Paraoxonase activity in the serum and hepatic mRNA levels decrease during the acute phase response
Introduction
The host response to infection and inflammation is associated with a wide array of metabolic changes including alterations in lipid metabolism [1]. The liver plays a central role in these changes, as the hepatic synthesis of many proteins increase (positive acute phase proteins) while the synthesis of other proteins decrease [negative acute phase proteins) [2]. The regulation of acute phase protein synthesis is mediated primarily by alterations in mRNA levels [2]. These metabolic changes can be induced by the administration of endotoxin (LPS), which mimics gram negative infections and by cytokines, such as TNF and IL-1, which mediate many of the pathophysiologic and metabolic responses that occur during infection and inflammation 1, 2. LPS and cytokines rapidly increase serum triglyceride levels due to an increase in VLDL 1, 3, 4. Both hepatic overproduction of VLDL and delayed clearance which is mediated by decreases in lipoprotein lipase activity account for the hypertriglyceridemia 1, 3, 4. In rodents, but not in primates, LPS and cytokines also increase serum cholesterol levels due to an increase in LDL cholesterol 1, 5. The increase in LDL is associated with a stimulation of hepatic cholesterol synthesis which is due to an increase in HMG CoA reductase activity and mRNA levels 5, 6. Additionally, LPS and cytokines decrease the activity and mRNA levels of cholesterol 7α-hydroxylase, the initial enzyme in bile synthesis [7]. A decrease in the conversion of cholesterol to bile acids could provide additional cholesterol for lipoprotein production.
In many species LPS and cytokines decrease HDL cholesterol levels [1]. Moreover, there are profound composition changes in HDL including increases in serum amyloid A, apolipoprotein J and free cholesterol and decreases in apolipoprotein A-I and esterified cholesterol 1, 8, 9, 10. Additionally, many of the enzymes/transfer proteins associated with HDL and/or which are important in HDL metabolism, such as LCAT, CETP, PLTP, and hepatic lipase, decrease following infection, LPS and cytokine treatment 1, 8, 11, 12, 13, 14, 15, 16. Alterations in hepatic mRNA levels for HDL related proteins account for many of these alterations in HDL composition including the levels of HDL associated enzymes/transfer proteins. These changes may have profound effects on HDL metabolism including, (1) decreasing cholesterol efflux from peripheral cells to HDL secondary to the decreased apolipoprotein A-I, LCAT, and hepatic lipase [17]; (2) decreasing cholesterol transfer from HDL to other lipoproteins by decreasing CETP and hepatic lipase [17]; and (3) redirecting HDL cholesterol away from the liver and to macrophages by increasing serum amyloid A and apolipoprotein J on HDL and decreasing hepatic lipase 17, 18, 19. While these changes may be beneficial in the host's response to infection, they could, if sustained, decrease reverse cholesterol transport and thus be potentially proatherogenic.
In addition to reverse cholesterol transport, the antioxidant properties of HDL may also be important in preventing atherosclerosis 20, 21, 22. During the host response to inflammation, HDL particles are no longer able to protect LDL from oxidation [23]. Rather, during inflammation HDL particles amplify the oxidative process 22, 23. A recent paper found that PAF acetylhydrolase and paraoxonase (PON) levels markedly decrease in HDL during the acute phase response; enrichment of acute phase HDL with purified PAF acetylhydrolase or PON restores its protective effect against LDL oxidation [23]. Studies have shown that PON metabolizes and detoxifies the active biological lipids in oxidized LDL 24, 25.
Given the potential importance of PON in preventing the oxidation of lipoproteins, in the present study we examined the effect of LPS and cytokines on serum PON activity and hepatic PON mRNA levels in Syrian hamsters. Additionally, we determined the effect of cytokines on PON mRNA levels in HepG2 cells in culture.
Section snippets
Materials
[α-32P]dCTP (3000 Ci/mmol, 10 mCi/ml) was purchased from New England Nuclear (Boston, MA). Endotoxin (E. coli 55:B5) was purchased from Difco Laboratories (Detroit, MI) and was freshly diluted to desired concentrations in pyrogen-free 0.9% saline (Kendall McGraw Laboratories, Irvine, CA). Human TNF-α with a specific activity of 5×107 U/mg was kindly provided by Genentech (So. San Francisco, CA). Recombinant human IL-1β with a specific activity of 1×109 U/mg was generously provided by Immunex
Results
The effect of LPS administration on serum PON activity is shown in Fig. 1. Following LPS treatment there is a progressive decrease in serum PON activity with a 67% reduction at 48 h (Fig. 1A). As shown in Fig. 1B, at 24 h, as little as 100 ng/100 g body weight of LPS produces a decrease in serum PON activity (36%) while 100 μg/100 g body weight decreases serum PON activity 62%. The effect of cytokines on serum PON activity is shown in Fig. 2. Both TNF and IL-1 decrease serum PON activity and
Discussion
PON is an HDL associated enzyme which has been shown to metabolize and detoxify biologically active lipids in oxidized LDL 22, 24, 25. A previous study by Van Lenten et al. demonstrated that PON activity in HDL decreased in rabbits following injection of croton oil and in humans post operatively [23]. In the present study, we demonstrate that serum PON activity decreases following LPS administration. This decrease is seen within 24 h following LPS treatment and low doses of LPS (100 ng/100 g
Acknowledgements
We appreciate the excellent secretarial assistance of Pamela Herranz. We would like to thank Dr C. Furlong (University of Washington, Seattle, WA) for providing cDNA for paraoxonase. This work was supported by grants from the Research Service of the Department of Veterans Affairs and the National Institute of Health (DK 40990 and DK 49448).
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