Role of cell cycle regulatory molecules in retinoic acid- and vitamin D3-induced differentiation of acute myeloid leukaemia cells.

The important role of cell cycle regulatory molecules in all trans-retinoic acid (ATRA)- and vitamin D3-induced growth inhibition and differentiation induction has been intensively studied in both acute myeloid leukaemia primary cells and a variety of leukaemia cell lines. Cyclin-dependent kinases (CDK)-activating kinase has been demonstrated to interact with retinoic acid receptor (RAR)α in acute promyelocytic leukaemia cells, and inhibition of CDK-activating kinase by ATRA causes hypophosphorylation of PML-RARα, leading to myeloid differentiation. In many cases, downregulation of CDK activity by ATRA and vitamin D3 is a result of elevated p21- and p27-bound CDKs. Activation of p21 is regulated at the transcriptional level, whereas elevated p27 results from both (indirectly) transcriptional activation and post-translational modifications. CDK inhibitors (CKIs) of the INK family, such as p15, p16 and p18, are mainly involved in inhibition of cell proliferation, whereas CIP/KIP members, such as p21, regulate both growth arrest and induction of differentiation. ATRA and vitamin D3 can also downregulate expression of G1 CDKs, especially CDK2 and CDK6. Inhibition of cyclin E expression has only been observed in ATRA- but not in vitamin D3-treated leukaemic cells. In vitro, not only dephosphorylation of pRb but also elevation of total pRb is required for ATRA and vitamin D3 to suppress growth and trigger their differentiation. Finally, sharp reduction in c-Myc has been observed in several leukaemia cell lines treated with ATRA, which may regulate expression of CDKs and CKIs.

TGFbeta inhibits GM-CSF-induced phosphorylation of ERK and MEK in human myeloid leukaemia cell lines via inhibition of phosphatidylinositol 3-kinase (PI3-k).

OBJECTIVES: Activation of SMAD-independent p44/42 MAPK (ERK1/2) signalling by TGFbeta has been recently reported in various cell types. However, the mechanisms for the linkage between the SMAD-dependent and -independent pathways are poorly understood. In this study, we investigated whether TGF-beta activates the ERK pathway and how TGFbeta communicates with the MAP kinase signals induced by a mitogen, in human myeloid leukaemia cells. MATERIALS AND METHODS AND RESULTS: TGFbeta dramatically suppressed proliferation of MV4-11 and TF-1 cells without detectable phosphorylation of ERK1/2 and MEK1/2 for the duration of 48 h, as detected by MTT assay and Western blot analysis, respectively. In contrast, GM-CSF induced rapid and transient phosphorylation of MEK1/2 and ERK1/2 and up-regulated cell proliferation. Both GM-CSF-induced ERK1/2 activation and cell proliferation were significantly inhibited by TGFbeta. GM-CSF also induced transient phosphorylation of the p85 subunit of PI3-kinase. Corresponding to this change, phosphorylated p85 was found to bind to the GM-CSF receptor-alpha subunit, as detected by immunoprecipitation and Western blot analysis. PD98059, a selective inhibitor of MEK, blocked GM-CSF-induced phosphorylation of MEK and ERK but not p85. However, TGFbeta and LY294002, a potent inhibitor of PI3-kinase, significantly inhibited phosphorylation of both p85 and ERK1/2. CONCLUSIONS: These studies thus indicate that TGFbeta does not activate the ERK pathway but turns off the GM-CSF-induced ERK signal via inhibition of the PI3-kinase-Akt pathway, in these human leukaemia cells.

Differential effects of transforming growth factor on cell cycle regulatory molecules in human myeloid leukemia cells.

In this report we have studied the mechanism by which Transforming Growth Factor beta (TGF beta) inhibits growth of human myeloid leukemia cell lines. TGF beta 1 arrested cells in G1 phase and significantly downregulated the expression of cyclin D2, cyclin D3, cdk4, cyclin A, and cdk2. The downregulation of the molecules resulted in approximately 50-90% decrease of the molecule-dependent kinase activity, varying with each molecule. Although treatment of cells with TGF beta 1 up-regulated accumulation of p27(kip1) in both nucleus and cytoplasm, the association of the p27(kip1) with cdk2, cyclin A, cyclin D2, cyclin D3, and cdk4 was markedly down-regulated, suggesting that p27(kip1) is not responsible for the downregulation of the kinase activity. In contrast, TGF beta 1 upregulated cyclin E-associated p27(kip1) with no effect on the expression of cyclin E. p27(kip1)-immunodepletion upregulated cyclin E-dependent kinase activity by more than 10-fold in TGF beta 1-treated cells but not in proliferating cells; whereas immunodepletion of p27(kip1) from cdk2-immunoprecipitates markedly downregulated cdk2 kinase activity in the lysates extracted from both proliferating and TGF beta-treated cells. Consistent with this observation, TGF beta 1 and p27(kip1) antisense cDNA had a synergistic or additive inhibitory effect on cdk2 but not cyclin E-dependent kinase activity. Our data suggest that (1) TGF beta 1-mediated growth inhibition is accomplished through multiple pathways and (2) p27(kip1) has opposing effects on cdk2 and cyclin E activity in response to TGF beta 1.

An IkappaBalpha inhibitor causes leukemia cell death through a p38 MAP kinase-dependent, NF-kappaB-independent mechanism.

Treatment of U937 cells with an IkappaBalpha phosphorylation inhibitor, Bay 11-7085, induced a rapid phosphorylation of p38 mitogen-activated protein (MAP) kinase, significant apoptosis, extensive necrosis, and a weak phosphorylation of MAP kinase kinase. Bay 11-7085 had no effect on the basal levels of phosphorylated IkappaBalpha but completely inhibited phorbol 12-myristate 13-acetate-induced phosphorylation of IkappaBalpha. Although Bay 11-7085 prevented phorbol 12-myristate 13-acetate-induced NF-kappaB nuclear translocation, SN50, a specific inhibitor of nuclear translocation and function of NF-kappaB, did not induce any significant nuclear/DNA fragmentation, caspase 3 activation, or cell death. The p38 MAP kinase-specific inhibitor, SB203580, completely inhibited the phosphorylation of p38 MAP kinase and significantly decreased Bay 11-7085-induced apoptosis. In contrast, the MAP kinase kinase-specific inhibitor PD98059 had no effect on Bay 11-7085-induced apoptosis. Caspase-specific inhibitor, z-Val-Ala-Asp-fluoromethyl ketone prevented Bay 11-7085-induced activation of caspase 3 but was not able to block Bay 11-7085-induced phosphorylation of p38 MAP kinase. These data suggest that Bay 11-7085 induces apoptosis through a p38 MAP kinase-dependent, NF-kappaB-independent mechanism.

Transforming growth factor: signal transduction pathways, cell cycle mediation, and effects on hematopoiesis.

Transforming growth factor-beta (TGF-beta) is a potent growth inhibitor of various cell types including hematopoietic cells. Two receptors, TGFbetaRI and TGFbetaRII, govern the interaction between the cell and the TGF-beta ligand. Primary binding of the ligand occurs with the RII receptor, promoting formation of a heterodimer with RI and activation of signaling. This induces transient association of Smad proteins with the receptors. Smad 3 and 4 may be involved in the TGF-beta-induced G(1) arrest. TGF-beta(1) down-regulates G(1) and G(2) cyclin-dependent kinases (cdks) and cyclins in terms of both kinase activity and protein amount. TGF- beta (1) also inhibits phosphorylation of the product of the retinoblastoma tumor suppressor gene (pRb) at multiple serine and threonine residues in human myeloid leukemia cells. The underphosphorylated pRb associates with transcription factor E2F-4 in G(1) phase, whereas the phosphorylated pRb mainly binds to E2F-1 and E2F-3. Because TGF-beta(1) up-regulates p130(pRb family member)/E2F-4 complex formation and down-regulates p107(pRb family member)/E2F-4 complex formation, with E2F-4 levels remaining constant, these results suggest that E2F-4 is switched from p107 to pRb and p130 when cells exit from the cell cycle and arrest in G(1) by the action of TGF-beta(1). The "cdk inhibitor" p27 is both a positive and a negative regulator of TGF-beta(1)-mediated cell cycle control. Although TGF-beta(1) has been reported to be a selected inhibitor of normal primitive hematopoietic stem cells, TGF-beta inhibits both primitive and more differentiated myeloid leukemia cell lines.

Transforming growth factor beta inhibits the phosphorylation of pRB at multiple serine/threonine sites and differentially regulates the formation of pRB family-E2F complexes in human myeloid leukemia cells.

Transforming growth factor beta (TGFbeta)1 induced dephosphorylation of pRb at multiple serine and threonine residues including Ser249/Thr252, Thr373, Ser780, and Ser807/811 in MV4-11 cells. Likewise, TGFbeta1 caused the dephosphorylation of p130, while inhibiting accumulation of p107 protein. Phosphorylated pRb was detected to bind E2F-1 and E2F-3, which appears to be a major form of pRb complexes in actively cycling cells. TGFbeta1 significantly downregulated pRb-E2F-1 and pRb-E2F-3 complexes as a result of inhibition of E2F-1 and E2F-3. In contrast, complexes of E2F-4 with pRb and with p130 were increased markedly upon TGFbeta1 treatment, whereas p107 associated E2F-4 was dramatically decreased. In agreement with these results, p130-E2F-4 DNA binding activity was dominant in TGFbeta1 treated cells, whereas p107-E2F-4 DNA binding activity was only found in proliferating cells. Our data strongly suggest that inhibition of E2Fs and differential regulation of pRb family-E2F-4 complexes are linked to TGFbeta1-induced growth inhibition. E2F-4 is switched from p107 to p130 and pRb when cells are arrested in G1 phase by TGFbeta1.

Adult Chediak-Higashi parkinsonian syndrome with dystonia.

Chediak-Higashi syndrome (CHS) is a rare autosomal-recessive disorder characterized by immune deficiency, partial oculocutaneous albinism, and large eosinophilic, peroxidase-positive inclusion bodies in granule-containing cells. The adult form of CHS manifests during late childhood to early adulthood and is marked by various neurologic sequelae, including parkinsonism, dementia, spinocerebellar degeneration, and peripheral neuropathy. We report the case of a 29-year-old man with adult CHS who exhibited a progressive asymmetric parkinsonism, including rest tremor, and axial, cervical, and appendicular dystonia. The diagnosis was confirmed by the presence of characteristic large peroxidase-positive granules within leukocytes and markedly decreased natural killer cell function. Levodopa/carbidopa and amantadine provided benefit for tremor. CHS, although rare, should be considered in the differential diagnosis of young adult parkinsonism.

Cell cycle and transcriptional control of human myeloid leukemic cells by transforming growth factor beta.

TGFbeta1 is a potent growth inhibitor of both primitive and more differentiated human myeloid leukemic cells. The extent of the growth inhibitory response to TGFbeta varies with cell type, and is not linked to stages of differentiation of cell lines. Downregulation of multiple cell cycle-regulatory molecules is a dominant event in TGFbeta1-mediated growth inhibition of human MV4-11 myeloid leukemia cells. Both G1-phase and G2-phase cyclins and cdks participate in the regulation of TGFbeta1-mediated growth inhibition of MV4-11 cells. By both depressing cdk2 synthesis and up-regulating cyclin E-associated p27, TGFbeta1 may magnify its inhibitory efficiency. TGFbeta1 also rapidly inhibits phosphorylation of pRb at several serine and threonine residues. The underphosphorylated pRb associates with E2F-4 in G1 phase, whereas the phosphorylated pRb mainly binds to E2F-1 and E2F-3 in proliferating MV4-11 cells. Since TGFbeta1 upregulates p130/E2F-4 complex formation and downregulates p107/E2F-4 complex formation, with E2F-4 levels remaining constant, our results suggest that E2F-4 is switched from p107 to pRb and p130 when cells exit from the cell cycle and arrest in G1 by TGFbeta1. In summary, TGFbeta1 inhibits growth of human myeloid leukemic cells through multiple pathways, whereas the "cdk inhibitor" p27 is both a positive and negative regulator.

Experience-dependent development of spinal motor neurons.

Locomotor activity in many species undergoes pronounced alterations in early postnatal life, and environmental cues may be responsible for modifying this process. To determine how these events are reflected in the nervous system, we studied rats reared under two different conditions-the presence or absence of gravity-in which the performance of motor operations differed. We found a significant effect of rearing environment on the size and complexity of dendritic architecture of spinal motor neurons, particularly those that are likely to participate in postural control. These results provide evidence that neurons subserving motor function undergo activity-dependent maturation in early postnatal life in a manner analogous to sensory systems.

Prolonged activation of the mitogen-activated protein kinase pathway is required for macrophage-like differentiation of a human myeloid leukemic cell line.

The role of the mitogen-activated protein kinase (MAPK) signal transduction pathway in the proliferation of mammalian cells has been well established. However, there are relatively few reports concerning cell differentiation being mediated by MAPK. The effect of phorbol 12-myristate 13-acetate (PMA) on cell differentiation and signal transduction in a human myeloid leukemia cell line, TF-1a, was investigated. When TF-1a cells were treated with 10(-6), 10(-7), 10(-8), and 10(-9) M PMA for 24 h, they underwent 98, 93, 91, and 51% macrophage-like differentiation, respectively. PMA treatment rapidly (10 min) induced phosphorylation of MAPK kinase (MEK and p44/42 MAPK), which persisted for at least 24 h. p44/42 MAPK immunoprecipitates from lysates of PMA-treated cells had increased ability to phosphorylate the transcription factor Elk-1. This is important because phosphorylated Elk-1 can be considered an "end-product" of the MAPK pathway. In contrast, treatment of TF-1a cells with granulocyte/macrophage-colony stimulating factor induced only transient activation of MEK and p44/42 MAPK (10-20 min) and an increase (approximately 50%) in cell proliferation, without any change in cellular differentiation. These results suggest that macrophage-like differentiation may be dependent on prolonged activation of the MAPK pathway. Additional support for this conclusion was obtained from experiments showing that treatment of TF-1a cells with antisense oligonucleotides for MEK1 coding sequences prior to adding PMA inhibited macrophage-like differentiation. Furthermore, transient transfection with an inactive, dominant-negative MEK mutant also inhibited PMA-induced differentiation, whereas transient transfection with a plasmid coding for constitutively activated MEK led to macrophage-like differentiation in the absence of PMA.

Activation of p38 mitogen-activated protein kinase is required for tumor necrosis factor-alpha -supported proliferation of leukemia and lymphoma cell lines.

To elucidate mechanisms of tumor necrosis factor alpha (TNF-alpha)-induced proliferation of a number of human leukemia and lymphoma cell lines, we examined the role of p38 mitogen-activated protein kinase (MAPK) in TNF-alpha signaling in Mo7e and Hut-78 cells. TNF-alpha-dependent p38 MAPK activation was detected in both Mo7e and Hut-78 cells and was blocked by the p38 MAPK inhibitor, SB203580. Ablation of p38 MAPK activity by SB203580 abrogated TNF-alpha-induced Mo7e cell proliferation and TNF-alpha-dependent autocrine growth of Hut-78. As we have shown previously that activation of the nuclear factor kappaB (NF-kappaB) is also required for TNF-alpha-induced Mo7e cell proliferation, the involvement of p38 MAPK in NF-kappaB activation was assessed. SB203580 did not affect TNF-alpha-signaled nuclear translocation and DNA-binding activity of NF-kappaB, and inhibition of NF-kappaB function did not affect TNF-alpha-induced p38 MAPK activation, indicating that these events are not dependent on each other. However, SB203580 depressed the expression of NF-kappaB-dependent genes, as monitored by a kappaB-driven reporter gene. Our findings demonstrate that activation of both p38 MAPK and NF-kappaB plays a critical role in TNF-alpha-mediated survival and proliferation of human leukemia and lymphoma cells, and p38 MAPK acts at least in part by facilitating the transcriptional activation function of NF-kappaB.

TNF-alpha-induced growth suppression of CD34+ myeloid leukemic cell lines signals through TNF receptor type I and is associated with NF-kappa B activation.

Conflicting results have been reported regarding the effect of TNF-alpha on the growth of human primitive hemopoietic cells. In this study, we have examined the effect of TNF-alpha on the proliferation of several CD34+/CD38+ (KG-1, TF-1) and CD34+/CD38- (KG-1a, TF-1a) myeloid leukemic progenitor cell lines. Our data show that TNF-alpha markedly inhibits the growth of these cells in both liquid and soft agar cultures. Addition of GM-CSF or IL-3 does not prevent TNF-alpha-induced growth inhibition. Flow cytometry analyses of propidium iodide-stained cells demonstrated cell death of all four cell lines, as judged by the presence of cells with hypodiploid DNA content after exposure of cells to TNF-alpha for 4 days. Annexin V assays detected apoptosis in TF-1, but not in TF-1a, KG-1, and KG-1a cells in terms of translocation of phosphatidylserine shortly after TNF-alpha treatment. Neutralizing anti-TNF receptor type I (TNFR-I; p55) Ab almost completely reversed TNF-alpha-induced growth inhibition in both liquid and soft agar cultures, whereas anti-TNFR-II (p75) Ab had only a marginal effect. TNF-alpha rapidly induced marked activation of nuclear transcription factor NF-kappa B in all 4 cell lines. The majority of this effect was abolished by the type I receptor Ab, whereas the type II receptor neutralizing Ab had no effect. Our data also show that TNF-alpha is incapable of inducing activation of the mitogen-activated protein kinase pathway in these leukemic cell lines.

Tumor necrosis factor-alpha-induced proliferation of human Mo7e leukemic cells occurs via activation of nuclear factor kappaB transcription factor.

Tumor necrosis factor-alpha (TNF-alpha) stimulates proliferation of Mo7e, CMK, HU-3, and M-MOK human leukemic cell lines. We report here the signal transduction pathway involved in TNF-alpha-induced Mo7e cell proliferation. Mo7e cells spontaneously die in the absence of growth factors, but treating the cells with interleukin (IL)-3, IL-6, thrombopoietin, granulocyte/macrophage colony-stimulating factor, or TNF-alpha promotes their survival and proliferation. Although most of these factors activate MAP kinase and Jun NH2-terminal kinase/signal transducer and activators of transcription signaling pathways, TNF-alpha fails to activate either pathway. When Mo7e cells were treated with TNF-alpha, nuclear factor kappaB (NF-kappaB) was activated transiently. The activated NF-kappaB consisted of heterodimers of p65 and p50 subunits. The degradation of IkappaBalpha coincided with activation of NF-kappaB in TNF-alpha-treated cells. To investigate the role of activated NF-kappaB in TNF-alpha-induced Mo7e proliferation, a cell-permeable peptide (SN50) carrying the nuclear localization sequence of p50 NF-kappaB was used to block nuclear translocation of activated NF-kappaB. Pretreating Mo7e cells with SN50 blocked TNF-alpha-induced nuclear translocation of NF-kappaB and inhibited TNF-alpha-induced Mo7e cell survival and proliferation. A mutant SN50 peptide did not affect TNF-alpha-induced Mo7e cell growth. SN50 had no effects on IL-3- or granulocyte/macrophage colony-stimulating factor-induced Mo7e cell proliferation. The results indicate that activation of NF-kappaB is involved in TNF-alpha-induced Mo7e cell survival and proliferation.

Transforming growth factor beta inhibits growth of more differentiated myeloid leukemia cells and retinoblastoma protein phosphorylation at serine 795.

Transforming growth factor beta (TGF-beta) has been shown to be a specific inhibitor of early human myeloid progenitors. We show here that TGF-beta1 potentially inhibited not only the growth of primitive but also more mature myeloid leukemic cells. Surprisingly, those apparently more mature progenitor cells, such as MV4-11 and Mo7e cells, are very sensitive to the action of TGF-beta. The addition of TGF-beta1 to liquid cultures of these cells significantly inhibited their proliferation, with as much as 72% inhibition of growth of MV4-11 cells. The suppressive effect by TGF-beta1 was not reversed or prevented by granulocyte-macrophage colony-stimulating factor or interleukin 3 used to promote cell growth in TF-1a and MV4-11 cells. TGF-beta1 completely abolished the clonal growth of MV4-11 cells in soft agar and inhibited Mo7e, KG-1, K562, TF-1, and TF-1a colony growth by 99%, 90%, 63%, 53%, and 43%, respectively. The cells treated with TGF-beta1 showed progressive accumulation in the G1 phase of cell cycle. Maximal G1 arrest (93%) was observed in MV4-11 cells. Using anti-retinoblastoma protein (pRb) and anti-specific phosphorylated-pRb antibodies, we demonstrated that TGF-beta1 greatly inhibited pRb phosphorylation at serine 795 in MV4-11 and Mo7e cells. Taken together, our data suggest that the sensitivity of myeloid leukemic progenitor cells to growth inhibition by TGF-beta may not be inversely correlated with their maturation stage, and the inhibition of the cells appeared to be linked to the suppression of pRb phosphorylation at serine 795.

Constitutive activation of the JAK2/STAT5 signal transduction pathway correlates with growth factor independence of megakaryocytic leukemic cell lines.

The factor-independent Dami/HEL and Meg-01 and factor-dependent Mo7e leukemic cell lines were used as models to investigate JAK/STAT signal transduction pathways in leukemic cell proliferation. Although Dami/HEL and Meg-01 cell proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis factor-alpha (TNF-alpha), the growth of Mo7e cells was dependent on hematopoietic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was detected in nuclear extracts from both Dami/HEL and Meg-01 cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activated transiently in Mo7e cells in response to cytokine treatments. The constitutively activated and cytokine-induced STAT-like DNA-binding factor in these three cell lines was identified as STAT5 by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assays. Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure. Meg-01 cells express a p185 BCR/ABL oncogene, which may be responsible for the constitutive activation of STAT5. Dami/HEL cells do not express the BCR/ABL oncogene, but increased constitutive phosphorylation of Raf-1 oncoprotein was detected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that the constitutive activation of JAK2/STAT5 correlates with the factor-independent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JAK2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein.

The role of nitric oxide and NMDA receptors in the development of motor neuron dendrites.

Nitric oxide (NO) has been implicated in the establishment of precise synaptic connectivity throughout the neuroaxis in several species. To determine the contribution of NO to NMDA receptor-dependent dendritic growth in motor neurons, we administered the NMDA antagonist MK-801 to wild-type mice and neuronal nitric oxide synthase (nNOS) knock-out mice between postnatal days 7 and 14. Compared to saline-treated wild-type animals the number of dendritic bifurcations was significantly reduced in nNOS knock-out animals and MK-801-treated wild-type animals. There was no significant difference in dendritic bifurcation between MK-801-treated wild-type, MK-801-treated nNOS knock-out, and saline-treated nNOS knock-out animals, suggesting that nNOS knock-out and NMDA receptor block had similar effects. The path of the longest dendrite and the number of primary dendrites was the same in all treatment groups, indicating an effect specific to bifurcation. Sholl analysis revealed that differences in bifurcation numbers occurred between 160 and 320 micrometers from the cell body, the distance at which second, third, and fourth order dendrites are most prevalent. Dendrite order analyses confirmed a significant reduction in numbers, but not lengths, of third and fourth order dendrites in nNOS knock-out and drug-treatment groups. Finally, immunohistochemical examination of the developing spinal cord indicated that NMDA receptors and nNOS are colocalized within interneurons surrounding the motor neuron pool. These results support the view that at least part of NMDA receptor-dependent arborization of motor neuron dendrites is mediated by the local production of NO within the developing spinal cord.

Characterization of a unique factor-independent variant derived from human factor-dependent TF-1 cells: a transformed event.

A factor-independent variant (TF-1a) has been isolated from the factor-dependent TF-1 cell line. The subline has been grown continuously in culture for > 1.5 years without added cytokines. The cells retain the ability to respond to multicytokines, with a different response pattern from its parental cell line. The TF-1 cells appeared singly in liquid culture. In contrast. TF-1a cells formed aggregates which increased markedly in size and in number upon TGFbeta1 treatment and showed a diminished TGFbeta-mediated growth inhibition. TF-1a, but not TF-1 cells, formed colonies in soft agar culture in the absence of any added growth factors, and developed the capacity to generate an invasive tumor(s) in nude mice. There was a constitutive activation of MAPK and MEK in TF-1a but not in TF-1 cells, which may be one of the mechanisms leading to factor-independent growth of TF-1a cells. Phenotypically, TF-1 cells were CD34+ /CD38+, whereas TF-1a cells were CD34+ /CD38-. This suggests that TF-1a may represent a less mature hematopoietic cell than TF-1. In conclusion, TF-1a is different from TF-1 in many important aspects which are associated with neoplastic transformation. The variant appears to be an excellent model for studying the process of progressive malignant transformation of myeloid cells and for studying signal pathways involved in the spontaneous and factor-induced growth of the cells.

Cloning and sequencing of an alternative splicing-derived cDNA variant of the GM-CSF receptor alpha subunit, which encodes a truncated protein.

GM-CSF interacts with the low affinity GM-CSF receptor alpha-subunit, which leads to high affinity association with the alpha-subunit/common beta-subunit complex and transduction of intracellular signals leading to proliferation, differentiation, and/or activation of hemopoietic cells, predominantly in the neutrophil and monocyte/macrophage lineages. Several alternative splicing-derived variants of the GM-CSF receptor alpha-subunit have been described previously by this and other laboratories. A newly discovered alternative-splicing derived variant was isolated from the peripheral blood mononuclear cells of a patient with juvenile myelomonocytic leukemia. This variant lacks 397 base pairs corresponding to exons 8-11 of the wild type GM-CSF receptor alpha-subunit cDNA and potentially encodes a 233 amino acid protein lacking a membrane-anchoring domain and creating the fourth known potential soluble isoform of the alpha-subunit of the GM-CSF receptor.