Fetal inhibition of inflammation improves disease phenotypes in harlequin ichthyosis

Harlequin ichthyosis (HI) is a severe skin disease which leads to neonatal death in ∼50% of cases. It is the result of mutations in ABCA12, a protein that transports lipids required to establish the protective skin barrier needed after birth. To better understand the life-threatening newborn HI phenotype, we analysed the developing epidermis for consequences of lipid dysregulation in mouse models. We observed a pro-inflammatory signature which was characterized by chemokine upregulation in embryonic skin which is distinct from that seen in other types of ichthyosis. Inflammation also persisted in grafted HI skin. To examine the contribution of inflammation to disease development, we overexpressed interleukin-37b to globally suppress fetal inflammation, observing considerable improvements in keratinocyte differentiation. These studies highlight inflammation as an unexpected contributor to HI disease development in utero, and suggest that inhibiting inflammation may reduce disease severity.

ABCA12 regulates ABCA1-dependent cholesterol efflux from macrophages and the development of atherosclerosis

ABCA12 is involved in the transport of ceramides in skin, but it may play a wider role in lipid metabolism. We show that, in Abca12-deficient macrophages, cholesterol efflux failed to respond to activation with LXR agonists. Abca12 deficiency caused a reduction in the abundance of Abca1, Abcg1, and Lxrβ. Overexpression of Lxrβ reversed the effects. Mechanistically, Abca12 deficiency did not affect expression of genes involved in cholesterol metabolism. Instead, a physical association between Abca1, Abca12, and Lxrβ proteins was established. Abca12 deficiency enhanced interaction between Abca1 and Lxrβ and the degradation of Abca1. Overexpression of ABCA12 in HeLa-ABCA1 cells increased the abundance and stability of ABCA1. Abca12 deficiency caused an accumulation of cholesterol in macrophages and the formation of foam cells, impaired reverse cholesterol transport in vivo, and increased the development of atherosclerosis in irradiated Apoe(-/-) mice reconstituted with Apoe(-/-)Abca12(-/-) bone marrow. Thus, ABCA12 regulates the cellular cholesterol metabolism via an LXRβ-dependent posttranscriptional mechanism.

A model for studying the hemostatic consumption or destruction of platelets

A fundamental issue in understanding homeostasis of the hematopoietic system is to what extent intrinsic and extrinsic factors regulate cell fate. We recently revisited this issue for the case of blood platelets and concluded that platelet life span is largely regulated by internal factors, in contrast to the long-held view that accumulated damage from the environment triggers clearance. However, it is known that in humans there is an ongoing fixed requirement for platelets to maintain hemostasis and prevent bleeding; hence a proportion of platelets may be consumed in such processes before the end of their natural life span. Whether it is possible to detect this random loss of platelets in normal individuals at steady-state is unknown. To address this question, we have developed a mathematical model that independently incorporates age-independent random loss and age-dependent natural senescent clearance. By fitting to population survival curves, we illustrate the application of the model in quantifying the fixed requirement for platelets to maintain hemostasis in mice, and discuss the relationship with previous work in humans. Our results suggest a higher requirement for platelets in mice than in humans, however experimental uncertainty in the data limits our ability to constrain this quantity. We then explored the relationship between experimental uncertainty and parameter constraint using simulated data. We conclude that in order to provide useful constraint on the random loss fraction the standard error in the mean of the data must be reduced substantially, either through improving experimental uncertainty or increasing the number of experimental replicates to impractical levels. Finally we find that parameter constraint is improved at higher values of the random loss fraction; thus the model find utility in situations where the random loss fraction is expected to be high, for example during active bleeding or some types of thrombocytopenia.

Translation inhibitors induce cell death by multiple mechanisms and Mcl-1 reduction is only a minor contributor

There is significant interest in treating cancers by blocking protein synthesis, to which hematological malignancies seem particularly sensitive. The translation elongation inhibitor homoharringtonine (Omacetaxine mepesuccinate) is undergoing clinical trials for chronic myeloid leukemia, whereas the translation initiation inhibitor silvestrol has shown promise in mouse models of cancer. Precisely how these compounds induce cell death is unclear, but reduction in Mcl-1, a labile pro-survival Bcl-2 family member, has been proposed to constitute the critical event. Moreover, the contribution of translation inhibitors to neutropenia and lymphopenia has not been precisely defined. Herein, we demonstrate that primary B cells and neutrophils are highly sensitive to translation inhibitors, which trigger the Bax/Bak-mediated apoptotic pathway. However, contrary to expectations, reduction of Mcl-1 did not significantly enhance cytotoxicity of these compounds, suggesting that it does not have a principal role and cautions that strong correlations do not always signify causality. On the other hand, the killing of T lymphocytes was less dependent on Bax and Bak, indicating that translation inhibitors can also induce cell death via alternative mechanisms. Indeed, loss of clonogenic survival proved to be independent of the Bax/Bak-mediated apoptosis altogether. Our findings warn of potential toxicity as these translation inhibitors are cytotoxic to many differentiated non-cycling cells.

Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Deletion of Bak and Bax, the effectors of mitochondrial apoptosis, does not affect platelet production, however, loss of prosurvival Bcl-x_L results in megakaryocyte apoptosis and failure of platelet shedding.

It is believed that megakaryocytes undergo a specialized form of apoptosis to shed platelets. Conversely, a range of pathophysiological insults, including chemotherapy, are thought to cause thrombocytopenia by inducing the apoptotic death of megakaryocytes and their progenitors. To resolve this paradox, we generated mice with hematopoietic- or megakaryocyte-specific deletions of the essential mediators of apoptosis, Bak and Bax. We found that platelet production was unperturbed. In stark contrast, deletion of the prosurvival protein Bcl-x_L resulted in megakaryocyte apoptosis and a failure of platelet shedding. This could be rescued by deletion of Bak and Bax. We examined the effect on megakaryocytes of three agents that activate the intrinsic apoptosis pathway in other cell types: etoposide, staurosporine, and the BH3 mimetic ABT-737. All three triggered mitochondrial damage, caspase activation, and cell death. Deletion of Bak and Bax rendered megakaryocytes resistant to etoposide and ABT-737. In vivo, mice with a Bak^−/− Bax^−/− hematopoietic system were protected against thrombocytopenia induced by the chemotherapeutic agent carboplatin. Thus, megakaryocytes do not activate the intrinsic pathway to generate platelets; rather, the opposite is true: they must restrain it to survive and progress safely through proplatelet formation and platelet shedding.

Abstract 2613: Histone deacetylase inhibitor induced thrombocytopenia occurs due to inhibition platelet shedding by megakaryocytes, via increased phosporylation of myosin light chain

Introduction: Histone deacetylase inhibitors (HDACi) are a novel group of anti-cancer drugs with marked efficacy in haematological malignancy, for which thrombocytopenia (TCP) is the predominant dose limiting toxicity, currently limiting their use in combination treatment strategies. Using the HDAC 1/2 selective HDACi romidepsin and the pan-HDACi panobinostat, we have investigated the processes which underscore this significant clinical problem. Experimental procedures and data: We have demonstrated TCP is not due to myelosuppression, but decreased platelet release from megakaryocytes (MK) based on: 1) platelet half-life studies utilizing NHS-biotin to label circulating platelets, and reticulated platelet assays using thiazole orange staining, 2) studies in Bak-/- and Bak-/-Bax-/- bone marrow reconstituted mice which excluded HDACi-induced platelet apoptosis, and 3) bone marrow (BM) sections showing increased MK number in mice treated with HDACi compared to controls. Increases in thrombopoietin (TPO) were seen in thrombocytopenic mice, and using c-Mpl-/- mice, we demonstrated that TPO is required for the MK hyperplasia and rebound thrombocyosis seen on treatment cessation. To further elucidate the pathway causing reduced platelet production, we used the human MK cell line Meg-01 and primary MK derived from fetal liver cells stimulated with TPO. Proplatelet assays of primary MK showed reductions in proplatelet extensions following HDACi exposure and a concomitant dose dependant increase in the phosphorylation of myosin light chain (MLC). The phosphorylation status of the MLC (pMLC) is regulated by the Rho GTPase family, of which Rac1/CDC42, acting via PAK1 are postulated to have opposing actions to RhoA which is known to increase pMLC and reduce proplatelet formation. Western blots of lysates from Meg-01 and primary cells showed a reduction in protein levels of all three family members following HDACi, however qRT PCR did not demonstrate HDACi to cause transcriptional repression of these proteins We were able to recapitulate alteration in pMLC levels using both pharmacological inhibitors of PAK1 and Rac1 as well as genetically, using constitutively active and dominant negative Rac1 constructs. Furthermore, by administering a murine TPO-mimetic, we were able to treat murine HDACi-induced TCP in-vivo in both non-tumor bearing mice, resulting in platelet numbers increasing to levels similar to vehicle treated controls in naïve mice. In mice with established Eμ-Myc lymphoma, platelet numbers were increased above all control groups. Conclusions: HDACi induced TCP not due to myeloablation or effects on platelet half life, but is rather due to impaired proplatelet formation, most likely via inhibition of the Rho/Rac1/CDC42 pathways. HDACi induced TCP my be overcome in a variety of settings using TPO-mimetics.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2613. doi:10.1158/1538-7445.AM2011-2613

The transcription factor Erg is essential for definitive hematopoiesis and the function of adult hematopoietic stem cells

Ets-related gene (ERG), which encodes a member of the Ets family of transcription factors, is a potent oncogene. Chromosomal rearrangements involving ERG are found in acute myeloid leukemia, acute lymphoblastic leukemia, Ewing's sarcoma and more than half of all prostate cancers; however, the normal physiological function of Erg is unknown. We did a sensitized genetic screen of the mouse for regulators of hematopoietic stem cell function and report here a germline mutation of Erg. We show that Erg is required for definitive hematopoiesis, adult hematopoietic stem cell function and the maintenance of normal peripheral blood platelet numbers.

Mutations in the cofilin partner Aip1/Wdr1 cause autoinflammatory disease and macrothrombocytopenia

A pivotal mediator of actin dynamics is the protein cofilin, which promotes filament severing and depolymerization, facilitating the breakdown of existing filaments, and the enhancement of filament growth from newly created barbed ends. It does so in concert with actin interacting protein 1 (Aip1), which serves to accelerate cofilin's activity. While progress has been made in understanding its biochemical functions, the physiologic processes the cofilin/Aip1 complex regulates, particularly in higher organisms, are yet to be determined. We have generated an allelic series for WD40 repeat protein 1 (Wdr1), the mammalian homolog of Aip1, and report that reductions in Wdr1 function produce a dramatic phenotype gradient. While severe loss of function at the Wdr1 locus causes embryonic lethality, macrothrombocytopenia and autoinflammatory disease develop in mice carrying hypomorphic alleles. Macrothrombocytopenia is the result of megakaryocyte maturation defects, which lead to a failure of normal platelet shedding. Autoinflammatory disease, which is bone marrow-derived yet nonlymphoid in origin, is characterized by a massive infiltration of neutrophils into inflammatory lesions. Cytoskeletal responses are impaired in Wdr1 mutant neutrophils. These studies establish an essential requirement for Wdr1 in megakaryocytes and neutrophils, indicating that cofilin-mediated actin dynamics are critically important to the development and function of both cell types.