12-O-tetradecanoylphorbol-13-acetate and staurosporine induce increased retinoblastoma tumor suppressor gene expression with megakaryocytic differentiation of leukemic cells

The phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), induced increased expression of the retinoblastoma (RB) tumor suppressor gene product in the course of megakaryocytic differentiation of the K562 human leukemia cell line, a differentiatively multipotent hematopoietic precursor cell. The induced increase in RB protein per cell occurred early, by 8 h of treatment, preceding any significant phenotypic differentiation evidenced by cellular expression of the CD41 differentiation-specific megakaryocytic cell surface marker, but not inhibition of cell cycle transit, leading to a cell population arrested with 2 n, 4 n, and 8 n DNA content. The increase in RB protein per cell occurred for cells in all cell cycle phases. Staurosporine (STSP) was found to induce a similar course of cell cycle arrest and differentiation. Furthermore, STSP caused an up-regulation of RB expression similar to that caused by TPA. Almost all of the RB protein is phosphorylated in untreated cells, but TPA and STSP both caused the late appearance of hypophosphorylated RB protein following cell cycle arrest. The STSP-caused hypophosphorylation was much later than the TPA effect. Hypophosphorylation of RB is, thus, not necessarily a prerequisite for cell cycle arrest but may be a consequence of G0. Given that TPA can be an activator and STSP an inhibitor of protein kinase C, it appears that the induced processes of tumor suppressor gene regulation and growth and differentiation control are not necessarily protein kinase C dependent in K562 cells. Furthermore, the findings that these two presumably divergent inducing agents caused a similar increase in RB gene expression suggests that the up-regulation of RB associated with differentiation is not a coincidence of just one specific inducer but may be a common essential feature of the induced differentiation. The amount of RB protein per cell increased within hours of exposure to TPA or STSP and may have a role in the induced metabolic cascade producing the new phenotype.

Dexamethasone enhances agonist induction of tissue factor in monocytes but not in endothelial cells

Stimulation of monocytic cells by inflammatory agents such as bacterial lipopolysaccharide or tumour necrosis factor-alpha leads to the rapid and transient expression of tissue factor, the major cellular initiator of the extrinsic coagulation cascade in both haemostasis and tissue inflammation. In this study we investigated whether the synthetic anti-inflammatory glucocorticoid, dexamethasone, would inhibit agonist induction of tissue factor expression in both monocytes and endothelial cells. Surprisingly, dexamethasone significantly enhanced the induction of tissue factor expression by peripheral blood mononuclear cells and an established monocytic cell line, THP-1, in response to lipopolysaccharide or tumour necrosis factor-alpha. However, unlike monocytic cells, dexamethasone did not enhance agonist induction of tissue factor in endothelial cells. Synergistic enhancement of tissue factor expression by dexamethasone was also reflected in tissue factor mRNA levels in THP-1 cells, but was not the result of improved TF mRNA stability. Synergism between bacterial lipopolysaccharide and glucocorticoid in the induction of monocyte effector function is extremely unusual and may help to explain the variable outcome of glucocorticoid treatment of septic shock.

Activation of the jun-D gene during treatment of human myeloid leukemia cells with 1-beta-D-arabinofuranosylcytosine

The jun-D gene is a member of the c-jun family of early response genes that code for DNA binding proteins. The present studies demonstrate that 1-beta-D-arabinofuranosylcytosine (ara-C) increases jun-D expression in HL-525 myeloid leukemia cells. This induction by ara-C was maximal at 6 hr and transient. In contrast, ara-C had no detectable effect on the gene coding for the cAMP-responsive element binding protein 1. Nuclear run-on assays demonstrated that ara-C treatment is associated with an increased rate of jun-D transcription. The results also show that jun-D transcripts are stabilized at a posttranscriptional level in ara-C-treated cells. Taken together, these results demonstrate that ara-C induces expression of the jun-D gene and that this effect is regulated by transcriptional and posttranscriptional mechanisms.

Inhibition of P210 expression in chronic myeloid leukaemia: oligonucleotides and/or transduced antisense sequences

There is now strong evidence that the BCR-ABL gene product (P210) of the Philadelphia chromosome plays a crucial role in the pathogenesis of chronic myeloid leukaemia (CML). That is why antisense strategies aiming at inhibiting P210 expression for research or therapeutic purposes are increasingly investigated. Two main tools are currently available in this respect: oligonucleotides and retrovirally transduced antisense sequences. In this paper, we discuss the potential advantages and drawbacks of each approaches and report experimental evidences showing the feasibility of the second one in a murine lymphoid cell line (BaF3) expressing P210 upon retroviral transduction of the complete BCR-ABL cDNA. A retroviral vector was used to introduce selected antisense and sense sequences into this cell line, that P210 expression had rendered Interleukin-3 (IL3) independent. The antisense transcripts generated under the control of MoMLV promoter specifically killed BaF3 cells in the absence of IL3 and stably inhibited P210 expression. Retrovirally transduced antisense sequences can thus successfully achieve stable suppression of P210 and may be used to study further the mechanisms by which P210 is transforming cells. The effect on CML cell lines and fresh CML cells, in bone marrow cultures, remains to be investigated before considering this technique for in vitro selective suppression of Philadelphia-positive haematopoiesis.

Differentiation and growth modulation of myeloid leukemia cells by the protein kinase C activating agent bryostatin-1

Bryostatin-1 (Bryo), a macrocyclic lactone of the sea water bryozoan Bugula neritina, is a potent activator of protein kinase C and was found to exhibit antineoplastic activity in several systems. We studied the effect of Bryo on differentiation and growth modulation of human myeloid leukemia cell lines and freshly explanted blood cells from patients with myeloid leukemia. Alterations at the molecular level and phenotypic changes triggered by Bryo were similar, but not identical, to those induced by phorbol esters. Bryo was able to inhibit cellular proliferation as evidenced by [3H]-thymidine uptake and induced morphological changes associated with monocytic differentiation. In studies using continuous cell lines, the glucocorticoid dexamethasone was unable to prevent the Bryo-induced growth inhibition or the induced phenotypic changes. However, in fresh myeloid blood cells dexamethasone attenuated these Bryo-triggered effects. Our own data taken together with reports from the literature reviewed here suggest the following conclusions: (i) Bryo, while lacking tumor promoting activity, is able to induce differentiation in maturation arrested leukemia cells; (ii) it exhibits selective antiproliferative properties in normal or malignant hematopoietic cells and supports growth of multipotent stem cells. These features might qualify Bryostatin-1 as a potential candidate for promising research and possibly for future clinical applications.

The serial study of c-myc expression in bone marrow biopsy specimens during treatment for acute myelogenous leukaemia

The AMEX method of fixation permitted the serial study of c-myc expression in bone-marrow (BM) biopsies obtained from 6 patients with acute myelogenous leukaemia (AML) and one with myelodysplastic syndrome (MDS) during therapy with various cytotoxic and bioactive agents. BM cytotoxic therapy and therapy with bioactive agents was capable of altering c-myc expression in vivo. While cytotoxic therapy was generally associated with a fall in myc expression, it did not produce a dramatic effect on myc expression. Recombinant human granulocyte-macrophage colony-stimulating factor (RhGM-CSF) can increase and retinoic acid/alpha-interferon can decrease c-myc expression in myeloid cells in vivo.

Proliferative advantage rather than classical drug resistance as the cause of treatment failure in chronic myelogenous leukemia

This presentation discusses the role that proliferative advantage plays in making both the chronic and blastic phases of CML resistant to therapy. A case is made for the addition of "regrowth" inhibitors between courses of chemotherapy as a means of increasing the efficacy of therapy by suppressing or reducing the proliferative advantage that the target cells enjoy over those cells which one would like to repopulate the hematopoietic system.

Potential mechanisms of action of interferon-alpha in CML

Treatment with interferon-alpha (IFN-alpha) adequately controls the leukemic cell mass in the majority of newly diagnosed patients with chronic myeloid leukemia (CML). However, the degree of response ranges from no 'hematologic' response to complete suppression of the leukemic clone. The mechanism(s) by which IFN-alpha elicits these responses is unknown, but in vitro studies have indicated that IFN-alpha might function by (1) selective toxicity against the leukemic clone, (2) enhancement of 'immune' regulation, and (3) modulation of bone marrow microenvironmental regulation of hematopoiesis. Using in vitro clonogenic assays we were unable to demonstrate that IFN-alpha selectively inhibited the proliferation of CML progenitor cells. We also found no difference in the expression of LFA-3 on normal or CML CD34+ cells. However, by panning and co-culturing hematopoietic cells on monolayers of bone marrow stromal cells, grown with and without IFN-alpha, we found that IFN-alpha enhanced the adhesion of CML progenitors to stromal cells, whereas adhesion by normal progenitor cells was essentially unaffected. This enhanced adhesion by CML progenitor cells was associated with a reduction in neuraminic acid levels in the extracellular matrix overlying stromal cells. Therefore, it is possible that one of the mechanisms by which IFN-alpha exerts its regulatory effect on the leukemic clone is through enhancement of hematopoietic cell-microenvironmental cell interactions.

An overview of some studies of chronic myelogenous leukemia: biological-clinical observations and viewing the disease as a chaotic system

While much is known about CML at both the clinical and molecular biological levels, the precise relationship between the disease at these two levels is unclear. The appearance of the fusion gene bcr-abl and disorders in the regulation of the myc gene, and perhaps other oncogenes which code for nucleoproteins, appear to play integral roles in the genesis of the chronic and blastic phases of the disease. The resistance of this disease to cytotoxic therapy appears to reflect both "classical" drug resistance and the ability of those cells which survive cytotoxic therapy to rapidly replace the killed cells thereby offsetting the effects of chemotherapy ("regrowth resistance"). The clinical evolution of the disease is compatible with two fundamentally different processes: one compatible with a deterministic chaotic model and the other involves two basically independent linear phenomena which overlap and intersect as the blastic phase appears and replaces the chronic phase.

Modulation of GM-CSF receptor beta-subunit and interleukin-6 receptor mRNA expression in a human megakaryocytic leukemia cell line

Our present study was designed to clarify the mechanism by which the same megakaryocyte progenitor cells respond to various cytokines at different stages of megakaryocyte development. We examined the changes in mRNA expression of granulocyte macrophage colony-stimulating factor receptor beta-subunit (GM-CSFR beta-subunit), which was a common subunit of a high-affinity interleukin-3 receptor (IL-3R) and a high-affinity GM-CSFR, and interleukin-6 receptor (IL-6R) during megakaryocyte development in a human megakaryocytic leukemia cell line (CMK) which could proliferate and/or differentiate in the presence of 12-O-tetradecanoylphorbol 13-acetate (TPA), IL-3, GM-CSF, and IL-6. We found that GM-CSFR beta-subunit mRNA was expressed constitutively in CMK cells and was transiently down-regulated by TPA and IL-6, while the expression of IL-6R mRNA was increased by TPA in association with the differentiation of megakaryocytes. Furthermore, the TPA-induced down-regulation of GM-CSFR beta-subunit mRNA expression and its recovery were blocked by cycloheximide (CHX), a protein synthesis inhibitor, suggesting that these modulations required de novo protein synthesis. These findings imply that multi-lineage cytokines such as GM-CSF and IL-3 may contribute preferentially to the regulation of the earlier development of megakaryocyte progenitor cells with high densities of multi-lineage cytokine receptors, while IL-6 may be limited in its action to supporting the maturation of more differentiated megakaryocyte progenitor cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of ras protooncogenes during in vitro differentiation of human erythroleukemia cells

We have compared the expression of the ras protooncogene family (H-, K-, and N-ras) in leukemia cell differentiation utilizing as a model K562 and HEL erythroleukemia cells treated either with 1-beta-arabinofuranosylcytosine or 12-O-tetradecanoylphorbol-13-acetate (TPA). 1-beta-D-Arabinofuranosylcytosine induced terminal erythroid differentiation of K562 cells, while TPA induced myeloid differentiation of K562 and HEL cells, resulting in myelomonocytic-like cells expressing macrophagic and megakaryocytic markers. H-ras mRNA levels showed a dramatic decrease in K562 cells subjected to erythroid and myelomonocytic differentiation. The same result was found at the protein level for p21H-ras. Expression of K-ras and N-ras in K562 cells also decreased with differentiation, although significant mRNA levels remained despite cessation of cell proliferation. The decrease in K-ras expression was greater for TPA-treated cells than for 1-beta-arabinofuranosylcytosine-treated cells. TPA-induced myelomonocytic differentiation in HEL cells also resulted in a dramatic down-regulation of H-ras mRNA levels. Thus, by using a leukemia cell line able to differentiate along two different lineages, our results reveal a lineage-specific modulation of ras gene family expression.

Inhibition of specific pathways of myeloid cell differentiation by an activated Hox-2.4 homeobox gene

Abnormal expression of homeobox genes is one of the abnormalities associated with the development of murine and human leukemia. Myeloid leukemic cells that can be induced to differentiate to mature cells by interleukin 6 were stably transfected with an activated Hox-2.4 homeobox gene. Expression of the Hox-2.4 gene in the transfected clones inhibited specific pathways of the myeloid differentiation program induced by interleukin 6. The expression of some genes associated with differentiation was almost completely blocked, and the expression of other genes was either partially inhibited or not affected. The results support the hypothesis that abnormal expression of Hox-2.4 may contribute to the development of leukemia by interfering with the differentiation program.

The differentiation pathway of HL60 cells is a model system for studying the specific regulation of some myeloid genes

During granulopoiesis, certain myeloid genes encoding products of azurophilic granules are specifically down-regulated. The myeloid specific enzyme myeloperoxidase belongs to this group of genes. It is responsible for the production of hypochlorous acid, a potent microbicidal agent which is involved in host defense. During induced differentiation of promyelocytic leukemic HL60 cells to granulocyte- or monocyte-like cells, myeloperoxidase RNA is depressed. We studied this depression process in more detail by limiting the exposure to the inducer phorbol 12-myristate 13-acetate to 24 h. During this time period, no significant decrease in cell number and cell viability could be observed. Analysis of these in vitro differentiated HL60 cells on the protein and RNA levels showed that they can be used under defined conditions as a cell system to study the specific depression of myeloid genes. Under the described conditions, both the transcriptional rate of the myeloperoxidase gene as well as the stability of its transcript was reduced.

Protein kinase C beta gene expression is associated with susceptibility of human promyelocytic leukemia cells to phorbol ester-induced differentiation

To study the signal transduction pathway leading to phorbol 12-myristate 13-acetate (PMA)-induced differentiation in human promyelocytic HL-60 leukemia cells, we examined the expression of protein kinase C (PKC) isozyme genes in HL-60 cells that are susceptible or resistant to PMA-induced differentiation. The PKC-alpha, -beta, -gamma, -delta, epsilon, and -zeta transcript levels were assessed by Northern blotting, and the PKC-alpha, -beta, and -gamma protein levels were examined by immunoblotting. The PMA-resistant cell variants HL-525 and HL-534 were found to be deficient in the PKC-beta isozyme RNA and protein as compared with the PMA-susceptible HL-60 and HL-205 cell lines. In addition, a "delta-like" PKC RNA species identified in these cells demonstrated a reduced abundance in the HL-525 and HL-534 cells. Southern blot analysis indicated that the observed reduction in PKC-beta gene expression does not appear to be due to a gross deletion or rearrangement of the gene. The expression of the early response genes junB, c-fos, and c-jun was attenuated in PMA-treated HL-525 and HL-534 cells as compared to the PMA-treated HL-60 and HL-205 cells. These results suggest that the signal transduction pathway that leads to PMA-induced differentiation in the HL-60 cell system requires PKC-beta and/or delta-like PKC for the proper expression of the early response genes, and ultimately the expression of genes that define the mature state.

Abnormal regulation of the myc gene in myeloid leukemia

To study the regulation of expression of the myc protooncogene, cells from normal individuals and patients with acute myelogenous leukemia (AML), and chronic phase and blastic crisis of chronic myeloid leukemia (CML) cells were put in overnight culture in the presence or absence of fetal calf serum. Myc expression in normal marrow cells and chronic phase CML cells fell after culture in vitro. In contrast, myc expression was maintained or increased in a majority of the AML and blastic crisis CML specimens. These data demonstrate that the regulation of myc expression is disordered in many AML and blastic crisis specimens but not in chronic phase CML cells.

Proliferation is required for induction of terminal differentiation of Friend erythroleukemia cells

The relationship between cell proliferation and differentiation has long been a source of controversy. Stimulation of normal erythroid maturation results in a finite number of cell divisions accompanied by a concomitant accumulation of hemoglobin. Friend erythroleukemia cells treated with hexamethylene bisacetamide differentiate in a similar manner, while agents such as hemin apparently induce differentiation without limiting cell proliferation. Aphidicolin, an inhibitor of DNA synthesis, has been reported to induce differentiation in the absence of cell proliferation. Using these three chemicals we have investigated the relationship between cell proliferation and erythrocytic maturation by exposing Friend erythroleukemia cells to either hexamethylene bisacetamide (5 mM), hemin (100 microM), or aphidicolin 1.2 microM) and examining the effects on cell growth, morphology, and hemoglobin production. Proliferation in the presence of hexamethylene bisacetamide is limited to four to five rounds of cell division, while hemin has no inhibitory effect. Hexamethylene bisacetamide initiates the complete erythrocytic maturation program, including cellular structural changes and hemoglobin synthesis. Hemin stimulates only globin gene transcription, not differentiation. Aphidicolin inhibits cell growth within 24 h, but does not induce differentiation. Furthermore, inhibition of proliferation by aphidicolin prevents subsequent hexamethylene bisacetamide induced differentiation. These results indicate that at least one round of cell division is required for initiation of erythrocytic differentiation.

The p21ras protein as an intermediate signaling molecule in the IL-4-induced HLA-DR expression on normal and leukemic human myeloid cells

We have previously demonstrated that the low number of interleukin-4 receptors (IL-4Rc) on HL-60 leukemia cells render this population susceptible to differentiation by IL-4. As it occurs with normal human monocytes, IL-4 induces the expression of HLA-DR surface antigens on HL-60 cells as well. The second messenger pathway(s) involved after the IL-4 stimulation leading to class II up-regulation has not been fully examined. Here we show that IL-4-induced class II antigen expression on the HL-60 cell line or normal human monocytes is calcium/calmodulin-independent since theophylline (TPH, a calmodulin inhibitor) does not block the IL-4 effect. In addition, the pyruvate kinase C (PKC) pathway does not seem to participate in the process either because in our system activation of PKC by 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) is insufficient by itself to induce HLA-DR. We found, however, that a second messenger pathway can be mediated by a G protein system since IL-4 concomitantly induces class II and p21ras expression which can be successfully blocked by a highly specific anti-p21ras monoclonal antibody. In addition, using another p21ras inducer, the 5-azacytidine C (5-AzaC), we showed that this agent can also induce the expression of p21ras and class II, both of which can be inhibited by the same antibody. Thus, it appears that IL-4 selects the G protein system as a signaling pathway in order to exert its action for the induction of HLA-DR on human normal monocytes or M2 leukemia target cells. Since monocytes and macrophages participate in virtually all immune reactions, the regulation of class II induction is of obvious importance.

Expression and phosphorylation of the retinoblastoma protein during induced differentiation of murine erythroleukemia cells

Phosphorylation and dephosphorylation of the retinoblastoma protein, pRB, play a role in the control of cell cycle progression and expression of differentiation in eukaryotic cells. The regulation of pRB level and phosphorylation state was investigated during the induction of differentiation of murine erythroleukemia cells (MELC) by the chemical agent hexamethylene bisacetamide (HMBA). In MELC, there is a critical time in G1 or early S phase when HMBA must be present in order to induce differentiation. This is followed by prolongation of the subsequent G1 phase, resumption of progression through the cell cycle for several generations, and then cell cycle arrest in G1-G0. Associated with HMBA-induced prolongation of G1, there is an increase in the amount of the underphosphorylated form of pRB. A variant cell line (DS19/VCR-C) with accelerated kinetics of HMBA-mediated differentiation shows a more marked increase in underphosphorylated pRB. In culture with HMBA, as MELC resume progression through the cell cycle, pRB is present in the phosphorylated form. The total amount of pRB increases approximately 3-fold over the succeeding cell divisions prior to terminal arrest in G1. This increase in pRB is inhibited by dexamethasone, which also blocks HMBA-induced MELC differentiation. During this period, RB mRNA also increases approximately 3- to 5-fold, which reflects an increase in the rate of transcription, with no change in mRNA stability. The state of phosphorylation and amount of pRB appear to be involved in the control of HMBA-induced terminal cell division of MELC.

Regulation of c-jun expression during induction of monocytic differentiation by okadaic acid

The present work has examined the effects of okadaic acid, an inhibitor of type 1 and 2A protein phosphatases, on the regulation of c-jun expression during monocytic differentiation of U-937 leukemia cells. The results demonstrate that okadaic acid treatment is associated with induction of a differentiated monocyte phenotype characterized by: (a) growth arrest; (b) increases in Mac-1 cell surface antigen expression; (c) down-regulation of c-myc transcripts; and (d) induction of tumor necrosis factor gene expression. This induction of monocytic differentiation was associated with transient increases in c-jun mRNA levels, which were maximal at 6 h. Similar effects were obtained for the c-fos gene. Run-on analysis demonstrated detectable levels of c-jun transcription in U-937 cells and that this rate is increased approximately 40-fold following okadaic acid exposure. c-jun mRNA levels were superinduced in cells treated with both okadaic acid and cycloheximide, whereas inhibition of protein synthesis had little, if any, effect on okadaic acid-induced c-jun transcription. The half-life of c-jun mRNA was similar (45-50 min) in both untreated and okadaic acid-induced cells. In contrast, treatment with both okadaic acid and cycloheximide was associated with stabilization (t 1/2 = 90 min) of c-jun transcripts. Taken together, these findings indicate that the induction of c-jun transcription by okadaic acid is controlled primarily by a transcriptional mechanism. Since previous studies have demonstrated that the c-jun gene is autoinduced by Jun/AP-1, we also studied transcription of c-jun promoter (positions -132/+170)-reporter gene constructs with and without a mutated AP-1 element.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of the fms proto-oncogene product in HL-60 cells by vitamin D: a flow cytometric analysis

Agents which induce monocytic characteristics in HL-60 human acute promyelocytic leukemia cells induce mRNA for the fms proto-oncogene, which encodes the receptor for M-CSF. Previous studies of fms expression in HL-60 cells have characterized chiefly induction by phorbol esters of fms mRNA. Our studies of fms expression in HI-60 cells have characterized induction by vitamin D3 of the fms protein. We have used flow cytometry to correlate fms antigen with a monocyte-specific differentiation antigen recognized by antibody MO2 (CD14), with DNA content, and with the nuclear antigen Ki-67, a marker of cell cycling. HL-60 cells were cultured with or without 1 microM vitamin D for 7 days. fms antigen was found on 42 +/- 5.8% of the cells cultured without vitamin D, but on 63 +/- 4.3% of the cells cultured with vitamin D. MO2 binding was detected on only 2 +/- 0.5% of the cells without vitamin D, but on 59 +/- 9% with vitamin D. Cells cultured with vitamin D that were fms-positive were also predominantly (83%) MO2-positive. Analysis of DNA content, measured by propidium iodide staining, showed that 57 +/- 1.5% of cells cultured without vitamin D, but 93 +/- 0.5% of cells cultured with vitamin D, were in the G0/G1 cell cycle phase. Analysis of nuclear antigen Ki-67 revealed that, of the vitamin D-treated cells that were fms-positive, a significant proportion (37%) were still cycling. We conclude that (1) fms is demonstrable on some uninduced HL-60 cells, (2) when HL-60 cells are induced to develop monocytic characteristics by vitamin D, fms induction is part of the program for monocytic differentiation that includes MO2 expression, yet (3) some induced cells expressing fms are still cycling.

Differentiation and retrodifferentiation of human myeloid leukemia cells is associated with reversible induction of cell cycle-regulatory genes

Treatment of human myeloid leukemia cells (HL-60, U-937, THP-1) with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with differentiation along the monocytic lineage. This induction by TPA is characterized in part by growth arrest and the appearance of differentiated monocytic phenotype. The present studies demonstrate that myeloid leukemia cells exit the cell cycle to G0-G1 between 24 and 36 h following TPA treatment. This G0-G1 arrest was accompanied by down-regulation of the cell cycle-regulatory genes cdc2, cyclin A, cyclin B, and cdc25. Similar findings were obtained for histones H1 and H4. Cell cycle progression of synchronized U-937 cells revealed low to undetectable mRNA levels for these genes in G1 and maximal transcription in G2-M phase. Results obtained from mRNA half-life studies demonstrate that the stability of cdc2, cyclin A, cyclin B, and cdc25 transcripts is similar in control and TPA-treated U-937 cells. Nuclear run-on assays demonstrated down-regulation of histone gene transcription, while there was no signal detectable for the cell cycle-regulatory genes. The present findings also demonstrate that long term culture of TPA-differentiated U-937 cells is associated with a decrease in G0-G1-arrested cells and an increase of cells in S and G2-M after 25 days. This reentry into the cell cycle was accompanied by loss of adherence, down-regulation of markers for the monocytic phenotype, and induction of the cell cycle-regulatory genes. This process of retrodifferentiation was completed after 36 days when patterns of cell cycle-regulatory and histone gene expression were identical to that in untreated U-937 cells.

Changes in IgG Fc receptor expression induced by phorbol 12-myristate 13-acetate treatment of THP-1 monocytic leukemia cells

We studied changes in the three types of Fc gamma receptor (FcR) on the THP-1 human monocytic leukemia cells, after incubation with the phorbol ester, PMA, which has been shown to alter the expression of several genes in these cells. THP-1 cells constitutively express FcRI and FcRII, and PMA down-regulated the expression of both FcRI and FcRII. The FcRIII expression was not detected on either untreated or PMA-treated cells. Addition of PMA to THP-1 cells also resulted in a dose-dependent decrease of CD4 expression, as well as in an increased expression of activation-associated antigens. PMA treatment was followed by a progressive decrease in the steady state level of FcRI mRNA, while FcRII mRNA levels did not change, pointing to different regulatory mechanisms at the pre- and post-transcriptional level respectively. The FcRIII mRNA was undetectable. In order to further delineate the mechanism by which PMA induces alterations in FcR expression, we treated cells with stimulators of protein kinase C, of Ca2+ calmodulin-dependent kinase, and of protein kinase A. Since stimulation of none of these second messenger systems induced similar alterations in FcR expression as PMA we next tested the effects of PMA on differentiation and arrest of proliferation. The changes in FcR only occurred at PMA concentrations capable of inducing cell adherence and an arrest of proliferation, and showed a relatively slow time pattern. This suggested that the alterations in FcR expression may be linked to partial differentiation into a more macrophage-like cell. The changes in FcR expression could furthermore be reproduced by 1,25(OH)2 vitamin D3, another agent capable of differenting monocytes. In conclusion, PMA treatment of THP-1 cells decreases FcRI gene transcription and membrane expression and reduces membrane expression of FcRII. Both changes might be linked with an arrest of cell growth and induction of differentiation.