This dimerization causes a cross-phosphorylation of the intracellular tyrosine kinases between the two receptors and ultimately activates a cell signaling pathway. HER2 is the only receptor within this family that is constitutively active being able to dimerize with other HER family members acting in a ligand-independent manner [2][8]. This continuous activation of the cell signal pathway causes an increase in cell division; thus, potentially causing a tumor. [8]  HerceptinHerceptin, generic trastuzumab, is a monoclonal antibody (refer to figure 3)[10]. Herceptin is an effective treatment for breast cancer for the reason that it binds to the extracellular domain of HER2 and by multiple mechanisms of action can prevent cell proliferation as well as target these HER2+ cells for destruction by the immune system [3][10]. Structures and InteractionsHER2 ERBB2 is located on the long arm of human chromosome 17 (17q12; refer to figure 4)[11]. The ERBB genes get their name from closely resembling the avian erythroblastosis virus. This gene codes for the protein HER2 and is known as a proto-oncogene due to its ability to become an oncogene from an over-expression. The HER family is composed of three domains: an extracellular domain further divided into four subdomains composed of about 630 amino acids, a transmembrane domain composed of a single membrane-spanning region, and an intracellular domain composed of a tyrosine kinase [3]. The four subdomains of the extracellular region include: In EGFR, HER3, and HER4 sub-domains I and III remain in a “closed” conformation until a ligand binds [2][12]. Subdomain II remains hidden making contact with subdomain IV creating a "tether" (See figure 6) [12]. The tether between subdomain II and IV can be temporarily broken in order for the receptor to become more available for a ligand to bind. Evidence suggests that the ligand binds to subdomain I and/or subdomain III; also, there’s some evidence of ligand-binding to subdomain IV[2][13]. Once ligand-binding is initiated, the receptor experiences a conformational change allowing subdomains I and III to become closer together resulting in the “open” conformation [2]. This open conformation exposes subdomain II which allows for dimerization between receptors. Although some evidence suggests that subdomain IV is necessary for ligand-binding, stabilizing the extracellular domain, and locking the receptor in an open conformation, the exact function is still unknown. Once the dimerization between the two receptors takes place, a cross-phosphorylation reaction between the two tyrosine kinases occurs following the activation of certain cell signaling pathways [8]. These pathways include:
 HER2 structure differs in that subdomains I and III are in constant contact resulting in a permanent open conformation [2][3]. Subdomains I and III in HER2 are stabilized by core hydrophobic residues surrounded by hydrophilic contacts facilitating the stabilization of this interaction [2]. This fixed conformation allows the receptor to act in a ligand-independent manner and permits the access of subdomain II for dimerization. Perhaps this may explain why a high-affinity ligand for HER2 has not yet been recognized. The absence of subdomain II and IV contact can be explained by the mutations of Gly 563 and His 565 found in the other members of the HER family to Pro and Phe respectively in HER2. Since subdomain II is always available for dimerization it is free to form dimers with any of the epidermal growth factor receptors. Asp 285 is the only residue in subdomain II not conserved between EGFR and HER2. Leu replaces Asp 285 in HER2 and can explain why HER2 forms homodimers to a lesser extent. Once dimerized, HER2 activates the MAPK pathway, which in turn initiates cell proliferation, migration, differentiation, and angiogenesis [3]. A particularly harmful form of HER2 can exist within the cell membrane along with the other HER members[14]. The juxtamembrane region of the extracellular domain is prone to proteolytic cleavage resulting in a protein called p95HER2. p95HER2 is the truncated form of HER2 and has been shown to have an increased ability to cross-phosphorylate with the other receptors of the HER family and has increased cellular signaling capabilities. One combination of epidermal growth factor receptors has the potential to be a potent inducer of tumorigenesis [3]. HER2:HER3 combination can be especially detrimental to a patient exhibiting over-expression of HER2. While HER2 activates a cell proliferation pathway, HER3 is the only receptor that can directly activate the PI3K pathway. The PI3K pathway recruits its messengers (Akt and mTOR) that allows for the activation of cell survival mechanisms and helps the cell resist apoptosis. This combination can become deadly resulting in metastasized cancer. HerceptinHerceptin is a monoclonal antibody proven to be clinically effective as a monotherapy in treating HER2+ breast cancer as well as some evidence showing that Herceptin can enhance the effects of chemotherapy[3] [14]. Herceptin binds to the juxtamembrane region of HER2 on the C-terminal portion of subdomain IV [2]. The interaction formed by Herceptin and HER2 is mediated by three regions on HER2 that form three loops: residues 557-561 (loop 1), 570-573 (loop 2), and 593-603 (loop 3). Loops 1 and 3 are formed primarily by electrostatic interactions and loop 2 is formed by hydrophobic contacts. The key amino acids involved in the interaction between HER2 and Herceptin can be seen here. Herceptin's key amino acids (green) include: Asn 30 and Thr 94 on the light chain and Tyr 33, Arg 50, Trp 99, Gly 103, and Tyr 105 on the heavy chain [6]. Key amino acids involved in HER2 are as follows: loop 1 (blue) which includes Glu 558 and Asp 560, loop 2 (red) which includes Asp 570 and Phe 573, and loop 3 (orange) which includes Gln 602. Herceptin acts by four different mechanisms of action [2][3][14]:
When Herceptin binds to HER2 it is able to flag these cells for destruction[2][3][14]. This works by attracting lymphocytes to these HER2+ cells. Lymphocytes are then stimulated to release substances that cause cellular apoptosis. An added significant mechanism of action of Herceptin is the prevention of the formation of p95HER2[2][3][14]. By binding to the juxtamembrane region Herceptin sterically hinders any cleavage of the site ultimately preventing the formation of the highly active p95HER2. The next two mechanisms of action for Herceptin involve intracellular inhibition[2][3][14]. By the binding of Herceptin to the extracellular region of HER2 Herceptin is able to prevent downstream activation of cell signals that induce cell proliferation and angiogenesis. By repression of the proangiogenic factors Herceptin is also able to induce antiangiogenic factors. Inhibiting these intracellular processes Herceptin prevents any further blood vessels from developing towards the tumor and prevents the tumor from growing any larger. One limitation with this therapy is that it does not prevent HER2 from dimerizing with HER3 [3]. This makes it possible for the activation of the PI3K pathway which increases the ability of the cell to survive. Further therapies targeting the prevention of HER2 to dimerize with other members of the HER family will be necessary for future investigational studies. SEE ALSO: Structural insight into a matured humanized monoclonal antibody HuA21 against HER2-overexpressing cancer cells[15]
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