Hormonal regulation of type I receptor tyrosine kinase expression in the mammary gland.

Hormones guide mammary gland development and differentiation by regulating the expression of local growth factors and their receptors at the cell surface. In line with this principle the expression of the epidermal growth factor receptor (EGFR)3 and ErbB2 receptors varies in the mammary gland during pregnancy, following the changing hormonal profile. In breast cancer, expression of EGFR and ErbB2 is clearly related to the absence of estrogen and progesterone receptors. In breast cancer cells in vitro, the expression of these receptors is modulated by hormones and other growth-modulatory reagents. Moreover, transcriptional regulation of both EGFR and ERBB2 by estrogens has been demonstrated. The action of hormones may therefore result in the differential availability of individual ErbB family members at the cell surface, in this way determining the specific response of the cell to EGF-like factors and heregulins.