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

Abstract

Mechanisms of tissue stem cell (SC) quiescence control are important for normal homeostasis and for preventing cancer. Cyclin-dependent kinase inhibitors (CDKis) are known inhibitors of cell cycle progression. We document CDKis expression in vivo during hair follicle stem cell (HFSC) homeostasis and find p21 (cyclin-dependent kinase inhibitor 1a, Cdkn1a), p57, and p15 up-regulated at quiescence onset. p21 appears important for HFSC timely onset of quiescence. Conversely, we find that Runx1 (runt related transcription factor 1), which is known for promoting HFSC proliferation, represses p21, p27, p57, and p15 transcription in HFSC in vivo. Intriguingly, in cell culture, tumors, and normal homeostasis, Runx1 and p21 interplay modulates proliferation in opposing directions under the different conditions. Unexpectedly, Runx1 and p21 synergistically limit the extent of HFSC quiescence in vivo, which antagonizes the role of p21 as a cell cycle inhibitor. Importantly, we find in cultured keratinocytes that Runx1 and p21 bind to the p15 promoter and synergistically repress p15 mRNA transcription, thereby restraining cell cycle arrest. This documents a surprising ability of a CDKi (p21) to act as a direct transcriptional repressor of another CDKi (p15). We unveil a robust in vivo mechanism that enforces quiescence of HFSCs, and a context-dependent role of a CDKi (p21) to limit quiescence of SCs, potentially by directly down-regulating mRNA levels of (an)other CDKi(s).AML1 | skin | malignancy | pulse-chase | H2B-GFP

High Runx1 Levels Promote a Reversible, More-Differentiated Cell State in Hair-Follicle Stem Cells during Quiescence

“…Previously we showed that Runx1, a transcription factor from the Runt family (Blyth et al, 2005) is highly expressed in HG cells and is essential for their activation/proliferation and subsequent anagen onset (Hoi et al, 2010; Lee et al, 2013; Osorio et al, 2008; Scheitz et al, 2012). Runx1 is even more highly expressed in the epithelial strand at late catagen (Fig.…”

Linking chromatin dynamics, cell fate plasticity, and tissue homeostasis in adult mouse hair follicle stem cells

“…For in vivo FRAP, tail explants of K5-tTA x pTRE-H2BGFP (13, 21) mice were imaged in PBS within 30 minutes from surgery. For in vitro FRAP, keratinocytes from K14-rtTA (14) × pTRE-H2BGFP (13) mice were cultured on MatTek dishes and treated with doxycycline (2 μg/mL) for at least 48 hours to induce H2B-GFP expression.…”

“…A previously described method was used for RNA extraction and qRT-PCR (21). To calculate the reprograming efficiency, we first used a standard ΔΔCT method to measure the relative fold enrichment of target over control ( Gapdh ) across different populations.…”

SOX9 accelerates ESC differentiation to three germ layer lineages by repressing SOX2 expression through P21 (WAF1/CIP1)

“…Variable outcomes are likely to be correlated with the relative activation of P21. P21 regulates functions of many progenitors and stem cells, including cancer stem cells, neural stem cells (NSCs), hematopoietic stem cells (HSCs), hair follicle stem cells, mesenchymal stem cells and ESC/iPSCs (van den Bos et al, 1998;Kippin et al, 2005;Orford and Scadden, 2008;Moriguchi et al, 2010;Dolezalova et al, 2012;Lee et al, 2013). P21 governs self-renewal of colon cancer-initiating cells through cell-cycle restriction and protection from DNA damage accumulation (O'Brien et al, 2012).…”