Tumor promoters: effects on proliferation and differentiation of cells in culture

Potential Role of Intercellular Communication in the Rate-Limiting Step in Carcinogenesis

In order to ascertain whether there might be a scientific basis for determining practical “thresholds” for “carcinogens,” the concepts of thresholds and carcinogens were examined in the context of some current ideas on cardnogenesis. The observation that cardnogenesis seems to involve the donal expansion of a pre-malignant cell through a series of pheno-typic changes was explained by the initiation/promotion model of cardnogenesis. Unrepaired DNA lesions, acting as substrates for mutations in dividing cells, were speculated to play a role in the initiation phase of cardnogenesis (and indirectly to the promotion phase if the lesions lead to significant cell killing, forcing “compensatory hyperplasia”). Inhibition of intercellular communication, either by cell removal, cell death, growth factors or chemical promoters, was speculated to allow the donal expansion of initiated cells to reach a “critical mass.” During that donal expansion of initiated cells, additional phenotypic changes were speculated to occur during cell replication by mutational and/or epigenetic events. Therefore, it was concluded, on the basis of this model, that conditions which prevented the inhibition of intercellular communication between normal cells and the initiated cell(s) contributed to the rate limiting step of cardnogenesis.

Assuming the initiation and promotion model of cardnogenesis, the classical concepts of “thresholds” and “carcinogens” were viewed as grossly inadequate because they did not symbolically represent the known determinants of the complex carcinogenic process. Unless genetic, developmental stage, tissue, nutritional, stress, life style, as well as concurrent antagonists and/or synergists, factors are known, extrapolation about the potential carcinogenicity of a given chemical from molecular, in vitro or even in vivo experiments or epidemiological data would be extremely risky. It was concluded that, at this stage of our understanding of the mech-anism(s) of carcinogenesis, attempts to determine “thresholds” for “carcinogens” naively assume “carcinogens” are the single determinants for carcinogenesis, and that all chemicals which might influence the appearance of tumors act the same way.

Building bridges toward invasion: tumor promoter treatment induces a novel protein kinase C-dependent phenotype in MCF10A mammary cell acini

The potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) alters many cellular processes through activation of its receptor protein kinase C (PKC), including gene expression, cell cycle, and the regulation of cell morphology, raising an important question for developing targeted methods to prevent cancer: which effects of TPA are crucial for carcinogenesis? To address this question, we studied TPA action in the 3-dimensional (3D) MCF10A human breast epithelial cell system, which models important features of in vivo epithelial tissue including growth constraints, structural organization of cells, and establishment of a basement membrane. MCF10A cells, which are immortalized but nontumorigenic, form hollow, spheroid structures in 3D culture referred to as acini. The development of normal acini requires the tight spatiotemporal regulation of cellular proliferation, polarization, apoptosis, and growth arrest. Treatment of MCF10A acini with TPA caused the appearance of multi-acinar structures. Surprisingly, this phenotype did not involve an increase in cell number or major changes in cell death, and polarization. Instead, live cell and confocal microscopy revealed that TPA stimulates MCF10A acini to aggregate. TPA induces the PKC-dependent production of actin-based protrusions, which leads to the formation of cellular bridges between acini, the clustering of acini, and allows cells to move into adjacent acini. During this process, the integrity of the laminin V basement membrane is disrupted, while E-cadherin-based cell-cell contacts remain intact. Altogether, our results show that under the biochemical and structural constraints of epithelial tissue, as modeled by the 3D MCF10A system, TPA induces a novel PKC-dependent phenotype that resembles local invasion. Of the many effects caused by TPA, these studies highlight the aggressive production of actin-based cellular protrusions as a potentially important event along the pathway to carcinogenesis.

Detection of initiating as well as promoting activity of chemicals by a novel cell transformation assay using v-Ha-ras-transfected BALB/c 3T3 cells (Bhas 42 cells)

Cell transformation assay using BALB/c 3T3 cells, C3H10T1/2 cells and others, can simulate the two-stage carcinogenesis utilized for formation of transformed foci. A sensitive cell transformation assay for tumor initiators as well as promoters has been developed using a v-Ha-ras-transfected BALB/c 3T3 cell line, Bhas 42; these cells are regarded as initiated in the two-stage paradigm of carcinogenesis. To distinguish between initiation and promotion, the initiation assay involves a 2-day treatment of low-density cells, obtained one day after plating, with a test chemical, and the promotion assay involves treatment of near-confluent cells with a test chemical for a period of 12 days (Day 3-14). When Bhas 42 cells were treated with tumor initiators, N-methyl-N'-nitro-N-nitrosoguanidine and 3-methylcholanthrene, transformed foci were induced in the initiation assay but not in the promotion assay. In contrast, tumor promoters, 12-O-tetradecanoylphorbol-13-acetate, lithocholic acid and okadaic acid, gave negative responses in the initiation assay but positive responses in the promotion assay. The results were reproducible with various treatment protocols. Sixteen polycyclic aromatic hydrocarbons were examined using both assays. Benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene induced focus formation only in the initiation assay. Increase of focus formation was observed in the promotion assay with benzo[e]pyrene, benzo[ghi]perylene, 1-nitropyrene and pyrene. Benz[a]anthracene, benz[b]anthracene, chrysene and perylene showed positive responses in both initiation and promotion assays. Results of initiation and promotion assays of acenaphthylene, anthracene, coronene, 9,10-diphenylanthracene, naphthalene and phenanthrene were negative or equivocal. The present Bhas assays for the detection of either/both initiating and promoting activities of chemicals are sensitive and of high performance compared with other cell transformation assays.

Comparison of the fatty acid compositions in intraepithelial and infiltrating lesions of the cervix: part II, free fatty acid profiles

In the second part of this study, the emphasis is on the free fatty acids during cervical carginogenesis, since they may reflect active cell metabolism during this disease process. Lipids were extracted from biopsies of normal epithelial tissue (N) (n=36), cervical intraepithelial lesions (CIL) (n=47), and infiltrating lesions (Ca) (n=47) of the cervix. Samples, from which the free fatty acid compositions were determined, were saponified, methylated and analysed by GLC. In accordance with results obtained on total fatty acid compositions, essential fatty acid deficiency (EFAD) in the intraepithelial lesions, compared with normal tissue (linoleic acid, P< 0.01), and infiltrating lesions compared with intraepithelial lesions (linoleic acid and arachidonic acid, P< 0.01) were observed. High levels of oleic acid were also observed when infiltrating lesions were compared with normal tissue (P < 0.01). As previously mentioned by us in part I of this study, with regard to possible disturbances in metabolic pathways based on the total fatty acid profiles during stages of cervical cancer, EFAD is prevalent during cervical carcinogenesis. This EFAD in cancer cells may result in many defective cell mechanisms, since fatty acids are associated with biochemical events such as lipid peroxidation, signal transduction and immune responses. The high level of oleic acid in cancer cells is known to activate PKC and thus contribute to the continous growth stimulus thought to exist in malignant cells. From a therapeutic viewpoint, substantial changes in the fatty acid composition of the membranes can be produced in cancer cells by selective fatty acid supplementation strategies. At present, modifications of the fatty acid compositions of cell membranes represent an experimental model that has promoted increased understanding of lipid transportation, membrane remodelling, and the relationship between membrane lipids and membrane function. By addressing factors responsible for insufficient essential fatty acid levels, carginogenesis may be prevented or treated. The clinical feasibility of using modification of fatty acids in tumours or cancer by diet or perfusion as an adjunct to standard therapies should be tested.

Decreased phosphorylation of a low molecular weight protein by cGMP-dependent protein kinase in variant HL-60 cells resistant to nitric oxide- and cGMP-induced differentiation

We previously described the isolation of a variant subline of HL-60 cells that does not differentiate in response to nitric oxide (NO)-generating agents or to cGMP analogs. The variant cells have normal guanylate cyclase activity and normal NO-induced increases in the intracellular cGMP concentration. We now show that the variant cells have normal cGMP-dependent protein kinase (G-kinase) activity, both by an in vitro and in vivo assay, and using two-dimensional gel electrophoresis we have identified six G-kinase substrates in the parental cells. Of these six proteins, we found considerably less phosphorylation of one of the proteins in the variant cells than in parental cells, both in vitro and in intact cells, and by 35S-methionine/35S-cysteine incorporation we found much less of this protein in the variant cells than in parental cells. The protein is a shared substrate of cAMP-dependent protein kinase (A-kinase); since cAMP analogs still induce differentiation of the variant cells, it appears that the NO/cGMP/G-kinase and cAMP/A-kinase signal transduction pathways share some but not all of the same target proteins in inducing differentiation of HL-60 cells.

Induction of differentiation and down-regulation of c-myb gene expression in ML-1 human myeloblastic leukemia cells by the clinically effective anti-leukemia agent meisoindigo

Meisoindigo, a second generation derivative of indirubin, is an effective chemotherapeutic agent with very low toxicity used in the treatment of chronic myeloid leukemia. To determine the nature of this activity, the effect of a nontoxic concentration (0.72 micrograms/mL) of this compound on ML-1 human myeloblastic leukemic cells was examined. At such a concentration, differentiation induction was found to be the most pronounced drug effect. During the 3-day drug incubation period, the viable cell number remained essentially constant, with approximately 48% of the cells demonstrating a mature phenotype with increased acid phosphatase activity and nitroblue tetrazolium dye reduction. As observed with other DNA-specific agents, induction of ML-1 differentiation by meisoindigo was accompanied by the down-regulation of c-myb gene expression. These data suggest that induction of leukemic cell differentiation associated with decreased c-myb expression may be one of the mechanisms of the antitumor action of meisoindigo.

Hypoxia-mediated impaired differentiation by LLC-PK1 cells: evidence based on the protein kinase C profile

We recently reported that mild hypoxia in LLC-PK1 cells, grown in standard fashion under a still layer of overlying medium at 5% CO2/18% O2 environment, result in decreased oxidative metabolism and impaired differentiated functions in comparison to adequately oxygenated cultures maintained either under a higher oxygen (36% O2) environment or conditions of continuous rocking of the media fluid. In the present study, subcellular distribution of a regulatory enzyme protein kinase C (PKC) was examined between hypoxic still and normoxic rocked LLC-PK1 cells. Subconfluent cultures of hypoxic LLC-PK1 cells exhibited significantly lower and predominantly membrane-bound PKC activity in comparison to mostly cytosolic localization of this enzyme in normoxic rocked cells. One hour of exposure of adequately oxygenated-rocked LLC-PK1 cells with the phorbol ester TPA, a dedifferentiating agent that did not effect the cell ATP content, resulted in significant inhibition of dome formation and sodium-dependent glucose transport activity, a partial loss of pH-responsive ammoniagenesis, and almost complete translocation of protein kinase C activity from cytosol to the membrane pool; all of which resembles the behavior of hypoxic still cultured cells. In addition, acute re-oxygenation of hypoxic still cultures by rocking the media fluid for one hour resulted in an increase in cell ATP content to the cellular levels of ATP observed in normoxic rocked cells. However, all the parameters of differentiation were unaffected by re-oxygenation. These studies support the notion that hypoxia can act in some primary fashion, independent of its effects on energy metabolism, to impair cellular differentiation in LLC-PK1 cells. They also raise the possibility that activation of protein kinase C may act as an important mediator in this process.

The treatment of intracranial malignant gliomas using orally administered tamoxifen therapy: preliminary results in a series of "failed" patients

In vitro studies have shown that the nonsteroidal antiestrogen tamoxifen can suppress deoxyribonucleic acid synthesis and cell proliferation in cultured human gliomas. This growth suppression is independent of its antiestrogenic properties. Tamoxifen may act through the inhibition of the enzyme protein kinase C, which transduces mitogenic signals from the cell surface to the nucleus. Based on these preclinical studies, we initiated a clinical trial of orally administered tamoxifen, 20 mg twice daily, to patients with recurrent, progressive malignant gliomas who were not candidates for other "failed" protocols, such as brachytherapy. No limits were placed on age, Karnofsky Performance Score (KPS), or expected survival. Thirty-two patients were entered in the study, 29 with a glioblastoma multiforme and 3 with an anaplastic astrocytoma. The mean age of the group was 48 years, and the mean KPS was 65. Median survival of the entire cohort from the onset of tamoxifen therapy was 17 weeks; the median survival of those patients with an initial KPS of 70 or more was 21 weeks. Seven patients survived for more than 6 months with no change in their baseline computed tomographic scans or KPS on tamoxifen, including 2 patients with computed tomographic evidence of regression during the course of therapy. There were no significant patient-reported side effects of the treatment. Three patients had thromboembolic complications during tamoxifen administration. We conclude that tamoxifen can be administered safely to these patients and may show some efficacy against glial neoplasms.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Potassium transfer assay for cell communication: effects of phorbol esters, retinoic acid, and furosemide

Using a mass culture assay for the contact-dependent transfer of potassium among cells with intrinsic differences in ability to concentrate it, we have investigated the ability of several drugs to influence this form of cell communication. We concentrated on 12-O-tetradecanoylphorbol-13-acetate (TPA), which is known to interfere with gap junction-mediated communication and ion transport in several other systems, and compared its effects with those of its inactive derivative, 4-O-methyl-TPA. We found that the communication between mouse BALB/c 3T3 cells and human diploid fibroblasts was reduced in the presence of TPA but not O-methyl-TPA and that this inhibition was not obscured by small but measurable influences of TPA on steady-state content and transport of 86Rb+. We confirmed these findings using an autoradiographic assay for transfer of uridine derivatives among cells in contact. We also showed that retinoic acid had no effect on communication in the ion transfer assay but that furosemide, an inhibitor of Na(+)-K(+)-2Cl- cotransport, stimulated ion transfer dramatically both in the presence and absence of TPA. These results indicate both the promise and the limitations of the potassium transfer assay for identifying potential modulators of gap junction-mediated cell communication.

Response of malignant glioma cell lines to activation and inhibition of protein kinase C-mediated pathways

To evaluate the role of protein kinase C-mediated pathways in the proliferation of malignant gliomas, this study examined the effect of a protein kinase C (PKC)-activating phorbol ester (12-O-tetradecanoyl-13-phorbol acetate or TPA) and a protein kinase C inhibitor (polymyxin B) on deoxyribonucleic acid (DNA) synthesis of malignant glioma cells in vitro. A serum-free chemically defined medium, MCDB 105, was employed for all studies. Two established human malignant glioma cell lines (T98G and U138), two rat glioma lines (9L and C6), and two low-passage human glioma lines (obtained from surgical specimens) were studied. With the exception of the C6 line, all tumors responded in a dose-dependent fashion to nanomolar concentrations of TPA with a median effective dose that varied from 0.5 ng/ml for the U138 glioma to 1 ng/ml for the T98G glioma. At optimal concentrations (5 to 10 ng/ml), TPA produced a two- to five-fold increase in the rate of DNA synthesis (p less than 0.05) as assessed by incorporation of 3H-thymidine. However, TPA had no additive effect on the mitogenic response produced by epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). Inhibition of PKC using the antibiotic polymyxin B (20 micrograms/ml) abolished the TPA-induced mitogenic response in the five responsive lines tested. In two tumors (U138 and 9L), polymyxin B also eliminated EGF-, PDGF-, and serum-induced DNA synthesis as well as abolishing baseline DNA synthesis. These cells remained viable, however, as assessed by trypan blue exclusion; after removal of polymyxin B from the medium, they were able to resume DNA synthesis in response to TPA and serum. In the three other tumors (T98G and the two low-passage human glioma lines), growth factor-induced and serum-induced DNA synthesis were inhibited by approximately 25% to 85%. It is concluded that PKC-mediated pathways affect DNA synthesis in the human malignant glial tumors studied. The response of the glioma cells to TPA is similar to the responses seen in fetal astrocytes, but differs significantly from those reported for normal adult glial cultures. Because the response of the 9L glioma to TPA is similar to the responses seen in the human tumors, the 9L rat glioma model may prove useful for examining the role of PKC-mediated pathways in controlling glioma growth in vivo.

The phorbol ester TPA strongly inhibits HIV-1-induced syncytia formation but enhances virus production: possible involvement of protein kinase C pathway

Cocultivation of MOLT-4 and MOLT-4/HIVHTLV-IIIB cells with more than 0.01 ng/ml of 12-O-tetradecanoylphorbol-13-acetate (TPA) for 20 hr strikingly inhibited HIV-induced syncytia formation resulting from cell to cell infection. Interestingly, the production of HIV-specific p24 antigen in the culture fluid was significantly enhanced by TPA. TPA down-modulated the expression of CD4. CD4 is essential for syncytia formation through interaction with viral envelope protein gp120 on the surface of MOLT-4 cells. The effects of TPA on syncytia formation and on CD4 expression were specifically interfered with by nontoxic doses of blockers of protein kinase C (PKC) such as staurosporine and H7. These data suggest that (1) TPA inhibits HIV-induced syncytia formation through down-modulation of CD4 molecules on the surface of MOLT-4 cells and (2) PKC may play an important role in cell to cell as well as in cell-free infection of HIV.

Expression of cell surface sialic acid and galactose by normal adult human melanocytes in culture

Normal adult human melanocytes grown either in the presence of phorbol ester or dialyzed hypothalamic extract were analyzed for their cell surface sialic acid and galactose content. In both cases, cells expressed large amounts of sialic acid, whereas they differed in their terminal nonreducing beta-D-galactosyl residues linked to N-acetyl galactosamine; such residues were accessible to peanut agglutinin and Bauhinia purpurea lectin on cells grown in phorbol ester and inaccessible on cells grown with dialyzed hypothalamic extract. In addition, striking differences in morphology and growth characteristics were observed between adult melanocytes grown with phorbol ester or with dialyzed hypothalamic extract. Thus, pure cultures of normal adult human melanocytes grown in the presence of dialyzed hypothalamic extract displayed cell surface properties different from those of melanocytes grown with phorbol ester. Cultures of melanocytes with dialyzed hypothalamic extract are likely to reflect known cell surface characteristics of human melanocytes in the skin. Such cultures could represent a useful model to study normal behavior and tumor progression of pigmented cells.

Effect of the ornithine decarboxylase inhibitor alpha, alpha-difluoromethylornithine on phorbol diester-induced inhibition of murine B lymphocyte differentiation

The tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits the differentiation of murine B lymphocytes to antibody-producing plasma cells, in unfractionated spleen cell cultures or enriched B lymphocyte cultures. To determine the role of polyamines in TPA-induced inhibition, unfractionated splenic lymphocytes, in culture with antigen, were incubated with alpha, alpha-difluoromethylornithine (DFMO, 0.10 mM), an irreversible inhibitor of ornithine decarboxylase (ODC). DFMO prevented the TPA-induced inhibition of antibody forming cell number in a 5-day in vitro immunization procedure as measured by a hemolytic plaque assay. In enriched B lymphocyte cultures, however, DFMO had no comparable effect. DFMO did not prevent TPA-induced inhibition of antibody production in unfractionated spleen cell cultures but itself inhibited the amount of antibody produced. Putrescine (0.1 mM), added on day 4 of immunization, reversed DFMO inhibition of antibody production but did not enable DFMO to prevent the TPA-induced inhibition. These findings suggest that TPA-induced inhibition of plasma cell number can be mediated indirectly through effects on T lymphocytes and/or macrophages or directly through effect on B lymphocytes.

A derivative of staurosporine (CGP 41 251) shows selectivity for protein kinase C inhibition and in vitro anti-proliferative as well as in vivo anti-tumor activity

Analogues of staurosporine were synthesized and their ability to inhibit protein kinases was examined. Staurosporine is a potent but non-selective inhibitor of in vitro protein kinase C(PKC) activity (IC50 6.0 nM). The derivative CGP 41 251 had reduced PKC activity with an IC50 of 50 nM but showed a high degree of selectivity when assayed for inhibition of cyclic AMP-dependent protein kinase (IC50 2.4 microM), S6 kinase (IC50 5.0 microM) and tyrosine-kinase-specific activity of epidermal growth factor receptor (IC50 3.0 microM). Staurosporine and CGP 41 251 exerted growth inhibition in the human bladder carcinoma line T-24, human promyelocytic leukemia line HL-60 and bovine corneal endothelial cells at concentrations which correlated well with in vitro PKC inhibition. In addition, both compounds inhibited the release of H2O2 from human monocytes pre-treated with 12-O-tetradecanoyl-phorbol-13-acetate at non-toxic concentrations. In vivo anti-tumor activity was examined in T-24 human bladder carcinoma xenografts in athymic nude mice. Tumor growth inhibition tests revealed significant anti-tumor activity (2p less than 0.001) at 1/10 of the maximum tolerated doses for both compounds. By contrast, a closely related derivative of staurosporine (CGP 42 700) was inactive at concentrations of over 100 microM in all in vitro enzyme and anti-proliferative assays as well as in animal tumor models. Our data suggest an association between PKC inhibition and anti-proliferative and anti-tumor activity.

The regulatory region of the human plasminogen activator inhibitor type-1 (PAI-1) gene

The human gene for plasminogen activator inhibitor type-1 (PAI-1) has been isolated and its promoter region characterized. PAI-1 regulation by glucocorticoids, transforming growth factor-beta (TGF-beta) and the phorbol ester PMA is shown to be exerted at the promoter level. A fragment spanning 805 nucleotides of the 5' flanking and 72 of the 5' untranslated region contain information enough to promote transcription and to respond to glucocorticoids when fused to a reporter gene and transfected into human fibrosarcoma cells. A moderately repetitive DNA sequence, containing a TATA box, a GRE consensus, a Z-DNA forming sequence and two imperfect direct repeats at the extremities, is present a few nucleotides 5' of the human PAI-1 gene transcription start site, raising the possibility that this gene could have been activated by DNA insertion during evolution.

Altered growth regulation and enhanced tumorigenicity of NIH 3T3 fibroblasts transfected with protein kinase C-I cDNA

Transfection of NIH 3T3 cells with plasmids containing rat brain protein kinase C-I (PKC-I) cDNA controlled by strong viral promoter/enhancer elements led to PKC-I gene expression as assessed by Northern analysis, cellular binding of phorbol ester, immunoblotting of cellular PKC, and membrane-associated PKC activity. While transfection did not induce foci, altered growth regulation was observed in established transfectant lines: transfectants displayed reduced dependence on serum for growth, grew to higher saturation densities, and displayed enhanced tumorigenicity when inoculated into nude mice. Continued high-level expression of PKC-I, however, may not be obligatory for the malignant phenotype in vivo. Tumors that retained transfected sequences had lower PKC-I transcript levels than the parental in vitro lines, suggesting an in vivo modulation. Our data show that PKC-I dysregulation leads to altered cell growth regulation and may be functionally equivalent to the action of tumor promoters.

Buoyant density characterization of neoplastic cell populations in patients with chronic B-lymphocytic leukemia

Leukemic cells from a series of patients with chronic B-lymphocytic leukemia (CLL) were analyzed for their buoyant density on discontinuous Percoll gradients. The density profile varied markedly between different patients and also between samples from different body compartments within the same patient. A good correlation was observed between buoyant density and maturation stage of the leukemic clones as judged by Ig-expression and their reactivity with a panel of monoclonal antibodies. Phorbol-ester-induced changes in the leukemic cells were found to be accompanied by a general decrease in their buoyant density. No correlation between density and clinical parameters such as cell counts, clinical stage and survival could be noted. Buoyant density characterization of leukemic B-cell populations is seen as a useful, rapid and simple marker of compartmentalization within the B-lymphocyte maturation spectrum but its clinical relevance remains to be established.

Effect of phorbol ester on prostaglandin regulation of proliferation in rabbit endometrial cells

We have proposed that two of the endogenously synthesized endometrial prostaglandins, prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E1 (PGE1), play a regulatory role in growth control of the endometrium. PGF2 alpha increases DNA synthesis and PGE1 inhibits that effect. Primary cultures of rabbit endometrial cells were used here to examine the effects of the tumor-promoting, diacylglycerol mimicking, phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), on the prostaglandin control of cell proliferation. TPA treatment of these cultures results in: a decrease in control levels of proliferation and complete inhibition by TPA of PGF2 alpha stimulated DNA synthesis; a reduction in [3H]PGF2 alpha binding with short term treatment but an increase to above control binding level with long term treatment; an inhibition of the normal PGF2 alpha stimulated inositol polyphosphate synthesis; and a small increase in accumulation of PGF2 alpha in the culture media. Furthermore, in this culture system, TPA does not down regulate [3H]PGE1 binding; it does not alter the normal PGE1 stimulation of cAMP synthesis; and it has no effect on the normal endogenous PGE1 synthesis by these cultures. The above results are consistent with our previous observations that PGF2 alpha works through the intracellular messengers inositol polyphosphate/diacylglycerol whereas PGE1 works through cAMP.

Activation of protein kinase C down-regulates IFN-gamma receptors

Treatment of mouse EL-4 cells with intracellular activators of protein kinase C, namely 4-phorbol 12-myristate 13-acetate (PMA) and diacylglycerol, resulted in 90% reduction in cell surface interferon-gamma (IFN-gamma) receptors as judged by iodinated-IFN-gamma binding. This did not seem to be due to a decreased in the receptor affinity, since that of the remaining surface receptors appeared to be significantly increased as shown in Scatchard plot analysis. Kinetics experiments revealed that a PMA treatment as short as 15 min was sufficient to induce a decrease of 30% of IFN-gamma receptors, whereas the highest levels of down-regulation were observed after 60-90 min. Treatment of EL-4 cells with calcium ionophore, A23187, although ineffective by itself, dramatically increased the ability of suboptimal PMA concentrations to mediate IFN-gamma receptor down-regulation. Finally, specificity studies revealed that PMA is particularly effective in decreasing the binding of IFN-gamma to T-lymphocytes. Altogether these results suggest a possible involvement of protein kinase C in the regulation of IFN-gamma receptor expression.

Direct inhibition of murine B lymphocyte differentiation by 12-O-tetradecanoylphorbol-13-acetate

The tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits the humoral immune response of lymphocytes to antigen. To test the hypothesis that this inhibition is due to a direct effect upon B lymphocytes, splenic lymphocytes or murine B lymphocytes, enriched by 'panning' splenic lymphocytes onto anti-IgM-coated petri dishes, were immunized in vitro with the thymus/accessory cell-independent antigen trinitrophenyl lipopolysaccharide (TNP-LPS) with or without TPA. The number of anti-TNP antibody-forming cells present in both lymphocyte populations after 5 days was almost completely inhibited to the same degree by TPA. These data unambiguously show that TPA can directly inhibit the differentiation of B lymphocytes to antibody-forming cells.

Two new cell lines from B-prolymphocytic leukaemia: characterization by morphology, immunological markers, karyotype and Ig gene rearrangement

Leukaemic cells from 2 patients with B-prolymphocytic leukaemia were immortalized in vitro by means of Epstein-Barr virus and phorbol-ester TPA. The resulting cell lines, named JVM-2 and JVM-3, have been growing continuously in liquid culture for more than one year. JVM-2 is characterized cytogenetically by t(11;14)(q13; q32), and JVM-3 by trisomy 12. The immunoglobulin (Ig) heavy- and light-chain genes showed the same pattern of rearrangement in both lines as in the original prolymphocytes from each case. The cells from these lines showed a spectrum of morphological and immunological features corresponding to different stages of B-cell maturation. The expression of Ig, IgM-lambda in JVM-2 and IgMD-K in JVM-3, changed from a predominantly membrane pattern in the original cells to a cytoplasmic one in the cell lines. By comparison with their original progenitors, the cells from both lines showed reduced reactivity with the monoclonal antibody (MAb) FMC7, and increased expression of the antigens recognized by the MAbs OKT10, alpha-Tac, FMC53 and Ki-I. The availability of cell lines from this rare type of lymphoid leukaemia offers a potential tool for the study of molecular events associated with the expression of Ig and other antigens by neoplastic cells.

Potentiation of specific association of insulin with HepG2 cells by phorbol esters

The effects of tumour-promoting phorbol esters on the receptor-mediated endocytosis of insulin were investigated in the human hepatoma cell line HepG2. Treatment of these cells with the biologically active phorbol 12-O-tetradecanoylphorbol 13-acetate (TPA), but not with the non-tumour-promoting analogue 4 alpha-phorbol 12,13-didecanoate, resulted in dramatic morphological changes, which were accompanied by a 1.5-2.5-fold increase in specific 125I-insulin association with the cells at 37 degrees C. This increase in insulin binding was not observed when the binding reaction was performed at 4 degrees C. The potentiation of 125I-insulin association with TPA-treated cells at 37 degrees C could be completely accounted for by an increase in the intracellular pool of internalized insulin; there was no concomitant increase in cell-surface insulin binding. Dissociation studies showed that the enhanced internalization of insulin by cells after treatment with TPA resulted from a decrease in the rate of intracellular processing of the insulin after receptor-mediated endocytosis. The phorbol-ester-induced enhancement of internalized insulin in HepG2 cells was additive with the potentiation of endocytosed insulin induced by both the lysosomotropic reagent chloroquine and the ionophore monensin; this indicates that TPA affects the intracellular processing of the insulin receptor at a point other than those disrupted by either of these two reagents. The potentiation of insulin receptor internalization by tumour-promoting phorbol esters could be completely mimicked by treatment with phospholipase C, but not with phospholipase A, and partially mimicked by treatment with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol. By these criteria, the effects of phorbol esters on the insulin receptor in HepG2 cells appear to be mediated through protein kinase C. These results support the concept that the activation of protein kinase C by treatment with phorbol esters causes a perturbation of the insulin-receptor-mediated endocytotic pathway in HepG2 cells, reflected in a long-term decreased rate of dissociation of internalized insulin by the phorbol-ester-treated cells.

Phorbol diesters inhibit enzymatic hydrolysis of diacylglycerols in vitro

The effect of phorbol 12-myristate 13-acetate (PMA) on diacylglycerol lipase activity was examined in rat serum, tissue, and cellular preparations by using di[14C]oleoylglycerol, [3H]palmitoylacetylglycerol, and membrane-resident phospholipase C-generated diacylglycerols as substrates. These experiments were conducted to address whether phorbol esters can mimic diacylglycerols in interacting with enzymes other than protein kinase C. Serum hydrolysis of palmitoylacetylglycerol, assayed by the formation of [3H]palmitic acid, was inhibited by PMA, 4-O-methyl-PMA, or phorbol 12,13-dibutyrate (in order of decreasing potency). The hydrolysis of palmitoylacetylglycerol was inhibited more than 40% by the addition of PMA at a 1:1 molar ratio with substrate. The inhibition resembled the competitive type, with a Ki of approximately 2.7 microM. PMA in the 10-60 microM range also inhibited hydrolysis of palmitoylacetylglycerol by lipases from rat brain microsomes and by homogenates of C3H/10T1/2 mouse fibroblasts. PMA was likewise inhibitory when assayed in an intramembrane enzyme-substrate milieu in which diacylglycerols were generated, in situ, by treatment of [3H]palmitate-labeled cell homogenates with phospholipase C. Collectively, these data demonstrate that PMA, which is now thought to act by mimicry of diacylglycerols, can inhibit the action of diacylglycerol lipase. It is possible that such a mechanism is linked to the multiplicity of responses elicited by phorbol diesters and that other agents may function by means of enzyme interactions (post-phospholipase C) to influence the levels of the cellular diacylglycerol mediators.

Intracellular activation of protein kinase C and regulation of the surface transferrin receptor by diacylglycerol is a spontaneously reversible process that is associated with rapid formation of phosphatidic acid

The effect of the synthetic diacylglycerol, sn-1,2-dioctanoylglycerol (diC8), on the expression of the surface transferrin receptor reveals that exogenous diC8 can act as an intracellular activator of protein kinase C and stimulate both down-regulation and increased receptor phosphorylation in a manner similar to that induced by the active tumor promotor, 4 beta-phorbol 12,13-dibutyrate. Unlike the spontaneously irreversible effect noted when 4 beta-phorbol 12, 13-dibutyrate is added, this same effect mediated by diC8 is brief, lasting only minutes, and is spontaneously reversible. The rate of reversibility is dependent on the concentration of diC8 added, and it is associated with rapid formation of a newly detected intracellular phospholipid that corresponds to sn-1,2-dioctanoyl phosphatidic acid. These data, in conjunction with findings that demonstrate that exogenous diacylglycerols (including diC8) when added to cells do not stimulate cellular phospholipase A2 or C, argue that protein kinase C is activated only briefly in this system since exogenous diC8 is subject to rapid intracellular metabolism to phosphatidic acid.

Correlation between hormone dependency and the regulation of epidermal growth factor receptor by tumor promoters in human mammary carcinoma cells

The effects of the tumor promoter phorbol 12-tetradecanoate 13-acetate (TPA) on the epidermal growth factor (EGF) receptor levels were investigated in hormone-dependent (MCF-7, T-47-D, and ZR-75-1) and hormone-independent (MDA-MB-231, HBL-100, and BT-20) human mammary carcinoma cell lines. In the absence of TPA, hormone-independent cell lines contained high concentrations of low-affinity EGF receptors (apparent Kd = 8 X 10(-10) M), whereas hormone-dependent cell lines exhibited low concentrations of high-affinity receptors (apparent Kd = 1 X 10(-10) M). TPA causes a change of the receptor from a high- to the low-affinity state in hormone-dependent cell lines (MCF-7, T-47-D, and ZR-75-1), as well as in the hormone-independent HBL-100, whereas the affinity remained unchanged in MDA-MB-231 and BT-20 cells. In addition, progesterone receptor levels are decreased after TPA treatment in the hormone-dependent cell lines MCF-7, T-47-D, and ZR-75-1, whereas the estrogen receptor levels remained unchanged. Tumor promoters such as TPA or teleocidin inhibited the proliferation of these cell lines at concentrations above 10 microM with the exception of the T-47-D cells. The most sensitive cell line towards growth inhibition by tumor promoter was the hormone-dependent MCF-7 cell line. Evaluation of different TPA analogs indicated a positive correlation between the growth-inhibitory effects and their ability to stimulate the subcellular redistribution of protein kinase C activity in MCF-7 cells. These data suggest a protein kinase C-mediated down-regulation of the progesterone receptor concentration and of the EGF receptor affinity, which is supposed to mediate the mitogenic response. Furthermore, these results support the hypothesis that the tumor-derived growth factors induced by estradiol act via the EGF receptor in hormone-dependent mammary carcinoma cells.

Differential effects of the tumor promoter phorbol-12-myristate-13-acetate on the morphological and biochemical differentiation of N-18 mouse neuroblastoma cells

The phorbol ester tumor promoter phorbol-12-myristate-13-acetate (PMA) was found to have differential inhibitory effects on the expression of morphological and biochemical differentiation of N-18 mouse neuroblastoma cells. PMA completely inhibited neurite extension and associated growth characteristics and partially inhibited the increased expression of R1 cAMP-binding protein; PMA had no effect on the induction of acetylcholinesterase activity in cells prompted to differentiate either by treatment with 1 mM dibutyryl cAMP or by serum deprivation. 4-alpha-Phorbol-12, 13-didecanoate, an inactive analogue of phorbol ester tumor promoter, was without effect. The implications of these findings concerning the mechanism of action of phorbol ester tumor promoters in the control of cell differentiation are discussed.

Role of intracellular calcium mobilization in the regulation of protein kinase C-mediated membrane processes

Phorbol esters are potent tumour-promoting agents that exert pleiotropic effects on cells. Among these are the control of growth, stimulation of release of stored bioactive constituents and regulation of growth-factor surface receptors. Phorbol esters bind to and activate protein kinase C, leading to the phosphorylation of specific protein substrates presumed to be necessary for eliciting the full response. Strong evidence exists that specific binding of tumour promoter occurs at the membrane level in intact cells, resulting in activation of protein kinase C. Recent evidence concerning the release of bioactive constituents from platelets and neutrophils has linked agonist-induced protein kinase C activation and Ca2+ mobilization in a synergistic mechanism. Here we present a novel model of synergism between Ca2+ and phorbol esters that leads to transferrin receptor phosphorylation and down-regulation in HL-60 human leukaemic cells. Raising intracellular Ca2+, although ineffective by itself, increases the potency and rate of action of phorbol ester for activating protein kinase C and mediating transferrin receptor phosphorylation and down-regulation. We propose a molecular model in which increased intracellular Ca2+ recruits protein kinase C to the plasma membrane, thus "priming' the system for activation by phorbol ester.

Mitogens trigger a calcium-independent signal for proliferation in phorbol-ester-treated lymphocytes

The activation of T lymphocytes by mitogens requires at least two signals; the first, delivered to T cells by a mitogen in conjunction with accessory cells (monocytes/macrophages), leads to the generation of the second signal, interleukin-2 (IL-2). The first signal also induces the expression of IL-2 receptors on the surface of a subpopulation of T cells; binding of IL-2 to its receptor then initiates a cascade of events culminating in DNA synthesis by these cells. Certain compounds act synergistically with mitogens in promoting T-cell proliferation by substituting for the activities of interacting cells or their products. For example, the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) has been shown to restore the ability of macrophage-depleted T-cell populations to respond to mitogenic lectins. Transmembrane fluxes of calcium, leading to increased free cytosolic calcium concentrations ([Ca2+]), have been demonstrated following mitogen binding to lymphocytes and have been implicated in the initiation of cell proliferation. We show here that the effect of TPA on lymphocyte proliferation occurs in the absence of extracellular Ca2+ or detectable changes in [Ca2+]i, but only in the presence of mitogens. This suggests that in cells which have been incubated with the phorbol ester, mitogens can induce proliferation by a calcium-independent signal.

Effects of n-butyrate and phorbol ester (TPA) on induction of Epstein-Barr virus antigens and cell differentiation

N-Butyrate, an effective inducer of synthesis of Epstein-Barr virus (EBV) antigens in virus-producer P3HR-1 cells, has recently been shown (2) to induce morphological differentiation towards plasma cell in nonproducer Raji cells. The effects of n-butyrate and 12-O-tetradecanoylphorbol-13-acetate (TPA) on both EBV-antigen induction and cell differentiation in two virus-nonproducer lymphoblastoid cell lines, Raji and NC37, were now studied. The following observations were made (1). On its own either drug induced 1-2 per cent of cells to EBV-early-antigen positivity in both lines; their mixture induced 35 and 15 per cent positive cells in Raji and NC37 respectively (2). In Raji, n-butyrate induced about 80 per cent of cells to differentiate to plasmablast or plasma cell morphology, whereas TPA only induced the early stages of differentiation in 8 per cent of cells; a mixture of both inducers produced a similar effect as TPA alone. The addition of TPA alone or butyrate-TPA mixture led to some cellular alterations resembling virus-specific changes in virus-producer cell lines. In NC37, either drug alone or their mixture drove 13 per cent of cells to differentiate into plasmablasts or earlier stages of differentiation. In the presence of TPA protrusions and "loops" were seen on cell surfaces. Evidently, the stage of differentiation at which B-lymphoblastoid cell lines have been arrested can be changed in vitro. However, cell-line dependent and inducer-dependent differences in the differentiation response were apparent.

Tumor promoting phorbol diesters: substrates for diacylglycerol lipase

Enzyme activity in rat serum was examined utilizing the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and various glycerolipids as substrates. The serum activity was specific for hydrolysis of the long chain tetradecanoate moiety of TPA, hydrolyzed mono- and diacylglycerols, but was not effective against triacylglycerols, cholesterylesters, or phospholipids. Heating the enzyme preparation at 56 degrees C for 1 min was dually effective in reducing the hydrolysis of both TPA and dioleoylglycerol by 83-86% of control levels. The potent diacylglycerol lipase inhibitor, RHC 80267, inhibited the hydrolysis of TPA in the 0.2-1.0 microM range and was also a potent blocker of monoacyl- and diacylglycerol hydrolysis. In substrate competition studies, exogenous unlabeled TPA was added to the [14C]dioleoylglycerol-containing reaction mixture, however, this produced an approximate 3-fold stimulation of [14]dioleoylglycerol hydrolysis. Although we have not established whether the hydrolysis of TPA and diacylglycerol is the work of one enzyme, the effectiveness of the specific lipase inhibitor, RHC 80267, demonstrates that diacylglycerol lipase can utilize TPA as substrate, a finding never before documented. This point is of interest in light of the theory that phorbol esters act by mimicry of the natural lipid mediator, diacylglycerols.

Phorbol esters rapidly stimulate amiloride-sensitive Na+/H+ exchange in a human leukemic cell line

The human, leukemic cell line, HL-60, undergoes differentiation in response to tumor-promoting phorbol esters. Recent studies have implicated stimulation of a Na+/H+ antiporter as an initial event in cellular differentiation and/or proliferation. The effects of phorbol esters on Na+-dependent H+ efflux from HL-60 cells were studied by pH-stat titration. Tumor-promoting phorbol diesters, but not the inactive parent alcohol, stimulated Na+-dependent H+ efflux in a rapid (within 1 min at 37 degrees C) and reversible manner. Stimulation was dependent on the concentration of extracellular sodium; lithium could substitute for sodium, but choline could not. Stimulation was dependent on the activity of extracellular protons and was inhibited completely by amiloride. The concentrations of phorbol diesters at which we observed half-maximal stimulation of Na+-dependent H+ efflux are very similar to the Kd reported in the literature for binding of these phorbol diesters to the phorbol ester receptor and the Km for phorbol diester activation of protein kinase C. Overall characterization of basal and phorbol ester-stimulated H+ efflux indicate that stimulation of a Na+/H+ antiporter constitutes a primary event in phorbol ester interaction with HL-60 cells.

Augmentation of mixed lymphocyte response. Stimulatory activity by phorbol ester

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces a variety of phenotypic changes on normal and malignant cells. In chronic and acute lymphocytic leukemia we have observed a marked augmentation in mixed lymphocyte reaction stimulatory capacity (MLRs) following pretreatment of leukemia blasts with TPA. We have now characterized the effects of TPA on MLRs utilizing the non-T, non-B lymphoblastic leukemia-cell line, HOON. Following pretreatment with TPA (optimal concentration, 1.6 X 10(-10) M), but not with other phorbol esters, there was a marked increase in MLRs, particularly at lower stimulator-cell concentrations. This effect was maximal following a 44-hr preincubation period and the enhancement in MLRs was not accompanied by changes in levels of Ia expression. This cell line provides a model for determining the molecular basis for the TPA-induced augmentation of stimulatory capacity in the mixed lymphocyte reaction.

Continuous suppression of globin gene expression and differentiation of Friend erythroleukemia cells by phorbol 12-myristate 13-acetate (PMA) despite the loss of PMA binding sites by down regulation

The tumor promoter phorbol 12-myristate 13-acetate (PMA) reversibly inhibits hexamethylene bisacetamide-induced terminal differentiation of Friend erythroleukemia cells (FELC). We were successful in continuously inhibiting FELC differentiation by PMA up to 125 weeks (about 240 serial passages of cells in the presence of PMA). During that period, FELC can be induced to differentiate and enter terminal cell division upon removal of PMA. PMA-mediated suppression of FELC differentiation was associated with only a low level of globin mRNA accumulation. However, a rapid accumulation of globin mRNA in the cytoplasm followed by hemoglobin accumulation occurred upon removal of PMA. A specific, saturable, high-affinity receptor for phorbol esters is present in FELC, as was shown by binding studies with [3H]phorbol 12,13-dibutyrate. A significant (80%) loss in the number of phorbol ester receptors of FELC was observed after a continuous inhibition of differentiation by PMA for as much as 125 weeks. Despite such a down regulation of phorbol ester receptors, these cells respond to PMA with a dose-response similar to that of their parent cells, which have the normal number of phorbol ester receptors. Thus, PMA can suppress reversibly the accumulation of globin-specific mRNA and terminal differentiation of FELC during prolonged periods, despite loss of receptor sites, and our results suggest that only few phorbol ester receptors may be necessary for complete inhibition of FELC differentiation by PMA.

A unifying concept for carcinogenic risk assessments

Carcinogens influence both the initiation of abnormal cells and the subsequent promotion of such cells into neoplasia. Certain other insults seem limited to the stimulation of cellular proliferation and of carcinogenic potentiation. Common examples include surgical, mechanical, chemical, and temperature wounding of tissue followed by healing. In addition, certain hyperplastic growth induced by some chemicals may also enhance tumorigenesis. We propose that the quantification of carcinogenic potentiation may derive from a common-index-quantity estimated according to enhanced cell proliferation resulting from cytotoxicity or toxic hyperplasia induced by a specific exposure. At this time, it is not possible to define, in a restrictive sense, the molecular events which are critical to potentiation but the processes of cell proliferation resulting from cytotoxicity/hyperplasia seem to serve as indices which contain the necessary (and perhaps several secondary) biological responses. The unique advantage is that cell-culture, animal, and human-level studies can be used to evaluate certain parameters of the mathematical model for an untested treatment protocol or chemical insult suspected to be a cofactor in tumorigenesis. The main thrust of this paper is to propose that tumorigenesis should be studied in terms of cellular-population kinetics in response to a biological challenge rather than according to chemical or energetic parameters of that challenge. This approach leads to mathematical equations which can serve as a unifying concept for carcinogenic risk assessments. Sample results, to illustrate the utility of this model, are given for polynuclear aromatic hydrocarbons, trace metals, ionizing radiations, CO, NO, SO2, O3, and NO2. Treatment, here, is for acute exposure conditions, but because the model is mechanistic, other exposure protocols can be addressed by simply adjusting some of the mathematical parameters according to factors estimated from a relative potency comparison of in vitro and in vivo studies best suited to the particular application of interest.

12-0-tetradecanoylphorbol-13-acetate and concanavalin A enhance glucose uptake in thymocytes by different mechanisms

The effects of the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and the plant lectin concanavalin A (Con A) on glucose uptake in murine thymocytes were studied. TPA induces a rapid dose-dependent increase in the uptake of 2-deoxyglucose and in the transport of 3-0-methylglucose. Con A also elicits a time- and dose-dependent enhancement of 2-deoxyglucose uptake. The effect of Con A, however, is less pronounced. The effect of combined treatment of thymocytes with Con A and TPA is not additive. Cytochalasin B completely inhibits the basal, as well as TPA- and Con A-enhanced, 2-deoxyglucose uptake. Dexamethasone markedly inhibits basal 2-deoxyglucose uptake, but is less inhibitory to enhanced 2-deoxyglucose uptake induced by TPA and Con A. The effect of TPA on 2-deoxyglucose uptake and 3-0-methylglucose transport is refractory to inhibition by isobutyl methyl xanthine, dibutyryl cyclic AMP, and ethyleneglycol tetraacetic acid. These agents markedly inhibit the enhancement of 2-deoxyglucose (2-DOG) uptake by Con A. p-Bromophenacyl bromide, an inhibitor of phospholipase A2, also selectively inhibits Con A enhancement of 2-DOG uptake. Taken together, the results suggest that Con A and TPA exert their stimulatory effect on glucose uptake by different activating mechanisms, but they may share a final common transport pathway.