Continued withdrawal from the cell cycle and regulation of cellular genes in mouse erythroleukemia cells blocked in differentiation by the c-myc oncogene

Mechanisms involved in the induced differentiation of leukemia cells

Despite the remarkable progress achieved in the treatment of leukemias over the last several years, many problems (multidrug resistance [MDR], cellular heterogeneity, heterogeneous molecular abnormalities, karyotypic instability, and lack of selective action of antineoplastic agents) still remain. The recent progress in tumor molecular biology has revealed that leukemias are likely to arise from disruption of differentiation of early hematopoietic progenitors that fail to give birth to cell lineage restricted phenotypes. Evidence supporting such mechanisms has been derived from studying bone marrow leukemiogenesis and analyzing differentiation of leukemic cell lines in culture that serve as models of erythroleukemic (murine erythroleukemia [MEL] and human leukemia [K562] cells) and myeloid (human promyelocytic leukemia [HL-60] cells) cell maturation. This paper reviews the current concepts of differentiation, the chemical/pharmacological inducing agents developed thus far, and the mechanisms involved in initiation of leukemic cell differentiation. Emphasis was given on commitment and the cell lineage transcriptional factors as key regulators of terminal differentiation as well as on membrane-mediated events and signaling pathways involved in hematopoietic cell differentiation. The developmental program of MEL cells was presented in considerable depth. It is quite remarkable that the erythrocytic maturation of these cells is orchestrated into specific subprograms and gene expression patterns, suggesting that leukemic cell differentiation represents a highly coordinated set of events that lead to irreversible growth arrest and expression of cell lineage restricted phenotypes. In MEL and other leukemic cells, differentiation appears to be accompanied by differentiation-dependent apoptosis (DDA), an event that can be exploited chemotherapeutically. The mechanisms by which the chemical inducers promote differentiation of leukemic cells have been discussed.

Transcriptional elongation of c-myb is regulated by NF-kappaB (p50/RelB)

High levels of c-myb expression are necessary for the proliferation of hematopoietic precursor cells whereas down-regulation of c-myb is required for terminal differentiation; this down-regulation occurs through a conditional block to transcriptional elongation in intron I. We previously observed that cAMP analogs prevented the late down-regulation of c-myb during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells and blocked differentiation; this correlated with the induction of NF-kappaB (p50/RelB) complexes which were shown to bind to NF-kappaB recognition sites flanking the transcriptional pause site of c-myb. We now selected stably-transfected MEL cells which overexpressed p50, RelB or both at levels similar to those induced by cAMP to determine whether these NF-kappaB proteins regulate c-myb expression in intact cells. We demonstrate that transcriptionally active NF-kappaB (p50/RelB) complexes, but not p50 or RelB alone, prevented the early and late down-regulation of c-myb mRNA and increased c-myb transcriptional elongation in HMBA-induced MEL cells. The increase in c-myb expression was sufficient to block erythroid differentiation and allow continuous proliferation of cells in the presence of HMBA. Steady-state c-myb mRNA levels in untreated cells were not affected by overexpression of NF-kappaB, suggesting that p50/RelB specifically modulated the efficiency of transcriptional attenuation during MEL cell differentiation.

MNU affects mouse erythroleukemia cell differentiation at sub-cytotoxic doses

MNU is a potent carcinogen and mutagen to various tissues. Molecular events during differentiation show particular sensitivity to MNU exposure. We have investigated the mouse erythroleukemia (MEL) cell differentiation in response to DMSO and the influence of subcytotoxic doses of MNU on this process to assess the role of MNU on the course of differentiation and specific gene expression in a single cell type. Differentiation was followed by determining the extent of hemoglobinization and beta-globin gene expression, which are representative measures of red cell maturation. In this study we have shown a delay and decrease in the extent of MEL cell differentiation by MNU exposure at the time of induction to differentiate, even at sub-lethal MNU concentrations. Once the differentiation process was initiated, exposure to MNU at sub-lethal doses showed a significantly smaller effect on the molecular course of events. Pre-treatment of MEL cells with MNU before DMSO induction did not affect differentiation. The MNU-induced delay in differentiation was reflected in the delayed appearance of beta-globin transcripts during the first 12 h post induction. However, transcription could not account for reduced hemoglobinization of the MNU-treated cells at 48 and 72 h post induction.

Endogenous activation of c-myc expression and DNA synthesis in serum-starved neonatal rat smooth muscle cells

Earlier studies have shown that smooth muscle cells (SMC) from arteries of neonatal and adult rats differ markedly in their in vitro growth characteristics. Since some of these differences may be relevant to the proliferation of SMC in atherosclerotic plaques we examined the expression of three proto-oncogenes (c-fos, c-jun, and c-myc) and an SMC-specific differentiation marker (alpha-actin) in cultured SMC. In presence of serum cultured adult SMC contained lower levels of alpha-actin mRNA than neonatal cells. In neonatal cells serum-starvation resulted in a distinct increase in both c-myc and alpha-actin mRNA levels, whereas the expression of these genes appeared to be unaffected in adult cells. Transfer of adult SMC proliferating in the presence of fetal calf serum to serum-free medium for 48 h almost completely inhibited DNA synthesis, whereas transfer of neonatal SMC to serum-free medium reduced DNA synthesis only to about 50%. Serum-starved adult and neonatal SMC did not contain c-fos or c-jun transcripts, but in both cell types serum-stimulation resulted in a comparable increase in the expression of both genes. The present results demonstrate clear differences in the mechanisms regulating gene expression in adult and neonatal SMC.

Murine erythroleukemia cells contain two distinct GATA-binding proteins that have different patterns of expression during cellular differentiation

GATA-1 is a major transcription factor of the erythroid lineage that has been implicated in the induced expression of a variety of red cell-specific genes during terminal differentiation of murine erythroleukemia cells. Although the GATA-1 protein is present at nearly equal levels before and after differentiation of murine erythroleukemia cells, in this study it was found that in the early commitment stages of the differentiation program there is a transient decrease in the GATA-1 mRNA and DNA binding activity levels due to a temporary block in transcription of the gene. Moreover, using a whole cell extraction procedure it was discovered that murine erythroleukemia cells contain a second GATA binding activity (denoted GATA-rel) which appears to be distinct from the GATA-1 factor based on its non-reactivity to two GATA-1 antisera. This protein has a limited tissue specificity, as it could not be detected in extracts from CHO, NIH 3T3, or COS cells. Similarly to the GATA-1 DNA-binding activity, the GATA-rel activity decreased during the early stages of differentiation. However, unlike GATA-1, GATA-rel activity did not return to pre-induced levels at later times. These results suggest that changes in gene expression during erythroid terminal differentiation may involve an interplay on levels of different GATA-binding factors.

Guanidine group specific ADP-ribosyltransferase in murine cells

We have identified a guanidine group specific ADP-ribosyltransferase activity, capable of transferring an ADP-ribose group from NAD to a low molecular weight guanidine compound [p-(nitrobenzylidine)amino]guanidine and proteins such as histone and poly-L-arginine, in a variety of murine cell lines. The enzyme activity appears to be associated with an integral membrane protein of apparent molecular weight 30-33 kDa. Incubation of the viable cells in isotonic phosphate buffered saline with [32P]NAD results in the incorporation of label into cellular proteins. Dimethyl sulfoxide treatment of the cells downregulates the transferase activity as well as the ADP-ribosylation of cell proteins with extracellular NAD.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)