Inactivation of the forkhead transcription factor FoxO3 is essential for PKB-mediated survival of hematopoietic progenitor cells by kit ligand

Cell Signaling Pathways That Promote Radioresistance of Cancer Cells

Radiation therapy (RT) is a standard treatment for solid tumors and about 50% of patients with cancer, including pediatric cancer, receive RT. While RT has significantly improved the overall survival and quality of life of cancer patients, its efficacy has still been markedly limited by radioresistance in a significant number of cancer patients (intrinsic or acquired), resulting in failure of the RT control of the disease. Radiation eradicates cancer cells mainly by causing DNA damage. However, radiation also concomitantly activates multiple prosurvival signaling pathways, which include those mediated by ATM, ATR, AKT, ERK, and NF-κB that promote DNA damage checkpoint activation/DNA repair, autophagy induction, and/or inhibition of apoptosis. Furthermore, emerging data support the role of YAP signaling in promoting the intrinsic radioresistance of cancer cells, which occurs through its activation of the transcription of many essential genes that support cell survival, DNA repair, proliferation, and the stemness of cancer stem cells. Together, these signaling pathways protect cancer cells by reducing the magnitude of radiation-induced cytotoxicity and promoting radioresistance. Thus, targeting these prosurvival signaling pathways could potentially improve the radiosensitivity of cancer cells. In this review, we summarize the contribution of these pathways to the radioresistance of cancer cells.

FoxO3a Gene Down-regulation in Pathogenesis of Pediatric Acute Lymphoblastic Leukemia

Introduction: Acute lymphoblastic leukemia (ALL) is the most common malignancy found in the pediatrics with the peak prevalence between the ages of 2 and 5 years. The constitutive activation of PI3K/AKT pathway inhibits the tumor-suppressor role of FoxO3a (a member of the forkhead class O [FoxO] transcription factor family) in a variety of cancers and leads to tumorigenesis. This study aims to investigate the expression of FoxO3a in three different stages of pediatric ALL in mRNA level. Subjects and Methods: In this case-control study, 70 patients with childhood ALL and 70 healthy age- and gender-matched as the control group were enrolled. Real-time quantitative RT-polymerase chain reaction (qRT-PCR) was used to detect the mRNA expression level of FoxO3a in children with different stages of ALL and healthy children as a control group. Results: Data showed that the expression of FoxO3a mRNA was lower in newly diagnosed ALL patients compared to controls (P < 0.0001), maintenance (P = 0.0342), and relapse (P = 0.0006) groups, while no difference was observed between other groups. In addition, T-ALL patients showed decreased expression of FoxO3a compared to Pre-B ALL ones (P < 0.0001). Conclusion: The study results suggest that FoxO3a plays a tumor-suppressor role in ALL. Thus, its up-regulation seems to be a plausible therapeutic strategy for this type of tumor.

The paradox role of caspase cascade in ionizing radiation therapy

Radiotherapy alone or in combination with chemotherapy/surgery is widely used for treatment of cancers. It reduces tumor growth and prevents metastasis. While ionizing radiation activates caspase cascade resulted in apoptosis in cancer cells, it also stimulates tumor cell re-population that leads to reduce the effectiveness of the radiation therapy. This review describes the mechanisms for paradox role of caspase cascade in cancer therapy and discusses the logical and practical strategies for improvement the therapeutic index of radiotherapy through enhancement of radiosensitivity and decreasing the rate of tumor recurrence.

Receptor tyrosine kinase (c-Kit) inhibitors: a potential therapeutic target in cancer cells

c-Kit, a receptor tyrosine kinase, is involved in intracellular signaling, and the mutated form of c-Kit plays a crucial role in occurrence of some cancers. The function of c-Kit has led to the concept that inhibiting c-Kit kinase activity can be a target for cancer therapy. The promising results of inhibition of c-Kit for treatment of cancers have been observed in some cancers such as gastrointestinal stromal tumor, acute myeloid leukemia, melanoma, and other tumors, and these results have encouraged attempts toward improvement of using c-Kit as a capable target for cancer therapy. This paper presents the findings of previous studies regarding c-Kit as a receptor tyrosine kinase and an oncogene, as well as its gene targets and signaling pathways in normal and cancer cells. The c-Kit gene location, protein structure, and the role of c-Kit in normal cell have been discussed. Comprehending the molecular mechanism underlying c-Kit-mediated tumorogenesis is consequently essential and may lead to the identification of future novel drug targets. The potential mechanisms by which c-Kit induces cellular transformation have been described. This study aims to elucidate the function of c-Kit for future cancer therapy. In addition, it has c-Kit inhibitor drug properties and their functions have been listed in tables and demonstrated in schematic pictures. This review also has collected previous studies that targeted c-Kit as a novel strategy for cancer therapy. This paper further emphasizes the advantages of this approach, as well as the limitations that must be addressed in the future. Finally, although c-Kit is an attractive target for cancer therapy, based on the outcomes of treatment of patients with c-Kit inhibitors, it is unlikely that Kit inhibitors alone can lead to cure. It seems that c-Kit mutations alone are not sufficient for tumorogenesis, but do play a crucial role in cancer occurrence.

From molecules to neural morphology: understanding neuroinflammation in autism spectrum condition

Growing evidence points toward a critical role for early (prenatal) atypical neurodevelopmental processes in the aetiology of autism spectrum condition (ASC). One such process that could impact early neural development is inflammation. We review the evidence for atypical expression of molecular markers in the amniotic fluid, serum, cerebrospinal fluid (CSF), and the brain parenchyma that suggest a role for inflammation in the emergence of ASC. This is complemented with a number of neuroimaging and neuropathological studies describing microglial activation. Implications for treatment are discussed.

Radiation-induced signaling pathways that promote cancer cell survival (review)

Radiation therapy is a staple cancer treatment approach that has significantly improved local disease control and the overall survival of cancer patients. However, its efficacy is still limited by the development of radiation resistance and the presence of residual disease after therapy that leads to cancer recurrence. Radiation impedes cancer cell growth by inducing cytotoxicity, mainly caused by DNA damage. However, radiation can also simultaneously induce multiple pro-survival signaling pathways, such as those mediated by AKT, ERK and ATM/ATR, which can lead to suppression of apoptosis, induction of cell cycle arrest and/or initiation of DNA repair. These signaling pathways act conjointly to reduce the magnitude of radiation-induced cytotoxicity and promote the development of radioresistance in cancer cells. Thus, targeting these pro-survival pathways has great potential for the radiosensitization of cancer cells. In the present review, we summarize the current literature on how these radiation‑activated signaling pathways promote cancer cell survival.

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

The PI3K/PKB signaling module as key regulator of hematopoiesis: implications for therapeutic strategies in leukemia

An important mediator of cytokine signaling implicated in regulation of hematopoiesis is the PI3K/protein kinase B (PKB/c-Akt) signaling module. Constitutive activation of this signaling module has been observed in a large group of leukemias. Because activation of this signaling pathway has been demonstrated to be sufficient to induce hematologic malignancies and is thought to correlate with poor prognosis and enhanced drug resistance, it is considered to be a promising target for therapy. A high number of pharmacologic inhibitors directed against either individual or multiple components of this pathway have already been developed to improve therapy. In this review, the safety and efficacy of both single and dual-specificity inhibitors will be discussed as well as the potential of combination therapy with either inhibitors directed against other signal transduction molecules or classic chemotherapy.

TNF-α/Fas-RIP-1-induced cell death signaling separates murine hematopoietic stem cells/progenitors into 2 distinct populations

We studied the effects of TNF-α and Fas-induced death signaling in hematopoietic stem and progenitor cells (HSPCs) by examining their contributions to the development of bone marrow failure syndromes in Tak1-knockout mice (Tak1(-/-)). We found that complete inactivation of TNF-α signaling by deleting both of its receptors, 1 and 2 (Tnfr1(-/-)r2(-/-)), can prevent the death of 30% to 40% of Tak1(-/-) HSPCs and partially repress the bone marrow failure phenotype of Tak1(-/-) mice. Fas deletion can prevent the death of 5% to 10% of Tak1(-/-) HSPCs but fails to further improve the survival of Tak1(-/-)Tnfr1(-/-)r2(-/-) HSPCs, suggesting that Fas might induce death within a subset of TNF-α-sensitive HSPCs. This TNF-α/Fas-induced cell death is a type of receptor-interacting protein-1 (RIP-1)-dependent programmed necrosis called necroptosis, which can be prevented by necrostatin-1, a specific RIP-1 inhibitor. In addition, we found that the remaining Tak1(-/-) HSPCs died of apoptosis mediated by the caspase-8-dependent extrinsic apoptotic pathway. This apoptosis can be converted into necroptosis by the inhibition of caspase-8 and prevented by inhibiting both caspase-8 and RIP-1 activities. We concluded that HSPCs are heterogeneous populations in response to death signaling stimulation. Tak1 mediates a critical survival signal, which protects against both TNF-α/Fas-RIP-1-dependent necroptosis and TNF-α/Fas-independent apoptosis in HSPCs.

Identification of a novel nonsense mutation on the Pax3 gene in ENU-derived white belly spotting mice and its genetic interaction with c-Kit

In the course of a large-scale screening program of N-ethyl-N-nitrosourea mutagenesis, we isolated two semidominant mutation lines with white belly spotting, named as wps and wbs. Direct sequencing detected a nucleotide G-to-A transversion in exon 2 of the c-Kit gene in wps, which resulted in a missense D60N mutation. Another mutant, wbs, was mapped to chromosome 1 by genome-wide linkage analysis. In 93 meioses, the wbs locus was confined to a 5.2-Mb region between D1Mit380 and D1Mit215, including the Pax3 gene. A nonsense mutation K107X on the Pax3 coding region in wbs mice was identified, causing the loss of Pax3 protein in the homozygous mutant. We further demonstrated that Pax3 exhibited genetic interaction with c-Kit by intercrossing the wps and wbs mice. Further, Pax3 transactivated the c-Kit promoter in different cell lines. However, electrophoretic mobility shift assays showed that Pax3 did not bind to the c-Kit promoter, indicating that Pax3 may interact with c-Kit in an indirect way. This expands our understanding of the intricate regulatory network governing the melanocyte development.

Kit and foxd3 genetically interact to regulate melanophore survival in zebrafish

We have investigated the role of foxd3 activity in conjunction with signaling by the kit tyrosine kinase receptor in zebrafish black pigment cell (melanophore) development. As loss-of-function of these molecules individually has distinct effects on melanophore number, we have examined the phenotype of double mutants. Individuals with a null mutation in kit have fewer melanophores than wild-type, with cells lost through death. When kit mutants are injected with foxd3 antisense morpholino oligonucleotides or crossed with a foxd3 zebrafish mutant, they have more melanophores than their uninjected or foxd3+ counterparts. Examination of foxd3 loss-of-function in two additional kit mutants that differentially alter kit-dependent migration and survival indicates a change in melanophore number in survival mutants only. Consistently, TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling) analysis confirms a partial rescue of melanophores from cell death. Ectopic expression of foxd3 indicates that foxd3 promotes early melanophore death only when kit is inactive. Taken together, these data suggest a kit-dependent role for foxd3 in the regulation of melanophore survival.

Stem cell factor prevents neuronal cell apoptosis after acute spinal cord injury

STUDY DESIGN

A rat spinal cord injury (SCI) model and immunohistochemistry were used to examine the levels of expression of stem cell factor and c-kit. In addition, we examined whether intraperitoneal administration of stem cell factor could prevent neural cells apoptosis after acute SCI.

OBJECTIVE

To evaluate the antiapoptotic effect of stem cell factor after SCI.

SUMMARY OF BACKGROUND DATA

It is well known that the mode of delayed neuronal and glial cell death after SCI is apoptosis. Inhibition of apoptosis might thus promote neurologic improvement after SCI. Stem cell factor and its receptor c-kit exhibit pleiotropic effects in early hematopoiesis, and are also known to prevent hematopoietic progenitor cell apoptosis. Stem cell factor has recently been reported to be a survival factor for neural stem cells in vitro. We examined the levels of expression of stem cell factor and c-kit in normal and injured rat spinal cord. In addition, we examined whether stem cell factor prevents neural cell apoptosis after acute SCI.

METHODS

We examined the expression of stem cell factor and c-kit in spinal cord after injury by quantitative RT-PCR and immunohistochemistry. Antiapoptotic effects of stem cell factor were examined using rats with SCI that received stem cell factor intraperitoneally, and were examined immunohistochemically with anticleaved PARP antibody and antiactive caspase-3 antibody between 1 and 3 days after injury.

RESULTS

Upregulation of stem cell factor and c-kit expression occured after SCI. We also found that neurons express stem cell factor, and neurons and oligodendrocytes express c-kit after SCI. In addition, intraperitoneal administration of stem cell factor prevented spinal neural cells apoptosis after injury.

CONCLUSION

These findings suggest the possibility that stem cell factor, a hematopoietic cytokine, could be useful as an agent for treatment of SCI.

Akt-FOXO3a signaling affects human endothelial progenitor cell differentiation

Here we address the effect of Akt signaling on endothelial progenitor cells (EPCs). Human peripheral blood mononuclear cells (PBMCs) were cultured on fibronectin-coated dishes in EPC differentiation medium. PBMCs differentiated in a series of three steps: proliferation for foci formation, tight attachment to the dishes in the early stages of differentiation, and maturation in the late stages. In Western blot analysis, Akt expression was attenuated in the early stages of differentiation and was gradually upregulated during EPC maturation. Forkhead box-containing protein, class O 3a (FOXO3a), an Akt downstream target, was downregulated through phosphorylation in the late stages of EPC differentiation. Adenovirus-mediated overexpression of activated FOXO3a in PBMCs markedly increased the number of cell foci but reduced the number of DiI-acetyl LDL EPCs that appear at later time points. These data suggest that Akt/FOXO3a signaling is an important regulator of EPC maturation.

The characterization of epithelial and stromal subsets of candidate stem/progenitor cells in the human adult prostate

OBJECTIVES

Questions regarding the cell source and mechanisms in the initiation and progression of prostate cancer are today still open for debate. Indeed, our knowledge regarding prostate cell regulation, self-renewal, and cytodifferentiation is presently rather limited. In this study, we investigated these processes in the normal adult human prostate.

METHODS

Dynamic expression patterns in prostate stem/progenitor cells, intermediate/transit-amplifying cells, and cell lineages were immunohistochemically identified in an in situ explant renewal model of the human normal/benign adult prostate (n=6).

RESULTS

Cells with a basal phenotype proliferated significantly in explant cultures, whereas luminal cells went into apoptosis. Results further show down-regulation in tissue cultures of the basal and hypothetical stem cell marker Bcl-2 in the majority of cells, except in rare putative epithelial stem cells. Investigation of established (AC133) and novel candidate prostate stem/progenitor markers, including the cell surface receptor tyrosine kinase KIT and its ligand stem cell factor (SCF), showed that these rare epithelial cells are AC133(+)/CD133(low)/Bcl-2(high)/cytokeratin(+)/vimentin(-)/KIT(low)/SCF(low). In addition, we report on a stromal population that expresses the mesenchymal marker vimentin and that is AC133(-)/CD133(high)/Bcl-2(-)/cytokeratin(-)/KIT(high)/SCF(high).

CONCLUSIONS

We provide evidence for epithelial renewal in response to tissue culture and for basal and epithelial stem/progenitor cell recruitment leading to an expansion of an intermediate luminal precursor phenotype. Data further suggest that SCF regulates prostate epithelial stem/progenitor cells in an autocrine manner and that all or a subset of the identified novel stromal phenotype represents prostate stromal progenitor cells or interstitial pacemaker cells or both.

The BH3-only protein Puma plays an essential role in cytokine deprivation induced apoptosis of mast cells

Mast cells play critical roles in the regulation of inflammation. One characteristic feature of mast cells is their relatively long lifespan in vivo. Members of the Bcl-2 protein family are regulators of cell survival and apoptosis, where the BH3-only proteins are critical proapoptotic proteins. In this study we investigated the role of the BH3-only proteins Noxa, Bad, Bim, Bmf, Bid, and Puma in apoptosis of mucosal-like mast cells (MLMCs) and connective tissue-like mast cells (CTLMCs). We demonstrate that Puma is critical for the induction of mast-cell death following cytokine deprivation and treatment with the DNA-damaging agent etoposide in MLMCs and CTLMCs. Using p53-/- mast cells, we found that cytokine deprivation-induced apoptosis, in contrast to that elicited by etoposide, is p53-independent. Interestingly, mast cells deficient in FOXO3a, previously proposed as a transcription factor for Puma induction in response to growth factor deprivation, were markedly resistant to cytokine withdrawal compared with wild-type cells. Moreover, overexpression of phosphorylation-deficient, constitutively active FOXO3a caused an up-regulation of Puma. In conclusion, our data demonstrate a pivotal role for Puma in the regulation of cytokine deprivation-induced mast-cell apoptosis and suggest a plausible role for Puma in the regulation of mast cell numbers in vivo.

Stem cell c-KIT and HOXB4 genes: critical roles and mechanisms in self-renewal, proliferation, and differentiation

Hematopoietic stem cells (HSCs) possess a distinct ability to perpetuate through self-renewal and to generate progeny that differentiate into mature cells of myeloid and lymphoid lineages. A better understanding of the molecular mechanisms by which HSCs replicate and differentiate from the perspective of developing new approaches for HSC transplantation is necessary for further advances. The interaction of the receptor tyrosine kinase--c-KIT--with its ligand stem cell factor plays a key role in HSC survival, mitogenesis, proliferation, differentiation, adhesion, homing, migration, and functional activation. Evidence that activating site-directed point mutations in the c-KIT gene contributes to its ligand-independent constitutive activation, which induces enhanced proliferation of HSCs, is accumulating. Similarly, and equally important, self-renewal is a process by which HSCs generate daughter cells via division. Self-renewal is necessary for retaining the HSC pool. Therefore, elucidating the molecular machinery that governs self-renewal is of key importance. The transcription factor, HOXB4 is a key molecule that has been reported to induce the in vitro expansion of HSCs via self-renewal. However, critical downstream effector molecules of HOXB4 remain to be determined. This concisely reviewed information on c-KIT and HOXB4 helps us to update our understanding of their function and mechanism of action in self-renewal, proliferation, and differentiation of HSCs, particularly modulation by c-KIT mutant interactions, and HOXB4 overexpression showing certain therapeutic implications.

c-Kit--a hematopoietic cell essential receptor tyrosine kinase

The receptor tyrosine kinase c-Kit is expressed in hematopoietic stem and progenitor cells and in several non-hematopoietic tissues. In the hematopoietic system, c-Kit is critical for proliferation, survival and differentiation. During recent years exploration of the signalling pathways downstream of this receptor has yielded significant new insights in the field. In this review, we will summarise the c-Kit background, structure, downstream signalling and medical significance with particular focus on its role in hematopoietic progenitor cells and mast cells.

Akt promotes increased cardiomyocyte cycling and expansion of the cardiac progenitor cell population

Activation of Akt is associated with enhanced cell cycling and cellular proliferation in nonmyocytes, but this effect of nuclear Akt accumulation has not been explored in the context of the myocardium. Cardiac-specific expression of nuclear-targeted Akt (Akt/nuc) in transgenics prolongs postnatal cell cycling as evidenced by increased numbers of Ki67+ cardiomyocytes at 2 to 3 weeks after birth. Similarly, nuclear-targeting of Akt promotes expansion of the presumptive cardiac progenitor cell population as assessed by immunolabeling for c-kit in combination with myocyte-specific markers Nkx 2.5 or MEF 2C. Increases in pro-proliferative cytokines, including tumor-necrosis superfamily 8, interleukin-17e, and hepatocyte growth factor, were found in nuclear-targeted Akt myocardial samples. Concurrent signaling mediated by paracrine factors downstream of Akt/nuc expression may be responsible for phenotypic effects of nuclear-targeted Akt in the myocardium, including enhanced cell proliferation and expansion of the stem cell population.

The survival of differentiating embryonic stem cells is dependent on the SCF-KIT pathway

The stem cell factor (SCF)-KIT signal transduction pathway plays a role in the proliferation, differentiation and survival of a range of stem and progenitor cell types but little is known about its function in embryonic stem (ES) cells. We generated ES cells carrying a null allele of Kit as well as a knock-in allele that encodes an SCF-independent hybrid KIT receptor that can be activated by the FKBP binding drug, AP20187. KIT null ES cells die when induced to differentiate upon withdrawal of leukaemia inhibitory factor in monolayer culture. This phenotype is recapitulated in wild-type ES cells treated with a KIT-neutralising antibody and reversed in mutant cells by activation of the hybrid KIT receptor. Differentiating KIT null ES cells exhibit elevated levels of DNA laddering and reduced BCL2 expression, indicative of apoptosis. We conclude that mouse ES cell differentiation in vitro is dependent on the SCF-KIT pathway contrasting with the apparently normal differentiation of KIT null inner cell mass or epiblast cells in vivo. This discrepancy could be explained by the presence of compensatory signals in the embryo or it could lend support to the idea of a phenotypic relationship between ES cells and early germ cells.

Neuroprotection by stem cell factor in rat cortical neurons involves AKT and NFκB

Stem cell factor (SCF) is a highly expressed cytokine in the central nervous system. In the present study, we demonstrate a neuroprotective role for SCF and its tyrosine kinase receptor, c-kit, against camptothecin-induced apoptosis and glutamate excitotoxicity in rat cortical neurons. This protection was blocked by pharmacological or molecular inhibition of either the MEK/ERK or PI3K/Akt signaling pathways. The importance of these pathways was further confirmed by the activation of both ERK, in a MEK-dependent manner, and Akt, via PI3K. Activation of Akt increased the binding of the p50 and p65 subunits of NFkappaB, which was also important for neuroprotection. Akt inhibition prevented NFkappaB binding, suggesting a role for Akt in SCF-induced NFkappaB. Pharmacological inhibition of NFkappaB or dominant negative IkappaB also prevented neuroprotection by SCF. SCF up-regulated the anti-apoptotic genes, bcl-2 and bcl-xL in an NFkappaB-dependent manner. Together, these findings demonstrate a neuroprotective role for SCF in cortical neurons, an effect that was mediated by Akt and ERK, as well as NFkappaB-mediated gene transcription. SCF represents a novel therapeutic target in the treatment of neurodegenerative disease.

Stem cell factor promotes mast cell survival via inactivation of FOXO3a-mediated transcriptional induction and MEK-regulated phosphorylation of the proapoptotic protein Bim

Mast cells are found in tissues throughout the body where they play important roles in the regulation of inflammatory responses. One characteristic feature of mast cells is their longevity. Although it is well established that mast cell survival is dependent on stem cell factor (SCF), it has not been described how this process is regulated. Herein, we report that SCF promotes mast cell survival through inactivation of the Forkhead transcription factor FOXO3a (forkhead box, class O3A) and down-regulation and phosphorylation of its target Bim (Bcl-2 [B-cell lymphoma-2] interacting modulator of cell death), a Bcl-2 homology 3 (BH3)-only proapoptotic protein. SCF induced a rapid and transient phosphorylation of Akt (protein kinase B) and FOXO3a. SCF treatment prevented up-regulation of Bim protein expression and led to increased Bim phosphorylation. Bim phosphorylation was inhibited by PD98059 and LY294002 treatment, suggesting the involvement of mitogen-activated protein kinase kinase/mitogen-activated protein kinase (MEK/MAPK) and phosphatidylinositol 3 (PI3)-kinase pathways in this process. Overexpression of phosphorylation-deficient FOXO3a caused an up-regulation of Bim and induced mast cell apoptosis even in the presence of SCF. Mast cell apoptosis induced by the phosphorylation-deficient FOXO3a was attenuated in bim-/- mast cells. Because apoptosis is abnormally reduced in bim-/- mast cells, these data provide evidence that Akt-mediated inhibition of FOXO3a and its transcription target Bim provides an important mechanism by which SCF acts to prevent apoptosis in mast cells.

Stem cells today: B1. Bone marrow stem cells

This review is the second in a series of four devoted to the analysis of recent studies on stem cells. The first considered embryo stem cells (ES). This review covers bone marrow stem cells. They are analysed initially in a historical perspective, and then in relation to foundation studies in the later 20th century before a detailed analysis is presented on very recent studies. Methods of identifying, culturing, expanding and grafting stem cells are described, including the separation of haemopoietic and mesenchyme cell lines (HSC and MSC) and recent more detailed analyses using numerous CD and other markers to identify very small subsets of stem cells such as multipotent adult progenitor cells (MAPC) and bone marrow stromal stem cells (BMSSC) from MSC. Queries arising on the immense potential of these stem cell lines due to the discovery of epigentic factors and cell fusions influencing their development and potency are described. A section on cord blood stem cells is followed by a detailed discussion on the modern situation regarding the clinical use of stem cells, its recent setbacks due to epigenetic factors, different approaches to the discovery of a highly multipotent bone marrow stem cell, and a brief description of embryological approaches to identifying the basic bone marrow stem cell in very early mammalian embryos.

Akt decreases lymphocyte apoptosis and improves survival in sepsis

Sepsis induces extensive death of lymphocytes that may contribute to the immunosuppression and mortality of the disorder. The serine/threonine kinase Akt is a key regulator of cell proliferation and death. The purpose of this study was to determine whether overexpression of Akt would prevent lymphocyte apoptosis and improve survival in sepsis. In addition, given the important role of Akt in cell signaling, T cell Th1 and Th2 cytokine production was determined. Mice that overexpress a constitutively active Akt in lymphocytes were made septic, and survival was recorded. Lymphocyte apoptosis and cytokine production were determined at 24 h after surgery. Mice with overexpression of Akt had a marked improvement in survival compared with wild-type littermates, i.e., 94 and 47% survival, respectively, p < 0.01. In wild-type littermates, sepsis caused a marked decrease in IFN-gamma production, while increasing IL-4 production >2-fold. In contrast, T cells from Akt transgenic mice had an elevated production of IFN-gamma at baseline that was maintained during sepsis, while IL-4 had little change. Akt overexpression also decreased sepsis-induced lymphocyte apoptosis via a non-Bcl-2 mechanism. In conclusion, Akt overexpression in lymphocytes prevents sepsis-induced apoptosis, causes a Th1 cytokine propensity, and improves survival. Findings from this study strengthen the concept that a major defect in sepsis is impairment of the adaptive immune system, and suggest that strategies to prevent lymphocyte apoptosis represent a potential important new therapy.

Anaplastic lymphoma kinase proteins in growth control and cancer

The normal functions of full-length anaplastic lymphoma kinase (ALK) remain to be completely elucidated. Although considered to be important in neural development, recent studies in Drosophila also highlight a role for ALK in gut muscle differentiation. Indeed, the Drosophila model offers a future arena for the study of ALK, its ligands and signalling cascades. The discovery of activated fusion forms of the ALK tyrosine kinase in anaplastic large cell lymphoma (ALCL) has dramatically improved our understanding of the pathogenesis of these lymphomas and enhanced the pathological diagnosis of this subtype of non-Hodgkin's lymphoma (NHL). Likewise, the realisation that a high percentage of inflammatory myofibroblastic tumours express activated-ALK fusion proteins has clarified the causation of these mesenchymal neoplasms and provided for their easier discrimination from other mesenchymal-derived inflammatory myofibroblastic tumour (IMT) mimics. Recent reports of ALK expression in a range of carcinoma-derived cell lines together with its apparent role as a receptor for PTN and MK, both of which have been implicated in tumourigenesis, raise the possibility that ALK-mediated signalling could play a role in the development and/or progression of a number of common solid tumours. The therapeutic targeting of ALK may prove to have efficacy in the treatment of many of these neoplasms.

FLT3 ligand regulates apoptosis through AKT-dependent inactivation of transcription factor FoxO3

Proliferation, differentiation, and survival of hematopoietic cells are regulated by cytokines, acting through specific receptors. FLT3 ligand (FL) is one of the most important cytokines for regulation of the hematopoietic system, and its receptor FLT3 is expressed on both stem cells and progenitors. Regulation of Forkhead transcription factors has been described as an important mechanism to control apoptosis and cell cycle progression in hematopoietic progenitors. Here we report that FL induces AKT/PKB activation, which in turn phosphorylates and thereby inactivates the Forkhead protein FoxO3 in the progenitor cell line FDC-P1 stably expressing murine FLT3 receptor. Phosphorylation of AKT and FoxO3 was blocked by the PI-3 kinase inhibitor LY294002 but not by the MAP kinase inhibitor PD98059. Expression of a mutated FoxO3, in which all three inhibitory phosphorylation sites were mutated to alanine, led to rapid increase of apoptotic cells in the presence of FL. These results suggest that FL-induced regulation of apoptosis is executed by FoxO3.

FLT3 receptors with internal tandem duplications promote cell viability and proliferation by signaling through Foxo proteins

In about 30% of the patients with acute myeloid leukemia, activating FLT3 receptor mutations have been identified, often as in-frame internal tandem duplications (ITD) at the juxtamembrane domain of the receptor. FLT3-ITD receptors exhibit constitutive tyrosine kinase activity in the absence of FLT3 ligand (FL) binding, and when expressed in cytokine-dependent cell lines and primary hematopoietic cells suppress programmed cell death and increase cell division. However, the signaling pathways important for transformation, in particular the nuclear targets, are unknown. Here we demonstrate that FLT3-ITD expression in Ba/F3 cells results in activation of Akt and concomitant phosphorylation of the Forkhead family member Foxo3a. Phosphorylation of Foxo proteins through FLT3-ITD signaling promotes their translocation from the nucleus into the cytoplasm, which requires the presence of conserved Akt phosphorylation sites in Forkhead transcription factors and PI3K activity. Induction of Foxo3a phosphorylation by FLT3-ITD receptors in Ba/F3 cells correlates with the suppression of Foxo-target genes p27Kip1 and the proapoptotic Bcl-2 family member Bim. Specifically, FLT3-ITD expression prevents Foxo3a-mediated apoptosis and upregulation of p27Kip1 and Bim gene expression. These data indicate that the oncogenic tyrosine kinase FLT3 can negatively regulate Foxo transcription factors, thereby promoting cell survival and proliferation.

The phosphoinositide 3-kinase/Akt pathway regulates cell cycle progression of HL60 human leukemia cells through cytoplasmic relocalization of the cyclin-dependent kinase inhibitor p27(Kip1) and control of cyclin D1 expression

The serine/threonine protein kinase Akt, a downstream effector of phosphoinositide 3-kinase (PI3K), plays a pivotal role in tumorigenesis because it affects the growth and survival of cancer cells. Several laboratories have demonstrated that Akt inhibits transcriptional activation of a number of related forkhead transcription factors now referred to as FoxO1, FoxO3, and FoxO4. Akt-regulated forkhead transcription factors are involved in the control of the expression of both the cyclin-dependent kinase (cdk) inhibitor p27(Kip1) and proapoptotic Bim protein. Very little information is available concerning the importance of the PI3K/Akt pathway in HL60 human leukemia cells. Here, we present our findings showing that the PI3K/Akt axis regulates cell cycle progression of HL60 cells through multiple mechanisms also involving the control of FoxO1 and FoxO3. To this end, we took advantage of a HL60 cell clone (HL60AR cells) with a constitutively activated PI3K/Akt axis. When compared with parental (PT) HL60 cells, HL60AR cells displayed higher levels of phosphorylated FoxO1 and FoxO3. In AR cells forkhead factors localized predominantly in the cytoplasm, whereas in PT cells they were mostly nuclear. AR cells proliferated faster than PT cells and showed a lower amount of the cdk inhibitor p27(Kip1), which was mainly found in the cytoplasm and was hyperphosphorylated on threonine residues. AR cells also displayed higher levels of cyclin D1 and phosphorylated p110 Retinoblastoma protein. The protein levels of cdk2, cdk4, and cdk6 were not altered in HL60AR cells, whereas the activities of both ckd2 and cdk6 were higher in AR than in PT cells. These results show that in HL60 cells the PI3K/Akt signaling pathway may be involved in the control of the cell cycle progression most likely through mechanisms involving the activation of forkhead transcription factors.

FoxO3a transcriptional regulation of Bim controls apoptosis in paclitaxel-treated breast cancer cell lines

Paclitaxel is used to treat breast cancers, but the mechanisms by which it induces apoptosis are poorly understood. Consequently, we have studied the role of the FoxO transcription factors in determining cellular response to paclitaxel. Western blotting revealed that in a panel of nine breast cancer cell lines expression of FoxO1a and FoxO3a correlated with the expression of the pro-apoptotic FoxO target Bim, which was associated with paclitaxel-induced apoptosis. In MCF-7 cells, which were paclitaxel-sensitive, the already high basal levels of FoxO3a and Bim protein increased dramatically after drug treatment, as did Bim mRNA, which correlated with apoptosis induction. This was not observed in MDA-231 cells, which expressed low levels of FoxOs and Bim. Gene reporter experiments demonstrated that in MCF-7 cells maximal induction of Bim promoter was dependent on a FoxO binding site, suggesting that FoxO3a is responsible for the transcriptional up-regulation of Bim. Gene silencing experiments showed that small interference RNA (siRNA) specific for FoxO3a reduced the levels of FoxO3a and Bim protein as well as inhibited apoptosis in paclitaxel-treated MCF-7 cells. Furthermore, siRNA specific for Bim reduced the levels of Bim protein and inhibited apoptosis in paclitaxel-treated MCF-7 cells. This is the first demonstration that up-regulation of FoxO3a by paclitaxel can result in increased levels of Bim mRNA and protein, which can be a direct cause of apoptosis in breast cancer cells.