Sphingolipids inhibit insulin and phorbol ester stimulated uptake of 2-deoxyglucose

Studies are presented demonstrating inhibition of both insulin and phorbol myristate acetate stimulated uptake of 2-deoxyglucose uptake by 3T3-L1 fibroblasts. Greatest inhibition of uptake was seen with sphinganine while sphingosine was also potent in this regard. Ceramide inhibited phorbol myristate acetate but not insulin stimulation of uptake. It is suggested that sphingolipid inhibition of glucose transport relates to the previously demonstrated effect of corticosteroids to increase membrane sphingomyelin and inhibit glucose transport.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced tumor promotion and epidermal ornithine decarboxylase activity in mouse skin by palmitoylcarnitine

Palmitoylcarnitine, which has been reported to be an inhibitor of calcium-activated, phospholipid-dependent protein kinase (protein kinase C), inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced epidermal ornithine decarboxylase in mouse skin in a dose-dependent manner. Neither acetylcarnitine nor palmitic acid inhibited TPA-caused ornithine decarboxylase induction. In addition, palmitoylcarnitine markedly inhibited skin tumor promotion induced by TPA. Palmitoylcarnitine inhibited epidermal protein kinase C activity which was stimulated by Ca2+ in the presence of phosphatidylserine but failed to inhibit the enzyme activity which was stimulated by TPA in the presence of either phosphatidylserine or Ca2+ plus phosphatidylserine. Therefore, it seems unlikely that the potent anti-tumor-promoting action of palmitoylcarnitine, which is shown in the present study, is explained solely by its effect on protein kinase C.

Search for possible antitumor promoters by inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced Epstein-Barr virus activation; ursolic acid and oleanolic acid from an anti-inflammatory Chinese medicinal plant, Glechoma hederaceae L

From an anti-inflammatory Chinese medicinal plant, Glechoma hederaceae L., two triterpene carboxylic acids, ursolic acid (UA) and oleanolic acid (OA) have been isolated as inhibitors of 12-O-tetradecanoylphorbol-13-acetate (TPA) induced Epstein-Barr virus (EBV) activation in Raji cells. Both acids significantly inhibited the activation at a 1000-fold molar ratio to TPA, and also teleocidin B-4. The dose responses of the acids were very similar to those of the antitumor promoters, retinoic acid (RA) and glycyrrhetinic acid (GA). However, a characteristic property that UA and OA possess, far higher cell viability to the Raji cells than RA to the Raji cells, has been pointed out. Furthermore, enhancement of the inhibitory activity was found in 3-keto derivatives of UA and OA, while either loss of oxygen functionality at C-3 position of UA or oxidation at C-3 of GA led to reduction of the activity. Binding assay suggested that the inhibitory activity should be exhibited by some event caused after binding of TPA to the receptor in the cells.

Inhibition of the specific binding of 12-O-tetradecanoylphorbol-13-acetate to mouse epidermal membrane fractions by glycyrrhetic acid

Glycyrrhetic acid inhibited the specific binding of 12-O-tetradecanoylphorbol-13-acetate (TPA) to mouse epidermal membrane fractions in a dose- and time-dependent manner. Glycyrrhizic acid, a glycoside of glycyrrhetic acid, has no inhibitory effect on TPA binding. The results of kinetic analysis suggested that glycyrrhetic acid directly binds to the TPA receptor, resulting in the competitive inhibition of the binding of TPA to its receptor without affecting the number of binding sites (Ki = 2.2 X 10(-4) M). The inhibitory effect of glycyrrhetic acid on TPA binding to the membrane receptor may play a role in its antitumor-promoting activity in vivo.

Polymyxin B inhibits phorbol 12-myristate 13-acetate, but not chemotactic factor, induced effects in rabbit neutrophils

The addition of the amphipathic polycationic antibiotic polymyxin B to a suspension of rabbit neutrophils results in inhibiton of the agonist (secretion of secondary granules) and antagonist (inhibition of chemotactic factor induced degranulation) properties of phorbol 12-myristate 13-acetate. On the other hand, polymyxin B does not inhibit the degranulation of the neutrophils that is induced by chemotactic factors. These results imply that the role of protein kinase C in the initiation of neutrophil functions in response to the addition of chemotactic factors is less critical than previously thought. In addition, the reversal of the inhibitory properties of phorbol esters by polymyxin B indicates that the former are mediated by the ability of the tumor promoters to activate protein kinase C. These results thus strengthen the hypothesis that protein kinase C plays important roles in the regulation (as contrasted to initiation) of neutrophil functions.

Inhibition of phorbol ester-caused synthesis of mouse epidermal ornithine decarboxylase by retinoic acid

The mechanisms by which topically applied retinoic acid to mouse skin inhibits tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced epidermal ornithine decarboxylase activity were analyzed. Retinoic acid inhibition of the induction of epidermal ornithine decarboxylic activity was not the result of nonspecific cytotoxicity, production of a soluble inhibitor of ornithine decarboxylase, or direct effect on its activity. In addition, inhibition of TPA-caused increased ornithine decarboxylase activity does not appear to be due to enhanced degradation and/or post-translational modification of ornithine decarboxylase by transglutaminase-mediated putrescine incorporation. We found that retinoic acid inhibits the synthesis of ornithine decarboxylase caused by TPA. Application of 10 nmol TPA to mouse skin led to a dramatic induction of epidermal ornithine decarboxylase activity which was paralled by increased [3H]difluoromethylornithine binding and an increased incorporation of [35S]methionine into the enzyme. Application of 17 nmol retinoic acid 1 h prior to application of 10 nmol TPA to skin resulted in inhibition of the induction of activity which accompanied inhibition of [3H]difluoromethylornithine binding and [35S]methionine incorporation into ornithine decarboxylase protein as determined by the tube-gel electrophoresis of the enzyme immunoprecipitated with monoclonal antibodies to it. Inhibition of ornithine decarboxylase synthesis was not the result of the inhibitory effect of retinoic acid on general protein synthesis. The results indicate that retinoic acid possibly inhibits TPA-caused synthesis of ornithine decarboxylase protein selectively.

Retinoids antagonize the induction of ornithine decarboxylase activity by phorbol esters and phospholipase C in rat tracheal epithelial cells

In this study we examined the action of phorbol esters, several phospholipases and retinoids on the induction of ornithine decarboxylase (ODC) activity in rat tracheal epithelial cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA) induces ODC activity in these cells in a dose-and time-dependent manner. This induction is inhibited by cycloheximide indicating a requirement for protein synthesis. Tracheal epithelial 2C5 cells contain two binding sites for phorbol esters, one with a high affinity KD,1 = 4.58 nM and one with a low affinity KD,2 = 344.8 nM. The ability of several phorbol esters to induce ODC correlates well with the described efficacy with which they bind to the receptor and is in agreement with the concept that phorbol ester receptors are involved in the induction of ODC. There is strong evidence that the phorbol ester receptor is the protein kinase C for which diacylglycerol is the physiological ligand. Treatment of cells with phospholipase C generates diacylglycerol and induces ODC activity in a dose- and time-dependent manner. Treatment with phospholipase A2 or D has no effect on ODC activity. These results support the concept that activation of protein kinase C is related to the induction of ODC activity. The induction of ODC by TPA as well as by phospholipase C is inhibited by retinoids. Specific cytosolic binding proteins for retinoids might be involved in at least some of the responses to these compounds. To examine whether the binding proteins are involved in the inhibition of ODC we determined the presence of these binding proteins and the structure-activity relationship of retinoids. Both retinol and retinoic acid-binding proteins can be detected in 2C5 cells, their levels are 1.06 and 3.36 pmoles/mg protein, respectively. The ability of several retinoids to inhibit ODC induction correlates well with their binding activity and support a role for these binding proteins in the action of retinoids on ODC induction.

Inhibition of the effects of 12-O-tetradecanoylphorbol-13-acetate on mouse epidermal glutathione peroxidase and ornithine decarboxylase activities by glutathione level-raising agents and selenium-containing compounds

The present study was undertaken to determine the effect of 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter known to inhibit superoxide dismutase (SOD) (superoxide: superoxide oxidoreductase, EC 1.15.1.1) and catalase (CAT) (H2O2: H2O2 oxidoreductase, EC 1.11.1.6) activities, on mouse epidermal glutathione (GSH) peroxidase (glutathione: H2O2 oxidoreductase, EC 1.11.1.9) activity in vivo and in vitro. TPA led to a rapid and transient increase in GSH peroxidase specific activity within 30 min followed by a decrease from 1 to 12 h. Incubation of isolated epidermal cells with GSH level-raising agents and/or selenium-containing compounds increased remarkably basal GSH peroxidase activity, and thus, abolished totally the prolonged inhibitory effects of TPA on this enzyme. The inhibitory effects of 0.2 mM cysteine (Cys) or 0.5 mM GSH and 2.5 microM Na2 SeO3 or 50 microM selenocystamine on TPA-decreased GSH peroxidase activity were additive, in relation with their additive inhibitory effects on TPA-induced ornithine decarboxylase (ODC) (L-ornithine carboxylase, EC 4.1.1.17) activity. These data support the hypothesis that the stimulators of the GSH-dependent antioxidant protective system of the epidermal cells may inhibit the oxidative challenge linked to skin tumor promotion by TPA.

Cyclosporin A inhibits biological effects of tumor promoting phorbol esters

The antilymphocytic and antiphlogistic agent cyclosporin A (CsA) inhibits in vivo various effects induced by the tumor promoting phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA). These include the edema of the mouse ear, the alkaline phosphatase (AP) activity and the ornithine decarboxylase (ODC) activity in mouse epidermis as well as the generation of a specific arachidonic acid (AA) metabolite in mouse epidermis. AA metabolism in an epidermal cell-free system of mouse epidermis was not suppressed by CsA. According to thin layer chromatography the TPA-induced and as yet unidentified AA metabolite exhibits a polarity between that of 5-HETE and 12-/15-HETE. Studies with inhibitors indicate it to be a lipoxygenase product.

Effects of agents known to antagonize the enhancement of in vitro transformation by 12-tetradecanoyl-phorbol-13-acetate (TPA) on the TPA suppression of metabolic cooperation

Utilizing the phenomenon of metabolic cooperation in Chinese hamster V79 cells, we have studied the effects of agents which suppress the 12-tetradecanoyl-phorbol-13-acetate (TPA) enhancement of transformation in vitro, on the TPA suppression of cell-cell communication. None of the agents tested, namely all-trans-retinoic acid, the trimethyl methoxyphenyl analogue of N-ethyl-retinamide, soybean trypsin inhibitor, antipain nor superoxide dismutase, decreased the enhanced recovery effect of TPA on metabolic cooperation. One of the compounds, retinoic acid, significantly increased the % recovery above that observed for TPA alone.

Effects of local anesthetics on phorbol ester-induced ornithine decarboxylase activity in mouse and human skin

In preparation for experiments to determine the effects of various orally administered compounds on human skin ornithine decarboxylase (ODC) activity, it was observed that intradermal lidocaine hydrochloride inhibited 12-0-tetradecanoylphorbol-13-acetate (TPA)-induced ODC. Topically applied ethyl chloride had variable and unpredictable effects on ODC induction. Adequate local anesthesia can be obtained by using intradermal 1.0% procaine hydrochloride or normal saline; neither of these agents inhibited TPA induction of epidermal ODC activity in incubated human skin punch biopsies.

Inhibitory effects of various drugs on phorbol myristate acetate and n-formyl methionyl leucyl phenylalanine induced O2- production in polymorphonuclear leukocytes

To clarify the mechanisms of O2- formation by polymorphonuclear leukocytes (PMNs), the effects of clinically employed drugs on PMNs were investigated by measuring changes in membrane potential and rates of O2- production. These variables were effectively diminished with antihistaminic agents, adrenergic beta-antagonists, and antiarrhythmic drugs when guinea pig peritoneal PMNs were stimulated by either phorbol myristate acetate (PMA) or n-formyl-methionyl-leucyl-phenylalanine (FMLP). The order of potency of the inhibitory effects of these chemicals on the PMA-induced O2- formation was as follows: azelastine (IC50 = 4.1 microM) less than clemastine less than dl-propranolol less than chlorpheniramine maleate less than dichlorisoproterenol less than quinidine less than diphenhydramine less than indomethacin (IC50 greater than 400 microM). Similar phenomena were observed when FMLP was employed instead of PMA, but the FMLP-stimulated O2- production was effectively inhibited by indomethacin. Changes in membrane potential, using the cyanin dye method, also indicated that most of these drugs cancelled functional changes of plasma membrane of PMNs. From these observations, it was demonstrated that changes in membrane potential by the stimuli were essential for the initiation of O2- generation from plasma membrane of PMNs, although the initiation mechanisms were not identical for the two stimuli.

Characterization of a human placental factor which inhibits specific binding of phorbol esters to cultured cells

Phorbol ester receptors have been demonstrated in a variety of cells and tissues using [3H]phorbol-12,13-dibutyrate (PDBu) as a ligand. In a search for possible endogenous ligand(s) for the receptor, we used the human placenta as a source. A factor that can inhibit the binding of [3H]PDBu on different types of cells was purified (133-fold) from an extract of human placenta. This factor, PEBIF ('phorbol ester binding inhibitory factor'), is sensitive to pepsin and resistant to trypsin treatment. It is heat- and acid (pH3)-stable and can be precipitated by 80% ethanol with no loss of activity. PEBIF inhibits binding whether it is added before or after incubation of [3H]PDBu with human amniotic membrane cells (FL). Inhibition occurs at both 37 degrees C and 4 degrees C and is rapid and reversible; it does not require intact cells, since it also occurs with membrane fractions. PEBIF does not act like a binding protein for PDBu, and the kinetics of the inhibition on FL cells is non-competitive. Inhibition was also observed in rat liver cells (IAR 6) and Friend erythroleukaemia cells (FELC). Differentiation of FELC induced by hexamethylene bisacetamide can be inhibited by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) only if TPA-sensitive cells (TS 19-101) are used; no inhibition is observed with TPA-resistant cells (TR 19-9). The same is true of PEBIF. It has been shown that these two clones have about the same number of receptors, with no change of affinity; and the extent of inhibition of PDBu binding by PEBIF was similar in the two clones. Like TPA, PEBIF can increase 2-deoxyglucose uptake in mouse fibroblasts (BALB/3T3 cells). These data suggest that this physiological factor may play a role in the regulation of cell differentiation and/or in the modulation of carcinogenesis.

Quercetin inhibits the action of 12-O-tetradecanoylphorbol-13-acetate, a tumor promoter

Quercetin, a mutagenic but noncarcinogenic flavonoid, inhibited the increased incorporation of inorganic phosphate into phospholipids of human embryo fibroblasts induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter. Quercetin also inhibited TPA-induced increases of sugar transport and RNA synthesis in chick embryo fibroblasts, and TPA-induced aggregation of human platelets. These results suggest that quercetin may have antitumor promoter activity, which provides a possible reason why quercetin does not develop malignant tumors despite its mutagenicity.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase activity in mouse epidermis by sweetening agents and related compounds

The effects of naturally occurring sweetening agents, which inhibited the induction of Epstein-Barr virus-associated early antigen (EBV-EA) induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), and related compounds on the induction of ornithine decarboxylase (ODC) by TPA is examined. Application of glycyrrhetinic acid or steviol to mouse skin 1 h before TPA treatment showed a remarkable decrease in TPA-induced ODC activity. Post-treatment with glycyrrhetinic acid or steviol 1 h after application of TPA also resulted in a considerable depression in the induction of ODC activity. Neither glycyrrhetinic acid nor steviol alone induced epidermal ODC activity. These results suggest that glycyrrhetinic acid and steviol interfere with the process of induction of epidermal ODC by TPA treatment of mouse skin. cis-Abienol, frullanolide and norambreinolide, which have a partially similar structure in the moiety with glycyrrhetinic acid or steviol, were tested. cis-Abienol and frullanolide showed an inhibitory effect when applied 1 h before TPA treatment, but norambreinolide was not effective. A relationship between suppression of ODC activity and inhibition of EBV-EA induction is discussed.

1 alpha, 25-Dihydroxyvitamin D3 inhibits phorbol ester-dependent chemical carcinogenesis in mouse skin

The effect of topical application of 1 alpha, 25-dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3) on the promotional phase of skin tumor formation in mice was evaluated using 7,12-dimethylbenz [a] anthracene as the tumor initiator and 12-0-tetradecanoylphorbol-13-acetate (TPA) as the tumor promoter. Fifteen weeks of twice weekly topical application of 1 alpha, 25-(OH)2D3 1 hour prior to topical treatment with 16 nmol of TPA inhibited tumor formation in a dose-dependent manner. Doses of 0.25-0.50 nmol of the vitamin D3 metabolite inhibited tumor formation approximately 50% and had no significant effect on the survival or weight gain of the mice. These results indicate that in addition to maintaining calcium homeostasis and affecting the growth and differentiation of certain neoplastic cells, 1 alpha, 25-(OH)2D3 can also suppress the formation of chemically induced tumors.

Diazepam inhibition of phorbol ester tumor promotion

Diazepam (Valium) had previously been shown to enhance the growth rate of some transplantable tumors although studies in other systems have suggested an inhibitory effect. Skin tumor promotion studies were therefore carried out to determine the effect of diazepam on 12-O-tetradecanoyl phorbol-13-acetate (TPA) promotion in SENCAR mice. A sex-independent dose response inhibition occurred over a dose range of 100 g to 5 mg, both in number of papillomas and percent of mice bearing tumors. Furthermore, a dose-response reduction in tumor size was observed. By itself, diazepam had no tumor promoting activity.

Glucocorticoids and prostaglandins inhibit the induction of macrophage DNA synthesis by macrophage growth factor and phorbol ester

The tumor-promoting phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), stimulates starch-elicited mouse peritoneal macrophages to undergo DNA synthesis in vitro, apparently without the generation of an endogenous macrophage growth factor (MGF). No evidence was found for any synergistic interaction between TPA and exogenous colony stimulating factors (CSFs) for macrophage DNA synthesis. Low concentrations of glucocorticoids and also prostaglandins E1 and E2 suppress both the CSF-1-stimulated and the TPA-stimulated macrophage DNA synthesis; these same drugs inhibit the CSF-1-mediated and TPA-mediated enhancement of macrophage plasminogen activator (PA) activity. Thus glucocorticoids and prostaglandins E1 and E2 oppose the action of growth factors and the tumor promoter on macrophage and precursor cell function.

Modifiers of free radicals inhibit in vitro the oncogenic actions of x-rays, bleomycin, and the tumor promoter 12-O-tetradecanoylphorbol 13-acetate

Using short-term cultures of hamster embryo cells, we have examined the effects of the free-radical scavenger superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) and the enzyme catalase (hydrogen-peroxide:hydrogenperoxide oxidoreductase, EC 1.11.1.6) on x-ray- and bleomycin-induced transformation and on the enhancement of radiogenic transformation by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA). We find that superoxide dismutase inhibits (i) transformation induced by x-ray and bleomycin and (ii) promotional action of TPA in vitro. The results suggest that the oncogenic action of x-rays and bleomycin and the enhancement of oncogenic transformation by TPA are mediated in part by free radicals. The findings also suggest that superoxide dismutase can serve as an inhibitor of oncogenesis and that its actions, as seen in this in vitro system, are most predominantly on inhibiting late events in the progression of cellular transformation--those associated with promotion.

Modulation of natural killer sensitivity of murine trophoblast cells by tumor promoter and interferon

The in vitro established lines of murine placental trophoblast cells showed little membrane expression of H-2 antigens and a high degree of sensitivity to natural killer (NK)-cell-mediated cytotoxicity. This NK-sensitivity was decreased either by short-term treatment of these cells with murine interferon (IFN) or by culture for more than 4 days in the presence of a tumor promotor, 12-O-tetradecanoylphorbol-13-acetate (TPA). This effect of TPA was reversible after removal of TPA from the culture medium. However, IFN treatment of trophoblast cells cultured previously in TPA medium resulted in restoration of NK-sensitivity to the original level exhibited by cells untreated with IFN or TPA. The mechanism for this antagonistic effect is not clear. The decrease in NK-sensitivity of IFN- or TPA-treated trophoblast cells was not due to the loss of NK-recognition membrane structures, or to induction of prostaglandin which elevates cyclic-AMP levels. Inhibition of protein synthesis by cycloheximide restored the NK-sensitivity or TPA-treated trophoblast cells, indicating the possible contribution of de novo synthesized protein(s) which could confer NK-resistance. The TPA-treated trophoblast cells became more tumorigenic, producing larger tumors and a higher mortality than the untreated controls when inoculated into mice. The possible roles of NK cells in trophoblastic diseases and feto-maternal relationships are discussed.

Reversal of the promotional effect of 12-O-tetradecanoylphorbol-13-acetate on morphological transformation of hamster embryo cells by glucocorticoids

The effects of glucocorticoids and cholesterol on morphological transformation have been studied using hamster embryo cells. The cells were exposed sequentially to benzo[a]pyrene (BP) (3 days) and the tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA) (4 days) before scoring for morphologically transformed colonies. Dexamethasone and hydrocortisone strongly inhibited the formation of morphologically transformed colonies when applied to the cells in the second period of exposure together with TPA, but had no effect when present with BP during the first period. Corticosterone had only a weak effect, while cortisone had no effect on the transformation frequency. Cholesterol gave rise to an enhanced number of transformed colonies. Dexamethasone, being the most potent compound tested, inhibited morphological transformation by more than 50% when present in a concentration of 0.25 nM. The finding that dexamethasone reduced the frequency of transformation even when added only 6 h prior to staining, indicates that the glucocorticoids may also reduce the transformation frequency by reversing morphologically transformed cell colonies to a normal appearance.

Effects of inhibitors of tumor promotion on 12-O-tetradecanoylphorbol-13-acetate-induced keratin modification in mouse epidermis

In order to investigate the role that hyperplasia plays in the induction of the keratin modifications by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), the effects of several inhibitors of tumor promotion and cycloheximide on the TPA-induced protein changes were studied. The antiinflammatory steroid fluocinolone acetonide and cycloheximide, which prevent the induction of hyperplasia, were found to prevent the appearance of the TPA-induced keratin modifications. Retinoic acid and tosylphenylalanylchloromethyl ketone, which have minor influence on hyperplasia, were found to have essentially no effect on these protein changes. These results provided further evidence that the TPA-induced epidermal keratin changes were associated with the induction of hyperplasia by TPA and not necessarily related to promoting ability.

Inhibition of mouse skin tumor promotion by several inhibitors of arachidonic acid metabolism

12-O-Tetradecanoylphorbol-13-acetate promotion of skin tumors in mice can be inhibited by topical application of either the phospholipase A2 inhibitor dibromoacetophenone or the cyclooxygenase-lipoxygenase inhibitors 5,8,11,14-eicosatetrayonic acid or 1-phenyl-3-pyrazolidinone. The phospholipase A2 inhibitors in particular appear to be among the most potent inhibitors of skin tumor promotion known. These results support the hypothesis that at least some of the products of arachidonic acid transformation are essential for tumor promotion.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced epidermal ornithine decarboxylase activity by lipoxygenase inhibitors: possible role of product(s) of lipoxygenase pathway

Induction of epidermal ornithine decarboxylase (ODC) by a topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA), a tumor promoter, was inhibited by treatment of mouse skin with phenidone (3-90 mumol/mouse), nordihydroguaiaretic acid (30 mumol/mouse) or 3-amino-1-[m-(trifluoromethyl)-phenyl]-2-pyrazoline (BW 755C, 30 mumol/mouse), which are well-known lipoxygenase inhibitors. Phenidone and BW 755C are also to be cyclooxygenase inhibitors. Inhibition of TPA-induced ODC by indomethacin (1.12 mumol/mouse), a selective cyclooxygenase inhibitor, was counteracted by prostaglandin E2 (PGE2) (140 nmol/mouse). This counteracting effect of PGE2 was reversed by the treatment of mice with nordihydroguaiaretic acid (30 mumol/mouse) or phenidone (30 mumol/mouse). ODC activity which was suppressed by nordihydroguaiaretic acid or phenidone at a dose of 180 mumol/mouse was not further inhibited by indomethacin (1.12 mumol/mouse). In addition, the counteracting action of PGE2 (140 nmol/mouse) was not observed in mice treated with nordihydroguaiaretic acid or phenidone at a dose of 180 mumol/mouse. Thus, the suppressive effect of nordihydroguaiaretic acid or phenidone on the ODC induction by TPA would be due to the inhibition of lipoxygenase. The above findings strongly suggest that not only cyclooxygenase product (i.e., PGE2) but also lipoxygenase product(s) are involved in the mechanism of ODC induction in mouse epidermis, and a lack of either cyclooxygenase product or lipoxygenase product(s) causes a failure of ODC induction by TPA.

Prevention of tumor promoter-mediated inhibition of preadipocyte differentiation by dexamethasone

A clonal cell line of murine fibroblast (ST 13) undergoes a terminal differentiation into adipose cells in culture. A tumor promoter, 12-O-tetradecanoyl phorbol 13-acetate (TPA), reversibly inhibited the differentiation of ST 13 preadipocytes. The inhibition of differentiation was accompanied by changes in cell morphology and growth properties and by suppression of cellular insulin binding activity. Dexamethasone (DXM) could prevent the inhibition by TPA of preadipocyte differentiation, concomitant with prevention of the morphological changes and an increase in insulin binding activity. TPA-induced plasminogen activator (PA) secretion and DXM-mediated inhibition of PA activity could not be correlated with inhibition/prevention-of-inhibition of differentiation.

Inhibiton of phorbol ester-stimulated chemiluminescence in human polymorphonuclear leukocytes by retinoic acid and 5,6-epoxyretinoic acid

Chemiluminescence (CL) is an index of both the generation of and reactions mediated by O2-. and 1O2. The tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), is a potent stimulator of CL by human polymorphonuclear leukocytes; treatment with TPA (100 ng/ml) provokes a CL response that peaks within five min and persists for over 30 min. The response is proportional to concentration over the range of one to 100 ng/ml. The ability of different phorbol diesters to stimulate both CL and O2-. production correlates with their relative activities as tumor promoters in vivo. Non-phorbol diester tumor promoters such as iodoacetic acid, anthralin, and tween 60 are inactive in this system. The TPA-mediated stimulation of CL can be inhibited by retinoids; cells preincubated for 15 min with 100 microM retinoic acid show only a marginal CL response to TPA. Addition of retinoic acid to resting polymorphonuclear leukocytes results in a transient burst of CL without concomitant O2-. release, observations indicative of an excitable substrate. A similar CL response is seen when retinoic acid is incubated with potassium superoxide in a cell-free system. 5,6-Epoxyretinoic acid, and even more effective inhibitor of TPA-stimulated CL than retinoic acid when added simultaneously with TPA, does not undergo these two CL reactions. Thus, it appears that retinoic acid may undergo oxidative activation to a species that exert enhanced antipromoter activities. Polymorphonuclear leukocytes provide a useful system for exploring the roles of reactive oxygen species in the mechanism of action of both TPA and retinoic acid.

Protease inhibitor antipain suppresses 12-O-tetradecanoyl-phorbol-13-acetate induction of plasminogen activator in transformable mouse embryo fibroblasts

Plasminogen activator (PA) activity was analyzed in normal and transformed 10T1/2 mouse fibroblasts treated with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and the protease inhibitors antipain, leupeptin, and soybean trypsin inhibitor (SBTI). TPA induced PA activity in normal 10T1/2 cells was inhibited by antipain. Transformed 10T1/2 cells maintained high levels of PA activity which were not further stimulated by the addition of TPA. Similarly, antipain inhibited the PA activity of the transformed cultures. Leupeptin and SBTI had no effect. These findings, in light of the fact that antipain has been shown to suppress the promotional effect of TPA in X-ray induced malignant transformation, may suggest a definite role for proteases in the transformational event or maintenance of the transformed state.

Phorbol ester-induced anchorage independence and its antagonism by retinoic acid correlates with altered expression of specific glycoproteins

Since tumor promoting phorbol esters produce a variety of glycoprotein synthesis changes and since retinoids act both as antipromoters and modulators of glycoprotein synthesis, we sought to ascertain whether specific changes in glycoprotein synthesis might be targets both for the promoting action of phorbol esters and for the antipromoting actin of retinoids. In this report we present evidence that tumor promoting but not nonpromoting phorbol esters produce decreased levels of 180,000 and 150,000 mol, wt, glycoproteins in mouse JB-6 cells which are promotable to tumor cell phenotype by phorbol esters. These relatively specific decreases are blocked by an antipromoting concentration of retinoic acid, thus suggesting that decreases in 180K and 150K glycoproteins may play a role in promotion of transformation.

Differential retinoic acid inhibition of ornithine decarboxylase induction by 12-O-tetradecanoylphorbol-13-acetate and by germicidal ultraviolet light

Several retinoids including retinoic acid effectively inhibit phorbol ester-mediated tumor promotion and ornithine decarboxylase (ODC) induction in mouse epidermis. To understand better the possible cellular site of action of retinoids, the inhibitory action of retinoic acid on the induction of ODC was compared for two distinctly different inducers, namely, 12-O-tetradecanoylphorbol-13-acetate (TPA) and germicidal ultraviolet light (UV), in primary mouse epidermal cell cultures. It was found that the induction of ODC by TPA is almost completely prevented by 0.1 to 1 microM retinoic acid while the induction by UV is only moderately inhibited. Maximum inhibition is achieved by treating cells continuously with retinoic acid from 4 hr after plating, although pretreatment or simultaneous treatment relative to either inducer is almost as effective. When added after the inducer, retinoic acid loses its effectiveness as an inhibitor more rapidly for TPA induction than for UV induction of ODC. The differential inhibition of enzyme induction cannot be accounted for by selective retinoid inhibition of DNA, RNA, or protein synthesis either alone or in concert with TPA or UV. Other agents known to modulate the induction of ODC by TPA (fluocinolone acetonide, tosyl-L-lysylchloromethane, and local anesthetics) do not act differentially on UV induction. These agents possibly act at transcription or translation, both of which are required for ODC induction by TPA or UV. The preferential inhibition by retinoic acid of ODC induction by TPA is interpreted to result from specific interference at a unique and early site of interaction of TPA with the cell.

Inhibition by putrescine of the induction of epidermal ornithine decarboxylase activity and tumor promotion caused by 12-O-tetradecanoylphorbol-13-acetate

The induction of epidermal ornithine decarboxylase (EC 4.1.1.17) (ODC) following topical application of the tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA) to mice can be inhibited by topical application of putrescine, the product of the enzyme. The degree of inhibition depended on both the dose and the time of putrescine application; application of 20 mumol of putrescine 2 hr after TPA treatment inhibited the induction of ODC activity by 50%. TPA-induced activity of another polyamine-biosynthetic enzyme, S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50), was unaffected by application of putrescine. Among several amines tested for their ability to inhibit the induction of ODC activity, spermidine, 1,7-diaminoheptane, and spermine were the most effective, causing a 90% inhibition at the 20-mumol dose. Putrescine, when added directly to the assay medium at a 100-mumol dose level inhibited by 97% the TPA-induced ODC activity, but the amount of putrescine (20 mumol) which gave 50% inhibition of the induction of ODC activity in vivo had no effect when added to the assay system. Mixing of soluble extracts from TPA-treated mouse epidermis posttreated either with acetone or putrescine or with mouse epidermis treated with putrescine alone gave essentially additive ODC activity. Furthermore, putrescine did not elicit production of detectable ODC-antizyme activity in mouse epidermis. Putrescine inhibited the formation of mouse skin papillomas promoted with TPA. Topical application of 20 and 100 mumol of putrescine 2 hr after each application of TPA to mice initiated with 7,12-dimethylbenz[a]anthracene resulted in a 30 and 80% inhibition, respectively, of papilloma formation compared to animals receiving no putrescine.

Local anesthetics inhibit induction of ornithine decarboxylase by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate

The induction of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) activity in mouse epidermal cells in vivo and in vitro occurs rapidly after exposure to the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA). This induction has characteristics of a cell surface receptor-mediated process. Local anesthetics modify a variety of cellular responses mediated by membrane receptors. When cultured mouse epidermal cells were exposed to the local anesthetics lidocaine, tetracaine, or procaine (0.1-1 mM), induction of the decarboxylase by TPA was inhibited by more than 90%. In vivo, lidocaine essentially abolishes the decarboxylase response of mouse epidermis when applied shortly after TPA. In contrast, local anesthetics have no effect on the enzyme's activity when added directly to the assay mixture and, in concert with TPA, have only a minimal effect on overall protein synthesis relative to controls. However, lidocaine has no effect on TPA-stimulated DNA synthesis in vitro (12-fold with or without lidocaine). Local anesthetics also markedly inhibit induction of the decarboxylase by ultraviolet light, which is probably not membrane mediated. Furthermore, in culture, lidocaine has only a small inhibitory effect on ornithine decarboxylase when given before TPA but is an effective inhibitor even when given up to 4-5 hr after the promoter, a time when decarboxylase activity has already increased. These findings suggest that local anesthetics, which are tertiary amines, do not act at the site of interaction of TPA and its putative receptor but may be acting specifically on polyamine biosynthesis. These drugs could be useful agents to determine the role of the polyamine pathway in tumor promotion.

alpha-Tocopherol inhibits 12-O-tetradecanoyl-phorbol-13-acetate-stimulated deacylation of cellular lipids, prostaglandin production, and changes in cell morphology of Madin-Darby canine kidney cells

Incubation of the tumor-promoting phorbol diester, 12-O-tetradecanoyl-phorbol-13-acetate with Madin-Darby canine kidney cells (MDCK) stimulated deacylation of phospholipids, prostaglandin production and altered cell morphology. alpha-Tocopherol, if present during the incubation, inhibited these effects. For inhibition, alpha-tocopherol had to be present during incubation of 12-O-tetradecanoyl-phorbol-13-acetate and cells; pretreatment or posttreatment of the cells with alpha-tocopherol was not effective. Inhibition of the 12-O-tetradecanoyl-phorbol-13-acetate effects was specific for the tumor promoter extracted from the Croton tiglium L. plant of the family Euphorbiaceae. Stimulated prostaglandin production by another tumor promotor (phorbol-12, 13-didecanoate), a semisynthetic product of that plant, was also inhibited by alpha-tocopherol. But that of structurally related diterpenoid esters isolated from plants of the family Thymelaeaceae, such as mezerein, gnidimacrin, gnilatimacrin, and gnilatimacrin-20-palmitate, were not. Stimulation of prostaglandin production in MDCK cells by compounds such as benzo(a)pyrene, adriamycin and 17 beta-estradiol or even the basal synthetic activity of MDCK cells was not affected by alpha-tocopherol.

Inhibition of skin tumor promotion by retinoic acid and its metabolite 5,6-epoxyretinoic acid

The ability of 5,6-epoxyretinoic acid, a biologically active metabolites of retinoic acid, to inhibit both the induction of ornithine decarboxylase (ODC) activity and skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA) was evaluated. Application of 5,6-epoxyretinoic acid either concurrently with or 1 hr after each application of TPA to the initiated mouse skin inhibited the formation of skin tumors as effectively as did retinoic acid. 5,6-Dihydroretinoic acid, which is a poor substrate for epoxidation, also inhibited skin tumor promotion. 5,6-Epoxyretinoic acid, 5,6-dihydroretinoic acid, and retinoic acid were equally effective in inhibiting the induction of ODC activity by TPA. Insect juvenile hormones inhibited neither the induction of ODC activity nor skin tumor promotion by TPA. These results indicate that (a) epoxidation of retinoic acid at the 5,6-position is not a rate-limiting modification for the anti-promoting activity of retinoic acid and that (b) inhibition of the induction by TPA of mouse epidermal ODC activity may be a simple test for screening the potential prophylactic activities of new retinoids.

Chemical inducers of differentiation in Friend leukaemia cells inhibit lymphocyte mitogenesis

Several phorbol esters, the potent tumour-promoting agents isolated from croton oil, induce proliferation of human lymphocytes and enhance the mitogenic effect of lectins on bovine lymphocytes. While studying the mitogenic properties of one of these agents, phorbol myristate acetate (PMA), we found that dimethyl sulphoxide (DMSO), frequently used as a solvent for PMA, markedly inhibits PMA-induced mitogenesis at DMSO concentrations that have little effect on phytohaemagglutinin (PHA)-induced responses. DMSO, as well as a variety of other organic compounds, induce erythroid differentiation in Friend leukaemia (FL) cells. Phorbol esters, on the other hand, are potent inhibitors of both spontaneous and induced cellular differentiation. We therefore investigated the relationship between the potency of compounds to induce erythroid differentiation in FL cells and their potency to inhibit lymphocyte proliferation induced by PMA and other mitogens. We report here that many of the compounds that induce erythroid differentiation in FL cells are similar to DMSO in selectively suppressing PMA-induced lymphocyte mitogenesis.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-accelerated phospholipid metabolism by 5,8,11,14-eicosatetraynoic acid

The tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate (TPA), accelerates choline phospholipid synthesis in bovine lymphocytes by an oxygen-dependent mechanism. This action is prevented by high concentrations of the cyclooxygenase inhibitor, indomethacin (1 to 3 mM), suggesting a possible involvement of lipid oxidation in the response. The acetylenic analog of arachidonic acid, 5,8,11,14-eicosatetraynoic acid (ETYA), at concentrations in the 10 to 50 microM range also prevents the acceleration of the incorporation of [methyl-3H]choline hydrochloride into choline phospholipids, and it appears more selective in its action. This antagonistic effect of ETYA, an agent which inhibits both the cyclooxygenase and lipoxygenase routes of arachidonic acid oxidation, can be precluded by arachidonic acid but not by other unsaturated or saturated fatty acids. If ETYA is added to lymphocyte cultures after 1 hr of TPA treatment, the established rate of [methyl-3H]choline hydrochloride incorporation is unaffected, but further acceleration is blocked. The inhibition by ETYA cannot be counteracted by any of the prostaglandins, Types A, B, D, E, or F alpha, or by the prostacyclin compounds, PGI1 and 6,9-thia-PGI2. The thromboxane pathway also appears not to be involved since 9,11-azoprosta-5,13-dienoic acid (Azo I), a selective inhibitor of thromboxane synthetase, does not affect the TPA response. These results suggest that TPS may activate the lipoxygenase rather than the cyclooxygenase pathway for lipid oxidation and that an arachidonic acid hydroperoxide or a subsequent metabolite plays a key role in the stimulation of choline phospholipid synthesis by the tumor-promoting phorbol ester, TPA.

Retinoids inhibit the mitogenic activity of tumour-promoting phorbol esters on human lymphocytes

The tumour-promoting agents 12-0-tetradecanoylphorbol-13-acetate (TPA) and phorbol-12, 13-dibutyrate (PDB) I are potent mitogens for human peripheral blood lymphocytes. In contrast, the non-cocarcinogenic substance phorbol lacks lymphocyte-activating properties. Non-toxic levels of retinoic acid (RA) or retinyl acetate (RAt) inhibit the phorbol-ester-stimulated lymphocyte blastogenesis required the near-concurrent addition of retinoids. Differences in the sensitivity of phorbol-ester-stimulated lymphocyte subpopulations to the antagonistic action of RA or RAt, respectively, suggest that the inhibitory effect of retinoids may not be due to a common mode of action. Lymphocyte cultures may provide a useful model system for studies of the mechanisms of action of both phorbol esters and retinoids.

Inhibition of phorbol myristate acetate and phytohemagglutinin stimulation of human lymphocytes by retinol

Retinol (vitamin A) in concentrations above (5 microgram/ml inhibits the stimulation of human lymphocytes by phytohemagglutinin and phorbol myristate acetate. This is in contrast to bovine lymphocytes in which retinol inhibits only phorbol myristate acetate stimulation of lymphocytes. The cause of the inhibition does not appear to be due to a direct toxic action because the viability and ultrastructure of lymphocytes exposed to a concentration of 20 microgram/ml for 72 hr were normal. The mechanism of action of the retinol is not known, but it may be mediated by a retinol-binding protein in the cells.

Retinoic acid inhibition of the comitogenic action of mezerein and phorbol esters in bovine lymphocytes

12-O-Tetradecanoylphorbol-13-acetate (TPA) is an effective comitogen in phytohemagglutinin-treated bovine lymphocytes. Concurrent addition of 10(-8) M TPA gives a greater than 6-fold increase in DNA synthesis over cultures treated with the lectin alone. The delayed addition of phorbol ester, relative to the start of the lectin treatment, eliminates this synergistic action. Structure-function studies show that the comitogenic activity of different phorbol diesters runs parallel to their tumor-promoting activity. A nontoxic level (50 micronM) of retinoic acid selectively antagonizes this synergistic effect of phorbol ester. This inhibitory action requires the near-concurrent addition of retinoic acid with TPA. In contrast, the TPA-mediated induction of RNA and protein synthesis is unaffected by retinolic acid. A number of natural and synthetic retinoids were evaluated; none were as inhibitory as was retinoic acid. Lymphocyte cultures appear to provide a useful system for exploring the mechanisms of action of both TPA and retinoic acid.

The effect of colchicine on the induction of ornithine decarboxylase by 12-O-tetradecanoyl-phorbol-13-acetate

The induction of mouse epidermal ornithine decarboxylase, 1 of the earliest and largest phenotypic changes following treatment of mouse skin with the tumor-promoting agent, 12-O-tetradecanoyl-phorbol-13-acetate, can be inhibited by prior administration of colchicine. Maximal inhibition of this enzyme induction was observed when colchicine was injected i.p. 90 or 120 min before promoter treatment, although time intervals up to 20 hr between colchicine and promoter treatment were effective. The effect of colchicine was dose dependent, with a dose as low as 25 nmoles/mouse causing an inhibition of 35%. Other microtubule-disrupting agents, vinblastine, vincristine, and Colcemid, had a similar effect on ornithine decarboxylase activity. However, beta, gamma-lumicolchicine, a photochemical derivative of colchicine with no antimitotic or microtubule-disrupting ability, and cytochalasin B, an inhibitor of microfilament-dependent processes, had no effect. N6, O2'-dibutyryl 3',5'-cyclic adenosine monophosphate, when administered just before colchicine, blocked the inhibitory action of colchicine. The results of these studies suggest that colchicine-sensitive structures, most likely containing microtubules, may be mediating elements between the binding of tumor promoters, perhaps to specific cell surface receptors, and the subsequent induction of ornithine decdaboxylase.

Inhibition of two-stage carcinogenesis in mouse skin with bis(2-chloroethyl)sulfide

ICR/Ha Swiss mice, 30 females/group, were given a single application of 20 mug 7,12-dimethylbenz(a)anthracene, followed 2 weeks later by three times weekly applications of phorbol myristate acetate (PMA), 2.5 mug/application. In the accompanying inhibition experiments, sulfur mustard [SM; bis(2-chloroethyl)sulfide] was applied at a dose of 20 mug/application during the promotion phase, either on the same day as PMA, i.e., three times weekly, or on alternate days, i.e., twice weekly. The experiments were continued for 385 days. The control group, i.e., without SM, resulted in 27 of 30 mice with a total of 281 papillomas. Sixteen of these mice also bore squamous cell carcinomas. In the two groups also receiving SM, two or three times weekly, 2 of 30 and 1 of 30 mice, respectively, bore papillomas. Carcinomas were not observed in these groups. The inflammatory response of mouse skin to the combined treatments of SM and PMA was no greater than with PMA alone.