Scientists from the University of Pennsylvania say they have discovered a new signaling exchange that mediates normal mammary gland development by regulating the mammary stem cell niche, which was not previously understood. The finding indicates that mammary gland stem cells communicate with macrophages using Delta-like-ligand 1 (Dll1), which is part of the Notch signaling pathway. The team found that this molecular chatter is essential for the survival of the mammary stem cells, which leads to mammary gland development.
Because the Notch pathway and other molecular components of the communications between mammary stem cells and macrophages have been implicated in breast cancer genesis and spread, future research on the pathway in the context of cancer may bear crucial information for diagnosis and treatment. The current study (“Notch Ligand Dll1 Mediates Cross-Talk between Mammary Stem Cells and the Macrophageal Niche”) appears in Science.
“The stem cell niche is a specialized environment that dictates stem cell function during development and homeostasis. Here, we show that Dll1, a Notch pathway ligand, is enriched in mammary gland stem cells (MaSCs) and mediates critical interactions with stromal macrophages in the surrounding niche. Conditional deletion of Dll1 reduced the number of MaSCs and impaired ductal morphogenesis in the mammary gland,” write the investigators.
“Moreover, MaSC-expressed Dll1 activates Notch signaling in stromal macrophages, increasing their expression of Wnt family ligands such as Wnt3, Wnt10A, and Wnt16, thereby initiating a feed back loop that promotes the function of Dll1+ MaSCs. Together, these findings reveal functionally important cross-talk between MaSCs and their macrophageal niche through Dll1/Notch-mediated signaling.”
“Cancer cells are smart,” says Rumela Chakrabarti, Ph.D., an assistant professor of biomedical sciences in Penn's School of Veterinary Medicine. “They can actually hijack the normal cellular machinery to use for their benefit. I'm interested in the cellular signaling that is having an impact in normal development as well as in the initiation and development of breast cancer.”
Dr. Chakrabarti and colleagues compared the gene expression profiles of mammary stem cells versus non–stem cells, and found Dll1 to be among the genes most differentially expressed between the two cell types. Honing in, they developed a mouse model that lacked Dll1 predominantly in the mammary gland. Through every life stage, these mice had problems with mammary gland development, and females did not produce milk after giving birth. They also had significantly fewer mammary stem cells and macrophages compared with normal mice.
“People have identified other genes that affect mammary gland development,” Dr. Chakrabarti says, “but usually the defect is temporary and the mammary gland can somehow bypass it over time. With this gene, we found it cannot be compensated, it affects every stage of development, including pregnancy.”
Using additional reporter mouse models, developed by collaborators at The Netherlands' Hubrecht Institute and New York University, the researchers tracked mammary stem cells based on associated fluorescent color, confirming that Dll1 was indeed a marker of stem cells that were able to give rise to every cell type in the mammary gland.
Because Dll1 was known to be a ligand of Notch signaling, or a molecule that binds to another molecule, the next step was to find its “receiving” molecule. Screening a variety of cell types that exist in the environment of the mammary gland, they narrowed in on macrophages. Working with Ming Li, Ph.D., of Memorial Sloan Kettering Cancer Center, who provided mouse models where Notch signaling in macrophages is deleted, they found that mammary stem cells depended on macrophages to function normally.
Further gene expression studies elucidated the relationship, showing that the stem cells used Dll1 to communicate with macrophages, and the macrophages released the signaling proteins, such as Wnt3, Wnt10, and Wnt16, to support the environment around mammary stem cells, allowing them to thrive.
The fact that both Wnt and Notch signaling are involved in supporting the mammary stem cell niche provides a strong clue that the relationship and signaling pathways that link stem cells and macrophages may play a role in breast cancer, as aberrant functioning of both of these pathways has previously been shown to present in breast cancer.
“That is where the lab is now looking,” says Dr. Chakrabarti. “How are these pathways functioning in breast cancer?”
If changes in Dll1 expression are found to play a role in the early stages of cancer, the molecule could offer a promising biomarker and a target for cancer therapy. As a ligand, it could be zeroed in on without the toxicity of some other types of drugs that operate in the same pathway, which function by inhibiting receptors and sometimes have problematic side effects.
“The lab is very interested in detecting early changes in cancer,” Dr, Chakrabarti says. “That's why we are looking very closely at normal development and physiology.”