Epithelial progenitor cell lines as models of normal breast morphogenesis and neoplasia

Abstract

The majority of human breast carcinomas exhibit luminal characteristics and as such, are most probably derived from progenitor cells within the luminal epithelial compartment. This has been subdivided recently into at least three luminal subtypes based on gene expression patterns. The value of knowing the cellular origin of individual tumours is clear and should aid in designing effective therapies. To do this, however, we need strategies aimed at defining the nature of stem and progenitor cell populations in the normal breast. In this review, we will discuss our technical approach for delineating the origin of the epithelial cell types. A major step forward was the purification of each cell type by the application of immunomagnetic cell sorting based on expression of lineage-specific surface antigens. We then developed chemically defined media that could support either the luminal epithelial or the myoepithelial cell phenotype in primary cultures. Having succeeded in continuous propagation presumably without loss of markers, we could show that a subset of the luminal epithelial cells could convert to myoepithelial cells, signifying the possible existence of a progenitor cell population. By combining the information on marker expression and in situ localization with immunomagnetic sorting and subsequent immortalization, we have identified and isolated a cytokeratin 19-positive suprabasal putative precursor cell in the luminal epithelial compartment and established representative cell lines. This suprabasal-derived epithelial cell line is able to generate both itself and differentiated luminal epithelial and myoepithelial cells, and in addition, is able to form elaborate terminal duct lobular unit (TDLU)-like structures within a reconstituted basement membrane. As more than 90% of breast cancers arise in TDLUs and more than 90% are also cytokeratin 19-positive, we suggest that this cell population contains a breast-cancer progenitor.

Figure 1

Patterns of maturation of the mammary gland. The frame indicates the part that best reflects the differentiation repertoire of MUC⁻/ESA⁺ cells and their progeny. Phase‐contrast figures illustrate the patterns of morphogenesis of these cells in a reconstituted basement membrane. SLC, small light cells; ULLC, undifferentiated large light cells; DLLC, differentiated large light cells; DSC, differentiated secretory cell; Premyo, precursor myoepithelial cell (Modified from Chepko & Smith 1997 and Gudjonsson et al. 2002b, with permission from Elsevier and Cold Spring Harbor Laboratory Press). An acinus is approximately 50 µm in diameter.

Figure 2

Primary cultures of MUC⁻/ESA⁺ cells formed elaborate TDLU‐like structures inside a reconstituted basement membrane. 100 µm = 10 mm.

Figure 3

Early passage MUC⁻/ESA⁺ cell line (TH69) stained with BA17 against keratin K19 (left) and 115D8 against MUC1 (right). The keratin staining is heterogeneous and the MUC staining is almost negative. 50 µm = 5 mm.

Figure 4

Cloned MUC⁻/ESA⁺ cells doublestained with keratin K19 (red) and keratin K14 (green). Note the heterogenous staining pattern (from Gudjonsson et al. 2002b, with permission from Cold Spring Harbor Laboratory Press). 20 µm = 8 mm.