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C/EBP
regulates cell cycle and self-renewal of human limbal stem cells
Correspondence to Michele De Luca: michele.deluca{at}unimore.it
Human limbal stem cells produce transit amplifying progenitors that migrate centripetally to regenerate the corneal epithelium. Coexpression of CCAAT enhancer binding protein
(C/EBP
), Bmi1, and
Np63
identifies mitotically quiescent limbal stem cells, which generate holoclones in culture. Upon corneal injury, a fraction of these cells switches off C/EBP
and Bmi1, proliferates, and differentiates into mature corneal cells. Forced expression of C/EBP
inhibits the growth of limbal colonies and increases the cell cycle length of primary limbal cells through the activity of p27Kip1 and p57Kip2. These effects are reversible; do not alter the limbal cell proliferative capacity; and are not due to apoptosis, senescence, or differentiation. C/EBP
, but not
Np63
, indefinitely promotes holoclone self-renewal and prevents clonal evolution, suggesting that self-renewal and proliferation are distinct, albeit related, processes in limbal stem cells. C/EBP
is recruited to the chromatin of positively (p27Kip1 and p57Kip2) and negatively (p16INK4A and involucrin) regulated gene loci, suggesting a direct role of this transcription factor in determining limbal stem cell identity.
Abbreviations used in this paper: 4OHT, 4-hydroxytamoxifen; C/EBP, CCAAT enhancer binding protein; CFE, colony forming efficiency; ChIP, chromatin immunoprecipitation; ER, estrogen receptor; IRES, internal ribosomal entry site; NGFr, NGF receptor; PGK, phosphoglycerokinase; TA, transit amplifying.
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