PPAR[alpha] and effects of TCE.

We would like to offer a different opinion on the ideas presented in the article by Keshava and Caldwell (2006). The authors indicated that their article summarized scientific literature published since an earlier U.S. Environmental Protection Agency (EPA) risk assessment of trichloroethylene (TCE), with an emphasis on the possible role of proliferator-activated receptor [alpha] (PPAR[alpha]) agonism relevant to TCE risk assessment. Interestingly, in the section on recent data on PPAR[alpha] agonism, Keshava and Caldwell failed to establish any gene expression signature relating TCE and PPAR[alpha].

Keshava and Caldwell (2006) contended that it is difficult to identify a clear pattern of common gene expression changes for TCE and PPAR[alpha] agonists in general. However, they did not consider numerous reports and reviews (e.g., Klaunig et al. 2003; Peters et al. 2005) illustrating that there are common and reproducible changes in gene expression associated with PPAR[alpha] agonists. Further, extensive characterization has definitively demonstrated specific, direct targets of PPAR[alpha]-retinoid X receptor heterodimers (reviewed by Klaunig et al. 2003). Keshava and Caldwell (2006) also did not discuss the possibility that the effect of TCE on gene expression could be mediated by mechanisms independent of PPAR[alpha], which likely explains the disparity described in their article. Keshava and Caldwell did not critically discuss the data summarized in their Table 2 (Keshava and Caldwell 2006), failing to note that many of these gene targets have no clear linkage with the PPAR[alpha] agonist mode of action (MOA) and may be mediated either via different ligand-receptor-coactivator complexes that form on the promoter regions of the regulated genes by secondary events downstream of the initial events associated with PPAR[alpha] activation, or by mechanisms that are independent of PPAR[alpha]. In addition, the authors failed to describe the limitations of the various gene array platforms and to correctly interpret the findings in the context of gene targets by other PPAR[alpha] agonists, especially when more comprehensive data sets exist but were not cited (Anderson et al. 2004a, 2004b).

Keshava and Caldwell (2006) further raised concerns regarding the use of PPAR[alpha]-null mice to evaluate the MOA of PPAR[alpha] by indicating that the physiologic differences observed in PPAR[alpha]-null mice relative to wild-type mice suggest that the null mouse is an inadequate model to study the PPAR[alpha] MOA. The data they cited, however, appears selective because they failed to mention that liver regeneration in PPAR[alpha]-null mice is reportedly unchanged compared with wildtype mice (Rao et al. 2002), and age-related, sexually dimorphic obesity has not been observed in congenic PPAR[alpha]-null mice (Akiyama et al. 2001). Thus, although the null mouse exhibits changes consistent with the critical role of PPAR[alpha] in modulating fatty acid catabolism, this phenotype does not preclude its application for determining the critical role of this receptor in the MOA of PPAR[alpha] agonists. Importantly, Keshava and Caldwell (2006) did not comprehensively discuss significant findings a) that PPAR[alpha]-null mice are refractory to liver tumors induced by two different PPAR[alpha] agonists (Hays et al. 2005; Peters et al. 1997); b) that they are refractory to increased markers of replicative DNA synthesis and suppression of apoptosis after exposure to numerous PPAR[alpha] ligands (summarized by Peters et al. 2005); or c) that PPAR[alpha]-null mice expressing the human PPAR[alpha] in the liver respond to PPAR[alpha] agonists by increasing expression of genes encoding proteins that catabolize lipids, but they fail to show increases in markers of cell proliferation and are resistant to liver cancer (Cheung et al. 2004; Morimura et al. 2006). To dismiss these findings through lack of discussion or citation does little to advance our understanding and suggests that Keshava and Caldwell's article is unbalanced.

Keshava and Caldwell (2006) also misrepresented an earlier review by Klaunig et al. (2003) regarding the MOA of PPAR[alpha] agonists. Keshava and Caldwell (2006) incorrectly suggested that Klaunig et al. (2003) placed substantial weight on the associative event of peroxisome proliferation with this MOA, when, in fact, peroxisome proliferation was strongly--but not causally--associated, as noted for sustained increased cell proliferation. Keshava and Caldwell (2006) also misconstrued this review (Klaunig et al. 2003), focusing on DNA damage as a possible contributor to the MOA. Citing one manuscript that examined the effect of one, nonspecific PPAR[alpha] ligand (DHEA) is not sufficient to refute the comprehensive review by Klaunig et al. (2003). Finally, Keshava and Caldwell (2006) also suggested that the effects of PPAR[alpha] ligands on mitochondrial function are part of the MOA, but they provided no direct evidence to support their contention that PPAR[alpha] agonists or TCE causes mitochondrial dysfunction.

In summary, Keshava and Caldwell (2006) missed an excellent opportunity to critically and objectively examine the data that support or refute the role of PPAR[alpha] in TCE-induced effects. In our opinion, their article did not advance our understanding of the MOA of PPAR[alpha] agonists or TCE.

The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the U.S. Consumer Product Safety Commission.

J.C.C. is employed by Pfizer, which is developing PPAR agonists for treatment of disease indications. R.M.D. is a member of the American Chemistry Council, Phthalate Ester Panel. J.G.D. is employed by Merck Research Laboratories, which has an interest in the development of PPAR agonists as therapeutic agents, and he owns stock and stock options in Merck. R.H.M. is employed by ExxonMobil, a manufacturer of PPAR agonists (but not TCE). R.A.R. is employed by AstraZeneca, which has an active research program in PPAR[alpha]/[gamma] agonists for potential treatment of lipid and glucose abnormalities associated with diabetes. The remaining authors declare they have no competing financial interests.

James E. Klaunig

Indiana University Indianapolis, Indiana

Michael A. Babich

U.S. Consumer Product Safety Commission Bethesda, Maryland

Jon C. Cook

Pfizer, Inc. Groton, Connecticut

Raymond M. David

K & D Scientific Consulting Inc. Pittsford, New York 

John G. DeLuca

Merck Research Laboratories West Point, Pennsylvania

Richard H. McKee

ExxonMobil Biomedical Sciences Inc. Annandale, New Jersey 

Jeffrey M. Peters

The Pennsylvania State University, University Park, Pennsylvania

E-mail: [e-mail]

Ruth A. Roberts

AstraZeneca UK Macclesfield, Cheshire, United Kingdom

Penelope A. Fenner-Crisp

Consultant North Garden, Virginia

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The correspondence section is a pubic forum and, as such, is not peer-reviewed. EHP is not responsible for the accuracy, currency, or reliability of personal opinion expressed herein; it is he sole responsibility of the authors. EHP neither endorses nor disputes their published commentary.