Involvement of PPAR nuclear receptors in tissue injury and wound repair (original) (raw)
PPARs in kidney I/R. The kidney is vulnerable to damage by toxins, infection, immune reactions, and ischemia. Acute renal failure (ARF) affects about 5% of hospitalized patients and carries a high mortality. Damage to renal tubules alters epithelial cells and is accompanied by the shedding of cells into the tubule lumen and the back-leakage of glomerular filtrate, further increasing epithelial apoptosis and necrosis. Surviving cells participate in regeneration of the epithelium and restoration of renal function. The prognosis for ARF is complicated by secondary injuries induced by free radicals formed during I/R, although inadequate renal cortical-medullary reperfusion may be more deleterious (10). Today, there is no treatment for this devastating clinical syndrome.
A role for PPARs in reducing renal injury and dysfunction is established in animal models. _PPAR_α-null mice subjected to I/R injury by arterial ligation show enhanced cortical necrosis and impaired renal function (11). Conversely, induction of FA oxidation enzymes by PPARα is thought to preserve kidney structure and function during renal I/R injury (11). In humans, nephrotoxicity is a common side effect of treatment with the antitumor agent cisplastin (12). In mice, PPARα ligands attenuate cisplatin-induced ARF by preventing the inhibition of FA oxidation, reducing apoptosis and necrosis in the proximal tubule (13), and repressing inflammation via inhibition of NF-κB binding activity, which attenuates neutrophil infiltration and cytokine/chemokine release (14).
Consistent with their defect in skin healing, _PPAR_δ-deficient mice exhibit greater kidney injury and dysfunction than wild-type counterparts after renal I/R. Conversely, wild-type mice pretreated with a PPARδ ligand are protected from I/R damage, with a reduction in medullary necrosis, apoptosis, and inflammation. Cell culture studies show that PPARδ ligands activate the PKB/Akt pathway, as they do in keratinocytes, and increase the spread of tubular epithelial cells. In vivo, these events may accelerate healing by suppressing tubular epithelial shedding and anoikis (15).
The PPARγ agonists rosiglitazone and pioglitazone have protective effects against not only I/R, but also various kidney injuries including diabetic nephropathy, hypertensive nephropathy, experimental glomerulonephritis, and cyclosporine-induced renal injury (reviewed in refs. 16, 17). This protection reflects both improved glucose metabolism and insulin resistance as well as the antiinflammatory, antifibrotic, and antiapoptotic effects of PPARγ ligands (18). The mechanisms underlying these beneficial properties are similar for synthetic agonists and the natural cyclopentenone prostaglandin 15d-PGJ2. The pathways involve inhibition of NF-κB activation, together with reduced expression and/or activity of AP-1, TGF-β1, monocyte chemoattractant protein-1 (MCP-1), ICAM-1, iNOS, fibronectin, and collagen I. The outcome of these signaling changes includes attenuated infiltration of polymorphonuclear cells into renal tissues, reducing oxidative stress and inflammation (19–23).
PPARs in lung I/R and fibrosis. Patients with end-stage pulmonary diseases are often treated with lung transplantation. Although improvements in techniques such as the preservation of vascular endothelium have significantly improved survival, I/R lung injury still occurs in over 20% of patients and remains the main cause of death during the first month after transplantation (24). Rodent models show that PPAR ligands, such as rosiglitazone and pioglitazone, can significantly attenuate I/R–induced lung injuries (17). Furthermore, treatment with the PPARγ agonist pioglitazone before ischemia reduces I/R–induced lung damage in rats. The mechanism involves inhibition of proinflammatory cytokines (TNF-α, cytokine-induced neutrophil chemoattractant 1) and polymorphonuclear cell infiltration into the lung interstitium, resulting in reduced pulmonary edema (25). Similarly, in a murine I/R model, pretreatment with the PPARγ agonist troglitazone prevents induction of the zinc finger transcription factor early growth response gene-1 (Egr-1), a master switch for the inflammatory response in ischemic vessels. Thus, PPARγ neutralizes the potential for harm caused by Egr-1 target genes such as _IL-1_β, MCP-1, and macrophage inflammatory protein-2. As a consequence of this protection, leukostasis is decreased, while oxygenation and overall survival are increased (26).
The term pulmonary fibrosis covers several life-threatening diseases for which no effective therapy exists. All have a similar pathology characterized by an immune response closely resembling a Th2-type phenotype, with proliferation and accumulation of myofibroblasts and excessive deposition of ECM proteins in the lung parenchyma. The clinical features are shortness of breath, evident diffuse pulmonary infiltrates, and varying degrees of inflammation and fibrosis (27). In humans, bleomycin treatment for cancer chemotherapy induces interstitial lung fibrosis (28). In human pulmonary fibroblast cultures, PPARγ agonists interrupt the profibrotic effects of TGF-β (29). Similarly, mice subjected to intratracheal administration of bleomycin develop lung fibrosis, which is significantly reduced by PPARγ agonists. As expected, this beneficial effect is attenuated by the PPARγ antagonist bisphenol A diglycidyl ether (BADGE), suggesting that PPARγ activity is required for protection (30).
PPARs in digestive tract I/R. Acute mesenteric ischemia, abdominal aortic aneurysm, hemorrhagic, traumatic, or septic shock, small bowel transplantation, and severe burns cause intestinal I/R, a severe condition characterized by endothelial cell swelling, capillary plugging, and mucosal barrier dysfunction (31). In rodent models of intestinal I/R injury, PPARγ activation downregulates _TNF-_α and ICAM-1 (probably via inhibition of NF-κB), and pretreatment with a PPARγ agonist before ischemia significantly reduces neutrophil infiltration (32, 33). These protective effects are attenuated by PPARγ antagonists or reduction of PPARγ levels in mutant _PPAR_γ heterozygous animals (32, 34). Similarly, activation of PPARα attenuates I/R injury by reducing ICAM-1 expression, peroxynitrite activity, and the production of proinflammatory cytokines (35). Additionally, enteral nutrition is beneficial when administered soon after severe gut I/R insults. For instance, the solute glutamine maintains small bowel function depending on cellular energetics and epithelial cell functions after I/R injury in rats. This effect is associated with PPARγ induction and, logically, abrogated by a PPARγ antagonist (36).
Evidence is also accumulating for a beneficial role of PPARγ agonists in healing gastric mucosal damage associated with I/R in animal models (37–40). Activation of PPARγ reduces gastric lesions and attenuates levels of lipid peroxidation, ICAM-1, TNF-α, COX-2, iNOS, and apoptosis after gastric I/R injury. As a result, PPARγ alleviates oxidative injury and inflammation (here again the mechanism likely involves inhibition of NF-κB). The protective and healing effects of all 3 PPAR isotypes on kidney, lung, and digestive tract epithelia after injury are summarized in Figure 2.
Epithelial repair pathways controlled by PPARs during kidney, digestive tract, and lung injury. Common to all tissue injury is a rapid increase in inflammation. PPAR activation, mediated by the binding of natural and synthetic ligands, restricts inflammation to prevent extensive tissue necrosis and chronic oxidative damage.
PPARs in liver injury (cirrhosis and fibrosis). Chronic liver disease remains an important cause of mortality and morbidity. Recurring or chronic injury and inflammation trigger tissue remodeling pathways that can lead to severe fibrosis and end-stage cirrhosis. Unfortunately, no effective treatment exists for these injuries except liver transplantation (41). The causes of liver fibrosis and cirrhosis include genetic abnormalities, toxic, alcoholic, and autoimmune-mediated damage, nonalcoholic steato-hepatitis associated with the metabolic syndrome, and viral hepatitis forms B and C. Liver fibrosis involves proliferation of myofibroblasts derived from hepatic stellate cells (HSCs, also called Ito cells or lipocytes). In the damaged areas, the transition of normally quiescent HSCs to proliferative myofibroblasts, through the action of cytokines and oxidative stress, increases ECM deposition. In the fibrotic and cirrhotic liver, matrix degradation by MMPs occurs but is restricted by tissue inhibitors of metalloproteinases (TIMPs). Encouraging results from experimental models suggest that fibrosis could be attenuated by enhancing apoptosis of stellate cells or blocking their transdifferentiation, as well as by manipulating the TIMP-MMP balance to facilitate matrix degradation and improve liver architecture (42).
PPARγ agonists suppress the growth and fibrotic activity of HSCs by the downregulation of proteins such as α1(I) collagen, fibronectin, α-SMAs, and MCP-1, which is consistent with reduced PPARγ levels in transdifferentiated HSCs (43–46). Experimental overexpression of PPARγ in myofibroblasts reverses their phenotype to quiescent cells, restores their ability to store retinyl esters, and represses activation markers such as α1(I) procollagen and TGF-β1 by suppressing AP-1 and nuclear factor-1 activities (47, 48). Interestingly, an analogy has been proposed between preadipocyte-adipocyte differentiation and HSC transdifferentiation. The high level of expression of adipogenic transcription factors in quiescent HSCs rapidly declines during their transdifferentiation to myofibroblastic HSCs. Similarly, treating these cells with an adipocyte differentiation “cocktail” or ectopic expression of SREBP-1c or PPARγ reverts them to quiescent HSCs (49). The COX-2 inhibitor SC-236 attenuates liver inflammation and fibrosis through PPARγ activation and downregulation of α-SMA expression and MMP-2 and -9 activities, as well as by the induction of Kupffer cells and HSC apoptosis (50). In addition, ligands of the farnesoid X receptor stimulate expression of PPARγ in HSCs and maintain its enhanced levels after injury, thereby promoting the antifibrotic action of PPARγ agonists (51). In addition to their action on HSCs, PPARγ ligands also reduce ductal proliferation and fibrosis after bile duct ligation in rats, showing that PPARγ attenuates fibrosis through not only direct action on matrix-producing cells, but also modulation of the epithelial-mesenchymal interactions in chronic obstructive cholestasis (52).
The function of PPARδ in fibrogenesis is less well studied, but PPARδ expression is strongly induced after HSC activation. In a model of carbon tetrachloride–induced acute liver damage, PPARδ activation induces HSC proliferation during early fibrogenesis and enhances expression of fibrotic markers (53). Thus, PPARγ and PPARδ appear to have antagonistic effects that require further investigation using _PPAR_γ- and _PPAR_δ-deficient mice. The possibility of manipulating the balance of PPARγ and PPARδ pharmacologically signifies a promising development for the attenuation of liver fibrosis. Future studies should improve our understanding of pathways regulating HSC survival, death, and clearance, leading to potential therapies to induce HSC apoptosis (41).
PPARα ligands also have antifibrotic effects in the rat thioacetemide model of liver cirrhosis, probably via their antioxidant action associated with enhanced catalase expression and activity (54). Interestingly, in a mouse model of I/R, PPARα regulates hepatic neutrophil accumulation and reduces iNOS expression after hepatocellular injury. This finding is important because activation of PPARα in hepatocytes protects against oxidant injury, indicating that parenchymal cells might impact the inflammatory response (55). Finally, the function of PPARα in liver regeneration after partial hepatectomy remains unclear. PPARα is not necessary for compensatory hyperplasia induced by partial hepatectomy, yet PPARα-dependent regulation of genes associated with cell cycle progression, cytokine signaling, and metabolic changes appears to be involved (56–59).
PPARs in ischemic brain injury and neurodegenerative disease. Brain injury resulting from insufficient blood (oxygen) supply can be transient (from syncope or ischemic attack) or permanent (from infarct or irreversible stroke). The latter is a major cause of disability and death in developed countries, and because of limited therapeutic strategies there is increasing interest in prophylactic pharmacological treatment (60). It was first observed that the fibrate gemfibrozil reduces stroke incidence in men with low HDL cholesterol and low LDL cholesterol who suffer from coronary heart disease (61). In mice this outcome is associated with improved endothelial relaxation, reduced brain oxidative stress, and decreased VCAM-1 and ICAM-1 expression and is thus independent of lipid metabolism (62). Similarly, the PPARα and PPARγ agonist resveratrol, a polyphenol found in grapes, protects the murine brain from stroke, in a PPARα-dependent manner (63). Thus, PPARα agonists may prevent or reduce the severity of ischemic stroke in humans. In rat hippocampal neurons, the PPARα agonist Wy-14,463 induces peroxisomal proliferation that attenuates β-amyloid peptide–dependent neurotoxicity and decreases intraneuronal oxidative stress (64). In addition, PPARγ ligands have neuroprotective effects in experimental models of ischemic injury, Alzheimer disease, multiple sclerosis, and autoimmune encephalomyelitis. The benefits result from suppressing inflammation (65–68). In addition, in a mouse model of amyotrophic lateral sclerosis for which neuroinflammation may contribute to motor neuron death, the PPARγ ligand pioglitazone improves muscle strength and body weight, delays disease onset, and increases lifespan (69).
PPARs in cardiac I/R. Myocardial I/R is a clinically relevant problem associated with reestablishment of blood flow by coronary bypass surgery, thrombolysis, and angioplasty, and with the need to minimize myocardial damage after heart infarct. Heart tissue normally uses FAs as the major energy source. However, hypoxia or pressure overload in the heart results in a substrate switch from FAs to glucose, caused by downregulation of PPARα (70). It is thought that partial inhibition of FA oxidation improves the functional recovery of the heart during reperfusion (71). In support of this idea, experimental overexpression of PPARα in the heart impairs cardiac recovery after ischemia (72). Thus, pharmacological treatments that stimulate glucose oxidation and repress FA oxidation appear to be beneficial for cardiac recovery (73). Along this line of thought, it has been proposed that downregulation of PPARγ coactivator-1 and PPARα may shift myocytes toward a more glycolytic metabolism (74). However, beneficial effects of PPARα agonists on I/R damage have been reported as well (75–77). Experimentally, this contradiction could be resolved by determination of whether PPARα agonists improve myocardial function via metabolic and antiinflammatory actions, and whether cardiac overexpression of PPARα has deleterious effects on the heart when circulating FA levels are high. Nevertheless, these observations suggest that cardiac PPARα antagonism could be a therapy for treating I/R damage (72).
In healthy, diabetic, or obese animals (76, 78–82), PPARγ agonists reduce myocardial infarct size. These effects are associated with increased glucose uptake and improved insulin sensitivity. In addition, PPARγ agonists reduce postischemic myocardial apoptosis (83). However, the role of PPARγ in heart failure is debated, particularly with regard to patients with type 2 diabetes mellitus. The Prospective Pioglitazone Clinical Trial in Macrovascular Events (PROactive) study concluded that pioglitazone may improve cardiovascular outcome (84), while a retrospective cohort study suggested that thiazolidinediones (TZDs) may increase the risk of heart failure. Since type 2 diabetes patients are at increased risk of heart failure, those with underlying myocardial disease may be especially vulnerable to the effects of TZDs (85). Exacerbation of heart failure is documented in animal studies. TZDs are associated with increased post–myocardial infarction mortality in rats (86), and increased susceptibility to ventricular fibrillation during myocardial I/R in pigs (87). Finally, PPARα or PPARγ stimulation prevents or attenuates cardiac fibrosis by reducing endothelin-I, collagen type I, and MMP-1 production, and by improving myocardial inflammation in anoxia/reoxygenation and pressure-overloaded hearts (88–91).
These experimental studies suggest that PPARs exert beneficial effects in reducing infarct size, myocardial reperfusion injury, and hypertrophic signaling and inflammatory responses. However, clinical applications have revealed some undesirable side effects, suggesting that TZDs should be used with caution in diabetic patients predisposed to heart failure (92). Obviously, there are uncertainties that require additional research (93). The protective effects of PPARs on liver, brain, and heart injury are summarized in Figure 3.
Role of PPARs in repair of liver, brain, and heart damage. Various systemic states such as shock or sepsis can lead to organ injury and failure. These injuries, as well as tissue-specific insults such as cirrhosis, fibrosis, and I/R injury, can be partially alleviated or prevented through the actions of PPARs.