mTORC1 is essential for leukemia propagation but not stem cell self-renewal
01 Jun 2012-Journal of Clinical Investigation (American Society for Clinical Investigation)-Vol. 122, Iss: 6, pp 2114-2129
TL;DR: Transplantation of Raptor-deficient undifferentiated AML cells in a limiting dilution revealed that mTORC1 is essential for leukemia initiation, and it was demonstrated that the reactivation of m TORC1 in those cells restored their leukemia-initiating capacity.
Abstract: Although dysregulation of mTOR complex 1 (mTORC1) promotes leukemogenesis, how mTORC1 affects established leukemia is unclear. We investigated the role of mTORC1 in mouse hematopoiesis using a mouse model of conditional deletion of Raptor, an essential component of mTORC1. Raptor deficiency impaired granulocyte and B cell development but did not alter survival or proliferation of hematopoietic progenitor cells. In a mouse model of acute myeloid leukemia (AML), Raptor deficiency significantly suppressed leukemia progression by causing apoptosis of differentiated, but not undifferentiated, leukemia cells. mTORC1 did not control cell cycle or cell growth in undifferentiated AML cells in vivo. Transplantation of Raptor-deficient undifferentiated AML cells in a limiting dilution revealed that mTORC1 is essential for leukemia initiation. Strikingly, a subset of AML cells with undifferentiated phenotypes survived long-term in the absence of mTORC1 activity. We further demonstrated that the reactivation of mTORC1 in those cells restored their leukemia-initiating capacity. Thus, AML cells lacking mTORC1 activity can self-renew as AML stem cells. Our findings provide mechanistic insight into how residual tumor cells circumvent anticancer therapies and drive tumor recurrence.
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TL;DR: The profound role of m TOR in embryogenesis and haemopoiesis is described, 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.
Abstract: 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.
10 citations
TL;DR: It is demonstrated that down modulating of SMC4 expression could prolong the survival time of AML mice and provide new insights into the mechanism of LSC maintenance.
Abstract: The gene, structural maintenance of chromosomes 4 (SMC4) plays important role in chromosomes condensing and mitotic sister chromatid segregation, which has been revealed in regulating multiple cancer development and carcinogenesis. However, the role of SMC4 in acute myeloid leukemia (AML) propagation and its function in regulation of leukemia stem cells (LSCs) is not yet clear. Using an MLL-AF9 induced AML mouse model, we demonstrated that down modulating of SMC4 expression could prolong the survival time of AML mice. Furthermore, we found that knockdown SMC4 expression decreased the proportion of LSCs and affected its leukemia-initiating capacity. Cell cycle assay demonstrated that more LSCs were arrested in G0 phase by SMC4 knockdown. This activity was accompanied by increased expression of the Cdkn1a (P21) and Cdkn1b (P27) as well as decreased expression of CDK4. Therefore, our study revealed the critical role of SMC4 during AML progression and provided new insights into the mechanism of LSC maintenance.
8 citations
TL;DR: An important attribute of rapamycin treatment during transduction is the preservation of HSC function, allowing reconstitution of long-term hematopoiesis in vivo in murine models.
Abstract: Purpose of review A major goal in repopulating hematopoietic stem cell (HSC) gene therapies is achieving high-efficacy gene transfer, while maintaining robust HSC engraftment and differentiation in vivo. Recent studies have documented that rapamycin treatment of HSC during lentiviral vector transduction enhances gene transfer to human and mouse HSCs and maintains engraftment capacity. In this review, we place into context the role of mammalian target of rapamycin (mTOR) pathways in HSC quiescence and function, endocytic regulation, and lentiviral gene delivery. Recent findings Lentiviral vector transduction of human and mouse HSCs is considerably enhanced by rapamycin treatment. Furthermore, rapamycin preserves long-term engraftment of human and mouse HSCs. Investigations of cellular mechanisms that contribute to increased transduction in HSCs uncover a role for mTOR inhibition-dependent activation of endocytosis. Summary Rapamycin enhances lentiviral vector transduction of HSCs through regulation of endocytic activity via mTOR inhibition. An important attribute of rapamycin treatment during transduction is the preservation of HSC function, allowing reconstitution of long-term hematopoiesis in vivo in murine models.
8 citations
Cites background from "mTORC1 is essential for leukemia pr..."
...Inhibition of mTORC1 through Raptor depletion causes pancytopenia and skewing of hematopoiesis toward myelomonocytic lineages through impaired granulocyte and B-cell development, but does not affect proliferation of hematopoietic and myeloid progenitors [36,38,39]....
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TL;DR: In this paper , the authors examined the role of alternative polyadenylation (APA) in acute myeloid leukemia (AML) by performing 3' region extraction and deep sequencing (3'READS) on a subset of AML patient samples.
8 citations
TL;DR: In this article, the authors investigated the effects of Atg5 deletion on stage-specific HSC functions, and compared the repopulating capacity of HSCs in Atg 5f/f;Vavi-cre mice from postnatal day (P) 0-7 weeks of age.
Abstract: 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 Atg5f/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 Atg5f/f;Rosa26-Cre-ERT2 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 Atg5f/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.
8 citations
References
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TL;DR: Stem cell biology has come of age: Unequivocal proof that stem cells exist in the haematopoietic system has given way to the prospective isolation of several tissue-specific stem and progenitor cells, the initial delineation of their properties and expressed genetic programmes, and the beginnings of their utility in regenerative medicine.
Abstract: Stem cell biology has come of age. Unequivocal proof that stem cells exist in the haematopoietic system has given way to the prospective isolation of several tissue-specific stem and progenitor cells, the initial delineation of their properties and expressed genetic programmes, and the beginnings of their utility in regenerative medicine. Perhaps the most important and useful property of stem cells is that of self-renewal. Through this property, striking parallels can be found between stem cells and cancer cells: tumours may often originate from the transformation of normal stem cells, similar signalling pathways may regulate self-renewal in stem cells and cancer cells, and cancer cells may include 'cancer stem cells' - rare cells with indefinite potential for self-renewal that drive tumorigenesis.
8,999 citations
TL;DR: It is reported that mTOR forms a stoichiometric complex with raptor, an evolutionarily conserved protein with at least two roles in the mTOR pathway that through its association with mTOR regulates cell size in response to nutrient levels.
2,902 citations
"mTORC1 is essential for leukemia pr..." refers result in this paper
...Although mTORC1 reportedly controls cell size (cell growth) (34), we did not observe a decrease in the size of our Raptor-deficient AML cells (Figure 6B), and we found that the amount of protein per cell was comparable in control and Raptor-deficient AML stem cells (Figure 6, C and D)....
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TL;DR: It is shown that rapamycin inhibits the assembly of mTORC2 and that, in many cell types, prolongedRapamycin treatment reduces the levels of m TORC2 below those needed to maintain Akt/PKB signaling.
2,621 citations
"mTORC1 is essential for leukemia pr..." refers background in this paper
...We found that Raptor deficiency did not affect the expression level of Rictor, an essential component of mTORC2, but it is possible that long-term inactivation of mTORC1 in Raptordeficient AML cells affects the assembly and activity of mTORC2, leading to inhibition of AKT, in the same manner as prolonged rapamycin treatment (49)....
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...On the other hand, prolonged rapamycin treatment is reported to suppress AKT via the disassembly of the mTORC2 complex in certain cell types (49)....
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TL;DR: The data suggest that feedback down-regulation of receptor tyrosine kinase signaling is a frequent event in tumor cells with constitutive mTOR activation, and reversal of this feedback loop by rapamycin may attenuate its therapeutic effects, whereas combination therapy that ablates mTOR function and prevents Akt activation may have improved antitumor activity.
Abstract: Stimulation of the insulin and insulin-like growth factor I (IGF-I) receptor activates the phosphoinositide-3-kinase/Akt/mTOR pathway causing pleiotropic cellular effects including an mTOR-dependent loss in insulin receptor substrate-1 expression leading to feedback down-regulation of signaling through the pathway. In model systems, tumors exhibiting mutational activation of phosphoinositide-3-kinase/Akt kinase, a common event in cancers, are hypersensitive to mTOR inhibitors, including rapamycin. Despite the activity in model systems, in patients, mTOR inhibitors exhibit more modest antitumor activity. We now show that mTOR inhibition induces insulin receptor substrate-1 expression and abrogates feedback inhibition of the pathway, resulting in Akt activation both in cancer cell lines and in patient tumors treated with the rapamycin derivative, RAD001. IGF-I receptor inhibition prevents rapamycin-induced Akt activation and sensitizes tumor cells to inhibition of mTOR. In contrast, IGF-I reverses the antiproliferative effects of rapamycin in serum-free medium. The data suggest that feedback down-regulation of receptor tyrosine kinase signaling is a frequent event in tumor cells with constitutive mTOR activation. Reversal of this feedback loop by rapamycin may attenuate its therapeutic effects, whereas combination therapy that ablates mTOR function and prevents Akt activation may have improved antitumor activity.
2,423 citations
"mTORC1 is essential for leukemia pr..." refers background in this paper
...org Volume 122 Number 6 June 2012 (32, 33), neither Raptor deficiency nor rapamycin resulted in hyperphosphorylation of AKT (S473) (Figure 6A and Supplemental Fig-...
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TL;DR: Recent findings on the regulators and effectors of mTOR are highlighted and specific cases that serve as paradigms for the different modes of m TOR regulation and its control of translation are discussed.
Abstract: The process of translation requires substantial cellular resources. Cells have therefore evolved complex mechanisms to control overall protein synthesis as well as the translation of specific mRNAs that are crucial for cell growth and proliferation. At the heart of this process is the mammalian target of rapamycin (mTOR) signalling pathway, which senses and responds to nutrient availability, energy sufficiency, stress, hormones and mitogens to modulate protein synthesis. Here, we highlight recent findings on the regulators and effectors of mTOR and discuss specific cases that serve as paradigms for the different modes of mTOR regulation and its control of translation.
2,328 citations
"mTORC1 is essential for leukemia pr..." refers background in this paper
...These target molecules control cell growth (size) and proliferation by modifying protein translation (1)....
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