Jeffrey M. Peters

Peroxisome proliferator–activated receptor α mediates the adaptive response to fasting

Prolonged deprivation of food induces dramatic changes in mammalian metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by their oxidation in the liver. The nuclear receptor known as peroxisome proliferator‐activated receptor α (PPARα) was found to play a role in regulating mitochondrial and peroxisomal fatty acid oxidation, suggesting that PPARα may be involved in the transcriptional response to fasting. To investigate this possibility, PPARα-null mice were subjected to a high fat diet or to fasting, and their responses were compared with those of wildtype mice. PPARα-null mice chronically fed a high fat diet showed a massive accumulation of lipid in their livers. A similar phenotype was noted in PPARα-null mice fasted for 24 hours, who also displayed severe hypoglycemia, hypoketonemia, hypothermia, and elevated plasma free fatty acid levels, indicating a dramatic inhibition of fatty acid uptake and oxidation. It is shown that to accommodate the increased requirement for hepatic fatty acid oxidation, PPARα mRNA is induced during fasting in wildtype mice. The data indicate that PPARα plays a pivotal role in the management of energy stores during fasting. By modulating gene expression, PPARα stimulates hepatic fatty acid oxidation to supply substrates that can be metabolized by other tissues.

Altered Constitutive Expression of Fatty Acid-metabolizing Enzymes in Mice Lacking the Peroxisome Proliferator-activated Receptor α (PPARα)

Peroxisome proliferator-activated receptor α (PPARα) is a member of the steroid/nuclear receptor superfamily and mediates the biological and toxicological effects of peroxisome proliferators. To determine the physiological role of PPARα in fatty acid metabolism, levels of peroxisomal and mitochondrial fatty acid metabolizing enzymes were determined in the PPARα null mouse. Constitutive liver β-oxidation of the long chain fatty acid, palmitic acid, was lower in the PPARα null mice as compared with wild type mice, indicating defective mitochondrial fatty acid catabolism. In contrast, constitutive oxidation of the very long chain fatty acid, lignoceric acid, was not different between wild type and PPARα null mice, suggesting that constitutive expression of enzymes involved in peroxisomal β-oxidation is independent of PPARα. Indeed, the PPARα null mice had normal levels of the peroxisomal acyl-CoA oxidase, bifunctional protein (hydratase + 3-hydroxyacyl-CoA dehydrogenase), and thiolase but lower constitutive expression of the D-type bifunctional protein (hydratase + 3-hydroxyacyl-CoA dehydrogenase). Several mitochondrial fatty acid metabolizing enzymes including very long chain acyl-CoA dehydrogenase, long chain acyl-CoA dehydrogenase, short chain-specific 3-ketoacyl-CoA thiolase, and long chain acyl-CoA synthetase are also expressed at lower levels in the untreated PPARα null mice, whereas other fatty acid metabolizing enzymes were not different between the untreated null mice and wild type mice. A lower constitutive expression of mRNAs encoding these enzymes was also found, suggesting that the effect was due to altered gene expression. In wild type mice, both peroxisomal and mitochondrial enzymes were induced by the peroxisome proliferator Wy-14,643; induction was not observed in the PPARα null animals. These data indicate that PPARα modulates constitutive expression of genes encoding several mitochondrial fatty acid-catabolizing enzymes in addition to mediating inducible mitochondrial and peroxisomal fatty acid β-oxidation, thus establishing a role for the receptor in fatty acid homeostasis.

Growth, Adipose, Brain, and Skin Alterations Resulting from Targeted Disruption of the Mouse Peroxisome Proliferator-Activated Receptor β(δ)

ABSTRACT To determine the physiological roles of peroxisome proliferator-activated receptor β (PPARβ), null mice were constructed by targeted disruption of the ligand binding domain of the murine PPARβ gene. Homozygous PPARβ-null term fetuses were smaller than controls, and this phenotype persisted postnatally. Gonadal adipose stores were smaller, and constitutive mRNA levels of CD36 were higher, in PPARβ-null mice than in controls. In the brain, myelination of the corpus callosum was altered in PPARβ-null mice. PPARβ was not required for induction of mRNAs involved in epidermal differentiation induced byO-tetradecanoylphorbol-13-acetate (TPA). The hyperplastic response observed in the epidermis after TPA application was significantly greater in the PPARβ-null mice than in controls. Inflammation induced by TPA in the skin was lower in wild-type mice fed sulindac than in similarly treated PPARβ-null mice. These results are the first to provide in vivo evidence of significant roles for PPARβ in development, myelination of the corpus callosum, lipid metabolism, and epidermal cell proliferation.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals—including certain herbicides, plasticizers, drugs, and natural products—induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARα), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARα agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARα agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues—rat and mouse liver, rat pancreas, and rat testis—lead to several different postulated MOAs, some beginning with PPARα activation as a causal first step. For example, studies in rodent liver identified seven “key events,” including three “causal events”—activation of PPARα, perturbation of cell proliferation and apoptosis, and selective clonal expansion—and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARα activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARα activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARα-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Alterations in lipoprotein metabolism in peroxisome proliferator-activated receptor alpha-deficient mice

The peroxisome proliferator-activated receptor-α (PPARα) controls gene expression in response to a diverse class of compounds collectively referred to as peroxisome proliferators. Whereas most known peroxisome proliferators are of exogenous origin and include hypolipidemic drugs and other industrial chemicals, several endogenous PPARα activators have been identified such as fatty acids and steroids. The latter finding and the fact that PPARα modulates target genes encoding enzymes involved in lipid metabolism suggest a role for PPARα in lipid metabolism. This was investigated in the PPARα-deficient mouse model. Basal levels of total serum cholesterol, high density lipoprotein cholesterol, hepatic apolipoprotein A-I mRNA, and serum apolipoprotein A-I in PPARα-deficient mice are significantly higher compared with wild-type controls. Treatment with the fibrate Wy 14,643 decreased apoA-I serum levels and hepatic mRNA levels in wild-type mice, whereas no effect was detected in the PPARα-deficient mice. Administration of the fibrate Wy 14,643 to wild-type mice results in marked depression of hepatic apolipoprotein C-III mRNA and serum triglycerides compared with untreated controls. In contrast, PPARα-deficient mice were unaffected by Wy 14,643 treatment. These studies demonstrate that PPARα modulates basal levels of serum cholesterol, in particular high density lipoprotein cholesterol, and establish that fibrate-induced modulation in hepatic apolipoprotein A-I, C-III mRNA, and serum triglycerides observed in wild-type mice is mediated by PPARα.

Impaired skin wound healing in peroxisome proliferator–activated receptor (PPAR)α and PPARβ mutant mice

We show here that the α, β, and γ isotypes of peroxisome proliferator–activated receptor (PPAR) are expressed in the mouse epidermis during fetal development and that they disappear progressively from the interfollicular epithelium after birth. Interestingly, PPARα and β expression is reactivated in the adult epidermis after various stimuli, resulting in keratinocyte proliferation and differentiation such as tetradecanoylphorbol acetate topical application, hair plucking, or skin wound healing. Using PPARα, β, and γ mutant mice, we demonstrate that PPARα and β are important for the rapid epithelialization of a skin wound and that each of them plays a specific role in this process. PPARα is mainly involved in the early inflammation phase of the healing, whereas PPARβ is implicated in the control of keratinocyte proliferation. In addition and very interestingly, PPARβ mutant primary keratinocytes show impaired adhesion and migration properties. Thus, the findings presented here reveal unpredicted roles for PPARα and β in adult mouse epidermal repair.

Polyunsaturated Fatty Acid Suppression of Hepatic Fatty Acid Synthase and S14 Gene Expression Does Not Require Peroxisome Proliferator-activated Receptor α

Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor α (PPARα) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARα-deficient mouse to examine the role of PPARα in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2(8-fold) and suppressed mRNAFAS and mRNAS14 by ≥80% in wild type mice. In PPARα-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARα in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARα-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARα and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARα and is involved in the PUFA-mediated suppression of lipogenic gene expression.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Peroxisome proliferator-activated receptor-α and liver cancer : where do we stand?

The peroxisome proliferator-activated receptor-α (PPARα), first identified in 1990 as a member of the nuclear receptor superfamily, has a central role in the regulation of numerous target genes encoding proteins that modulate fatty acid transport and catabolism. PPARα is the molecular target for the widely prescribed lipid-lowering fibrate drugs and the diverse class of chemicals collectively referred to as peroxisome proliferators. The lipid-lowering function of PPARα occurs across a number of mammalian species, thus demonstrating the essential role of this nuclear receptor in lipid homeostasis. In contrast, prolonged administration of PPARα agonists causes hepatocarcinogenesis, specifically in rats and mice, indicating that PPARα also mediates this effect. There is no strong evidence that the low-affinity fibrate ligands are associated with cancer in humans, but it still remains a possibility that chronic activation with high-affinity ligands could be carcinogenic in humans. It is now established that the species difference between rodents and humans in response to peroxisome proliferators is due in part to PPARα. The cascade of molecular events leading to liver cancer in rodents involves hepatocyte proliferation and oxidative stress, but the PPARα target genes that mediate this response are unknown. This review focuses on the current understanding of the role of PPARα in hepatocarcinogenesis and identifies future research directions that should be taken to delineate the mechanisms underlying PPARα agonist-induced hepatocarcinogenesis.

Peroxisome proliferator-activated receptor-delta attenuates colon carcinogenesis.

Peroxisome proliferator–activated receptor-δ (PPAR-δ; also known as PPAR-β) is expressed at high levels in colon tumors, but its contribution to colon cancer is unclear. We examined the role of PPAR-δ in colon carcinogenesis using PPAR-δ-deficient (Ppard−/−) mice. In both the Min mutant and chemically induced mouse models, colon polyp formation was significantly greater in mice nullizygous for PPAR-δ. In contrast to previous reports suggesting that activation of PPAR-δ potentiates colon polyp formation, here we show that PPAR-δ attenuates colon carcinogenesis.