Mitogenic actions of peroxidase proliferators: involvement of protein kinase C and tumor necrosis factor alpha

Impaired Ras membrane association and activation in PPARalpha knockout mice after partial hepatectomy

Liver regeneration after partial hepatectomy (PH) involves several signaling mechanisms including activation of the small GTPases Ras and RhoA in response to mitogens leading to DNA synthesis and cell proliferation. Peroxisome proliferator-activated receptor-alpha (PPARalpha) regulates the expression of several key enzymes in isoprenoid synthesis, which are key events for membrane association of Ras and RhoA. Thus the role of PPARalpha in cell proliferation after PH was tested. After PH, an increase in PPARalpha DNA binding was observed in wild-type mice, correlating with an increase in the PPARalpha-regulated enzyme acyl-CoA oxidase. In addition, the PPARalpha-regulated genes farnesyl pyrophosphate synthase and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase were significantly increased in wild-type mice. However, these increases were not observed in PPARalpha knockout (PPARalpha -/-) mice. The peak in DNA synthesis observed 42 h after PH was reduced by approximately 60% in PPARalpha -/- mice, despite increases in TNF-alpha and IL-1. Also, under these conditions, membrane association of Ras was high in wild-type mice after PH but was impaired in PPARalpha -/- mice. Accordingly, Ras was significantly elevated in the cytosol in PPARalpha -/- mice. This observation correlated with lower levels of active GTP-bound Ras after PH in PPARalpha -/- mice compared with wild-type mice. Similar observations were made for RhoA. Moreover, deletion of PPARalpha blunted the activation of cyclin-dependent kinase (cdk)2/cyclin E and cdk4/cyclin D complexes. Collectively, these results support the hypothesis that PPARalpha is necessary for cell cycle progression in regenerating mouse liver via mechanisms involving prenylation of small GTPases Ras and RhoA.

The transcription of the peroxisome proliferator-activated receptor alpha gene is regulated by protein kinase C

The transcriptional regulation of peroxisome proliferator-activated receptor alpha (PPARalpha) by a variety of peroxisome proliferators was investigated. The treatment of primary cultures of rat hepatocytes with Wy14,643 or clofibrate increased mRNA steady state levels of both PPARalpha and acyl coenzyme A oxidase (ACOX). In contrast, fenofibrate and ciprofibrate increased the expression of ACOX without affecting that of PPARalpha. Inhibition of protein kinase C (PKC) activity using bisindolylmaleimide or calphostin C abolished the increased PPARalpha expression by the peroxisome proliferators whereas the expression of the ACOX gene remained unaffected. Phorbol-12-myristate-13-acetate increased PPARalpha mRNA levels without altering ACOX mRNA levels. It can thus be concluded that a number of peroxisome proliferators activate a PKC-dependent signalling pathway in addition to the PPARalpha pathway. The PKC signal transduction pathway contributes to the regulation of PPARalpha expression but does not influence the transcriptional activity of PPARalpha.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.

Effects of peroxisome proliferators on rat liver phospholipids: sphingomyelin degradation may be involved in hepatotoxic mechanism of perfluorodecanoic acid

Perfluorooctanoic acid (PFOA), perfluorodecanoic acid (PFDA), clofibrate, di(2-ethylhexyl)phthalate (DEHP), and Wy-14,643 represent a class of compounds known as peroxisome proliferators (PPs). Such compounds induce biogenesis of liver peroxisomes and cause a varying degree of hepatotoxicity and carcinogenesis in rodents. We examined the effects of these PPs on rat hepatic lipids and phospholipid profiles using phosphorus-31 NMR spectroscopy. All PPs caused a 25-57% increase in hepatic phospholipid content, while all but clofibrate increased the total lipid content by 26-156%. Treatments also influenced the composition of liver phospholipids. Phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEth) contents were significantly increased in all treatment groups. Most notably, PFDA caused the largest increase in PtdCho and PtdEth content (ca. 70%), while PFOA and Wy-14,643 were the only test compounds that influenced the PtdCho:PtdEth ratio. PFDA also caused an ca. 30% decrease in sphingomyelin (SphM) from 24 to 120 h postdose. SphM is a key lipid in signal transduction processes involved in apoptosis. Hydrolysis of SphM can be mediated through the action of tumor necrosis factor (TNF-alpha). We measured the TNF-alpha concentrations in rat sera at 24 h post-PFDA-exposure and found an 8-fold increase relative to vehicle-treated controls. These data demonstrate that an increase in the serum TNF-alpha level correlates with the time frame for the observed reduction in hepatic SphM. PFOA, a structurally similar compound, had no effect on hepatic SphM content, nor did it affect the serum TNF-alpha concentration. These effects may be related to differences in the tumorigenicity associated with these compounds. We postulate that PFDA activates the SphM signal transduction pathway via the release of TNF-alpha. This then stimulates cytotoxic responses and processes of apoptosis and may suppress cell proliferative and mitogenic responses.