A member of Forkhead family transcription factor, FKHRL1, is one of the downstream molecules of phosphatidylinositol 3-kinase-Akt activation pathway in erythropoietin signal transduction

The second transferrin receptor regulates red blood cell production in mice

Transferrin receptor 2 (TFR2) contributes to hepcidin regulation in the liver and associates with erythropoietin receptor in erythroid cells. Nevertheless, TFR2 mutations cause iron overload (hemochromatosis type 3) without overt erythroid abnormalities. To clarify TFR2 erythroid function, we generated a mouse lacking Tfr2 exclusively in the bone marrow (Tfr2(BMKO)). Tfr2(BMKO) mice have normal iron parameters, reduced hepcidin levels, higher hemoglobin and red blood cell counts, and lower mean corpuscular volume than normal control mice, a phenotype that becomes more evident in iron deficiency. In Tfr2(BMKO) mice, the proportion of nucleated erythroid cells in the bone marrow is higher and the apoptosis lower than in controls, irrespective of comparable erythropoietin levels. Induction of moderate iron deficiency increases erythroblasts number, reduces apoptosis, and enhances erythropoietin (Epo) levels in controls, but not in Tfr2(BMKO) mice. Epo-target genes such as Bcl-xL and Epor are highly expressed in the spleen and in isolated erythroblasts from Tfr2(BMKO) mice. Low hepcidin expression in Tfr2(BMKO) is accounted for by erythroid expansion and production of the erythroid regulator erythroferrone. We suggest that Tfr2 is a component of a novel iron-sensing mechanism that adjusts erythrocyte production according to iron availability, likely by modulating the erythroblast Epo sensitivity.

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.

Erythropoietin promotes survival of primary human endothelial cells through PI3K-dependent, NF-kappaB-independent upregulation of Bcl-xL

Erythropoietin (EPO) regulates the production of red blood cells primarily by preventing apoptosis of erythroid progenitors. More recently, however, EPO has emerged as a major cytoprotective cytokine in several nonhemopoietic tissues in the setting of stress or injury. The underlying mechanisms of the protective responses of EPO have not been fully defined. Here we show that EPO triggers a phosphatidylinositol 3-kinase-(PI3K)-dependent survival pathway that counteracts endothelial cell death. The protection conferred by PI3K relies on the subsequent induction of Bcl-x(L), a prosurvival member of the Bcl-2 protein family. In addition, EPO counteracts the upregulation of the pro-apoptotic BH3-only protein BIM, which is induced by serum withdrawal. EPO also activates extracellular signal-regulated kinase 1 and 2 (ERK1/2), which are involved in a Bcl-x(L)-independent cytoprotective pathway. EPO caused a prolonged activation of nuclear factor (NF)-kappaB, which was blocked by inhibition of PI3K, but not by inhibition of mitogen-activated protein (MAP)/ERK kinase (MEK), suggesting that EPO-activated NF-kappaB requires PI3K activity. However, the activation of the NF-kappaB pathway was not required for the ability of EPO to counteract endothelial apoptosis. Thus EPO promotes survival of endothelial cells through PI3K-dependent Bcl-x(L)-induction and BIM regulation, as well as through a separate mechanism involving the ERK pathway.

FOXO transcription factors at the interface between longevity and tumor suppression

A wide range of human diseases, including cancer, has a striking age-dependent onset. However, the molecular mechanisms that connect aging and cancer are just beginning to be unraveled. FOXO transcription factors are promising candidates to serve as molecular links between longevity and tumor suppression. These factors are major substrates of the protein kinase Akt. In the presence of insulin and growth factors, FOXO proteins are relocalized from the nucleus to the cytoplasm and degraded via the ubiquitin-proteasome pathway. In the absence of growth factors, FOXO proteins translocate to the nucleus and upregulate a series of target genes, thereby promoting cell cycle arrest, stress resistance, or apoptosis. Stress stimuli also trigger the relocalization of FOXO factors into the nucleus, thus allowing an adaptive response to stress stimuli. Consistent with the notion that stress resistance is highly coupled with lifespan extension, activation of FOXO transcription factors in worms and flies increases longevity. Emerging evidence also suggests that FOXO factors play a tumor suppressor role in a variety of cancers. Thus, FOXO proteins translate environmental stimuli into changes in gene expression programs that may coordinate organismal longevity and tumor suppression.

Amiloride augments TRAIL-induced apoptotic death by inhibiting phosphorylation of kinases and phosphatases associated with the P13K-Akt pathway

We have previously shown that low extracellular pH (pHe) promotes cell killing by the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study, we examined whether amiloride, an inhibitor of the Na(+)/H(+) antiporter capable of lowering the intracellular pH (pHi), can potentiate TRAIL-induced apoptotic death. Human prostate adenocarcinoma DU-145 cells were treated with various concentrations of TRAIL (10-200 ng/ml) and/or amiloride (0.1-1 mM) for 4 h. Amiloride, which caused little or no cytotoxicity by itself, enhanced TRAIL-induced apoptosis. The TRAIL-mediated activation of caspase, and PARP (poly (ADP-ribose) polymerase) cleavage were both promoted by amiloride. Western blot analysis showed that combined treatment with TRAIL and amiloride did not change the levels of TRAIL receptors (death receptor (DR)4, DR5, and DcR2 (decoy recepter 2) or antiapoptotic proteins (FLICE-inhibitory protein (FLIP), inhibitor of apoptosis (IAP), and Bcl-2). However, unlike pHe, amiloride promoted the dephosphorylation of Akt. Interestingly, amiloride also induced the dephosphorylation of P13K (phosphatidylinositol 3-kinase) and PDK-1 (phosphoinositide-dependent kinase-1) kinases along with PTEN (phosphatase and tensin homolog deleted on chromosome 10) and PP1alpha phosphatases. In vitro kinase assays revealed that amiloride inhibited phosphorylation of kinases and phosphatases by competing with ATP. Taken together, the present studies suggest that amiloride enhances TRAIL-induced cytotoxicity by inhibiting phosphorylation of the PI3K-Akt pathway-associated kinases and phosphatases.

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.

Proteolytic regulation of Forkhead transcription factor FOXO3a by caspase-3-like proteases

Forkhead family transcription factors are critical regulators of cell cycle progression and apoptosis in hematopoietic cells. Here, we show that FOXO3a (also known as FKHRL1) is a new substrate of caspase-3-like proteases during apoptosis in T lymphocytes. FOXO3a was cleaved in vivo by caspases in leukemic Jurkat cells following engagement of Fas (CD95) receptor, staurosporine, and etoposide treatment, but not following engagement of CD99, a caspase-independent cell death inducer. Caspase-mediated cleavage of FOXO3a was also observed in CD4+ peripheral T cells subjected to activation-induced cell death. The expression of the death adapter FADD and caspase-8 was required for Fas-induced FOXO3a cleavage, but activation of survival pathways by overexpression of FLICE-inhibitory protein or phorbol myristate acetate treatment prevented it. FOXO3a was cleaved in vitro by caspase-3-like proteases at the consensus sequence DELD304A, releasing the N-terminal DNA-binding domain of FOXO3a from its C-terminal transactivating domain. Whereas full-length FOXO3a enhanced Forkhead response element-dependent transcription and apoptosis in Jurkat cells, both fragments were inactive to promote gene activation and cell death. In contrast, a caspase-resistant FOXO3a mutant exhibited enhanced transcriptional and proapoptotic activities. Together, these results indicate that the proteolytic cleavage of FOXO3a by caspases may represent a novel regulatory mechanism of FOXO3a activity during death receptors signaling.

Critical role for PI 3-kinase in the control of erythropoietin-induced erythroid progenitor proliferation

The production of red blood cells is tightly regulated by erythropoietin (Epo). The phosphoinositide 3-kinase (PI 3-kinase) pathway was previously shown to be activated in response to Epo. We studied the role of this pathway in the control of Epo-induced survival and proliferation of primary human erythroid progenitors. We show that phosphoinositide 3 (PI 3)-kinase associates with 4 tyrosine-phosphorylated proteins in primary human erythroid progenitors, namely insulin receptor substrate-2 (IRS2), Src homology 2 domain-containing inositol 5'-phosphatase (SHIP), Grb2-associated binder-1 (Gab1), and the Epo receptor (EpoR). Using different in vitro systems, we demonstrate that 3 alternative pathways independently lead to Epo-induced activation of PI 3-kinase and phosphorylation of its downstream effectors, Akt, FKHRL1, and P70S6 kinase: through direct association of PI 3-kinase with the last tyrosine residue (Tyr479) of the Epo receptor (EpoR), through recruitment and phosphorylation of Gab proteins via either Tyr343 or Tyr401 of the EpoR, or through phosphorylation of IRS2 adaptor protein. The mitogen-activated protein (MAP) kinase pathway was also activated by Epo in erythroid progenitors, but we found that this process is independent of PI 3-kinase activation. In erythroid progenitors, the functional role of PI 3-kinase was both to prevent apoptosis and to stimulate cell proliferation in response to Epo stimulation. Finally, our results show that PI 3-kinase-mediated proliferation of erythroid progenitors in response to Epo occurs mainly through modulation of the E3 ligase SCF(SKP2), which, in turn, down-regulates p27(Kip1) cyclin-dependent kinase (CDK) inhibitor via proteasome degradation.

Phosphorylation of forkhead transcription factors by erythropoietin and stem cell factor prevents acetylation and their interaction with coactivator p300 in erythroid progenitor cells

The mammalian forkhead transcription factors, FOXO3a (FKHRL1), FOXO1a (FKHR) and FOXO4 (AFX) are negatively regulated by PKB/Akt kinase. In the present study we examined the engagement of forkhead family of transcription factors in erythropoietin (Epo)- and stem cell factor (SCF)-mediated signal transduction. Our data show that all three forkhead family members, FOXO3a, FOXO1a and FOXO4 are phosphorylated in human primary erythroid progenitors. Experiments performed to determine various upstream signaling pathways contributing to phosphorylation of forkhead family members show that only PI-3-kinase pathway is required for inactivation of FOXO3a. Our data also demonstrate that during Epo deprivation FOXO3a interacts with the transcriptional coactivator p300 and such interaction is disrupted by stimulation of cells with Epo. To determine the domains in FOXO3a, mediating its interaction with p300, we performed GST pull-down assays and found that the N-terminus region containing the first 52 amino acids was sufficient for binding p300. Finally, our data demonstrate that FOXO3a and FOXO1a are acetylated during growth factor deprivation and such acetylation is reversed by stimulation with Epo. Thus mammalian forkhead transcription factors are involved in Epo and SCF signaling in primary erythroid progenitors and may play a role in the induction of apoptotic and mitogenic signals.