Wnt4 is not sufficient to induce lobuloalveolar mammary development

We found that despite pregnancy-associated expression levels of Wnt4, mammary glands did not display the side-branching typical of early pregnancy. Control experiments designed to test the Wnt4 construct in zebrafish reproduced other studies that demonstrated Wnt4-specific phenotypes distinct from Wnt1-induced phenotypes. Indeed, using qPCR-based array analyses, we found that a specific transcriptional target of Wnt4, namely Wnt16, was induced in Wnt4-expressing transgenic glands, to levels equivalent to that of early pregnant glands.Taken together, we propose that Wnt4 is necessary, but not sufficient, to induce side-branch development.Wnt signaling has been implicated in mammary gland development [1], particularly during the specification of mammary placodes from ectoderm (revealed by LEF1-deficient, and K5-dkk1 transgenic glands) [2-4]. Our data have shown that LRP-5, one of two canonical Wnt signaling receptors, is required for ductal stem cell maintenance [5]. Since several Wnt family genes are differentially expressed in the virgin, pregnant, and lactating mammary gland, some of them regulated by ovarian hormones, it would not be surprising if these Wnt proteins have other, as yet unidentified functions [6,7].During pregnancy, mammary glands undergo substantial changes induced by steroid and peptide hormones. Progesterone signaling is predominantly responsible for the growth and differentiation of the lobuloalveolar lineage that is characteristic of early pregnancy [8]. Brisken et al (2000) concluded that Wnt4 expression was important to side-branching and lobuloalveolar development during pregnancy, and was regulated by progesterone/PR signaling. Thus, Wnt4 null epithelium failed to form side-branches and lobuloalveoli in transplanted mammary fat pads during early pregnancy (12 days of pregnancy). Early developmental defects were overcome in late stage of pregnancy [9]. In addition, Wnt4 expression was induced in vitro in cultured mammary epithelial cells by