Role of c-Myc haploinsufficiency in the maintenance of HSCs.

Here we report a dosage effect of c-Myc on hematopoiesis and its distinct role in mediating the Wnt/b-catenin pathway in hematopoietic stem cell (HSC) and bone marrow niche cells. We showed that c-Myc haploinsufficiency led to ineffective hematopoiesis by inhibiting HSC self-renewal and quiescence, and promoting its apoptosis. We have identified Nr4a1, Nr4a2 and Jmjd3, which are critical for the maintenance of HSC functions, as previously unrecognized downstream targets of c-Myc in HSCs. c-Myc directly binds to the promoter regions of Nr4a1, Nr4a2 and Jmjd3 and regulates their expression. Our results revealed that Nr4a1 and Nr4a2 mediated the function of c-Myc in regulating HSC quiescence while all three genes contribute to the function of c-Myc in the maintenance of HSC survival. Adenomatous polyposis coli (Apc) is a negative regulator of the Wnt/b-catenin pathway. We have provided the first evidence that Apc haploinsufficiency induced a blockage of erythroid lineage differentiation through promoting expression and secretion of IL6 in bone marrow endothelial cells. We found that c-Myc haploinsufficiency failed to rescue defective function of Apc-deficient HSCs in vivo but it was sufficient to prevent the development of severe anemia in Apc heterozygous mice, and to significantly prolong survival of such mice. Furthermore, we showed that c-Myc mediated Apc loss-induced IL6 secretion in endothelial cells, and c-Myc haploinsufficiency reversed the negative effect of Apc deficient endothelial cells on erythroid cell differentiation. Our studies indicate that c-Myc has a context-dependent role in mediating the function of Apc in hematopoiesis.