Dysbalance between fatty acid and glucose uptake in cardiac diseases: the role of substrate transporter trafficking.

Long-chain fatty acids (FA) and glucose are the predominant substrates for cardiac metabolic energy production. Under normal conditions there is a distinctive and very finely tuned balance between the utilization of these metabolic substrates. However, in chronic cardiac disease this balance is upset. While the development of hypertrophy and cardiac failure is characterized by a gradual decrease in FA utilization, partly compensated by increased glucose utilization, the diabetic heart suffers from impaired glucose uptake, and relies almost completely on FA. Conversely, a shift of this substrate balance through other causes elicits cardiac malfunctioning. For instance, a genetic limitation in FA utilization results in hypertrophic cardiomyopathy, while genetic alterations in cardiac glucose uptake also adversely affect cardiac function. Thus, tilting of the substrate balance and cardiac pathology appear invariably linked. Why a substrate dysbalance leads to cardiac malfunctioning is not yet understood, neither is the mechanism responsible for the substrate dysbalance in these cardiac diseases. With respect to the latter we have obtained very strong evidence for the involvement of substrate transporters, especially their translocation from intracellular stores to the sarcolemma. We discovered that (the bulk of) FA uptake occurs by a protein-mediated process, involving plasma-lemmal fatty acid-binding protein (FABPpm) and fatty acid translocase (FAT/CD36), and is acutely regulated by the reversible translocation of FAT/CD36 from endosomes to the sarcolemma. In analogy to the regulation of cardiac glucose uptake by the glucose transporter GLUT4, recruitment occurs upon either myocyte contractions or insulin treatment. The latter appears a novel role for insulin in cardiac FA utilization. Thus, the notion arises that cardiac substrate preference is regulated at the site of the sarcolemma by an interplay between FA and glucose transporters recruited from intracellular depots. This level of regulation differs from the classical concept that cardiac substrate metabolism is primarily regulated by enzymes of intermediairy metabolism. The central hypothesis to be studied is: (i) The dysbalance in cardiac FA and glucose utilization seen in hypertrophy/failure and in type-2 diabetic cardiomyopathy is caused by a dysbalance in cellular distribution of substrate transporters. (ii) Selective modulation of the sarcolemmal localization of FA and/or glucose transporters can be applied as therapeutic tool to rectify the cardiac substrate balance, thereby remedy cardiac malfunctioning occurring in these diseases. Disclosure of the signalling and trafficking routes involved in substrate transporter recruitment. Investigating the changes in FAT/CD36 recycling, in relation to that of GLUT4, in cardiac hypertrophy/-failure and in diabetes by analysing the signalling pathways recently discovered to be involved (PI-3 kinase and AMP-kinase cascades) and analysing the trafficking pathways. This knowledge will give clues to design strategies for selective modulation of substrate transporter localization (see B and C). Investigation of the causal relationship between malfunctioning transporter recruitment and cardiac disease. Studying whether selective modulation of FAT/CD36 and of GLUT4 recruitment to the sarcolemma, either by influencing (i) the signal transduction pathway(s) or (ii) the trafficking pathway(s) involved, affects cardiac substrate preference and elicits the development of disease. Both pharmacological agents and genetic modifications will be applied to isolated cells and to experimental animals (transgenic mice) and the functional consequences of a dysbalance between FA and glucose utilization investigated. Evaluation of substrate transporter trafficking as therapeutic target in cardiac disease. Testing of the suitability and efficacy of the recruitment of substrate transporters as therapeutic target to rectify the cardiac substrate balance and contractile performance in hypertrophy/failure and in diabetes. Therapeutic approaches will be tested by subjecting transgenic mice with heart-specific (inducible) alterations in substrate transporter recruitment to interventions eliciting cardiac hypertrophy/ failure or insulin resistance and diabetes. Studies will primarily be directed towards manipulating the mobilization of FAT/CD36, because for cardiac energy production FA are preferred over glucose, so that forced changes in FA uptake generally will be followed by secondary (reciprocal) changes in glucose uptake. All investigations will be performed on mice models. Results from recent pilot studies indicate that (i) the phosphodiesterase inhibitor dipyridamole is a powerful stimulator to alter the signalling cascade leading to the selective recruitment of FAT/CD36 to the sarcolemma, and (ii) caveolae, especially the cardiac-specific protein caveolin-3, play a pivotal role in the intracellular trafficking of FAT/CD36. Therefore, dipyridamole, caveolae-disrupting agents, and transgenic mice either null for the target protein of dipyridamole (soon to be identified) or for caveolin-3, or overexpressing these proteins in cardiac muscle, will be studied to unravel the consequences of alterations in FAT/CD36 recruitment for cardiac substrate selection and cardiac functioning, and whether these interventions elicit cardiac disease (especially hypertrophy/ failure and insulin resistance and type-2 diabetes). For the target protein of dipyridamole, heart-specific inducible (aMHC-MerCreMer) mice will be generated, allowing to conditionally induce the disruption of this gene. Finally, these transgenic animals will be subjected to experimental myocardial infarction to evoke cardiac hypertrophy/ failure, and to high-fat feeding to evoke insulin resistance and type-2 diabetes, and the development of these diseases studied as a function of the (inducible) deficiency or overexpression of these genes. This will yield insight into the suitability of substrate transporters as target for normalizing the cardiac substrate dysbalance seen in these diseases. Of pivotal importance for detailed evaluation of cardiac substrate preference and its regulation, and the role of the substrate transporters FAT/CD36 and GLUT4 therein, is an in