The Regulation of Signaling in Hematopietic Stem Cell Maintenance.

Abstract

Hematopoietic stem cells (HSCs) maintain themselves throughout life by undergoing self-renewing divisions and by differentiating to generate all the blood and immune system cells in the body. Tumor suppressors play an important role in regulating signaling pathways that maintain HSCs while avoiding leukemogenesis. Deficiency in the tumor suppressor Pten depletes HSCs but expands leukemia-initiating cells. Understanding this mechanistic difference could lead to anti-leukemia therapies with less toxicity to HSCs. Indeed, the mTOR inhibitor, rapamycin, blocks HSC depletion and leukemogenesis in Pten-deficient cells, raising the question of how mTOR activation depletes HSCs. In contrast to what occurs after FoxO1/3/4 deletion, we found that the depletion of Pten-deficient HSCs was not caused by oxidative stress and could not be blocked by N-acetyl-cysteine. Instead, Pten deletion induced the expression of p16Ink4a and p53 in HSCs, and p19Arf and p53 in other hematopoietic cells. Rapamycin treatment attenuated these increases. Analysis of compound mutant mice indicated that p53 suppressed leukemogenesis and promoted HSC depletion after Pten deletion. p16Ink4a also promoted HSC depletion but had a limited role suppressing leukemogenesis. p19Arf suppressed leukemogenesis but did not deplete HSCs. Pten deficiency and FoxO deficiency therefore deplete HSCs by different mechanisms. These results provide functional evidence that mTOR activation depletes stem cells by inducing a tumor suppressor response. Little is known about whether autophagic processes are active in HSCs and whether they contribute to HSC maintenance. FIP200 plays important roles in mammalian autophagy and other cellular functions, but its role in hematopoiesis has not been examined. We found that conditional deletion of FIP200 in hematopoietic cells led to impaired autophagy in the fetal liver, severe anemia, and perinatal lethality. FIP200 was also cell-autonomously required for the maintenance of fetal HSCs as FIP200-deleted HSCs were unable to reconstitute lethally irradiated recipients. FIP200 ablation increased the rate of cell-cycling in HSCs, which may have contributed to HSC depletion. Interestingly, FIP200-deleted HSCs exhibited increased mitochondrial mass and elevated reactive oxygen species levels. Our data identify FIP200 as a key intrinsic regulator of fetal HSCs and implicate a potential role for autophagy in the maintenance of fetal hematopoiesis and HSCs.