Expression of the erbB-2 proto-oncogene during differentiation of the mammary gland in the rat

Abstract.

The erbB-2 proto-oncogene encodes a transmembrane protein (p185) that is a tyrosine kinase sharing high homology with the epidermal growth factor receptor. Its expression in mammary cell lines is modulated by estrogens, epidermal growth factor, and several other factors at the RNA and/or protein levels. We have used in situ hybridization, immunoblot, and immunohistochemistry to study the expression of erbB-2 in the rat mammary gland at various stages of differentiation. erbB-2 RNA is present at low levels in mammary glands from virgin, mid-pregnant, and lactating female rats. Increased RNA levels can be detected in early and late pregnancy. In all samples, erbB-2 RNA has been found in all epithelial cells. Immunohistochemistry with antisera directed against the intracellular domain of p185 has shown that only a minority of cells are stained in virgin and early pregnant samples, whereas no staining is seen in late pregnant and lactating mammary glands. In contrast, immunoblot analysis has detected the highest levels of p185 in late pregnancy and during lactation. This may reflect either that the cellular content of p185 is too low to be detected by immunohistochemistry, or that the epitopes are not accessible to the antisera in situ. Taken together, our data indicate that erbB-2 is expressed by mammary epithelial cells at all physiological stages and suggest that erbB-2 expression is modulated at both the RNA and protein level in vivo.