The Role of Free Radicals in Tumor Promotion: Oxy Radical Production by Murine Peritoneal Macrophages In Vivo in Response to Tumor Promoters

Enhancement of N-Nitrosodiethylamine-lnitiated Hepatocarcinogenesis by Phenytoin in Male F344/NCr Rats at a Dose Causing Maximal Induction of CYP2B

The effect of the clinically important anticonvulsant phenytoin (DPH) on hepatocarcinogenesis of male F344/NCr rats initiated with a single i.p. dose of N-nitrosodiethylamine (75 mg/kg b.w.) was studied. Beginning 2 weeks post-initiation, the rats received control diet or diet containing 500 or 1500 ppm DPH or 500 ppm phenobarbital. At 52 weeks age, the incidences (and multiplicities, in units of tumors per tumor-bearing rat) of hepatocellular adenomas were 0%, 17% (1 ± 0), 42% (1.8 ± 0.8), or 67% (2.5 ± 1.9) in rats exposed to N-nitrosodiethylamine alone, or the carcinogen followed by 500 ppm DPH, 1500 ppm DPH, or 500 ppm phenobarbital, respectively. Between 53 and 79 weeks of age, 39% of rats receiving N-nitrosodiethylamine alone developed multiple (1.5 ± 0.8) hepatocellular adenomas. A similar incidence (41%) occurred in the rats administered the carcinogen followed by 500 ppm DPH. The incidence of hepatocellular adenomas (88% and 89%) was significantly greater in rats exposed to N-nitrosodiethylamine followed by 1500 ppm DPH or 500 ppm phenobarbital, respectively. Multiplicities of hepatocellular adenomas were significantly greater than the control value in rats fed 1500 ppm DPH or 500 ppm phenobarbital (5.9 ± 4.8 and 10.1 ± 6.7, respectively), but not in the rats receiving 500 ppm DPH (2.3 ± 1.6). No rats exposed to N-nitrosodiethylamine alone or the carcinogen followed by 500 ppm DPH developed hepatocellular carcinomas, while hepatocellular carcinomas occurred in 29% or 67% of the rats given 1500 ppm DPH or 500 ppm phenobarbital, respectively, following initiation. Increases in hepatic CYP2B-mediated benzyloxyresorufin O-dealkylation activity in rats exposed to 500 and 1500 ppm DPH for 2 or 23 weeks were ∼50% and ∼100%, respectively, of the maximal induction caused by 500 ppm phenobarbital. Thus, in the rat model, DPH enhanced N-nitrosodiethyl-amine-initiated hepatocarcinogenesis when administered at a dose causing maximal CYP2B induction.

Transforming growth factor-alpha expression in peritoneal macrophages elicited from SENCAR and B6C3F1 mice: responses to lipopolysaccharide and 12-O-tetradecanoylphorbol-13-acetate

Recent findings in our laboratory indicated that peritoneal macrophages (MPs) elicited from phorbol ester-sensitive SENCAR mice generated significant amounts of superoxide when stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) in vitro; negligible responses were observed for MPs derived from relatively resistant B6C3F1 mice. We hypothesized a similar strain-dependent secretion of transforming growth factor-alpha (TGF-alpha) by TPA-stimulated MPs. TGF-alpha secreted by MPs was quantitated by competitive enzyme-linked immunosorbent assay. After 72 h (maximal secretion), for MPs derived from B6C3F1 mice, in vitro exposure to 10 microgram/mL lipopolysaccharide (LPS; non-lipid-A-detoxified) resulted in maximal induction (708 pg/mL), in vivo exposure to intraperitoneally (i.p.) administered TPA (2 micrograms/mouse) alone resulted in a minimal response (32 pg/mL), and prior in vivo exposure to TPA significantly inhibited (more than 90% suppression) the LPS-stimulated MP response in culture (i.e., to 62 pg/mL). Although significant amounts of TGF-alpha could be detected in both SENCAR- and B6C3F1-derived MPs (i.e., approximately 2-3 ng/5 x 10(6) cells), SENCAR MPs did not secrete TGF-alpha in response to either TPA or LPS. In addition, the use of the semiquantitative reverse transcription-polymerase chain reaction to detect TGF-alpha-specific mRNA did not support the strain dependency observed for LPS-stimulated TGF-alpha secretion, i.e., detectable transcripts were observed in MP RNA derived from both strains. In conclusion, although TPA itself demonstrated negligible effects on TGF-alpha expression in murine MPs, prior in vivo exposure inhibited LPS-stimulated transcriptional activation and intracellular TGF-alpha production. The negligible TGF-alpha secretion determined for LPS-stimulated SENCAR-derived MPs suggested the possibility of a strain-specific defect in the posttranslational processing of the proTGF-alpha molecule.

Alterations in the morphology and functional activity of bone marrow phagocytes following benzene treatment of mice

Benzene is a well-established hematotoxin that affects developing leukocytes and erythrocytes as well as bone marrow stromal cells. In the present studies we analyzed the effects of benzene on the morphology and functional activity of bone marrow phagocytes. Male Balb/c mice were treated with benzene (660 mg/kg) once per day for 3 days. Bone marrow cells were then isolated and fractionated by density gradient centrifugation. Using highly sensitive techniques in flow cytometry/cell sorting, we found that we could separate three distinct populations of bone marrow cells that differed with respect to size and density. Monoclonal antibody binding and cell sorting revealed a large, dense population that consisted predominantly of granulocytes, a smaller, less dense population of lymphocytes, and a population of intermediate size and density consisting of mononuclear phagocytes and precursor cells. Differential staining of sorted mononuclear phagocytes revealed that benzene treatment of mice caused a marked increase in the number of mature, morphologically activated macrophages in the bone marrow. Benzene treatment of mice also resulted in enhanced chemotaxis and production of hydrogen peroxide by bone marrow granulocytes and mononuclear phagocytes. In contrast, treatment of mice with the combination of hydroquinone and phenol (50 mg/kg each, 1 x/day, 3 days), two metabolites of benzene, resulted in a significant (p < or = 0.02) depression of granulocyte chemotaxis and had no effect on hydrogen peroxide production by bone marrow phagocytes compared to cells from control animals. Taken together these results demonstrate that benzene causes increased differentiation and/or activation of phagocytes in the bone marrow.

In vivo and in vitro studies on the effects of vitamin A on the chemiluminescent response of murine peritoneal exudate cells

Retinoids are known to enhance macrophage function and enhance bacterial clearance during experimental infection. The purpose of these experiments was to determine if one indicator of macrophage activation, chemiluminescence (CL), was enhanced by retinoids. Peritoneal exudate cells (PEC) harvested from mice injected intraperitoneally for 5 days with retinol palmitate (25, 125, or 250 U/g body wt./d) showed significantly enhanced chemiluminescence (CL) when exposed to opsonized zymosan. A direct dose-response effect was observed, in that the more retinol palmitate was injected, the more CL was observed. The same dose of retinol palmitate injected subcutaneously did not result in enhanced CL. In vitro incubation of murine PEC with physiological concentrations of retinol palmitate or retinoic acid for 1, 6 or 24 h did not result in enhanced CL. The reasons for the effect of the route of administration of retinol palmitate are unknown, but may include poor absorption from the site of subcutaneous injection or an adjuvant effect when injected intraperitoneally due to the particulate nature of the water-dispersible retinol palmitate preparation.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

Free radicals and carcinogenesis

The role of free radicals and active states of oxygen in human cancer is as yet unresolved. Various lines of evidence provide strong but inferential evidence that free radical reactions can be of crucial importance in certain carcinogenic mechanisms. A central point in considering free radical reactions in carcinogenesis is that human cancer is really a group of highly diverse diseases for which the initial causation and the progression to clinical disease occur through a wide variety of mechanisms. Furthermore, for many human cancers it appears that there are alternate pathways capable of tumor initiation and tumor progression. While for certain of these pathways free radical reactions appear necessary, it is unlikely that there are human cancers for which free radicals, or any other mechanism, are sufficient for the entire process beginning with the genetic alteration leading to a somatic mutation and eventually resulting in clinically overt disease. It is crucial that we view free radical reactions as among a panoply of mechanisms leading to human cancer, and consider research about the role of free radicals in cancer as opportunities to prevent the initiation or progression of human cancer.

Comparative effects of pair-feeding and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on various biochemical parameters in female rats

Hypophagia is a common characteristic of the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and may be responsible for some of the toxic manifestations. Pair-feeding has been used in control animals to compensate for the hypophagia, but relatively few studies have assessed biochemical changes associated with pair-feeding versus weight loss induced by TCDD. Rats were treated with TCDD and killed 7 days post-treatment while pair-fed animals received an amount of diet equivalent to TCDD-treated partner animals. Ad libitum-fed rats were also used. No correlations were seen in altered calcium and iron homeostasis between pair-feeding and TCDD administration relative to ad libitum-fed animals. Pair-feeding resulted in greater alterations than TCDD administration in the subcellular distribution of iron in mitochondria, microsomes and cytosol. Pair-feeding also resulted in greater accumulation of calcium in mitochondria and microsomes in pair-fed as compared to TCDD-treated animals. Greater lipid peroxidation was observed in whole liver and nuclei of rats receiving TCDD relative to pair-fed animals. A significantly greater incidence of DNA single strand breaks occurred in hepatic nuclei of TCDD-treated animals as compared to pair-fed and ad libitum-fed animals. Significantly greater inhibition of hepatic glutathione peroxidase activity and thymic involution were observed in TCDD treated animals as compared to the pair-fed group. Although some similarities existed between TCDD-treated animals and pair-fed rats, the overall biochemical changes which were observed following TCDD administration cannot be attributed to weight loss associated with hypophagia.