Parallel determination of glucose metabolism and fractional synthesis rates of individual hepatic proteins during hyperdynamic septic shock

Introduction

Proteomic platforms allow one to detect changes in the proteome of tissues. They can be combined with the use of stable isotope-labelled tracer amino acids to detect changes in the synthesis or breakdown rate of individual proteins. An ideal complement to these data would be a characterization of the metabolism. We focus on the defense against oxidative stress, which is closely related to energy metabolism. The latter can be assessed using isotope-labelled 1,2,3,4,5,6-¹³C6-glucose and following the metabolic fate of the labelled carbons. We therefore investigated whether an in-vivo septic animal model with labelled glucose would provide useful data about glucose and energy metabolism and whether the same tracer set up could be used to reliably estimate the turnover of individual proteins.

Methods

In a cecal ligation and puncture (CLP) model anaesthetized and mechanically ventilated mice were infused over 8–10 hours with labelled glucose and then liver samples were taken. Samples from five septic and five sham-operated mice were analysed by 2D-Page gel electrophoresis/Maldi TOF mass spectrometry, and the turnover of individual proteins was determined [1]. The same samples were also used for isotopomer analysis of glutamate and glucose to assess glucose production and oxidation via the Krebs or citric acid cycle [2]. Comparisons were based on Mann-Whitney rank sum tests.

Results

Endogenous glucose production significantly decreased from 3.1 (CLP) to 1.6 (sham) mg/g/hour. Glucose oxidation via acetyl-CoA and subsequent use in the Krebs cycle was significantly reduced under sepsis. Metabolic labelling via glucose resulted in extensive labelling of glucose, glutamate and alanine such that 30–70% of the molecules had more than two carbons simultaneously labelled. It was sufficient to detect sepsis-induced changes in the protein labelling and their fractional synthesis rate: HSC71, a constitutive chaperone, had a reduced rate, the heat shock proteins HSP60 and HSP70 showed a minor increase, and protein disulfide isomerase, a protein repair enzyme, showed a significant increase.

Conclusion

Metabolic labelling with glucose allows one to simultaneously assess key parameters of glucose metabolism and changes in the dynamics of individual proteins. The combined measurements allow a fine-grained characterization of sepsis and can be used to link changes in the energy metabolism to changes in the dynamics of individual proteins.