Regulation of the activation of heat shock factor 1: one mechanism of glutamine-mediated protection during cellular stress
Patients that are given intravenous glutamine pre-operatively have a reduced length of hospital stay and rate of sepsis. Glutamine administration prior to a variety of stresses improves survival in vitro as well. The mechanism of glutamine-mediated protection during stress is unknown. However, in mouse embryonic fibroblasts containing a knockout of the gene encoding the transcription factor heat shock factor 1 (HSF-1), glutamine-mediated protection is ablated, suggesting that HSF-1 is an important part of the glutamine-mediated stress response. HSF-1 is a transcription factor for heat shock protein 70 (HSP 70), one of the major stress-inducible molecular chaperones. During stressful conditions, HSF-1 becomes able to induce gene transcription after trimerization, hyperphosphorylation and localization to the nucleus. Our hypothesis is that glutamine-mediated protection is due to increased activation of HSF-1 and elevated HSP 70 production after glutamine treatment. In this research we demonstrated that two of the crucial steps in the activation of HSF-1, hyperphosphorylation and nuclear localization, are decreased in heat-stressed glutamine-starved mouse embryonic fibroblasts (Mefs) compared to heat-stressed Mefs maintained in 2 mM L-glutamine. HSP 70 levels are also decreased in the glutaminestarved heat stressed Mefs in a manner that is consistent with decreased HSF-1 activation in these cells. The manner in which glutamine treatment increases HSF-1 hyperphosphorylation and nuclear localization remains unclear. We found that the glutamine-starved Mefs have decreased levels of the sugar Olinked B N-acetylglucosamine (O-GlcNAc) relative to the glutamine-fed Mefs. OGlcNAc is attached post-translationally to over 500 intracellular proteins and increases in the level of O-GlcNAc are correlated with improved survival after a stressor in vitro. Glutamine-mediated protection may be linked to O-GlcNAc-mediated protection because glutamine is required for the metabolic pathway that produces O-GlcNAc, the hexosamine biosynthetic pathway (HBP), and a partial knockout of the enzyme that attaches this sugar to proteins leads to a decrease in glutamine-mediated protection. OGlcNAc modifications regulate protein function in a manner similar to phosphorylation. A future direction of this project is to determine if the glutamine-mediated changes in HSF-1 hyperphosphorylation and nuclear localization are moderated through O-GlcNAcmediated regulation of kinases, phosphatases and other proteins that affect HSF-1 activation.
© Copyright 2008 Sarah Haserodt