CardioNet: a human metabolic network suited for the study of cardiomyocyte metabolism.

BACKGROUND: Availability of oxygen and nutrients in the coronary circulation is a crucial determinant of cardiac performance. Nutrient composition of coronary blood may significantly vary in specific physiological and pathological conditions, for example, administration of special diets, long-term starvation, physical exercise or diabetes. Quantitative analysis of cardiac metabolism from a systems biology perspective may help to a better understanding of the relationship between nutrient supply and efficiency of metabolic processes required for an adequate cardiac output. RESULTS: Here we present CardioNet, the first large-scale reconstruction of the metabolic network of the human cardiomyocyte comprising 1793 metabolic reactions, including 560 transport processes in six compartments. We use flux-balance analysis to demonstrate the capability of the network to accomplish a set of 368 metabolic functions required for maintaining the structural and functional integrity of the cell. Taking the maintenance of ATP, biosynthesis of ceramide, cardiolipin and further important phospholipids as examples, we analyse how a changed supply of glucose, lactate, fatty acids and ketone bodies may influence the efficiency of these essential processes. CONCLUSIONS: CardioNet is a functionally validated metabolic network of the human cardiomyocyte that enables theorectical studies of cellular metabolic processes crucial for the accomplishment of an adequate cardiac output.

Activin A stimulates vascular endothelial growth factor gene transcription in human hepatocellular carcinoma cells.

BACKGROUND & AIMS: Up-regulation of vascular endothelial growth factor is known to play a critical role in hepatocellular tumor biology. In an attempt to identify factors responsible for vascular endothelial growth factor induction in human hepatocellular carcinoma, we evaluated the effects of activin A, a member of the transforming growth factor-beta cytokine superfamily, on vascular endothelial growth factor gene expression. METHODS: Expression of vascular endothelial growth factor, activin A, and its receptors was analyzed by immunohistochemistry, polymerase chain reaction, and enzyme-linked immunosorbent assay. Functional vascular endothelial growth factor promoter analysis and gel shift assays were performed to define minimal promoter requirements and potential transcription factors. Nuclear expression and biochemical modifications of Sp1, as well as subcellular distribution, expression, and physical interaction of Smad proteins with Sp1, were assessed with immunoprecipitation and Western blot analysis. RESULTS: Hepatocellular carcinoma tumors and cell lines expressed activin A and its receptors. Activin A stimulated vascular endothelial growth factor gene transcription through Sp1-dependent induction of vascular endothelial growth factor promoter activity. Furthermore, activin A stimulated the DNA-binding and transactivation potential of Sp1. Immunoprecipitation showed activin A-dependent nuclear translocation of Smad2 and induction of Sp1/Smad2 interaction. The functional relevance of Sp1/Smad2 interaction was confirmed by transient transfection experiments, which showed that overexpression of Smad2 increased vascular endothelial growth factor promoter activity and endogenous vascular endothelial growth factor protein expression, whereas dominant negative Smad2 blocked activin A responsiveness. CONCLUSIONS: This study identifies activin A as a novel stimulus of vascular endothelial growth factor gene expression in hepatocellular carcinoma and delineates physical and functional cooperation of Sp1 and Smad2 as the underlying mechanism.