A role for stem cells or distinct progenitor cells as the cells of origin in many types of cancer has been proposed [29
]. These studies have led to the hypothesis that in many types of cancers, including breast cancer, distinct progenitors and their abrogated self-renewal pathways may ultimately underlie the mechanisms of heterogeneity observed among subtypes of human breast cancer [33
]. The identification and further characterization of distinct mammary progenitors are necessary prerequisites for testing this hypothesis. Single-cell cloning of an immortalized mammary cell line, CDβ, was utilized to isolate mammary-specific multipotent, ductal and alveolar progenitor clones. Molecular phenotyping showed that mammary progenitors express significantly higher levels of CK5.
Sca-1 expression was not associated with outgrowth potential of the clones. The unique expression of Sca-1 in stem and progenitor subpopulations has been proposed for many cell types. Sca-1 (Ly-6A), a member of Ly6
gene family, is a glycosylphosphatidylinositol-anchored cell surface protein. Sca-1, when combined with other markers, c-kit and the hematopoietic lineage, identifies hematopoietic stem cells in the bone marrow [34
]. However, its role as a stem or progenitor cell marker in the mammary gland has been controversial [35
CDβ clones that possess outgrowth potential contain significantly higher levels of CK5 expression. The expression of CK5 in mammary stem cells has been proposed previously [25
]. In human breast tissue, there is a small population of CK5-expressing cells (5%) that lack the expression of other markers of glandular, CK8, CK18, CK19 or myoepithelial cells (SMA) [25
]. Additionally, the stem cell subpopulation in normal mouse mammary gland (Lin-
) is enriched in the expression of CK5 and displays a basal phenotype [27
]. Dontu and colleagues [26
] showed that mammospheres were enriched in undifferentiated cells and expressed the basal markers CK5 and CK14. These data demonstrate that high-level CK5-expressing cells may represent an enriched population of stem and progenitor cells. It will be interesting to explore whether CK5 serves only as a stem cell marker or perhaps is coregulated with a critical stem cell gene.
In light of the important role of estrogen signaling during normal mammary development, it was surprising that the CDβ-derived outgrowths lacked ER expression. This may be because the CDβ-derived outgrowths express mutated p53
. It has been demonstrated that some of the p53
-null preneoplastic mammary outgrowths lack ER expression, although they do develop normally [37
]. Therefore, the requirement for ER expression may apply only to wild-type p53
-expressing mammary glands. Furthermore, ER-
mammary glands develop normally until puberty. However, there is stunted ductal outgrowth after puberty. Therefore, the outgrowths generated by the CDβ-derived clones may develop without the expression of ER because the CDβ cells lack a functional p53
gene and/or the outgrowths grow in response to prepubertal growth hormones such as the parathyroid hormone signaling pathway [18
]. The progesterone signaling pathway has also been shown to play an important role in mammary ductal branching morphogenesis and alveolar development, as well as in mammary tumorigenesis [18
]. Interestingly, the expression of PR was extranuclear in the outgrowths derived from the progenitor clones. It has been demonstrated that PR may bind and activate Src-1 and MAPK signaling pathway and thus promote the proliferation of breast cancer cells. These extranuclear effects of PR may mediate mammary hyperplasia and tumor progression in the outgrowths derived from the CDβ cells.
The CDβ cell line is unique in that transplantation of cells into the epithelium-free fat pads of syngeneic female mice generates mammary outgrowths that eventually progress to tumors over time in vivo. Mammary tumors eventually form because the CDβ cell line harbors two distinct p53 mutations. Thus, the model provides a unique opportunity to study the cellular and molecular mechanisms underlying mammary tumor progression beginning at the normal stages.
Figure shows a hypothetical model of mammary development based on the findings in this study. As illustrated, mammary development begins by asymmetric self-renewal in a stem cell, which generates multipotent and bipotent ductal and alveolar progenitor cells. Each ductal and alveolar progenitor cell is bipotent, expresses high CK5 levels and may give rise to luminal- and myoepithelial-restricted progenitors. The luminal and myoepithelial progenitors give rise to the ductal and alveolar structures. Upon commitment to transient amplification and differentiation, restricted progenitor cells may downregulate CK5 expression.
Figure 6 Hypothetical model of stem cell differentiation during mammary gland development. Mammary gland development begins by asymmetric self-renewal in a stem cell, which generates multipotent and bipotent ductal and alveolar progenitor cells. Ductal and alveolar (more ...)