Multistage chemical carcinogenesis in mouse skin

Skin tumors in mice can be induced by the sequential application of a subthreshold dose of a carcinogen (initiation phase) followed by repetitive treatment with a noncarcinogenic tumor promoter. The initiation phase requires only a single application of either a direct-acting carcinogen or a procarcinogen which has to be metabolized before being active; it is essentially an irreversible step which probably involves a somatic cell mutation as evidenced by a good correlation between the carcinogenicity of many chemical carcinogens and their mutagenic activities. There is a good correlation between the skin-tumor-initiating activities of several polycyclic aromatic hydrocarbons (PAH) and their ability to bind covalently to epidermal DNA. Results from our laboratory as well as others suggest that "bay region" diol-epoxides are the ultimate carcinogenic form of PAH carcinogens. Potent inhibitors and stimulators of PAH tumor initiation appear to affect the level of the PAH diol-epoxide reacting with specific DNA bases. REcent data suggest that the tumor-promotion stage involves at least 3 important steps: (1) the induction of embryonic-looking cells (dark cells) in adult epidermis; (2) an increased production of epidermal prostaglandins and polyamines; (3) sustained proliferation of dark cells. Retinoic acid specifically inhibits step 2, whereas the anti-inflammatory steroid fluocinolone acetonide is a potent inhibitor of steps 1 and 3. The mechanism and the importance of a specific sequence for each step in chemical carcinogenesis in mouse skin will be discussed in detail.

Protein kinase inhibitors selectively block phorbol ester- or forskolin-induced changes in excitability of Aplysia neurons

Exposure of the bag cell neurons of Aplysia to activators of protein kinase C, such as phorbol esters, enhances electrically evoked action potentials by increasing the voltage-dependent calcium current. We have hypothesized that this effect is mediated by the activation of protein kinase C (PKC). An important prediction of this hypothesis is that inhibitors of PKC should inhibit these phorbol ester-induced changes in bag cell neuronal excitability. We have now found that treatment of bag cell neurons with the protein kinase inhibitor 1-[5-isoquinolinesulfonyl]-2-methyl piperazine (H-7) inhibits the phorbol ester-induced enhancement of bag cell action potentials and prevents the enhancement of calcium current by phorbol esters. The height and width of electrically evoked action potentials in bag cell neurons can also be enhanced by cAMP analogs or agents that elevate cAMP. These agents do not influence the major voltage-dependent calcium current in the bag cell neurons but may act by modulating potassium currents and other voltage-dependent currents. We have found that microinjection of a protein inhibitor of cAMP-PK (PKA-I) into isolated bag cell neurons prevents and reverses the effect of the adenylate cyclase activator forskolin on action potentials of these cells. In contrast, H-7 does not inhibit the effects of forskolin on a variety of responses in these cells, including its effects on action potentials, granule movement, and 32P incorporation into phosphoproteins. This suggests that H-7 is selective for PKC relative to cAMP-PK in intact bag cell neurons.

Staurosporine counteracts the phorbol ester-induced enhancement of neurotransmitter release in hippocampus

The effects of staurosporine, introduced as a very potent inhibitor of protein kinase C (PKC), on evoked neurotransmitter release were investigated and compared with those of the other PKC inhibitors: 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) and polymyxin B (PMB). Slices of rabbit hippocampus, prelabelled with either [3H]noradrenaline, [3H]5-hydroxytryptamine or [3H]choline were superfused with physiological medium. During superfusion the slices were stimulated either electrically (3 Hz, 5 V/cm, 24 mA, 2 msec) or by high K+ (30 mM) for 2 min, respectively. Both the electrically and potassium evoked overflow were increased by the PKC activator 4 beta-phorbol 12,13-dibutyrate (PDB). The degree of the enhancement by PDB was dependent on the transmitter and the stimulation conditions used. These results may be explained by differences in the extent of activation of PKC during electrically or potassium evoked release. The PDB-induced enhancement of electrically or potassium evoked release of the 3 transmitters was counteracted by staurosporine (1 microM) in concentrations much lower than those required for H7 (100 microM) and PMB (100 microM). PMB, which has been shown to decrease electrically evoked transmitter release, similarly diminished K+-evoked release. In contrast, only the potassium evoked [3H]acetylcholine release was significantly diminished by staurosporine (1 microM) and H7 (100 microM). In conclusion, these results show again that facilitation of neurotransmitter release by phorbol esters is due to activation of PKC.

Differential effects of bryostatins and phorbol esters on arachidonic acid metabolite release and epidermal growth factor binding in C3H 10T1/2 cells

The bryostatins, a group of macrocyclic lactones isolated on the basis of their antineoplastic activity, protein kinase C in vitro and block phorbol ester binding to this enzyme. In some cellular systems, bryostatins mimic phorbol ester action. In other systems, however, the bryostatins display only marginal agonistic action and, instead, inhibit phorbol ester-induced responses. At least in primary mouse epidermal cells, a transient duration of action of bryostatin 1 could rationalize these differences. To determine whether this model of transient activation could explain the dual actions of bryostatin 1 in other cell systems, we have examined the effects of bryostatin 1 on short-term responses in C3H 10T1/2 mouse fibroblasts. Even at very short exposures (30 min), bryostatin 1 blocked phorbol ester-induced arachidonic acid metabolite release and induced only minimal release when assayed alone. In contrast, epidermal growth factor binding was markedly and rapidly decreased in bryostatin 1-treated C3H 10T1/2 cells, and this decrease showed only limited reversal 16 h after initial exposure. Bryostatins 2, 3, 4, 10, and several of their derivatives caused variable arachidonic acid metabolite release (10 to 60% of phorbol ester control) and correspondingly variable inhibition of phorbol ester action. Our findings on arachidonic acid metabolite release argue against transient activation of the protein kinase C pathway as the sole explanation of bryostatin 1 action. They indicate, moreover, differences in the structure-activity relations of the bryostatins for the phorbol ester-mimetic and phorbol ester-inhibitory actions.

Cytogenetic effects caused by phorbol ester tumor promoters in primary mouse keratinocyte cultures: correlation with the convertogenic activity of TPA in multistage skin carcinogenesis

The effects of the convertogenic ('first-stage') tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), the non-convertogenic ('second stage') tumor promoter 12-O-retinoyl-phorbol-13-acetate (RPA) and the non-promoting phorbol esters 4-O-MeTPA and 4-alpha-PDD on the chromosomes of mouse keratinocytes in primary cultures were investigated. In these target cells of tumor promotion TPA caused severe numerical and structural chromosomal aberrations, which were evident after two cell cycles and accumulated after multiple applications. Numerical aberrations were visible as hypo- and hyperdiploidy, with non-random loss or gain of specific chromosomes. The clastogenic effects were evident as simple alterations such as gaps and breaks, but more severe alterations such as tri- and quadriradial chromatid interchanges and ring chromosomes, as well as translocations could be observed. The structural aberrations were nonrandomly distributed in the genome and chromosomes no. 1, 2, 5, 6 and 18 were preferentially involved in rearrangements. In addition to the aneuploidogenic, clastogenic and recombinogenic effects induced by TPA, short treatment with this tumor promoter was efficient in producing cytogenetic equivalents of gene amplification, i.e. double minute chromosomes. The cytogenetic effects were not merely due to cytotoxicity since they occurred after a low TPA dose (10(-8) M) and did not considerably increase with a higher dose (10(-6) M). Moreover, at both dose levels cell cycle traverse of mouse keratinocytes was not drastically altered. In contrast, the non-convertogenic tumor promoter RPA and the non-promoting phorbol esters 4-alpha-PDD and 4-O-MeTPA (at the same dose level) did not cause any substantial chromosomal alterations. This discrepancy between the action of TPA and RPA suggests that effects which result in chromosomal alterations in the target cells may be critical for the conversion stage of skin tumor promotion. This conclusion is supported by experiments with substances such as antipain and eicosa-5,8,11,14-tetraynoic acid which inhibit both tumor induction in initiated skin and the cytogenetic alterations induced by TPA in cultured keratinocytes. These studies provide for the first time the possibility of differentiating between convertogenic and non-convertogenic tumor promoters in an in vitro assay using the target cells of mouse skin carcinogenesis.

Modulation of cellular phorbol ester binding and/or protein kinase C activity by human placental fractions

We describe two factors in human placenta that modulate the interaction of phorbol ester tumor promoters with cell membranes or with protein kinase C. One, phorbol ester binding inhibitory factor, can inhibit binding of [3H]phorbol-12,13-dibutyrate to cultured cells or to a membrane fraction but does not inhibit its binding to a homogeneous C kinase preparation (phorbol ester binding sites). The other, C kinase activating factor, stimulates C kinase activity in a calcium-dependent manner. We separated these two biochemical activities from a crude human placental fraction by gel filtration.

Inhibition by bryostatin 1 of the phorbol ester-induced blockage of differentiation in hexamethylene bisacetamide-treated Friend erythroleukemia cells

Phorbol esters inhibit chemically induced differentiation in Friend erythroleukemia cells. This study examines the effect of the macrocyclic lactone bryostatin 1 on phorbol ester responses in a Friend erythroleukemia cell clone, PS 7. In several biological systems, bryostatin 1 was reported to mimic phorbol ester action, including activation of protein kinase C, but in HL-60 cells it blocked phorbol ester-induced differentiation. We report here that bryostatin 1 blocks phorbol ester action in Friend cells (clone PS 7), a second differentiating system. In this system, in contrast to HL-60 cells, the phorbol esters inhibit rather than induce differentiation. Bryostatin 1 restores the differentiation response [50% effective dose, 15 +/- 3.5 nM (SEM)] as well as blocks a second phorbol ester effect, induction of cellular adherence. The inhibition of erythroid differentiation by dexamethasone, a nonphorbol compound whose action presumably is not protein kinase C mediated, is unaffected by bryostatin 1. Although bryostatin 1 inhibits [3H]phorbol 12,13-dibutyrate binding in intact Friend erythroleukemia cell clone PS 7, the mechanism for the antagonism of phorbol ester action by bryostatin 1 in Friend cells cannot be explained by simple competition at the binding site.

Partial parallelism and partial blockade by bryostatin 1 of effects of phorbol ester tumor promoters on primary mouse epidermal cells

Bryostatin 1, a macrocyclic lactone, functions like the phorbol esters biochemically in binding to and activating protein kinase C. Biologically, however, although it induces some phorbol ester responses such as mitogenesis in Swiss 3T3 cells, it paradoxically blocks the effects of the phorbol esters on differentiation in HL-60 promyelocytic leukemia cells and Friend erythroleukemia cells. Since the phorbol esters induce proliferation and terminal differentiation in distinct subpopulations of epidermal basal cells, we have now examined the action of bryostatin 1 in that system. Bryostatin 1 decreased epidermal growth factor binding and induced ornithine decarboxylase activity, the latter a marker of proliferation. The magnitude of the maximal induction of ornithine decarboxylase was less than for phorbol 12,13-dibutyrate. Bryostatin 1 only transiently caused the morphological change typical of phorbol ester treatment and did not induce transglutaminase or cornified envelope production, markers of the differentiative pathway. Combined treatment with bryostatin 1 and phorbol 12,13-dibutyrate gave similar results to treatment with bryostatin 1 alone, i.e., slight reduction to complete inhibition of phorbol ester action, depending on the response. The mechanism may reflect time dependent block of the protein kinase C pathway by bryostatin 1 in this system; although bryostatin 1 inhibited epidermal growth factor binding at short incubation times (1-2 h), by 4 h of incubation its inhibition was markedly reduced and it correspondingly blocked inhibition of epidermal growth factor binding by phorbol 12,13-dibutyrate. Since induction of terminal differentiation is proposed to be an essential component of phorbol ester mediated tumor promotion in skin, our findings suggest that bryostatin 1 may function as an inhibitor of phorbol ester promotion.

Inhibition of phorbol ester stimulated interleukin 2 production by copper(II) complexes

Superoxide dismutase mimetic copper(II) complexes, such as copper(II)(3,5-diisopropylsalicylate)2 (CuDIPS), inhibit phorbol ester stimulated tumor promotion in mouse skin. Therefore, CuDIPS was tested as a potential inhibitor of another effect of phorbol esters, induction of interleukin 2 (IL2) synthesis, in the mouse thymoma cell line EL4. CuDIPS inhibited phorbol ester induced IL2 production in a concentration dependent manner with a 50% inhibitory concentration of about 10 microM. However, the ligand 3,5-diisopropylsalicylic acid also inhibited the induction of IL2 by phorbol esters (50% inhibitory concentration, 15 microM). Since the superoxide dismutase mimetic activity of CuDIPS is not stable in the presence of ethylenediaminetetraacetic acid, the effects of CuDIPS could be due to the free ligand and not to the intact metallocomplex. Consequently, a series of extremely stable copper(II) macrocyclic compounds was synthesized, and the reduction potential, superoxide dismutase mimetic activity, and ability to inhibit phorbol ester induced IL2 production were determined for each. Of the copper(II) macrocyclic complexes studied, only the most potent superoxide dismutase mimetic compound was found to inhibit phorbol ester induced IL2 production. Copper(II) complexes had to be added no later than 4 following phorbol ester administration to be effective inhibitors of the IL response, suggesting that these compounds act subsequent to the binding of phorbol esters but prior to the transcription of IL2 messenger RNA. Adherence of EL4 cells to substrate in response to phorbol esters was unaffected by copper(II) compounds. In summary, copper(II) compounds with appropriate reduction potentials can act within a defined time period to inhibit some, but not all, of the effects of phorbol esters on EL4 cells.

Bryostatin, an activator of the calcium phospholipid-dependent protein kinase, blocks phorbol ester-induced differentiation of human promyelocytic leukemia cells HL-60

Phorbol esters bind to and activate a calcium phospholipid-dependent protein kinase (C kinase). Some researchers believe that activation of C kinase is necessary for the induction of phorbol ester biologic effects. Our research indicates that bryostatin, a macrocyclic lactone that binds to the phorbol ester receptor in human polymorphonuclear leukocytes, also binds to this receptor in the human promyelocytic leukemia cell line, HL-60. Bryostatin activates partially purified C kinase from HL-60 cells in vitro, and when applied to HL-60 cells in vivo, it decreases measurable cytoplasmic C kinase activity. Unlike the phorbol esters, bryostatin is unable to induce a macrophage-like differentiation of HL-60 cells; however, bryostatin, in a dose-dependent fashion, blocks phorbol ester-induced differentiation of HL-60 cells and, if applied within 48 hr of phorbol esters, halts further differentiation. These results suggest that activation of the C kinase by some agents is not sufficient for induction of HL-60 cell differentiation and imply that some of the biologic effects of phorbol esters may occur through a more complex mechanism than previously thought.

Inhibition of phorbol ester-induced phenotypic differentiation of HL-60 cells by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a protein kinase inhibitor

To identify the possible role of calcium ions in cell differentiation, we studied the extracellular Ca2+ requirement and the effect of Ca2+/phospholipid-dependent protein kinase (protein kinase C) inhibitor on proliferation and differentiation of human promyelocytic leukemic HL-60 cells. HL-60 cells grew equally well in 0.1 and 1.0 mM Ca2+ media. The addition of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), 1,25-dihydroxyvitamin D3, and all-trans-beta-retinoic acid inhibited the cell growth and induced mature macrophage and granulocyte phenotypes in 1.0 mM Ca2+ medium. 1,25-Dihydroxyvitamin D3 and all-trans-beta-retinoic acid induced HL-60 differentiation to the same degree in 0.1 mM Ca2+ and 1.0 mM Ca2+ media. However, TPA failed to induce HL-60 differentiation or to inhibit proliferation in a 0.1 mM Ca2+ medium. The decrease of extracellular Ca2+ from 1.0 to 0.1 mM caused a significant drop in the intracellular Ca2+ level in undifferentiated and TPA-treated HL-60 cells, although no rapid change in cytosolic Ca2+ was detected in response to TPA addition. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor, inhibited proliferation of HL-60 cells in a dose-dependent manner. In contrast, H-7 selectively restored the proliferation of TPA-treated HL-60 cells and inhibited TPA-induced phenotypic differentiation. However, the same concentrations of 1-(5-isoquinolinylsulfonyl)-2,3-dimethylpiperazin and N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, analogues of H-7 that inhibit protein kinase C more weakly, had no effect on the proliferation or differentiation induction. H-7 also suppressed 1,25-dihydroxyvitamin D3- and all-trans-beta-retinoic acid-induced phenotypic changes of HL-60 cells but did not eliminate the growth inhibition by these inducers. These results demonstrate the Ca2+ requirement and the protein kinase C involvement in phorbol ester-induced phenotypic differentiation of HL-60 cells.

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.

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.

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.

Induction of chemotaxis in mouse peritoneal macrophages by phorbol ester tumor promoters

The ability of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), to induce chemotaxis in three different populations of mouse peritoneal macrophages was studied. TPA in the range of 10(-9) to 10(-7) M produced a dose- and time-related increase in chemotaxis in resident, thioglycollate-elicited, and divinyl ether maleic anhydride copolymer-activated macrophages. A maximal response was obtained after 4 hr incubation with 10(-7) M TPA, and this concentration of TPA was as effective as inducing chemotaxis as was endotoxin-activated mouse serum. Orientation of macrophages towards TPA was also observed by microscopy. Within 2 hr, cells exposed to TPA sent out cytoplasmic processes along the TPA gradient. Parallel arrays of cells oriented towards the TPA were observed after 4 hr incubation. Two other diterpene tumor promoters, phorbol-12,13-didecanoate and mezerein, were also chemotactic for the macrophages, as was the peptide epidermal growth factor, which shares a number of effects with TPA on cells in culture. On the other hand, two phorbol esters inactive as tumor promoters, 4-alpha-phorbol-12,13-didecanoate and phorbol, were not chemotactic for macrophages. Retinoic acid, which inhibits tumor promotion, inhibited TPA-induced, but not endotoxin-activated mouse serum-induced chemotaxis. These findings, taken together with previous studies, indicate that phorbol ester tumor promoters are potent modulators of macrophage function.

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.

Inhibition of erythema induced by pro-inflammatory esters of 12-deoxyphorbol

The pro-inflammatory tigliane esters 12-deoxyphorbolphenylacetate (12-DOPPA) and 12-deoxyphorbolphenylacetate-20-acetate (12-DOPPAA) at a dose of 0.1 microgram induced erythema in the mouse ear. Observations of ear redness were made both two and four hours after application. Indomethacin was only partly successful as an antagonist since 10% inhibition of 12-DOPPA and no inhibition of 12-DOPPAA induced erythema was produced four hours after application. The free radical scavengers, phenol, thioanisole and sodium benzoate all produced less than 30% inhibition of 12-DOPPA induced erythema and less than 15% inhibition of 12-DOPPAA, whereas aminopyrine produced 70% and 25% inhibition of 12-DOPPA and 12-DOPPAA respectively. The fact that free radical scavengers (with the exception of aminopyrine) and indomethacin, failed to markedly change the mouse ear reaction to 12-deoxyphorbol esters, indicated that this erythema is not entirely mediated via cyclooxygenase products. Mepyramine and cyproheptadine also failed to inhibit the erythema, whereas hydrocortisone produced a 55% inhibition of the 12-DOPPA and a 20% inhibition of the 12-DOPPAA reaction. The membrane stabilising agents trifluoperazine, promethazine, imipramine and desmethylimipramine were the most successful compounds used in inhibiting both 12-DOPPA and 12-DOPPAA induced erythema. In addition propranolol, which inhibits stimulus activation of phospholipase A2, produced 70% and 55% inhibition of the reaction of mice ears to 12-DOPPA and 12-DOPPAA.

Effect of phorbol ester application and other mitogenic treatments on 3',5'-cyclic-nucleotide phosphodiesterase activity in mouse epidermis in vivo

The effects of phorbol ester application and of other mitogenic treatments on the activity of 3',5'-cyclic-nucleotide phosphodiesterase were investigated in dorsal mouse epidermis in vivo. Local treatment with either the weak tumor promoter phorbol 12,13-dibenzoate or the strong promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) increased the activity of the high affinity enzyme (Km = 4 microM). The enzymic changes began within the first hour after application, and lasted for about 5 days. maximal stimulations of approximately 300--400% were reached after 3--6 h with TPA application, whereas with phorbol dibenzoate the maximum could only be reached after 1--2 days. TPA stimulation of the enzyme depended on doses within the range of 0.2 to 20 nmol and could be completely prevented by cycloheximide, but not by 5-azacytidine, actinomycin D, 5,8,11,14-eicosatetraynoic acid or indomethacin. No evidence could be found for cAMP participation in enzyme induction. An increase in enzyme activity could also be observed after other mitogenic treatments such as local application of the weakly promoting phorbol esters C14:4-phorbol acetate ("Ti8") and 4.O-methyl-TPA, or of the non-promoting divalent cation ionophore A 23187, as well as after treatment with a depilatory cream. Skin massage or removal of the horny layer, which also stimulate mitosis, did not evoke a significant increase in enzyme activity. No apparent correlation exists between the hyperplasiogenic and tumor-promoting effectiveness of a manipulation and its effect on epidermal 3',5'-cyclic-nucleotide phosphodiesterase.

Inhibition of intercellular communication by tumor-promoting phorbol esters

Cocultures were established of mouse epidermal cells (HEL/37) and mouse fibroblast cells (PG-19) deficient in the enzyme hypoxanthine-guanine phosphoribosyltransferase. Metabolic cooperation between the cocultured cells was detected as labeling of PG-19 cells on incubation of cocultures with [3H]-hypoxanthine. The transfer of label from HEL/37 cells to PG-19 cells was inhibited by the tumor prmoters 12-O-tetra-decanoylphorbol-13-acetate (10(-8) M) and phorbol-12,13-di-decanoate (10(-7) M) but not by nonpromoting derivatives of these phorbol esters. The inhibition was partially prevented by the antiinflammatory steroid fluocinolone acetonide, which is an antagonist of mouse skin tumor promotion, and by prolonged exposure of the cocultures to 12-O-tetradecanoylphorbol-13-acetate.

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.

Studies on mechanism of action of anti-tumor-promoting agents: their specificity in two-stage promotion

The effects of fluocinolone acetonide (FA), retinoic acid (RA), and tosylphenylalanine chloromethyl ketone (TPCK) on two-stage promotion after 7,12-dimethylbenz[a]-anthracene (DMBA) initiation in female Sencar mice were investigated. The two-stage promotion protocol was achieved by twice weekly applications of 2 microgram of 12-O-tetradecanoylphorbol 13-acetate (TPA) for 2 weeks (stage I) followed by twice weekly applications of mezerein for 18 weeks (stage II). Separately stage I and II do not cause any tumors to develop after DMBA initiation. FA was found to be a potent inhibitor of stages I and II but to a greater degree for stage I than for stage II. RA was ineffective in stage I but was a potent inhibitor of stage II; TPCK specifically inhibited stage I but not stage II. FA and TPCK effectively counteract the appearance of the dark basal keratinocytes, whereas RA has no effect. These results provide additional evidence for the importance of dark basal keratinocytes in stage I of promotion and indicate that most of the other biochemical and morphological responses normally associated with promotion (such as polyamines) are actually associated with stage II of promotion.

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.

Stimulation of prostaglandin production in bone by phorbol diesters and melittin

The production of prostaglandin E2 (PGE2) and bone resorption were studied in neonatal mouse calvaria in organ culture. Two tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and phorbol-12, 13-di-decanoate, but not the non-tumor promoters 4alpha-phorbol-12,13-didecanoate and phorbol, stimulated both PGE2 synthesis in bone and bone resorption. The effect of TPA was maximum at about 25 ng/ml, and half-maximum stimulation occurred at about 8 ng/ml TPA. The effects of TPA on the production of PGE2 and bone resorption were inhibited completely by indomethacin (5.6 X 10(-8) to 5.6 X 10(-7) M). The been venom toxin, melittin, was also a potent stimulator of prostaglandin synthesis in bone and bone resorption. The effect of melittin was maximum at about 25 ng/ml, and the dose-response curve was biphasic. The effects of melittin on the production of PGE2 and bone resorption were also inhbited by indomethacin. Indomethacin did not inhibit the bone resorption-stimulating activity of exogenously added PGE2. We conclude that phorbol diesters, which have irritant and tumor-promoting activity in mouse skin, and the polypeptide melittin can act directly on bone to stimulate resorption by a mechanism involving the local production of PGE2 or possible other indomethacin-inhibited metabolites odonic acid.

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.