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Ultimate success of hematopoietic stem cell transplantation (HSCT) depends not only on donor HSCs themselves but also on the host environment. Total body irradiation is a component in various host conditioning regimens for HSCT. It is known that ionizing radiation exerts "bystander effects" on nontargeted cells and that HSCs transplanted into irradiated recipients undergo proliferative exhaustion. However, whether irradiated recipients pose a proliferation-independent bystander effect on transplanted HSCs is unclear. In this study, we found that irradiated mouse recipients significantly impaired the long-term repopulating ability of transplanted mouse HSCs shortly (ϳ 17 hours) after exposure to irradiated hosts and before the cells began to divide. There was an increase of acute cell death associated with accelerated proliferation of the bystander hematopoietic cells. This effect was marked by dramatic down-regulation of c-Kit, apparently because of elevated reactive oxygen species. Administration of an antioxidant chemical, N-acetylcysteine, or ectopically overexpressing a reactive oxygen species scavenging enzyme, catalase, improved the function of transplanted HSCs in irradiated hosts. Together, this study provides evidence for an acute negative, yet proliferation-independent, bystander effect of irradiated recipients on transplanted HSCs, thereby having implications for HSCT in both experimental and clinical scenarios in which total body irradiation is involved. (Blood. 2012;119(15):3629-3637) IntroductionHematopoietic stem cell transplantation (HSCT) may provide cures for many congenital diseases, immunodeficiencies, and hematopoietic or nonhematopoietic malignancies. 1 However, the broader use of HSCT in patients is hindered in part by the limited number of HSCs contained in each harvest. This issue may prove especially problematic in cases when HSCs are collected from human cord blood, where the number of HSCs per harvest is not sufficient for an adult patient. 2 Successful HSC engraftment depends not only on the dosage and quality of transplanted HSCs but also the conditioning of recipients. Given the current unsatisfactory status of ex vivo HSC expansion, manipulating the bone marrow (BM) microenvironment or the stem cell niche has been thought to be an attractive strategy [3][4][5][6] to improve the outcome of HSCT.On transplantation, HSCs go through a series of processes, including homing, lodgment, proliferation, and differentiation, all of which are important determinants for short-and long-term engraftment. One of the major factors that may impact these processes is the conditioning regimen administered to the HSCT recipient before transplantation to reduce the chances of graft rejection and in the case of cancer treatment, to reduce the burden of disease and to make niches available for donor stem cells. Of particular interest is the effects of total body irradiation (TBI) on these processes as this remains a component in many HSCT conditioning regimens and has been associated with a number of eff...

Mechanisms underlying the reprogramming process of induced pluripotent stem cells remain poorly defined. Like tumorigenesis, generation of induced pluripotent stem cells was shown to be suppressed by the Trp53 (p53) pathway, at least in part via p21Cdkn1a (p21)-mediated cell cycle arrest. Here we examine the role of PUMA, a pro-apoptotic mediator of p53, during somatic reprogramming in comparison to p21 in the p53 pathway. Using mouse strains deficient in these molecules, we demonstrate that PUMA is an independent mediator of the negative effect of p53 on induced pluripotent stem cell induction. PUMA deficiency leads to a better survival rate associated with reduced DNA damage and fewer chromosomal aberrations in induced pluripotent stem cells, whereas loss of p21 or p53 results in an opposite outcome. Given these new findings, PUMA may serve as a distinct and more desirable target in the p53 pathway for induced pluripotent stem cell generation, thereby having important implications for potential therapeutic applications of induced pluripotent stem cells.

Among cyclin-dependent kinase inhibitors that control the G1 phase in cell cycle, only p18 and p27 can negatively regulate haematopoietic stem cell (HSC) self-renewal. In this manuscript, we demonstrate that p18 protein is a more potent inhibitor of HSC self-renewal than p27 in mouse models and its deficiency promoted HSC expansion in long-term culture. Single-cell analysis indicated that deleting p18 gene favoured self-renewing division of HSC in vitro. Based on the structure of p18 protein and in-silico screening, we further identified novel small-molecule inhibitors that can specifically block the activity of p18 protein. Our selected lead compounds were able to expand functional HSCs in a short-term culture. Thus, these putative small-molecule inhibitors for p18 protein are valuable for further dissecting the signalling pathways of stem cell self-renewal and may help develop more effective chemical agents for therapeutic expansion of HSC.