mTORC1 is essential for leukemia propagation but not stem cell self-renewal

ONC212 enhances YM155 cytotoxicity by triggering SLC35F2 expression and NOXA-dependent MCL1 degradation in acute myeloid leukemia cells

Although the anticancer activity of ONC212 has been reported, the precise mechanism underlying its apoptotic effects remains unclear. In this study, we investigated the apoptotic mechanism of ONC212 in acute myeloid leukemia (AML) cells. ONC212 induces apoptosis, MCL1 downregulation, and mitochondrial depolarization in AML U937 cells. Ectopic MCL1 expression alleviates mitochondria-mediated apoptosis in ONC212-treated U937 cells. ONC212 triggers AKT phosphorylation, inducing NOX4-dependent ROS production and promoting HuR transcription. HuR-mediated ATF4 mRNA stabilization stimulates NOXA and SLC35F2 expression; ONC212-induced upregulation of NOXA leads to MCL1 degradation. The synergistic effect of ONC212 on YM155 cytotoxicity was dependent on increased SLC35F2 expression. In addition, YM155 feedback facilitated the activation of the ONC212-induced signaling pathway. A similar mechanism explains ONC212- and ONC212/YM155-induced AML HL-60 cell death. The continuous treatment of U937 cells with the benzene metabolite hydroquinone (HQ) generated U937/HQ cells, exhibiting enhanced responsiveness to the cytotoxic effects of ONC212. In U937/HQ cells, ONC212 triggered apoptosis through NOXA-mediated MCL1 downregulation, enhancing YM155 cytotoxicity. Collectively, our data suggested that ONC212 upregulated SLC35F2 expression and triggered NOXA-mediated MCL1 degradation in U937, U937/HQ, and HL-60 cells by activating the AKT/NOX4/HuR/ATF4 pathway. The ONC212-induced signaling pathway showed anti-AML activity and enhanced YM155 cytotoxicity.

Overcoming adaptive resistance in AML by synergistically targeting FOXO3A-GNG7-mTOR axis with FOXO3A inhibitor Gardenoside and rapamycin

Therapeutic targeting FOXO3A (a forkhead transcription factor) represents a promising strategy to suppress acute myeloid leukemia (AML). However, the effective inhibitors that target FOXO3A are lacking and the adaptive response signaling weakens the cytotoxic effect of FOXO3A depletion on AML cells. Here, we show that FOXO3A deficiency induces a compensatory response involved in the reactive activation of mTOR that leads to signaling rebound and adaptive resistance. Mitochondrial metabolism acts downstream of mTOR to provoke activation of JNK/c-JUN via reactive oxygen species (ROS). At the molecular level, FOXO3A directly binds to the promoter of G protein gamma subunit 7 (GNG7) and preserves its expression, while GNG7 interacts with mTOR and restricts phosphorylated activation of mTOR. Consequently, combinatorial inhibition of FOXO3A and mTOR show a synergistic cytotoxic effect on AML cells and prolongs survival in a mouse model of AML. Through a structure-based virtual screening, we report one potent small-molecule FOXO3A inhibitor (Gardenoside) that exhibits a strong effect of anti-FOXO3A DNA binding. Gardenoside synergizes with rapamycin to substantially reduce tumor burden and extend survival in AML patient-derived xenograft model. These results demonstrate that mTOR can mediate adaptive resistance to FOXO3A inhibition and validate a combinatorial approach for treating AML.

Dienediamine: A safe surrogate for the herbicide paraquat

Paraquat (PQ) has been used as an herbicide worldwide because of its potent activity against weeds. However, it is highly toxic to humans. The very high fatality of PQ poisoning is due to its inherent toxicity and the lack of any effective treatment. Consequently, developing a non-toxic herbicide with comparable efficacy to PQ will contribute to global food security and help prevent PQ-related fatalities. Herein, we report a new herbicide called dienediamine, which was discovered from how to intervene the redox cycle of PQ, an inherent toxicity nature. Dienediamine, the "reduced" form of PQ with no function as an electron transfer agent, was shown to be non-toxic through comprehensive in vivo and in vitro experiments at molar concentrations equivalent to PQ's absolute lethal dose. Remarkably, dienediamine can undergo conversion to PQ under natural sunlight and ambient air conditions, exhibiting herbicidal activities that are comparable to those of PQ. The conversion of dienediamine to PQ, which is toxic to chloroplasts, is the key mechanism underlying its potent herbicidal activity. Our study discovers that dienediamine is a safe and superior alternative to PQ, possessing significant potential for application in sustainable agriculture globally.

mTORC1-selective activation of translation elongation promotes disease progression in chronic lymphocytic leukemia

Targeted deletion of Raptor, a component of mechanistic target of rapamycin complex 1 (mTORC1), reveals an essential role for mTORC1 in initiation/maintenance of leukemia in a CLL model, resulting from a failure for haemopoietic stem/progenitor cells (HSPCs) to commit to the B cell lineage. Induction of Raptor-deficiency in NSG mice transplanted with Mx1-Raptor CLL progenitor cells (PKCα-KR-transduced HSPCs) after disease establishment revealed a reduction in CLL-like disease load and a significant increase in survival in the mice. Interestingly in an aggressive CLL-like disease model, rapamycin treatment reduced disease burden more effectively than AZD2014 (dual mTORC1/2 inhibitor), indicating a skew towards mTORC1 sensitivity with more aggressive disease. Rapamycin, but not ibrutinib, efficiently targeted the eEF2/eEF2K translation elongation regulatory axis, downstream of mTORC1, resulting in eEF2 inactivation through induction of eEF2T56 phosphorylation. mTOR inhibitor treatment of primary patient CLL cells halted proliferation, at least in part through modulation of eEF2K/eEF2 phosphorylation and expression, reduced protein synthesis and inhibited expression of MCL1, Cyclin A and Cyclin D2. Our studies highlight the importance of translation elongation as a driver of disease progression and identify inactivation of eEF2 activity as a novel therapeutic target for blocking CLL progression.

Multifaceted role of mTOR (mammalian target of rapamycin) signaling pathway in human health and disease

The mammalian target of rapamycin (mTOR) is a protein kinase that controls cellular metabolism, catabolism, immune responses, autophagy, survival, proliferation, and migration, to maintain cellular homeostasis. The mTOR signaling cascade consists of two distinct multi-subunit complexes named mTOR complex 1/2 (mTORC1/2). mTOR catalyzes the phosphorylation of several critical proteins like AKT, protein kinase C, insulin growth factor receptor (IGF-1R), 4E binding protein 1 (4E-BP1), ribosomal protein S6 kinase (S6K), transcription factor EB (TFEB), sterol-responsive element-binding proteins (SREBPs), Lipin-1, and Unc-51-like autophagy-activating kinases. mTOR signaling plays a central role in regulating translation, lipid synthesis, nucleotide synthesis, biogenesis of lysosomes, nutrient sensing, and growth factor signaling. The emerging pieces of evidence have revealed that the constitutive activation of the mTOR pathway due to mutations/amplification/deletion in either mTOR and its complexes (mTORC1 and mTORC2) or upstream targets is responsible for aging, neurological diseases, and human malignancies. Here, we provide the detailed structure of mTOR, its complexes, and the comprehensive role of upstream regulators, as well as downstream effectors of mTOR signaling cascades in the metabolism, biogenesis of biomolecules, immune responses, and autophagy. Additionally, we summarize the potential of long noncoding RNAs (lncRNAs) as an important modulator of mTOR signaling. Importantly, we have highlighted the potential of mTOR signaling in aging, neurological disorders, human cancers, cancer stem cells, and drug resistance. Here, we discuss the developments for the therapeutic targeting of mTOR signaling with improved anticancer efficacy for the benefit of cancer patients in clinics.

Integrative Analyses Reveal the Anticancer Mechanisms and Sensitivity Markers of the Next-Generation Hypomethylating Agent NTX-301

Epigenetic dysregulation characterized by aberrant DNA hypermethylation is a hallmark of cancer, and it can be targeted by hypomethylating agents (HMAs). Recently, we described the superior therapeutic efficacy of a novel HMA, namely, NTX-301, when used as a monotherapy and in combination with venetoclax in the treatment of acute myeloid leukemia. Following a previous study, we further explored the therapeutic properties of NTX-301 based on experimental investigations and integrative data analyses. Comprehensive sensitivity profiling revealed that NTX-301 primarily exerted anticancer effects against blood cancers and exhibited improved potency against a wide range of solid cancers. Subsequent assays showed that the superior efficacy of NTX-301 depended on its strong effects on cell cycle arrest, apoptosis, and differentiation. Due to its superior efficacy, low doses of NTX-301 achieved sufficiently substantial tumor regression in vivo. Multiomics analyses revealed the mechanisms of action (MoAs) of NTX-301 and linked these MoAs to markers of sensitivity to NTX-301 and to the demethylation activity of NTX-301 with high concordance. In conclusion, our findings provide a rationale for currently ongoing clinical trials of NTX-301 and will help guide the development of novel therapeutic options for cancer patients.

Mitochondrial fusion is a therapeutic vulnerability of acute myeloid leukemia

Mitochondrial metabolism recently emerged as a critical dependency in acute myeloid leukemia (AML). The shape of mitochondria is tightly regulated by dynamin GTPase proteins, which drive opposing fusion and fission forces to consistently adapt bioenergetics to the cellular context. Here, we showed that targeting mitochondrial fusion was a new vulnerability of AML cells, when assayed in patient-derived xenograft (PDX) models. Genetic depletion of mitofusin 2 (MFN2) or optic atrophy 1 (OPA1) or pharmacological inhibition of OPA1 (MYLS22) blocked mitochondrial fusion and had significant anti-leukemic activity, while having limited impact on normal hematopoietic cells ex vivo and in vivo. Mechanistically, inhibition of mitochondrial fusion disrupted mitochondrial respiration and reactive oxygen species production, leading to cell cycle arrest at the G₀/G₁ transition. These results nominate the inhibition of mitochondrial fusion as a promising therapeutic approach for AML.

Mechanism of cystogenesis by Cd79a-driven, conditional mTOR activation in developing mouse nephrons

Polycystic kidney disease (PKD) is a common genetic disorder arising from developmental and postnatal processes. Defects in primary cilia and their signaling (eg, mTOR) underlie the pathogenesis. However, how mTOR regulates tubular integrity remains unclear. The paucity of faithful models has limited our understanding of pathogenesis and, therefore, the refinement of therapeutic targets. To understand the role of mTOR in early cystogenesis, we studied an in-house mouse model, Cd79a-Cre;Tsc1ff. (Cd79a-Tsc1 KO hereafter), recapitulating human autosomal-dominant PKD histology. Cre-mediated Tsc1 depletion driven by the promoter for Cd79a, a known B-cell receptor, activated mTORC1 exclusively along the distal nephron from embryonic day 16 onward. Cysts appeared in the distal nephron at 1 weeks of age and mice developed definite PKD by 4 weeks. Cd79a-Tsc1 KO tubule cells proliferated at a rate comparable to controls after birth but continued to divide even after postnatal day 14 when tubulogenesis is normally completed. Apoptosis occurred only after 9 weeks. During postnatal days 7-11, pre-cystic Cd79a-Tsc1 KO tubule cells showed cilia elongation, aberrant cell intercalation, and mitotic division, suggesting that defective cell planar polarity (PCP) may underlie cystogenesis. mTORC1 was activated in a portion of cyst-lining cells and occasionally even when Tsc1 was not depleted, implying a non-autonomous mechanism. Our results indicate that mTORC1 overactivation in developing distal tubules impairs their postnatal narrowing by disrupting morphogenesis, which orients an actively proliferating cell toward the elongating axis. The interplay between mTOR and cilium signaling, which coordinate cell proliferation with PCP, may be essential for cystogenesis.

mTORC2 Is Activated under Hypoxia and Could Support Chronic Myeloid Leukemia Stem Cells

Hypoxia is a critical condition that governs survival, self-renewal, quiescence, metabolic shift and refractoriness to leukemic stem cell (LSC) therapy. The present study aims to investigate the hypoxia-driven regulation of the mammalian Target of the Rapamycin-2 (mTORC2) complex to unravel it as a novel potential target in chronic myeloid leukemia (CML) therapeutic strategies. After inducing hypoxia in a CML cell line model, we investigated the activities of mTORC1 and mTORC2. Surprisingly, we detected a significant activation of mTORC2 at the expense of mTORC1, accompanied by the nuclear localization of the main substrate phospho-Akt (Ser473). Moreover, the Gene Ontology analysis of CML patients' CD34+ cells showed enrichment in the mTORC2 signature, further strengthening our data. The deregulation of mTOR complexes highlights how hypoxia could be crucial in CML development. In conclusion, we propose a mechanism by which CML cells residing under a low-oxygen tension, i.e., in the leukemia quiescent LSCs, singularly regulate the mTORC2 and its downstream effectors.

Splicing factor-mediated regulation patterns reveals biological characteristics and aid in predicting prognosis in acute myeloid leukemia

BACKGROUND

Alternative splicing (AS) of RNA is a fundamental biological process that shapes protein diversity. Many non-characteristic AS events are involved in the onset and development of acute myeloid leukemia (AML). Abnormal alterations in splicing factors (SFs), which regulate the onset of AS events, affect the process of splicing regulation. Hence, it is important to explore the relationship between SFs and the clinical features and biological processes of patients with AML.

METHODS

This study focused on SFs of the classical heterogeneous nuclear ribonucleoprotein (hnRNP) family and arginine and serine/arginine-rich (SR) splicing factor family. We explored the relationship between the regulation patterns associated with the expression of SFs and clinicopathological factors and biological behaviors of AML based on a multi-omics approach. The biological functions of SRSF10 in AML were further analyzed using clinical samples and in vitro experiments.

RESULTS

Most SFs were upregulated in AML samples and were associated with poor prognosis. The four splicing regulation patterns were characterized by differences in immune function, tumor mutation, signaling pathway activity, prognosis, and predicted response to chemotherapy and immunotherapy. A risk score model was constructed and validated as an independent prognostic factor for AML. Overall survival was significantly shorter in the high-risk score group. In addition, we confirmed that SRSF10 expression was significantly up-regulated in clinical samples of AML, and knockdown of SRSF10 inhibited the proliferation of AML cells and promoted apoptosis and G1 phase arrest during the cell cycle.

CONCLUSION

The analysis of splicing regulation patterns can help us better understand the differences in the tumor microenvironment of patients with AML and guide clinical decision-making and prognosis prediction. SRSF10 can be a potential therapeutic target and biomarker for AML.

RHEB is a potential therapeutic target in T cell acute lymphoblastic leukemia

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of immature T lymphocytes. Although various therapeutic approaches have been developed, refractoriness of chemotherapy and relapse cause a poor prognosis of the disease and further therapeutic strategies are required. Here, we report that Ras homolog enriched in brain (RHEB), a critical regulator of mTOR complex 1 activity, is a potential target for T-ALL therapy. In this study, we established an sgRNA library that comprehensively targeted mTOR upstream and downstream pathways, including autophagy. CRISPR/Cas9 dropout screening revealed critical roles of mTOR-related molecules in T-ALL cell survival. Among the regulators, we focused on RHEB because we previously found that it is dispensable for normal hematopoiesis in mice. Transcriptome and metabolic analyses revealed that RHEB deficiency suppressed de novo nucleotide biosynthesis, leading to human T-ALL cell death. Importantly, RHEB deficiency suppressed tumor growth in both mouse and xenograft models. Our data provide a potential strategy for efficient therapy of T-ALL by RHEB-specific inhibition.

Autophagy is required for spermatogonial differentiation in the Drosophila testis

Autophagy is a conserved, lysosome-dependent catabolic process of eukaryotic cells which is involved in cellular differentiation. Here, we studied its specific role in the differentiation of spermatogonial cells in the Drosophila testis. In the apical part of the Drosophila testis, there is a niche of germline stem cells (GSCs), which are connected to hub cells. Hub cells emit a ligand for bone morhphogenetic protein (BMP)-mediated signalling that represses Bam (bag of marbles) expression in GSCs to maintain them in an undifferentiated state. GSCs divide asymmetrically, and one of the daughter cells differentiates into a gonialblast, which eventually generates a cluster of spermatogonia (SG) by mitoses. Bam is active in SG, and defects in Bam function arrest these cells at mitosis. We show that BMP signalling represses autophagy in GSCs, but upregulates the process in SG. Inhibiting autophagy in SG results in an overproliferating phenotype similar to that caused by bam mutations. Furthermore, Bam deficiency leads to a failure in downstream mechanisms of the autophagic breakdown. These results suggest that the BMP-Bam signalling axis regulates developmental autophagy in the Drosophila testis, and that acidic breakdown of cellular materials is required for spermatogonial differentiation.

Targeting metabolism: A potential strategy for hematological cancer therapy

Most hematological cancer-related relapses and deaths are caused by metastasis; thus, the importance of this process as a target of therapy should be considered. Hematological cancer is a type of cancer in which metabolism plays an essential role in progression. Therefore, we are required to block fundamental metastatic processes and develop specific preclinical and clinical strategies against those biomarkers involved in the metabolic regulation of hematological cancer cells, which do not rely on primary tumor responses. To understand progress in this field, we provide a summary of recent developments in the understanding of metabolism in hematological cancer and a general understanding of biomarkers currently used and under investigation for clinical and preclinical applications involving drug development. The signaling pathways involved in cancer cell metabolism are highlighted and shed light on how we could identify novel biomarkers involved in cancer development and treatment. This review provides new insights into biomolecular carriers that could be targeted as anticancer biomarkers.

Lysosomes in Stem Cell Quiescence: A Potential Therapeutic Target in Acute Myeloid Leukemia

Lysosomes are cellular organelles that regulate essential biological processes such as cellular homeostasis, development, and aging. They are primarily connected to the degradation/recycling of cellular macromolecules and participate in cellular trafficking, nutritional signaling, energy metabolism, and immune regulation. Therefore, lysosomes connect cellular metabolism and signaling pathways. Lysosome's involvement in the critical biological processes has rekindled clinical interest towards this organelle for treating various diseases, including cancer. Recent research advancements have demonstrated that lysosomes also regulate the maintenance and hemostasis of hematopoietic stem cells (HSCs), which play a critical role in the progression of acute myeloid leukemia (AML) and other types of cancer. Lysosomes regulate both HSCs' metabolic networks and identity transition. AML is a lethal type of blood cancer with a poor prognosis that is particularly associated with aging. Although the genetic landscape of AML has been extensively described, only a few targeted therapies have been produced, warranting the need for further research. This review summarizes the functions and importance of targeting lysosomes in AML, while highlighting the significance of lysosomes in HSCs maintenance.

The Role of Nutrients in Maintaining Hematopoietic Stem Cells and Healthy Hematopoiesis for Life

Nutrients are converted by the body to smaller molecules, which are utilized for both anabolic and catabolic metabolic reactions. Cooperative regulation of these processes is critical for life-sustaining activities. In this review, we focus on how the regulation of nutrient-driven metabolism maintains healthy hematopoietic stem cells (HSCs). For this purpose, we have examined the metabolic regulation of HSCs from two perspectives: (1) the control of intracellular metabolism by the balance of anabolic and catabolic reactions; and (2) the control of organismal metabolic status and hematopoiesis by dietary intake of nutrients. Critical roles of catabolic regulators in stem cell homeostasis are conserved in several types of tissues, including hematopoiesis. These catabolic signals are also major regulators of organismal lifespan in multiple species. In parallel, changes to nutrients via alterations to dietary intake affect not only an organism’s metabolic state but also the behavior of its stem cells. While the molecular mechanisms involved in these two aspects of nutrient function may not necessarily overlap, a deeper understanding of these phenomena will point to new avenues of medical research and may furnish new agents for improving human health care.

Modulating mTOR Signaling as a Promising Therapeutic Strategy for Atherosclerosis

For more than a decade, atherosclerosis has been one of the leading causes of death in developed countries. The issue of treatment and prevention of the disease is especially acute. Despite the huge amount of basic and clinical research, a significant number of gaps remain in our understanding of the pathogenesis of atherosclerosis, and only their closure will bring us closer to understanding the causes of the disease at the cellular and molecular levels and, accordingly, to the development of an effective treatment. One of the seemingly well-studied elements of atherogenesis is the mTOR signaling pathway. However, more and more new details are still being clarified. Therapeutic strategies associated with rapamycin have worked well in a number of different diseases, and there is every reason to believe that targeting components of the mTOR pathway may pay off in atherosclerosis as well.

Alternative polyadenylation dysregulation contributes to the differentiation block of acute myeloid leukemia

Posttranscriptional regulation has emerged as a driver for leukemia development and an avenue for therapeutic targeting. Among posttranscriptional processes, alternative polyadenylation (APA) is globally dysregulated across cancer types. However, limited studies have focused on the prevalence and role of APA in myeloid leukemia. Furthermore, it is poorly understood how altered poly(A) site usage of individual genes contributes to malignancy or whether targeting global APA patterns might alter oncogenic potential. In this study, we examined global APA dysregulation in patients with acute myeloid leukemia (AML) by performing 3' region extraction and deep sequencing (3'READS) on a subset of AML patient samples along with healthy hematopoietic stem and progenitor cells (HSPCs) and by analyzing publicly available data from a broad AML patient cohort. We show that patient cells exhibit global 3' untranslated region (UTR) shortening and coding sequence lengthening due to differences in poly(A) site (PAS) usage. Among APA regulators, expression of FIP1L1, one of the core cleavage and polyadenylation factors, correlated with the degree of APA dysregulation in our 3'READS data set. Targeting global APA by FIP1L1 knockdown reversed the global trends seen in patients. Importantly, FIP1L1 knockdown induced differentiation of t(8;21) cells by promoting 3'UTR lengthening and downregulation of the fusion oncoprotein AML1-ETO. In non-t(8;21) cells, FIP1L1 knockdown also promoted differentiation by attenuating mechanistic target of rapamycin complex 1 (mTORC1) signaling and reducing MYC protein levels. Our study provides mechanistic insights into the role of APA in AML pathogenesis and indicates that targeting global APA patterns can overcome the differentiation block in patients with AML.

Investigating increased hematopoietic stem cell fitness in a novel mouse model

T-cell acute lymphoblastic leukaemia (T-ALL) is a bone marrow (BM) malignancy affecting children and adults. Typically treated with chemotherapy, leukaemia remains a major death cause in people under 20 years old. Understanding molecularly altered pathways in T-ALL may lead to new therapeutic avenues in the future. Ras pathway dysregulation is common in T-ALL. We have shown elevated expression levels of the Ras guanine nucleotide exchange factor RasGRP1 in T-ALL patients, which results in constant production of active Ras (RasGTP). When leukaemia cell lines are exposed to cytokines, RasGTP levels further increase in a RasGRP1-dependent manner. How overexpressed RasGRP1 may impact primary BM cells has remained unknown. We recently published a new RoLoRiG mouse model that allows for pIpC-induced overexpression of RasGRP1 in haematopoietic cells, which can be traced with an ires-EGFP cassette. This novel model revealed that RasGRP1 overexpression bestows a fitness advantage to haematopoietic stem cells (HSCs) over wild-type cells. Intriguingly, this increased fitness only manifests in native Hematopoiesis, and not in BM transplantation (BMT) assays. In this commentary, we summarize key features of our RoLoRiG model, elaborate on BM niche importance, and discuss differences between native Hematopoiesis and BMT in the context of stem cell metabolism.

Bone marrow microenvironment drives AML cell OXPHOS addiction and AMPK inhibition to resist chemotherapy

The stromal niche plays a pivotal role in AML chemoresistance and energy metabolism reprogramming is a hallmark of a tumor. 5′‐Adenosine monophosphate‐activated protein kinase (AMPK) is an important energy sensor suppressing mammalian target of rapamycin complex 1 (mTORC1) activity. However, the role of AMPK‐mTORC1 pathway on connecting AML cell energy metabolism reprogramming and chemoresistance induced by the bone marrow microenvironment (BMM) is not defined. Here, with a co‐culture system that simulates the interaction between BMM and AML cells, it is shown that stromal contact led to a decreased sensitivity to chemotherapy accompanied by an increase of oxidative phosphorylation (OXPHOS) activity and mitochondrial ATP synthesis in AML cells. The increased OXPHOS activity and excessive ATP production promoted chemoresistance of AML cells through inhibiting AMPK activity and in turn activating mTORC1 activity. In an in vivo AML mouse model, depletion of AMPK activity with genetic targeting promoted AML progression and reduced their sensitivity to chemotherapeutic drugs. Collectively, AML cells’ acquired increased OXPHOS activity as well as AMPK inhibition could be therapeutically exploited in an effort to overcome BMM‐mediated chemoresistance.

The Akt-mTOR network at the interface of hematopoietic stem cell homeostasis

Hematopoietic stem cells (HSCs) are immature blood cells that exhibit multilineage differentiation capacity. Homeostasis is critical for HSC potential and lifelong hematopoiesis, and HSC homeostasis is tightly governed by both intrinsic molecular networks and microenvironmental signals. The evolutionarily conserved serine/threonine protein kinase B (PKB, also referred to as Akt)-mammalian target of rapamycin (mTOR) pathway is universal to nearly all multicellular organisms and plays an integral role in most cellular processes. Emerging evidence has revealed a central role of the Akt-mTOR network in HSC homeostasis, because it responds to multiple intracellular and extracellular signals and regulates various downstream targets, eventually affecting several cellular processes, including the cell cycle, mitochondrial metabolism, and protein synthesis. Dysregulated Akt-mTOR signaling greatly affects HSC self-renewal, maintenance, differentiation, survival, autophagy, and aging, as well as transformation of HSCs to leukemia stem cells. Here, we review recent works and provide an advanced understanding of how the Akt-mTOR network regulates HSC homeostasis, thus offering insights into future clinical applications.

Reducing FASN expression sensitizes acute myeloid leukemia cells to differentiation therapy

Fatty acid synthase (FASN) is the only human lipogenic enzyme available for de novo fatty acid synthesis and is often highly expressed in cancer cells. We found that FASN mRNA levels were significantly higher in acute myeloid leukemia (AML) patients than in healthy granulocytes or CD34⁺ hematopoietic progenitors. Accordingly, FASN levels decreased during all-trans retinoic acid (ATRA)-mediated granulocytic differentiation of acute promyelocytic leukemia (APL) cells, partially via autophagic degradation. Furthermore, our data suggest that inhibition of FASN expression levels using RNAi or (-)-epigallocatechin-3-gallate (EGCG) accelerated the differentiation of APL cell lines and significantly re-sensitized ATRA refractory non-APL AML cells. FASN reduction promoted translocation of transcription factor EB (TFEB) to the nucleus, paralleled by activation of CLEAR network genes and lysosomal biogenesis. Together, our data demonstrate that inhibition of FASN expression in combination with ATRA treatment facilitates granulocytic differentiation of APL cells and may extend differentiation therapy to non-APL AML cells.

Hypoxia favors chemoresistance in T-ALL through an HIF1α-mediated mTORC1 inhibition loop

Resistance to chemotherapy, a major therapeutic challenge in the treatment of T-cell acute lymphoblastic leukemia (T-ALL), can be driven by interactions between leukemic cells and the microenvironment that promote survival of leukemic cells. The bone marrow, an important leukemia niche, has low oxygen partial pressures that highly participate in the regulation of normal hematopoiesis. Here we show that hypoxia inhibits T-ALL cell growth by slowing down cell cycle progression, decreasing mitochondria activity, and increasing glycolysis, making them less sensitive to antileukemic drugs and preserving their ability to initiate leukemia after treatment. Activation of the mammalian target of rapamycin (mTOR) was diminished in hypoxic leukemic cells, and treatment of T-ALL with the mTOR inhibitor rapamycin in normoxia mimicked the hypoxia effects, namely decreased cell growth and increased quiescence and drug resistance. Knocking down (KD) hypoxia-induced factor 1α (HIF-1α), a key regulator of the cellular response to hypoxia, antagonized the effects observed in hypoxic T-ALL and restored chemosensitivity. HIF-1α KD also restored mTOR activation in low O2 concentrations, and inhibiting mTOR in HIF1α KD T-ALL protected leukemic cells from chemotherapy. Thus, hypoxic niches play a protective role of T-ALL during treatments. Inhibition of HIF-1α and activation of the mTORC1 pathway may help suppress the drug resistance of T-ALL in hypoxic niches.

New Insights into the Pathogenesis of Systemic Mastocytosis

Mastocytosis is a type of myeloid neoplasm characterized by the clonal, neoplastic proliferation of morphologically and immunophenotypically abnormal mast cells that infiltrate one or more organ systems. Systemic mastocytosis (SM) is a more aggressive variant of mastocytosis with extracutaneous involvement, which might be associated with multi-organ dysfunction or failure and shortened survival. Over 80% of patients with SM carry the KIT D816V mutation. However, the KIT D816V mutation serves as a weak oncogene and appears to be a late event in the pathogenesis of mastocytosis. The management of SM is highly individualized and was largely palliative for patients without a targeted form of therapy in past decades. Targeted therapy with midostaurin, a multiple kinase inhibitor that inhibits KIT, has demonstrated efficacy in patients with advanced SM. This led to the recent approval of midostaurin by the United States Food and Drug Administration and European Medicines Agency. However, the overall survival of patients treated with midostaurin remains unsatisfactory. The identification of genetic and epigenetic alterations and understanding their interactions and the molecular mechanisms involved in mastocytosis is necessary to develop rationally targeted therapeutic strategies. This review briefly summarizes recent developments in the understanding of SM pathogenesis and potential treatment strategies for patients with SM.

Cytoplasmic DNA accumulation preferentially triggers cell death of myeloid leukemia cells by interacting with intracellular DNA sensing pathway

Accumulating evidence indicates the presence of cytoplasmic DNAs in various types of malignant cells, and its involvement in anti-cancer drug- or radiotherapy-mediated DNA damage response and replication stress. However, the pathophysiological roles of cytoplasmic DNAs in leukemias remain largely unknown. We observed that during hematopoietic stem cell transplantation (HSCT) in mouse myeloid leukemia models, double-stranded (ds)DNAs were constitutively secreted in the form of extracellular vesicles (EVs) from myeloid leukemia cells and were transferred to the donor cells to dampen their hematopoietic capabilities. Subsequent analysis of cytoplasmic DNA dynamics in leukemia cells revealed that autophagy regulated cytoplasmic dsDNA accumulation and subsequent redistribution into EVs. Moreover, accumulated cytoplasmic dsDNAs activated STING pathway, thereby reducing leukemia cell viability through reactive oxygen species (ROS) generation. Pharmaceutical inhibition of autophagosome formation induced cytoplasmic DNA accumulation, eventually triggering cytoplasmic DNA sensing pathways to exert cytotoxicity, preferentially in leukemia cells. Thus, manipulation of cytoplasmic dsDNA dynamics can be a novel and potent therapeutic strategy for myeloid leukemias.

Cell signaling pathways as molecular targets to eliminate AML stem cells

Acute myeloid leukemia (AML) remains the most lethal of leukemias and a small population of cells called leukemic stem cells (LSCs) has been associated with disease relapses. Some cell signaling pathways play an important role in AML survival, proliferation and self-renewal properties and are abnormally activated or suppressed in LSCs. This includes the NF-κB, Wnt/β-catenin, Hedgehog, Notch, EGFR, JAK/STAT, PI3K/AKT/mTOR, TGF/SMAD and PPAR pathways. This review aimed to discuss these pathways as molecular targets for eliminating AML LSCs. Herein, inhibitors/activators of these pathways were summarized as a potential new anti-AML therapy capable of eliminating LSCs to guide future researches. The clinical use of cell signaling pathways data can be useful to enhance the anti-AML therapy.

Essential role of autophagy in protecting neonatal haematopoietic stem cells from oxidative stress in a p62-independent manner

Autophagy is a cellular degradation system contributing to homeostasis of tissue stem cells including haematopoietic stem cells (HSCs). It plays pleiotropic roles in HSC characteristics throughout life, but its stage-specific roles in HSC self-renewal are unclear. To investigate the effects of Atg5 deletion on stage-specific HSC functions, we compared the repopulating capacity of HSCs in Atg5^f/f;Vavi-cre mice from postnatal day (P) 0-7 weeks of age. Interestingly, Atg5 deficiency led to no remarkable abnormality in the HSC self-renewal capacity at P0, but significant defects at P7, followed by severe defects. Induction of Atg5 deletion at P5 by tamoxifen administration to Atg5^f/f;Rosa26-Cre-ER^T2 mice resulted in normal haematopoiesis, including the HSC population, until around 1 year, suggesting that Atg5 in the early neonatal period was critical for haematopoiesis in adults. Mitochondrial oxidative stress was increased by Atg5 loss in neonatal HSC/progenitor cells. Although p62 had accumulated in immature bone marrow cells of Atg5^f/f;Vavi-cre mice, p62 deletion did not restore defective HSC functions, indicating that Atg5-dependent haematopoietic regulation in the developmental period was independent of p62. This study proposes a critical role of autophagy in HSC protection against harsh environments in the early neonatal stage, which is essential for healthy long-term haematopoiesis.

Imaging dynamic mTORC1 pathway activity in vivo reveals marked shifts that support time-specific inhibitor therapy in AML

Acute myeloid leukemia (AML) is a high remission, high relapse fatal blood cancer. Although mTORC1 is a master regulator of cell proliferation and survival, its inhibitors have not performed well as AML treatments. To uncover the dynamics of mTORC1 activity in vivo, fluorescent probes are developed to track single cell proliferation, apoptosis and mTORC1 activity of AML cells in the bone marrow of live animals and to quantify these activities in the context of microanatomical localization and intra-tumoral heterogeneity. When chemotherapy drugs commonly used clinically are given to mice with AML, apoptosis is rapid, diffuse and not preferentially restricted to anatomic sites. Dynamic measurement of mTORC1 activity indicated a decline in mTORC1 activity with AML progression. However, at the time of maximal chemotherapy response, mTORC1 signaling is high and positively correlated with a leukemia stemness transcriptional profile. Cell barcoding reveals the induction of mTORC1 activity rather than selection of mTORC1 high cells and timed inhibition of mTORC1 improved the killing of AML cells. These data define the real-time dynamics of AML and the mTORC1 pathway in association with AML growth, response to and relapse after chemotherapy. They provide guidance for timed intervention with pathway-specific inhibitors.

The role of mTORC1 in AML has not yet been proven due to the mixed results of its inhibitors in clinical trials. Here the authors show the real-time dynamics of the mTORC1 pathway in association with AML growth and response to chemotherapy with fluorescent markers, providing guidance for timed intervention with pathway-specific inhibitors.

Signaling Pathways Involved in Nutrient Sensing Control in Cancer Stem Cells: An Overview

Cancer cells characteristically have a high proliferation rate. Because tumor growth depends on energy-consuming anabolic processes, including biosynthesis of protein, lipid, and nucleotides, many tumor-associated conditions, including intermittent oxygen deficiency due to insufficient vascularization, oxidative stress, and nutrient deprivation, results from fast growth. To cope with these environmental stressors, cancer cells, including cancer stem cells, must adapt their metabolism to maintain cellular homeostasis. It is well- known that cancer stem cells (CSC) reprogram their metabolism to adapt to live in hypoxic niches. They usually change from oxidative phosphorylation to increased aerobic glycolysis even in the presence of oxygen. However, as opposed to most differentiated cancer cells relying on glycolysis, CSCs can be highly glycolytic or oxidative phosphorylation-dependent, displaying high metabolic plasticity. Although the influence of the metabolic and nutrient-sensing pathways on the maintenance of stemness has been recognized, the molecular mechanisms that link these pathways to stemness are not well known. Here in this review, we describe the most relevant signaling pathways involved in nutrient sensing and cancer cell survival. Among them, Adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway, mTOR pathway, and Hexosamine Biosynthetic Pathway (HBP) are critical sensors of cellular energy and nutrient status in cancer cells and interact in complex and dynamic ways.

Lysosomes in acute myeloid leukemia: potential therapeutic targets?

Lysosomes, since their discovery, have been primarily known for degrading cellular macromolecules. However, in recent studies, they have begun to emerge as crucial regulators of cell homeostasis. They are at the crossroads of catabolic and anabolic pathways and are intricately involved in cellular trafficking, nutrient signaling, energy metabolism, and immune regulation. Their involvement in such essential cellular functions has renewed clinical interest in targeting the lysosome as a novel way to treat disease, particularly cancer. Acute myeloid leukemia (AML) is an aggressive blood cancer with a low survival probability, particularly in older patients. The genomic landscape of AML has been extensively characterized but few targeted therapies (with the exception of differentiation therapy) can achieve a long-term cure. Therefore, there is an unmet need for less intensive and more tolerable therapeutic interventions. In this review, we will give an overview on the myriad of functions performed by lysosomes and their importance in malignant disease. Furthermore, we will discuss their relevance in hematopoietic cells and different ways to potentially target them in AML.

Dual Kinase Targeting in Leukemia

Simple Summary

A new option to treat cancer is based on the use of so-called multi-targeting drugs. This strategy can replace the standard treatment based on the co-administration of several drugs. An increased and uncontrolled activity of kinases (enzymes devoted to the regulation of several cell pathways) is often seen in hematological malignancies. The development of multi-kinase inhibitors is having a great impact on the treatment of this kind of cancer. Here, we review the most recent findings on this novel class of drugs.

Abstract

Pharmacological cancer therapy is often based on the concurrent inhibition of different survival pathways to improve treatment outcomes and to reduce the risk of relapses. While this strategy is traditionally pursued only through the co-administration of several drugs, the recent development of multi-targeting drugs (i.e., compounds intrinsically able to simultaneously target several macromolecules involved in cancer onset) has had a dramatic impact on cancer treatment. This review focuses on the most recent developments in dual-kinase inhibitors used in acute myeloid leukemia (AML), chronic myelogenous leukemia (CML), and lymphoid tumors, giving details on preclinical studies as well as ongoing clinical trials. A brief overview of dual-targeting inhibitors (kinase/histone deacetylase (HDAC) and kinase/tubulin polymerization inhibitors) applied to leukemia is also given. Finally, the very recently developed Proteolysis Targeting Chimeras (PROTAC)-based kinase inhibitors are presented.

A cell-type deconvolution meta-analysis of whole blood EWAS reveals lineage-specific smoking-associated DNA methylation changes

Highly reproducible smoking-associated DNA methylation changes in whole blood have been reported by many Epigenome-Wide-Association Studies (EWAS). These epigenetic alterations could have important implications for understanding and predicting the risk of smoking-related diseases. To this end, it is important to establish if these DNA methylation changes happen in all blood cell subtypes or if they are cell-type specific. Here, we apply a cell-type deconvolution algorithm to identify cell-type specific DNA methylation signals in seven large EWAS. We find that most of the highly reproducible smoking-associated hypomethylation signatures are more prominent in the myeloid lineage. A meta-analysis further identifies a myeloid-specific smoking-associated hypermethylation signature enriched for DNase Hypersensitive Sites in acute myeloid leukemia. These results may guide the design of future smoking EWAS and have important implications for our understanding of how smoking affects immune-cell subtypes and how this may influence the risk of smoking related diseases.

Smoking-associated DNA methylation changes in whole blood have been reported by many EWAS. Here, the authors use a cell-type deconvolution algorithm to identify cell-type specific DNA methylation signals in seven EWAS, identifying lineage-specific smoking-associated DNA methylation changes.

B-cell lymphoma-2 inhibition and resistance in acute myeloid leukemia

Spurred by better understanding of disease biology, improvements in molecular diagnostics, and the development of targeted therapies, the treatment of acute myeloid leukemia (AML) has undergone significant evolution in recent years. Arguably, the most exciting shift has come from the success of treatment with the B-cell lymphoma-2 inhibitor venetoclax. When given in combination with a hypomethylating agent or low dose cytarabine, venetoclax demonstrates high response rates, some of which are durable. In spite of this, relapses after venetoclax treatment are common, and much interest exists in elucidating the mechanisms of resistance to the drug. Alterations in leukemic stem cell metabolism have been identified as a possible escape route, and clinical trials focusing on targeting metabolism in AML are ongoing. This review article highlights current research regarding venetoclax treatment and resistance in AML with a focus on cellular metabolism.

Next Generation Strategies for Geroprotection via mTORC1 Inhibition

Inhibition of mTORC1 (mechanistic Target Of Rapamycin Complex 1) with the pharmaceutical rapamycin prolongs the lifespan and healthspan of model organisms including rodents, with evidence now emerging that rapamycin and its analogs may also have rejuvenative effects in dogs and humans. However, the side effects associated with long-term rapamycin treatment, many of which are due to inhibition of a second mTOR complex, mTORC2, have seemed to preclude the routine use of rapamycin as a therapy for age-related diseases. Here, we discuss recent findings suggesting that strong, chronic inhibition of both mTOR complexes may not be necessary to realize the geroprotective effects of rapamycin. Instead, modestly but specifically inhibiting mTORC1 via a variety of emerging techniques, including intermittent or transient treatment with rapamycin derivatives, or specific dietary regimens, may be sufficient to promote health and longevity with reduced side effects. We will also discuss prospects for the development of new molecules that, by harnessing the detailed molecular understanding of mTORC1 signaling developed over the last decade, will provide new routes to the selective inhibition of mTORC1. We conclude that therapies based on the selective inhibition of mTORC1 may soon permit the safer treatment of diseases of aging.

Non-Thermal Plasma Induces Antileukemic Effect Through mTOR Ubiquitination

Non-thermal plasma (NTP) has been studied as a novel therapeutic tool for cancer that does not damage healthy cells. In this study, we show that NTP-treated solutions (NTS) can induce death in various leukemia cells through mechanistic target of rapamycin (mTOR) ubiquitination. Previously, we manufactured and demonstrated the efficacy of NTS in solid cancers. NTS did not exhibit any deleterious side effects, such as acute death or weight loss in nude mice. In the present study, NTS induced cell death in myeloid leukemia cells, including acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). We found that mTOR was downregulated in NTS-treated cells via the ubiquitin-proteasome system (UPS). We also identified ‘really interesting new gene’ finger protein 126 (RNF126) as a novel binding protein for mTOR through protein arrays and determined the role of E3 ligase in NTS-induced mTOR ubiquitination. NTS-derived reactive oxygen species (ROS) affected RNF126 expression and lysosomal dysfunction. These findings suggest that NTS has potential antileukemic effects through RNF126-mediated mTOR ubiquitination with no deleterious side effects. Thus, NTS may represent a new therapeutic method for chemotherapy-resistant leukemia.

mTORC1 as a cell-intrinsic rheostat that shapes development, preimmune repertoire, and function of B lymphocytes

Ample evidence indicates that nutrient concentrations in extracellular milieux affect signaling mediated by environmental sensor proteins. For instance, the mechanistic target of rapamycin (mTOR) is reduced during protein malnutrition and is known to be modulated by concentrations of several amino acids when in a multiprotein signaling complex that contains regulatory-associated protein of mTOR. We hypothesized that a partial decrease in mTOR complex 1 (mTORC1) activity intrinsic to B-lineage cells would perturb lymphocyte development or function, or both. We show that a cell-intrinsic decrease in mTORC1 activity impacted developmental progression, antigen receptor repertoire, and function along the B lineage. Thus, preimmune repertoires of B-lineage cells were altered in the marrow and periphery in a genetic model of regulatory-associated protein of mTOR haplo-insufficiency. An additional role for mTORC1 was revealed when a B-cell antigen receptor transgene was found to circumvent the abnormal B-cell development: haploinsufficient B cells were profoundly impaired in responses to antigen in vivo. Collectively, our findings indicate that mTORC1 serves as a rheostat that shapes differentiation along the B lineage, the preimmune repertoire, and antigen-driven selection of mature B cells. The findings also reveal a range in the impact of this nutrient sensor on activity-response relationships for distinct endpoints.-Raybuck, A. L., Lee, K., Cho, S. H., Li, J., Thomas, J. W., Boothby, M. R. mTORC1 as a cell-intrinsic rheostat that shapes development, preimmune repertoire, and function of B lymphocytes.

mTORC1 activity is essential for erythropoiesis and B cell lineage commitment

Mechanistic target of rapamycin (mTOR) is a serine/threonine protein kinase that mediates phosphoinositide-3-kinase (PI3K)/AKT signalling. This pathway is involved in a plethora of cellular functions including protein and lipid synthesis, cell migration, cell proliferation and apoptosis. In this study, we proposed to delineate the role of mTORC1 in haemopoietic lineage commitment using knock out (KO) mouse and cell line models. Mx1-cre and Vav-cre expression systems were used to specifically target Raptorfl/fl (mTORC1), either in all tissues upon poly(I:C) inoculation, or specifically in haemopoietic stem cells, respectively. Assessment of the role of mTORC1 during the early stages of development in Vav-cre+Raptorfl/fl mice, revealed that these mice do not survive post birth due to aberrations in erythropoiesis resulting from an arrest in development at the megakaryocyte-erythrocyte progenitor stage. Furthermore, Raptor-deficient mice exhibited a block in B cell lineage commitment. The essential role of Raptor (mTORC1) in erythrocyte and B lineage commitment was confirmed in adult Mx1-cre+Raptorfl/fl mice upon cre-recombinase induction. These studies were supported by results showing that the expression of key lineage commitment regulators, GATA1, GATA2 and PAX5 were dysregulated in the absence of mTORC1-mediated signals. The regulatory role of mTOR during erythropoiesis was confirmed in vitro by demonstrating a reduction of K562 cell differentiation towards RBCs in the presence of established mTOR inhibitors. While mTORC1 plays a fundamental role in promoting RBC development, we showed that mTORC2 has an opposing role, as Rictor-deficient progenitor cells exhibited an elevation in RBC colony formation ex vivo. Collectively, our data demonstrate a critical role played by mTORC1 in regulating the haemopoietic cell lineage commitment.

Tuning mTORC1 activity dictates the response of acute myeloid leukemia to LSD1 inhibition

Lysine specific demethylase-1 (LSD1) has been shown to be critical in acute myeloid leukemia (AML) pathogenesis and this has led to the development of LSD1 inhibitors (LSD1i) which are currently tested in clinical trials. Nonetheless, preclinical studies reported that AML cells frequently exhibit intrinsic resistance to LSD1 inhibition, and the molecular basis for this phenomenon is largely unknown. We explored the potential involvement of mammalian target of rapamycin (mTOR) in mediating the resistance of leukemic cells to LSD1i. Strikingly, unlike sensitive leukemias, mTOR complex 1 (mTORC1) signaling was robustly triggered in resistant leukemias following LSD1 inhibition. Transcriptomic, chromatin immunoprecipitation and functional studies revealed that insulin receptor substrate 1(IRS1)/extracellular-signal regulated kinases (ERK1/2) signaling critically controls LSD1i induced mTORC1 activation. Notably, inhibiting mTOR unlocked the resistance of AML cell lines and primary patient-derived blasts to LSD1i both in vitro and in vivo. In conclusion, mTOR activation might act as a novel pro-survival mechanism of intrinsic as well as acquired resistance to LSD1i, and combination regimens co-targeting LSD1/mTOR could represent a rational approach in AML therapy.

The Role of AMPK/mTOR Modulators in the Therapy of Acute Myeloid Leukemia

Differentiation therapy of acute promyelocytic leukemia with all-trans retinoic acid represents the most successful pharmacological therapy of acute myeloid leukemia (AML). Numerous studies demonstrate that drugs that inhibit mechanistic target of rapamycin (mTOR) and activate AMP-kinase (AMPK) have beneficial effects in promoting differentiation and blocking proliferation of AML. Most of these drugs are already in use for other purposes; rapalogs as immunosuppressants, biguanides as oral antidiabetics, and 5-amino-4-imidazolecarboxamide ribonucleoside (AICAr, acadesine) as an exercise mimetic. Although most of these pharmacological modulators have been widely used for decades, their mechanism of action is only partially understood. In this review, we summarize the role of AMPK and mTOR in hematological malignancies and discuss the possible role of pharmacological modulators in proliferation and differentiation of leukemia cells.

mTOR as a central regulator of lifespan and aging

The mammalian/mechanistic target of rapamycin (mTOR) is a key component of cellular metabolism that integrates nutrient sensing with cellular processes that fuel cell growth and proliferation. Although the involvement of the mTOR pathway in regulating life span and aging has been studied extensively in the last decade, the underpinning mechanisms remain elusive. In this review, we highlight the emerging insights that link mTOR to various processes related to aging, such as nutrient sensing, maintenance of proteostasis, autophagy, mitochondrial dysfunction, cellular senescence, and decline in stem cell function.

Autophagy regulation and its role in normal and malignant hematopoiesis

Autophagy, the molecular machinery of self-eating, plays a dual role of a tumor promoter and tumor suppressor. This mechanism affects different clinical responses in cancer cells. Autophagy is targeted for treating patients resistant to chemotherapy or radiation. Limited reports investigate the significance of autophagy in cancer therapy, the regulation of hematopoietic and leukemic stem cells and leukemia formation. In the current review, the role of autophagy is discussed in various stages of hematopoiesis including quiescence, self-renewal, and differentiation.

Signaling Pathways in Leukemic Stem Cells

Hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) utilize many of the same signaling pathways for their maintenance and survival. In this review, we will focus on several signaling pathways whose roles have been extensively studied in both HSCs and LSCs. Our main focus will be on the PI3K/AKT/mTOR pathway and several of its regulators and downstream effectors. We will also discuss several other signaling pathways of particular importance in LSCs, including the WNT/β-catenin pathway, the NOTCH pathway, and the TGFβ pathway. For each of these pathways, we will emphasize differences in how these pathways operate in LSCs, compared to their function in HSCs, to highlight opportunities for the specific therapeutic targeting of LSCs. We will also highlight areas of crosstalk between multiple signaling pathways that may affect LSC function.

Autophagy in Development and Differentiation

Autophagy is crucial in the differentiation and development of both mammals and invertebrates, as a rapid response to environmental and hormonal stimuli. Autophagy is also important for intracellular renewal, maintaining the health of terminally differentiated cells. Studies of Drosophila, Caenorhabditis elegans, and other species revealed abnormal autophagy lead to developmental and differential abnormality, including those in salivary glands and midgut development, protein aggregation, removal of apoptotic cell corpses, and development of dauer and synapse. Autophagy also participates in the development of mammalian embryos before implantation into the uterus, adaption to the nascent hunger environment, blood cells production, and cell differentiation in adipogenesis. Autophagy found in various stem cells, like hematopoietic stem cells, bone marrow mesenchymal stem cells and neural stem cells (NSCs), is tightly associated with their self-renewal, directed differentiation, and senescence.

Hif-1α Deletion May Lead to Adverse Treatment Effect in a Mouse Model of MLL-AF9-Driven AML

Relapse of acute myeloid leukemia (AML) remains a significant clinical challenge due to limited therapeutic options and poor prognosis. Leukemic stem cells (LSCs) are the cellular units responsible for relapse in AML, and strategies that target LSCs are thus critical. One proposed potential strategy to this end is to break the quiescent state of LSCs, thereby sensitizing LSCs to conventional cytostatics. The hypoxia-inducible factor (HIF) pathway is a main driver of cellular quiescence and a potential therapeutic target, with precedence from both solid cancers and leukemias. Here, we used a conditional knockout Hif-1α mouse model together with a standard chemotherapy regimen to evaluate LSC targeting in AML. Contrary to expectation, our studies revealed that Hif-1α-deleted-leukemias displayed a faster disease progression after chemotherapy. Our studies thereby challenge the general notion of cancer stem cell sensitization by inhibition of the HIF pathway, and warrant caution when applying HIF inhibition in combination with chemotherapy in AML.

•

Deletion of Hif-1α accelerates the progression of chemotherapy-treated MLL-AF9-AML

•

Deletion of Hif-1α does not decrease LSC frequency after chemotherapy

•

Chemotherapy targets more mature cells indicated by transcriptional analysis

•

Hif-1α deletion affects few transcriptional pathways in AML cells

The role of mTOR-mediated signals during haemopoiesis and lineage commitment

The serine/threonine protein kinase mechanistic target of rapamycin (mTOR) has been implicated in the regulation of an array of cellular functions including protein and lipid synthesis, proliferation, cell size and survival. Here, we describe the role of mTOR during haemopoiesis within the context of mTORC1 and mTORC2, the distinct complexes in which it functions. The use of conditional transgenic mouse models specifically targeting individual mTOR signalling components, together with selective inhibitors, have generated a significant body of research emphasising the critical roles played by mTOR, and individual mTOR complexes, in haemopoietic lineage commitment and development. This review will describe the profound role of mTOR in embryogenesis and haemopoiesis, underscoring the importance of mTORC1 at the early stages of haemopoietic cell development, through modulation of stem cell potentiation and self-renewal, and erythroid and B cell lineage commitment. Furthermore, the relatively discrete role of mTORC2 in haemopoiesis will be explored during T cell development and B cell maturation. Collectively, this review aims to highlight the functional diversity of mTOR signalling and underline the importance of this pathway in haemopoiesis.

A Western Blotting Protocol for Small Numbers of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) are rare cells, with the mouse bone marrow containing only ~25,000 phenotypic long term repopulating HSCs. A Western blotting protocol was optimized and suitable for the analysis of small numbers of HSCs (500 - 15,000 cells). Phenotypic HSCs were purified, accurately counted, and directly lysed in Laemmli sample buffer. Lysates containing equal numbers of cells were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and the blot was prepared and processed following standard Western blotting protocols. Using this protocol, 2,000 - 5,000 HSCs can be routinely analyzed, and in some cases data can be obtained from as few as 500 cells, compared to the 20,000 to 40,000 cells reported in most publications. This protocol should be generally applicable to other hematopoietic cells, and enables the routine analysis of small numbers of cells using standard laboratory procedures.

Biological Aspects of mTOR in Leukemia

The mammalian target of rapamycin (mTOR) is a central processor of intra- and extracellular signals, regulating many fundamental cellular processes such as metabolism, growth, proliferation, and survival. Strong evidences have indicated that mTOR dysregulation is deeply implicated in leukemogenesis. This has led to growing interest in the development of modulators of its activity for leukemia treatment. This review intends to provide an outline of the principal biological and molecular functions of mTOR. We summarize the current understanding of how mTOR interacts with microRNAs, with components of cell metabolism, and with controllers of apoptotic machinery. Lastly, from a clinical/translational perspective, we recapitulate the therapeutic results in leukemia, obtained by using mTOR inhibitors as single agents and in combination with other compounds.

An ERK-Dependent Feedback Mechanism Prevents Hematopoietic Stem Cell Exhaustion

Hematopoietic stem cells (HSCs) sustain hematopoiesis throughout life. HSCs exit dormancy to restore hemostasis in response to stressful events, such as acute blood loss, and must return to a quiescent state to prevent their exhaustion and resulting bone marrow failure. HSC activation is driven in part through the phosphatidylinositol 3-kinase (PI3K)/AKT/mTORC1 signaling pathway, but less is known about the cell-intrinsic pathways that control HSC dormancy. Here, we delineate an ERK-dependent, rate-limiting feedback mechanism that controls HSC fitness and their re-entry into quiescence. We show that the MEK/ERK and PI3K pathways are synchronously activated in HSCs during emergency hematopoiesis and that feedback phosphorylation of MEK1 by activated ERK counterbalances AKT/mTORC1 activation. Genetic or chemical ablation of this feedback loop tilts the balance between HSC dormancy and activation, increasing differentiated cell output and accelerating HSC exhaustion. These results suggest that MEK inhibitors developed for cancer therapy may find additional utility in controlling HSC activation.

•

MEK/ERK and AKT/mTORC1 are reversibly activated during hematopoiesis

•

MEK1 prevents HSC exhaustion during stress hematopoiesis

•

Feedback phosphorylation of MEK1 by ERK limits AKT/mTORC1 activation

•

ERK-mediated MEK1 phosphorylation returns activated HSCs to quiescence

Genetic Variants Within MTORC1 Genes Predict Gastric Cancer Prognosis in Chinese Populations

Objective: Mammalian target of rapamycin complex 1 (mTORC1) plays an important role in maintaining proper cellular functions in gastric cancer (GC). Previous studies demonstrated genetic variants within mTORC1 genes were associated with GC risk. However, no studies reported the associations between genetic variants within mTORC1 genes and GC prognosis. Herein, we firstly assessed the associations of genetic variants of mTORC1 genes with overall survival (OS) of GC in Chinese populations.

Methods: We genotyped eight single nucleotide polymorphisms (SNPs) in mTORC1 genes (i.e., rs2536 T>C and rs1883965 G>A for mTOR, rs3160 T>C and rs26865 A>G for MLST8, rs3751934 C>A, rs1062935 T>C, rs3751932 T>C and rs12602885 G>A for RPTOR) by the TaqMan method in 197 Chinese GC patients who had surgical resection in Xinhua Hospital. We conducted Kaplan-Meier survival plots and Cox hazards regression analysis to explore the associations of these SNPs with OS.

Results: The single-locus analysis indicated that RPTOR rs1062935 T>C was associated with an increased risk of poor GC prognosis (CC vs. TT/TC: adjusted Hazard ratio (HR) = 1.71, 95% confidence interval (CI) = 1.04-2.82). The combined analysis of all eight SNPs showed that patients with more than three risk genotypes significantly increased risk of death (adjusted HR = 2.44, 95% CI = 1.30-4.58), when compared to those with three or less risk genotypes.

Conclusions: Our findings indicated that genetic variants within mTORC1 genes may predict GC prognosis in Chinese populations. The results need to be validated in future studies with larger sample sizes.

A population of innate myelolymphoblastoid effector cell expanded by inactivation of mTOR complex 1 in mice

Adaptive autoimmunity is restrained by controlling population sizes and pathogenicity of harmful clones, while innate destruction is controlled at effector phase. We report here that deletion of Rptor in mouse hematopoietic stem/progenitor cells causes self-destructive innate immunity by massively increasing the population of previously uncharacterized innate myelolymphoblastoid effector cells (IMLECs). Mouse IMLECs are CD3-B220-NK1.1-Ter119- CD11clow/-CD115-F4/80low/-Gr-1- CD11b+, but surprisingly express high levels of PD-L1. Although they morphologically resemble lymphocytes and actively produce transcripts from Immunoglobulin loci, IMLECs have non-rearranged Ig loci, are phenotypically distinguishable from all known lymphocytes, and have a gene signature that bridges lymphoid and myeloid leukocytes. Rptor deletion unleashes differentiation of IMLECs from common myeloid progenitor cells by reducing expression of Myb. Importantly, IMLECs broadly overexpress pattern-recognition receptors and their expansion causes systemic inflammation in response to Toll-like receptor ligands in mice. Our data unveil a novel leukocyte population and an unrecognized role of Raptor/mTORC1 in innate immune tolerance.

Dueling for dual inhibition: Means to enhance effectiveness of PI3K/Akt/mTOR inhibitors in AML

The phosphatidylinositol 3-kinase/protein kinase B (Akt)/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathway is amplified in 60-80% of patients with acute myelogenous leukemia (AML). Since this complex pathway is crucial to cell functions such as growth, proliferation, and survival, inhibition of this pathway would be postulated to inhibit leukemia initiation and propagation. Inhibition of the mTORC1 pathway has met with limited success in AML due to multiple resistance mechanisms including direct insensitivity of the mTORC1 complex, feedback activation of the PI3k/Akt signaling network, insulin growth factor-1 (IGF-1) activation of PI3K, and others. This review explores the role of mTOR inhibition in AML, mechanisms of resistance, and means to improve outcomes through use of dual mTORC1/2 inhibitors or dual TORC/PI3K inhibitors. How these inhibitors interface with currently available therapies in AML will require additional preclinical experiments and conduct of well-designed clinical trials.