CDK6 levels regulate quiescence exit in human hematopoietic stem cells

Essential Roles of Cyclin Y-Like 1 and Cyclin Y in Dividing Wnt-Responsive Mammary Stem/Progenitor Cells

Cyclin Y family can enhance Wnt/β-catenin signaling in mitosis. Their physiological roles in mammalian development are yet unknown. Here we show that Cyclin Y-like 1 (Ccnyl1) and Cyclin Y (Ccny) have overlapping function and are crucial for mouse embryonic development and mammary stem/progenitor cell functions. Double knockout of Ccnys results in embryonic lethality at E16.5. In pubertal development, mammary terminal end buds robustly express Ccnyl1. Depletion of Ccnys leads to reduction of Lrp6 phosphorylation, hampering β-catenin activities and abolishing mammary stem/progenitor cell expansion in vitro. In lineage tracing experiments, Ccnys-deficient mammary cells lose their competitiveness and cease to contribute to mammary development. In transplantation assays, Ccnys-deficient mammary cells fail to reconstitute, whereas constitutively active β-catenin restores their regeneration abilities. Together, our results demonstrate the physiological significance of Ccnys-mediated mitotic Wnt signaling in embryonic development and mammary stem/progenitor cells, and reveal insights in the molecular mechanisms orchestrating cell cycle progression and maintenance of stem cell properties.

Stem cell self-renewal has two essential elements, cell division and at least of one of the daughter cells retaining stem cell properties, so-called stemness. The interconnections between cell cycle and cell fate specification have been explored in embryonic stem cells. However, less is known about how cell cycle affects the cell fate decision in tissue stem cells. In this study, we explore the function of particular mitotic factors Ccny and Ccnyl1 in regulating the dividing tissue stem cells. The development of the mammary gland occurs mostly in postnatal pubertal stage. At the time, the robustly dividing stem/progenitor cells reside at the forefront of the mammary epithelium extension, underlining the mammary gland as a good model to study the interconnection of cell cycle and tissue stem cells. In this study, we show that in dividing mammary stem/progenitor cells, Ccny and Ccnyl1 enhance Wnt signaling activities in mitosis. The signaling enhancement in this time window is essential for the stem/progenitor cell property maintenance during division. Deletion of Ccnys results in diminishing their competitiveness and developmental potential.

Enhanced thrombopoietin but not G-CSF receptor stimulation induces self-renewing hematopoietic stem cell divisions in vivo

In steady-state adult hematopoiesis, most hematopoietic stem cells (HSCs) are in the resting phase of the cell cycle. Upon enhanced hematopoietic demand, HSCs can be induced to divide and self-renew or differentiate. However, the cell-extrinsic signals inducing HSC cycling remain to be elucidated. Using in vivo high-resolution single HSC divisional tracking, we directly demonstrate that clinically applied thrombopoietin receptor but not granulocyte colony-stimulating factor (G-CSF) receptor agonists drive HSCs into self-renewing divisions leading to quantitative expansion of functional HSC as defined by their in vivo serial multilineage and long-term repopulating potential. These results suggest that thrombopoietin mimetics might be applicable to expand HSCs in vivo and to sensitize thrombopoietin receptor-expressing HSCs to cell cycle-dependent cytotoxic agents.

CD82/KAI1 Maintains the Dormancy of Long-Term Hematopoietic Stem Cells through Interaction with DARC-Expressing Macrophages

Hematopoiesis is regulated by crosstalk between long-term repopulating hematopoietic stem cells (LT-HSCs) and supporting niche cells in the bone marrow (BM). Here, we examine the role of CD82/KAI1 in niche-mediated LT-HSC maintenance. We found that CD82/KAI1 is expressed predominantly on LT-HSCs and rarely on other hematopoietic stem-progenitor cells (HSPCs). In Cd82(-/-) mice, LT-HSCs were selectively lost as they exited from quiescence and differentiated. Mechanistically, CD82-based TGF-β1/Smad3 signaling leads to induction of CDK inhibitors and cell-cycle inhibition. The CD82 binding partner DARC/CD234 is expressed on macrophages and stabilizes CD82 on LT-HSCs, promoting their quiescence. When DARC(+) BM macrophages were ablated, the level of surface CD82 on LT-HSCs decreased, leading to cell-cycle entry, proliferation, and differentiation. A similar interaction appears to be relevant for human HSPCs. Thus, CD82 is a functional surface marker of LT-HSCs that maintains quiescence through interaction with DARC-expressing macrophages in the BM stem cell niche.

Molecular landscapes of human hematopoietic stem cells in health and leukemia

Blood cells are organized as a hierarchy with hematopoietic stem cells (HSCs) at the root. The advent of genomic technologies has opened the way for global characterization of the molecular landscape of HSCs and their progeny, both in mouse and human models, at the genetic, transcriptomic, epigenetic, and proteomics levels. Here, we outline our current understanding of the molecular programs that govern human HSCs and how dynamic changes occurring during HSC differentiation are necessary for well-regulated blood formation under homeostasis and upon injury. A large body of evidence is accumulating on how the programs of normal hematopoiesis are modified in acute myeloid leukemia, an aggressive adult malignancy driven by leukemic stem cells. We summarize these findings and their clinical implications.

Phenotypically Dormant and Immature Leukaemia Cells Display Increased Ribosomal Protein S6 Phosphorylation

Mechanistic/mammalian target of rapamycin (mTOR) activity drives a number of key metabolic processes including growth and protein synthesis. Inhibition of the mTOR pathway promotes cellular dormancy. Since cells from patients with acute myeloid leukaemia (AML) can be phenotypically dormant (quiescent), we examined biomarkers of their mTOR pathway activity concurrently with Ki-67 and CD71 (indicators of cycling cells) by quantitative flow cytometry. Using antibodies to phosphorylated epitopes of mTOR (S2448) and its downstream targets ribosomal protein S6 (rpS6, S235/236) and 4E-BP1 (T36/45), we documented that these phosphorylations were negligible in lymphocytes, but evident in dormant as well as proliferating subsets of both mobilised normal stem cell harvest CD34+ cells and AML blasts. Although mTOR phosphorylation in AML blasts was lower than that of the normal CD34+ cells, p-4E-BP1 was 2.6-fold higher and p-rpS6 was 22-fold higher. Moreover, in contrast to 4E-BP1, rpS6 phosphorylation was higher in dormant than proliferating AML blasts, and was also higher in the immature CD34+CD38- blast subset. Data from the Cancer Genome Atlas show that rpS6 expression is associated with that of respiratory chain enzymes in AML. We conclude that phenotypic quiescence markers do not necessarily predict metabolic dormancy and that elevated rpS6 ser235/236 phosphorylation is characteristic of AML.

Targeting CDK4/6 in patients with cancer

The cyclin D-cyclin dependent kinase (CDK) 4/6-inhibitor of CDK4 (INK4)-retinoblastoma (Rb) pathway controls cell cycle progression by regulating the G1-S checkpoint. Dysregulation of the cyclin D-CDK4/6-INK4-Rb pathway results in increased proliferation, and is frequently observed in many types of cancer. Pathway activation can occur through a variety of mechanisms, including gene amplification or rearrangement, loss of negative regulators, epigenetic alterations, and point mutations in key pathway components. Due to the importance of CDK4/6 activity in cancer cells, CDK4/6 inhibitors have emerged as promising candidates for cancer treatment. Moreover, combination of a CDK4/6 inhibitor with other targeted therapies may help overcome acquired or de novo treatment resistance. Ongoing studies include combinations of CDK4/6 inhibitors with endocrine therapy and phosphatidylinositol 3-kinase (PI3K) pathway inhibitors for hormone receptor-positive (HR+) breast cancers, and with selective RAF and MEK inhibitors for tumors with alterations in the mitogen activated protein kinase (MAPK) pathway such as melanoma. In particular, the combination of CDK4/6 inhibitors with endocrine therapy, such as palbociclib's recent first-line approval in combination with letrozole, is expected to transform the treatment of HR+ breast cancer. Currently, three selective CDK4/6 inhibitors have been approved or are in late-stage development: palbociclib (PD-0332991), ribociclib (LEE011), and abemaciclib (LY2835219). Here we describe the current preclinical and clinical data for these novel agents and discuss combination strategies with other agents for the treatment of cancer.

Myc Depletion Induces a Pluripotent Dormant State Mimicking Diapause

Mouse embryonic stem cells (ESCs) are maintained in a naive ground state of pluripotency in the presence of MEK and GSK3 inhibitors. Here, we show that ground-state ESCs express low Myc levels. Deletion of both c-myc and N-myc (dKO) or pharmacological inhibition of Myc activity strongly decreases transcription, splicing, and protein synthesis, leading to proliferation arrest. This process is reversible and occurs without affecting pluripotency, suggesting that Myc-depleted stem cells enter a state of dormancy similar to embryonic diapause. Indeed, c-Myc is depleted in diapaused blastocysts, and the differential expression signatures of dKO ESCs and diapaused epiblasts are remarkably similar. Following Myc inhibition, pre-implantation blastocysts enter biosynthetic dormancy but can progress through their normal developmental program after transfer into pseudo-pregnant recipients. Our study shows that Myc controls the biosynthetic machinery of stem cells without affecting their potency, thus regulating their entry and exit from the dormant state.

miR-126 Regulates Distinct Self-Renewal Outcomes in Normal and Malignant Hematopoietic Stem Cells

To investigate miRNA function in human acute myeloid leukemia (AML) stem cells (LSC), we generated a prognostic LSC-associated miRNA signature derived from functionally validated subpopulations of AML samples. For one signature miRNA, miR-126, high bioactivity aggregated all in vivo patient sample LSC activity into a single sorted population, tightly coupling miR-126 expression to LSC function. Through functional studies, miR-126 was found to restrain cell cycle progression, prevent differentiation, and increase self-renewal of primary LSC in vivo. Compared with prior results showing miR-126 regulation of normal hematopoietic stem cell (HSC) cycling, these functional stem effects are opposite between LSC and HSC. Combined transcriptome and proteome analysis demonstrates that miR-126 targets the PI3K/AKT/MTOR signaling pathway, preserving LSC quiescence and promoting chemotherapy resistance.

Genetic and Epigenetic Mechanisms That Maintain Hematopoietic Stem Cell Function

All hematopoiesis cells develop from multipotent progenitor cells. Hematopoietic stem cells (HSC) have the ability to develop into all blood lineages but also maintain their stemness. Different molecular mechanisms have been identified that are crucial for regulating quiescence and self-renewal to maintain the stem cell pool and for inducing proliferation and lineage differentiation. The stem cell niche provides the microenvironment to keep HSC in a quiescent state. Furthermore, several transcription factors and epigenetic modifiers are involved in this process. These create modifications that regulate the cell fate in a more or less reversible and dynamic way and contribute to HSC homeostasis. In addition, HSC respond in a unique way to DNA damage. These mechanisms also contribute to the regulation of HSC function and are essential to ensure viability after DNA damage. How HSC maintain their quiescent stage during the entire life is still matter of ongoing research. Here we will focus on the molecular mechanisms that regulate HSC function.

CDK6-a review of the past and a glimpse into the future: from cell-cycle control to transcriptional regulation

The G1 cell-cycle kinase CDK6 has long been thought of as a redundant homolog of CDK4. Although the two kinases have very similar roles in cell-cycle progression, it has recently become apparent that they differ in tissue-specific functions and contribute differently to tumor development. CDK6 is directly involved in transcription in tumor cells and in hematopoietic stem cells. These functions point to a role of CDK6 in tissue homeostasis and differentiation that is partially independent of CDK6's kinase activity and is not shared with CDK4. We review the literature on the contribution of CDK6 to transcription in an attempt to link the new findings on CDK6's transcriptional activity to cell-cycle progression. Finally, we note that anticancer therapies based on the inhibition of CDK6 kinase activity fail to take into account its kinase-independent role in tumor development.

CD150(-) Side Population Defines Leukemia Stem Cells in a BALB/c Mouse Model of CML and Is Depleted by Genetic Loss of SIRT1

Leukemia stem cells (LSCs) of chronic myeloid leukemia (CML) are refractory to tyrosine kinase inhibitor treatment, persist in the residual disease, and are important source for disease recurrence. Better understanding CML LSCs will help devise new strategies to eradicate these cells. The BALB/c mouse model of CML using retroviral bone marrow transduction and transplantation is a widely used mouse model system for CML, but LSCs in this model are poorly characterized. Here, we show that lineage negative CD150(-) side population (CD150(-)SP), but not CD150(+)SP, are CML LSCs in this model, although both CD150(-)SP and CD150(+)SP cells are enriched for long-term hematopoietic stem cells in normal BALB/c mice. We previously showed that BCR-ABL transformation activates protein lysine deacetylase SIRT1 and inhibition of SIRT1 sensitizes CML stem/progenitor cells to tyrosine kinase inhibitors by acetylating and activating p53. In this study, we demonstrate that SIRT1 homozygous knockout substantially reduces CD150(-)SP CML LSCs, and compromises the maintenance of CML LSCs in the BALB/c model. We identified several molecular alterations in CD150(-)SP LSCs that included the elevated expression of cyclin-dependent kinase Cdk6 facilitating LSC activation and significantly reduced p53 expression. SIRT1 knockout suppressed Cdk6 expression and likely increases p53 protein functions through deacetylation without increasing its expression. Our results shed novel insight into CML LSCs and support a crucial role of SIRT1 in CML LSCs. Our study also provides a novel means for assessing new agents to eradicate CML LSCs.

Targeting LSCs through membrane antigens selectively or preferentially expressed on these cells

Studies of xenotransplantation of bone marrow and blood cells of AML patients have supported the existence of rare leukemic stem cells, able to initiate and maintain the leukemic process and bearing the typical leukemic abnormalities. LSCs possess self-renewal capacity and are responsible for the growth of the more differentiated leukemic progeny in the bone marrow and in the blood. These cells are more resistant than bulk leukemic cells to anti-leukemic drugs, thus survive to treatment and are, at a large extent, responsible for leukemia relapse. During the last two decades, considerable progresses have been made in the understanding of the peculiar cellular and molecular properties of LSCs. In this context, particularly relevant was the discovery of several membrane markers, selectively or preferentially expressed on LSCs. These membrane markers offer now unique opportunities to identify LSCs and to distinguish them from normal HSCs, to monitor the response of the various anti-leukemic treatments at the level of the LSC compartment, to identify relevant therapeutic targets. Concerning this last point, the most promising therapeutic targets are CD33 and CD123.

CCND1-CDK4-mediated cell cycle progression provides a competitive advantage for human hematopoietic stem cells in vivo

Maintenance of stem cell properties is associated with reduced proliferation but it is unknown whether the transition kinetics through distinct cell cycle phases influences the function of HSCs. Mende et al examine the effects of increasing two cell cycle complexes CCND1–CDK4 and CCNE1–CDK2 on the transition kinetics of human HSCs and their maintenance and functional alterations in vivo.

Maintenance of stem cell properties is associated with reduced proliferation. However, in mouse hematopoietic stem cells (HSCs), loss of quiescence results in a wide range of phenotypes, ranging from functional failure to extensive self-renewal. It remains unknown whether the function of human HSCs is controlled by the kinetics of cell cycle progression. Using human HSCs and human progenitor cells (HSPCs), we report here that elevated levels of CCND1–CDK4 complexes promoted the transit from G0 to G1 and shortened the G1 cell cycle phase, resulting in protection from differentiation-inducing signals in vitro and increasing human leukocyte engraftment in vivo. Further, CCND1–CDK4 overexpression conferred a competitive advantage without impacting HSPC numbers. In contrast, accelerated cell cycle progression mediated by elevated levels of CCNE1–CDK2 led to the loss of functional HSPCs in vivo. Collectively, these data suggest that the transition kinetics through the early cell cycle phases are key regulators of human HSPC function and important for lifelong hematopoiesis.