Myc and mammary cancer: Myc is a downstream effector of the ErbB2 receptortyrosine kinase

The proto-oncogene c-myc encodes a transcription factor which plays a major role in the regulation of normal cellular proliferation and is aberrantly expressed in many breast tumors. In a normal cell Myc expression levels are tightly regulated being subject to many layers of control. Errantly expres sed Myc collaborates with other oncogenes to promote transformation. In thi s review we will focus on the association between abnormal Myc expression a nd mammary cancer. In particular, we will discuss the role of Myc as a down stream effector of the ErbB2 receptor tyrosine kinase which is overexpresse d and constitutively activate in many mammary tumors. The cooperation betwe en Myc and ErbB2 in transformation will be discussed in relation to clinica l studies on Myc in human cancer and with consideration of transgenic model s of Myc-induced mammary cancer. Data from our laboratory will be presented showing that deregulated ErbB2 activity strongly stimulates cytoplasmic si gnaling pathways which in turn impinge on Myc at multiple levels causing it s deregulated expression.