Extracellular matrix regulates whey acidic protein gene expression by suppression of TGF-alpha in mouse mammary epithelial cells: studies in culture and in transgenic mice

Whey acidic protein (WAP) is an abundant rodent milk protein. Its expression in mouse mammary epithelial cell cultures was previously found to require the formation of an extracellular matrix (ECM)-induced three-dimensional alveolar structure. In the absence of such structures, cells were shown to secrete diffusible factors leading to suppression of WAP expression. We demonstrate here that (a) TGF-alpha production and secretion by mammary cells is downregulated by the basement membrane-dependent alveolar structure, and (b) compared with beta-casein, WAP expression is preferentially inhibited both in culture and in transgenic mice when TGF-alpha is added or overexpressed. Thus, (c) the enhanced TGF-alpha production when cells are not in three- dimensional structures largely accounts for the WAP-inhibitory activity found in the conditioned medium. Since this activity can be abolished by incubating the conditioned medium with a function blocking antibody to TGF-alpha. The data suggest that ECM upregulates WAP by downregulating TGF-alpha production. We also propose that changes in TGF-alpha activity during mouse gestation and lactation could contribute to the pattern of temporal expression of WAP in the gland. These results provide a clear example of cooperation among lactogenic hormones, ECM, and locally acting growth factors in regulation of tissue-specific gene expression.