Hepatocellular proliferation in response to agonists of peroxisome proliferator-activated receptor alpha: a role for Kupffer cells?

Abstract

Background: It has been proposed that PPARalpha agonists stimulate Kupffer cells in rodents which in turn, release mitogenic factors leading to hepatic hyperplasia, and eventually cancer. However, Kupffer cells do not express PPARalpha receptors, and PPARalpha agonists stimulate hepatocellular proliferation in both TNFalpha- and TNFalpha receptor-null mice, casting doubt on the involvement of Kupffer cells in the mitogenic response to PPARalpha agonists. This study was therefore designed to investigate whether the PPARalpha agonist PFOA and the Kupffer cell inhibitor methylpalmitate produce opposing effects on hepatocellular proliferation and Kupffer cell activity in vivo, in a manner that would implicate these cells in the mitogenic effects of PPARalpha agonists.

Methods: Male Sprague-Dawley rats were treated intravenously via the tail vein with methylpalmitate 24 hrs prior to perfluorooctanoic acid (PFOA), and were sacrificed 24 hrs later, one hr after an intraperitoneal injection of bromodeoxyuridine (BrdU). Sera were analyzed for TNFalpha and IL-1beta. Liver sections were stained immunohistochemically and quantified for BrdU incorporated into DNA.

Results: Data show that PFOA remarkably stimulated hepatocellular proliferation in the absence of significant changes in the serum levels of either TNFalpha or IL-1beta. In addition, methylpalmitate did not alter the levels of these mitogens in PFOA-treated animals, despite the fact that it significantly blocked the hepatocellular proliferative effect of PFOA. Correlation between hepatocellular proliferation and serum levels of TNFalpha or IL-1beta was extremely poor.

Conclusion: It is unlikely that mechanisms involving Kupffer cells play an eminent role in the hepatic hyperplasia, and consequently hepatocarcinogenicity attributed to PPARalpha agonists. This conclusion is based on the above mentioned published data and the current findings showing animals treated with PFOA alone or in combination with methylpalmitate to have similar levels of serum TNFalpha and IL-1beta, which are reliable indicators of Kupffer cell activity, despite a remarkable difference in hepatocellular proliferation.

Figure 1

Effect of methylpalmitate on liver/body weight ratios, hepatocellular proliferation and peroxisomal β-oxidation in response to the PPARα agonist PFOA. Animals were treated, livers were excised, weighed, and liver/body weight ratios were calculated (A). Hepatocyte BrdU labeling indices (B), and peroxisomal β-oxiation activity (C) were determined as described under "Methods". Data are means ± SEM of 5 animals per group. *p < 0.05 compared to the control group. **p < .01 compared to the PFOA group.

Figure 2

Effect of methylpalmitate on serum levels of TNFα and IL-1β. Animals were treated, and sera were collected and analyzed for TNFα (A) and IL-1β (B) as described under "Methods". Data are means ± SEM of 5 animals per group.

Figure 3

Lack of correlation between BrdU labeling indices and serum mitogen levels. Linear regression analyses were performed on data generated from animal groups depicted in Fig 1, for TNFα (A) and IL-1β (B).