Changes in the expression of oncogenes encoding nuclear phosphoproteins but not c-Ha-ras have a relationship to monocytic differentiation of HL 60 cells

DNA Repair in Despair-Vitamin D Is Not Fair

The role of vitamin D as a treatment option for neoplastic diseases, once considered to have a bright future, remains controversial. The preclinical studies discussed herein show compelling evidence that Vitamin D Derivatives (VDDs) can convert some cancer and leukemia cells to a benign phenotype, by differentiation/maturation, cell cycle arrest, or induction of apoptosis. Furthermore, there is considerable, though still evolving, knowledge of the molecular mechanisms underlying these changes. However, the attempts to clearly document that the treatment outcomes of human neoplastic diseases can be positively influenced by VDDs have been, so far, disappointing. The clinical trials to date of VDDs, alone or combined with other agents, have not shown consistent results. It is our contention, shared by others, that there were limitations in the design or execution of these trials which have not yet been fully addressed. Based on the connection between upregulation of JNK by VDDs and DNA repair, we propose a new avenue of attack on cancer cells by increasing the toxicity of the current, only partially effective, cancer chemotherapeutic drugs by combining them with VDDs. This can impair DNA repair and thus kill the malignant cells, warranting a comprehensive study of this novel concept. J. Cell. Biochem. 117: 1733-1744, 2016. © 2016 Wiley Periodicals, Inc.

The Potential of Vitamin D-Regulated Intracellular Signaling Pathways as Targets for Myeloid Leukemia Therapy

The current standard regimens for the treatment of acute myeloid leukemia (AML) are curative in less than half of patients; therefore, there is a great need for innovative new approaches to this problem. One approach is to target new treatments to the pathways that are instrumental to cell growth and survival with drugs that are less harmful to normal cells than to neoplastic cells. In this review, we focus on the MAPK family of signaling pathways and those that are known to, or potentially can, interact with MAPKs, such as PI3K/AKT/FOXO and JAK/STAT. We exemplify the recent studies in this field with specific relevance to vitamin D and its derivatives, since they have featured prominently in recent scientific literature as having anti-cancer properties. Since microRNAs also are known to be regulated by activated vitamin D, this is also briefly discussed here, as are the implications of the emerging acquisition of transcriptosome data and potentiation of the biological effects of vitamin D by other compounds. While there are ongoing clinical trials of various compounds that affect signaling pathways, more studies are needed to establish the clinical utility of vitamin D in the treatment of cancer.

A possible interaction of thioredoxin with VDUP1 in HeLa cells detected in a yeast two-hybrid system

Human thioredoxin (hTrx), a small ubiquitous protein with strong reducing potential, has multiple biological functions, including signal transduction and regulation of the activity of transcription factors. hTrx expression is enhanced in HPV-transformed cancer cells; however, the role of hTrx in the malignant cells is not fully understood. We employed a yeast two-hybrid system to search for proteins that bind to hTrx in HeLa cells, a type of HPV-transformed human cervical cancer cell. In a screen of 1.62 x 10(6) yeast cotransformed with a HeLa cDNA library and an hTrx vector, 13 clones were identified as candidates for hTrx-binding proteins. Among them, 3 clones were found to code in frame for the carboxyl-terminal portion of VDUP1 protein, lacking at most the first 155 residues from the start codon. A reconstructed clone carrying the full-length VDUP1 coding sequence also showed the ability to bind to an hTrx fusion protein. Loss of interaction between VDUP1 and hTrx was observed either when two cysteines (Cys 32 and 35) in hTrx were substituted by serines or when the deletion in VDUP1 was extended from amino acid position 155 to 225 or beyond. The 71-mer peptide fragment (position 155-225) of VDUP-1 alone did not bind to hTrx.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Uncoupling of cell cycle arrest from the expression of monocytic differentiation markers in HL60 cell variants

Differentiation generally leads to cell cycle arrest. Human leukemia HL60 cells respond to the presence of 1,25-dihydroxyvitamin D3 (1,25D3) by expressing a number of markers of the monocyte/macrophage phenotype and become arrested predominantly in the G1 phase of the cell cycle. We have recently reported a series (A) of 1,25D3-resistant variants of HL60 cells which proliferate in the presence of 1,25D3 and do not express differentiation markers (Exp. Cell Res. 224, 312, 1996). We now describe another series (B) of such variants, which differ from A series cells grown in similar concentrations of 1,25D3 in that they express the CD14 antigen and nonspecific esterase, characteristic of the monocyte, while continuing to proliferate and they develop hypotetraploid DNA (4C) content at higher concentrations of ambient 1,25D3 than the A series cells. Cells in the B series with 4C DNA content (100B and 200B) also differed from the A series 4C cells by the absence of DNA binding by the full-length Sp1 transcription factor. However, B series cells resembled the A series cells in exhibiting faster growth rates than the parental HL60 cells and showed high levels of vitamin D receptor and retinoid receptor X proteins. These results show that the initial steps in the 1,25D3 signaling pathway are intact in B series resistant cells and lead to the appearance of early markers of monocytic differentiation. However, the progression to subsequent events which comprise terminal differentiation and cell cycle arrest is halted during the adaptation to the presence of 1,25D3 in these cells. Thus, the availability of these variant cells should provide a system for studying the link between differentiation and cell cycle arrest.

Increased stringency of the 1,25-dihydroxyvitamin D3-induced G1 to S phase block in polyploid HL60 cells

Treatment of mammalian cells with 1,25-dihydroxyvitamin D3 (1,25D3) produces a G1 to S (G1/S) phase cell cycle block. In addition, it has been noted that a smaller proportion of cells accumulates in the G2/M compartment in 1,25D3-treated cultures. Since cyclins have a major influence on the regulation of cell cycle progression, we determined the expression of cyclins A and B as markers of the G2 phase and of cyclin E as the marker of G1/S transition. No increase in the steady-state levels of cyclin A or cyclin B mRNA was detected in the total cell population or in the cyclin B1 protein in the G2/M cell cycle compartment. In contrast, immunodetectable cyclin E protein was increased in cell cultures as a whole and specifically in the G2/M compartment cells. Determination of BrdU incorporation into DNA by flow cytometry showed marked inhibition of DNA replication in cells with DNA content higher than 4C, and autoradiography of 3H-TdR-pulsed cells showed that polynucleated cells did not replicate DNA after 96 h of treatment with 1,25D3 or analogs. Taken together, these experiments show that at least a portion of the G2/M compartment in 1,25D3-arrested cultures of HL60 cells represents G1 cells at a higher ploidy level, which are blocked from entering the high ploidy S phase.

Inhibition of vitamin D3-induced cell differentiation by interferon-gamma in HL-60 cells determined by a nitroblue tetrazolium reduction test

The combined effects of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] and interferon-gamma (IFN-gamma) or tumor necrosis factor alpha (TNF-alpha) on cell differentiation in HL-60 human promyelocytic leukemia cells were examined by a nitroblue tetrazolium (NBT) reduction test. 1,25(OH)2D3 at the concentrations of 7-70 nM induced NBT-positive cells, which was used as a criterion of cell differentiation. IFN-gamma itself showed little effect on induction of NBT-positive cells or on cell growth at a concentration up to 1000 U/ml. However, in a combination of 1,25(OH)2D3 with IFN-gamma (100 and 300 U/ml), cell differentiation was strongly inhibited and was accompanied by growth inhibition. Treatment with a combination of 1,25(OH)2D3 and TNF-alpha or IFN-gamma and TNF-alpha showed an additive effect on cell differentiation. IFN-gamma seems to act as a specific inhibitor for 1,25(OH)2D3-induced cell differentiation. To elucidate the cause of the inhibition of cell differentiation by IFN-gamma, the ability of the cells to produce superoxide (O2-) was examined after culture for 5 days in the presence of 1,25(OH)2D3 and IFN-gamma. The results indicated that the inhibition of IFN-gamma was caused by a reduction in the ability of the cells to produce O2- in response to stimulation by 12-O-tetradecanoylphorbol-13-acetate (TPA).

c-myc protein expression during cell cycle phases in differentiating HL-60 cells

We examined c-myc protein expression in cell cycle phases during differentiation induction of HL-60 cells by flow cytometry using an indirect immunofluorescence method. In exponentially proliferating HL-60 cells, c-myc protein was expressed in a cell cycle dependent manner. During the differentiation induction of HL-60 cells with dimethylsulfoxide (DMSO), c-myc protein was rapidly down-regulated in the G1/0 specific phase prior to the appearance of differentiation associated markers. Our results indicate that c-myc protein functions in the G1/0 specific phase in cellular differentiation, and the rapid down-regulation of c-myc protein in G1/0 phase is closely associated with initial differentiation programs.

Vitamin D and breast cancer

1. 1,25-Dihydroxyvitamin D3, the biologically active form of vitamin D, in addition to regulating calcium homeostasis, also has antiproliferative and prodifferentiating effects. 2. Most studies concerning the therapeutic potential of analogs of 1,25(OH)2D3, which are antiproliferative and prodifferentiating but do not cause hypercalcemia, have been done using leukemic cells. 3. Recent evidence from both in vivo and in vitro studies has indicated that 1,25(OH)2D3 or analogs of 1,25(OH)2D3 can inhibit the growth of breast cancer cells, thus suggesting the therapeutic potential of analogs of 1,25(OH)2D3 in the treatment of breast cancer.

Altered gene expression in human leukemia K562 cells selected for resistance to etoposide

Sublines of K562 human leukemia cells were selected for resistance (30- to 80-fold) to etoposide by continuous exposure to 0.5 microM VP-16. Two etoposide-resistant cell lines, K/VP.5 and K/VP.5-1, showed a 5-fold reduction in levels of topoisomerase II alpha protein compared with K562 cells. Northern analysis indicated a 2.5-fold reduction in topoisomerase II alpha mRNA in etoposide-resistant cell lines, due in part to a 1.7-fold decrease in topoisomerase II mRNA stability with no change in transcription rate. Immunoblotting assays of electrophoresed cell lysates from VP-16-treated cells revealed less drug-induced covalent topoisomerase II/DNA adducts in resistant than in sensitive cells, suggesting a functional alteration in resistant cell topoisomerase II. Recent reports of specific topoisomerase II DNA binding sites near the promoter sites of growth response genes and alterations of gene expression in cells treated with topoisomerase II inhibitory drugs led to experiments to determine if the apparent functional alterations of topoisomerase II were accompanied by changes in the regulation of these genes. Therefore, the expression of several growth response genes was compared by northern analysis in parental K562 and both VP-16-resistant cell lines. Basal levels of c-myc were comparable for all three cell lines, but levels of c-jun and c-fos were elevated 2- to 4-fold in VP-16-resistant cell lines. Increased levels of c-fos and c-jun were not a result of altered rates of transcription, as determined by nuclear run-off assays. Exposure of both sensitive and resistant cells to 200 microM VP-16 for 5 hr resulted in no further changes in topoisomerase II mRNA levels but caused an additional 2- to 3-fold elevation in the level of c-jun mRNA, indicating that altered basal levels of this gene were not due to deregulation of this gene. Acquired VP-16 resistance in K/VP.5 and K/VP.5-1 cells was accompanied by reduced levels and altered activities of DNA topoisomerase II as well as changes affecting the expression of genes important for growth and differentiation.

Differences in expression of transcription factor AP-1 in human promyelocytic HL-60 cells during differentiation towards macrophages versus granulocytes

Commitment of HL-60 cells to macrophage or granulocytic differentiation was achieved by incubation with 4 beta-phorbol 12-myristate 13-acetate (PMA) for 30-60 min or with dimethyl sulphoxide (DMSO) for 24 h respectively. The commitment stage towards PMA-induced macrophage differentiation was associated with increases in jun B and c-fos mRNA levels, as well as with an increase in the binding activity of transcription factor AP-1. Nevertheless, gel retardation analysis indicated that the AP-1 activity detected in untreated cells was drastically reduced during the commitment stage of DMSO-induced HL-60 differentiation towards granulocytes. When HL-60 cells were treated with sodium butyrate, which induced monocytic differentiation, a remarkable increase in AP-1 binding activity was detected. Treatment of HL-60 cells with 1 alpha,25-dihydroxyvitamin D3, another monocytic differentiation agent, induced a weak, but appreciable, increase in AP-1 activity. Furthermore, addition of sodium butyrate or 1 alpha,25-dihydroxyvitamin D3 to HL-60 cells induced the expression of c-fos, c-jun, jun B and jun D proto-oncogenes. In contrast, when HL-60 cells were treated with retinoic acid, a granulocytic differentiation inducer, no enhanced AP-1 binding activity was observed, and only a weak increase in jun D mRNA level was detected. These data indicate that formation of AP-1 is not required for the induction of HL-60 differentiation towards granulocytes, whereas induction of monocytic differentiation is correlated with an increase in AP-1 activity. The differential expression of AP-1 activity may be critical in the differentiation of HL-60 cells towards monocytic or granulocytic lineages.

Resolution of multiple AP-1 complexes in HL-60 cells induced to differentiate by 1,25-dihydroxyvitamin D3

Activator protein-1 (AP-1) complex plays a central role in the regulation of both growth and differentiation in many cell types. Monocytic differentiation of HL-60 cells by TPA (12-0-tetradecanoyl phorbol-13-acetate) has been reported to be paralleled by increased AP-1 binding to DNA and by elevated c-jun expression, suggesting transcriptional level of control. We show that two forms of AP-1 complex, designated AP-1/1 and AP-1/2, can be demonstrated in logarithmically growing HL-60 cells, that the exposure of these cells to 10(-8) M 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) results in increased binding of these complexes to the AP-1 DNA element, and that the AP-1 complex can be resolved into at least three forms in differentiated cells. Binding to, or competition with, a mutated form of the AP-1 binding site shows that the most slowly migrating complex (AP-1/3) binds to DNA with greater specificity than do complexes AP-1/1 and AP-1/2, while antibody inhibition and binding studies performed at 37 degrees C indicate that jun proteins predominate in AP-1/2 complexes. Exposure of extracts from differentiated, but not untreated, HL-60 cells to 2 mM ATP increases the prominence of AP-1/3 complexes, and reduces the DNA binding of AP-1/1 complexes. Treatment of the extracts with phosphatases abolishes the binding of AP-1/2 and AP-1/3 to DNA, and increases the binding intensity of AP-1/1. When extracts from differentiated cells are mixed with extracts from undifferentiated cells the AP-1/3 complexes become less prominent, suggesting than an inhibitory activity in undifferentiated cells prevents the formation of AP-1/3 complexes. These studies show the association of multiple forms of AP-1 complex with the mature monocytic phenotype, and suggest several levels of control of monocytic differentiation.

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.

Resistance to 1,25-dihydroxyvitamin D3 of a deoxycytidine kinase-deficient variant of human leukemia HL60 cells

A deoxycytidine kinase-deficient variant of HL60 cells (HL60-araC), isolated by its resistance to 1-beta-D-arabinofuranosyl cytosine (ara-C), shows cross-resistance to the differentiation-inducing and growth-inhibitory effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This is not due to the lack of uptake of 1,25(OH)2D3 by HL60-araC cells, shown by an increased rate of intracellular accumulation of [3H]-1,25(OH)2D3, or to the lack of expression of the gene for the vitamin D3 receptor. However, down-modulation of the expression of this gene by 1,25(OH)2D3 is markedly delayed in HL60-araC cells, and the down-regulation of the expression of the c-myc gene is also delayed. In contrast, the expression of the constitutively expressed 16S mitochondrial rRNA gene is unchanged by 1,25(OH)2D3 treatment of either cell subline. These findings suggest that some cases of drug resistance may be associated with defective functioning of a differentiation pathway.

Inhibition by 1,25 dihydroxyvitamin D3 of c-myc down-regulation and DNA fragmentation in cytosine arabinoside-induced erythroid differentiation of K562 cells

The effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on DNA fragmentation, altered expression of the heat shock protein (hsp) 70 gene, and protooncogenes c-myc and c-myb was studied during chemical induction of erythroid differentiation in K562 cells. Preincubation of K562 cells with 1,25(OH)2D3 did not alter the concentration of hemoglobin in cells which did differentiate, but led to a reduction in the accumulation of low molecular weight DNA generated by Ara-C administration. The extent of this reduction was similar to the degree of inhibition of hemoglobin formation in the culture as the whole. Preincubation with 1,25(OH)2D3 had no effect on the increase of hsp 70 gene expression induced by a 48-hr treatment with Ara-C, but prevented the Ara-C-induced down-regulation of the protooncogene c-myc. The protooncogene c-myb was down-regulated after 15 min of treatment with Ara-C, and exposure to 1,25(OH)2D3 prior to Ara-C caused a further down-regulation of its expression. The data suggest that the events associated with erythroid differentiation may be separable into at least two groups; one of these may have an influence on the kinetics of the cell cycle traverse, and the other may be related to the expression of the erythroid phenotype.

Mitochondrial nucleic acids as internal standards for blot hybridization analyses

A plasmid, designated p72, constructed from human lung carcinoma DNA inserted into the promoterless herpes simplex virus thymidine kinase gene pML-TK-Bgl II vector, hybridizes strongly to human nucleic acids on Southern and Northern blots. The portion of the DNA insert responsible for the strong signal following hybridization to human DNA or RNA is a 167-bp 3' terminal portion of the mitochondrial 16S ribosomal RNA gene. The expression of this gene is constitutive in the several human cell lines that were tested and is unaffected by exposure to cytotoxic chemicals that alter the expression of nuclear genes. This plasmid offers an excellent tool for studies of perturbations of gene expression and for controlling for the variations in sample preparation, loading, and transfer in Southern or Northern analysis of nucleic acids.

1 alpha,25-dihydroxyvitamin D3 regulates the transcription of carbonic anhydrase II mRNA in avian myelomonocytes

Carbonic anhydrase II (CAII) is highly expressed in the osteoclast, where it is involved in the process of extracellular acidification required for bone resorption. We have previously shown that 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3], a steroid hormone that regulates the differentiation of macrophages and osteoclasts, induces the expression of CAII mRNA and protein in avian bone marrow cells. To determine whether this regulation occurred at the gene level, we have studied the effects of 1,25(OH)2D3 on CAII expression in a transformed myelomonocytic avian cell line (BM2). As observed in nontransformed cells, 1,25(OH)2D3 markedly increased CAII biosynthesis and mRNA levels. The increase in CAII mRNA was detected as early as 3 hr after adding the hormone (1.9-fold) and reached 4.7-fold by 48 hr. These effects were completely blocked by actinomycin D, and nuclear run-on analysis confirmed that 1,25(OH)2D3 increased the rate of CAII gene transcription. In contrast, induction of CAII mRNA expression was not affected by inhibition of protein synthesis with cycloheximide, and no significant changes in mRNA stability were seen. Thus, 1,25(OH)2D3 modulates CAII gene expression at the transcriptional level, and this effect does not require de novo synthesis of other gene products. These results suggest that activation of the CAII gene occurs early in the differentiation events triggered by vitamin D3 in myelomonocytic cells.

Antisense inhibition of c-myc expression reveals common and distinct mechanisms of growth inhibition by TGF beta and TNF alpha

Downregulation of the c-myc gene in HL-60 cells is associated with growth inhibition and induction of differentiation. Previous studies have reported that the growth inhibitors TGF beta and TNF alpha downregulate c-myc mRNA levels, suggesting the possibility that these agents may exert some of their phenotypic effects via c-myc downregulation. Our study demonstrates that although both growth inhibitors produce a similar decrease in c-myc protein synthesis, TNF alpha produces a greater growth inhibition and differentiation induction in HL-60 cells. Combined addition of anti-myc oligomer with either growth inhibitor produces no additive effect. In fact, 4 microM anti-myc oligomer produces the same growth and differentiation effects as does 10 ng/ml TGF beta 1. We conclude that downregulation of c-myc expression represents a common mechanism of growth inhibition by TGF beta and TNF alpha, but that TNF alpha possesses an additional effect that is independent of c-myc expression.

Analysis of nuclear sugar-binding components in undifferentiated and in vitro differentiated human promyelocytic leukemia cells (HL60)

The nuclear sugar-binding components (i.e., lectin-like molecules) were analyzed using isolated and membrane-depleted nuclei after incubation in the presence of fluorescein-labeled neoglycoproteins. This analysis was performed before and during the in vitro differentiation of HL60 cells into monocytes by PMA treatment and into granulocytes by DMSO treatment. The nucleoli of undifferentiated and differentiated HL60 cells were not labeled, unlike the nucleoli of other mammalian cells studied so far. This peculiarity allowed us to quantitatively analyze by flow cytometry the changes in the lectin activity associated with the extranucleolar territories enriched in ribonucleoprotein complexes. The neoglycoprotein binding was found to be significantly lower in differentiated than in undifferentiated cells. The decrease in neoglycoprotein binding was observed within the first 24 h of DMSO or PMA treatment, just before the arrest of DNA synthesis. Taking into account that the granulocytic differentiation required 72 h of chemical treatment, the extra-nucleolar lectins might be involved in modulation of the DNA synthesis rather than in phenotypic differentiation. These data are discussed in an attempt to reconcile the association of lectins with RNP complexes and their possible involvement in modulation of HL60 cell proliferation.

Rapid actions of 1 alpha,25-dihydroxyvitamin D3 on Ca2+ and phospholipids in isolated rat liver nuclei

The effects of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) on Ca2+ levels and phospholipid metabolism were studied in isolated nuclei prepared from rat liver. Nuclear Ca2+ concentration was estimated with the fluorescent indicator Fura 2. In agreement with previous reports, ATP (1 mM) produced a rapid increase in nuclear Ca2+ from 188 +/- 25 to 593 +/- 121 nM. Exposure to 1 alpha,25-(OH)2D3 (20 nM) also produced a rapid increase in nuclear Ca2+ to 402 +/- 71 nM. The 1 beta epimer of 1 alpha,25-(OH)2D3 had no effect. Nuclear phosphatidylinositol was labeled by incubation with [gamma-32P]ATP for 3 h. 1 alpha,25-(OH)2D3 produced a two-fold increase in [32P]lysophosphatidylinositol (LPI) within 5 min from 44 +/- 11 to 87 +/- 19 cpm/2.5 x 10(7) nuclei. 1 beta,25-(OH)2D3 had no effect on [32P]LPI production. Exposure of nuclei to exogenous LPI (15 microM) produced an instantaneous increase in nuclear Ca2+ to 372 +/- 81 nM, comparable to ATP and 1 alpha,25-(OH)2D3. The rapid effects of 1 alpha,25-(OH)2D3 on phospholipid metabolism and Ca2+ in isolated nuclei suggest that the steroid may exert effects distinct from the well-characterized receptor-mediated changes in gene expression.

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)

Enhanced messenger RNA stability and differentiation of HL 60 cells treated with 1,25-dihydroxyvitamin D3 and cordycepin

The effect of inhibitors of RNA synthesis on 1,25(OH)2 vitamin D3-induced monocytic differentiation was studied in a well-differentiating clone AB 47 of HL 60 cells. The concentrations of these inhibitors were chosen to permit the maintenance of cell viability for at least 48 hours, and resulted in 40-60% inhibition of total cellular RNA synthesis. No impairment of 1,25(OH)2 vitamin D3-induced monocytic differentiation was observed with all inhibitors tested, and the presence of cordycepin actually enhanced differentiation. The phenotypic evidence of monocytic differentiation correlated with the increased levels of mRNA for c-fos and c-fms measured by hybridization to appropriate nick-translated cDNA probes. In contrast, nuclear run-on experiments showed the expected inhibition of transcription of these genes by the compounds used. The data suggest that interference by external agents with transcription of genes essential for a differentiation program brings into play compensatory mechanisms which permit the program to continue. Thus, differentiation appears to have a high priority among various competing intracellular pathways in 1,25(OH)2 vitamin D3-treated HL 60 cells. Stabilization of messenger RNA levels evident in this study may therefore represent a general cellular mechanism for the correction of unwanted effects of xenobiotics on the cell.

Growth and differentiation signals as determinants of cancer cell proliferation

The development and the maintenance of a tissue involves the sequential differentiation of precursor cells along multiple intermediate stages of maturity. At these stages the cells can proliferate or differentiate depending upon the growth or differentiation signals they receive. Terminally mature cells lose their ability to proliferate. Alterations can occur at the level of the cell or the whole organism that lead to the sustained proliferation of incompletely mature cells. This process constitutes an oncogenic event, and the cellular and molecular changes that lead to its occurrence are surveyed in this review.

c-myc protein kinetics during growth and differentiation of CD34 (MY10)-positive blast cells from normal human marrow

We have used flow cytometry to quantitate nuclear c-myc protein, at each phase of the cell cycle, during in-vitro differentiation of CD34-positive stem cells isolated from normal human bone marrow by the monoclonal antibody, MY10. Mean c-myc protein levels in CD34-positive cells, consisting of greater than 70% blasts, are lower than a marrow fraction containing myeloid cells of intermediate maturation, but have an invariant proportional relationship, with regard to nuclear mass, over the cell cycle. The majority of these primitive cells are non-cycling, as revealed by DNA content. Under our assay conditions, nuclear c-myc protein distribution over the cell cycle did not change as these progenitors entered a proliferative phase in culture. In cultures containing factors supporting myeloid maturation, mean G0/G1 p62c-myc levels initially decline, then rise above starting values as promyelocytes and myelocytes differentiate from CD34-positive cells, and as proliferation begins. With further myeloid maturation, and while cell numbers are increasing, c-myc protein continues to increase. C-myc protein kinetics differ in cultures in which macrophages, rather than myeloid cells, predominate. These data indicate that a complex relationship exists between c-myc gene expression and proliferation, maturation and lineage in haemopoietic cells, and lend support to the notion that early down regulation may be causally associated with the differentiation process.

Differentiation of human leukaemic cell lines and fresh leukaemia cells by low dose Ara-C: monitoring by expression of c-myc and c-fos oncogenes

The effects of low dose Ara-C on cellular oncogene expression were measured in HL-60 and U-937 cell lines and in primary cultures from leukaemic patients. Expression of c-myc was completely abolished in U-937 and greatly reduced in HL-60 after a 3 day exposure to the drug, whereas specific c-fos transcripts were increased. In fresh myeloid leukaemia samples, growth and DNA synthesis were reduced as in the two cell lines. Conversely, changes compatible with the induction of differentiation along the myelomonocytic pathway were much less pronounced than in cell lines treated with the same dose of Ara-C. Cells from one patient did not show any appreciable morphological change. The same sample displayed a greatly reduced expression of c-myc accompanied by a concurrent 10-fold increased expression of c-fos. The data suggest that the action of low dose Ara-C on oncogene expression is comparable to that of other differentiation-inducing agents that display both cytostatic and maturation promoting effects. Evaluation of cellular oncogene expression may therefore represent a useful tool for monitoring effects of low dose Ara-C on leukaemia cells.

Coordinate expression of c-myc, c-myb, and histone H4 genes in reversibly differentiating HL 60 cells

The expression of oncogenes c-myc and c-myb in human leukemic cells HL 60 was compared to the expression of histone H4 gene, known to be cell-cycle dependent. Steady-state levels of mRNA transcribed from these genes were determined by simultaneous hybridization of Northern transfers with four probes, and the rates of gene expression were measured by nuclear transcription ("run-on") assays. Expression of genes c-myc, c-myb and histone H4 varied coordinately and in parallel with the rates of DNA synthesis, while the rates of total and ribosomal RNA synthesis, the expression of gene c-Ha-ras, unrelated to proliferation of these cells, and gene p 72, a constitutively expressed human gene, were unchanged. Further, the levels of c-myc and c-myb mRNA but not p 72 mRNA were higher in cell populations enriched for S phase cells. Thus, transcription of genes c-myc and c-myb in HL 60 cells appears to be linked to DNA replication in a manner previously demonstrated for core histone gene expression.

Participation of c-myc protein in DNA synthesis of human cells

The protein product of oncogene c-myc is believed to be important in regulation of the cell cycle. However, its direct role in DNA synthesis has not been explored. Experiments presented here show that the addition of affinity-purified antibodies against the human c-myc protein to nuclei isolated from several types of human cells reversibly inhibited DNA synthesis and DNA polymerase activity of these nuclei. This suggests that c-myc encodes a protein that is functionally involved in DNA synthesis.