Runx1 Directly Promotes Proliferation of Hair Follicle Stem Cells and Epithelial Tumor Formation in Mouse Skin

Abstract

Runx1/AML1 is a transcription factor implicated in tissue stem cell regulation and belongs to the small Runx family of cancer genes. In the hair follicle (HF), Runx1 epithelial deletion in morphogenesis impairs normal adult hair homeostasis (cycle) and blocks adult hair follicle stem cells (HFSCs) in quiescence. Here, we show that these effects are overcome later in adulthood. By deleting Runx1 after the end of morphogenesis, we demonstrate its direct role in promoting anagen onset and HFSC proliferation. Runx1 deletion resulted in cyclin-dependent kinase inhibitor Cdkn1a (p21) upregulation. Interfering with Runx1 function in cultured HFSCs impaired their proliferation and normal G 0 /G1 and G 1 /S cell cycle progression. The proliferation defect could be rescued by Runx1 readdition or by p21 deletion. Chemically induced skin tumorigenesis in mice turned on broad Runx1 expression in regions of the skin epithelium, papillomas, and squamous cell carcinomas. In addition, it revealed reduced rates of tumor formation in the absence of Runx1 that were accompanied by decreased epithelial levels of phospho-Stat3. Runx1 protein expression was similar in normal human and mouse hair cycles. We propose that Runx1 may act as a skin oncogene by directly promoting proliferation of the epithelial cells.

Runx1 and p21 synergistically limit the extent of hair follicle stem cell quiescence in vivo

“…Loss of Runx1 in cell culture induced a severe proliferation defect and p21 loss rescued this phenotype (16), suggesting an important genetic interaction between Runx1 and p21 in vitro. To ask whether this interaction works by the direct binding of Runx1 to the p21 promoter, which Runx1 may inhibit, we performed chromatin immunoprecipitation (ChIP) on mouse keratinocytes.…”

“…1E). Indeed, p21 was up-regulated in the bulge when we previously deleted Runx1 at anagen (16). Here we isolated CD34 + /α6 + bulge cells from Runx1 KO mice at quiescence [postnatal day (PD) 21] before the onset of the Runx1 phenotype.…”

“…In particular Runx1, a transcription factor of the Runt family member of cancer genes (14), promotes HFSC activation from quiescence to migrate, proliferate, and differentiate at telogen (15) and self-renew at anagen (16) and controls timely emergence of HFSCs and proper maturation during embryogenesis (17). Moreover, Runx1 is indispensable for skin tumorigenesis and for keratinocyte growth in culture.…”

“…Moreover, Runx1 is indispensable for skin tumorigenesis and for keratinocyte growth in culture. Interestingly, Runx1 deletion in mouse skin epithelium up-regulates p21 in the HFSCs (16). p21 is a tumor supressor downstream of p53 known to arrest the cell cycle in response to DNA damage and telomere shortening (18).…”

“…Moreover, p21 KO mouse keratinocytes showed increased proliferation in vitro and enhanced short-term engraftment ability in vivo (19). Runx1 interacts genetically with p21 in cultured keratinocytes (16), but the significance of this interaction in the normal tissue or in skin tumorigenesis is unknown.…”