Identifying Quiescent Stem Cells in Hair Follicles.

Hair follicle stem cells (HFSCs) are noted for their relative quiescence and therefore can be distinguished from other cells by their differential history of cell division. Replicating cells can be labeled by pulsing the animals repeatedly with 5-bromo-2'-deoxyuridine (BrdU) or tritiated thymidine ([3H]TdR), thymidine analogs that get incorporated into DNA during DNA synthesis. Because dividing cells dilute the label after each cell division, frequently dividing cells will lose the label over time while slow cycling cells will retain the label and thus are termed label retaining cells (LRCs). [3H]TdR can be visualized by autoradiography and BrdU can be detected by immunofluorescence with anti-BrdU antibodies. Alternatively, a well-established tet-regulatable transgenic mouse model can be used to express histone H2B-GFP in epithelial proliferative cells and their dilution and retention of the GFP signal can be followed. In this chapter, we detail the steps to perform BrdU pulse-chase and H2B-GFP pulse-chase experiments to identify quiescent cells in the hair follicle.

TCF7L1 promotes skin tumorigenesis independently of ß-catenin through induction of LCN2.

The transcription factor TCF7L1 is an embryonic stem cell signature gene that is upregulated in multiple aggressive cancer types, but its role in skin tumorigenesis has not yet been defined. Here we document TCF7L1 upregulation in skin squamous cell carcinoma (SCC) and demonstrate that TCF7L1 overexpression increases tumor incidence, tumor multiplicity, and malignant progression in the chemically induced mouse model of skin SCC. Additionally, we show that downregulation of TCF7L1 and its paralogue TCF7L2 reduces tumor growth in a xenograft model of human skin SCC. Using separation-of-function mutants, we show that TCF7L1 promotes tumor growth, enhances cell migration, and overrides oncogenic RAS-induced senescence independently of its interaction with ß-catenin. Through transcriptome profiling and combined gain- and loss-of-function studies, we identified LCN2 as a major downstream effector of TCF7L1 that drives tumor growth. Our findings establish a tumor-promoting role for TCF7L1 in skin and elucidate the mechanisms underlying its tumorigenic capacity.