Non-phorbol mouse skin tumor promoters do not mimic phorbol myristate acetate in its effects on chick embryo fibroblasts

Modulation of immune responses in mice by d-limonene

The immunotoxicologic effects of d-limonene were determined. This naturally occurring substance is widely used in food flavorings and is a common additive in cosmetics. In the present study, BALB/c mice were treated with d-limonene for 9 wk. Effects on T- and B-cell responses were determined after 4 and 8 wk of treatment. Concanavalin-A responses at 8 wk, but not 4 wk, were suppressed in treated mice. A similar trend was observed for phytohemagglutinin and lipopolysaccharide responses. Evidence was presented that d-limonene had polyclonal activator action. Mice primed with keyhole limpet hemocyanin (KLH) prior to initiation of d-limonene treatment had suppressed primary and secondary anti-KLH responses. Mice treated with d-limonene prior to KLH priming produced significant increased antibody responses. Additional evidence for polyclonal stimulation was obtained by histopathologic examination of secondary lymphoreticular tissue. Significant secondary follicle development and prominent lymphoid nodules and aggregates were found in the pancreas and intestinal mucosa, particularly apparent in mice receiving the highest d-limonene dosage. A subchronic LD50 study was conducted wherein BALB/c mice received 16 daily doses of d-limonene. An LD50 of approximately 0.0850 mg d-limonene/kg (corrected for 82% purity) was determined.

Phorbol ester receptors and the in vitro effects of tumor promoters

The evidence for the multistage nature of tumor promotion in vivo and for multiple subclasses of phorbol ester receptors in vitro argues that multiple mechanisms of tumor promotion exist. The existence of multiple mechanisms suggests that brute force assay for tumor promoters in vivo may be inadequate and that understanding of mechanisms may be essential. The interest in the phorbol esters is not primarily that they are environmental hazards for man, but rather that they provide a probe for phorbol ester receptors. These receptors are found in people, and modulation of their activity may play a role in tumor promotion in man.

Activation of monocyte and granulocyte antibody-dependent cytotoxicity by phorbol myristate acetate

We have characterized the effects of phorbol myristate acetate (PMA) on human monocyte and neutrophil oxidative metabolism and antibody-dependent cytotoxicity toward anti-D sensitized human erythrocytes (RBC) and a human lymphoblastoid cell line (CEM). Hexose monophosphate shunt activity was measured by [1-(14)C]glucose oxidation and target lysis by (51)Cr release. PMA produced a dose-dependent stimulation of hexose monophosphate shunt activity. Neutrophils responded with higher hexose monophosphate shunt activity and at a lower PMA concentration than did monocytes. PMA increased monocyte lysis of antibody-sensitized RBC by two-thirds, but did not affect lysis of CEM targets. Neutrophils were unable to lyse either antibody-sensitized or nonsensitized RBC without the addition of PMA. When PMA was added, lysis of both targets increased markedly. Neutrophils without PMA were able to lyse a small number of both antibody-sensitized and nonsensitized CEM targets. PMA also increased neutrophil lysis of these targets. Target lysis by neutrophils from a patient with chronic granulomatous disease, cells unable to produce reactive oxygen species, was not increased by PMA. Chronic granulomatous disease monocytes, however, responded to PMA by more than doubling lysis of antibody-sensitized RBC. Hypoxia inhibited PMA augmentation of antibody-sensitized RBC lysis by neutrophils, but not by monocytes. Generation of reactive oxygen species by the xanthine-xanthine oxidase system inhibited CEM growth, but did not cause lysis, indicating that in some cases oxidative injury may be nonlytic. We suggest that PMA augments neutrophil cytotoxicity to tumor and RBC targets by stimulating reactive oxygen species-mediated lysis, but in monocytes augmentation of lysis is due to activation of a nonoxidative mechanism of lysis.

Inhibition by zinc of hepatic lysosomal release of beta-glucuronidase induced by phorbol-12-myristate-13-acetate (PMA)

The tumor promoter, phorbol-12-myristate-13-acetate (PMA), elicited a significant increase in lysosomal membrane permeability, as measured by release of free beta-glucurnidase, in guinea pig and hamster isolated hepatic lysosomes in vitro. This response was antagonized by zinc sulfate; zinc compounds having previously been shown to inhibit tumor growth at multiple sites in several species. Since phorbol ester tumor promotion is likely associated in part with actions on cellular membrane systems, its inhibition by zinc may relate to the membrane stabilizing activity of zinc compounds.

Specific binding of phorbol ester tumor promoters

[20-(3)H]Phorbol 12,13-dibutyrate bound to particulate preparations from chicken embryo fibroblasts in a specific, saturable, reversible fashion. Equilibrium binding occurred with a K(d) of 25 nM; this value is very close to the 50% effective dose (ED(50)), 50 nM, previously determined for the biological response (induction of fibronectin loss) in growing chicken embryo fibroblasts. At saturation, 1.4 pmol of [20-(3)H]phorbol 12,13-dibutyrate was bound per mg of protein (approximately 7 x 10(4) molecules per cell). Binding was inhibited by phorbol 12-myristate 13-acetate (K(i) = 2 nM), mezerein (K(i) = 180 nM), phorbol 12,13-dibenzoate (K(i) = 180 nM), phorbol 12,13-diacetate (K(i) = 1.7 muM), phorbol 12,13,20-triacetate (K(i) = 39 muM), and phorbol 13-acetate (K(i) = 120 muM). The measured K(i) values are all within a factor of 3.5 of the ED(50) values of these derivatives for inducing loss of fibronectin in intact cells. Binding was not inhibited by the inactive compounds phorbol (10 mug/ml) and 4alpha-phorbol 12,13-didecanoate (10 mug/ml) or by the inflammatory but nonpromoting phorbol-related diterpene esters resiniferatoxin (100 ng/ml) and 12-deoxyphorbol 13-isobutyrate 20-acetate (100 ng/ml). These data suggest that biological responses to the phorbol esters in chicken embryo fibroblasts are mediated by this binding activity and that the binding activity corresponds to the phorbol ester target in mouse skin involved in tumor promotion. Binding was not inhibited by the nonphorbol promoters anthralin (1 muM), phenol (1 mM), iodoacetic acid (1.7 muM), and cantharidin (75 muM), or by epidermal growth factor (100 ng/ml), dexamethasone acetate (2 muM), retinoic acid (10 muM), or prostaglandin E(2) (1 muM). These agents thus appear to act at a target distinct from that of the phorbol esters.

Induction of differentiation in human promyelocytic leukemia cells by tumor promoters

Phorbol diester tumor promoters and the promoter mezerein convert human promyelocytic leukemia cells in culture into adherent, nonproliferating cells with many of the characteristics of macrophages. Other types of promoters such as anthralin, phenobarbital, and saccharin do not have this effect. Various compounds that can inhibit some of the biological and biochemical effects of tumor promoters do not interfere with the induction of cell adherence and differentiation by the effective promoters.