Regulation of the expression of enzymes involved in the replication of DNA in chemically-induced granulocytic differentiation of HL-60 leukemia cells.

The expression of seven enzymes involved in the biosynthesis of DNA was measured in HL-60 promyelocytic leukemia cells treated with dimethylsulfoxide (DMSO) or all-trans retinoic acid (RA) to gain information on their role in the termination of proliferation in cells undergoing granulocytic differentiation. The steady-state levels of the mRNAs for topoisomerase I, topoisomerase II. DNA polymerase-alpha, thymidylate synthase, thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase progressively declined from day 3 to day 7 of exposure to the polar solvent or the retinoid suggesting that the expression of these enzymes is coordinately regulated. In contrast, a pronounced difference between the two inducers of differentiation occurred in the expression of the mRNA of the M2 subunit of ribonucleotide reductase, with DMSO causing virtually complete inhibition of the expression of the M2 subunit of the enzyme from day 5 through day 7, with no change in the steady-state levels of the mRNA being produced by retinoic acid. Measurement of the enzymatic activities of two of these catalysts, thymidylate synthase and thymidine kinase, in cells exposed to the two inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The findings collectively demonstrate that the down-regulation of the expression of a relatively wide variety of enzymes involved in DNA replication occurs as late events in the granulocytic differentiation of HL-60 cells, ensuring that cellular replication cannot occur in terminally differentiated cells.

Regulation of the expression of enzymes involved in the replication of DNA in chemically induced monocytic/macrophagic differentiation of HL-60 leukemia cells.

The expression of a number of housekeeping enzymes of DNA biosynthesis was measured in HL-60 promyelocytic leukemia cells undergoing monocytic/macrophagic differentiation following treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1alpha,25-dihydroxyvitamin D3 (vitamin D3). Progressive decreases in the steady-state levels of the mRNAs for thymidylate synthase, topoisomerase II, and hypoxanthine guanine phosphoribosyltransferase occurred following exposure to TPA or vitamin D3. In contrast, the steady-state levels of the mRNAs for thymidine kinase, topoisomerase I, and DNA polymerase-alpha did not decrease until days 3-5 of treatment with vitamin D3 and then progressively declined thereafter. The mRNAs for thymidine kinase and topoisomerase I decreased slightly and the mRNA for DNA polymerase-alpha by 30-40%, and then remained constant between days 1 to 3 of treatment with the phorbol ester. The M2 subunit of ribonucleotide reductase exhibited an even greater difference, with no change in the steady-state concentration of mRNA over 3 days of exposure to TPA or vitamin D3. On days 5-7 of treatment with vitamin D3, essentially complete loss of the expression of the mRNA for the M2 subunit of ribonucleotide reductase occurred. Measurement of the enzymatic activities of thymidylate synthase and thymidine kinase in cells exposed to either of the inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The results indicate that the down-regulation of the expression of housekeeping enzymes of DNA replication occurs as late events in HL-60 cells undergoing monocytic/macrophagic differentiation, implying that the decreases in their gene expression are the result of the termination of proliferation rather than an initiating event in the cessation of DNA biosynthesis.

Induction of the differentiation of HL-60 promyelocytic leukemia cells by nonsteroidal anti-inflammatory agents in combination with low levels of vitamin D3.

Previous experiments have shown that a variety of agents that interfere with the activity of the transcription factor NF-kB significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of the monocytic/macrophagic differentiating agent vitamin D3. These include an antisense phosphorothioate oligonucleotide to the Rel A subunit of NF-kB, vitamin E and other antioxidants, and curcumin. Acetylsalicylic acid and other nonsteroidal anti-inflammatory agents represent another group of agents that have been reported to inhibit NF-kB at serum levels approximating those obtained during long-term therapy of chronic inflammatory states. To determine whether nonsteroidal anti-inflammatory agents also were capable of enhancing the differentiation of HL-60 leukemia cells produced by vitamin D3, we measured the effects of a variety of nonsteroidal anti-inflammatory agents on the maturation of HL-60 cells produced by low levels of vitamin D3. Acetylsalicylic acid by itself had no significant effect on the differentiation of HL-60 cells; however, this agent markedly increased the degree of differentiation produced by low levels of vitamin D3. Furthermore, a variety of other nonsteroidal anti-inflammatory agents of different chemical classes exhibited similar enhancements of the maturation of HL-60 cells when combined with vitamin D3. An analogous increase in the differentiation of HL-60 cells was also obtained by combination of several nonsteroidal anti-inflammatory agents with the granulocytic inducing agent, retinoic acid, but not with dimethylsulfoxide. The nonsteroidal anti-inflammatory agents also enhanced the differentiation of HL-60 cells when combined with vitamin D analogs which share the receptor binding properties of vitamin D3; however, a vitamin D analog which caused significant calcium mobilization, but was less effective in receptor binding than vitamin D3, did not induce the differentiation of HL-60 cells in the presence or absence of anti-inflammatory agents. The findings suggest that the nonsteroidal anti-inflammatory agents may have utility in the treatment of acute promyelocytic leukemia when used with the D vitamins or retinoic acid.

Enhancement by antisense oligonucleotides to NF-kappaB of the differentiation of HL-60 promyelocytic leukemia cells induced by vitamin D3.

We have demonstrated previously that a phosphorothioate antisense oligonucleotide to the p65 subunit of the inducible transcription factor NF-kappaB produced rapid changes in the expression of leukocyte integrin CD11b (Mo 1) and in the adhesion of dimethylsulfoxide (DMSO)-differentiated HL-60 cells stimulated by 12-O-tetradecanoylphorbol 13-acetate. We have also shown that a variety of agents which inhibit NF-kappaB, including vitamin E and related antioxidants, curcumin and several non-steroidal anti-inflammatory agents, significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of 1,25-dihydroxyvitamin D3 (vitamin D3). To provide further evidence that interference with the activation of NF-kappaB affects the maturation of HL-60 leukemia cells by creating an environment conducive to terminal differentiation, we measured the effects of phosphorothioate antisense oligonucleotides to the various subunits of NF-kappaB on the differentiation of HL-60 cells produced by low levels of vitamin D3. When used alone these oligonucleotides had no significant effect on the differentiation of HL-60 cells. However, the antisense oligomer to the Rel A subunit of NF-kappaB markedly increased the extent of differentiation produced by low levels of vitamin D3. An enhancement of the differentiation of HL-60 cells induced by vitamin D3 was also obtained by several transcription factor decoys designed to mimic the consensus sequences of genes activated by Rel A. The findings provide additional support for the concept that inhibition of the activation of NF-kappaB may be involved in regulating the entry of promyelocytic leukemia cells into a differentiation pathway.

Induction of the differentiation of HL-60 promyelocytic leukemia cells by vitamin E and other antioxidants in combination with low levels of vitamin D3: possible relationship to NF-kappaB.

Epidemiological studies have provided evidence that diets rich in antioxidant nutrients may reduce the risk of cancer. To evaluate the possibility that dietary phytochemicals with antioxidant potential would create an environment capable of affecting the differentiation of HL-60 leukemia cells, we measured the effects of vitamin E and other dietary antioxidants on the differentiation produced by low levels of vitamin D3 and analogs thereof. Vitamin E succinate and other antioxidant compounds (ie butylated hydroxyanisole, beta-carotene and lipoic acid) used alone had no significant effect on the differentiation of HL-60 cells; however, these agents markedly increased the differentiation produced by vitamin D3. Previous studies from this laboratory have shown that a sequence-specific antisense phosphorothioate oligonucleotide to the Rel A subunit of NF-kappaB enhanced the differentiation of HL-60 cells produced by several inducing agents. Consistent with these observations, vitamin E succinate caused a marked reduction in the nuclear content of NF-kappaB both in the presence and absence of vitamin D3. These findings suggest that NF-kappaB may be a factor in regulating the differentiation of myeloid leukemia cells. The results also indicate that combinations of vitamin D3 and analogs thereof with dietary antioxidants may be useful in overcoming the differentiation block present in acute promyelocytic leukemia cells.

Induction of the differentiation of HL-60 promyelocytic leukemia cells by curcumin in combination with low levels of vitamin D3.

Previous studies have shown that an antisense phosphorothioate oligonucleotide to the Rel A subunit of NF- kappa B, as well as vitamin E and related antioxidants, significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of 1 alpha, 25-dihydroxyvitamin D3 (vitamin D3) an effect accompanied by a marked inhibition of the transcription factor, NF-kappa B. Curcumin, a potent inhibitor of tumor promotion and of tumor cell growth, has also been shown to have antioxidant properties and to inhibit NF-kappa b. to ascertain whether curcumin would also enhance the differentiation of HL-60 leukemia cells produced by vitamin D3, presumably by interfering with NF- kappa B activity, the effects of curcumin on the differentiation of HL-60 cells produced by low levels of vitamin D3 were measured. Curcumin used alone did not produce a significant degree of differentiation of HL-60 cells; however, this agent markedly enhanced the expression of differentiation markers induced by low levels of vitamin D3. Curcumin also increased the differentiation of HL-60 cells when combined with vitamin D analogues (1,25-dihydroxy-16-ene-23-yne vitamin D3 and 1,25-dihydroxy-16-ene vitamin D3) that share the receptor binding properties of vitamin D3, whereas as vitamin D analogue (1,25-dihydroxy-16,23-diene vitamin D3) that caused significant calcium mobilization, but was less effective than vitamin d3 in binding the receptor, did not cause the differentiation of HL-60 cells in the presence or absence of curcumin. Several dietary compounds structurally related to curcumin (i.e., caffeic acid, chlorogenic acid, and ferulic acid) did not increase the differentiation of HL-60 cells produced by vitamin D3. However, the more lipophilic ethyl of ferulic and caffeic acid were capable of inducing the differentiation of HL-60 cells, as well as enhancing the maturation produced by vitamin D3. Curcumin caused a marked reduction in NF-kappa B activity in nuclear extracts of HL-60 cells exposed to this agent in the presence or absence of vitamin D3, supporting the possibility that NF-kappa B may be a factor in the regulation of the state of differentiation of leukemia cells.

Cell cycle-dependent modulation of telomerase activity in tumor cells.

Telomerase is a ribonucleoprotein complex that is thought to add telomeric repeats onto the ends of chromosomes during the replicative phase of the cell cycle. We tested this hypothesis by arresting human tumor cell lines at different stages of the cell cycle. Induction of quiescence by serum deprivation did not affect telomerase activity. Cells arrested at the G1/S phase of the cell cycle showed similar levels of telomerase to asynchronous cultures; progression through the S phase was associated with increased telomerase activity. The highest level of telomerase activity was detected in S-phase cells. In contrast, cells arrested at G2/M phase of the cell cycle were almost devoid of telomerase activity. Diverse cell cycle blockers, including transforming growth factor beta1 and cytotoxic agents, also caused inhibition of telomerase activity. These results establish a direct link between telomerase activity and progression through the cell cycle.

Differentiation of immortal cells inhibits telomerase activity.

Telomerase, a ribonucleic acid-protein complex, adds hexameric repeats of 5'-TTAGGG-3' to the ends of mammalian chromosomal DNA (telomeres) to compensate for the progressive loss that occurs with successive rounds of DNA replication. Although somatic cells do not express telomerase, germ cells and immortalized cells, including neoplastic cells, express this activity. To determine whether the phenotypic differentiation of immortalized cells is linked to the regulation of telomerase activity, terminal differentiation was induced in leukemic cell lines by diverse agents. A pronounced downregulation of telomerase activity was produced as a consequence of the differentiated status. The differentiation-inducing agents did not directly inhibit telomerase activity, suggesting that the inhibition of telomerase activity is in response to induction of differentiation. The loss of telomerase activity was not due to the production of an inhibitor, since extracts from differentiated cells did not cause inhibition of telomerase activity. By using additional cell lineages including epithelial and embryonal stem cells, down-regulation of telomerase activity was found to be a general response to the induction of differentiation. These findings provide the first direct link between telomerase activity and terminal differentiation and may provide a model to study regulation of telomerase activity.

Differentiation of HL-60 cells by dimethylsulfoxide activates a Na(+)-dependent nucleoside transport system.

Uridine transport in undifferentiated HL-60 cells occurs primarily by facilitated diffusion, but a limited Na(+)-dependent process can be demonstrated (Km = 44 +/- 4.4 microM, Vmax = 0.13 +/- 0.01 microM/s). This latter transport system was inhibited by adenosine and inosine (Ki = 110 and 260 microM, respectively), whereas guanosine and thymidine were less effective (Ki = 1600 and 1200 microM, respectively). Dimethylsulfoxide (DMSO) caused a concentration-dependent decrease in facilitated uridine transport. This change was attributable to a decrease in the number of transporter molecules as determined by the binding of [3H]nitrobenzylthioinosine to cell membranes. Moreover, the Na(+)-dependent transport of uridine was enhanced by DMSO at a concentration of the polar solvent as low as 0.4%. When HL-60 cells were exposed to 1.0% DMSO, a marked increase in Na(+)-dependent uridine transport occurred within 72 hr, a time preceding maximum granulocytic differentiation. This change was attributable to an increase in transport affinity (Km = 1.54 +/- 0.65 microM), with no change in Vmax (Vmax = 0.13 +/- 0.02 microM/s). The consequence of these changes was the generation of a 3- to 4-fold increase in the intracellular concentration of uridine relative to the medium at a physiological concentration of 5 microM uridine. Similar increases in transport affinity were observed for adenosine, inosine, guanosine and thymidine in DMSO-differentiated HL-60 cells (Km values of 2 to 5 microM). These results complement our previous studies with phorbol 12-myristate 13-acetate, in which differentiation to a monocytic phenotype was also associated with enhanced Na(+)-dependent nucleoside transport.

Enhancement of pertussis-toxin-sensitive Na(+)-dependent uridine transporter activity in HL-60 granulocytes by N-formylmethionyl-leucyl-phenylalanine.

N-Formyl-Met-Leu-Phe (FMLP), at concentrations as low as 5 nM, caused an increase in intracellular uridine pools in dimethyl sulphoxide (Me2SO)-differentiated HL-60 cells. Intracellular uridine pools were elevated rapidly and reached a maximum within 10 min of exposure to 10 microM FMLP, followed by a gradual decline. This enhancement by FMLP was a consequence of a 3-fold increase in the Vmax of pertussis-toxin-sensitive Na(+)-dependent uridine transport system, with no change in the apparent Km. Km values of 2.67 +/- 0.45 and 3.85 +/- 0.52 microM and Vmax. values of 0.046 +/- 0.017 and 0.125 +/- 0.020 microM/s were obtained for untreated and FMLP-treated Me2SO-differentiated cells respectively. The effect of FMLP on the Na(+)-dependent transport of uridine in Me2SO-differentiated HL-60 cells was specific, as the facilitated transport of uridine was unaffected. Furthermore, this phenomenon was not observed in undifferentiated, phorbol 12-myristate 13-acetate (PMA)-differentiated or pertussis-toxin-treated Me2SO-differentiated HL-60 cells. Removal of extracellular Ca2+ with EGTA abolished the FMLP enhancement of uridine transport in a reversible manner, suggesting the involvement of Ca2+. However, the Ca2+ ionophore A23187 only partially mimicked the effect of FMLP. Similarly, with PMA the transport was sub-optimally enhanced, but a full activation was observed in cells treated with both A23187 and PMA. These findings suggest that activation of the Na(+)-dependent uridine transporter by FMLP in Me2SO-differentiated HL-60 cells involves a pertussis-toxin-sensitive G-protein with a bifurcating signal-transduction pathway.

Antisense oligonucleotides to the p65 subunit of NF-kappa B block CD11b expression and alter adhesion properties of differentiated HL-60 granulocytes.

NF-kappa B is a pleiotropic regulator of a variety of genes implicated in the cellular response to injury. This function has been attributed to the coordinated binding of subunits of NF-kappa B to distinct regions of the promoter elements of numerous genes, including cytokines, growth factor receptors, and adhesion molecules. Antisense phosphorothioate oligonucleotides to the p50 and p65 subunits of the NF-kappa B complex were used to define the physiologic role of this transcription factor in resting and stimulated granulocytes. A reduction in the expression of p65 was produced by treatment with the phosphorothioate antisense oligodeoxynucleotide. This reduction was accompanied by rapid changes in the cellular adhesion of dimethyl sulfoxide-differentiated HL-60 leukemia cells stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA). These effects were characterized by a marked reduction in CD11b integrin expression on the surface of treated cells. Furthermore, the p65 antisense oligomer effectively abolished an upregulation of CD11b that was produced by formyl-met-leu-phe and TPA. However, the p65 antisense phosphorothioate oligodeoxynucleotide had no significant effect on the production of reactive oxygen intermediates or on phagocytosis by these cells. These findings indicate that antisense oligomers to p65 can be used to define the role of NF-kappa B in the activation pathways of neutrophils.

Induction of the differentiation of HL-60 and WEHI-3B D+ leukemia cells by lithium chloride.

The use of lithium chloride in manic-depressive patients and in patients receiving myelo-suppressive cancer chemotherapeutic agents is accompanied by a sustained leukocytosis due to an increase in granulocyte production. This property suggests that lithium chloride may have effects on hematopoietic differentiation. Treatment of cultured WEHI-3B D+ murine myelomonocytic and HL-60 human promyelocytic leukemia cells with millimolar concentrations of lithium chloride resulted in concentration-dependent increases in the number of differentiated myeloid cells, as determined by the ability of the cells to reduce nitroblue tetrazolium and by the binding of myeloid specific antibodies, and was associated with an inhibition of cellular proliferation. The effects of lithium chloride on growth and differentiation were antagonized by KCl, whereas NaCl had little effect. The induction of leukemic cell maturation by lithium chloride was markedly enhanced by the addition of low levels of retinoic acid. In contrast, other differentiation inducing agents (i.e. dimethyl sulfoxide and selenazofurin) had no effect on the degree of maturation induced by lithium. These findings suggest that the combination of lithium chloride and retinoic acid may have clinical utility in the treatment of leukemia through the induction of terminal differentiation.

Evidence for a relationship between intracellular GTP levels and the induction of HL-60 leukemia cell differentiation by 5,10-dideazatetrahydrofolic acid (DDATHF).

5,10-Dideazatetrahydrofolic acid (DDATHF) is an inhibitor of glycinamide ribonucleotide transformylase, the first of two tetrahydrofolate requiring enzymes in the de novo purine nucleotide biosynthetic pathway, and is a potent inducer of the maturation of HL-60 promyelocytic leukemia cells. The inhibition of cellular growth by DDATHF was effectively prevented by adenosine or deoxyadenosine, whereas guanosine or deoxyguanosine only partially prevented the growth inhibition produced by this folate antimetabolite, implying that the depletion of both ATP and GTP, which occurs with this agent, was responsible for its growth inhibitory effects. In contrast, the induction of differentiation by DDATHF was completely abolished by the presence of guanosine or deoxyguanosine, suggesting that the depletion of intracellular guanine nucleotides by DDATHF represents the event that is essential to the induction of differentiation by this folate analog. This possibility was supported by the observation that the concentration of dGTP was not decreased in cells treated with DDATHF under the conditions employed. Both guanine nucleosides selectively restored intracellular GTP pools depleted by the treatment with DDATHF to their normal level, whereas only adenine nucleosides completely restored the levels of both ATP and GTP to their normal intracellular concentrations. The relationship between guanine nucleotide pools and the induction of HL-60 differentiation by DDATHF was further supported by the finding that maturation and the depletion of intracellular GTP by DDATHF were not reversed by guanine nucleosides in HL-60 cells deficient in hypoxanthine-guanine phosphoribosyltransferase activity. The findings provide support for the hypothesis that the terminal differentiation of these leukemic cells by DDATHF is the result of the depletion of intracellular GTP pools.

Changes in microtubules, microtubule-associated proteins, and intermediate filaments during the differentiation of HL-60 leukemia cells.

The cytoskeleton is composed mainly of microtubules (MT), microfilaments, and intermediate filaments (IF) that form a structural network which connects cellular membranes, cytoplasmic organelles, and the nucleus. Since the cytoskeleton may be involved in modulating signal transduction and in the morphological and structural changes that occur during cellular proliferation and differentiation, cytoskeletal changes were measured by immunofluorescence microscopy and fluorescence-activated cell sorter analysis during the differentiation of HL-60 leukemia cells induced by retinoic acid (RA). Differentiated HL-60 cells exhibited increased staining intensity and altered organization of MT and IF, as visualized by immunofluorescence microscopy with anti-tubulin monoclonal antibody and anti-vimentin antibody, respectively. A new procedure was developed and used to measure the content of the cytoskeletal components of HL-60 cells during the process of maturation. HL-60 cells were fixed with formaldehyde in an MT-stabilizing buffer, permeabilized using L-lysophosphatidylcholine, stained for immunofluorescent measurement with antibodies specific for particular cytoskeletal components, and analyzed by flow cytometry. Terminally differentiated cells produced by exposure to RA contained larger amounts of MT and the IF vimentin. During the course of the maturation process, a transient increase in the amounts of the microtubule-associated proteins, (MAPs) MAP2 and tau, occurred. An RA-supersensitive clone, designated HL-60/S4, and an RA-resistant clone, designated HL-60/R3, were developed by mutagenization and selection. Use of these clones supported the concept that the observed changes in MT, MAPs, and vimentin were associated with the differentiation process rather than being due to other effects produced by the retinoid. Thus, the findings suggest that changes in MT, MAPs, and IF are important to the terminal maturation of leukemia cells.

Inhibition by pertussis toxin of the activation of Na(+)-dependent uridine transport in dimethyl-sulphoxide-induced HL-60 leukaemia cells.

The effects of pertussis toxin on the Na(+)-dependent transport of uridine were studied in HL-60 leukaemia cells induced to differentiate along the granulocytic or monocytic pathways by dimethyl sulphoxide (DMSO) or phorbol 12-myristate 13-acetate (PMA) respectively. Pertussis toxin at 50 ng/ml completely inhibited the activation of Na(+)-dependent uridine transport and consequently prevented the formation of intracellular pools of free uridine which occurs in HL-60 cells induced to differentiate by DMSO. The inhibition of Na(+)-dependent uridine transport by pertussis toxin in cells exposed to DMSO was associated with a 14-fold decrease in affinity, with no change in Vmax. Pertussis toxin, however, had no effect on Na(+)-dependent uridine transport in PMA-induced HL-60 cells. Furthermore, 500 ng of cholera toxin/ml had no effect on the Na(+)-dependent uptake of uridine in DMSO-treated HL-60 cells. These results suggest that the activation of the Na(+)-dependent transport of uridine in HL-60 cells induced to differentiate along the granulocytic pathway by DMSO is coupled to a pertussis-toxin-sensitive guanine-nucleotide binding protein (G-protein).

Induction of the differentiation of HL-60 cells by phorbol 12-myristate 13-acetate activates a Na(+)-dependent uridine-transport system. Involvement of protein kinase C.

The Na(+)-dependent transport and facilitated diffusion of uridine were measured after differentiation of HL-60 leukaemia cells along the monocytic pathway by phorbol 12-myristate 13-acetate (PMA). PMA (200 ng/ml) caused a marked increase in Na(+)-dependent uridine transport within 48 h of exposure that was attributable to an increase in transport affinity (apparent Km values of 1.15 +/- 0.22 and 44 +/- 4.4 microM for PMA-induced and uninduced cells respectively), with no change in Vmax. (0.15 +/- 0.02 and 0.13 +/- 0.01 pmol/s per microliter of cell water for PMA-induced and uninduced cells respectively). A corresponding rapid decrease in both the rate of facilitated diffusion and the formation of uracil nucleotides occurred in PMA-induced cells. As a consequence of these changes, intracellular pools of uridine 3-4-fold greater than those in the medium were generated. A similar increase in Na(+)-dependent transport of adenosine, inosine, guanosine, thymidine and cytidine (Km values of 1-4 microM) was observed. The effects of PMA on the activation of the Na(+)-dependent uridine transporter were inhibited by staurosporine, suggesting the involvement of protein kinase C. The findings indicate that a change in the balance of the cellular mechanisms employed for nucleoside transport occurs during the monocytic differentiation of HL-60 leukaemia cells.

Expression of memory to the terminal differentiation inducing activity of tiazofurin in HL-60 leukemia cells.

Tiazofurin, a potent inhibitor of inosine 5'-phosphate dehydrogenase, depletes guanine nucleotide pools and induces granulocytic maturation of HL-60 leukemia cells. These effects are reversed when cells exposed to this agent for 24 h are washed and placed in tiazofurin-free medium. HL-60 cells treated with tiazofurin for a 24 h period, retain a precommitment memory that lessens the time interval necessary for cells to express the mature phenotype upon re-exposure. That protein synthesis was required for maintaining the expression of memory was demonstrated by the finding that memory was blocked when primed cells were exposed to cycloheximide during the intervening inducer-free interval, but not during the priming or subsequent drug exposure periods. The findings have significance with respect to the sequence of events required for commitment to a differentiation pathway.

Effects of uridine on the growth and differentiation of HL-60 leukemia cells.

HL-60 leukemia cells, induced to differentiate, activate a Na(+)-dependent nucleoside transport system, concomitant with a reduction in the nitrobenzylthioinosine (NBMPR)-sensitive facilitated transport of nucleosides. The consequence of these changes lead to the formation of intracellular pools of uridine. To examine the possible role of accumulated uridine in the commitment of HL-60 leukemia cells to undergo maturation, the effects of uridine on the growth and differentiation of HL-60 cells were monitored. Uridine at millimolar levels caused a concentration-dependent inhibition of cellular growth, resulting in the accumulation of cells in the G2/M phases of the cell cycle, phenomena that preceded the formation of differentiated cells. These effects of uridine were reduced by 10 microM NBMPR, an inhibitor of the facilitated transport of nucleosides. The effects of 24 mM uridine on growth and differentiation of HL-60 cells were also prevented by 5 mM inosine, and partially prevented by either 2 mM hypoxanthine or 20 microM adenosine. Pretreatment of HL-60 cells with 24 mM uridine for 6 days, followed by a 2 h exposure to TPA, resulted in the rapid attachment of cells to the tissue culture dish, and the extension of long processes. Although the concentrations of uridine required for the above effects are greater than those achieved during differentiation, these observations suggest that uridine may play a role in regulating the maturation process.

Induction of HL-60 leukemia cell differentiation by tetrahydrofolate inhibitors of de novo purine nucleotide biosynthesis.

5,10-Dideazatetrahydrofolic acid (DDATHF) is a folate antimetabolite that shows activity against glycinamide ribonucleotide (GAR) transformylase, a folate-requiring enzyme in the de novo purine nucleotide biosynthetic pathway. Previous studies from our laboratory have shown that DDATHF is an effective inducer of the maturation of HL-60 promyelocytic leukemia. In solution, DDATHF is a mixture of two diastereomers due to an asymmetric configuration at carbon 6. Incubation of HL-60 cells with 1 microM of each diastereomer resulted in an inhibition of cellular proliferation after 48 h that preceded an increase in the number of differentiated myeloid cells, as determined by the ability of cells to reduce nitroblue tetrazolium (NBT) and by the binding of the myeloid-specific antibody Mo 1. Several analogs of DDATHF were also tested as inducers of the differentiation of HL-60 cells. With the exception of the 10-acetyl analog of 5-deazatetrahydrofolic acid, all compounds displayed similar activities as inducers of maturation. The finding that both stereoisomers of DDATHF, as well as the analogs tested, could selectively reduce intracellular purine nucleotide levels suggested that these compounds inhibited purine nucleotide biosynthesis de novo. This possibility was confirmed by the finding that hypoxanthine completely prevented the reduction of intracellular purine nucleotide levels, as well as the induction of differentiation and the inhibition of cellular growth, by these folate analogs. The results suggest that GAR transformylase is a target for a series of compounds whose structures resemble that of tetrahydrofolate and indicate that the inhibition of GAR transformylase by these compounds is sufficient to induce the maturation of HL-60 leukemia cells.

Intracellular metabolism of 5,10-dideazatetrahydrofolic acid in human leukemia cell lines.

5,10-Dideazatetrahydrofolic acid (DDATHF) is a new potent antitumor agent that specifically inhibits purine biosynthesis, primarily through inhibition of glycinamide ribonucleotide transformylase, the first of the tetrahydrofolate-requiring enzymes in the de novo synthesis pathway. DDATHF has been shown to be an excellent substrate for mouse liver folylpolyglutamate synthetase in vitro, suggesting that intracellular conversion to polyglutamates could play an important role in the action of this antifolate. In this report, metabolic studies of the 6R-diastereomer of DDATHF in the cultured human leukemia cell lines CCRF-CEM and HL-60 are presented. At both 1 and 10 microM (6R)-DDATHF was rapidly converted to polyglutamates in both cell lines. DDATHF(Glu)5 and DDATHF(Glu)6 were the main intracellular metabolites. After incubation in drug-free medium, (6R)-DDATHF polyglutamates were better retained intracellularly with increasing glutamate chain length. (6R)-DDATHF showed reduced cytotoxicity toward a folylpolyglutamate synthetase-deficient cell line, CCRF-CEM30/6 related to a dramatically diminished accumulation of polyglutamates. The activity of (6R)-DDATHF in CCRF-CEM30/6 cells was decreased after both short and prolonged exposures. These results suggest that polyglutamylation of (6R)-DDATHF not only represents a mechanism for trapping the drug inside the cells but also produces a more potent inhibitor of the target enzyme.