Sweet Nothings: Sensing of sugar metabolites controls T cell function

Abstract

Signal transduction and metabolism cooperate to control cell fate, but mechanisms that link metabolic substrates to functional decisions are elusive. Now, Chang et al. in Cell provide a mechanism whereby available sugars dictate metabolic pathways in activated T cells and direct a non-metabolic regulatory function of glyceraldehyde-3-phosphate dehydrogenase.

A resurgence of interest in cellular metabolism has emerged with the realization that signaling pathways directly influence metabolic reprogramming. In cancer, oncogenic pathways directly promote a metabolic program termed aerobic glycolysis (glycolysis even in the presence of abundant oxygen). Likewise, activation of T cells to exit quiescence, proliferate, and produce cytokines, such as interferon-γ, also induces aerobic glycolysis. The benefit of this form of metabolism has been a point of debate since Otto Warburg first described the phenomenon in cancers in the 1920s. While inefficient at generating ATP, consensus has emerged that aerobic glycolysis acts to provide biosynthetic building blocks and support cell growth. Nevertheless, the broad cellular physiological implications of aerobic glycolysis for cell fate decisions have been poorly understood. Now, in a recent paper by Chang and colleagues (2013), a link is established in activated T cells between a glycolytic substrate and a non-enzymatic function of a glycolytic enzyme, influencing the functional consequences of activation.

Several checkpoints now link the availability of substrates, oxygen, or ATP to signaling. For example, AMPK and TORC1 “sense” ATP and amino acid levels, respectively, and some metabolic enzymes have been implicated in signaling events, independent of their metabolic functions. The question persists, however, as to whether metabolic enzymes can act as “sensors” of their substrates to alter cell fate through non-metabolic processes (Wang and Green, 2012). Chang and colleagues (2013) describe such a convergence of signaling and metabolism in the function of activated T lymphocytes: a metabolic enzyme directly regulates the translation of specific mRNAs in a manner controlled by the availability of its substrate. Therefore the mode of energy production by an activated T cell directly impacts on the function of the cell, and we are beginning to understand how.

To address the role of aerobic glycolysis in the proliferation and signaling of activated T cells, Chang et al. (2013) modified the timing and capacity of T cells to perform glycolysis or mitochondrial electron transport. They found that electron transport becomes dispensable in proliferating T lymphocytes, as rotenone and antimycin A (complex I and III inhibitors, respectively) did not prevent cell cycle progression once initiated. This is in contrast to recent observations showing that the function of complex III is essential for T cell activation prior to proliferation (Sena, et al., 2013). Although Chang, et al. (2013) did not examine glutamine utilization under these conditions, it is possible that α-ketoglutarate (αKG) enters the TCA cycle in reverse to directly provide citrate as a source of lipid production, as has been shown in activated T cells under hypoxia (Mullen, et al., 2011). Chang et al. (2013) went on to show by replacing glucose with galactose that even aerobic glycolysis is dispensable for proliferation of activated T cells. Since the conversion of galactose to glucose “costs” two ATP, ATP cannot be gained by glycolysis alone (Figure 1) making electron transport essential for the energetic demands of proliferation. Somewhat unexpectedly, galactose carbons did not appear to enter the glycolytic pathway in activated T cells, and therefore the intermediary metabolites of glycolysis presumably decline. Rather, it is possible that galactose-derived glucose is shuttled into the pentose phosphate pathway for de novo production of nucleotides (Figure 1).

Figure 1. Glyceraldehyde 3-Phosphate determines GAPDH suppression of IFN-γ translation.

Glucose and galactose were found to differentially enter the glycolytic pathway and regulate GAPDH. Galactose requires additional ATP to enter metabolic pathways and did not appear to undergo glycolysis but rather enther the Pentose Phosphate Pathway (PPP). When G3P was limiting with galactose, GAPDH associated with the 3′UTR of IFNγ mRNA and prevented translation. Increased glycolytic flux and G3P provided by glucose availability, however, relieved GAPDH inhibition of IFNγ mRNA translation. Thus aerobic glycolysis provides a nutrient signal through G3P to directly modify cytokine production.

But here is where things get particularly interesting. While activated T cells grown in galactose proliferated normally, their ability to produce IFNγ was severely compromised. Levels of IFNγ mRNA were normal, but IFNγ mRNA was not found in polysomes of galactose-cultured cells, and this was a function of a 3′AU-rich element (ARE) in the IFNγ mRNA (Figure 1). The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) had previously been found to bind to this 3′ARE and inhibit the translation of IFNγ, and indeed, the authors found that GAPDH was bound under galactose-fueled conditions. Remarkably, this binding and inhibition of IFNγ translation was blocked by the presence of the GAPDH substrate, glyceraldehyde 3-phosphate (G3P), either provided directly or by re-addition of glucose. Presumably, G3P becomes limiting when activated T cells are fueled with galactose and GAPDH is released to block IFNγ translation and T cell function.

While GAPDH is generally considered a metabolic enzyme (its “day job”), it is known to have other non-metabolic activities (its “night jobs”). GAPDH has previously been identified as a component of the gamma interferon-activated inhibitor of translation (GAIT) complex, which regulates selective translation in myeloid cells (Jia, et al., 2012). While Chang, et al. (2013) did not find any evidence for the GAIT complex in galactose-fueled T cells, G3P may influence the activity of this complex as well by controlling the availability of GAPDH. Other “night jobs” of GAPDH are also described, including roles in promoting (Hara, et al., 2005) or inhibiting (Colell, et al., 2007) some forms of cell death, regulation of nuclear protein nitrosylation (Kornberg, et al., 2010), and possibly control of autophagy/mitophagy (Colell, et al., 2007). Again, the possible role for G3P in controlling the availability of GAPDH for these processes remains to be tested

Intriguingly, the activated, galactose-fueled T cells expressed PD1, a marker of T cell “exhaustion,” that when ligated further inhibits T cell function (Barber, et al., 2006). Whether G3P and GAPDH control Programmed Death-1 (PD-1) expression is not known. However, it may be important that tumor infiltrating T cells express PD-1, and blockade of the PD-L1 (present on tumors and inflamed tissues)-PD-1 interaction enhances anti-cancer immunity (Pardoll, 2012). Therefore, the metabolic milieu of tumors (among other conditions) may have a profound impact on T cell biology and function, perhaps via this GAPDH mechanism. If so, then development of GAPDH agonists might have promise for enhancing immunotherapies through regulating the availability of GAPDH for its night job activity in controlling transcription, and in turn, T cell function

Night jobs for other glycolytic enzymes have been suggested for hexokinase-2 (Majewski, et al., 2004) and PKM2 (Yang, et al., 2011), and additional functions of other metabolic enzymes may very well emerge. The insights from Chang, et al. (2013) suggest that nutrients and substrates in aerobic glycolysis support not only cell growth, but also may directly modify cell signaling and other functional aspects of cell physiology by influencing non-metabolic functions of metabolic enzymes. The roles of specific substates (or products) should be examined, possibly shedding new light on the emerging connections between metabolism, signal transduction, and cell fate. We can envision the development of novel substrate mimetics that stimulate or inhibit the night jobs of GAPDH or other metabolic enzymes to influence cell fate decisions. Our emerging understanding of the night jobs of some of these, such as GAPDH, holds terrific promise for integrating and ultimately controlling the connections between metabolism and cell functions.