Department of Pharmacology and Toxicology, B-440 Life Sciences Building, Michigan State University, East Lansing, Michigan 48824
ABSTRACT
The article highlighted in this issue is "Reversibility and Persistence of Di-2-Ethylhexyl Phthalate (DEHP)- and Phenobarbital-Induced Hepatocellular Changes In Rodents," by Jason S. Isenberg, Lisa M. Kamendulis, David C. Ackley, Jacqueline H. Smith, George Pugh, Jr., Arthur W. Lington, Richard H. McKee, and James E. Klaunig (pp. 192199).
Cancer cells manifest six essential alterations in cell physiology that collectively dictate malignant growth: self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis (Hanahan and Weinberg, 2000). How do these cells arise? It was more than six decades ago that a key set of experimental observations was synthesized into the fundamental notion that carcinogenesis involves a series of independent processes and that particular chemicals could affect one or more of these (Berenblum, 1941a
,b
). The transformation of a normal cell into a frank malignancy entails a clonal evolution of tumor cell populations involving a stepwise selection of sublines that are increasingly abnormal and have a selective growth advantage over adjacent normal cells, and most of the variants are eliminated (Nowell, 1976
). Carcinogenesis is a multistage, multistep process consisting of at least three experimentally defined stages: initiation, promotion and progression (Pitot and Dragan, 1991
; Trosko, 2001
). Operational reversibility is a hallmark of the promotion stage (Pitot and Dragan, 1991
).
There is, particularly among toxicologists, an excessive focus upon mutagenesis as the (read the one and only) mechanism underlying carcinogenesis. An increased emphasis on epigenetic mechanisms is appropriate (Weisburger and Williams, 1981; Counts and Goodman, 1995
; Jones and Laird, 1999
; Feinberg, 2001
; Klaunig et al., 2000
; Trosko, 2001
), and DNA methylation is a prime example of an epigenetic mechanism (Holliday, 1987
; Counts and Goodman, 1995
; Goodman and Watson, 2001). In this context, it is important to note that aspects of altered methylation are reversible; thus alterations in methylation status may play a key role in the promotion stage of tumorigenesis (Counts et al., 1996
; Goodman and Watson, 2001). Indeed, susceptibility to tumorigenesis may be related inversely to the capacity to maintain normal DNA methylation patterns (Counts et al., 1996
).
Cell proliferation plays an important role in the evolution of tumor cell populations (Cohen and Ellwein, 1991). Additionally, inhibition of gap junction-mediated intercellular communication is a mechanism underlying carcinogenesis (Klaunig and Rusch, 1990; Trosko, 2001
). This view is supported strongly by the observations that hepatocarcinogenesis is enhanced in mice deficient in connexin32 (the major gap junction forming protein in liver) and functional connexin32 is required for promotion by phenobarbital (Moennikes et al., 2000
).
The highlighted study by Isenberg et al. in this issue (pp. xx-xx) extends our understanding of mechanisms underlying carcinogenesis by demonstrating that inhibition of gap junction-mediated intercellular communication along with indicators of peroxisomal proliferation persist during promotion of rat and mouse liver tumorigenesis and reverse when treatment is stopped. These authors employed nongenotoxic compounds, di-2-ethylhexylphthalate and phenobarbital, as promoters. Furthermore, the data are consistent with the observation that phenobarbital-induced decreases in methylation in mouse liver are reversible (Counts et al., 1996).
Two key issues for future experimentation are: (1) additional studies of dose-response relationships for the effects of tumor promoters on the various mechanisms involved in tumorigenesis, with an emphasis on sensitive vs. resistant animals, and discerning possible threshold doses; and (2) the use of nuclear receptor xenobiotic constitutive androstane receptor (CAR) knockout mice as an experimental model (Wei et al., 2000). The former orphan nuclear receptor CAR, a constitutive activator of transcription in the absence of ligands, mediates the activation of the Cyp2b gene expression and the liver hypertropic and hyperplastic responses to the phenobarbital-like inducers of drug metabolism (Wei et al., 2000
). Therefore, it would be most informative to test the hypothesis that CAR mediates the tumor promotion effects of phenobarbital, e.g., would CAR knockout mice be resistant to phenobarbital-induced alterations of DNA methylation, inhibition of gap junction-mediated intercellular communication, and liver tumorigenesis?
NOTES
1 For correspondence via fax: (517) 353-8915. E-mail: goodman3{at}pilot.msu.edu.
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