Multiple physical stresses induce γ-globin gene expression and fetal hemoglobin production in erythroid cells.

Increased fetal hemoglobin (HbF) expression is beneficial for ß-hemoglobinopathy patients; however, current inducing agents do not possess the ideal combination of efficacy, safety and ease of use. Better understanding the mechanisms involved in γ-globin gene induction is critical for designing improved therapies, as no complete mechanism for any inducing agent has been identified. Given the cytotoxic nature of most known inducing drugs, we hypothesized that γ-globin is a cell stress response gene, and that induction occurs via activation of cell stress signaling pathways. We tested this hypothesis by investigating the ability of physical stresses including heat-shock (HS), UV- and X-irradiation and osmotic shock to increase γ-globin gene expression in erythroid cells. Experiments in K562 and KU812 cells showed that each of these stresses increased steady-state γ-globin mRNA levels, but only after 3-5days of treatments. HS and UV also increased γ-globin mRNA and HbF levels in differentiating primary human erythroid cells. Mechanistic studies showed that HS affects γ-globin mRNA at multiple levels, including nascent transcription and transcript stability, and that induction is dependent on neither the master regulator of the canonical HS response, HSF1, nor p38 MAPK. Inhibitor panel testing identified PI3K inhibitor LY294002 as a novel inducing agent and revealed potential roles for NFκB and VEGFR/PDGFR/Raf kinases in HS-mediated γ-globin gene induction. These findings suggest that cell stress signaling pathways play an important role in γ-globin gene induction and may provide novel targets for the pharmacologic induction of fetal hemoglobin.

A cell stress signaling model of fetal hemoglobin induction: what doesn't kill red blood cells may make them stronger.

A major goal of hemoglobinopathy research is to develop treatments that correct the underlying molecular defects responsible for sickle cell disease and beta-thalassemia. One approach to achieving this goal is the pharmacologic induction of fetal hemoglobin (HbF). This strategy is capable of inhibiting the polymerization of sickle hemoglobin and correcting the globin chain imbalance of beta-thalassemia. Despite this promise, none of the currently available HbF-inducing agents exhibit the combination of efficacy, safety, and convenience of use that would make them applicable to most patients. The recent success of targeted drug therapies for malignant diseases suggests that this approach could be effective for developing optimal HbF-inducing agents. A first step in applying this approach is the identification of specific molecular targets. However, while >70 HbF-inducing agents have been described, neither molecular mechanisms nor target molecules have been definitively verified for any of these compounds. To help focus investigation in this area, we have reviewed known HbF-inducing agents and their proposed mechanisms of action. We find that in many cases, current models inadequately explain key experimental results. By integrating features of the erythropoietic stress model of HbF induction with data from recent intracellular signaling experiments, we have developed a new model that has the potential to explain several findings that are inconsistent with previous models and to unify most HbF-inducing agents under a common mechanism: cell stress signaling. If correct, this or related models could lead to new opportunities for development of targeted therapies for the beta-hemoglobinopathies.

MIF deficiency does not alter glucose homeostasis or adipose tissue inflammatory cell infiltrates during diet-induced obesity.

OBJECTIVE: Circulating macrophage migration inhibitory factor (MIF) levels have been shown to positively correlate with body mass index (BMI) in humans. Our objective in this study was to determine the effects of MIF deficiency in a model of high-fat diet-induced obesity. DESIGN AND METHODS: MIF wild type (MIF WT) and MIF deficient (MIF(-/-)) C57Bl/6J mice were fed a high-fat diet (HFD) for up to 15 weeks. Weight and metabolic responses were measured over the course of the disease. Immune cell infiltrates in visceral and subcutaneous adipose tissue were examined by flow cytometry. RESULTS: There was no difference in weight gain or adipose tissue mass in MIF(-/-) mice compared to MIF WT mice. Both groups fed HFD developed glucose intolerance at the same rate and had similar elevations in fasted blood insulin. MDSC abundance was evaluated and showed no MIF-dependent differences. Macrophages were elevated in the visceral adipose tissue of obese mice, but there was no difference between the two groups. CONCLUSIONS: While HFD feeding induced obesity with the expected perturbations in glucose homeostasis and adipose tissue inflammation, the presence or absence of MIF had no effect on any parameter examined.

Neither DNA hypomethylation nor changes in the kinetics of erythroid differentiation explain 5-azacytidine's ability to induce human fetal hemoglobin.

5-azacytidine (5-Aza) is a potent inducer of fetal hemoglobin (HbF) in people with beta-thalassemia and sickle cell disease. Two models have been proposed to explain this activity. The first is based on the drug's ability to inhibit global DNA methylation, including the fetal globin genes, resulting in their activation. The second is based on 5-Aza's cytotoxicity and observations that HbF production is enhanced during marrow recovery. We tested these models using human primary cells in an in vitro erythroid differentiation system. We found that doses of 5-Aza that produce near maximal induction of gamma-globin mRNA and HbF do not alter cell growth, differentiation kinetics, or cell cycle, but do cause a localized demethylation of the gamma promoter. However, when we reduced gamma promoter methylation to levels equivalent to those seen with 5-Aza or to the lower levels seen in primary fetal erythroid cells using DNMT1 siRNA and shRNA, we observed no induction of gamma-globin mRNA or HbF. These results suggest that 5-Aza induction of HbF is not the result of global DNA demethylation or of changes in differentiation kinetics, but involves an alternative, previously unrecognized mechanism. Other results suggest that posttranscriptional regulation plays an important role in the 5-Aza response.