Down-regulation of activin, activin receptors, and Smads in high-grade breast cancer

Activin and transforming growth factor (TGF) -β, members of the TGF-β superfamily of growth factors, have been implicated in both mammary gland development and breast carcinogenesis. TGF-β is thought to be involved in the maintenance of mammary gland ductal architecture and postlactational involution. TGF-β acts as both a tumor suppressor and has oncogenic capacities in breast cancer tissue. Activin is associated with growth modulation in glandular organs, and its receptors and signaling proteins are present and regulated during postnatal mammary gland development, primarily during the lactational phase. The presence of the major components of the activin signal transduction pathway in different pathologic grades of breast cancer tissue has not been described thoroughly, despite evidence from in vitro studies suggesting that activin can inhibit proliferation in breast cancer-derived cells. On the basis of the growth regulatory capacity of activin, we hypothesized that the components of this signal transduction system would be deregulated as breast cancer becomes more aggressive. To test this hypothesis, breast cancer samples were substratified by pathologic grade, a known prognostic marker for breast cancer, and then examined for the presence and cellular localization of activin ligand subunits (βA- and βB-), receptors (Act RIIA, Act RIIB, and Act RIB), and signaling proteins, Smads 2, 3, and 4, by immunohistochemistry and immunofluorescent analysis. Breast tissue from healthy patients undergoing reduction mammoplasty was also studied. The activin βA-subunit was present in all of the tissues examined, whereas the βB-subunit, activin type II receptors, and Smads were less evident in high-grade cancers. Significant correlations were made in breast cancer specimens between a decrease in nuclear Smad 3 abundance and high tumor grade, high architectural grade, larger tumor size, and hormone receptor negativity. Thus, activin signal transduction components are present in normal tissue and grade 1 cancer but down-regulated in high-grade cancer. The deregulation of this signal transduction system may be relevant to advancing oncogenic progression.