Differential regulation of the Wnt gene family during pregnancy and lactation suggests a role in postnatal development of the mammary gland.

The mouse Wnt family comprises at least 10 members sharing substantial amino acid identity with the secreted glycoprotein Wnt-1/int-1. Two of these, Wnt-1 and Wnt-3, are implicated in mouse mammary tumor virus-associated adenocarcinomas, although neither member is normally expressed in the mammary gland. These results suggest the presence of active cellular pathways which mediate the action of Wnt-1 and Wnt-3 signals. An understanding of the normal role of these signalling pathways is clearly necessary to comprehend the involvement of Wnt-1 and Wnt-3 in mammary tumorigenesis. We demonstrate here that five Wnt family members are expressed and differentially regulated in the normal mouse mammary gland. In addition, some of these genes are also expressed in both Wnt-1-responsive and nonresponsive mammary epithelial cell lines. We propose that Wnt-mediated signalling is involved in normal regulation of mammary development and that inappropriate expression of Wnt-1, Wnt-3, and possibly other family members can interfere with these signalling pathways.