Ring1B promotes hepatic stem/progenitor cell expansion through simultaneous suppression of Cdkn1a and Cdkn2a in mice.
Polycomb-group (PcG) proteins play crucial roles in self-renewal of stem cells by suppressing a host of genes through histone modifications. Identification of the downstream genes of PcG proteins is essential for elucidation of the molecular mechanisms of stem cell self-renewal. However, little is known about the PcG target genes in tissue stem cells. We found that the PcG protein, Ring1B, which regulates expression of various genes through monoubiquitination of histone H2AK119, is essential for expansion of hepatic stem/progenitor cells. In mouse embryos with a conditional knockout of Ring1B, we found that the lack of Ring1B inhibited proliferation and differentiation of hepatic stem/progenitor cells and thereby inhibited hepatic organogenesis. These events were characterized by derepression of cyclin-dependent kinase inhibitors (CDKIs) Cdkn1a and Cdkn2a, known negative regulators of cell proliferation. We conducted clonal culture experiments with hepatic stem/progenitor cells to investigate the individual genetic functions of Ring1B, Cdkn1a, and Cdkn2a. The data showed that the cell-cycle inhibition caused by Ring1B depletion was reversed when Cdkn1a and Cdkn2a were suppressed simultaneously, but not when they were suppressed individually.


CONCLUSIONS
Our results show that expansion of hepatic stem/progenitor cells requires Ring1B-mediated epigenetic silencing of Cdkn1a and Cdkn2a, demonstrating that Ring1B simultaneously regulates multiple CDKIs in tissue stem/progenitor cells.
Department of Regenerative Medicine, Yokohama City University Graduate School of Medicine, Yokohama, Japan.
Hiroyuki
H
Koike
Yasuharu
Y
Ueno
Takako
T
Naito
Tomoya
T
Shiina
Susumu
S
Nakata
Rie
R
Ouchi
Yuta
Y
Obana
Keisuke
K
Sekine
Yun-Wen
YW
Zheng
Takanori
T
Takebe
Kyo-ichi
K
Isono
Haruhiko
H
Koseki
Hideki
H
Taniguchi
D000818:Animals; D002454:Cell Differentiation / Q000502:physiology; D049109:Cell Proliferation; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000235:genetics / Q000378:metabolism*; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D053595:Embryonic Stem Cells / Q000166:cytology*; D044127:Epigenesis, Genetic / Q000502:physiology; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000502:physiology; D008099:Liver / Q000166:cytology* / Q000196:embryology / Q000502:physiology; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D038081:Organogenesis / Q000502:physiology; D063150:Polycomb Repressive Complex 1 / Q000235:genetics / Q000378:metabolism*; D011247:Pregnancy; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism*
C493957:Cdkn1a protein, mouse; C417743:Cdkn2a protein, mouse; D019941:Cyclin-Dependent Kinase Inhibitor p16; D050759:Cyclin-Dependent Kinase Inhibitor p21; D063150:Polycomb Repressive Complex 1; C489842:Rnf2 protein, mouse; D044767:Ubiquitin-Protein Ligases