K-Ras is essential for normal fetal liver erythropoiesis

RASA3 is a candidate gene in sickle cell disease-associated pulmonary hypertension and pulmonary arterial hypertension

Pulmonary hypertension (PH) is associated with significant morbidity and mortality. RASA3 is a GTPase activating protein integral to angiogenesis and endothelial barrier function. In this study, we explore the association of RASA3 genetic variation with PH risk in patients with sickle cell disease (SCD)-associated PH and pulmonary arterial hypertension (PAH). Cis-expression quantitative trait loci (eQTL) were queried for RASA3 using whole genome genotype arrays and gene expression profiles derived from peripheral blood mononuclear cells (PBMC) of three SCD cohorts. Genome-wide single nucleotide polymorphisms (SNPs) near or in the RASA3 gene that may associate with lung RASA3 expression were identified, reduced to 9 tagging SNPs for RASA3 and associated with markers of PH. Associations between the top RASA3 SNP and PAH severity were corroborated using data from the PAH Biobank and analyzed based on European or African ancestry (EA, AA). We found that PBMC RASA3 expression was lower in patients with SCD-associated PH as defined by echocardiography and right heart catheterization and was associated with higher mortality. One eQTL for RASA3 (rs9525228) was identified, with the risk allele correlating with PH risk, higher tricuspid regurgitant jet velocity and higher pulmonary vascular resistance in patients with SCD-associated PH. rs9525228 associated with markers of precapillary PH and decreased survival in individuals of EA but not AA. In conclusion, RASA3 is a novel candidate gene in SCD-associated PH and PAH, with RASA3 expression appearing to be protective. Further studies are ongoing to delineate the role of RASA3 in PH.

Differential effects of RASA3 mutations on hematopoiesis are profoundly influenced by genetic background and molecular variant

Studies of the severely pancytopenic scat mouse model first demonstrated the crucial role of RASA3, a dual RAS and RAP GTPase activating protein (GAP), in hematopoiesis. RASA3 is required for survival in utero; germline deletion is lethal at E12.5–13.5 due to severe hemorrhage. Here, conditional deletion in hematopoietic stem and progenitor cells (HSPCs) using Vav-iCre recapitulates the null phenotype demonstrating that RASA3 is required at the stem and progenitor level to maintain blood vessel development and integrity and effective blood production. In adults, bone marrow blood cell production and spleen stress erythropoiesis are suppressed significantly upon induction of RASA3 deficiency, leading to pancytopenia and death within two weeks. Notably, RASA3 missense mutations in two mouse models, scat (G125V) and hlb381 (H794L), show dramatically different hematopoietic consequences specific to both genetic background and molecular variant. The mutation effect is mediated at least in part by differential effects on RAS and RAP activation. In addition, we show that the role of RASA3 is conserved during human terminal erythropoiesis, highlighting a potential function for the RASA3-RAS axis in disordered erythropoiesis in humans. Finally, global transcriptomic studies in scat suggest potential targets to ameliorate disease progression.

Hematopoiesis is the process by which blood cells are formed. An individual must have a normal complement of red blood cells to prevent anemia, platelets to control bleeding, and white blood cells to maintain immune functions. All blood cells are derived from hematopoietic stem cells that differentiate into progenitor cells that then develop into mature circulating cells. We studied several mouse strains carrying different mutations in the gene encoding RASA3 and human CD34+ cells, which can be induced to produce blood cells in culture. We show that RASA3 is required at the earliest stages of blood formation, the stem and progenitor cells, and that the complement of genes other than RASA3, or the genetic background, profoundly alters the overall effect on blood formation. Further, the molecular nature of the mutation in RASA3 also has a profound and independent effect on overall blood formation. One mutant mouse strain, designated scat, suffers cyclic anemia characterized by severe anemic crisis episodes interspersed with remissions where the anemia significantly improves. Comparison of scat crisis and remission hematopoietic stem and progenitor cells reveals striking differences in gene expression. Analyses of these expression differences provide clues to processes that potentially drive improvement of anemia in scat and provide new avenues to pursue in future studies to identify novel therapeutics for anemia.

EPAS1 regulates proliferation of erythroblasts in chronic mountain sickness

Excessive erythrocytosis (EE) is a characteristic of chronic mountain sickness (CMS). Currently, the pathogenesis of CMS remains unclear. This study was intended to investigate the role of EPAS1 in the proliferation of erythroblasts in CMS. Changes of HIF-1α and EPAS1/HIF-2α in the bone marrow erythroblasts of 21 patients with CMS and 14 control subjects residing at the same altitudes were determined by RT-qPCR and western blotting. We also developed a lentiviral vector, Lv-EPAS1/sh-EPAS1, to over-express/silence EPAS1 in K562 cells. Cells cycle and proliferation were detected by flow cytometry. Transcriptome analyses were carried out on Illumina. CMS patients showed a higher expression of EPAS1/HIF-2α in the bone marrow erythroblasts than those of controls. Variations in EPAS1 expression in CMS patients were positively correlated with RBC levels, and negatively correlated with SaO₂. Over-expressing of EPAS1 in K562 cells accelerated the erythroid cells cycle progression and promoted the erythroid cells proliferation-and vice versa. Transcriptome data indicated that proliferation-related DEGs were significantly enriched in EPAS1 overexpression/silencing K562 cells. Our results suggest that EPAS1 might participate in the pathogenesis of EE by regulating the proliferation of erythroblasts.

ERK Dephosphorylation through MKP1 Deacetylation by SIRT1 Attenuates RAS-Driven Tumorigenesis

The role of Situin 1 (SIRT1) in tumorigenesis is still controversial due to its wide range of substrates, including both oncoproteins and tumor suppressors. A recent study has demonstrated that SIRT1 interferes in the Kirsten rat sarcoma viral oncogene homolog (KRAS)-driven activation of the Raf-mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK pathway, thereby inhibiting tumorigenesis. However, the molecular mechanism of SIRT1 as a tumor suppressor in RAS-driven tumorigenesis has been less clearly determined. This study presents evidence that the ectopic expression of SIRT1 attenuates RAS- or MEK-driven ERK activation and reduces cellular proliferation and transformation in vitro. The attenuation of ERK activation by SIRT1 results from prompt dephosphorylation of ERK, while MEK activity remains unchanged. We identified that MKP1, a dual specific phosphatase for MAPK, was deacetylated by SIRT1. Deacetylation of MKP1 by direct interaction with SIRT1 increased the binding affinity to ERK which in turn facilitated inactivation of ERK. Taken together, these results suggest that SIRT1 would act as a tumor suppressor by modulating RAS-driven ERK activity through MKP1 deacetylation.

Emerging EPO and EPO receptor regulators and signal transducers

As essential mediators of red cell production, erythropoietin (EPO) and its cell surface receptor (EPO receptor [EPOR]) have been intensely studied. Early investigations defined basic mechanisms for hypoxia-inducible factor induction of EPO expression, and within erythroid progenitors EPOR engagement of canonical Janus kinase 2/signal transducer and activator of transcription 5 (JAK2/STAT5), rat sarcoma/mitogen-activated protein kinase/extracellular signal-regulated kinase (RAS/MEK/ERK), and phosphatidylinositol 3-kinase (PI3K) pathways. Contemporary genetic, bioinformatic, and proteomic approaches continue to uncover new clinically relevant modulators of EPO and EPOR expression, and EPO's biological effects. This Spotlight review highlights such factors and their emerging roles during erythropoiesis and anemia.

KRAS mutation testing in clinical practice

Activating mutation of KRAS plays a significant role in the pathogenesis of common human malignancies and molecular testing of KRAS mutation has emerged as an essential biomarker in the current practice of clinical oncology. The presence of KRAS mutation is generally associated with clinical aggressiveness of the cancer and reduced survival of the patient. Therapeutically, KRAS mutation testing has maximum utility in stratifying metastatic colorectal carcinoma and lung cancer patients for treatment with targeted therapy. Diagnostically, KRAS mutation testing is useful in the workup of pancreaticobiliary and thyroid cancers, particularly using cytological specimens. In the era of precision medicine, the role of KRAS mutation testing is poised to expand, likely in a setting of combinatorial therapeutic strategy and requiring additional mutation testing of its upstream and/or downstream effectors.

Functional redundancy of Sos1 and Sos2 for lymphopoiesis and organismal homeostasis and survival

Sos1 and Sos2 are ubiquitously expressed, universal Ras guanine nucleotide exchange factors (Ras-GEFs) acting in multiple signal transduction pathways activated by upstream cellular kinases. The embryonic lethality of Sos1 null mutants has hampered ascertaining the specific in vivo contributions of Sos1 and Sos2 to processes controlling adult organism survival or development of hematopoietic and nonhematopoietic organs, tissues, and cell lineages. Here, we generated a tamoxifen-inducible Sos1-null mouse strain allowing analysis of the combined disruption of Sos1 and Sos2 (Sos1/2) during adulthood. Sos1/2 double-knockout (DKO) animals died precipitously, whereas individual Sos1 and Sos2 knockout (KO) mice were perfectly viable. A reduced percentage of total bone marrow precursors occurred in single-KO animals, but a dramatic depletion of B-cell progenitors was specifically detected in Sos1/2 DKO mice. We also confirmed a dominant role of Sos1 over Sos2 in early thymocyte maturation, with almost complete thymus disappearance and dramatically higher reduction of absolute thymocyte counts in Sos1/2 DKO animals. Absolute counts of mature B and T cells in spleen and peripheral blood were unchanged in single-KO mutants, while significantly reduced in Sos1/2 DKO mice. Our data demonstrate functional redundancy between Sos1 and Sos2 for homeostasis and survival of the full organism and for development and maturation of T and B lymphocytes.

MASL1 induces erythroid differentiation in human erythropoietin-dependent CD34+ cells through the Raf/MEK/ERK pathway

Human erythropoiesis is a dynamic and complex multistep process involving differentiation of early erythroid progenitors into enucleated RBCs. The mechanisms underlying erythropoiesis still remain incompletely understood. We previously demonstrated that erythropoietin-stimulated clone-1, which is selectively expressed in normal human erythroid-lineage cells, shares 99.5% identity with malignant fibrous histiocytoma-amplified sequences with leucine-rich tandem repeats 1 (MASL1). In this study, we hypothesized that the MASL1 gene plays a role in erythroid differentiation, and used a human erythroid cell culture system to explore this concept. MASL1 mRNA and protein expression levels were significantly increased during the erythroid differentiation of CD34(+) cells following erythropoietin (EPO) treatment. Conversely, MASL1 knockdown reduced erythroid differentiation in EPO-treated CD34(+) cells. In addition, MASL1 knockdown interrupted the Raf/MEK/ERK signaling pathway in CD34(+) cells. MASL1 mutant-transfected CD34(+) cells also showed decreased erythroid differentiation. Furthermore, inhibition of the SH3 domain of Son of Sevenless, which is an upstream adapter protein in EPO-induced erythroid differentiation, also reduced MASL1 expression and phosphorylation of Raf/MEK/ERK kinases that consequently reduced erythroid differentiation of EPO-induced CD34(+) cells. Importantly, we also demonstrated that MASL1 interacts physically with Raf1. Taken together, our data provide novel insights into MASL1 regulation of erythropoiesis through the Raf/MEK/ERK pathway.

Critical function for the Ras-GTPase activating protein RASA3 in vertebrate erythropoiesis and megakaryopoiesis

Phenotype-driven approaches to gene discovery using inbred mice have been instrumental in identifying genetic determinants of inherited blood dyscrasias. The recessive mutant scat (severe combined anemia and thrombocytopenia) alternates between crisis and remission episodes, indicating an aberrant regulatory feedback mechanism common to erythrocyte and platelet formation. Here, we identify a missense mutation (G125V) in the scat Rasa3 gene, encoding a Ras GTPase activating protein (RasGAP), and elucidate the mechanism producing crisis episodes. The mutation causes mislocalization of RASA3 to the cytosol in scat red cells where it is inactive, leading to increased GTP-bound Ras. Erythropoiesis is severely blocked in scat crisis mice, and ~94% succumb during the second crisis (~30 d of age) from catastrophic hematopoietic failure in the spleen and bone marrow. Megakaryopoiesis is also defective during crisis. Notably, the scat phenotype is recapitulated in zebrafish when rasa3 is silenced. These results highlight a critical, conserved, and nonredundant role for RASA3 in vertebrate hematopoiesis.

RAS Interaction with PI3K: More Than Just Another Effector Pathway

RAS PROTEINS ARE SMALL GTPASES KNOWN FOR THEIR INVOLVEMENT IN ONCOGENESIS: around 25% of human tumors present mutations in a member of this family. RAS operates in a complex signaling network with multiple activators and effectors, which allows them to regulate many cellular functions such as cell proliferation, differentiation, apoptosis, and senescence. Phosphatidylinositol 3-kinase (PI3K) is one of the main effector pathways of RAS, regulating cell growth, cell cycle entry, cell survival, cytoskeleton reorganization, and metabolism. However, it is the involvement of this pathway in human tumors that has attracted most attention. PI3K has proven to be necessary for RAS-induced transformation in vitro, and more importantly, mice with mutations in the PI3K catalytic subunit p110α that block its ability to interact with RAS are highly resistant to endogenous oncogenic KRAS-induced lung tumorigenesis and HRAS-induced skin carcinogenesis. These animals also have a delayed development of the lymphatic vasculature. Many PI3K inhibitors have been developed that are now in clinical trials. However, it is a complex pathway with many feedback loops, and interactions with other pathways make the results of its inhibition hard to predict. Combined therapy with another RAS-regulated pathway such as RAF/MEK/ERK may be the most effective way to treat cancer, at least in animal models mimicking the human disease. In this review, we will summarize current knowledge about how RAS regulates one of its best-known effectors, PI3K.

Nf1-/- Schwann cell-conditioned medium modulates mast cell degranulation by c-Kit-mediated hyperactivation of phosphatidylinositol 3-kinase

Neurofibromatosis type 1 (NF1) is a common genetic disorder and is characterized by both malignant and nonmalignant neurofibromas, which are composed of Schwann cells, degranulating mast cells, fibroblasts, and extracellular matrix. We and others have previously shown that hyperactivation of the c-Kit pathway in an Nf1 haploinsufficient microenvironment is required for both tumor formation and progression. Mast cells play a key role in both tumorigenesis and neoangiogenesis via the production of matrix metalloproteinases, heparin, and a range of different growth factors. In the present study, we show that tumorigenic Schwann cells derived from Nf1(-/-) embryos promote increased degranulation of Nf1(+/-) mast cells compared with wild-type mast cells via the secretion of the Kit ligand. Furthermore, we used genetic intercrosses as well as pharmacological agents to link the hyperactivation of the p21(Ras)-phosphatidylinositol 3-kinase (PI3K) pathway to the increased degranulation of Nf1(+/-) mast cells both in vitro and in vivo. These studies identify the p21(Ras)-PI3K pathway as a major regulator of the gain in Nf1(+/-) mast cell degranulation in neurofibromas. Collectively, these studies identify both c-Kit and PI3K as molecular targets that modulate mast cell functions in cases of NF1.

BCR-ABL but not JAK2 V617F inhibits erythropoiesis through the Ras signal by inducing p21CIP1/WAF1

BCR-ABL is a causative tyrosine kinase (TK) of chronic myelogenous leukemia (CML). In CML patients, although myeloid cells are remarkably proliferating, erythroid cells are rather decreased and anemia is commonly observed. This phenotype is quite different from that observed in polycythemia vera (PV) caused by JAK2 V617F, whereas both oncogenic TKs activate common downstream molecules at the level of hematopoietic stem cells (HSCs). To clarify this mechanism, we investigated the effects of BCR-ABL and JAK2 V617F on erythropoiesis. Enforced expression of BCR-ABL but not of JAK2 V617F in murine LSK (Lineage(-)Sca-1(hi)CD117(hi)) cells inhibited the development of erythroid cells. Among several signaling molecules downstream of BCR-ABL, an active mutant of N-Ras (N-RasE12) but not of STAT5 or phosphatidylinositol 3-kinase (PI3-K) inhibited erythropoiesis, while N-RasE12 enhanced the development of myeloid cells. BCR-ABL activated Ras signal more intensely than JAK2 V617F, and inhibition of Ras by manumycin A, a farnesyltransferase inhibitor, ameliorated erythroid colony formation of CML cells. As for the mechanisms of Ras-induced suppression of erythropoiesis, we found that GATA-1, an erythroid-specific transcription factor, blocked Ras-mediated mitogenic signaling at the level of MEK through the direct interaction. Furthermore, enforced expression of N-RasE12 in LSK cells derived from p53-, p16(INK4a)/p19(ARF)-, and p21(CIP1/WAF1)-null/wild-type mice revealed that suppressed erythroid cell growth by N-RasE12 was restored only by p21(CIP1/WAF1) deficiency, indicating that a cyclin-dependent kinase (CDK) inhibitor, p21(CIP1/WAF1), plays crucial roles in Ras-induced suppression of erythropoiesis. These data would, at least partly, explain why respective oncogenic TKs cause different disease phenotypes.

H-, N- and Kras cooperatively regulate lymphatic vessel growth by modulating VEGFR3 expression in lymphatic endothelial cells in mice

Mammalian Ras, which is encoded by three independent genes, has been thought to be a versatile component of intracellular signalling. However, when, where and how Ras signalling plays essential roles in development and whether the three Ras genes have overlapping functions in particular cells remain unclear. Here, we show that the three Ras proteins dose-dependently regulate lymphatic vessel growth in mice. We find that lymphatic vessel hypoplasia is a common phenotype in Ras compound knockout mice and that overexpressed normal Ras in an endothelial cell lineage selectively causes lymphatic vessel hyperplasia in vivo. Overexpression of normal Ras in lymphatic endothelial cells leads to sustained MAPK activation, cellular viability and enhanced endothelial network formation under serum-depleted culture conditions in vitro, and knockdown of endogenous Ras in lymphatic endothelial cells impairs cell proliferation, MAPK activation, cell migration and endothelial network formation. Ras overexpression and knockdown result in up- and downregulation of vascular endothelial growth factor receptor (VEGFR) 3 expression, respectively, in lymphatic endothelial cells in vitro. The close link between Ras and VEGFR3 in vitro is consistent with the result that Ras knockout and transgenic alleles are genetic modifiers in lymphatic vessel hypoplasia caused by Vegfr3 haploinsufficiency. Our findings demonstrate a cooperative function of the three Ras proteins in normal development, and also provide a novel aspect of VEGFR3 signalling modulated by Ras in lymphangiogenesis.

Overexpression of Rheb2 enhances mouse hematopoietic progenitor cell growth while impairing stem cell repopulation

Molecular mechanisms preserving hematopoietic stem cell (HSC) self-renewal by maintaining a balance between proliferation, differentiation, and other processes are not fully understood. Hyperactivation of the mammalian target of rapamycin (mTOR) pathway, causing sustained proliferative signals, can lead to exhaustion of HSC repopulating ability. We examined the role of the novel ras gene Rheb2, an activator of the mTOR kinase, in colony-forming ability, survival, and repopulation of immature mouse hematopoietic cells. In a cell line model of mouse hematopoietic progenitor cells (HPCs), we found enhanced proliferation and mTOR signaling in cells overexpressing Rheb2. In addition, overexpression of Rheb2 enhanced colony-forming ability and survival of primary mouse bone marrow HPCs. Expansion of phenotypic HSCs in vitro was enhanced by Rheb2 overexpression. Consistent with these findings, Rheb2 overexpression transiently expanded phenotypically defined immature hematopoietic cells after in vivo transplantation; however, these Rheb2-transduced cells were significantly impaired in overall repopulation of primary and secondary congenic transplantation recipients. Our findings suggest that HPCs and HSCs behave differently in response to growth-promoting signals stimulated by Rheb2. These results may have value in elucidating mechanisms controlling the balance between proliferation and repopulating ability, a finding of importance in clinical uses of HPCs/HSCs.

Tumor dormancy of primary and secondary cancers

Tumor dormancy is a phenomenon whereby cancer cells persist below the threshold of diagnostic detection for months to decades. This condition may arise due to either cell cycle arrest or a dynamic equilibrium state in which cell proliferation is in balance with cells undergoing apoptosis. Tumor dormancy is usually a reference to occult cancer cells that persist for an extended period of time after treatment, but primary cancers can also exhibit extended growth plateaus below the limits of detection. For example, autopsies of individuals who died of trauma reveal that most individuals harbor microscopic primary cancers. Mechanisms that operate independently or successively may restrict tumor expansion throughout tumor progression from incipiency to late-stage cancer. Proposed mechanisms include cell cycle withdrawal, immune surveillance, and blocked angiogenesis. The precise mechanisms underlying dormancy remain to be established, and relevant models will have an important impact on diagnostic and therapeutic strategies for treating cancer. This review summarizes the phenomenon of tumor dormancy, experimental models, and potential mechanisms.

K-ras/PI3K-Akt signaling is essential for zebrafish hematopoiesis and angiogenesis

The RAS small GTPases orchestrate multiple cellular processes. Studies on knock-out mice showed the essential and sufficient role of K-RAS, but not N-RAS and H-RAS in embryonic development. However, many physiological functions of K-RAS in vivo remain unclear. Using wild-type and fli1:GFP transgenic zebrafish, we showed that K-ras-knockdown resulted in specific hematopoietic and angiogenic defects, including the impaired expression of erythroid-specific gene gata1 and ße3-hemoglobin, reduced blood circulation and disorganized blood vessels. Expression of either K-rasC40 that links to phosphoinositide 3-kinase (PI3K) activation, or Akt2 that acts downstream of PI3K, could rescue both hematopoietic and angiogenic defects in the K-ras knockdown. Consistently, the functional rescue by k-ras mRNA was significantly suppressed by wortmannin, a PI3K-specific inhibitor. Our results provide direct evidence that PI3K-Akt plays a crucial role in mediating K-ras signaling during hematopoiesis and angiogenesis in vivo, thus offering new targets and alternative vertebrate model for studying these processes and their related diseases.

Erythroid defects in TRα−/− mice

Thyroid hormone and its cognate receptor (TR) have been implicated in the production of red blood cells. Here, we show mice deficient for TRα have compromised fetal and adult erythropoiesis. Erythroid progenitor numbers were significantly reduced in TRα−/− fetal livers, and transit through the final stages of maturation was impeded. In addition, immortalized TRα−/− erythroblasts displayed increased apoptosis and reduced capacity for proliferation and differentiation. Adult TRα−/− mice had lower hematocrit levels, elevated glucocorticoid levels, and an altered stress erythropoiesis response to hemolytic anemia. Most TRα−/− animals contained markedly altered progenitor numbers in their spleens. Strikingly, 20% of TRα−/− mice failed to elicit a stress erythropoiesis response and recovered very poorly from hemolytic anemia. We conclude that an underlying erythroid defect exists in TRα−/− mice, demon-strating the importance of TRα to the erythroid compartment.

Disruption of palladin leads to defects in definitive erythropoiesis by interfering with erythroblastic island formation in mouse fetal liver

Palladin was originally found up-regulated with NB4 cell differentiation induced by all-trans retinoic acid. Disruption of palladin results in neural tube closure defects, liver herniation, and embryonic lethality. Here we further report that Palld(-/-) embryos exhibit a significant defect in erythropoiesis characterized by a dramatic reduction in definitive erythrocytes derived from fetal liver but not primitive erythrocytes from yolk sac. The reduction of erythrocytes is accompanied by increased apoptosis of erythroblasts and partial blockage of erythroid differentiation. However, colony-forming assay shows no differences between wild-type (wt) and mutant fetal liver or yolk sac in the number and size of colonies tested. In addition, Palld(-/-) fetal liver cells can reconstitute hematopoiesis in lethally irradiated mice. These data strongly suggest that deficient erythropoiesis in Palld(-/-) fetal liver is mainly due to a compromised erythropoietic microenvironment. As expected, erythroblastic island in Palld(-/-) fetal liver was found disorganized. Palld(-/-) fetal liver cells fail to form erythroblastic island in vitro. Interestingly, wt macrophages can form such units with either wt or mutant erythroblasts, while mutant macrophages lose their ability to bind wt or mutant erythroblasts. These data demonstrate that palladin is crucial for definitive erythropoiesis and erythroblastic island formation and, especially, required for normal function of macrophages in fetal liver.

Hyperactive Ras in developmental disorders and cancer

Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

Primitive erythroblasts (EryPs) are the first hematopoietic cell type to form during mammalian embryogenesis and emerge within the blood islands of the yolk sac. Large, nucleated EryPs begin to circulate around midgestation, when connections between yolk sac and embryonic vasculature mature. Two to 3 days later, small cells of the definitive erythroid lineage (EryD) begin to differentiate within the fetal liver and rapidly outnumber EryPs in the circulation. The development and maturation of EryPs remain poorly defined. Our analysis of embryonic blood at different stages reveals a stepwise developmental progression within the EryP lineage from E9.5 to E12.5. Thereafter, EryDs are also present in the bloodstream, and the 2 lineages are not easily distinguished. We have generated a transgenic mouse line in which the human epsilon-globin gene promoter drives expression of green fluorescent protein exclusively within the EryP lineage. Here, we have used this line to characterize changes in cell morphology and surface-marker expression as EryPs mature and to track EryP numbers and enucleation throughout gestation. This study identifies previously unrecognized synchronous developmental stages leading to the maturation of EryPs in the mouse embryo. Unexpectedly, we find that EryPs are a stable cell population that persists through the end of gestation.

Genetic reduction of class IA PI-3 kinase activity alters fetal hematopoiesis and competitive repopulating ability of hematopoietic stem cells in vivo

Class I(A) phosphatidylinositol-3 kinase (PI-3K) is a lipid kinase, which is activated in blood cells by hematopoietic growth factors. In vitro experiments using chemical inhibitors of PI-3K suggest that this kinase is potentially important for hematopoietic stem and progenitor cell (HSC/P) function, and recent studies identify PI-3K as a therapeutic target in treating different leukemias and lymphomas. However, the role of PI-3K in regulating fetal liver or adult hematopoiesis in vivo is unknown. Therefore, we examined PI-3K-deficient embryos generated by a targeted deletion of the p85alpha and p85beta regulatory subunits of PI-3K (p85alpha-/-p85beta+/-). The absolute frequency and number of hematopoietic progenitor cells were reduced in p85alpha-/- p85beta+/- fetal livers compared with wild-type (WT) controls. Further, p85alpha-/-p85beta+/- fetal liver hematopoietic stem cells (HSCs) had decreased multilineage repopulating ability in vivo compared with WT controls in competitive repopulation assays. Finally, purified p85alpha-/-p85beta+/- c-kit+ cells had a decrease in proliferation in response to kit ligand (kitL), a growth factor important for controlling HSC function in vivo. Collectively, these data identify PI-3K as an important regulator of HSC function and potential therapeutic target in treating leukemic stem cells.

Identification of K-ras as the major regulator for cytokine-dependent Akt activation in erythroid progenitors in vivo

Despite intensive investigation, controversial results have been obtained concerning the precise signaling pathway(s) regulated by K-ras in different cell types. We show that in primary fetal liver erythroid progenitors, erythropoietin activates all three Ras isoforms, but preferentially N- and K-ras. In K-ras(-/-) fetal liver cells (FLC), erythropoietin- or stem cell factor-dependent Akt activation is greatly reduced, whereas other pathways including Stat5 and p44/p42 MAP kinase are activated normally. We further studied the effects of reduced cytokine-dependent Akt activation in erythroid differentiation. We find that freshly isolated K-ras(-/-) FLC show an approximately 7-fold increase of apoptosis and delayed erythroid differentiation, but only at the stage of erythroid progenitors and very early erythroblasts. When K-ras(-/-) erythroid progenitors are cultured in vitro, there is a significant delay in erythroid differentiation but little increase in apoptosis. Furthermore, we show that partial pharmacologic inhibition of the phosphatidylinositol 3-kinase/Akt pathway in wild-type erythroid progenitors leads to a delay in erythroid differentiation similar to that observed in K-ras(-/-) FLC. Taken together, our data identify K-ras as the major regulator for cytokine-dependent Akt activation, which is important for erythroid differentiation in vivo.