Nature and specificity of lymphokine independence induced by a selectable retroviral vector expressing v-src

Src-family kinases in the development and therapy of Philadelphia chromosome-positive chronic myeloid leukemia and acute lymphoblastic leukemia

The BCR-ABL kinase inhibitor imatinib has shown significant efficacy in chronic myeloid leukemia (CML) and is the standard front-line therapy for patients in chronic phase. However, a substantial number of patients are either primarily refractory or acquire resistance to imatinib. While a number of mechanisms are known to confer resistance to imatinib, increasing evidence has demonstrated a role for BCR-ABL–independent pathways. The Src-family kinases (SFKs) are one such pathway and have been implicated in imatinib resistance. Additionally, these kinases are key to the progression of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). The dual SFK/BCR-ABL inhibitor dasatinib is now clinically available and has markedly greater potency compared with imatinib against native BCR-ABL and the majority of imatinib resistant BCR-ABL mutants. Therefore, this agent, as well as other dual SFK/BCR-ABL inhibitors under development, could provide added therapeutic advantages by overcoming both BCR-ABL– dependent (i.e., BCR-ABL mutations) and – independent forms of imatinib resistance and delaying transition to advanced phase disease. In this review, we discuss the preclinical and clinical evidence demonstrating the involvement of SFKs in imatinib resistance and the progression of CML and Ph+ ALL, as well as the potential role of dual SFK/BCR-ABL inhibition in the management of these diseases.

Contributions of autocrine and non-autocrine mechanisms to tumorigenicity in a murine model for leukaemia

We have surveyed the possible mechanisms by which factor-dependent FDC-P1 cells can be rendered leukaemogenic by exposure of cells to the chemical mutagen, ethyl methane sulphonate. Cell lines established on the basis of an ability to proliferate in the absence of exogenous colony-stimulating factors (CSFs) fall into two classes; those that are maximally stimulated and show no evidence of production of CSFs and others that grow in a density-dependent manner and express granulocyte-macrophage CSF (GM-CSF). That the growth of this latter class can be suppressed by the inclusion of antisense GM-CSF oligonucleotides in the growth medium indicates that the basis for their in vitro proliferation, and probably their ability to initiate the formation of transplantable leukaemias, is autocrine stimulation by GM-CSF. The ability of low levels of CSF to sustain autocrine stimulation, as we have shown, raises the possibility of an autocrine basis for the proliferation of certain human leukaemic cells. The ability to detect low concentrations of CSFs and develop in vitro assays that closely mimic the conditions that exist in vivo will be important aids in the classification of human leukaemias.

Isolation and characterization of dominant and recessive IL-3-independent hematopoietic transformants

Retroviral integration mutagenesis and treatment with the frameshift mutagen ICR191 were used to transform v-H-ras expressing PB-3c cells to interleukin-3 (IL-3) independence. Six clones displayed viral integrations into the 3' region of the IL-3 gene thus acting post-transcriptionally by disrupting the AU-rich instability element. Two clones contained reverse orientation integration into the raf-1 gene revealing an enhancer insertion mechanism. Growth by this mechanism was sensitive to the Raf-1 inhibitor BAY 43-9006 and the Mek inhibitor U0126. Following treatment with ICR191, IL-3-independent clones were recovered and studied by cell fusion. With 21/22 clones, IL-3 independence resulted from a recessive mechanism as cellular hybrids with parental cells reverted to IL-3 dependence. Recessive clone D2c displayed increased phospho-Erk1/2 levels and was growth sensitive to U0126, but not to BAY43-9006. The single dominant clone, D5a, showed no signs of mitogen-activated protein kinases pathway activation but displayed constitutive phosphorylation of Stat5. We conclude that PB-3c has several options to acquire IL-3 growth autonomy involving transcriptional or post-transcriptional mechanisms affecting the distal regulators Erk or Stat5. The reported panel of independent dominant and recessive transformants should provide a useful tool for inhibitor profiling.

An increase in the expression and total activity of endogenous p60(c-Src) in several factor-independent mutants of a human GM-CSF-dependent leukemia cell line (TF-1)

Growth factor independence of hematopoietic cells can be induced by ectopic expression of a variety of oncogenes encoding receptor or cytoplasmic tyrosine kinases. To examine whether the activation of tyrosine kinases occurs in factor-independent mutants in vivo, the tyrosine-phosphorylated proteins from 14 factor-independent mutants of a GM-CSF-dependent cell line (TF-1) were analysed. These mutants did not secrete any growth-stimulating activity for TF-1 cells, suggesting that activation of intracellular signaling rather than an autocrine stimulation by secreted growth factors is responsible for their factor-independent growth. In 11 out of 14 GM-CSF-independent mutants analysed, a constitutively tyrosine-phosphorylated protein of 60 kDa was detected, which was subsequently identified as p60(c-Src). The kinase activity of p60(c-Src) was increased up to 12-fold in these mutants, which was at least in part due to overexpression of the c-src gene on the RNA and protein level. The Src substrate Sam68 showed an increased phosphorylation in mutants with high Src activity, suggesting that p60(c-Src) triggers downstream signaling in these cells. Treatment of the factor-independent mutants with the Src kinase inhibitor PP2 resulted in a reduced proliferation, demonstrating that Src kinases are essential for these cells for maximal proliferation. Further analysis of factor-independent mutants with low or undetectable Src activity revealed a constitutive phosphorylation of the common beta chain of the GM-CSF receptor and STAT5. Our data indicate an increase in the expression and total activity of endogenous p60(c-Src) in several GM-CSF-independent TF-1 mutants, further underlining the role of Src in the process of autonomous growth of hematopoietic cells.

Activation of the macrophage respiratory burst by phorbol myristate acetate: evidence for both tyrosine-kinase-dependent and -independent pathways

We have investigated the relationship between tyrosine phosphorylation and respiratory-burst activity in murine bone-marrow-derived macrophages (BMM) stimulated with phorbol myristate acetate (PMA). In unprimed BMM, a good correlation was observed between the net level of tyrosine phosphorylation and the activity of the respiratory burst. The phosphotyrosine phosphatase inhibitor, vanadate, enhanced both tyrosine phosphorylation and respiratory-burst activity triggered by PMA. Furthermore, the tyrosine kinase inhibitor, ST638, abolished both tyrosine phosphorylation and respiratory-burst activity stimulated by PMA. However, in BMM primed by preexposure to TNF alpha, the correlation between net tyrosine phosphorylation and respiratory-burst activity triggered by PMA was not maintained. ST638 was found to only partially inhibit the PMA-triggered respiratory burst under conditions where PMA-stimulated tyrosine phosphorylation was abolished. We conclude that PMA can activate the macrophage respiratory burst by both tyrosine-kinase-dependent and -independent pathways.

Increased viral titer through concentration of viral harvests from retroviral packaging lines

Dependent on the viral vector and the specific assay used, viral titers produced from commonly used retroviral packaging cell lines have an upper limit in the range of 10(5) to 10(7) infectious units/ml. We have developed a generally applicable method, using hollow-fiber filtration technology, which allows for the concentration of infectious virus derived from packaging lines. This method resulted in a reproducible 10- to 30-fold increase in viral titer and can readily be scaled to accommodate larger input volumes. Over 80% of the input virus is recovered in an infectious form in the concentrate. Concentrated virus containing media was seen to produce higher infection frequencies in Jurkat T cells as compared to unconcentrated virus containing media; however, this was not proportional to the differences in viral titer observed by limiting dilution analysis on NIH-3T3 cells. These results are discussed in relation to the importance of factors other than viral titer in determining transduction frequencies.

A retrovirus encoding the v-fps protein-tyrosine kinase induces factor-independent growth and tumorigenicity in FDC-P1 cells

There is increasing evidence that protein-tyrosine kinases play pivotal roles in the response to growth-factor signals. The cytoplasmic tyrosine kinase c-fps/fes, due to its restricted expression in hematopoietic tissue, is likely to participate in hematopoietic growth-factor signalling. We have introduced a retrovirus containing an activated fps gene (encoding P130gag-fps) into the growth factor-dependent myeloid cell line FDC-P1. Clonal cell lines were derived by selection for a marker gene coding for G418 resistance in the absence or presence of the hematopoietic growth factor IL-3. G418 resistant clones expressed P130gag-fps and its associated protein-tyrosine kinase activity and displayed either a factor-independent or IL-3 hypersensitive phenotype and were tumorigenic in syngeneic recipients. Thus, introduction of the activated v-fps gene was able to circumvent the requirement for exogenous growth factors by FDC-P1 cells. Bioassay of conditioned medium from the various clones did not detect hematopoietic growth factor activity and PCR analysis for IL-3 transcripts were negative, suggesting that growth-factor independence was achieved by a mechanism other than autocrine production of a growth factor. We suggest that P130gag-fps is acting to directly stimulate a hematopoietic growth-factor signalling pathway, perhaps one that normally involves the endogenous c-fps/fes protein-tyrosine kinase of FDC-P1 cells.

Abrogation of IL-2 dependence by recombinant murine retrovirus containing v-myb

Biological consequences of expression of v-myb gene in murine long-term T cell lines (CTLL-2 and HT-2) were studied using murine retroviral vectors. Expression of high levels of v-myb altered responses to interleukin (IL)-2, expression of surface markers, and growth characteristics of these cells. Interestingly, v-myb oncogene brings about growth-factor independence in IL-2-dependent T cell lines with concomitant induction of IL-2 mRNA and downregulation of IL-2 receptor synthesis. These results suggest a role for myb gene products in IL-2 gene expression.

A pair of selectable herpesvirus vectors for simultaneous gene expression in human lymphoid cells

The genome of Herpesvirus saimiri, a lymphotropic virus of non-human primates, was used to develop a vector system for transducing foreign genes into primary human T-cells and T-lymphoid cell lines. Recombinant viruses were obtained by homologous recombination of the viral genome with linearized plasmid DNA. The plasmid used contained a fragment of virion DNA, a hygromycin-B-resistance marker (HyR), and a multiple cloning site for the insertion of additional expression cassettes. The resulting recombinants were efficiently enriched and were plaque-purified. The virus mediating HyR and a H. saimiri strain carrying the Geneticin-resistance marker were used to infect the human T-lymphoid cell line Jurkat. Lymphocytes with a double-resistant phenotype were shown to contain the two different H. saimiri recombinants persisting as episomes at high multiplicity. The H. saimiri vector system will be suitable to study cooperating regulatory genes in T-lymphocytes.

Lymphokine signal transduction

Lymphokines are a group of signalling molecules involved in communication between cells, mainly those of the immune system. The lymphokines are multi-functional and most of them have mitogenic or co-mitogenic activity. An understanding of lymphokine biology is essential to understand how the immune system develops and functions and to provide a rationale for their use in immunotherapy. The potential to understand the cell biology of the lymphokines has recently become more apparent as molecular biological techniques have first of all produced recombinant factors and secondly have provided clues to the signal transduction pathways by cloning receptors, applying site-directed mutational analysis and also probing for specific promoters and enhancers that are activated along the signal pathway. This review discusses the information that has come from the recent analyses which blends with the biochemical analysis of the second messenger systems in an effort to understand the signalling pathways of the lymphokines.

Origins and properties of hematopoietic growth factor-dependent cell lines

Studies of the growth regulation, differentiation and transformation of myeloid cells have been greatly facilitated by the availability of a variety of hematopoietic growth factor-dependent cell lines. These cell lines have been isolated from long-term bone marrow cultures and myeloid tumors using interleukin 3 (IL-3) as a growth factor. Using growth factor-dependent cells, it has been shown that growth regulation by IL-3 involves binding to a high-affinity receptor of 140 Kd and activation of tyrosine phosphorylation. IL-3 binding is associated with a number of cellular responses which are required for maintenance of viability, including induction of transcription of the c-myc and ornithine decarboxylase (ODC) genes. In addition, IL-3 regulates the expression of transcription of the gamma T cell receptor locus. The properties of the IL-3-dependent lines are consistent with the hypothesis that they are transformed in their ability to terminally differentiate. In some of the cell lines, this transformation may terminally differentiate. In other of the cell lines, this transformation may be due to the altered expression of the c-myb gene. In other cell lines, transformation is associated with the activation of the expression of a novel gene, termed Evi-1, of the zinc finger family of transcriptional factors. Comparable transformation of erythroid lineage cells is speculated to be due to the activation of the expression of another novel gene termed spi-1. These studies have emphasized the value of well-characterized hematopoietic growth factor-dependent cell lines in advancing our understanding in the basic biology of myeloid cells.

Direct demonstration of an autocrine mechanism in EMS-induced, tumorigenic mutants of the growth factor-dependent hemopoietic cell line, FDC-P1

A growth factor-dependent hemopoietic cell-line, FDC-P1, was treated with a chemical mutagen, EMS, and a number of growth factor-independent variants isolated. Six lines have been extensively analyzed with respect to their growth kinetics, morphology, karyotype, tumorigenicity, and hemopoietic growth factor production. Four lines produced at least one growth factor, subsequently demonstrated to be GM-CSF, while two lines showed no evidence of hemopoietic growth factor production. The observation that the autonomous proliferation of those EMS-derived cell lines that produced GM-CSF can be inhibited by incubation in growth media containing 10-50 microM anti-sense GM-CSF oligonucleotides demonstrated directly that the autonomous behavior of these cells is based on an autocrine mechanism. The induction of the expression of the GM-CSF gene represents a rare class of EMS-induced mutants, and is strongly suggestive of repressor inactivation rather than promoter activation.

Retroviral activation of a novel gene encoding a zinc finger protein in IL-3-dependent myeloid leukemia cell lines

Normal hematopoietic stem cells proliferate and differentiate in the presence of growth factors such as interleukin-3 (IL-3). Transformation can alter their growth factor requirements, the ability of the cells to differentiate, or both. To identify genes that are capable of transforming hematopoietic cells, IL-3-dependent cell lines, isolated from retrovirus induced myeloid leukemias, were examined for viral insertions in proto-oncogenes and in common sites of viral integration. Five of 37 cell lines contained proviruses in a common viral integration site termed the ecotropic virus integration 1 site (Evi-1). The integrations were correlated with the activation of transcription from the locus. Sequencing of cDNA clones and genomic clones demonstrated that the integrations had occurred near or in 5' noncoding exons of a novel gene. The sequence of the cDNA clones predicts that the gene product is a 120 kd protein that contains two domains with seven and three repeats of a DNA binding consensus sequence (zinc finger) initially described in the Xenopus transcription factor III A (TFIIIA). This represents the first demonstration of the retroviral activation of a gene encoding a zinc finger protein and the first implication for a member of this gene family in the transformation of hematopoietic cells.