The transcription factor c‐MYC functions as the master transcription factor for establishing highly active metabolic states in proliferating cells. c‐Myc is essential for rapid proliferation of normal cells and has causal relationship with many cancers, including leukemia and lymphoma. While the expression of c‐MYC can be aberrantly driven by genetic abnormalities, such as chromosomal translocations directly involving the MYC locus or mutations of its upstream regulators, how c‐MYC expression is induced and amplified in normal lymphocytes in response to antigen stimulation remains elusive. In this issue of The EMBO Journal, Preston et al (2015) report how c‐MYC is selectively induced and amplified in the antigen‐specific T cells that undergo massive clonal expansion for host protection against pathogen infection.
See also: GC Preston et al
During immune responses to microbial challenge, activated T lymphocytes undergo rapid clonal expansion and progressively acquire effector functions (Pearce, 2010; Kaech & Cui, 2012). These processes are highly energy‐consuming due to the high demands of biogenesis and elevated levels of gene expression and protein translation. Therefore, activated T cells must establish a robust genetic program to facilitate an increase in nutrient uptake and metabolic reprogramming that activates aerobic glycolysis and glutaminolysis (Pearce et al, 2013). One of the most critically important transcriptional regulators required for these drastic changes in cellular activity is the proto‐oncogene c‐MYC (Wang & Green, 2012). c‐MYC not only specifically activates the expression of genes responsible for this metabolic reprogramming but also globally amplifies gene transcription through the regulation of RNA polymerase II complexes (Lin et al, 2012; Nie et al, 2012; Wang & Green, 2012). These aspects of c‐MYC functions have strong causal relationship with tumorigenesis. While the importance of rapid induction of c‐MYC protein in activated T cells by antigen receptor stimulation or cytokine receptor signals has been recognized, it remains unclear how the expression of c‐MYC is regulated by distinct environmental cues in activated T cells. Preston et al (2015) revealed a unique stepwise regulation of c‐MYC expression, which is differentially regulated by antigen receptor and cytokine signals (Fig 1).
Several studies have established that antigen receptor stimuli via TCR induce c‐Myc mRNA and protein expression at the population level (Wang et al, 2011). However, the impact of differential signaling strength on c‐MYC protein expression at the single‐cell level remains unknown. Preston et al (2015) first used a Myc‐GFP fusion reporter mouse to show that, shortly after activation, only a fraction of stimulated T cells upregulate c‐MYC protein. Using a fixed TCR and altered peptide ligands with varying affinity for that TCR, the authors further demonstrated that stronger TCR stimuli with a high‐affinity ligand induced the expression of c‐MYC protein at a higher frequency. Interestingly, levels of c‐MYC protein in cells that acquire c‐MYC expression are comparable regardless of the ligand affinities for the fixed TCR. These findings suggest that induction of c‐MYC requires an intrinsic threshold defined by the strength of TCR stimulation. Thus, antigen receptor stimuli alone determine whether cells turn on c‐MYC but not how much c‐MYC is expressed, hence functioning as a “digital” switch for c‐MYC induction.
Beside antigen receptor stimulation, cytokines can amplify T‐cell responses. Among a number of cytokines that act on T cells, the pro‐inflammatory cytokine IL‐2 enhances proliferation and has been shown to control expression of c‐MYC (Sinclair et al, 2013). Preston et al (2015) next showed that IL‐2 receptor signals provide a distinct mode of c‐MYC regulation in activated T cells. Following c‐MYC induction by TCR signals, IL‐2, or its closely related cytokine IL‐15, is necessary to sustain the expression of c‐MYC protein in activated T cells. In contrast to the digital effect of TCR stimulation, IL‐2R signals regulate levels of c‐MYC protein in a dose‐dependent manner without altering frequencies of c‐MYC‐positive cells. c‐MYC levels in activated T cells are highly correlated with the expression of CD25, a component of the high‐affinity IL‐2 receptor, which is upregulated during T‐cell priming. The requirement for IL‐2R signals for the modulation of c‐MYC levels was further validated by various experiments, including those using an inhibitor for a Janus kinase (JAK), tofacitinib, that blocks the signal transduction pathway activated by the ligation of IL‐2R. Collectively, data presented in the article highlighted “analogue” regulation of c‐MYC levels by a key cytokine, IL‐2.
So far, this study has convincingly demonstrated that TCR and IL‐2R signals orchestrate dynamic regulation of c‐MYC levels in activated T cells. The authors’ next experiments further highlighted the distinct mechanisms that are employed to increase c‐MYC by these two stimulatory cues. Transcription of Myc is primarily induced by TCR stimuli, as c‐MYC‐expressing cells that are stimulated with TCR signals have higher transcript levels compared to c‐MYC‐negative cells. This finding suggests that TCR signals above a certain threshold license T cells to be recruited to immune responses. Surprisingly, however, IL‐2R signal‐dependent regulation of c‐MYC occurs independent of its transcript levels. c‐MYC protein is short‐lived and constitutively degraded by the ubiquitin–proteasome system with an approximate half‐life of 20 minutes in some cells (Gregory & Hann, 2000). Therefore, to sustain c‐MYC levels, activated T cells must upregulate translation of c‐MYC and overcome the rapid degradation. The current study demonstrated that IL‐2R signals induce the expression of the L‐amino acid transporter SLC7A5 that facilitates amino acid uptake, and consequently increase c‐MYC translation. It was previously shown that SLC7A5 is induced by TCR signals and is essential for translation of c‐MYC in activated T cells (Sinclair et al, 2013). This new study revealed a sequential requirement for this amino acid transporter for translation of c‐MYC initially in TCR signal‐dependent and subsequently IL‐2R signal‐dependent manners.
This study has unveiled the mechanisms for a fine control of c‐MYC levels in activated T cells. But how are these discoveries linked with unsolved questions in T‐cell immune responses in vivo? For example, it is known that a single antigen can be recognized by naïve T‐cell clones with a broad range of reactivity. While very weak antigen stimuli are sufficient for initial activation, high‐affinity clones finally dominate the pool of expanding T cells over the course of the immune response and contribute to memory populations (Zehn et al, 2009). How does this selection happen? This enrichment of high‐affinity clones in vivo during antimicrobial responses now can be explained by the findings by Preston et al (2015). Equal amounts of c‐MYC protein in primed T cells, regardless of their affinity to antigen, allow various clones to initiate their clonal expansion. Yet, only clones with high‐affinity TCRs are presumably able to express IL‐2R at a level and thus sustain c‐MYC levels to continue their prolonged clonal expansion. The study also raises several new questions. Why is c‐MYC rapidly upregulated after T‐cell priming and yet downregulated even in the presence of constitutive stimulation (Chou et al, 2014)? How are kinetics of c‐MYC expression differently regulated between CD4+ and CD8+ T cells? Do other inflammatory cytokines, such as interferons, regulate c‐MYC levels? Is c‐MYC stability also modulated in response to extrinsic stimuli? Regardless of these questions, the current study by Preston et al (2015) uncovers the important molecular mechanisms by which c‐MYC is dynamically regulated to meet the high metabolic demand of activated T cells.
- © 2015 The Authors