Differential Antiatherogenic Effects of PPAR Versus PPAR Agonists
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动脉硬化血栓血管生物学 2005年第9期
From AstraZeneca, Discovery, M?lndal, Sweden.
Correspondence to Germán Camejo, AstraZeneca Discovery, S-431 83, M?lndal, Sweden. E-mail german.camejo@astrazeneca.com
Research during the past decade about the role of the transcription factors known as peroxisome proliferator-activated receptors (PPARs) subtype alpha (-), delta/beta (-/?), and gamma (-) indicates that they are master switches controlling tissue-specific physiological adaptations in the use of fatty acids, glucose, and amino acids during feeding and fasting.1,2 These nuclear receptors modulate in concert the expression of genes associated with specific metabolic pathways together with the signaling of hormones involved in energy homeostasis like insulin, glucagon, growth hormone (GH), and cortisol. In this capacity they seamlessly adjust the reversible transitions from storage to use of fatty acids and triglycerides, of glucose and glycogen, and of the more complex ones of amino acids and proteins. Moreover, enzymes and transporters required for the interpathway conversions of glucose into fatty acids and of some amino acids into glucose also are target genes for PPARs, at least when rodents and humans are maintained on a high carbohydrate supply. Endowed with such central functions it is not surprising that the PPARs have impact on metabolic alterations contributing to insulin resistance and diabetes, as well as on abnormalities of lipid metabolism causing dyslipidemias and atherosclerosis. Therefore is also logical that intense research is dedicated to evaluate how modulation of the PPARs could improve the dyslipidema associated with insulin resistance, insulin resistance itself, diabetes, and atherosclerosis.
See page 1897
The PPAR agonists, known generically as fibrates, reduce the risk of cardiovascular events especially in patients with the dyslipidemia of insulin resistance.3 At present, no data are available about the effect on cardiovascular events of the two PPAR agonists in clinical use, pioglitazone, and rosiglitazone. However studies are in progress and we will soon know whether these agents reduce coronary events in patients with type 2 diabetes, especially when used in addition to statins of proven efficacy. There are grounds to be optimistic about these trials because mouse models have shown that PPAR agonists do reduce atherosclerotic lesions4–7 and because in patients with type 2 diabetes PPAR agonists have been shown to reduce the carotid intima-media thickness as evaluated with ultrasound.8,9
In the midst of all the above positive lines of evidence indicating that PAR and PPAR agonists may exert antiatherosclerotic effects, there are apparent surprises: in this issue of Atherosclerosis, Thrombosis, and Vascular Biology, Hennuyer et al using a gene-manipulated mouse model show that although fenofibrate reduces macrophage-laden lesions the PPARg agonists, pioglitazone and rosiglitazone, do not.10 The mouse model used in this report is a knock-in that expresses human apoE2 instead of the murine apoE (E2-KI) developed by Sullivan et al.11,12 In humans, expression of the apoE2 isoform, ApoE2(Arg158>Cys158) can induce a dyslipidemia characterized by increased chylomicrons- and VLDL-remnants because the particles with such isoform have a lower affinity for the heparan sulfate of hepatic cells and for the remnant receptor, the LDL receptor-related protein (LRP). In addition these particles have a long circulating half-life because they are resistant to lipolysis by lipoprotein lipase.13 Therefore when the E2-KI mouse is fed a Western diet it accumulates large amounts of "VLDL-", "IDL-," and "LDL-like" cholesterol-rich remnant particles and has reduced amounts of HDL.
The finding that PPAR agonists have no antiatherosclerotic effects in this mouse model is apparently at odds with several studies in which troglitazone, rosiglitazone, and GW7845 (also a PPAR agonist) did show reduction of macrophage-rich lesions in LDL receptor–deficient and apoE-defective mice.4–7 On the other hand, Hennuyer et al present in this issue of ATVB data showing reduction of macrophage-laden lesions by the PPAR activator fenofibrate in agreement with previous studies in LDL receptor–deficient and apoE–deficient mice. Can we reconcile this paradox or does a paradox exist? Dosage or pharmacokinetics cannot be the explanation because similar rosiglitazone doses to those used in the present study, 25 to 50 times those prescribed to humans, have been shown to reduce lesions in LDL receptor–deficient mice. Are we left without explanations? Not likely, because probably the most straightforward explanation is put forward by the authors. This is that fenofibrate improves the dyslipidemia of the E2-KI mice whereas the PPARg agonists do not. However, is this unexpected? Not really, because these animals are neither insulin-resistant nor diabetic, and the dyslipidemia is probably entirely associated with the defective apoE2 isoform and not to defects of fatty acid use or storage that are modulated by PPAR and insulin signaling.
One of the most interesting findings in the article that motivates this editorial is how fenofibrate, an agonist that is not very potent toward the mouse PPAR, clearly reduces the plasma levels of remnants and increases HDL. Stimulation of PPAR in liver, and probably also the intestine, reduces the secretion of VLDL and consequently the intravascular formation of remnants, at the same time improving their lipolysis and removal by the liver via diminishing their apoCIII content. Decreased VLDL secretion and increased lipolysis on PPAR activation are well known effects of fibrates.3 At the other end, treatment of human hepatic cells with PPAR agonists, but not with PPAR agonists, increases the expression of genes and gene products for extracellular heparan-containing proteoglycans that bind more efficiently VLDL remnants.14 This is an important first step in remnant hepatic uptake that could have been enhanced in the fenofibrate-treated E2-KI mouse. It would be very interesting to test the effects of PPAR agonist in the remnant binding capacity of hepatocytes in this mouse.13
The article by Hennuyer et al presents a balanced discussion about the protective effects of the PPAR agonists on the development of macrophage-rich lesions in the E2-KI mouse and quotes published data mostly obtained by treatment of macrophages in cell cultures. One should exercise caution in the interpretation of some of the published work on the in vitro effects of PPAR agonists on cells. A common experimental design has been the use of cells cultured to the desired stage in 5% to 10% serum and, after washing them, 1 to 10 μmol/L of the PPAR agonists are added in media without albumin or at most with 1% serum. This common practice does not take into consideration that PPAR agonists are molecules that fit a hydrophobic pocket in the nuclear receptor similar to those pockets in albumin that bind fatty acids and other nonpolar molecules. Therefore, after oral administration most PPAR agonists are more than 90% bound to albumin, thus tissues are exposed to very low levels of free substance. The second reason why many of the in vitro observed pleiotropic effects of PPAR agonists may not translate fully in vivo is that the molecules were selected because they improve insulin action in models with defective insulin signaling. Thus many PPAR agonists at pharmacological doses show only modest effects in cells cultured at near to physiological conditions or in animals with normal response to insulin. This is probably the case of the E2-KI mouse used in the commented article that had normal plasma insulin and glucose and whose atherogenic dyslipidemia was entirely caused by the defective apoE2. This dyslipidemia apparently is not dependent of insulin resistance and should be therefore less responsive to PPAR agonists.
In conclusion the findings of Hunneyer et al provide interesting data about how PPAR agonists can improve a remnant-linked dyslipidemia that is particularly atherogenic and that is not associated with insulin resistance. In addition, they have further documented the important notion that the atherogenicity of a functionally-deficient apoE can be counteracted by PPAR stimulation, probably acting at the level of hepatic VLDL production and directly on the vascular tissue response to the dyslipidemia. Finally, the absence of effects of the PPAR agonists observed in this work supports the hypothesis that these drugs could be antiatherogenic only in a background of insulin-resistance and its associated defects of lipid metabolism.
Acknowledgments
I thank my colleagues Bengt Ljung and Nick Oakes from AstraZeneca, Discovery for useful discussions and suggestion about this editorial.
References
Kersten S, Desvergene B, Wahli W. Roles of PPARS in health and disease. Nature. 2000; 405: 421–424.
Francis G, Fayard E, Picard F, Auwerx J. Nuclear receptors and the control of metabolism. Ann Rev Physiol. 2003; 65: 261–311.
Fazio S, Linton M. The role of fibrates in managing hyperlipidemia: mechanisms of action and clinical efficacy. Curr Ather Rep. 2004; 6: 148–157.
Collins AR, Meehan WP, Kintscher U, Jackson S, Wakino S, Noh G, Palinski W, Hsue W, Law R. Troglitazone inhibits formation of early atherosclerotic lesions in diabetic and nondiabetic low density lipoprotein receptor-deficient mice. Arteriocler Thromb Vasc Biol. 2001; 21: 365–371.
Chen Z, Ishibashi S, Perrey S, Osuga J, Gotoda T, Kitamine T, Tamura Y, Okazaki H, Yahagi N, Iizuka Y, Shionoiri F, Ohashi K, Harada K, Shimano H, Nagai R, Yamada N. Troglitazone inhibits atherosclerosis in apolipoprotein E-knockout mice: pleiotropic effects on CD36 expression and HDL. Arteriocler Thromb Vasc Biol. 21: 372–377.
Li A, Brown K, Silvestre M, Wilson T, Palinski W, Glass K. Peroxisome proliferator-activated receptor gamma ligands inhibit development of atherosclerosis in LDL receptor-defcient mice. J Clin Invest. 2000; 106: 523–531.
Li AC, Binder CJ, Gutierrez A, Brown KK, Plotkin CR, Pattison JW, Valledor AF, Davis RA, Willson TM, Witztum JL, Palinski W, Glass CK. Differential inhibition of macrophage foam-cell formation and atherosclerosis in mice by PPAR, ?/, and . J Clin Invest. 2004; 114: 1564–1576.
Sidhu J, Kaposzta Z, Markus H, Kaski J. Effect of rosiglitazone on common carotid intima-media thickness in coronary artery disease patients without diabetes mellitus. Arterioscler Thromb Vasc Biol. 2004; 24: 930–943.
Langenfeld MR, Forst T, Hohberg C, Kann P, Lübben G, Konrad T, Füllert SD, Sachara C, Pfützner A. Pioglitazone decreases carotid intima-media thickness independently of glycemic control in patients with type 2 diabetes mellitus: results from a controlled randomized study. Circulation. 2005; 111: 2525–2531.
Hennuyer N, Tailleux A, Torpier G, Mezdour H, Fruchart JC, Staels B, Fievet C. PPAR, but not PPAR, activators decrease macrophage-laden atherosclerotic lesions in a nondiabetic mouse model of mixed dyslipidemia. Arterioscler Thromb Vasc Biol. 2005; 25: 1897–1902.
Sullivan PM, Mezdour H, Quarfordt SH, Maeda N. Type III hyperlipoproteinemia and spontaneous atherosclerosis in mice resulting from gene replacement of mouse apoE with human ApoE 2. J Clin Invest. 1998; 102: 130–135.
Knouff C, Hinsdale ME, Mezdour H, Altenburg MK, Watanabe M, Quafordt SH, Sullivan PM, Maeda N. Apo E structure determines VLDL clearance and atherosclerosis risk in mice. J Clin Invest. 1999; 103: 1579–1586.
Mahley R, Ji Z-S. Remnant lipoprotein metabolism: key pathways involving cell-surface heparan sulfate proteoglycans and apolipoprotein E. J Lipid Res. 1999; 40: 1–16.
Olsson U, Egnell AC, Rodríguez M, ?stergren G, Lorentzon M, Salmivirta M, Bondjers G, Camejo G. Changes in matrix proteoglycans induced by insulin and fatty acids in hepatic cells may contribute to dyslipidemia of insulin resistance. Diabetes. 2001; 50: 2126–2132.(Germán Camejo)
Correspondence to Germán Camejo, AstraZeneca Discovery, S-431 83, M?lndal, Sweden. E-mail german.camejo@astrazeneca.com
Research during the past decade about the role of the transcription factors known as peroxisome proliferator-activated receptors (PPARs) subtype alpha (-), delta/beta (-/?), and gamma (-) indicates that they are master switches controlling tissue-specific physiological adaptations in the use of fatty acids, glucose, and amino acids during feeding and fasting.1,2 These nuclear receptors modulate in concert the expression of genes associated with specific metabolic pathways together with the signaling of hormones involved in energy homeostasis like insulin, glucagon, growth hormone (GH), and cortisol. In this capacity they seamlessly adjust the reversible transitions from storage to use of fatty acids and triglycerides, of glucose and glycogen, and of the more complex ones of amino acids and proteins. Moreover, enzymes and transporters required for the interpathway conversions of glucose into fatty acids and of some amino acids into glucose also are target genes for PPARs, at least when rodents and humans are maintained on a high carbohydrate supply. Endowed with such central functions it is not surprising that the PPARs have impact on metabolic alterations contributing to insulin resistance and diabetes, as well as on abnormalities of lipid metabolism causing dyslipidemias and atherosclerosis. Therefore is also logical that intense research is dedicated to evaluate how modulation of the PPARs could improve the dyslipidema associated with insulin resistance, insulin resistance itself, diabetes, and atherosclerosis.
See page 1897
The PPAR agonists, known generically as fibrates, reduce the risk of cardiovascular events especially in patients with the dyslipidemia of insulin resistance.3 At present, no data are available about the effect on cardiovascular events of the two PPAR agonists in clinical use, pioglitazone, and rosiglitazone. However studies are in progress and we will soon know whether these agents reduce coronary events in patients with type 2 diabetes, especially when used in addition to statins of proven efficacy. There are grounds to be optimistic about these trials because mouse models have shown that PPAR agonists do reduce atherosclerotic lesions4–7 and because in patients with type 2 diabetes PPAR agonists have been shown to reduce the carotid intima-media thickness as evaluated with ultrasound.8,9
In the midst of all the above positive lines of evidence indicating that PAR and PPAR agonists may exert antiatherosclerotic effects, there are apparent surprises: in this issue of Atherosclerosis, Thrombosis, and Vascular Biology, Hennuyer et al using a gene-manipulated mouse model show that although fenofibrate reduces macrophage-laden lesions the PPARg agonists, pioglitazone and rosiglitazone, do not.10 The mouse model used in this report is a knock-in that expresses human apoE2 instead of the murine apoE (E2-KI) developed by Sullivan et al.11,12 In humans, expression of the apoE2 isoform, ApoE2(Arg158>Cys158) can induce a dyslipidemia characterized by increased chylomicrons- and VLDL-remnants because the particles with such isoform have a lower affinity for the heparan sulfate of hepatic cells and for the remnant receptor, the LDL receptor-related protein (LRP). In addition these particles have a long circulating half-life because they are resistant to lipolysis by lipoprotein lipase.13 Therefore when the E2-KI mouse is fed a Western diet it accumulates large amounts of "VLDL-", "IDL-," and "LDL-like" cholesterol-rich remnant particles and has reduced amounts of HDL.
The finding that PPAR agonists have no antiatherosclerotic effects in this mouse model is apparently at odds with several studies in which troglitazone, rosiglitazone, and GW7845 (also a PPAR agonist) did show reduction of macrophage-rich lesions in LDL receptor–deficient and apoE-defective mice.4–7 On the other hand, Hennuyer et al present in this issue of ATVB data showing reduction of macrophage-laden lesions by the PPAR activator fenofibrate in agreement with previous studies in LDL receptor–deficient and apoE–deficient mice. Can we reconcile this paradox or does a paradox exist? Dosage or pharmacokinetics cannot be the explanation because similar rosiglitazone doses to those used in the present study, 25 to 50 times those prescribed to humans, have been shown to reduce lesions in LDL receptor–deficient mice. Are we left without explanations? Not likely, because probably the most straightforward explanation is put forward by the authors. This is that fenofibrate improves the dyslipidemia of the E2-KI mice whereas the PPARg agonists do not. However, is this unexpected? Not really, because these animals are neither insulin-resistant nor diabetic, and the dyslipidemia is probably entirely associated with the defective apoE2 isoform and not to defects of fatty acid use or storage that are modulated by PPAR and insulin signaling.
One of the most interesting findings in the article that motivates this editorial is how fenofibrate, an agonist that is not very potent toward the mouse PPAR, clearly reduces the plasma levels of remnants and increases HDL. Stimulation of PPAR in liver, and probably also the intestine, reduces the secretion of VLDL and consequently the intravascular formation of remnants, at the same time improving their lipolysis and removal by the liver via diminishing their apoCIII content. Decreased VLDL secretion and increased lipolysis on PPAR activation are well known effects of fibrates.3 At the other end, treatment of human hepatic cells with PPAR agonists, but not with PPAR agonists, increases the expression of genes and gene products for extracellular heparan-containing proteoglycans that bind more efficiently VLDL remnants.14 This is an important first step in remnant hepatic uptake that could have been enhanced in the fenofibrate-treated E2-KI mouse. It would be very interesting to test the effects of PPAR agonist in the remnant binding capacity of hepatocytes in this mouse.13
The article by Hennuyer et al presents a balanced discussion about the protective effects of the PPAR agonists on the development of macrophage-rich lesions in the E2-KI mouse and quotes published data mostly obtained by treatment of macrophages in cell cultures. One should exercise caution in the interpretation of some of the published work on the in vitro effects of PPAR agonists on cells. A common experimental design has been the use of cells cultured to the desired stage in 5% to 10% serum and, after washing them, 1 to 10 μmol/L of the PPAR agonists are added in media without albumin or at most with 1% serum. This common practice does not take into consideration that PPAR agonists are molecules that fit a hydrophobic pocket in the nuclear receptor similar to those pockets in albumin that bind fatty acids and other nonpolar molecules. Therefore, after oral administration most PPAR agonists are more than 90% bound to albumin, thus tissues are exposed to very low levels of free substance. The second reason why many of the in vitro observed pleiotropic effects of PPAR agonists may not translate fully in vivo is that the molecules were selected because they improve insulin action in models with defective insulin signaling. Thus many PPAR agonists at pharmacological doses show only modest effects in cells cultured at near to physiological conditions or in animals with normal response to insulin. This is probably the case of the E2-KI mouse used in the commented article that had normal plasma insulin and glucose and whose atherogenic dyslipidemia was entirely caused by the defective apoE2. This dyslipidemia apparently is not dependent of insulin resistance and should be therefore less responsive to PPAR agonists.
In conclusion the findings of Hunneyer et al provide interesting data about how PPAR agonists can improve a remnant-linked dyslipidemia that is particularly atherogenic and that is not associated with insulin resistance. In addition, they have further documented the important notion that the atherogenicity of a functionally-deficient apoE can be counteracted by PPAR stimulation, probably acting at the level of hepatic VLDL production and directly on the vascular tissue response to the dyslipidemia. Finally, the absence of effects of the PPAR agonists observed in this work supports the hypothesis that these drugs could be antiatherogenic only in a background of insulin-resistance and its associated defects of lipid metabolism.
Acknowledgments
I thank my colleagues Bengt Ljung and Nick Oakes from AstraZeneca, Discovery for useful discussions and suggestion about this editorial.
References
Kersten S, Desvergene B, Wahli W. Roles of PPARS in health and disease. Nature. 2000; 405: 421–424.
Francis G, Fayard E, Picard F, Auwerx J. Nuclear receptors and the control of metabolism. Ann Rev Physiol. 2003; 65: 261–311.
Fazio S, Linton M. The role of fibrates in managing hyperlipidemia: mechanisms of action and clinical efficacy. Curr Ather Rep. 2004; 6: 148–157.
Collins AR, Meehan WP, Kintscher U, Jackson S, Wakino S, Noh G, Palinski W, Hsue W, Law R. Troglitazone inhibits formation of early atherosclerotic lesions in diabetic and nondiabetic low density lipoprotein receptor-deficient mice. Arteriocler Thromb Vasc Biol. 2001; 21: 365–371.
Chen Z, Ishibashi S, Perrey S, Osuga J, Gotoda T, Kitamine T, Tamura Y, Okazaki H, Yahagi N, Iizuka Y, Shionoiri F, Ohashi K, Harada K, Shimano H, Nagai R, Yamada N. Troglitazone inhibits atherosclerosis in apolipoprotein E-knockout mice: pleiotropic effects on CD36 expression and HDL. Arteriocler Thromb Vasc Biol. 21: 372–377.
Li A, Brown K, Silvestre M, Wilson T, Palinski W, Glass K. Peroxisome proliferator-activated receptor gamma ligands inhibit development of atherosclerosis in LDL receptor-defcient mice. J Clin Invest. 2000; 106: 523–531.
Li AC, Binder CJ, Gutierrez A, Brown KK, Plotkin CR, Pattison JW, Valledor AF, Davis RA, Willson TM, Witztum JL, Palinski W, Glass CK. Differential inhibition of macrophage foam-cell formation and atherosclerosis in mice by PPAR, ?/, and . J Clin Invest. 2004; 114: 1564–1576.
Sidhu J, Kaposzta Z, Markus H, Kaski J. Effect of rosiglitazone on common carotid intima-media thickness in coronary artery disease patients without diabetes mellitus. Arterioscler Thromb Vasc Biol. 2004; 24: 930–943.
Langenfeld MR, Forst T, Hohberg C, Kann P, Lübben G, Konrad T, Füllert SD, Sachara C, Pfützner A. Pioglitazone decreases carotid intima-media thickness independently of glycemic control in patients with type 2 diabetes mellitus: results from a controlled randomized study. Circulation. 2005; 111: 2525–2531.
Hennuyer N, Tailleux A, Torpier G, Mezdour H, Fruchart JC, Staels B, Fievet C. PPAR, but not PPAR, activators decrease macrophage-laden atherosclerotic lesions in a nondiabetic mouse model of mixed dyslipidemia. Arterioscler Thromb Vasc Biol. 2005; 25: 1897–1902.
Sullivan PM, Mezdour H, Quarfordt SH, Maeda N. Type III hyperlipoproteinemia and spontaneous atherosclerosis in mice resulting from gene replacement of mouse apoE with human ApoE 2. J Clin Invest. 1998; 102: 130–135.
Knouff C, Hinsdale ME, Mezdour H, Altenburg MK, Watanabe M, Quafordt SH, Sullivan PM, Maeda N. Apo E structure determines VLDL clearance and atherosclerosis risk in mice. J Clin Invest. 1999; 103: 1579–1586.
Mahley R, Ji Z-S. Remnant lipoprotein metabolism: key pathways involving cell-surface heparan sulfate proteoglycans and apolipoprotein E. J Lipid Res. 1999; 40: 1–16.
Olsson U, Egnell AC, Rodríguez M, ?stergren G, Lorentzon M, Salmivirta M, Bondjers G, Camejo G. Changes in matrix proteoglycans induced by insulin and fatty acids in hepatic cells may contribute to dyslipidemia of insulin resistance. Diabetes. 2001; 50: 2126–2132.(Germán Camejo)