C/EBPδ regulates cell cycle and self-renewal of human limbal stem cells.

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.