In this issue A Systems Approach to Tox The kidney is a major site for the excretion of drugs and toxicants, and as such, it can also be a site of toxic damage. Thus, acute or chronic renal failure is a potential side effect of many drugs. Clinicians usually monitor renal failure by testing levels of creatinine and urea nitrogen in the serum, but these tests lack both sensitivity and specificity. They also provide no information regarding the mechanism of the kidney damage. To develop better biomarkers of toxic renal failure and to learn more about the exact nature of the toxic damage, scientists have turned to more complex approaches. Now, Xu et al. (p 1548) combine transcriptomicswithanNMRbased metabolomic analysis to study the etiology of acute renal failure resulting from exposure to the antitumor agent cisplatin and the antibiotic gentamicin. Xu et al. studied the time- and dose-dependent changes in levels of urinary metabolites (43 in the cisplatin study and 38 in the gentamicin study) and in renal expression profiles of over 7000 genes. They used these data to identify metabolic pathways that were most affected by the drug exposures and to search for correlations between changes in the levels of individual metabolites and specific genes. Their combination of urine metabolomic and kidney transcriptomic data in functional enrichment analysis provided a more objective ranking of canonical biochemical pathways affected by
drug treatments, with the most significant pathways involving the transport and metabolism of glucose and neutral amino acids. Specific findings suggested that marked increases in the levels of certain urinary metabolites were best explained by a decrease in the correspondingsodium-dependenttransporters (SLC5A1, SLC5A2 for glucose, SLC6A18 for amino acids,andSLC16A7formonocarboxylic acids). In addition, decreased expression of the gene for collectrin was correlated with reduced levels of the transcription factors HNF1R and HNF1β. Collectrin normally associateswithSLC6A18and similar amino acid transporters, and its reduced expression would be expected to exacerbate the decreased levels of those proteins. Finally, increased expression of the glucose transporters GLUT2 and GLUT9 was explained on the basis of augmented expression of the hypoxia inducible transcription factor HIF-1.
cisplatin and gentamicin toxicity. Detoxification by Albumin The enzyme paraoxonase is named for its ability to hydrolyze the organophosphorus insecticide paraoxon. A lipoprotein-associated enzyme, the physiologic function of paraoxonase may be the hydrolysis of oxidized lipids in LDL. In addition, paraoxonase has long been thought to be primarily responsible for the detoxification of a large number of organophosphorus-based acetylcholinesterase (AchE) inhibitors. However, recent studies using knockout mice revealed that genetic deletion of paraoxonase resulted in increased sensitivity to chlorpyrifos-oxon and diazoxon toxicity but not to paraoxon toxicity. Now, Sogorb et al. (p 1524) address this conundrum through a detailed study of the paraoxon-hydrolyzing activities in serum. Sogorb et al. demonstrated that serum albumin has paraoxon-hydrolyzing activity. Whereas paraoxonase is calcium-dependent and EDTA is inhibitable, the activity of albumin is unaffected by the presence of EDTA but is inhibited by palmitate. Thus, assay in the
presence of EDTA vs palmitate plus calcium provided the means to determine the fraction of hydrolyzing activity associated with each protein. Sogorb et al. showed that in an intoxicated individual, serum albumin contributes equally with paraoxonase to the hydrolysis of paraoxon. In contrast, paraoxonase is almost entirely responsible for the hydrolysis of chlorpyrifos-oxon. Kinetic studies showed that the efficiency of hydrolysis of chlorpyrifosoxon by serum albumin, as determined by Kcat/Km values, is 3-4-fold higher than its efficiency of paraoxon hydrolysis.However,thisgreater efficiency results from a 26-fold higher Kcat value coupledwitha6.8-foldhigher Km for chlorpyrifos-oxon as compared to paraoxon. It isalbumin’slowKm forparaoxon that allows it to hydrolyze paraoxon at the concentrations found in vivo. In contrast, chlorpyrifos-oxon is not an efficient substrate for albumin at those concentrations. These results explain why paraoxonase knockout mice do not demonstrate increased sensitivity to paraoxon as compared to
Special Features Together, these data suggest a major role for changes in levels of key proximal tubule transporters as opposed to metabolic pathways in the kidney malfunction associated with toxic renal failure. They also provide new insight into transcription factors that may play a critical role in the kidney’s response to damage, at least in the case of
Published online 08/18/2008 • DOI: 10.1021/tx800233z © 2008 American Chemical Society
$40.75
So far this year, CRT has been running a series of Guest Editorials on the state of the field of toxicology around the world. One goal of this series was to incite discussion of varying points of view among our readers. In this issue, we continue that discussion, with a response to May’s Guest Editorial by Philip Burcham on toxicology in Australia. Do not miss the Letter to the Editor by Brian Priestly and Michael Moore and the reply from Dr. Burcham concerning the activities of toxicologists down under. Vol. 21,
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In this issue wild-type mice. They also demonstrate that detoxification of xenobiotics may occurviaunexpectedroutes. Chemistry of Skin Allergy Allergic contact dermatitis (ACD) is a common ailment that results from the reaction of chemicals with proteins in the skin, producing an altered structure that is recognized as foreign by the immune system. After processing of the modified protein by antigen-presenting cells, it is presented in conjunction with a major histocompatability complex class II molecule to responsive T cell subsets. The immune response generated by these T cells leads to the allergic reaction.
Terpenes, which are used as fragrances in numerous consumerproducts,arecommon ACD allergens. These compounds are subject to autoxidation, to yield hydroperoxides that could react with cellular constituents via a free radical mechanism. However, relatively little is known about the exact mechanism by which hydroperoxides react with proteins to generate allergic sensitizers. Consequently, Johansson et al.
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(p 1536) have performed careful studies of the free radicalsgeneratedfrom(4R)4-isopropenyl-1-methyl-2-cyclohexene-1-hydroperoxide(1)and(5R)-5-isopropenyl2-methyl-2-cyclohexene-1hydroperoxide (2), the two major autoxidation products of the terpene limonene. Johansson et al. used 5,10,15,20-tetraphenyl-21H, 23H-porphine iron(III) chloride to initiate homolytic cleavage of the oxygen to oxygen bond in the limonene hydroperoxides. The resulting allyloxyl radicals werethensubjectedtothree possible fates, including (i) hydrogenabstractiontoyield the corresponding alcohol, (ii) a 1,2-shift to form a 1-hydroxyallyl radical, and (iii) 1,3-cyclization to form an oxiranylcarbinyl radical. The products formed from each hydroperoxide were identified by chemical radical trapping and EPR spin-trapping experiments. The results showed that hydroperoxide (2) could undergo all three pathways with the 1,2-shift predominating. Because the 1-methyl group prevents the 1,2shift in hydroperoxide (1), it reacted via the other two pathways, approximately equally. Similarly, a structural analogue of (2) bearing a 1-methyl group (5), which could not undergo the 1,2-shift, reacted predominantly by the 1,3-cyclization reaction and by hydrogen abstraction to a lesser degree.
CHEMICAL RESEARCH IN TOXICOLOGY
Together, the results indicate that terpene hydroperoxides can react via either carbon- or oxygencentered radicals. Both hydroperoxides (1 and 2) showed high levels of potency in a skin sensitization assay. Thus, it is likely that sensitization occurs through reactions of one or more of the radical species identified by Johansson et al. Arsenite-InducedApoptosis Arsenic is an important environmental toxicant that is known to cause genetic damage and to induce apoptosis in cultured cells. Despite numerous studies of the mechanism of arsenic toxicity in cell culture, little is known about the mechanism by which arsenic induces apoptosis in an intact organism. Pei et al. (p 1530) have used the availability of numerous knockout strains of the nematode Caenorhabditis elegans to address this question. Incubation of wild-type nematodes on arsenitecontaining medium followed by incubation on
medium containing acridine orange resulted in increased staining of apoptoticgermcellsinthegonads of treated worms by fluorescence microscopy. Pei et al. found that nematodes bearing genetic deletions of key proteins of the apoptotic pathway (ced-3 and ced-4) showed no increase in the number of apoptotic germ cells after arsenic exposure. In contrast, deletion of the p53 homologuecep-1orofgenes involved in the DNA damage response (hus-1, clk-1, and egl-1) resulted in an apoptotic response to arsenic similar to that observed in wild-type nematodes. Mitogen-activated protein kinases have been implicated in the apoptotic response to arsenic, and Pei et al. found that deletion of key genes in the ERK pathway (lin-45, mek-2, and mpk-1), the JNK pathway (jkk-1, mek-1, jnk-1, and mkk4), and the p38 pathway (nsy-1, sek-1, and pmk-1) all produced a diminished apoptotic response to arsenite. Together, the results suggest that arsenite-induced apoptosis in intact C. elegans is caspase-dependent and requires participation from all three major mitogen-activated protein kinase branches. It does not, however, require the DNA damage response pathway. Thus, at least in this species, arsenite-induced apoptosis does not appear to be the direct result of a response to genetic damage. TX800233Z
Published online 08/18/2008 •
DOI: 10.1021/tx800233z $40.75 © 2008 American Chemical Society
