Retinoid metabolism in the prostate: effects of administration of the synthetic retinoid N-(4-hydroxyphenyl)retinamide.

We have carried out a series of complementary in vivo and in vitro studies to better understand the metabolism of vitamin A by the prostate gland. Male Sprague-Dawley rats were fed either a control diet sufficient in vitamin A [CON group; 0.8 microg retinol equivalents (RE)/g diet] or a CON diet supplemented with the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR; CON+/-4HPR group; 1,173 )microg of 4-HPR/g diet). After an i.v. injection of a physiological radiolabeled dose of retinol, the vitamin A content and radioactivity of plasma and a number of tissues, including the prostate glands, were monitored for time periods ranging between 30 min and 41 days. On the basis of the results of these vitamin A turnover studies, we developed tissue subsystem models to describe vitamin A dynamics in the prostates of both the CON and CON+4HPR groups. There was a gradual decrease in the vitamin A content of the prostates of the 4-HPR-treated group as compared with the control, such that by the end of the study period, the CON+4HPR group averaged 0.166 +/- 0.0827 (mean +/- SD) REs, whereas the CON group was 0.732 +/- 0.190 REs. The fraction of vitamin A exiting the prostate each day was not significantly different in the CON as compared with the CON+4HPR group [0.149 +/- 0.103 versus 0.155 +/- 0.191 h(-1) (mean +/- FSD), respectively]; however, the average amount of vitamin A turning over from the CON+4HPR group prostates (0.0885 /microg/day) was nearly three times less than that of the CON group (0.243 microg/day). To obtain more detailed information on the mechanisms that might be involved in the changes in vitamin A kinetics observed in our in vivo studies, we used both a normal human prostate cell line (PrEC) and a human prostate adenocarcinoma cell line (LNCaP) to monitor in vitro retinol and 4-HPR dynamics. Cells were treated with 4-HPR for different time periods up to 48 h (PrEC) or 96 h (LNCaP). Retinol in the media was taken up readily by both PrEC and LNCaP cells, and there was conversion of retinol to the major storage esters of vitamin A, retinyl palmitate and retinyl stearate, as well as several minor retinyl esters, in a pattern indicative of normal retinoid esterification activity. Although 4-HPR was taken up readily and over time accumulated in both cell lines, conversion of 4-HPR to its major metabolite, N-[4-methoxyphenyl]retinamide, as well as several other metabolites of 4-HPR was apparent only in the LNCaP cells. Our findings would suggest that a study design that includes appropriately designed complementary in vivo and in vitro experimental systems represents a useful approach to better understanding possible mechanisms involved in basic retinoid functioning and interactions in the prostate as well as in other organs and related tissue culture systems.

Effect of glutathione depletion and metallothionein gene expression on arsenic-induced cytotoxicity and c-myc expression in vitro.

Arsenic exposure is clearly linked to human cancer. In rodent cells, arsenic has been reported to induce aberrant gene expression, including activation of the proto-oncogene c-myc. Abnormal or altered expression of such oncogenes can be involved in the acquisition of a malignant phenotype. Although its mechanism of action is unclear, arsenic is known to exert at least some of its toxic effects through interaction with sulfhydryl groups, and the non-protein sulfhydryl glutathione (GSH) appears to play an important role in detoxication of arsenic. Similarly, metallothionein (MT), a metal-binding protein with high sulfhydryl content, often functions in defense against metal-induced or oxidative cellular injury. Therefore, we examined the relationship among GSH, MT gene expression, and arsenic-induced toxicity or c-myc expression in cultured rat myoblast (L6) cells. In initial toxicity studies, arsenic was used in both the trivalent (arsenite) and pentavalent (arsenate) forms. The role of GSH was studied by pretreating cells with L-buthionine sulfoximine (BSO), which induces a marked depletion of GSH. In vitro exposure of L6 cells to BSO (1 to 25 microM) resulted in dose-dependent decreases in GSH. GSH depletion sensitized cells to both arsenite and arsenate. Zinc pretreatment, at levels which highly activated MT expression, had no effect on arsenite-induced cytotoxicity. Arsenite (1 microM) alone modestly increased c-myc expression from 1 to 4 h after treatment (maximum of 2.0-fold over control). After GSH depletion cells responded to arsenite exposure with much larger increases in c-myc transcription (3.2-fold over control). Zinc pretreatment had no reductive effect on arsenite-induced c-myc expression despite markedly activating the MT gene. Thus, it appears that the cellular levels of GSH, but not MT gene expression, play an important role in resistance to arsenic toxicity and aberrant gene activation. Moreover, depletion of GSH enhances arsenic-induced proto-oncogene activation, which might contribute to subsequent transformation.

Epidermal growth factor and transforming growth factor-alpha-associated overexpression of cyclin D1, Cdk4, and c-Myc during hepatocarcinogenesis in Helicobacter hepaticus-infected A/JCr mice.

Helicobacter hepaticus is a new bacterial species that is homologous to Helicobacter pylori, a human gastric carcinogen. H. hepaticus causes chronic active hepatitis, with progression to hepatocellular tumors. We hypothesized that chronic up-regulation of epidermal growth factor (EGF), transforming growth factor-alpha, and nuclear oncogenes (cyclin D1 and c-Myc), all known to transform by overexpression, might contribute to tumorigenesis. Livers from mice that were 6-18 months old were analyzed, including nonneoplastic and preneoplastic tissues and tumors, along with age-matched controls, by immunohistochemistry and immunoblotting. EGF and transforming growth factor-alpha were increased at the earliest stage, with a further increase in EGF in tumors. Cyclin D1, cyclin-dependent kinase 4, and c-Myc were strongly increased in all infected livers, with even greater increases in tumors. An increase in cyclin D1/cyclin-dependent kinase 4 complex was also demonstrated in tumors, and its functionality was confirmed by an increase in the hyperphosphorylated:hypophosphorylated retinoblastoma protein ratio. Our findings suggest a possible cooperation of growth factors, cell cycle proteins, and transcription factors during the development of H. hepaticus-associated liver tumors and may have relevance to human cancers associated with bacterial, viral, or parasitic infections.

Effects of glutathione depletion on cadmium-induced metallothionein synthesis, cytotoxicity, and proto-oncogene expression in cultured rat myoblasts.

Cadmium (Cd) is a highly toxic metal and a known carcinogen. Although the carcinogenic mechanism of action is unknown, Cd will induce transcriptional activation of c-myc and c-jun. We have previously found that the extent of Cd-induced oncogene expression is limited by the presence of cellular metallothionein (MT) in rat L6 myoblasts. Glutathione (GSH) is thought to play an important role in protection against Cd before the onset of MT synthesis. Thus, this study examined the effects of GSH depletion on Cd-induced MT synthesis, cytotoxicity, and proto-oncogene expression in rat L6 myoblasts after pretreatment with L-buthionine sulfoximine (BSO), a potent inhibitor of gamma-glutamyl-cysteine synthetase, which effectively depletes GSH. Exposure of L6 cells to BSO (5 or 25 microM) resulted in a dose-dependent decrease in cellular GSH levels. GSH depletion had no effect on Cd- or zinc-induced MT synthesis. Although the depletion of GSH was not itself cytotoxic in L6 cells, BSO pretreatment, particularly at the higher dose (25 microM), resulted in a dose-dependent increase in the sensitivity to Cd cytotoxicity, as assessed by a tetrazolium-based dye (MTT) assay. Low levels of Cd (1 microM) slightly increased the expression of both c-myc and c-jun as assessed by increases in gene-specific mRNA levels, in accordance with previous studies. GSH depletion (5 muM BSO) likewise caused an increase in expression of c-myc and c-jun. However, combined GSH depletion and Cd exposure decreased levels of c-myc and c-jun transcription well below control levels. These results suggest that increased cytotoxicity resulting from exposure to Cd after BSO depletion of cellular GSH abrogates the oncogene activation observed after either treatment alone. Thus proto-oncogene expression induced by Cd appears to be dependent on the absence of over Cd-induced cytotoxicity.

Sequence of exposure to cadmium and arsenic determines the extent of toxic effects in male Fischer rats.

Arsenic and cadmium are both priority hazardous substances and human carcinogens. Although there is the potential for simultaneous exposure to both metals, the interactions of cadmium and arsenic are not well defined. We examined the toxicity of these metals when given alone or in alternating sequence to adult male Fischer rats. In the first study, a non-toxic dose of arsenic (22.5 micromol NaAsO2/kg, s.c.) was given 24 h before cadmium (10, 20, or 30 micromol CdCl2/kg, s.c.) and toxicity was assessed 24 h later. Arsenic pretreatment markedly reduced mortality in rats given the high dose of cadmium (9 survivors/10 treated) compared to rats given cadmium alone (2/10). Arsenic pretreatment also reduced cadmium-induced hepatotoxicity, as indicated by serum glutamic oxalacetic transaminase (SGOT) activity, and markedly reduced cadmium-induced testicular hemorrhagic necrosis. Arsenic pretreatment produced an 8-fold increase in hepatic levels of metallothionein (MT), a metal-binding protein often associated with cadmium tolerance. In the second study, a non-toxic dose of cadmium (3 micromol CdCl2/kg, s.c.) was given 24 h before arsenic (68, 79, 84, or 90 micro/mol NaAsO2/kg. s.c.) and toxicity was assessed 24 h later. Cadmium pretreatment did not alter the lethality of the high dose of arsenic and had no effect on arsenic-induced hepatotoxicity. Although cadmium pretreatment had no effect on arsenic toxicity, it produced large increases in hepatic MT (26-fold) before the arsenic challenge and greatly enhanced MT induction after the challenge. Thus, even though both arsenic and cadmium induce MT synthesis, only arsenic pretreatment protects against cadmium intoxication, and cadmium pretreatment does not effect arsenic toxicity. Thus, toxic interactions of arsenic and cadmium appear to depend on the sequence of exposure.

Testosterone pretreatment mitigates cadmium toxicity in male C57 mice but not in C3H mice.

Previous work has indicated that testosterone pretreatment protects against cadmium-induced toxicity in male rats while other data indicate that pretreatment of mice with testosterone offers no such protection against cadmium. Since cadmium toxicity may vary widely with species and strain, we examined the effect of testosterone pretreatment on cadmium toxicity in two strains of mice, one that is sensitive (C3H) and one that is resistant (C57) to cadmium toxicity. A single sc injection of 20 micromol CdCl2/kg to C3H mice or 45 micromol CdCl2/kg to C57 mice proved very toxic, causing 50%, and 44% mortalities, respectively. However, when C57 mice were pretreated with testosterone (5 mg/kg, s.c., at - 48, - 24, and 0 h) prior to cadmium (45 micromol/kg), complete resistance to cadmium-induced lethality developed. Testosterone had no effect on cadmium-induced lethality in C3H mice. Testosterone prevented extensive hepatocellular damage caused by cadmium in C57 mice and also significantly reduced cadmium-induced elevations in serum lactate dehydrogenase (LDH) activity and blood urea nitrogen (BUN), which are indicators of hepatic and renal function, respectively. The toxicokinetics of cadmium were apparently not affected by testosterone pretreatment, as the distribution of cadmium to liver in either strain was unchanged by the steroid. Cadmium-induced metallothionein (MT) levels in liver and kidney of C57 mice were increased in testosterone-pretreated mice given the higher doses of metal but no such enhancement of MT synthesis occurred in C3H mice. This increase in MT may provide some level of protection against cadmium toxicity in the C57 mice. These results indicate that testosterone pretreatment prevents toxicity of cadmium in male C57 mice, possibly through enhancement of MT synthesis, but has no effect in male C3H mice.

Progesterone pretreatment enhances cellular sensitivity to cadmium despite a marked activation of the metallothionein gene.

Previously, we found that in vivo pretreatment with progesterone markedly increased cadmium lethality in rats, apparently by enhancing cadmium-induced hepatonecrosis. Therefore, the present study was designed to investigate this phenomenon at the molecular level in an in vitro system. TRL-1215 rat liver cells were exposed to various concentrations of progesterone (0, 1, 10, and 100 microM) for 24 hr and subsequently exposed to cadmium (0, 1, 5, 10, and 50 microM; as CdCl2) for an additional 24 hr. Although the levels of progesterone used were essentially nontoxic, progesterone pretreatment resulted in a concentration-dependent increase in sensitivity to cadmium as assessed by loss of mitochondrial enzyme activity (tetrazolium-based dye assay) and loss of cytosolic enzyme activity (glutamic oxaloacetic transaminase). The effects of progesterone treatment on intracellular levels of metallothionein (MT), an inducible metal-binding protein generally associated with cadmium tolerance, were also measured. Progesterone (100 microM) alone increased MT levels 2.4-fold, while cadmium (10 microM) alone resulted in a 7-fold increase over control. Progesterone pretreatment followed by cadmium exposure caused a marked, 16-fold induction in MT synthesis, a level of activity that has been associated with acquired tolerance to cadmium. In addition, progesterone pretreatment clearly induced transcription of the MT gene as evidenced by enhanced cadmium-induced accumulation of cellular MT mRNA. Progesterone pretreatment had no effect on the level of glutathione, a cellular thiol thought to be important in detoxication of cadmium prior to MT gene activation and MT protein accumulation, or on cellular accumulation of cadmium during the initial 3 hr of exposure to the metal. The proportion of total cellular cadmium bound to MT in cells pretreated with progesterone was greater than that in the cells treated with cadmium alone, indicating an enhanced sequestration of the metal by MT after pretreatment. These results indicate that progesterone, at nontoxic levels, markedly exacerbates cadmium toxicity at the cellular level in liver cells. This is in accord with the observed progesterone-induced enhancement of the hepatotoxic effects of cadmium in vivo. The observed facilitation of cytotoxicity is not based in altered toxicokinetics of cadmium and occurs despite a pronounced activation of the MT gene resulting in an enhanced sequestration of cadmium by MT. The mechanism by which progesterone enhances cadmium toxicity deserves further study.

Evidence that nitric oxide enhances cadmium toxicity by displacing the metal from metallothionein.

Cadmium is carcinogenic in humans and rodents. Although extensive evidence indicates that the toxicity and genotoxicity of Cd is ameliorated by binding to cysteine clusters in metallothionein (MT), the factors governing Cd release at intracellular target sites remain unknown. Nitric oxide is a pollutant gas and an important intercellular messenger in the inflammatory immune response. When growing Chinese hamster ovary cells were treated for 24 h with 0.5, 0.75, or 1.0 mM CdCl2 followed by a 1-h exposure to 1.0, 1.5, or 2.0 mM 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO), an NO-generating sodium salt, NO enhanced Cd-induced inhibition of colony forming ability without affecting Cd-induced cytolethality. In experiments designed to determine whether NO acts by displacing Cd from cellular MT, cells treated with 2.0 mM CdCl2 followed by 1.5 or 3.0 mM DEA/NO exhibited 29 and 38% reductions, respectively, in the amount of Cd bound to MT. When purified rat liver MT was used to further characterize NO-induced release of Cd from MT, dose-related increases in Cd displacement were observed at DEA/NO concentrations between 0.1 and 0.5 mM, and a plateau was reached at 3 mol of Cd displaced/mol of MT at higher DEA/NO concentrations. Compared to cells exposed to Cd or DEA/NO alone, cells treated with Cd followed by DEA/NO also exhibited a transient 2-3-fold decrease in c-myc proto-oncogene expression. Taken together, our results support the hypothesis that NO mediates Cd release from MT in vivo and suggest that intracellular generation of free Cd may induce DNA damage and force cells into a period of growth arrest. Such findings may have particular relevance with regard to the etiology of Cd-induced carcinogenesis in human populations.

Sensitivity to cadmium-induced genotoxicity in rat testicular cells is associated with minimal expression of the metallothionein gene.

Cadmium is a carcinogenic metal. Although the mechanism of tumor induction is unknown, DNA/metal interactions may be involved. Metallothionein can protect against cadmium toxicity in our previous work it was shown to reduce cadmium genotoxicity in cultured cells. To extend these results, the genotoxicity of cadmium was studied in R2C cells, a rat testicular Leydig cell line. The R2C cells were very sensitive to cadmium-induced single-strand DNA damage (SSD), as measured by alkaline elution. SSD occurred in R2C cells after treatment with 25 and 50 microM CdCl2 for 2 hr. Prior work showed other cells required much higher levels of cadmium (approximately 500 microM) to induce genotoxicity. The genotoxic levels of cadmium (25-50 microM) were not cytotoxic in R2C cells as assessed by a metabolic activity (MTT) assay. Pretreatment of R2C cells with a low cadmium dose (2 microM, 24 hr) had no effect on cadmium-induced SSD, in contrast to prior work in other cells where such pretreatments reduced SSD through metallothionein gene activation. In fact, cadmium or zinc treatments resulted in little or no increase in metallothionein gene expression in R2C cells as determined by Northern blot analysis for metallothionein mRNA using cDNA or oligonucleotide probes and radioimmunoassay for metallothionein protein production. Basal metallothionein mRNA was essentially nondetectable. Induction of a cadmium-binding protein in R2C cells did occur, as determined by Cd-heme assay, but did not induce tolerance to SSD. In vivo, the Leydig cell is a target for cadmium carcinogenicity and its cadmium-binding protein is thought not to be a true metallothionein. These results indicate that R2C cells are sensitive to cadmium-induced genotoxicity and that this sensitivity is associated with minimal expression of the metallothionein gene.

Protective effects of passively transferred immune T- or B-lymphocytes in mice infected with Salmonella typhimurium.

Purified populations of bone marrow-derived (B-) lymphocytes and thymus-derived (T-) lymphocytes were obtained from C3D2F1 hybrid mice shown to be immune to Salmonella typhimurium. These subpopulations of lymphocytes were injected into normal mice; four days later the animals were challenged with 50 50% lethal doses of S. typhimurium, and viable bacteria in livers, spleens, and blood were counted at various intervals after challenge. On day 8 after challenge, the mice supplemented with B-lymphocytes showed a significant decrease in the number of organisms recovered from all three sites, compared with that seen in recipients of T-lymphocytes and in controls. The mice given B-lymphocytes showed a better rate of survival (65%) than mice that received only T-lymphocytes (21%) or T-lymphocyte fractions contaminated 10%-30% with B-lymphocytes (49%). These data indicate that, although the humoral response is not totally protective, it does play an important role in the suppression of the infection during its early stages.