Stem cell homing and repopulation are not well understood. The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 were found to be critical for murine bone marrow engraftment by human severe combined immunodeficient (SCID) repopulating stem cells. Treatment of human cells with antibodies to CXCR4 prevented engraftment. In vitro CXCR4-dependent migration to SDF-1 of CD34+CD38-/low cells correlated with in vivo engraftment and stem cell function. Stem cell factor and interleukin-6 induced CXCR4 expression on CD34+ cells, which potentiated migration to SDF-1 and engraftment in primary and secondary transplanted mice. Thus, up-regulation of CXCR4 expression may be useful for improving engraftment of repopulating stem cells in clinical transplantation.

Elevated expression of soluble vascular endothelial growth factor receptor-1 (sFlt-1) in preeclampsia plays a major role in the pathogenesis of this serious disorder of human pregnancy. Although reduced placental oxygenation is thought to be involved in the pathogenesis of preeclampsia, it is unclear how oxygen regulates placental sFlt-1 expression. The aims herein were to investigate sFlt-1 expression in in vivo and in vitro physiological and pathological models of human placental hypoxia and to understand the role of hypoxia inducible factor-1 (HIF-1) in regulating the expression of this molecule. sFlt-1 expression in placental villi was significantly increased under physiological low oxygen conditions in early first-trimester and in high-altitude placentae, as well as in pathological low oxygen conditions, such as preeclampsia. In high-altitude and in preeclamptic tissue, sFlt-1 localized within villi to perivascular regions, the syncytiotrophoblast layer, and syncytial knots. In first-trimester villous explants, low oxygen, but not hypoxia-reoxygenation (HR), increased sFlt-1 expression. Moreover, exposure of villous explants to dimethyloxalyl-glycin, a pharmacological inhibitor of prolyl-hydroxylases, which mimics hypoxia by increasing HIF-1alpha stability, increased sFlt-1 expression. Conversely, HIF-1alpha knockdown using antisense oligonucleotides, decreased sFlt-1 expression. In conclusion, placental sFlt-1 expression is increased by both physiologically and pathologically low levels of oxygen. This oxygen-induced effect is mediated via the transcription factor HIF-1. Low oxygen levels, as opposed to intermittent oxygen tension (HR) changes, play an important role in regulating sFlt-1 expression in the developing human placenta and hence may contribute to the development of preeclampsia.

For clarity, the authors are updating Figure 1, the flow chart for this study, to better illustrate the excluded samples and criteria for exclusion. The HTML and PDF versions of the figure and its legend have been updated.

In preeclampsia, poor placental perfusion may result in maternal endothelial dysfunction, but the pathways involved are largely unknown. Candidate placental mediators include products of oxidative stress released into the maternal circulation. Xanthine oxidase has been implicated in postischemic-reperfusion injury via the generation of superoxide anion radicals (superoxide; O 2 .؊ ) and hydrogen peroxide. We examined placentas and placental bed curettings and/or biopsies from preeclamptic control pregnant women to test the hypothesis that xanthine oxidase is a mediator of oxidative stress in placentas from women with preeclampsia. The expression of xanthine dehydrogenase/xanthine oxidase holoenzyme and the activity of xanthine oxidase, the isoform known to generate reactive oxygen species, were increased in a subpopulation of cytotrophoblasts of preeclamptic women. Additionally, the expression of superoxide dismutase, which would scavenge superoxide produced by xanthine oxidase, was reduced in the same cells. Preeclampsia, a pregnancy-specific disorder, is the leading cause of maternal mortality in the Western world and increases perinatal mortality fivefold. The clinical diagnosis is based on the new onset of hypertension and the appearance of proteinuria and edema during pregnancy. It is evident, however, that these findings are a small component of a multisystemic disease that reduces perfusion to virtually all of the organs in the body. 1 The hypothesis has been advanced that much of the disease could be explained by alterations in the function of vascular endothelium. Increasing data support this hypothesis. 2 The more complete hypothesis posits that the placenta, likely in response to reduced perfusion, produces a circulating factor(s) that alters endothelial cell function. 2 Among the candidate molecules are products resulting from oxidative stress. 3 Oxidative stress, the result of freeradical generation in excess of protective mechanisms, is suggested as an important component of other disorders that affect endothelial function, including diabetes and atherosclerosis. 4,5 Evidence for oxidative stress in preeclampsia includes elevated lipid hydroperoxides or their metabolites and reduced plasma antioxidant activity. 3,6 -8 Xanthine oxidase, which metabolizes xanthine and hypoxanthine to uric acid with production of superoxide and hydrogen peroxide, is an integral mediator of reactive oxygen species generation in many settings. Usually, this enzyme is present as the holoenzyme xanthine dehydrogenase/xanthine oxidase (XDH/XO). The dehydrogenase isoform (XDH) converts purines to uric acid with reducton of nicotinamide-adenine dinucleotide (NAD) to reduced NAD (NADH). With hypoxia and in response to several cytokines, XDH/XO synthesis increases, and the conversion of the enzyme to the XO form is enhanced. 9 Reduced organ perfusion and subsequent reperfusion are proposed to result in increased XO during reduced flow and in subsequent formation of reactive oxygen species with reperfusion. The inadequate pla...