ErbB

Overview
The ErbB protein family or epidermal growth factor receptor (EGFR) family is a family of four structurally related receptor tyrosine kinases. Insufficient ErbB signaling in humans is associated with the development of neurodegenerative diseases, such as multiple sclerosis and Alzheimer's Disease. In mice loss of signaling by any member of the ErbB family results in embryonic lethality with defects in organs including the lungs, skin, heart and brain. Excessive ErbB signaling is associated with the development of a wide variety of types of solid tumor. ErbB-1 and ErbB-2 are found in many human cancers and their excessive signaling may be critical factors in the development and malignancy of these tumors.

Family members
The ErbB protein family consists of 4 members
 * ErbB-1, also named epidermal growth factor receptor (EGFR)
 * ErbB-2, also named HER2 in humans and neu in rodents
 * ErbB-3, also named HER3 and
 * ErbB-4, also named HER4

Structure
ErbB receptors are made up of an extracellular region which contains approximately 620 amino acids, a single transmembrane spanning region and a cytoplasmic tyrosine kinase domain. The extracellular region of each family member is made up of four subdomains, L1, CR1, L2 and CR2, were "L" signifies a leucine-rich repeat domain and "CR" a cysteine-rich region. These subdomains are shown in blue (L1), green (CR1), yellow (L2) and red (CR2) in the figure below. These subdomains are also referred to as domains I-IV respectively. The figure below was created using the pdb files 1NQL (ErbB-1), 1S78 (ErbB-2), 1M6B (ErbB-3) and 2AHX (ErbB-4).



Kinase activation
The four members of the ErbB protein family are capable of forming homodimers, heterodimers, and possibly higher order oligomers upon activation by a subset of potential growth factor ligands. There are 11 growth factors that activate ErbB receptors. The ability of each growth factor to activate each of the ErbB receptors is shown in the table below, + and - signifying ability and inability to activate each of the ErbB receptors respectively.

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Role in cancer
ErbB-1 is overexpressed in many cancers. Drugs such as cetuximab, gefitinib, erlotinib are used to inhibit it. It has recently been shown that acquired resistance to the two first can be linked to hyperactivity of ErbB-3. This is linked to an acquired overexpression of c-MET which phosphorylates ErbB-3, which in turn activates the Akt pathway.