Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment

Twists in views on RB functions in cellular signaling, metabolism and stem cells

One-quarter of a century ago, identification of the human retinoblastoma gene (RB) loci proved Knudson's 'two-hit theory' that tumor suppressor genes exist. Since then, numerous works delineated crucial roles for the RB protein (pRB)-E2F transcription factor complex in G1-S phase transition. In addition, discovering the relationship between pRB and tissue-specific transcription factors enabled a better understanding of how cell cycle exit and terminal differentiation are coupled. Recent works provoked many exciting twists in views on pRB functions during cancer initiation and progression beyond its previously well-appreciated roles. Various mitogenic and cytostatic cellular signals appeared to modulate pRB functions and thus affect a wide variety of effector molecules. In addition, genetic studies in mice as well as other creatures incessantly force us to revise our views on pRB functions. This review will focus particularly on the roles of pRB in regulating intracellular signaling, cell metabolism, chromatin function, stem cells and cancer stem cells.

Myeloma as a model for the process of metastasis: implications for therapy

Multiple myeloma (MM) is a plasma cell dyscrasia characterized by the presence of multiple myelomatous "omas" throughout the skeleton, indicating that there is continuous trafficking of tumor cells to multiple areas in the bone marrow niches. MM may therefore represent one of the best models to study cell trafficking or cell metastasis. The process of cell metastasis is described as a multistep process, the invasion-metastasis cascade. This involves cell invasion, intravasation into nearby blood vessels, passage into the circulation, followed by homing into predetermined distant tissues, the formation of new foci of micrometastases, and finally the growth of micrometastasis into macroscopic tumors. This review discusses the significant advances that have been discovered in the complex process of invasion-metastasis in epithelial carcinomas and cell trafficking in hematopoietic stem cells and how this process relates to progression in MM. This progression is mediated by clonal intrinsic factors that mediate tumor invasiveness as well as factors present in the tumor microenvironment that are permissive to oncogenic proliferation. Therapeutic agents that target the different steps of cell dissemination and progression are discussed. Despite the significant advances in the treatment of MM, better therapeutic agents that target this metastatic cascade are urgently needed.

Concise review: Multidimensional regulation of the hematopoietic stem cell state

Hematopoietic stem cells (HSCs) are characterized by their unique function to produce all lineages of blood cells throughout life. Such tissue-specific function of HSC is attributed to their ability to execute self-renewal and multilineage differentiation. Accumulating evidence indicates that the undifferentiated state of HSC is characterized by dynamic maintenance of chromatin structures and epigenetic plasticity. Conversely, quiescence, self-renewal, and differentiation of HSCs are dictated by complex regulatory mechanisms involving specific transcription factors and microenvironmental crosstalk between stem cells and multiple compartments of niches in bone marrows. Thus, multidimensional regulatory inputs are integrated into two opposing characters of HSCs-maintenance of undifferentiated state analogous to pluripotent stem cells but execution of tissue-specific hematopoietic functions. Further studies on the interplay of such regulatory forces as "cell fate determinant" will likely shed the light on diverse spectrums of tissue-specific stem cells.

Adipose-derived mesenchymal stem cells enhance healing of mandibular defects in the ramus of swine

PURPOSE

This study investigated the effect of adipose-derived mesenchymal stem cells (ASCs) injected locally or systemically on the bone regeneration of a 10-mm-diameter cylindrical noncritical-size defect in the ramus of the pig mandible.

MATERIALS AND METHODS

Fifteen Yorkshire pigs, weighing 60 to 80 kg, received bilateral 10-mm-diameter cylindrical surgical defects in each ramus of the mandible. Pigs received 1) a direct injection into the defect of 2.5 million carboxy-fluorescein diacetate succinimidyl ester-labeled ASCs from 1 of 2 pig donors (n = 6); 2) an ear vein injection of 5 million carboxy-fluorescein diacetate succinimidyl ester-labeled ASCs from 1 of 2 pig donors (n = 6); or 3) an ear vein injection of culture Dulbecco's Modified Eagle's Medium without stem cells (control; n = 3). Pigs from each treatment were sacrificed at 1 hour, 2 weeks, or 4 weeks after surgery. Healing of the defect was evaluated by dual-energy x-ray absorptiometry, micro-computed tomography, fluorescent microscopy, and histology.

RESULTS

Bone healing was accelerated in the ASC-injected treatment groups at 2 and 4 weeks after surgery compared with the control pigs.

CONCLUSIONS

Results from this animal model provide evidence that the injection of ASC locally into a bone defect or systemically can accelerate the healing of bone.

Hematopoietic stem cells lacking Ott1 display aspects associated with aging and are unable to maintain quiescence during proliferative stress

Aging degrades hematopoietic stem cell (HSC) functions, including stress response; however, the involved molecular pathways are incompletely defined. Murine BM conditionally deleted for One-Twenty-Two-1 (Ott1), is able to maintain lifelong hematopoiesis and has preserved numbers of long-term HSCs, yet cannot repopulate nor sustain itself after transplantation against a competitor even when Ott1 is excised after engraftment. We show, specifically under replicative stress, that Ott1-deleted HSCs have a significant reduction of the G(0) cell-cycle fraction associated with self-renewal and undergo early failure. Therefore, Ott1 is required to preserve HSC quiescence during stress but not steady-state hematopoiesis. Reduced tolerance of replicative stress, increased myeloid potential, and greater absolute numbers are mutual characteristics of both Ott1-deleted and aged HSCs, and comparison of their gene expression profiles reveals a shared signature. Ott1-deleted HSCs share multiple aging-associated physiologic changes, including increases in NF-κB activation and DNA damage. Loss of Ott1 causes increased reactive oxygen species; however, antioxidant treatment does not rescue the competitive defect, indicating the existence of additional essential Ott1-dependent HSC pathways. In conclusion, our data establish a requirement for Ott1 in stress hematopoiesis and suggest that Ott1-dependent processes may converge with those affected by aging.

Concomitant inactivation of Rb and E2f8 in hematopoietic stem cells synergizes to induce severe anemia

The retinoblastoma (Rb) tumor suppressor plays important roles in regulating hematopoiesis, particularly erythropoiesis. In an effort to understand whether Rb function can be mediated by E2F transcription factors in a BM-derived hematopoietic system in mice, we uncovered a functional synergy between Rb and E2F8 to promote erythropoiesis and to prevent anemia. Specifically, whereas Mx1-Cre-mediated inactivation of Rb or E2f8 in hematopoietic stem cells only led to mild erythropoietic defects, concomitant inactivation of both genes resulted in marked ineffective erythropoiesis and mild hemolysis, leading to severe anemia despite the presence of enhanced extramedullary erythropoiesis. Interestingly, although ineffective erythropoiesis was already present in the RbΔ/Δ mice and exacerbated in the RbΔ/Δ;E2f8Δ/Δ mice, hemolysis was exclusively manifested in the double-knockout mice. Using an adoptive transfer system and an erythroid-specific knockout system, we have shown that the synergy of Rb and E2f8 deficiency in triggering severe anemia is intrinsic to the erythroid lineage. Surprisingly, concomitant inactivation of Rb and E2f7, a close family member of E2f8, did not substantially worsen the erythropoietic defect resulted from Rb deficiency. The results of the present study reveal the specificity of E2F8 in mediating Rb function in erythropoiesis and suggest critical and overlapping roles of Rb and E2f8 in maintaining normal erythropoiesis and in preventing hemolysis.

Bone marrow stromal cells create a permissive microenvironment for myeloma development: a new stromal role for Wnt inhibitor Dkk1

The rapid progression of multiple myeloma is dependent upon cellular interactions within the bone marrow microenvironment. In vitro studies suggest that bone marrow stromal cells (BMSC) can promote myeloma growth and survival and osteolytic bone disease. However, it is not possible to recreate all cellular aspects of the bone marrow microenvironment in an in vitro system, and the contributions of BMSCs to myeloma pathogenesis in an intact, immune competent, in vivo system are unknown. To investigate this, we used a murine myeloma model that replicates many features of the human disease. Coinoculation of myeloma cells and a BMSC line, isolated from myeloma-permissive mice, into otherwise nonpermissive mice resulted in myeloma development, associated with tumor growth within bone marrow and osteolytic bone disease. In contrast, inoculation of myeloma cells alone did not result in myeloma. BMSCs inoculated alone induced osteoblast suppression, associated with an increase in serum concentrations of the Wnt signaling inhibitor, Dkk1. Dkk1 was highly expressed in BMSCs and in myeloma-permissive bone marrow. Knockdown of Dkk1 expression in BMSCs decreased their ability to promote myeloma and the associated bone disease in mice. Collectively, our results show novel roles of BMSCs and BMSC-derived Dkk1 in the pathogenesis of multiple myeloma in vivo.

Cell cycle regulation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) give rise to all lineages of blood cells. Because HSCs must persist for a lifetime, the balance between their proliferation and quiescence is carefully regulated to ensure blood homeostasis while limiting cellular damage. Cell cycle regulation therefore plays a critical role in controlling HSC function during both fetal life and in the adult. The cell cycle activity of HSCs is carefully modulated by a complex interplay between cell-intrinsic mechanisms and cell-extrinsic factors produced by the microenvironment. This fine-tuned regulatory network may become altered with age, leading to aberrant HSC cell cycle regulation, degraded HSC function, and hematological malignancy.

Rac signaling in osteoblastic cells is required for normal bone development but is dispensable for hematopoietic development

Hematopoietic stem cells (HSCs) interact with osteoblastic, stromal, and vascular components of the BM hematopoietic microenvironment (HM) that are required for the maintenance of long-term self-renewal in vivo. Osteoblasts have been reported to be a critical cell type making up the HSC niche in vivo. Rac1 GTPase has been implicated in adhesion, spreading, and differentiation of osteoblast cell lines and is critical for HSC engraftment and retention. Recent data suggest a differential role of GTPases in endosteal/osteoblastic versus perivascular niche function. However, whether Rac signaling pathways are also necessary in the cell-extrinsic control of HSC function within the HM has not been examined. In the present study, genetic and inducible models of Rac deletion were used to demonstrate that Rac depletion causes impaired proliferation and induction of apoptosis in the OP9 cell line and in primary BM stromal cells. Deletion of Rac proteins caused reduced trabecular and cortical long bone growth in vivo. Surprisingly, HSC function and maintenance of hematopoiesis in vivo was preserved despite these substantial cell-extrinsic changes. These data have implications for therapeutic strategies to target Rac signaling in HSC mobilization and in the treatment of leukemia and provide clarification to our evolving concepts of HSC-HM interactions.

Extramedullary hematopoiesis: a new look at the underlying stem cell niche, theories of development, and occurrence in animals

Extramedullary hematopoiesis (EMH) is the formation and development of blood cells outside the medullary spaces of the bone marrow. Although widely considered an epiphenomenon, secondary to underlying primary disease and lacking serious clinical or diagnostic implications, the presence of EMH is far from incidental on a molecular basis; rather, it reflects a well-choreographed suite of changes involving stem cells and their microenvironment (the stem cell niche). The goals of this review are to reconsider the molecular basis of EMH based on current knowledge of stem cell niches and to examine its role in the pathophysiologic mechanisms of EMH in animals. The ability of blood cells to home, proliferate, and mature in extramedullary tissues of adult animals reflects embryonic patterns of hematopoiesis and establishment or reactivation of a stem cell niche. This involves pathophysiologic alterations in hematopoietic stem cells, extracellular matrix, stromal cells, and local and systemic chemokines. Four major theories involving changes in stem cells and/or their microenvironment can explain the development of most occurrences of EMH: (1) severe bone marrow failure; (2) myelostimulation; (3) tissue inflammation, injury, and repair; and (4) abnormal chemokine production. EMH has also been reported within many types of neoplasms. Understanding the concepts and factors involved in stem cell niches enhances our understanding of the occurrence of EMH in animals and its relationship to underlying disease. In turn, a better understanding of the prevalence and distribution of EMH in animals and its molecular basis could further inform our understanding of the hematopoietic stem cell niche.

Mesenchymal niches of bone marrow in cancer

Over the last decade, genetic and cell biology studies have indicated that tumour growth is not only determined by malignant cancer cells themselves, but also by the tumour microenvironment. Cells present in the tumour microenvironment include fibroblasts, vascular, smooth muscle, adipocytes, immune cells and mesenchymal stem cells (MSC). The nature of the relationship between MSC and tumour cells appears dual and whether MSC are pro- or anti-tumorigenic is a subject of controversial reports. This review is focused on the role of MSC and bone marrow (BM) niches in cancer.

Newly identified aspects of tumor suppression by RB

The retinoblastoma (RB) tumor suppressor belongs to a cellular pathway that plays a crucial role in restricting the G1-S transition of the cell cycle in response to a large number of extracellular and intracellular cues. Research in the last decade has highlighted the complexity of regulatory networks that ensure proper cell cycle progression, and has also identified multiple cellular functions beyond cell cycle regulation for RB and its two family members, p107 and p130. Here we review some of the recent evidence pointing to a role of RB as a molecular adaptor at the crossroads of multiple pathways, ensuring cellular homeostasis in different contexts. In particular, we discuss the pro- and anti-tumorigenic roles of RB during the early stages of cancer, as well as the importance of the RB pathway in stem cells and cell fate decisions.

The bone marrow at the crossroads of blood and immunity

Progenitor cells that are the basis for all blood cell production share the bone marrow with more mature elements of the adaptive immune system. Specialized niches within the bone marrow guide and, at times, constrain the development of haematopoietic stem and progenitor cells (HSPCs) and lineage-restricted immune progenitor cells. Specific niche components are organized into distinct domains to create a diversified landscape in which specialized cell differentiation or population expansion programmes proceed. Local cues that reflect the tissue and organismal state affect cellular interactions to alter the production of a range of cell types. Here, we review the organization of regulatory elements in the bone marrow and discuss how these elements provide a dynamic means for the host to modulate stem cell and adaptive immune cell responses to physiological challenges.

The bone marrow at the crossroads of blood and immunity

Progenitor cells that are the basis for all blood cell production share the bone marrow with more mature elements of the adaptive immune system. Specialized niches within the bone marrow guide and, at times, constrain the development of haematopoietic stem and progenitor cells (HSPCs) and lineage-restricted immune progenitor cells. Specific niche components are organized into distinct domains to create a diversified landscape in which specialized cell differentiation or population expansion programmes proceed. Local cues that reflect the tissue and organismal state affect cellular interactions to alter the production of a range of cell types. Here, we review the organization of regulatory elements in the bone marrow and discuss how these elements provide a dynamic means for the host to modulate stem cell and adaptive immune cell responses to physiological challenges.

Fak depletion in both hematopoietic and nonhematopoietic niche cells leads to hematopoietic stem cell expansion

Hematopoietic stem cells (HSCs) reside in complex bone marrow microenvironments, where niche-induced signals regulate hematopoiesis. Focal adhesion kinase (Fak) is a nonreceptor protein tyrosine kinase that plays an essential role in many cell types, where its activation controls adhesion, motility, and survival. Fak expression is relatively increased in HSCs compared to progenitors and mature blood cells. Therefore, we explored its role in HSC homeostasis. We have used the Mx1-Cre-inducible conditional knockout mouse model to investigate the effects of Fak deletion in bone marrow compartments. The total number as well as the fraction of cycling Lin(-)Sca-1(+)c-kit(+) (LSK) cells is increased in Fak(-/-) mice compared to controls, while hematopoietic progenitors and mature blood cells are unaffected. Bone marrow cells from Fak(-/-) mice exhibit enhanced, long-term (i.e., 20-week duration) engraftment in competitive transplantation assays. Intrinsic Fak function was assessed in serial transplantation assays, which showed that HSCs (Lin(-)Sca-1(+)c-kit(+)CD34(-)Flk-2(-) cells) sorted from Fak(-/-) mice have similar self-renewal and engraftment ability on a per-cell basis as wild-type HSCs. When Fak deletion is induced after engraftment of Fak(fl/fl)Mx1-Cre(+) bone marrow cells into wild-type recipient mice, the number of LSKs is unchanged. In conclusion, Fak inactivation does not intrinsically regulate HSC behavior and is not essential for steady-state hematopoiesis. However, widespread Fak inactivation in the hematopoietic system induces an increased and activated HSC pool size, potentially as a result of altered reciprocal interactions between HSCs and their microenvironment.

Osteoblastic and vascular endothelial niches, their control on normal hematopoietic stem cells, and their consequences on the development of leukemia

Stem cell self-renewal is regulated by intrinsic mechanisms and extrinsic signals mediated via specialized microenvironments called “niches.” The best-characterized stem cell is the hematopoietic stem cell (HSC). Self-renewal and differentiation ability of HSC are regulated by two major elements: endosteal and vascular regulatory elements. The osteoblastic niche localized at the inner surface of the bone cavity might serve as a reservoir for long-term HSC storage in a quiescent state. Whereas the vascular niche, which consists of sinusoidal endothelial cell lining blood vessel, provides an environment for short-term HSC proliferation and differentiation. Both niches act together to maintain hematopoietic homeostasis. In this paper, we provide some principles applying to the hematopoietic niches, which will be useful in the study and understanding of other stem cell niches. We will discuss altered microenvironment signaling leading to myeloid lineage disease. And finally, we will review some data on the development of acute myeloid leukemia from a subpopulation called leukemia-initiating cells (LIC), and we will discuss on the emerging evidences supporting the influence of the microenvironment on chemotherapy resistance.

Targeting the bone marrow in Waldenstrom macroglobulinemia

Waldenstrom macroglobulinemia (WM) is a low-grade B-cell lymphoma characterized by widespread involvement of the bone marrow with lymphoplasmacytic cells. In approximately 20% of patients, the malignant clone also involves the lymph nodes and induces hepatosplenomegaly. The mechanisms by which the tumor cells home to the bone marrow and preferentially reside in the marrow niches are not fully elucidated. In this review, we examine the role of the bone marrow microenvironment in the regulation of cell growth, survival and cell dissemination in WM. We also summarize specific regulators of niche-dependent tumor proliferation in WM. These include chemokines, adhesion molecules, Src/PI3K/Akt/mTOR signaling, NF-kB activation, and micro-RNA regulation in WM. Targeting these pathways in clinical trials could lead to significant responses in this rare disease.

Mesenchymal stromal cells of myelodysplastic syndrome and acute myeloid leukemia patients have distinct genetic abnormalities compared with leukemic blasts

Mesenchymal stromal cells (MSCs) are an essential cell type of the hematopoietic microenvironment. Concerns have been raised about the possibility that MSCs undergo malignant transformation. Several studies, including one from our own group, have shown the presence of cytogenetic abnormalities in MSCs from leukemia patients. The aim of the present study was to compare genetic aberrations in hematopoietic cells (HCs) and MSCs of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) patients. Cytogenetic aberrations were detected in HCs from 25 of 51 AML patients (49%) and 16 of 43 MDS patients (37%). Mutations of the FLT3 and NPM1 genes were detected in leukemic blasts in 12 (23%) and 8 (16%) AML patients, respectively. Chromosomal aberrations in MSCs were detected in 15 of 94 MDS/AML patients (16%). No chromosomal abnormalities were identified in MSCs of 36 healthy subjects. We demonstrate herein that MSCs have distinct genetic abnormalities compared with leukemic blasts. We also analyzed the main characteristics of patients with MSCs carrying chromosomal aberrations. In view of these data, the genetic alterations in MSCs may constitute a particular mechanism of leukemogenesis.

Dynamic niches in the origination and differentiation of haematopoietic stem cells

Haematopoietic stem cells (HSCs) are multipotent, self-renewing progenitors that generate all mature blood cells. HSC function is tightly controlled to maintain haematopoietic homeostasis, and this regulation relies on specialized cells and factors that constitute the haematopoietic 'niche', or microenvironment. Recent discoveries, aided in part by technological advances in in vivo imaging, have engendered a new appreciation for the dynamic nature of the niche, identifying novel cellular and acellular niche components and uncovering fluctuations in the relative importance of these components over time. These new insights significantly improve our understanding of haematopoiesis and raise fundamental questions about what truly constitutes a stem cell niche.

Rb and nucleolin antagonize in controlling human CD34 gene expression

Retinoblastoma protein (Rb) controls cell proliferation, differentiation, survival and gene expression and it has a central role in the signaling network that provides a cell cycle checkpoint in the G1 phase of the cell cycle. Studies in mice have shown that Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment and it acts as a critical regulator of hematopoietic stem and progenitor cells under stress. In human hematopoiesis, the CD34 protein is expressed on a subset of progenitor cells capable of self-renewal, multilineage differentiation, and hematopoietic reconstitution, and CD34 has a role in the differentiation of hematopoietic cells. Here we find that, in CD34-positive hematopoietic cells, Rb controls the human CD34 promoter region by antagonizing the CD34 promoter factor nucleolin to provide a mechanism that links expression of endogenous CD34 to cell cycle progression. Our study suggests a direct involvement of Rb in the transcriptional program of human CD34-positive hematopoietic stem/progenitor cells, thus providing further insights into the molecular network relevant to the features of these cells.

Adult hematopoiesis is regulated by TIF1γ, a repressor of TAL1 and PU.1 transcriptional activity

Crosstalk between transcription factors and cytokines precisely regulates tissue homeostasis. Transcriptional intermediary factor 1γ (TIF1γ) regulates vertebrate hematopoietic development, can control transcription elongation, and is a component of the TGF-β signaling pathway. Here we show that deletion of TIF1γ in adult hematopoiesis is compatible with life and long-term maintenance of essential blood cell lineages. However, loss of TIF1γ results in deficient long-term hematopoietic stem cell (LT-HSC) transplantation activity, deficient short-term HSC (ST-HSC) bone marrow retention, and priming ST-HSCs to myelomonocytic lineage. These defects are hematopoietic cell-autonomous, and priming of TIF1γ-deficient ST-HSCs can be partially rescued by wild-type hematopoietic cells. TIF1γ can form complexes with TAL1 or PU.1-two essential DNA-binding proteins in hematopoiesis-occupy specific subsets of their DNA binding sites in vivo, and repress their transcriptional activity. These results suggest a regulation of adult hematopoiesis through TIF1γ-mediated transcriptional repression of TAL1 and PU.1 target genes.

The Er71 is an important regulator of hematopoietic stem cells in adult mice

The Ets transcription factor Er71 is an important regulator of endothelial and hematopoietic development during mammalian embryogenesis. However, the role of Er71 in adult hematopoiesis has remained unknown. We now first show that conditional deletion of Er71 in the hematopoietic system of adult mice results in a marked reduction (55%) in the number of hematopoietic stem cells (HSCs) that is likely due to increased cell death. Bone marrow transplantation (BMT) experiments further confirmed that Er71 is required for repopulation of HSCs. In addition, Er71(+/-) mice exhibited a slight decrease (37%) in the number of HSCs than those of Er71(+/+) mice, indicating that the function of Er71 in HSC maintenance is dependent on gene dosage. Moreover, Er71 was shown to be required for Tie2 expression, which contributes to HSC maintenance. Our results thus suggest the role of a single transcription factor in controlling HSCs through regulation of Tie2 expression in adult animals.

Functional and structural insights into ASB2alpha, a novel regulator of integrin-dependent adhesion of hematopoietic cells

By providing contacts between hematopoietic cells and the bone marrow microenvironment, integrins are implicated in cell adhesion and thereby in control of cell fate of normal and leukemia cells. The ASB2 gene, initially identified as a retinoic acid responsive gene and a target of the promyelocytic leukemia retinoic acid receptor α oncoprotein in acute promyelocytic leukemia cells, encodes two isoforms, a hematopoietic-type (ASB2α) and a muscle-type (ASB2β) that are involved in hematopoietic and myogenic differentiation, respectively. ASB2α is the specificity subunit of an E3 ubiquitin ligase complex that targets filamins to proteasomal degradation. To examine the relationship of the ASB2α structure to E3 ubiquitin ligase function, functional assays and molecular modeling were performed. We show that ASB2α, through filamin A degradation, enhances adhesion of hematopoietic cells to fibronectin, the main ligand of β1 integrins. Furthermore, we demonstrate that a short N-terminal region specific to ASB2α, together with ankyrin repeats 1 to 10, is necessary for association of ASB2α with filamin A. Importantly, the ASB2α N-terminal region comprises a 9-residue segment with predicted structural homology to the filamin-binding motifs of migfilin and β integrins. Together, these data provide new insights into the molecular mechanisms of ASB2α binding to filamin.

Telomere dysfunction and cell cycle checkpoints in hematopoietic stem cell aging

Stem cells are believed to be closely associated with tissue degeneration during aging. Studies of human genetic diseases and gene-targeted animal models have provided evidence that functional decline of telomeres and deregulation of cell cycle checkpoints contribute to the aging process of tissue stem cells. Telomere dysfunction can induce DNA damage response via key cell cycle checkpoints, leading to cellular senescence or apoptosis depending on the tissue type and developmental stage of a specific stem cell compartment. Telomerase mutation and telomere shortening have been observed in a variety of hematological disorders, such as dyskeratosis congenital, aplastic anemia, myelodysplastic syndromes and leukemia, in which the hematopoietic stem cells (HSC) are a major target during the pathogenesis. Moreover, telomere dysfunction is able to induce both cell-intrinsic checkpoints and environmental factors limiting the self-renewal capacity and differentiation potential of HSCs. Crucial components in the cascade of DNA damage response, including ataxia telangiectasia mutated, CHK2, p53, p21 and p16/p19(ARF), play important roles in HSC maintenance and self-renewal in the scenarios of both sufficient telomere reserve and dysfunctional telomere. Therefore, a further understanding of the molecular mechanisms underlying HSC aging may help identity new therapeutic targets for stem cell-based regenerative medicine.

Transcription intermediary factor 1γ is a tumor suppressor in mouse and human chronic myelomonocytic leukemia

Transcription intermediary factor 1γ (TIF1γ) was suggested to play a role in erythropoiesis. However, how TIF1γ regulates the development of different blood cell lineages and whether TIF1γ is involved in human hematological malignancies remain to be determined. Here we have shown that TIF1γ was a tumor suppressor in mouse and human chronic myelomonocytic leukemia (CMML). Loss of Tif1g in mouse HSCs favored the expansion of the granulo-monocytic progenitor compartment. Furthermore, Tif1g deletion induced the age-dependent appearance of a cell-autonomous myeloproliferative disorder in mice that recapitulated essential characteristics of human CMML. TIF1γ was almost undetectable in leukemic cells of 35% of CMML patients. This downregulation was related to the hypermethylation of CpG sequences and specific histone modifications in the gene promoter. A demethylating agent restored the normal epigenetic status of the TIF1G promoter in human cells, which correlated with a reestablishment of TIF1γ expression. Together, these results demonstrate that TIF1G is an epigenetically regulated tumor suppressor gene in hematopoietic cells and suggest that changes in TIF1γ expression may be a biomarker of response to demethylating agents in CMML.

Bidirectional interactions between bone metabolism and hematopoiesis

Interactions between hematopoiesis and bone metabolism have been described in various developmental and pathological situations. Here we review this evidence from the literature with a focus on microenvironmental regulation of hematopoiesis and bone metabolism. Our hypothesis is that this process occurs by bidirectional signaling between hematopoietic and mesenchymal cells through cell adhesion molecules, membrane-bound growth factors, and secreted matrix proteins. Examples of steady-state hematopoiesis and pathologies are presented and support our view that hematopoietic and mesenchymal cell functions are modulated by specific microenvironments in the bone marrow.

Crebbp haploinsufficiency in mice alters the bone marrow microenvironment, leading to loss of stem cells and excessive myelopoiesis

CREB-binding protein (CREBBP) is important for the cell-autonomous regulation of hematopoiesis, including the stem cell compartment. In the present study, we show that CREBBP plays an equally pivotal role in microenvironment-mediated regulation of hematopoiesis. We found that the BM microenvironment of Crebbp(+/-) mice was unable to properly maintain the immature stem cell and progenitor cell pools. Instead, it stimulates myeloid differentiation, which progresses into a myeloproliferation phenotype. Alterations in the BM microenvironment resulting from haploinsufficiency of Crebbp included a marked decrease in trabecular bone that was predominantly caused by increased osteoclastogenesis. Although CFU-fibroblast (CFU-F) and total osteoblast numbers were decreased, the bone formation rate was similar to that found in wild-type mice. At the molecular level, we found that the known hematopoietic modulators matrix metallopeptidase-9 (MMP9) and kit ligand (KITL) were decreased with heterozygous levels of Crebbp. Lastly, potentially important regulatory proteins, endothelial cell adhesion molecule 1 (ESAM1) and cadherin 5 (CDH5), were increased on Crebbp(+/-) endothelial cells. Our findings reveal that a full dose of Crebbp is essential in the BM microenvironment to maintain proper hematopoiesis and to prevent excessive myeloproliferation.

Bone marrow stroma-secreted cytokines protect JAK2(V617F)-mutated cells from the effects of a JAK2 inhibitor

Signals emanating from the bone marrow microenvironment, such as stromal cells, are thought to support the survival and proliferation of the malignant cells in patients with myeloproliferative neoplasms (MPN). To examine this hypothesis, we established a coculture platform [cells cocultured directly (cell-on-cell) or indirectly (separated by micropore membrane)] designed to interrogate the interplay between Janus activated kinase 2-V617F (JAK2(V617F))-positive cells and the stromal cells. Treatment with atiprimod, a potent JAK2 inhibitor, caused marked growth inhibition and apoptosis of human (SET-2) and mouse (FDCP-EpoR) JAK2(V617F)-positive cells as well as primary blood or bone marrow mononuclear cells from patients with polycythemia vera; however, these effects were attenuated when any of these cell types were cocultured (cell-on-cell) with human marrow stromal cell lines (e.g., HS5, NK.tert, TM-R1). Coculture with stromal cells hampered the ability of atiprimod to inhibit phosphorylation of JAK2 and the downstream STAT3 and STAT5 pathways. This protective effect was maintained in noncontact coculture assays (JAK2(V617F)-positive cells separated by 0.4-μm-thick micropore membranes from stromal cells), indicating a paracrine effect. Cytokine profiling of supernatants from noncontact coculture assays detected distinctly high levels of interleukin 6 (IL-6), fibroblast growth factor (FGF), and chemokine C-X-C-motif ligand 10 (CXCL-10)/IFN-γ-inducible 10-kD protein (IP-10). Anti-IL-6, -FGF, or -CXCL-10/IP-10 neutralizing antibodies ablated the protective effect of stromal cells and restored atiprimod-induced apoptosis of JAK2(V617F)-positive cells. Therefore, our results indicate that humoral factors secreted by stromal cells protect MPN clones from JAK2 inhibitor therapy, thus underscoring the importance of targeting the marrow niche in MPN for therapeutic purposes.

Leukemia stem cells and microenvironment: biology and therapeutic targeting

Acute myelogenous leukemia is propagated by a subpopulation of leukemia stem cells (LSCs). In this article, we review both the intrinsic and extrinsic components that are known to influence the survival of human LSCs. The intrinsic factors encompass regulators of cell cycle and prosurvival pathways (such as nuclear factor kappa B [NF-κB], AKT), pathways regulating oxidative stress, and specific molecular components promoting self-renewal. The extrinsic components are generated by the bone marrow microenvironment and include chemokine receptors (CXCR4), adhesion molecules (VLA-4 and CD44), and hypoxia-related proteins. New strategies that exploit potentially unique properties of the LSCs and their microenvironment are discussed.

Regulatory Interactions in the Bone Marrow Microenvironment

Hematopoietic stem cells (HSCs) are the immature, pluripotent cells from which all myeloid and lymphoid cell types originate. As stem cells, HSCs are capable of two very different fate choices: self-renewal, ensuring they will persist throughout the lifetime of an organism, and differentiation to mature progeny. Therapeutic applications of HSCs include their routine use in stem cell transplantation to treat hematopoietic malignancies or bone marrow failure. Research and clinical experience have provided tools for the immunophenotypic identification and functional analysis of HSCs and there is increasing evidence suggesting that HSC regulation is greatly influenced by signals from their niches in the bone marrow. Although they represent one of the most rigorously studied stem cell types, still more remains to be known about how HSCs are regulated and respond to stress conditions.

Dynamics of epigenetic modifications in leukemia

Chromatin modifications at both histones and DNA are critical for regulating gene expression. Mis-regulation of such epigenetic marks can lead to pathological states; indeed, cancer affecting the hematopoietic system is frequently linked to epigenetic abnormalities. Here, we discuss the different types of modifications and their general impact on transcription, as well as the polycomb group of proteins, which effect transcriptional repression and are often mis-regulated. Further, we discuss how chromosomal translocations leading to fusion proteins can aberrantly regulate gene transcription through chromatin modifications within the hematopoietic system. PML-RARa, AML1-ETO and MLL-fusions are examples of fusion proteins that mis-regulate epigenetic modifications (either directly or indirectly), which can lead to acute myeloblastic leukemia (AML). An in-depth understanding of the mechanisms behind the mis-regulation of epigenetic modifications that lead to the development and progression of AMLs could be critical for designing effective treatments.

Fucose-deficient hematopoietic stem cells have decreased self-renewal and aberrant marrow niche occupancy

BACKGROUND

Modification of Notch receptors by O-linked fucose and its further elongation by the Fringe family of glycosyltransferase has been shown to be important for Notch signaling activation. Our recent studies disclose a myeloproliferative phenotype, hematopoietic stem cell (HSC) dysfunction, and abnormal Notch signaling in mice deficient in FX, which is required for fucosylation of a number of proteins including Notch. The purpose of this study was to assess the self-renewal and stem cell niche features of fucose-deficient HSCs.

STUDY DESIGN AND METHODS

Homeostasis and maintenance of HSCs derived from FX(-/-) mice were studied by serial bone marrow transplantation, homing assay, and cell cycle analysis. Two-photon intravital microscopy was performed to visualize and compare the in vivo marrow niche occupancy by fucose-deficient and wild-type (WT) HSCs.

RESULTS

Marrow progenitors from FX(-/-) mice had mild homing defects that could be partially prevented by exogenous fucose supplementation. Fucose-deficient HSCs from FX(-/-) mice displayed decreased self-renewal capability compared with the WT controls. This is accompanied with their increased cell cycling activity and suppressed Notch ligand binding. When tracked in vivo by two-photon intravital imaging, the fucose-deficient HSCs were found localized farther from the endosteum of the calvarium marrow than the WT HSCs.

CONCLUSIONS

The current reported aberrant niche occupancy by HSCs from FX(-/-) mice, in the context of a faulty blood lineage homeostasis and HSC dysfunction in mice expressing Notch receptors deficient in O-fucosylation, suggests that fucosylation-modified Notch receptor may represent a novel extrinsic regulator for HSC engraftment and HSC niche maintenance.

Suppression of leukemia development caused by PTEN loss

Multiple genetic or molecular alterations are known to be associated with cancer stem cell formation and cancer development. Targeting such alterations, therefore, may lead to cancer prevention. By crossing our previously established phosphatase and tensin homolog (Pten)-null acute T-lymphoblastic leukemia (T-ALL) model onto the recombination-activating gene 1(-/-) background, we show that the lack of variable, diversity and joining [V(D)J] recombination completely abolishes the Tcrα/δ-c-myc translocation and T-ALL development, regardless of β-catenin activation. We identify mammalian target of rapamycin (mTOR) as a regulator of β-selection. Rapamycin, an mTOR-specific inhibitor, alters nutrient sensing and blocks T-cell differentiation from CD4(-)CD8(-) to CD4(+)CD8(+), the stage where the Tcrα/δ-c-myc translocation occurs. Long-term rapamycin treatment of preleukemic Pten-null mice prevents Tcrα/δ-c-myc translocation and leukemia stem cell (LSC) formation, and it halts T-ALL development. However, rapamycin alone fails to inhibit mTOR signaling in the c-Kit(mid)CD3(+)Lin(-) population enriched for LSCs and eliminate these cells. Our results support the idea that preventing LSC formation and selectively targeting LSCs are promising approaches for antileukemia therapies.

Effects of the bone marrow microenvironment on hematopoietic malignancy

The bone marrow (BM) is contained within the bone cavity and is the main site of hematopoiesis, the continuous development of blood cells from immature hematopoietic stem and progenitor cells. The bone marrow consists of developing hematopoietic cells and non-hematopoietic cells, the latter collectively termed the bone marrow microenvironment. These non-hematopoietic cells include cells of the osteoblast lineage, adipocytes and endothelial cells. For many years these bone marrow microenvironment cells were predicted to play active roles in regulating hematopoiesis, and recent studies have confirmed such roles. Importantly, more recent data has indicated that cells of the BM microenvironment may also contribute to hematopoietic diseases. In this review we provide an overview of the roles of the data suggesting that the cells of the bone marrow microenvironment may play an active role in the initiation and progression of hematopoietic malignancy.

E2f1-3 are critical for myeloid development

Hematopoietic development involves the coordinated activity of differentiation and cell cycle regulators. In current models of mammalian cell cycle control, E2f activators (E2f1, E2f2, and E2f3) are portrayed as the ultimate transcriptional effectors that commit cells to enter and progress through S phase. Using conditional gene knock-out strategies, we show that E2f1-3 are not required for the proliferation of early myeloid progenitors. Rather, these E2fs are critical for cell survival and proliferation at two distinct steps of myeloid development. First, E2f1-3 are required as transcriptional repressors for the survival of CD11b(+) myeloid progenitors, and then they are required as activators for the proliferation of CD11b(+) macrophages. In bone marrow macrophages, we show that E2f1-3 respond to CSF1-Myc mitogenic signals and serve to activate E2f target genes and promote their proliferation. Together, these findings expose dual functions for E2f1-3 at distinct stages of myeloid development in vivo, first as repressors in cell survival and then as activators in cell proliferation. In summary, this work places E2f1-3 in a specific signaling cascade that is critical for myeloid development in vivo.

Concise review: multiple niches for hematopoietic stem cell regulations

Two types of stem cell niches in bone marrow (BM), endosteal osteoblastic, and vascular niches are involved in the microenvironmental regulation of hematopoietic stem cells (HSCs). Recently, redundant features of the two niches were identified, based on their common cellular origins or chemical mediators being produced in each niche. In contrast, studies have also revealed that HSCs are localized differentially in the niches with respect to their distinct functional status, and that the biological activity of each niche is differentially influenced by extrinsic conditions. An important question is, therefore, whether these two niches play distinct roles in regulating HSCs and whether they respond differentially to environmental stimuli/stress for "compartmentalized" niche organization in BM. In this review, recent discoveries related to the characteristics of each type of niche and their common or unique features are discussed, along with the possibility of multiniche regulation of HSCs in BM.

The endosteal 'osteoblastic' niche and its role in hematopoietic stem cell homing and mobilization

The concept of hematopoietic stem cell (HSC) niche was formulated in 1978, but HSC niches remained unidentified for the following two decades largely owing to technical limitations. Sophisticated live microscopy techniques and genetic manipulations have identified the endosteal region of the bone marrow (BM) as a preferential site of residence for the most potent HSC - able to reconstitute in serial transplants - with osteoblasts and their progenitors as critical cellular elements of these endosteal niches. This article reviews the path to the discovery of these endosteal niches (often called 'osteoblastic' niches) for HSC, what cell types contribute to these niches with their known physical and biochemical features. In the past decade, a first wave of research uncovered many mechanisms responsible for HSC homing to, and mobilization from, the whole BM tissue. However, the recent discovery of endosteal HSC niches has initiated a second wave of research focusing on the mechanisms by which most primitive HSC lodge into and migrate out of their endosteal niches. The second part of this article reviews the current knowledge of the mechanisms of HSC lodgment into, retention in and mobilization from osteoblastic niches.

Cancer: evolutionary, genetic and epigenetic aspects

There exist two paradigms about the nature of cancer. According to the generally accepted one, cancer is a by-product of design limitations of a multi-cellular organism (Greaves, Nat Rev Cancer 7:213–221, 2007). The essence of the second resides in the question “Does cancer kill the individual and save the species?” (Sommer, Hum Mutat 3:166–169, 1994). Recent data on genetic and epigenetic mechanisms of cell transformation summarized in this review support the latter point of view, namely that carcinogenesis is an evolutionary conserved phenomenon—a programmed death of an organism. It is assumed that cancer possesses an important function of altruistic nature: as a mediator of negative selection, it serves to preserve integrity of species gene pool and to mediate its evolutionary adjustment. Cancer fulfills its task due apparently to specific killer function, understanding mechanism of which may suggest new therapeutic strategy.

Glomerular epithelial stem cells: the good, the bad, and the ugly

Global glomerulosclerosis with loss of podocytes in humans is typical of end-stage renal pathology. Although mature podocytes are highly differentiated and nondividing, converging evidence from experimental and clinical data suggests adult stem cells within Bowman's capsule can rescue some of this loss. Glomerular epithelial stem cells generate podocytes during kidney growth and regenerate podocytes after injury, thus explaining why various glomerular disorders undergo remission occasionally. This regenerative process, however, is often inadequate because of inefficient proliferative responses by glomerular epithelial stem cells with aging or in the setting of focal segmental glomerulosclerosis. Alternatively, an excessive proliferative response by glomerular epithelial stem cells after podocyte injury can generate new lesions such as extracapillary crescentic glomerulonephritis, collapsing glomerulopathy and tip lesions. Better understanding of the mechanisms that regulate growth and differentiation of glomerular epithelial stem cells may provide new clues for prevention and treatment of glomerulosclerosis.

FIP200 is required for the cell-autonomous maintenance of fetal hematopoietic stem cells

Little is known about whether autophagic mechanisms are active in hematopoietic stem cells (HSCs) or how they are regulated. FIP200 (200-kDa FAK-family interacting protein) plays important roles in mammalian autophagy and other cellular functions, but its role in hematopoietic cells has not been examined. Here we show that conditional deletion of FIP200 in hematopoietic cells leads to perinatal lethality and severe anemia. FIP200 was cell-autonomously required for the maintenance and function of fetal HSCs. FIP200-deficient HSC were unable to reconstitute lethally irradiated recipients. FIP200 ablation did not result in increased HSC apoptosis, but it did increase the rate of HSC proliferation. Consistent with an essential role for FIP200 in autophagy, FIP200-null fetal HSCs exhibited both increased mitochondrial mass and reactive oxygen species. These data identify FIP200 as a key intrinsic regulator of fetal HSCs and implicate a potential role for autophagy in the maintenance of fetal hematopoiesis and HSCs.

Vitamin D receptor deletion leads to increased hematopoietic stem and progenitor cells residing in the spleen

Bone components participate in the regulation of hematopoietic stem cells in the adult mammal. Vitamin D regulates bone mineralization and is associated with pleiotropic effects in many cell types, including putative roles in hematopoietic differentiation. We report that deletion of the vitamin D receptor (VDR) in hematopoietic cells did not result in cell autonomous perturbation of hematopoietic stem cell or progenitor function. However, deletion of VDR in the microenvironment resulted in a marked accumulation of hematopoietic stem cells in the spleen that could be reversed by calcium dietary supplementation. These data suggest that VDR participates in restricting splenic hematopoiesis through maintenance of bone calcium homeostasis and are consistent with the concept that calcium regulation through VDR is a central participant in localizing adult hematopoiesis preferentially to bone marrow.

A germline gain-of-function mutation in Ptpn11 (Shp-2) phosphatase induces myeloproliferative disease by aberrant activation of hematopoietic stem cells

Germline and somatic gain-of-function mutations in tyrosine phosphatase PTPN11 (SHP-2) are associated with juvenile myelomonocytic leukemia (JMML), a myeloproliferative disease (MPD) of early childhood. The mechanism by which PTPN11 mutations induce this disease is not fully understood. Signaling partners that mediate the pathogenic effects of PTPN11 mutations have not been explored. Here we report that germ line mutation Ptpn11(D61G) in mice aberrantly accelerates hematopoietic stem cell (HSC) cycling, increases the stem cell pool, and elevates short-term and long-term repopulating capabilities, leading to the development of MPD. MPD is reproduced in primary and secondary recipient mice transplanted with Ptpn11(D61G/+) whole bone marrow cells or purified Lineage(-)Sca-1(+)c-Kit(+) cells, but not lineage committed progenitors. The deleterious effects of Ptpn11(D61G) mutation on HSCs are attributable to enhancing cytokine/growth factor signaling. The aberrant HSC activities caused by Ptpn11(D61G) mutation are largely corrected by deletion of Gab2, a prominent interacting protein and target of Shp-2 in cell signaling. As a result, MPD phenotypes are markedly ameliorated in Ptpn11(D61G/+)/Gab2(-/-) double mutant mice. Collectively, our data suggest that oncogenic Ptpn11 induces MPD by aberrant activation of HSCs. This study also identifies Gab2 as an important mediator for the pathogenic effects of Ptpn11 mutations.

Disruption of bis leads to the deterioration of the vascular niche for hematopoietic stem cells

The stem cell niche plays an important role in the microenvironmental regulation of hematopoietic stem cells, but the integration of niche activity remains poorly understood. In this study, we show that a functional deficiency of Bis/BAG-3/CAIR-1, a protein related to apoptosis and the response to cellular stress, results in perturbation of the vascular stem cell niche, causing a series of hematopoietic derangements. Mice with a targeted disruption of bis (bis(-/-)) exhibited a loss of hematopoietic stem cells and defective B-cell development. However, this hematological defect of bis(-/-) mice was not reproduced when bis(-/-) bone marrow cells were transplanted into bis(+/+) recipients. Moreover, bis(+/+) bone marrow cells, when transplanted into bis(-/-) mice, reproduced the same defect as bis(-/-) cells, pointing to the microenvironmental origin of the phenotypes. Subsequent analysis of bis(-/-) mice bone marrow revealed a characteristic defect in the vascular stem cell niche that included the defective growth of stromal progenitor cells in colony forming unit-fibroblasts, the defect in sinusoidal endothelium, and the loss of stromal cells expressing CXCL-12 or IL-7 in the bone marrow. In contrast, no abnormalities were observed in the growth and hematopoietic supporting activities of osteoblasts from bis(-/-) mice bone marrows. Collectively, these results indicate that Bis functions to mediate cellular regulation of the stem cell niche on the vascular compartment and suggest that the vascular and osteoblastic compartments of the stem cell niche can be independently regulated during the in vivo orchestration of hematopoiesis.

Evidence for organ-specific stem cell microenvironments

The X-linked Gata1(low) mutation in mice induces strain-restricted myeloproliferative disorders characterized by extramedullary hematopoiesis in spleen (CD1 and DBA/2) and liver (CD1 only). To assess the role of the microenvironment in establishing this myeloproliferative trait, progenitor cell compartments of spleen and marrow from wild-type and Gata1(low) mice were compared. Phenotype and clonal assay of non-fractionated cells indicated that Gata1(low) mice contain progenitor cell numbers 4-fold lower and 10-fold higher than normal in marrow and spleen, respectively. However, progenitor cells prospectively isolated from spleen, but not from marrow, of Gata1(low) mice expressed colony-forming function in vitro. Therefore, calculation of cloning activity of purified cells demonstrated that the total number of Gata1(low) progenitor cells was 10- to 100-fold lower than normal in marrow and >1,000 times higher than normal in spleen. This observation indicates that Gata1(low) hematopoiesis is favored by the spleen and is in agreement with our previous report that removal of this organ induces wild-type hematopoiesis in heterozygous Gata1(low/+) females (Migliaccio et al., 2009, Blood 114:2107). To clarify if rescue of wild-type hematopoiesis by splenectomy prevented extramedullary hematopoiesis in liver, marrow cytokine expression profile and liver histopathology of splenectomized Gata1(low/+) females were investigated. After splenectomy, the marrow expression levels of TGF-beta, VEGF, osteocalcin, PDGF-alpha, and SDF-1 remained abnormally high while Gata1(low) hematopoiesis was detectable in liver of both CD1 and DBA/2 mutants. Therefore, in the absence of the spleen, Gata1(low) hematopoiesis is supported by the liver suggesting that treatment of myelofibrosis in these animals requires the rescue of both stem cell and microenvironmental functions.

Knockdown of N-cadherin suppresses the long-term engraftment of hematopoietic stem cells

During postnatal life, the bone marrow (BM) supports both self-renewal and differentiation of hematopoietic stem cells (HSCs) in specialized microenvironments termed stem cell niches. Cell-cell and cell-extracellular matrix interactions between HSCs and their niches are critical for the maintenance of HSC properties. Here, we analyzed the function of N-cadherin in the regulation of the proliferation and long-term repopulation activity of hematopoietic stem/progenitor cells (HSPCs) by the transduction of N-cadherin shRNA. Inhibition of N-cadherin expression accelerated cell division in vitro and reduced the lodgment of donor HSPCs to the endosteal surface, resulting in a significant reduction in long-term engraftment. Cotransduction of N-cadherin shRNA and a mutant N-cadherin that introduced the silent mutations to shRNA target sequences rescued the accelerated cell division and reconstitution phenotypes. In addition, the requirement of N-cadherin for HSPC engraftment appears to be niche specific, as shN-cad-transduced lineage(-)Sca-1(+)c-Kit(+) cells successfully engrafted in spleen, which lacks an osteoblastic niche. These findings suggest that N-cad-mediated cell adhesion is functionally required for the establishment of hematopoiesis in the BM niche after BM transplantation.

Combined inactivation of pRB and hippo pathways induces dedifferentiation in the Drosophila retina

Functional inactivation of the Retinoblastoma (pRB) pathway is an early and obligatory event in tumorigenesis. The importance of pRB is usually explained by its ability to promote cell cycle exit. Here, we demonstrate that, independently of cell cycle exit control, in cooperation with the Hippo tumor suppressor pathway, pRB functions to maintain the terminally differentiated state. We show that mutations in the Hippo signaling pathway, wts or hpo, trigger widespread dedifferentiation of rbf mutant cells in the Drosophila eye. Initially, rbf wts or rbf hpo double mutant cells are morphologically indistinguishable from their wild-type counterparts as they properly differentiate into photoreceptors, form axonal projections, and express late neuronal markers. However, the double mutant cells cannot maintain their neuronal identity, dedifferentiate, and thus become uncommitted eye specific cells. Surprisingly, this dedifferentiation is fully independent of cell cycle exit defects and occurs even when inappropriate proliferation is fully blocked by a de2f1 mutation. Thus, our results reveal the novel involvement of the pRB pathway during the maintenance of a differentiated state and suggest that terminally differentiated Rb mutant cells are intrinsically prone to dedifferentiation, can be converted to progenitor cells, and thus contribute to cancer advancement.