Differentiation of simian virus 40 transformed human mammary epithelial stem cell lines to myoepithelial-like cells is associated with increased expression of viral large T antigen

In vitro multipotent differentiation and barrier function of a human mammary epithelium

As demonstrated by a variety of animal studies, barrier function in the mammary epithelium is essential for a fully functioning and differentiated gland. However, there is a paucity of information on barrier function in human mammary epithelium. Here, we have established characteristics of a polarizing differentiating model of human mammary epithelial cells capable of forming a high-resistance/low-conductance barrier in a predictable manner, viz., by using MCF10A cells on permeable membranes. Inulin flux decreased and transepithelial electrical resistance (TEER) increased over the course of several days after seeding MCF10A cells on permeable membranes. MCF10A cells exhibited multipotent phenotypic differentiation into layers expressing basal and lumenal markers when placed on permeable membranes, with at least two distinct cell phenotypes. A clonal subline of MCF10A, generated by culturing stem-like cells under non-adherent conditions, also generated a barrier-forming epithelial membrane with cells expressing markers of both basal and lumenal differentiation (CD10 and MUC1, respectively). Progressive changes associated with differentiation, including wholesale inhibition of cell-cycle genes and stimulation of cell and tissue morphogenic genes, were observed by gene expression profiling. Clustering and gene ontology categorization of significantly altered genes revealed a pattern of lumenal epithelial-cell-specific differentiation.

Secretion of transforming growth factor alpha and expression of its receptor in human mammary cell lines

The secretion of transforming growth factor alpha (TGF alpha) and the expression of cell-surface receptors for epidermal growth factor (EGF) were measured in a series of human mammary cell lines. The amount of TGF alpha secreted by the cells did not correlate with the phenotype of the cells (epithelial or myoepithelial), the mechanism of immortalization of the cells (SV40 or spontaneous) or the source of the cells (normal mammary gland, benign hyperplastic lesion, malignant tumour). The level of expression of cell-surface receptors for EGF was markedly increased as a consequence of SV40-immortalization of mammary cells, but otherwise did not correlate with the phenotype of the cells or the source of the cells. Much of the increase was accounted for by the appearance of a large number of low-affinity receptors for EGF in the SV40-immortalized cells. It is suggested that one of the mechanisms whereby SV40-immortalization suppresses the senescence of primary cultures of human mammary epithelial cells involves increasing the level of expression of receptors for EGF. In contrast the level of secretion of TGF alpha by cells in culture is probably a consequence of the mechanisms of adaptation of each cell line to culture conditions, and does not reflect the level of secretion of TGF alpha by cells in vivo.

Differential expression of annexins I and II in normal and malignant human mammary epithelial cells

Collagen-binding proteins (CBPs) of rat mammary tumors are identical to Ca(2+)-binding annexins. We have now isolated a protein of 38 kDa from the human mammary tumor cell line ALAB by collagen type I affinity chromatography as well as by extraction of calcium-binding proteins. The 38-kDa band of both preparations was identified as annexin II (calpactin I) by its reaction with an annexin II-specific monoclonal antibody in Western blot analysis. Annexin I (lipocortin I) was not detectable in these cells. Two other human cell lines, the SV40-transformed cell line SV3 and cell line HBL-100, both established from normal mammary glands, were also positive for annexin II and negative for annexin I. In vivo expression of annexins was investigated by immunohistological staining of normal and malignant human mammary tissue. The annexin II-specific mAb reacted with normal and tumor parenchyma whereas the annexin I-specific mAb reacted with acini and ductal myoepithelium of the normal mammary gland but showed no reaction with tumor tissue. Immunolocalization studies also showed annexin II expression in both normal and tumor stroma while only tumor stromal cells were found to be reactive with the antibody against annexin I. The differential expression of annexins in normal and malignant human mammary tissue suggests special functions of these proteins in the mammary gland.

Differential effects of the simian virus 40 early genes on mammary epithelial cell growth, morphology, and gene expression

To study the effect of SV40 T-antigen in mammary epithelial cells, a rat beta-casein promoter-driven SV40 early-region construct was stably introduced into the clonal mouse mammary epithelial cell line HC11. With the expression of the viral T-antigens under the control of a hormone-inducible promoter, it was possible to dissociate the effects of different levels of T-antigen expression on cell growth, morphology, and gene expression. Following hormonal induction, a rapid but transient induction of T-antigen was observed, followed by a delayed induction of H4 histone mRNA. In T-antigen-positive HC11 cells cultured in the absence of EGF, the expression of basal levels of T-antigen (in the absence of hormonal induction) led to a decreased doubling time and an increased cell density. In the presence of EGF, T-antigen expression resulted additionally in an altered cell morphology. Despite the effects of T-antigen on cell growth and gene expression, the cells were unable to form colonies in soft agar and were nontumorigenic when transplanted into cleared mammary fat pads. They were, however, weakly tumorigenic in nude mice. Relatively high levels of p53 protein synthesis were observed in both the transfected HC11 cells and the parental COMMA-D cells, as compared to 3T3E fibroblasts and another mammary epithelial cell line. The HC11 and COMMA-D cells synthesized approximately equal levels of wild-type and mutated p53 proteins as defined by their reactivities with monoclonal antibodies PAb246 and PAb240, respectively. Interactions between excess p53 and T-antigen may, in part, explain the failure of these cells to display a completely transformed phenotype.

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

Transformation of primary cultures of human breast cells with simian virus 40 and clonal selection has yielded single-cell-cloned, epithelial cell lines, as well as myoepithelial-related cell lines. When grown on floating collagen gels, the epithelial cell lines give rise to branching rays of cells, thick fingerlike protrusions, saclike structures, and degenerating areas. The myoepithelial-related cell lines give rise only to the branching rays. Epidermal growth factor stimulates the production of the thick protrusions, whereas cholera toxin stimulates the production of the degenerating areas. Immunocytochemical staining of these cultures using reagents directed against the cell surface-extracellular matrix or the cellular cytoskeleton confirms the epithelial and myoepithelial nature of the cells, and demonstrates that the degenerating areas are undergoing squamous metaplasia. The fingerlike protrusions consist of cords of cells composed of inner, epithelial and outer, myoepithelial-related cells sometimes surrounding a central lumen reminiscent of ducts. The saclike structures resemble alveoli. Ultrastructural analysis confirms the identification of the basic cell types and also identifies indeterminate cells possessing features of both epithelial and myoepithelial cells. It is suggested that the epithelial cell lines represent human mammary stem cells that can undergo processes of morphogenesis and differentiation in vitro to form many of the three-dimensional structures found within the breast.

Stem cells and the development of mammary cancers in experimental rats and in humans

Evidence based on immunocytochemical staining and ultrastructure suggests that morphological gradations between epithelial and myoepithelial cells, and possibly between epithelial cells and alveolar-like cells occur in terminal ductal structures of rat and human mammary glands. The benign carcinogen-induced rat and benign human mammary tumors can contain epithelial, myoepithelial-like and alveolar-like cells, whereas the malignant counterparts mainly contain only epithelial-like cells. Clonal epithelial cell lines from normal rat mammary glands, benign tumors, and SV40-transformed human mammary glands can differentiate to either myoepithelial-like or alveolar-like cells. In those of the rat, the differentiation processes occur in steps: intermediate cells along the myoepithelial-like pathway resemble intermediates in terminal ductal structures in vivo, and can also generate certain well-differentiated mesenchymal elements of the original tumours. Differentiation of the benign rat cells to alveolar-like cells with mammatrophic hormones and retinoids in vitro leads to a reduction in their tumor-forming ability in vivo. Cell lines from malignant rat mammary tumors of increasing metastatic potential and from human ductal carcinomas largely fail to yield myoepithelial-like or alveolar-like cells and are relatively slow-growing. Growth of the rat mammary epithelial cells in culture is stimulated by a pituitary-derived mammatrophic growth factor (PMGF), prostaglandin E2, and alpha-transforming growth factor; the response of the malignant cell lines to PMGF is reduced. It is suggested that stem cells exist in the rat and human glands that are capable of differentiating to the other major cell types of the mammary parenchyma, and that during the carcinogenic process they generate genetically unstable cells which lose their ability to differentiate and attempt to maximise their intrinsically slow growth rate.