Increasing C/Ebpα Expression Regulates the Transition of Hematopoietic Stem Cells During Development.

Abstract 1506

Poster Board I-529

During ontogeny, hematopoietic stem cells (HSCs) undergo a change from rapid dividing cells with high reconstitution ability to mainly quiescent cells with lower repopulation capacity. However, little is known about how this switch is regulated. Here we report that levels of C/EBPα, a transcription factor that is frequently disrupted in human acute myeloid leukemia, regulate the proliferation and self-renewal transition of HSCs during development. Loss of C/EBPα in adult mice resulted in a profound expansion of phenotypic HSCs and elevated proliferation rates. Limiting dilution transplantation to measure the frequency of competitive repopulation units (CRU) revealed an increase in the number of functional HSC in C/EBPα-/- mice. Serial transplantation of C/EBPα-/- bone marrow showed advanced reconstitution ability, indicating enhanced self-renewal ability. Interestingly, levels of C/EBPα in HSCs were significantly up-regulated 3 weeks after birth during which HSCs change from an actively cycling state to quiescence in bone marrow. When we conditionally inactivated C/EBPα in mice of different age, we observed a tight correlation between the age-specific levels of C/EBPα expression and the expansion of HSCs. Gene profiling analysis of C/EBPα-/- adult HSCs showed the up-regulation of oncogenes c-myc and n-myc, whose expression can regulate pluripotency and self-renewal of stem cells, as shown by recent induced pluripotent stem cell studies. Knocking down n-myc and c-myc expression in C/EBPα-/- HSCs using shRNA, we observed reduced proliferation and decreased colony formation in serial replating assay, which assesses the preservation of “self-renewal” in the progenitor cell compartment. Consistently, we observed down-regulation of n-myc in HSCs during the transition time, which is reciprocal to C/EBPα expression. Together, our data indicate C/EBPα as a key regulator of HSC self-renewal and proliferation during development, whose levels of expression might control the fetal to adult switch of HSC properties through regulating myc genes.