Dynamic expression and activation of ERBB receptors in the developing mouse mammary gland

The complex system of ERBB receptors and ligands is implicated in growth and differentiation of the mammary gland. However, it has not been comprehensively examined in this dynamic tissue. Combined RNA and protein analyses of glands in different stages from virgin to involution revealed differential expression of the four ERBB receptors, as well as distinctive patterns of ERBB ligand expression that suggested specialized function. ERBB localization was linked to mammary gland function. Thus, in the virgin gland, ERBB1 and ERBB2 were colocalized to all major cell types during ductal morphogenesis but differentially localized in the mature gland. All four ERBB receptors were restricted to epithelia in the differentiated gland. Analyses of ERBB tyrosine phosphorylation provided strong evidence of interaction between the four receptors in this physiological context. Thus, exogenous EGF induced stage-dependent transphosphorylation of ERBB2-4 as well as ERBB1, whereas endogenous phosphorylation of all four receptors peaked in late pregnancy and lactation.