Chemoprevention of pancreatic cancer: a review of the molecular pathways involved, and evidence for the potential for chemoprevention.

BACKGROUND: Pancreatic cancer has a poor prognosis. The use of drugs or natural agents which inhibit or slow down tumour growth therefore has important potential in the development of future therapies. METHODS: A literature search of the PubMed and ISI Web of Science databases was undertaken to review the current data available on the alterations in signalling pathways found in pancreatic carcinogenesis, in order to identify sites that could be targeted by chemopreventive agents. Several agents of particular relevance to pancreatic cancer were identified, and their possible mechanisms of action reviewed. RESULTS: Chemopreventive agents such as non-steroidal anti-inflammatory drugs, green tea constituents, and antioxidants have been shown to target various steps in intracellular signalling pathways, particularly those controlling cell proliferation and survival. Work on cell lines and animal models has shown that some of these agents may be able to modulate the growth of pancreatic tumours. Initial clinical trials of some chemopreventives in pancreatic cancer have been undertaken, and have yielded mixed results, prompting the need for further studies. CONCLUSION: As the molecular pathology of pancreatic cancer becomes better understood, sites of action of chemopreventive substances have been uncovered. Several agents have shown promising results by their ability to inhibit pancreatic carcinogenesis in laboratory studies. If these effects can be successfully translated into human studies then these agents may prove to be valuable adjuvant therapies in the future.

Prospects for plant-derived chemopreventive agents exhibiting multiple mechanisms of action.

There is great potential for the use of plant-derived agents in the fight to prevent onset or delay progression of the carcinogenic process. Epidemiological evidence for their chemopreventive action is compelling, but even though many of these compounds have an extensive history of use within the human populace, it is of increasing importance to determine more precisely the primary targets contributing to their efficacy, prior to embarking on large-scale clinical trials. This rapidly moving field now concentrates in particular, on the modulating effects these agents can have on cellular signalling pathways involved in the apoptotic, proliferative and angiogenic processes, perturbances to which, are common in many cancers. It is perhaps the ability of these agents to exhibit multi-site mechanisms of action that offers their key to success where conventional single-site agents have disappointed in the past. As well as being promising chemopreventive agents, there is also an exciting role for these compounds in combinatorial therapy with more traditional chemotherapeutics, potentially in lowering of toxicity and enhancing efficacy for treatment of more advanced cancers. This review will summarise known and proposed mechanisms of action for various chemopreventive agents of interest highlighting their potential in combination therapy, and will address benefits and problems of using such multi-site agents in long-term prevention/therapeutic regimes.

Pharmacodynamic and pharmacokinetic study of oral Curcuma extract in patients with colorectal cancer.

Curcuma spp. extracts, particularly the dietary polyphenol curcumin, prevent colon cancer in rodents. In view of the sparse information on the pharmacodynamics and pharmacokinetics of curcumin in humans, a dose-escalation pilot study of a novel standardized Curcuma extract in proprietary capsule form was performed at doses between 440 and 2200 mg/day, containing 36-180 mg of curcumin. Fifteen patients with advanced colorectal cancer refractory to standard chemotherapies received Curcuma extract daily for up to 4 months. Activity of glutathione S-transferase and levels of a DNA adduct (M(1)G) formed by malondialdehyde, a product of lipid peroxidation and prostaglandin biosynthesis, were measured in patients' blood cells. Oral Curcuma extract was well tolerated, and dose-limiting toxicity was not observed. Neither curcumin nor its metabolites were detected in blood or urine, but curcumin was recovered from feces. Curcumin sulfate was identified in the feces of one patient. Ingestion of 440 mg of Curcuma extract for 29 days was accompanied by a 59% decrease in lymphocytic glutathione S-transferase activity. At higher dose levels, this effect was not observed. Leukocytic M(1)G levels were constant within each patient and unaffected by treatment. Radiologically stable disease was demonstrated in five patients for 2-4 months of treatment. The results suggest that (a) Curcuma extract can be administered safely to patients at doses of up to 2.2 g daily, equivalent to 180 mg of curcumin; (b) curcumin has low oral bioavailability in humans and may undergo intestinal metabolism; and (c) larger clinical trials of Curcuma extract are merited.

Increased bioactivation of dihaloalkanes in rat liver due to induction of class theta glutathione S-transferase T1-1.

A characteristic feature of the class Theta glutathione S-transferase (GST) T1-1 is its ability to activate dichloromethane and dibromoethane by catalysing the formation of mutagenic conjugates. The level of the GSTT1 subunit within tissues is an important determinant of susceptibility to the carcinogenic effects of these dihaloalkanes. In the present study it is demonstrated that hepatic GST activity towards these compounds can be elevated significantly in female and male Fischer-344 rats by feeding these animals on diets supplemented with cancer chemopreventive agents. Immunoblotting experiments showed that increased activity towards the dihaloalkanes is associated with elevated levels of the GSTT1 subunit in rat liver. Sex-specific effects were observed in the induction of GSTT1 protein. Amongst the chemopreventive agents tested, indole-3-carbinol proved to be the most potent inducer of hepatic GSTT1 in male rats (6.2-fold), whereas coumarin was the most potent inducer of this subunit in the livers of female rats (3. 5-fold). Phenobarbital showed significant induction of GSTT1 only in male rat liver and had little effect in female rat liver. Western blotting showed that class Alpha, Mu and Pi GST subunits are not co-ordinately induced with GSTT1, indicating that the expression of GSTT1 is determined, at least in part, by mechanisms distinct from those that regulate levels of other transferases. The increase in amount of hepatic GSTT1 protein was also reflected by an increase in the steady-state level of mRNA in response to treatment with chemopreventive agents and model inducers. Immunohistochemical detection of GSTT1 in rat liver supported the Western blotting data, but showed, in addition to cytoplasmic staining, significant nuclear localization of the enzyme in hepatocytes from some treated animals, including those fed on an oltipraz-containing diet. Significantly, the hepatic level of cytochrome P-450 2E1, an enzyme which offers a detoxification pathway for dihaloalkanes, was unchanged by the various inducing agents studied. It is concluded that the induction of GSTT1 by dietary components and its localization within cells are important factors that should be considered when assessing the risk dihaloalkanes pose to human health.

Suppression of tumour development by substances derived from the diet--mechanisms and clinical implications.

The concept that cancer can be prevented, or its onset postponed, by certain diet-derived substances is currently eliciting considerable interest. Agents which interfere with tumour development at the stage of promotion and progression in particular are of potential clinical value. As chemopreventive agents have to be administered over a long period of time in order to establish whether they possess efficacy in humans, it is of paramount importance to establish their lack of toxicity. The desire to select the best chemopreventive drug candidates for clinical trial, and the necessity to monitor efficacy in the short and intermediate term, render the identification of specific mechanism-based in vivo markers of biological activity a high priority. Antioxidation, inhibition of arachidonic acid metabolism, modulation of cellular signal transduction pathways, inhibition of hormone and growth factor activity and inhibition of oncogene activity are discussed as mechanisms by which the soya constituent genistein, the curry ingredient curcumin and the vitamin A analogue 13-cis retinoic acid exert tumour suppression. A better understanding of these mechanisms will help the establishment of screens for the discovery of new and better chemopreventive agents and the identification of surrogate markers to assess the outcome of clinical chemoprevention trials.

Protection conferred by selenium deficiency against aflatoxin B1 in the rat is associated with the hepatic expression of an aldo-keto reductase and a glutathione S-transferase subunit that metabolize the mycotoxin.

Fischer 344 rats fed on a diet that is deficient in selenium are more resistant to the hepatocarcinogen aflatoxin B1 (AFB1) than those fed on a selenium-sufficient diet. Hepatic cytosol from either selenium-deficient Fischer 344 rats or Hooded Lister rats possesses a marked increase in both reductase activity toward AFB1-dialdehyde and glutathione S-transferase (GST) activity toward AFB(1)-8,9-epoxide than hepatic cytosol from selenium-sufficient rats. The elevation in hepatic AFB1-aldehyde reductase (AFAR) activity in selenium-deficient animals is accompanied by an increase of 11- and 15-fold in the levels of AFAR protein in liver cytosol from Fischer 344 and Hooded Lister rats, respectively. The amount of AFAR protein in selenium-sufficient and -deficient Fischer rats was modulated by treatment with N-acetylcysteine; this antioxidant reduced basal expression of AFAR but did not modulate the relative overexpression of AFAR during selenium deficiency. The enhanced capacity to conjugate glutathione with AFB(1)-8,9-epoxide in selenium-deficient livers from Fischer 344 and Hooded Lister rats is associated with a 5- and 7-fold increase, respectively, in the hepatic levels of the AFB1-metabolizing alpha-class GSTA5 subunit. The elevated levels of AFAR and GSTA5 protein in the selenium-deficient animals coincided with increases in the steady-state levels of their mRNAs. In selenium-deficient Fischer 344 rats, AFAR and GSTA5 were both found to be expressed throughout the centrilobular and midzonal areas of the liver lobule but were essentially absent from periportal hepatocytes. The effect of selenium insufficiency is pleiotropic, and it was also noted that the theta-class GSTT1 is overexpressed 3- and 10-fold in livers of selenium-deficient Hooded Lister and Fischer 344 rats. Inasmuch as GSTT1 is responsible for the metabolic activation of dihaloalkanes, selenium deficiency may increase the susceptibility of rats to mutagens such as dichloromethane.

Comparison of metabolic effects of vegetables and teas with colorectal proliferation and with tumour development in DMH-treated F344 rats.

The aim of this study was to screen potentially chemopreventive vegetables and teas for their effects as human dietary components for the colorectal epithelium and also to seek biomarkers of preventive efficacy. Groups of F344 rats were adapted to a human basal diet supplemented with vegetables or teas, having known contents of glucosinolates, polyphenols and anti-oxidants. Both inductions and suppressions were found for overall glutathione S-transferase (GST) and quinone reductase activities. The mitotic index (MI) showed a three-fold range between groups, with substantial reductions by black tea, spinach, petit pois and peppers. Changes to PCNA labelling index and proliferation zone were marginal. No correlation was found between colonic and hepatic enzyme activities, nor with glucosinolate intake. Colonic MI was associated with the activity ratio GST(hepatic)/GST(colonic) (r = 0.49, P < 0.002), possibly reflecting a need for direct induction rather than exposure to products of hepatic conjugation.

Modulation by iron of hepatic microsomal and nuclear cytochrome P450, and cytosolic glutathione S-transferase and peroxidase in C57BL/10ScSn mice induced with polychlorinated biphenyls (Aroclor 1254).

Exposure of iron-loaded C57BL/10ScSn mice to the polychlorinated biphenyls (PCBs) mixture Aroclor 1254 in the diet (0.01%) for 5 weeks caused massive hepatic porphyria far greater than occurred with PCBs alone. This regime eventually causes hepatocellular carcinoma. Hepatic microsomal ethoxy-, pentoxy-, and benzyloxyresorufin dealkylase activities (respectively EROD, PROD, and BROD) catalyzed primarily by cytochrome P4501A1 and 2B isoenzymes were markedly induced after 2 weeks of diet (when no porphyria had developed) but showed little effect of iron. EROD activity in the nuclear membrane was also induced by the PCBs as was CYP1A1 protein when shown by immunoblotting. Nuclear dealkylase activities of PCBs-treated mice were considerably less than microsomal activities but were stimulated by iron pretreatment. The mechanism of the iron-enhanced toxicity may be due to oxidative damage associated with chronic induction of CYP1A1 isoforms. Lucigenin-enhanced chemiluminescence (CL) by microsomes and nuclear membranes was used as a method to estimate their potential to form reactive oxygen species. Despite CL being induced by PCBs it was less with microsomes from iron-treated mice. In a comparison of a variety of inducers of microsomal cytochrome P450 there was no correlation between inducer, uroporphyrogenic agent, and intensity of CL. On the other hand, cytosolic glutathione S-transferase (GST) activities with 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene (DCNB) as substrates, were also induced by the PCBs mixture, the induction with DCNB being synergistically potentiated by iron pretreatment. Complementary results were observed by immunocytochemistry using anti alpha-GST antibody. In contrast, total glutathione peroxidase activity and selenium-dependent glutathione peroxidase activity were depressed by PCBs but particularly in mice also administered iron. The results illustrate that PCBs not only induce CYP1A1 in microsomes but also in the nuclear membrane, which may be of significance in the mechanism of the iron-enhanced carcinogenicity of these chemicals. The iron-enhanced induction of GST with accompanying depletion of glutathione peroxidase provides evidence for oxidative processes induced in vivo by the PCBs.

The effect of feeding aflatoxin-contaminated groundnut meal, with or without ammoniation, on the development of gamma-glutamyl transferase positive lesions in the livers of Fischer 344 rats.

The detoxification of aflatoxin-containing groundnut meal by ammoniation has been investigated using weanling male and female Fischer 344 rats. Rats were fed a pelleted diet consisting of 75% groundnut meal and 25% powdered rat diet, with vitamin supplement, for a period of 90 days. The groundnut meal was either aflatoxin-free or naturally contaminated with aflatoxins, and was used untreated or after ammoniation. Body weights and food consumptions were monitored throughout the experimental feeding period. After 90 days the animals were sacrificed and the gamma-glutamyl transferase (GGT) levels in the livers assayed fluorimetrically, and the distribution of the enzyme activity determined histochemically. The results indicated that ammoniation of the aflatoxin-containing meal eliminated the development of focal GGT positive lesions in both male and female animals. However, in the female, histochemical GGT staining of hepatocytes in the periportal areas was increased by all the experimental diets compared with the unammoniated, aflatoxin-free diet. This observation was supported by the fluorimetric assays. Ammoniation of the diet led to a decreased body weight gain in all cases compared with the corresponding unammoniated diet-fed groups.