William Doerrler

LDL subclass phenotypes and triglyceride metabolism in non-insulin-dependent diabetes.

Plasma low density lipoprotein (LDL) comprises multiple discrete subclasses differing in size, density, and chemical composition. A common, heritable phenotype characterized by the predominance of small, dense LDL particles (LDL subclass phenotype B) is associated with relatively increased concentrations of plasma triglycerides, reduced levels of high density lipoprotein, and increased risk of coronary artery disease in comparison with subjects with larger LDL (LDL subclass phenotype A). Population studies have indicated that approximately 20-30% of adult men have phenotype B, and another 15-20% have LDL of intermediate size. The lipid changes in phenotype B are similar to those that have been observed in patients with non-insulin-dependent diabetes mellitus (NIDDM). In the present study, we have assessed LDL subclass phenotypes in normolipidemic men with NIDDM and in age-matched control subjects who had similar lipid levels. There was a greater than twofold increase in the percentage of individuals with the LDL B phenotype in the NIDDM subjects. The LDL B phenotype was associated with higher plasma triglyceride levels and a trend toward lower high density lipoprotein cholesterol levels compared with the LDL A phenotype in the NIDDM subjects, as has been previously observed in control groups. Indices of diabetic control, such as fasting and hemoglobin A1 levels, were similar regardless of LDL phenotype pattern, suggesting that glycemic control was not likely to account for the increase in the LDL B phenotype. In both control and NIDDM subjects, the clearance of triglyceride-rich lipoproteins was slowed in the subjects with the LDL phenotype B compared with those with the A phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokines induce catabolic effects in cultured adipocytes by multiple mechanisms

Previous studies have demonstrated that some cytokines induce a coordinate catabolic response in adipose cells which leads to decreased fat storage. The mechanisms by which cytokines cause these effects are unknown. The primary purpose of the present study was to determine the effects of TNF, IL-1, IFN-alpha and IFN-alpha on the mRNA levels of the key enzymes involved in fat metabolism in 3T3-F442A adipocytes. TNF, IL-1, IFN-alpha and IFN-gamma decreased lipoprotein lipase activity and increased lipolysis in adipocytes. TNF, IFN-alpha and IFN-gamma decreased fatty acid synthesis while IL-1 increased fatty acid synthesis. However, the cytokine effects on mRNA levels were not always consistent with the observed changes in activity and were unique for each cytokine. Specifically, while all cytokines decreased LPL activity, only TNF and IFN-gamma decreased LPL mRNA levels. In addition, while TNF, IFN-alpha and IFN-gamma decreased fatty acid synthesis, only TNF significantly decreased the mRNA levels of both acetyl CoA carboxylase and fatty acid synthase, the key enzymes in fatty acid synthesis. IFN-alpha and IFN-gamma decreased fatty acid synthase mRNA levels without significantly altering acetyl CoA carboxylase mRNA. IL-1 caused a slight increase in fatty acid synthesis and increased acetyl CoA carboxylase mRNA levels. Finally, while all cytokines increased lipolysis, hormone sensitive lipase mRNA levels were decreased by TNF, IFN-alpha and IFN-gamma treatment. These results indicate that the regulation of adipocyte lipid metabolism by cytokines is complex and that coordinate changes in mRNA levels cannot account for the observed metabolic changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of interleukin-1 on lipid metabolism in the rat. Similarities to and differences from tumor necrosis factor.

Infection and inflammation are associated with hypertriglyceridemia, which is thought to be mediated by cytokines. Previous studies at our laboratory and others have shown that tumor necrosis factor acutely increases serum triglyceride levels primarily by stimulating hepatic lipid synthesis and secretion. The role of interleukin-1 (IL-1), a cytokine that is also secreted by stimulated macrophages and that has many actions that overlap those of tumor necrosis factor, has not been studied in depth. The present study demonstrates that IL-1, at doses similar to those that cause fever and anorexia and that stimulate adrenocorticotropic hormone secretion, rapidly increases serum triglyceride levels; this elevation persists for at least 17 hours. Serum cholesterol levels are not altered by IL-1. Neither is the clearance of triglyceride-rich lipoproteins affected by IL-1. However, hepatic triglyceride secretion, measured by the Triton WR-1339 technique, is increased in IL-1-treated animals. Accompanying this stimulation in hepatic lipid secretion is an increase in de novo fatty acid synthesis in the liver. IL-1 does not increase serum free fatty acid and glycerol levels, suggesting that IL-1 does not stimulate lipolysis in vivo. Additionally, inhibition of lipolysis does not prevent the increase in serum triglyceride levels, providing further evidence that lipolysis does not play a crucial role in the increased hepatic lipid synthesis and secretion induced by IL-1. In contrast, tumor necrosis factor increases lipolysis, which contributes to the increase in serum triglycerides. That multiple cytokines rapidly elevate plasma triglyceride levels suggest that these changes in lipid metabolism may play an important role in the organisms response to infection and inflammation.

Endotoxin and cytokines increase hepatic messenger RNA levels and serum concentrations of apolipoprotein J (clusterin) in Syrian hamsters.

Infection and inflammation induce alterations in hepatic synthesis and plasma concentrations of the acute phase proteins. Our results show that apolipoprotein (apo) J is a positive acute phase protein. Endotoxin (LPS), tumor necrosis factor (TNF), and interleukin (IL)-1 increased hepatic mRNA and serum protein levels of apo J in Syrian hamsters. Hepatic apo J mRNA levels increased 10- to 15-fold with doses of LPS from 0.1 to 100 micrograms/100 g body weight within 4 h and were elevated for > or = 24 h. Serum apo J concentrations were significantly increased by 16 h and further elevated to 3.3 times that of control, 24 h after LPS administration. Serum apo J was associated with high density lipoprotein and increased fivefold in this fraction, after LPS administration. Hepatic apo J mRNA levels increased 3.5- and 4.6-fold, with TNF and IL-1, respectively, and 8.2-fold with a combination of TNF and IL-1. Serum apo J concentrations were increased 2.3-fold by TNF, 79% by IL-1, and 2.9-fold with a combination of TNF and IL-1. These results demonstrate that apo J is a positive acute phase protein.

Cytokines stimulate lipolysis and decrease lipoprotein lipase activity in cultured fat cells by a prostaglandin independent mechanism.

We previously showed that indomethacin blocked the effect of tumor necrosis factor (TNF) and other cytokines on lipolysis. We now show that TNF stimulates prostaglandin (PG) production, enhances lipolysis and decreases lipoprotein lipase (LPL) activity in 3T3-F442A adipocytes and indomethacin blocks these activities, suggesting that the actions of TNF are mediated by PGs. However, exogenous PGE2 at the levels induced by TNF is not sufficient to affect lipolysis or LPL activity and low doses of indomethacin and flurbiprofen block PG production without affecting TNFs action. Interleukin-1 and interferon-alpha and gamma induce lipolysis and decrease LPL activity but do not stimulate much PG production. These results demonstrate that cytokines enhance lipolysis and decrease LPL activity in 3T3 adipocytes by a PG independent mechanism.

Tumor Necrosis Factor–Increased Hepatic Very-Low-Density Lipoprotein Production and Increased Serum Triglyceride Levels in Diabetic Rats

Previous studies demonstrated that administration of tumor necrosis factor (TNF) to diabetic rats rapidly increases serum triglyceride levels and stimulates hepatic lipogenesis without affecting the activity of adipose tissue lipoprotein lipase or serum insulin levels. The purpose of this study was to determine the mechanism by which TNF increases serum triglyceride levels and stimulates hepatic fatty acid synthesis in diabetic animals. The maximal increase (∼2-fold) in serum triglyceride levels in diabetic rats is seen with a dose of 10 μg TNF/200 g body wt, and the halfmaximal effect is observed with 5 μg TNF/200 g body wt. The clearance of labeled triglyceride-rich lipoproteins from the circulation is not affected by TNF administration (triglyceride t½: diabetic vs. TNFadministered diabetic, 3.5 ± 0.7 vs. 4.0 ± 0.6 min, respectively; NS). The production of triglyceride, measured by the Triton WR-1339 technique, is increased twofold in diabetic animals after TNF administration. These results indicate that the rapid increase in serum triglyceride levels after TNF treatment is accounted for by increased hepatic lipoprotein secretion. TNF administration did not alter either the amount or activation state of hepatic acetyl-CoA carboxylase, a key regulatory enzyme in fatty acid synthesis. There was also no change in the hepatic levels of fatty acyl-CoA, an allosteric inhibitor of acetyl-CoA carboxylase. However, there was a 71% increase in hepatic citrate concentrations. Citrate is an allosteric activator of acetyl-CoA carboxylase, and changes in hepatic citrate concentrations have been shown to mediate changes in the rates of fatty acid synthesis. These results suggest that the TNF-induced stimulation of hepatic lipogenesis is mediated by citrate activation of acetyl-CoA carboxylase. At 2 h after TNF administration, neither serum glucose nor p-hydroxybutyrate levels were adversely altered in the TNF group, indicating that the disturbances in lipid metabolism are not dependent on alterations in glycemic control. The increases in serum triglyceride levels that occur during infections or stress in diabetes may be secondary to TNF.