REGULATORY
TOXICOLOGY
AND
PHARMACOLOGY
13, 1-2 (1991)
EDITORIAL Risk Assessment
of Chemicals Causing cx,,-Globulin Nephropathy
An excellent review entitled “The Human Relevance of the Renal Tumor-Inducing Potential of d-Limonene in Male Rats: Implications for Risk Assessment” by Drs. Plamm and Lehman-McKeeman is published in this issue of Regulatory Toxicology and Pharmacology. Research conducted in several laboratories during the past 8 years on a large number of chemicals has elucidated a detailed understanding of the mechanisms involved in cu2,-globulin nephropathy and its associated carcinogenicity in male rats. At present, it represents one of the best characterized nongenotoxic carcinogenic mechanisms known. The weight.of evidence indicating that this syndrome is a male rat-specific phenomenon is compelling. The entire scientific literature related to cY2,-globulin nephropathy supports the concept that reversible binding of chemicals or their metabolites to this abundant male rat-specific protein is causally related to the disease syndrome. The chemical-protein complex is less easily degraded than free Lu&obulin by renal lysosomal enzymes, leading to accumulation, cell death, and compensatory cell proliferation. On chronic exposure to such agents, there is a marked increase in cell proliferation which promotes the development of renal neoplasms from spontaneously initiated cells of the proximal tubule. No research that argues against this mechanism has been published. All cells have spontaneous DNA damaging events occurring continuously (Loeb. Cancer Res. 49, 5489, 1989; Drinking Water and Health, Nat. Acad. Press, 1989). If cell replication takes place prior to repair of these lesions, some of them will be converted to mutations. Chronic increases in cell proliferation decrease the time available for DNA repair prior to DNA replication, thus increasing the probability of mutations. When these mutations occur at critical sites in the genome, cells become initiated for carcinogenesis. Chronic increases in cell proliferation can result in clonal expansion of initiated cells, which in turn increases the probability of additional mutational events associated with neoplastic progression. Species that do not synthesize +,-globulin are not at increased risk of developing cancer, since they do not form the chemicalprotein complex that causes the increase in cell proliferation and renal tumors. The specificity of this response for cY2,-globulin is discussed by Drs. Flamm and Lehman-McKeeman. In addition, molecular modeling studies are providing important clues relevant to understanding the mechanisms responsible (Borghoff et al., Toxicol. Appl. Pharmacol. 107,228, 199 1). a*,-Globulin, like other members of the superfamily, is a transport protein that binds hydrophobic chemicals deep in a pocket. Chemicals that cause the nephropathy also have structures that permit hydrogen bonding with a specific amino acid deep in the pocket of Luz,-globulin. It is this hydrogen bonding that appears to be necessary for the decreased digestibility of the chemical-protein
0273-2300/91 $3.00 Copyright 0 199 1 by Academic Press, Inc. All rights of reproduction in any form reserved.
2
EDITORIAL
complex in lysosomes of the P2 segment renal epithelial cells (Lehman-McKeeman et al., Toxicol. Appl. Pharmacol. 103,539, 1990; Borghoff et al., Annu. Rev. Pharmacol. Toxicol. 30, 349, 1990) and the resultant cell death and subsequent increases in cell proliferation. The fact that female rats do not develop disease clearly demonstrates that this is a threshold phenomenon, since the same metabolites and - l/lOOth the amount of cu&obulin present in male rats are present in their urine. The human members of the superfamily are excreted in much lower amounts in urine and are unlikely to share the same critical amino acid sequence required by Lu&obulin for hydrogen bonding. d-Limonene is a naturally occurring chemical which is widely used as a flavor and fragrance and as an industrial solvent. Moreover, d-limonene has been shown to inhibit chemical carcinogenesis and tumor progression in a variety of animal models. Whereas many compounds can modulate the initiation phase of carcinogenesis by affecting biotransformation, there are few chemicals that can inhibit progression. Several studies by Gould and co-workers (Elegbede et al., Carcinogenesis 5, 661, 1984, and J. Natl. Cancer Inst. 76, 323, 1986; Elson et al., Carcinogenesis 9, 331, 1988; Maltzman et al., Carcinogenesis 10, 78 1, 1989) have clearly demonstrated this for d-limonene. Because of this unique property and its general lack of toxicity, d-limonene has been proposed for clinical trials. The major roadblock for its approval has been the finding of kidney tumors in male rats in the NTP bioassay. Given its natural occurrence, its broad industrial uses, and its potential anticarcinogenic activity, we need a clear statement that the kidney tumors induced by d-limonene in male rats are not relevant for humans. In conclusion, it is clear that the renal tumors associated with chemicals that cause az,-globulin nephropathy are the result of secondary mechanisms that do not operate in humans. The mechanisms involved are among the best characterized of any carcinogen known. The time has come to recognize that not all animal carcinogens are relevant for man. New guidelines for carcinogen risk assessment should have a category for substances that are “carcinogenic for animals, but unlikely to be carcinogenic for humans.” JAMES A. SWENBERG Departments of Environmental Sciences and Engineering and Pathology The University of North Carolina at Chapel Hill CB 7095, Rosenau Hall Chapel Hill, North Carolina 27599-7400
TOXICOLOGY
AND
PHARMACOLOGY
13, 1-2 (1991)
EDITORIAL Risk Assessment
of Chemicals Causing cx,,-Globulin Nephropathy
An excellent review entitled “The Human Relevance of the Renal Tumor-Inducing Potential of d-Limonene in Male Rats: Implications for Risk Assessment” by Drs. Plamm and Lehman-McKeeman is published in this issue of Regulatory Toxicology and Pharmacology. Research conducted in several laboratories during the past 8 years on a large number of chemicals has elucidated a detailed understanding of the mechanisms involved in cu2,-globulin nephropathy and its associated carcinogenicity in male rats. At present, it represents one of the best characterized nongenotoxic carcinogenic mechanisms known. The weight.of evidence indicating that this syndrome is a male rat-specific phenomenon is compelling. The entire scientific literature related to cY2,-globulin nephropathy supports the concept that reversible binding of chemicals or their metabolites to this abundant male rat-specific protein is causally related to the disease syndrome. The chemical-protein complex is less easily degraded than free Lu&obulin by renal lysosomal enzymes, leading to accumulation, cell death, and compensatory cell proliferation. On chronic exposure to such agents, there is a marked increase in cell proliferation which promotes the development of renal neoplasms from spontaneously initiated cells of the proximal tubule. No research that argues against this mechanism has been published. All cells have spontaneous DNA damaging events occurring continuously (Loeb. Cancer Res. 49, 5489, 1989; Drinking Water and Health, Nat. Acad. Press, 1989). If cell replication takes place prior to repair of these lesions, some of them will be converted to mutations. Chronic increases in cell proliferation decrease the time available for DNA repair prior to DNA replication, thus increasing the probability of mutations. When these mutations occur at critical sites in the genome, cells become initiated for carcinogenesis. Chronic increases in cell proliferation can result in clonal expansion of initiated cells, which in turn increases the probability of additional mutational events associated with neoplastic progression. Species that do not synthesize +,-globulin are not at increased risk of developing cancer, since they do not form the chemicalprotein complex that causes the increase in cell proliferation and renal tumors. The specificity of this response for cY2,-globulin is discussed by Drs. Flamm and Lehman-McKeeman. In addition, molecular modeling studies are providing important clues relevant to understanding the mechanisms responsible (Borghoff et al., Toxicol. Appl. Pharmacol. 107,228, 199 1). a*,-Globulin, like other members of the superfamily, is a transport protein that binds hydrophobic chemicals deep in a pocket. Chemicals that cause the nephropathy also have structures that permit hydrogen bonding with a specific amino acid deep in the pocket of Luz,-globulin. It is this hydrogen bonding that appears to be necessary for the decreased digestibility of the chemical-protein
0273-2300/91 $3.00 Copyright 0 199 1 by Academic Press, Inc. All rights of reproduction in any form reserved.
2
EDITORIAL
complex in lysosomes of the P2 segment renal epithelial cells (Lehman-McKeeman et al., Toxicol. Appl. Pharmacol. 103,539, 1990; Borghoff et al., Annu. Rev. Pharmacol. Toxicol. 30, 349, 1990) and the resultant cell death and subsequent increases in cell proliferation. The fact that female rats do not develop disease clearly demonstrates that this is a threshold phenomenon, since the same metabolites and - l/lOOth the amount of cu&obulin present in male rats are present in their urine. The human members of the superfamily are excreted in much lower amounts in urine and are unlikely to share the same critical amino acid sequence required by Lu&obulin for hydrogen bonding. d-Limonene is a naturally occurring chemical which is widely used as a flavor and fragrance and as an industrial solvent. Moreover, d-limonene has been shown to inhibit chemical carcinogenesis and tumor progression in a variety of animal models. Whereas many compounds can modulate the initiation phase of carcinogenesis by affecting biotransformation, there are few chemicals that can inhibit progression. Several studies by Gould and co-workers (Elegbede et al., Carcinogenesis 5, 661, 1984, and J. Natl. Cancer Inst. 76, 323, 1986; Elson et al., Carcinogenesis 9, 331, 1988; Maltzman et al., Carcinogenesis 10, 78 1, 1989) have clearly demonstrated this for d-limonene. Because of this unique property and its general lack of toxicity, d-limonene has been proposed for clinical trials. The major roadblock for its approval has been the finding of kidney tumors in male rats in the NTP bioassay. Given its natural occurrence, its broad industrial uses, and its potential anticarcinogenic activity, we need a clear statement that the kidney tumors induced by d-limonene in male rats are not relevant for humans. In conclusion, it is clear that the renal tumors associated with chemicals that cause az,-globulin nephropathy are the result of secondary mechanisms that do not operate in humans. The mechanisms involved are among the best characterized of any carcinogen known. The time has come to recognize that not all animal carcinogens are relevant for man. New guidelines for carcinogen risk assessment should have a category for substances that are “carcinogenic for animals, but unlikely to be carcinogenic for humans.” JAMES A. SWENBERG Departments of Environmental Sciences and Engineering and Pathology The University of North Carolina at Chapel Hill CB 7095, Rosenau Hall Chapel Hill, North Carolina 27599-7400
