Temporal metabolic profiling of plasma during endotoxemia in humans

Endotoxemia induced by the administration of low-dose lipopolysaccharide (LPS) to healthy human volunteers is a well-established experimental protocol and has served as a reproducible platform for investigating the responses to systemic inflammation. Because metabolic composition of a tissue or body fluid is uniquely altered by stimuli and provides information about the dominant regulatory mechanisms at various cellular processes, understanding the global metabolic response to systemic inflammation constitutes a major part in this investigation complementing the studies undertaken so far in both clinical and systems biology fields. This article communicates the first proof-of-principle metabonomic analysis, which comprised global biochemical profiles in human plasma samples from healthy subjects given intravenous endotoxin at 2 ng/kg. Concentrations of a total of 366 plasma biochemicals were determined in archived blood samples collected from 15 endotoxin-treated subjects at five time points within 24 h after treatment and compared with control samples collected from four saline-treated subjects. Principal component analysis within this data set determined the sixth hour as a critical time point separating development and recovery phases of the LPS-induced metabolic changes. Consensus clustering of the differential metabolites identified two distinct subsets of metabolites that displayed common coherent profiles with opposing directionality. The first group of metabolites, which were mostly associated with pathways related to lipid metabolism, was upregulated within the first 6 h and downregulated by the 24th hour following LPS administration. The second group of metabolites, in contrast, was first downregulated until the sixth hour, then upregulated. Metabolites in this group were predominantly amino acids or their derivatives. In summary, nontargeted biochemical profiling and unsupervised multivariate analyses highlighted the prominent roles of lipid and protein metabolism in regulating the response to systemic inflammation while also revealing their dynamics in opposite directions.