Free radical-derived quinone methide mediates skin tumor promotion by butylated hydroxytoluene hydroperoxide: expanded role for electrophiles in multistage carcinogenesis.

Abstract

Free radical derivatives of peroxides, hydroperoxides, and anthrones are thought to mediate tumor promotion by these compounds. Further, the promoting activity of phorbol esters is attributed, in part, to their ability'to stimulate the cellular generation of oxygen radicals. A hydroperoxide metabolite of butylated hydroxytoluene, 2,6-di-tert-butyl4-hydroperoxyl4-methyl-2,5-cyclohexadienone (BHTOOH), has previously been shown to be a tumor promoter in mouse skin.BHTOOH is extensively metabolized by murine keratinocytes to several radical species. The primary radical generated from BHTOOH is a phenoxyl radical that can disproportionate to form butylated hydroxytoluene quinone methide, a reactive electrophile. Since electrophilic species have not been previously postulated to mediate tumor promotion, the present study was undertaken to examine the role of this electrophile in the promoting activity of BHTOOH. The biological activities of two chemicalanalogs of BHTOOH, 4-trideuteromethyl-BHTOOH and 4-terl-butyl-BHTOOH, were compared with that of the parent compound. 4-Trideuteromethyl-BHTOOH and 4-tertbutyl-BHTOOH have a reduced ability or inability, respectively, to form a quinone methide; however, like the parent compound, they both generate a phenoxyl radical when incubated with keratinocyte cytosol. The potency of BHTOOH, 4-trideuteromethyl-BHTOOH, and 4-tert-butyl-BHTOOH as inducers of ornithine decarboxyiase, a marker of tumor promotion, was commensurate with their capacity for generating butylated hydroxytoluene quinone metbide. These initial results were confirmed in a two-stage tumor promotion protocol in female SENCAR mice. Together, these data indicate that a quinone methide is mediating tumor promotion by BHTOOH, providing direct evidence that an electrophilic intermediate can elicit this stage of carcinogenesis.Butylated hydroxytoluene (BHT; 2,6-di-tert-butyl4methyl-phenol) has found wide commercial application because of its excellent antioxidant properties. Although BHT has attained generally regarded as safe (GRAS) status as afood additive, this phenolic antioxidant has toxic as well as carcinogenic properties (1,2). For example, BHT is toxic in both liver and lung and has been reported to increase tumorformation in the progeny ofrats that had high lifetime feeding of BHT (3). BHT is also a weak hepatocarcinogen in male mice (4). The most notable carcinogenic property of BHT, however, lies in its ability to act as a tumor promoter in a variety oftissues, including the liver, lung, colon, bladder, and thyroid (5). BHT is known to be extensively metabolized in its target tissues, and the toxic as well as tumor-promoting activities of BHT are thought to be mediated by metabolites of the parent compound. The role of the metabolism of BHT in its actions as a promoter and toxin is highlighted by the ability of several inhibitors of cytochrome P450 to suppress BHT toxicity (6, 7) and the observation that a hydroxylated metabolite is more effective than BHT as either a tumor promoter or toxin in mous...

Mechanistic Basis for Inflammation and Tumor Promotion in Lungs of 2,6-Di-tert-butyl-4-methylphenol-Treated Mice: Electrophilic Metabolites Alkylate and Inactivate Antioxidant Enzymes

“…The data presently in hand, however, support a state of persistent oxidative stress over several weeks as the major cause of cell proliferation. Other than QMs derived from BHT or structurallysimilar alkylphenols (8,10) and an electrophilic metabolite of safrole (45), there is a paucity of specific examples of tumor promotion by electrophiles. Nevertheless, it is probable that other electrophiles of exogenous or endogenous origin influence tumor development by epigenetic mechanisms, especially if exposures occur over a prolonged period of time.…”

“…In their work, the severity of BHTinduced inflammation correlated with promotion susceptibility in several inbred strains of mice. An unusual feature of this model is that the promoting agent undergoes metabolic activation in the target organ to form electrophilic metabolites that mediate its effects (7)(8)(9)(10). The para-quinone methides BHTQM and BHTOH-QM shown in Figure 1 undergo 1,6-Michael additions to thiol groups of Cys and (less rapidly) to amino groups of Lys and His residues (11,12), but specific protein targets in lungs are unknown.…”

Butylated Hydroxytoluene Toxicity

“…It has been suggested that the BHT quinone methide (QM) is formed from BHT-OOH and may be the ultimate tumor-promoting species derived from the latter (Guyton et al, 1991). During metabolism studies, the formation of QM along with BHT has been itself has, in spite of its high reactivity, been detected in a variety of studies.…”

Evaluation of the antioxidant effects of polysaccharides extracted from Lycium barbarum

“…There are two basic categories of antioxidants: synthetic and natural. The use of synthetic antioxidants is restricted because of their carcinogenicity (Kimmel and Kimmel, 1986;Guyton et al, 1991). Therefore, the development and utilization of more effective antioxidants of natural origin are desired.…”