GastrointestinalStressing Out Over Survival: Glutamine as an Apoptotic Modulator
Introduction
In the world of academic surgery, a desire to improve postoperative patient care and outcomes drives much of the research efforts. A major theme that has developed in this clinical realm is the concept of “nutritional pharmacology,” where specific nutrients are added in excess to patient nutritional regimens in an effort to enhance convalescence and reduce morbidity, mortality, and hospitalization time during critical illness. One of the more highly studied nutrients for this purpose is the amino acid glutamine (GLN). Classified as a “nonessential” amino acid by most biochemistry texts because of the ability of most cells to produce it, GLN has been reclassified as “conditionally essential” during the last decade because physiological demand often exceeds the cellular capacity to produce it endogenously during critical illness. The resulting GLN deficit can adversely affect cells that rely heavily on this amino acid for normal function, such as those of the gastrointestinal tract and immune system. The merits of GLN-supplemented nutritional regimens in improving outcomes has been evaluated and roundly debated. Indeed, most roundtable discussions on this topic end with the consensus that “we need more randomized prospective clinical trials” [1, 2]. Evidence for the clinical efficacy of GLN in critical illness will not be further discussed here; for very good recent reviews on this topic, the reader is referred to other sources [3, 4, 5, 6]. Instead, GLN supplementation in patient care was broached to provide context for a more academic topic: What are the cellular mechanisms by which GLN may potentially impact clinical outcomes? For clinicians, demonstrated efficacy of “glutamine therapy” would be sufficient, but as scientists, this is a topic that we are compelled to pursue.
The study of GLN in cellular physiology historically has focused on its anabolic effects, namely, its role as a metabolic precursor and physiological regulator of DNA and protein synthesis in cellular growth. GLN is particularly important for the growth, survival, and physiological health of actively dividing cells in the body such as fibroblasts [7, 8], enterocytes (intestinal epithelia) [9, 10], and lymphocytes [11, 12]. Indeed, most of these cell types have been shown to possess a GLN-intensive metabolic profile, almost to the point of auxotrophy. Not surprisingly, clinical conditions for which GLN therapy has been proposed and tested, such as after bone marrow transplant, maintenance of a “healthy bowel” after radiation therapy or resection, after burn injury and during chemotherapy, involve these cell types. Cancer cells also are avid GLN consumers [13, 14, 15, 16, 17]. In an effort to better understand the impact of GLN provision on energy metabolism, a recent study examined the effects of GLN depletion and subsequent repletion on metabolic and gene expression profiles in mouse hepatoma cells via microarray analysis and found that GLN depletion globally down-regulated metabolism [18], which is not surprising.
Just as cancer biologists have refocused their attention from growth (oncogenes) to evasion of programmed cell death (apoptosis and tumor suppressor genes) and the integration of the two processes during the last decade, recent studies have suggested that GLN may act not only to promote growth but also to suppress apoptosis and to evoke and modulate stress responses. This review therefore focuses on a new twist to an old theme, namely, the merits of GLN in supporting cellular physiology, not from an anabolic perspective, but rather as a survival factor. Hereafter, GLN metabolism and apoptosis are briefly reviewed, followed by a retrospective analysis of studies in specific cell types (enterocytes, cells of the immune system and cancer cells) showing a role for GLN in cell survival and in eliciting and modulating cellular stress responses, including implicated mechanisms for GLN effects.
Section snippets
GLN metabolism
As the most abundant amino acid in the plasma at levels around 0.6 mm, GLN exhibits the most rapid intracellular turnover rate of all amino acids [19]. Because of its abundance and rapid metabolism, GLN has been described as the major intercellular nontoxic ammonia shuttle in the body. GLN also serves as a metabolic intermediate contributing carbon and nitrogen for the synthesis of other amino acids, nucleic acids, fatty acids and proteins [20, 21]. Because GLN is a major source for cellular
Apoptosis
Programmed cell death (PCD) is an evolutionarily conserved biochemical pathway resulting in a characteristic morphological cell death termed apoptosis [30]. Hallmarks of apoptosis include membrane blebbing, cell shrinkage, chromatin condensation, and endonucleolytic cleavage of DNA [31]. Unlike necrosis, apoptosis is an energy (ATP)-dependent process that is highly regulated and avoids eliciting an inflammatory response from cell death. Apoptosis is required for successful organogenesis during
Enterocytes
The importance of GLN in maintaining gut homeostasis and health has long been established [58]. GLN is the major oxidative energy source for intestinal epithelial cells [59, 60]. Animal studies have shown the necessity of GLN for the synthesis of enterocyte nucleotides [61] and maintenance of intestinal glutathione levels [62]. Prolonged total parenteral nutrition (TPN) without GLN is known to result in whole-body GLN depletion and gut mucosal atrophy which can be ameliorated with GLN
Immune system-derived cells
The importance of GLN to cells of the immune system is well established [76, 77]. For example, GLN is required for the late events of T-cell activation, lymphocyte progression through the cell cycle [78], and protection of activated human T cells from apoptosis [79]. To determine the role of GLN in activation-induced T-cell death, Chang et al. stimulated Jurkat T cells, a CD4+ human lymphoblastoid cell line, with phorbol myristate acetate (PMA, 20 ng/mL), a protein kinase C activator, and
Hybridoma cells
Fusion of immortalized myeloma cells with spleen-derived lymphocytes to create monoclonal antibody – producing hybridoma cells has been standard practice and a watershed to the biotechnology industry since this innovative technique was conceived in the 1970s [90]. The exhaustion of nutrients, especially GLN, leads to apoptotic cell death during large-scale mammalian cell culture in bioreactors [91, 92]. Apoptosis can have devastating effects on the production of biopharmaceuticals like
Cancer cells
Some of the studies discussed earlier regarding “enterocytes” and “immune system-derived cells” used cancerous cell lines (HT-29, lymphoma, leukemia, myeloma), so those results may also be interpreted as cancer-related responses to GLN deprivation in addition to the underlying premise that they retain properties of the normal parent tissue. The propensity of cancerous cells to exhibit heightened GLN consumption has been long established, and has led to reluctance to use GLN-supplemented
Conclusions and future directions
In summary, overt GLN deprivation ultimately elicits apoptosis by intrinsic and/or extrinsic pathways, depending on cell type. Conversely, an enhanced GLN supply curbs death receptor-mediated apoptosis in certain cell types, but may actually enhance it in some cancer cells. Prior to the onset of apoptosis, GLN limitation promotes adaptive stress response pathways that aid in survival such as cell cycle arrest, ER stress (also known as the unfolded protein response) and angiogenesis, again
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