HER2 (ERBB2)
HER2 belongs to a family of receptor tyrosine kinases (RTKs) that includes EGFR/ERBB1, HER2/ERBB2/NEU, HER3/ERBB3, and HER4/ERBB4. The gene for HER2 is located on chromosome 17 and has been found to be amplified with an increased copy number in several cancers (Jorgensen 2010). Amplification of HER2 has been found to promote tumorigenesis and to be involved in the pathogenesis of several human cancers (Moasser 2007).
To date, no ligand has been identified for HER2. However, HER2 appears to be the preferential dimerization partner for all members of the ERBB family (Graus-Porta et al. 1997). The binding of ligand followed by HER2 hetero-dimerization results in activation of HER2 tyrosine kinase activity. Activated HER2 then phosphorylates its substrates, leading to activation of multiple downstream pathways within the cell, including the PI3K-AKT-mTOR pathway, which is involved in cell survival, and the RAS-RAF-MEK-ERK pathway, which is involved in cell proliferation (Figure 1).
Figure 1. Schematic of HER2 signaling pathway. Growth factor binding results HER2 heterodimerization and activation of the MAPK signaling pathway (RAS-RAF-MEK-ERK) and the PI3K pathway (PI3K-AKT-mTOR). The letter "K" within the schema denotes the tyrosine kinase domain.