The role of HER2 in angiogenesis.

Recent studies have established that growth factors and their receptors play an essential role in regulating the proliferation of epithelial cells. Abnormalities in the expression, structure, or activity of their proto-oncogene products contribute to the development and maintenance of the malignant phenotype. For example, c-erbB2 encodes the human epidermal growth factor receptor 2 (HER2), which is overexpressed or amplified or both in several human malignancies including breast, ovarian, and colon cancers. Tumor cells must use the process of vascularization (angiogenesis) for productive growth and metastasis. Overexpression of HER2 in human tumor cells is closely associated with increased angiogenesis and expression of vascular endothelial growth factor (VEGF). Indeed, when the VEGF pathway is inhibited, tumor growth is suppressed. The anti-HER2 blocking antibody trastuzumab has been shown to inhibit tumor cell growth and VEGF expression. Cancer cell invasiveness can be promoted, even in the absence of HER2 overexpression, by transregulation of HER2 by heregulins that bind to HER3 and HER4. Accordingly, heregulin beta1 regulates the expression and secretion of VEGF in breast cancer cells, and trastuzumab inhibits heregulin-mediated angiogenesis both in vitro and in vivo. Thus, potential upregulation of VEGF in cancer epithelial cells likely supports angiogenesis, sustaining and promoting survival and metastasis of tumor cells.