Expression of Hoxa-1 and Hoxb-7 is regulated by extracellular matrix-dependent signals in mammary epithelial cells.

Homeobox-containing genes encode transcriptional regulators involved in cell fate and pattern formation during embryogenesis. Recently, it has become clear that their expression in continuously developing adult tissues, as well as in tumorigenesis, may be of equal importance. In the mouse mammary gland, expression patterns of several homeobox genes suggest a role in epithelial-stromal interactions. Because the stroma and the extracellular matrix (ECM) are known to influence both functional and morphological development of the mammary gland, we asked whether these genes would be expressed postnatally in the gland and also in cell lines in culture and whether they could be modulated by ECM. Using a polymerase chain reaction-base strategy five members of the Hox gene clusters a and b were shown to be expressed in cultured mouse mammary cells. Hoxa-1 and Hoxb-7 were chosen for further analysis. Hoxb-7 was chosen because it had not been described previously in the mammary gland and was modulated at different stages of gland development. Hoxa-1 was chosen because it was reported previously to be expressed only in mammary tumors, and not in normal glands. We showed that culturing the mammary epithelial cell lines SCp2 and CID-9 on a basement membrane (BM) that was previously shown to induce a lactational phenotype was necessary to turn off Hoxb-7, but a change in cell shape, brought about by culturing the cells on an inert substratum such as polyHEMA, was sufficient to downregulate Hoxa-1. This is the first report of modulation of homeobox genes by ECM. The results provide a rationale for the differential pattern of expression in vivo of Hoxa-1 and Hoxb-7 during different stages of development. The culture model should permit further in-depth analysis of the molecular mechanisms involved in how ECM signaling and homeobox genes may interact to bring about tissue organization.