Mammary stem cells, self-renewal pathways, and carcinogenesis

Abstract

The mammary gland epithelial components are thought to arise from stem cells that undergo both self-renewal and differentiation. Self-renewal has been shown to be regulated by the Hedgehog, Notch, and Wnt pathways and the transcription factor B lymphoma Mo-MLV insertion region 1 (Bmi-1). We review data about the existence of stem cells in the mammary gland and the pathways regulating the self-renewal of these cells. We present evidence that deregulation of the self-renewal in stem cells/progenitors might be a key event in mammary carcinogenesis. If 'tumor stem cells' are inherently resistant to current therapies, targeting stem cell self-renewal pathways might provide a novel approach for breast cancer treatment.

Figure 1

Experimental design for assessing self-renewal and differentiation potential of cells grown as mammospheres. (a) Self-renewal is assessed by evaluating the ability of mammosphere-derived cells to form new spheres, containing multipotent cells. (b) Differentiation into all the three mammary lineage types on collagen in the presence of serum [immunostained with lineage-specific markers: brown, ductal epithelial (ESA); purple, myoepithelial (CD10); red, alveolar (β-casein)]. (c) Generate complex ductal-alveolar structures in three-dimensional Matrigel culture. (d) Differentiation and self-renewal in vivo are tested by implanting human mammary epithelial cells into the cleared mammary fat pads of immunodeficient mice (NOD/SCID mice). EGF, epidermal growth factor.

Figure 2

A schematic diagram for the hedgehog (HH) signaling pathway. Ligands, such as Sonic Hedgehog (Shh), Indian Hedgehog (Ihh), and Desert Hedgehog (Dhh), are secreted by signaling cells and bind the transmembrane receptor patched (Ptch) in HH responding cells. In the absence of ligands, Ptch binds to Smoothened (Smo) and blocks Smo's function, whereas this inhibition is relieved in the presence of ligands, and Smo initiates a signaling cascade that results in the release of transcription factors Glis from cytoplasmic proteins fused (Fu) and suppressor of fused (SuFu). In the inactive situation, SuFu prevents Glis from translocating to the nucleus; in the active situation, Fu inhibits SuFu and Glis are released. Gli proteins translocate into the nucleus and control target gene transcription. The red lines and the agents in red show the inhibitors of this pathway with potential therapeutic value.

Figure 3

A schematic diagram for the Notch signaling pathway. Upon binding of the DSL ligand, Notch signaling is modulated by fringe, and Notch receptors are activated by serial cleavage events involving members of the ADAM (for 'a disintegrin and metalloproteinase') protease family, as well as an intramembrane cleavage regulated by γ-secretase (presenilin). This intramembrane cleavage is followed by translocation of the intracellular domain on Notch to the nucleus, where it acts on downstream targets. CBF, C promoter binding factor; HDAC, histone deacetylase; HAT, histone acetyltransferase.

Figure 4

A schematic diagram for the Wnt signaling pathway. (a) The canonical Wnt/β-catenin pathway. Canonical Wnt signaling requires the Frizzled (Fz) and low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) co-receptors to activate Dishevelled (Dsh). Then Dsh inhibits the activity of the β-catenin destruction complex (adenomatous polyposis coli (APC), axin, and glycogen synthase kinase-3 (GSK-3)), which phosphorylates β-catenin in the absence of the ligands. β-Catenin is stabilized and translocated to the nucleus, where it recruits transactivators to high mobility group (HMG)-box DNA-binding proteins of the lymphoid enhancer factor/T cell factor (LEF/TCF) family. (b) The noncanonical Wnt signaling pathway. Noncanonical Wnt signaling requires Frizzled receptors and the proteoglycan co-receptor Knypek. In this pathway, Dsh localizes to the cell membrane through its DEP domain. A main branch downstream of Dsh involves the small GTPases of the Rho family. Dsh activation of Rho requires the bridging molecule Daam1. Dsh can also stimulate calcium flux and the activation of the calcium-sensitive kinases protein kinase C (PKC) and calmodulin-dependent protein kinase II (CanKII). At the end, the activation of this pathway induces the complex and dynamic cellular response.

Figure 5

A hypothetic interacting model in the regulation of stem cell self-renewal by the Hedgehog signaling pathway, the Notch signaling pathway, the Wnt signaling pathway, and B lymphoma Mo-MLV insertion region 1 (Bmi-1). Interactions between the Hedgehog, Notch, and Wnt pathways and Bmi-1 are shown by solid arrows; interactions between stem cell self-renewal regulation by the pathways and Bmi-1 are shown by dashed arrows; the question marks represent the postulated interactions.