Oct4, along with Sox2 and Klf4 (SK), can induce pluripotency but structurally similar factors like Oct6 cannot. To decode why Oct4 has this unique ability, we compare Oct4-binding, accessibility patterns and transcriptional waves with Oct6 and an Oct4 mutant defective in the dimerization with Sox2 (Oct4
defSox2
). We find that initial silencing of the somatic program proceeds indistinguishably with or without Oct4. Oct6 mitigates the mesenchymal-to-epithelial transition and derails reprogramming. These effects are a consequence of differences in genome-wide binding, as the early binding profile of Oct4
defSox2
resembles Oct4, whilst Oct6 does not bind pluripotency enhancers. Nevertheless, in the Oct6-SK condition many otherwise Oct4-bound locations become accessible but chromatin opening is compromised when Oct4
defSox2
occupies these sites. We find that Sox2 predominantly facilitates chromatin opening, whilst Oct4 serves an accessory role. Formation of Oct4/Sox2 heterodimers is essential for pluripotency establishment; however, reliance on Oct4/Sox2 heterodimers declines during pluripotency maintenance.
Loss of CDKN2A/p16INK4A in hematopoietic stem cells is associated with enhanced self-renewal capacity and might facilitate progression of damaged stem cells into pre-cancerous cells that give rise to leukemia. This is also reflected by the frequent loss of the INK4A locus in acute lymphoblastic T-cell leukemia. T-cell acute lymphoblastic leukemia cells designed to conditionally express p16INK4A arrest in the G 0 /G 1 phase of the cell cycle and show increased sensitivity to glucocorticoid-and tumor necrosis factor receptor superfamily 6-induced apoptosis. To investigate the underlying molecular mechanism for increased death sensitivity, we interfered with specific steps of apoptosis signaling by expression of anti-apoptotic proteins. We found that alterations in cell death susceptibility resulted from changes in the composition of pro-and anti-apoptotic BCL2 proteins, i.e. repression of MCL1, BCL2, and PMAIP1/ Noxa and the induction of pro-apoptotic BBC3/Puma. Interference with Puma induction by short hairpin RNA technology or retroviral expression of MCL1 or BCL2 significantly reduced both glucocorticoid-and FAS-induced cell death in p16INK4A -reconstituted leukemia cells. These results suggest that Puma, in concert with MCL1 and BCL2 repression, critically mediates p16 INK4A -induced death sensitization and that in human T-cell leukemia the deletion of p16 INK4A confers apoptosis resistance by shifting the balance of pro-and anti-apoptotic BCL2 proteins toward apoptosis protection.
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