Post-initiation chlorophyllin exposure does not modulate aflatoxin-induced foci in the liver and colon of rats

Chlorophyllin (CHL) is a promising chemopreventive agent believed to block cancer primarily by inhibiting carcinogen uptake through the formation of molecular complexes with the carcinogens. However, recent studies suggest that CHL may have additional biological effects particularly when given after the period of carcinogen treatment. This study examines the post-initiation effects of CHL towards aflatoxin B1 (AFB(1))-induced preneoplastic foci of the liver and colon. The single concentration of CHL tested in this study (0.1% in the drinking water) had no significant effects on AFB(1)-induced foci of the liver and colons of rats

Chemical genomics of cancer chemopreventive dithiolethiones

3H-1,2-dithiole-3-thione (D3T) and its analogues 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (OLT) and 5-tert-butyl-3H-1,2-dithiole-3-thione (TBD) are chemopreventive agents that block or diminish early stages of carcinogenesis by inducing activities of detoxication enzymes. While OLT has been used in clinical trials, TBD has been shown to be more efficacious and possibly less toxic than OLT in animals. Here, we utilize a robust and high-resolution chemical genomics procedure to examine the pharmacological structure–activity relationships of these compounds in livers of male rats by microarray analyses. We identified 226 differentially expressed genes that were common to all treatments. Functional analysis identified the relation of these genes to glutathione metabolism and the nuclear factor, erythroid derived 2-related factor 2 pathway (Nrf2) that is known to regulate many of the protective actions of dithiolethiones. OLT and TBD were shown to have similar efficacies and both were weaker than D3T. In addition, we identified 40 genes whose responses were common to OLT and TBD, yet distinct from D3T. As inhibition of cytochrome P450 (CYP) has been associated with the effects of OLT on CYP expression, we determined the half maximal inhibitory concentration (IC50) values for inhibition of CYP1A2. The rank order of inhibitor potency was OLT ≫ TBD ≫ D3T, with IC50 values estimated as 0.2, 12.8 and >100 μM, respectively. Functional analysis revealed that OLT and TBD, in addition to their effects on CYP, modulate liver lipid metabolism, especially fatty acids. Together, these findings provide new insight into the actions of clinically relevant and lead dithiolethione analogues

Evaluation of the cancer chemopreventive potency of dithiolethione analogs of oltipraz

Oltipraz and related dithiolethiones constitute an important class of chemopreventive agents that enhance the expression of carcinogen detoxication and antioxidant genes. Dose-response studies were undertaken to characterize the cancer chemopreventive activities of several dithiolethiones that are at least as active as oltipraz as inducers. Inhibition of formation of pre-neoplastic lesions and formation of DNA adducts in livers of rats exposed to aflatoxin B1 (AFB1) was monitored. In the tumorigenesis experiment, the dithiolethiones were orally gavaged 3 days/week for 3 successive weeks and at four doses ranging from 0.03 to 0.3 mmol/kg body wt. AFB1 was gavaged beginning 1 week after the start of the dithiolethiones and for two successive weeks. The burden of AFB1-induced putative pre-neoplastic lesions (glutathione S-transferase-placental isoform positive foci) was quantified by light microscopy. Reduction in AFB-DNA adduct burden was assessed 24 h following the first dose of AFB1. Both the parent 1,2-dithiole-3-thione (D3T) and its 5-tert-butyl derivative were more potent inhibitors than oltipraz against these endpoints, while two of the seven tested analogs were slightly less inhibitory. D3T, the most potent dithiolethione of this series, was examined by microarray analysis for induction of hepatic genes at an intermediate chemopreventive dose (0.1 mmol/kg). Transcript levels of eight genes, including two known to detoxify aflatoxin, namely, glutathione S-transferase A5 (GSTA5) and AFB1 aldehyde reductase (AFAR) were elevated. Western analysis indicated that induction of hepatic GSTA5 and AFAR were directly related to the dose of D3T. At the highest dose of D3T (0.3 mmol/kg), protein levels of GSTA5 and AFAR were induced by 7- and 27-fold, respectively. While efficacy in humans has yet to be tested, D3T is clearly more potent than oltipraz and serves as a useful molecular probe for determining the key events associated with protection by this class of agents

Transgenic expression of aflatoxin aldehyde reductase (AKR7A1) modulates aflatoxin B1 metabolism but not hepatic carcinogenesis in the rat

In both experimental animals and humans, aflatoxin B1 (AFB1) is a potent hepatic toxin and carcinogen against which a variety of antioxidants and experimental or therapeutic drugs (e.g., oltipraz, related dithiolethiones, and various triterpenoids) protect from both acute toxicity and carcinogenesis. These agents induce several hepatic glutathione S-transferases (GST) as well as aldo-keto reductases (AKR) which are thought to contribute to protection. Studies were undertaken in transgenic rats to examine the role of one inducible enzyme, AKR7A1, for protection against acute and chronic actions of AFB1 by enhancing detoxication of a reactive metabolite, AFB1dialdehyde, by reduction to alcohols. The AFB1 dialdehyde forms adducts with protein amino groups by a Schiff base mechanism and these adducts have been theorized to be at least one cause of the acute toxicity of AFB1 and to enhance carcinogenesis. A liver-specific AKR7A1 transgenic rat was constructed in the Sprague-Dawley strain and two lines, AKR7A1Tg2 and AKR7A1Tg5, were found to overexpress AKR7A1 by 18- and 8-fold, respectively. Rates of formation of AFB1 alcohols, both in hepatic cytosols and as urinary excretion products, increased in the transgenic lines with AKR7A1Tg2 being the highest. Neither line offered protection against acute AFB1-induced bile duct proliferation, a functional assessment of acute hepatotoxicity by AFB1, nor did they protect against the formation of GST-P positive putative preneoplastic foci as a result of chronic exposure to AFB1. These results imply that the prevention of protein adducts mediated by AKR are not critical to protection against AFB1 tumorigenicity. © The Author 2009. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved

Aldo-Keto Reductase-7A Protects Liver Cells and Tissues From Acetaminophen-Induced Oxidative Stress and Hepatotoxicity

esponsive transcription factor nuclear factor erythroid 2 p45-related factor 2 (Nrf2), a protein highly responsive to acetaminophen (APAP) or its intermediate metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). This study was, therefore, carried out to investigate whether Akr7a is involved in the protection against APAP-induced oxidative stress and hepatotoxicity. We found that in response to APAP or NAPQI exposure, Akr7a3 mRNA and protein were significantly up-regulated in vitro in human HepG2 and LO2 cells. Similarly, strong induction was observed for Akr7a5 in mouse AML12 hepatocytes exposed to APAP. In vivo in wild-type rats, significant up-regulation of hepatic AKR7A1 protein was observed after administration of APAP. On the other hand, depletion of Nrf2 reduced the expression of Akr7a3, suggesting that Nrf2, indeed, contributes significantly to the induction of Akr7a. Moreover, loss of cell viability in Nrf2-depleted cells was significantly rescued by coexpression of AKR7A3. Furthermore, increased AKR7A3 in HepG2 cells was associated with the up-regulation of oxidative stress-related enzymes to enhance cellular antioxidant defense, which appeared to contribute significantly to protection against APAP-induced toxicity. In a line of transgenic rats overexpressing AKR7A1, increased AKR7A1 stimulated the expression of Nrf2 and other Nrf2-regulated genes, but did not better protect rats from APAP insults. In contrast, depletion of Akr7a5 in vitro in cultured AML12 cells or depletion of Akr7a1 in vivo in rat liver greatly increased APAP-induced hepatotoxicity. Conclusion: AKR7A proteins are significantly up-regulated in response to APAP/NAPQI exposure to contribute significantly to protection against APAP-induced hepatotoxicity. AKR7A mediates this protection, in part, through enhancing hepatocellular antioxidant defense. (HEPATOLOGY 2011;54:1322-1332)National Science Foundation of China[30970649]; 973 Program of China[2009CB941601]; Fujian Provincial Department of Science and Technology[2010L0002]; Science Planning Program of Fujian Province[2010J1008