Morphogenetic behavior of simian virus 40-transformed human mammary epithelial stem cell lines on collagen gels

The plasticity of human breast carcinoma cells is more than epithelial to mesenchymal conversion

The human breast comprises three lineages: the luminal epithelial lineage, the myoepithelial lineage, and the mesenchymal lineage. It has been widely accepted that human breast neoplasia pertains only to the luminal epithelial lineage. In recent years, however, evidence has accumulated that neoplastic breast epithelial cells may be substantially more plastic in their differentiation repertoire than previously anticipated. Thus, along with an increasing availability of markers for the myoepithelial lineage, at least a partial differentiation towards this lineage is being revealed frequently. It has also become clear that conversions towards the mesenchymal lineage actually occur, referred to as epithelial to mesenchymal transitions. Indeed, some of the so-called myofibroblasts surrounding the tumor may have an epithelial origin rather than a mesenchymal origin. Because myoepithelial cells, epithelial to mesenchymal transition-derived cells, genuine stromal cells and myofibroblasts share common markers, we now need to define a more ambitious set of markers to distinguish these cell types in the microenvironment of the tumors. This is necessary because the different microenvironments may confer different clinical outcomes. The aim of this commentary is to describe some of the inherent complexities in defining cellular phenotypes in the microenvironment of breast cancer and to expand wherever possible on the implications for tumor suppression and progression.

Estrogen responsiveness and control of normal human breast proliferation

Our understanding of the hormonal control of the proliferation of normal human breast epithelium is still surprisingly meager. However, the results of a number of recent studies have confirmed that estrogen is the major steroid mitogen for the luminal epithelial cell population (the usual targets for neoplastic transformation). Estrogen seemingly exerts its effects on cell division indirectly as there is complete dissociation between the population of luminal epithelial cells expressing the estrogen receptor (ER)4 and those that proliferate. We suggest that the ER-negative proliferating cells represent a precursor or stem cell population that differentiates to ER-containing, nonproliferative cells. In turn, these ER-positive cells act as 'estrogen sensors' and transmit positive or negative paracrine growth signals to the precursor cells depending on the prevailing hormonal environment. As yet there is no direct evidence supporting this hypothesis but we suggest ways in which it may be obtained. The implication of these studies is that inhibition of luminal epithelial proliferation with tamoxifen or pure antiestrogens or by preventing ovarian steroid secretion should be an effective strategy for the prevention of breast cancer. In addition, we may be able to predict the risk of breast cancer in an individual by measuring the intrinsic estrogen sensitivity of her breast epithelium. Finally, study of the paracrine mechanisms of growth control in the normal human breast may provide new, more specific, therapeutic targets for breast cancer prevention.

Establishment and characterization of a bovine mammary myoepithelial cell line

The thermolabile large T-antigen, encoded by the simian virus 40 early region mutant tsA58, was used to establish clonal cell lines (BMM-UV) from primary bovine myoepithelial cells. The BMM-UV cells have undergone more than 300 population doublings without any signs of senescence, and they contain the intranuclear large T antigen. At low confluency, they grow in a spindlelike manner and develop very long projections that most likely allow for communication of cells at a distance from each other. Establishment results in a decrease in the number of cells that contract in response to oxytocin compared with the parental nontransfected cells (20% versus 45%). Oxytocin responsiveness of BMM-UV cells increases when the cells are cultured in a medium supplemented with staphylococcal proteases. Proliferation of BMM-UV cells increases when they are cultured in the presence of epidermal growth factor (10 ng/ml) or insulinlike growth factor I (50 ng/ml). The BMM-UV cells may become a useful model to study growth properties, cell-to-cell communication, and the function of bovine mammary myoepithelial cells.

Relative promoter activity in human mammary epithelial cells assayed by transient expression

Chimeric DNA expression vectors containing regulatory sequences proximal to the 5' end of coding sequences for mammalian genes provide valuable tools to study gene expression. Genes coding for easily measured products (reporter genes) can be used to study promoter strength and regulation of gene expression after transient expression of promoter-reporter constructs in mammalian cells. To determine the strength of a variety of mammalian and viral promoter-enhancer sequences in primary cultures of human mammary epithelial cells (HMEC), these sequences were fused to the bacterial chloramphenicol acetyltransferase (CAT) gene and transfected into HMEC using strontium phosphate. The long terminal repeat (LTR) of the endogenous murine leukemia virus AKR-623 was the most potent promoter of transient CAT expression in HMEC. A number of commonly available promoter sequences displayed a wide range of activities in these cells. The glucocorticoid responsive LTR promoter from the murine mammary tumor virus modulated expression of CAT and was sensitive to the concentration of dexamethasone in the growth media. In a similar fashion, the regulatory sequences from the murine metallothionein-1 gene retained responsiveness to zinc concentration in the growth media.