Abstract
Previously, we identified calcium-independent phospholipase A2gamma (iPLA2gamma) with multiple translation initiation sites and dual mitochondrial and peroxisomal localization motifs. To determine the role of iPLA2gamma in integrating lipid and energy metabolism, we generated transgenic mice containing the alpha-myosin heavy chain promoter (alphaMHC) placed proximally to the human iPLA2gamma coding sequence that resulted in cardiac myocyte-restricted expression of iPLA2gamma (TGiPLA2gamma). TGiPLA2gamma mice possessed multiple phenotypes including: 1) a dramatic approximately 35% reduction in myocardial phospholipid mass in both the fed and mildly fasted states; 2) a marked accumulation of triglycerides during brief caloric restriction that represented 50% of total myocardial lipid mass; and 3) acute fasting-induced hemodynamic dysfunction. Biochemical characterization of the TGiPLA2gamma protein expressed in cardiac myocytes demonstrated over 25 distinct isoforms by two-dimensional SDS-PAGE Western analysis. Immunohistochemistry identified iPLA2gamma in the peroxisomal and mitochondrial compartments in both wild type and transgenic myocardium. Electron microscopy revealed the presence of loosely packed and disorganized mitochondrial cristae in TGiPLA2gamma mice that were accompanied by defects in mitochondrial function. Moreover, markedly elevated levels of 1-hydroxyl-2-arachidonoyl-sn-glycero-3-phosphocholine and 1-hydroxyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine were prominent in the TGiPLA2gamma myocardium identifying the production of signaling metabolites by this enzyme in vivo. Collectively, these results identified the participation of iPLA2gamma in the remarkable lipid plasticity of myocardium, its role in generating signaling metabolites, and its prominent effects in modulating energy storage and utilization in myocardium in different metabolic contexts.
Highlights
Maladaptive changes in lipid metabolism leading to the intracellular accumulation of triglycerides are increasingly recognized as the likely cause of the multiple end organ sequelae of the metabolic syndrome and diabetes (1– 6)
Transgenic mice selectively expressing iPLA2␥ in cardiac myocytes were generated, and their resultant phenotypes were characterized demonstrating the following: 1) the profound depletion of specific phospholipid classes and individual molecular species; 2) gross morphologic abnormalities in mitochondrial cristae; 3) multiple defects in mitochondrial function, including changes in state 3 and state 4 respiration; and 4) marked augmentation of fasting-induced accumulation of triglycerides that was accompanied by the acute precipitation of cardiac hemodynamic dysfunction
Myocardium actively extracts fatty acids from serum resulting in increases in the intracellular triglyceride content of cardiac myocytes from Ϸ2–3 mol % in the fed state to Ϸ5– 6 mol % in the fasted state
Summary
Materials—Radionucleotide 1-palmitoyl-2-[1-14C]arachidonyl phosphatidylcholine was purchased from PerkinElmer. Two-dimensional mass spectrometric profiles of lipids extracted from calorically restricted TGiPLA2␥ myocardium demonstrated remarkable differences in the acyl chain composition of individual TAG molecular species relative to those from TG mice fed ad libitum (Fig. 4). Examination of TAG molecular species containing 16:0 fatty acid in fed (Fig. 4A) and fasted (Fig. 4B) TGiPLA2␥ transgenic myocardium (NL 256.2 mass spectra in Fig. 4) revealed a shift in the relative intensity of the ion peaks at m/z 837.7– 839.7 and from m/z 863.7 to 865.7 These alterations indicate a remodeling of TAG, increasing the content of saturated acyl chains (i.e. decreasing the relative unsaturated acyl chain content) after 16 h of caloric deprivation. Comparing WT and TGiPLA2␥ after fasting, TGiPLA2␥ shows a dramatic increase in left ventricular
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