Cardiac expression of microsomal triglyceride transfer protein is increased in obesity and serves to attenuate cardiac triglyceride accumulation

Abstract

Obesity causes lipid accumulation in the heart and may lead to lipotoxic heart disease. Traditionally, the size of the cardiac triglyceride pool is thought to reflect the balance between uptake and beta-oxidation of fatty acids. However, triglycerides can also be exported from cardiomyocytes via secretion of apolipoproteinB-containing (apoB) lipoproteins. Lipoprotein formation depends on expression of microsomal triglyceride transfer protein (MTP); the mouse expresses two isoforms of MTP, A and B. Since many aspects of the link between obesity-induced cardiac disease and cardiac lipid metabolism remain unknown, we investigated how cardiac lipoprotein synthesis affects cardiac expression of triglyceride metabolism-controlling genes, insulin sensitivity, and function in obese mice. Heart-specific ablation of MTP-A in mice using Cre-loxP technology impaired upregulation of MTP expression in response to increased fatty acid availability during fasting and fat feeding. This resulted in cardiac triglyceride accumulation but unaffected cardiac insulin-stimulated glucose uptake. Long-term fat-feeding of male C57Bl/6 mice increased cardiac triglycerides, induced cardiac expression of triglyceride metabolism-controlling genes and attenuated heart function. Abolishing cardiac triglyceride accumulation in fat-fed mice by overexpression of an apoB transgene in the heart prevented the induction of triglyceride metabolism-controlling genes and improved heart function. The results suggest that in obesity, the physiological increase of cardiac MTP expression serves to attenuate cardiac triglyceride accumulation albeit without major effects on cardiac insulin sensitivity. Nevertheless, the data suggest that genetically increased lipoprotein secretion prevents development of obesity-induced lipotoxic heart disease.

Figure 1. Effect of heart-specific MTP-A-deficiency in chow-fed mice.

Mttp^flox/flox mice were bred with Mck-Cre^+/o mice to generate mice with heart-specific MTP-A-deficiency. A) Cre mRNA expression in liver (n = 4) and heart (n = 4) of Mttp^flox/floxMck-Cre^+/o mice, B) MTP-A mRNA expression in the liver (n = 4 in each group) and heart (n = 8 in each group) of Mttp^flox/flox and Mttp^flox/floxMck-Cre^+/o mice, C) Total MTP mRNA expression in the heart of Mttp^flox/flox (n = 8) and Mttp^flox/floxMck-Cre^+/o mice (n = 8), D) Cardiac MTP-B mRNA expression in Mttp^flox/flox (n = 8) and Mttp^flox/floxMck-Cre^+/o mice (n = 8), E) Cardiac MTP activity Mttp^flox/flox (n = 6) and Mttp^flox/floxMck-Cre^+/o mice (n = 6), F) Cardiac triglycerides in Mttp^flox/flox (n = 8) and Mttp^flox/floxMck-Cre^+/o mice (n = 8). Open bars: Mttp^flox/flox mice and closed bars: Mttp^flox/floxMck-Cre^+/o mice. Values are mean±SEM. The p values for two-group comparisons are: ** P<0.01; *** P<0.005 compared to chow-fed controls.

Figure 2. Effect of heart-specific MTP-A deficiency on cardiac lipid accumulation and MTP activity in fasted mice and fat-fed obese mice.

A) cardiac triglycerides in mice that were fasted for 18 hours and mice that were fat-fed for three months (n = 7–9 in each group). *P<0.05 compared to Mttp^flox/flox mice. B) MTP activity in fasted versus chow-fed mice (n = 6 in each group). **P<0.01 compared to fed mice, ‡ P<0.01 fasted Mttp^flox/flox compared to fasted Mttp^flox/floxMck-Cre^+/o mice A+B) Open bars: Mttp^flox/flox mice and closed bars: Mttp^flox/floxMck-Cre^+/o mice. C) MTP-A and MTP-B mRNA expression in fat-fed (closed bars) versus chow-fed (open bars) Mttp^flox/flox mice (n = 7 in each group). **P<0.01 and ***P<0.005 compared to chow-fed Mttp^flox/flox mice. D) MTP-A and MTP-B mRNA expression in fat-fed (closed bars) versus chow-fed (open bars) Mttp^flox/floxMck-Cre^+/o mice (n = 7 in each group). Values are mean±SEM.

Figure 3. Effect of heart-specific MTP-A deficiency on insulin-stimulated glucose uptake and expression of lipid metabolizing genes in obese mouse heart.

The effect of heart-specific MTP-A deletion on A) insulin-stimulated glucose uptake in cardiac ventricles, B) skeletal muscle, and C) epididymal fat was determined after 3 months of fat-feeding. D) Cardiac mRNA expression was quantified with real-time PCR in fat-fed male Mttp^flox/flox mice, Mttp^flox/floxMck-Cre^+/o mice and their lean controls. Values are after 3 months of diet. Open bars: chow-fed Mttp^flox/flox mice (n = 7–8), closed bars: fat-fed Mttp^flox/flox mice (n = 7–8), hatched bars: chow-fed Mttp^flox/floxMck-Cre^+/o mice (n = 7–8), squared bars: fat-fed Mttp^flox/floxMck-Cre^+/o mice (n = 7). Values are mean±SEM. The p values for two-group comparisons are: * P<0.05, ** P<0.01; *** P<0.005 compared to chow-fed controls; ‡ P<0.01 compared to fat-fed Mttp^flox/floxMck-Cre^+/o mice.

Figure 4. Overexpression of human apoB in the heart affects cardiac triglyceride levels as well as expression of genes involved in cardiac stress and metabolism of free fatty acids in fat-fed C57Bl/6 mice.

The effect of cardiac overexpression of a human apoB-transgene in obese mice on A) cardiac triglycerides were determined using TLC, B) cardiac mRNA expression of lipid metabolising genes and C) cardiac mRNA expression of stress-related genes was quantified with real-time PCR in male fat-fed C57Bl/6 (n = 11) and C57Bl/6-apoB-Tg mice (n = 9) and their lean controls (n = 6). Values are after 12 months of diet. Open bars: chow-fed C57Bl/6 mice, closed bars: fat-fed C57Bl/6 mice, hatched bars: chow-fed C57Bl/6-apoB-Tg mice, squared bars: fat-fed C57Bl/6-apoB-Tg mice. Values are mean±SEM. The p values for two-group comparisons are: * P<0.05, ** P<0.01; *** P<0.005 compared to chow-fed controls; ‡ P<0.05 Fat-fed C57Bl/6 compared to fat-fed C57Bl/6-apoB-Tg.

Figure 5. Overexpression of human apoB in the heart attenuates deterioration of cardiac function in fat-fed obese mice.

Heart function was determined with a conductance catheter placed in the left cardiac ventricle A) EF, ejection fraction, B) PRSW, preload recruitable stroke work. Open bars: chow-fed C57Bl/6 mice (n = 7), closed bars: fat-fed C57Bl/6 mice (n = 7), hatched bars: chow-fed C57Bl/6-apoB-Tg mice (n = 11), squared bars: fat-fed C57Bl/6-apoB-Tg mice (n = 7). Values are mean±SEM after 11 months on the diets. The p values for two-group comparisons are: * P<0.05, ** P≤0.01.