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HER2 is a member of the human epidermal growth factor receptor (HER/EGFR/ERBB) family. But contrary to other members of the ERBB family, HER2 does not directly bind ligand. HER2 activation results from heterodimerization with another ERBB member or by homodimerization when HER2 concentration are high, for instance in cancer. [8]
These downstream signaling proteins initiate several signal transduction cascades, principally the MAPK, Akt and JNK pathways, leading to DNA synthesis and cell proliferation. [12] Such proteins modulate phenotypes such as cell migration, adhesion, and proliferation. Activation of the receptor is important for the innate immune response in ...
The Ras-Raf-MAPK pathway is a major signalling route for the ErbB family, as is the PI3-K/AKT pathway, both of which lead to increased cell proliferation and inhibition of apoptosis. [ 25 ] Genetic Ras mutations are infrequent in breast cancer but Ras may be pathologically activated in breast cancer by overexpression of ErbB receptors. [ 26 ]
This leads to constitutive activation of the pathway, which may or may not be overturned by compensation mechanisms. In the case of HER2, which acts as a dimerization partner of other EGFRs, constitutive activation leads to hyperproliferation and cancer. [14]
The human ERBB3 gene is located on the long arm of chromosome 12 (12q13). It is encoded by 23,651 base pairs and translates into 1342 amino acids. [5]During human development, ERBB3 is expressed in skin, bone, muscle, nervous system, heart, lungs, and intestinal epithelium. [6]
Recent research revealed that IL-6 secretion induced by HER2 overexpression activated STAT3 and altered gene expression, resulting in an autocrine loop of IL-6/STAT3 expression. Both mouse and human in vivo models of HER2-overexpressing breast cancers relied critically on this HER2–IL-6–STAT3 signaling pathway. [6]
The second pathway is a result of ERK activation leading to the accumulation of active complexes of Cyclin D and Cdk4/6 which destabilize Rb via phosphorylation and further serve to activate E2F and promote expression of its targets.
HER2 is an established therapeutic target within breast cancer, and the activation of HER2 is observed in approximately 20% of breast cancers as a result of overexpression. [ 19 ] [ 20 ] Trastuzumab , the first HER2-targeted drug developed in 1990, interferes with HER2 signalling.