Immunomodulatory Effects of Glutamine
While glutamine’s role in limiting oxidative injury remains unclear, its anti-inflammatory effects are well established and dependent on levels of heat shock proteins (HSP), acute phase reactants that assist in the refolding of proteins denatured by incipient insults (Wischmeyer et al. 2001a). Three members of the HSP family that play a vital role in cellular protection are HSP-70, HSP-72, and HSP-25 (rat equivalent of human HSP-27). Glutamine’s attenuation of cellular metabolic dysfunction and cell death appear to be HSP-dependent, as heat shock transcription factor-1 (HSF-1) knockout models eliminate glutamine’s effect (Peng et al. 2006). In vitro work suggests that glutamine induces a concentration-dependent increase in HSP-70 in intestinal epithelial cell lines, conferring a survival advantage against thermal and oxidative injury that is reversed by quercetin, an inhibitor of HSP-70 expression (Wischmeyer et al. 1997). Quercetin administration decreased HSP-70 expression and prevented glutamine-induced reduction of plasma TNF-a, chemokines, and neutrophil infiltration, and abolished glutamine’s renal-protective effects in murine acute kidney injury models (Peng et al. 2013). Likewise, glutamine’s protective effect against injury in a rodent model of smoke inhalation was associated with increased HSP-70 levels, increased levels of heme oxygenase 1, and decreased levels of NF-kB (Driks et al. 2013). Evidence also suggests that glutamine depletion results in decreased ability of human leukocytes to boost their HSP-70 expression in response to increased temperature. Glutamine also inhibits production of inducible nitric oxide synthetase (iNOS) and reduces nitric oxide (NO) levels, and these effects are abrogated in an HSF-1 knockout model incorporating murine embryonic fibroblast cells (Peng et al. 2006).
Rat endotoxemia models have helped shed light on the relationship between glutamine and both HSPs (Wischmeyer et al. 2001a). Wischmeyer and colleagues demonstrated that a single dose of glutamine significantly increased HSP-25 and HSP-72 levels in multiple organs, including the heart, lung, colon, kidney, liver, and ileum, in the unstressed rats. Sprague-Dawley rats injected with lipo- polysaccharide (LPS) had reduced mortality when treated with glutamine and fluid resuscitation; rats receiving glutamine and fluid resuscitation had reduced cellular infiltrates and improved morphology in alveolar and ileal tissues (Wischmeyer et al. 2001a). In a rat model of sepsis involving a controlled cecal puncture, animals receiving glutamine had increased levels of HSF-1 phosphorylation, HSP-25, and HSP-70. Metabolic parameters including ATP/ADP ratio and nicotinamide adenine (NAD) levels were also improved in the group receiving glutamine. The glutamine group also had a lower mortality (33% vs. 78%); quercetin reversed the glutamine-induced increases in HSP levels and improvements in mortality. There was no effect observed on GSH, potentially indicating that the HSP-mediated effects were more significant than those mediated by oxidation and reduction pathways (Singleton et al. 2005).
Glutamine increased HSP-72 levels in human peripheral blood mononuclear cells in response to thermal injury and attenuated increases in TNF-a levels in response to LPS stimulation (Wischmeyer et al. 2003). Similar findings were demonstrated with glutamate treatment in intestinal epithelial cells; 6-diazo-5-oxo-L-norleucine (DON), an inhibitor of the glutaminase enzyme responsible for converting glutamine to glutamate (Phanvijhitsiri 2006), prevented the increase in HSP-25 levels. On the other hand, the administration of buthionine sulfoximine, an inhibitor of the transformation of glutamine and glutamate to GSH, had no effect; the authors inferred that the increase in HSP-25 levels was predominantly due to conversion of glutamate to glutamine and not via a redox pathway.
Rodent models of Gram-negative sepsis simulated by LPS administration have also proved instructive. Administration of glutamine in conjunction with fluid resuscitation resulted in attenuated release of TNF-a and IL-10, and rats receiving glutamine had significantly decreased mortality (Wischmeyer et al. 2001b). The effect of glutamine was manifested by improvements in oxygenation and base deficit 6 h after LPS administration. Autopsy data indicated that the rats receiving glutamine had better preservation of tissue structure and decreased amounts of inflammatory cell infiltration into the lung and small intestine. Similarly, glutamine administration resulted in decreased serum levels of TNF-a and decreased leukocyte adherence and plasma extravasation compared to controls (Scheibe et al. 2009).
Taken together, the cellular and animal model data suggest that glutamine may exert significant protective effects during critical illness. The weight of the data suggests that beneficial effects are mediated through anti-inflammatory mechanisms rather than redox pathways.