Decoding the crosstalk between mevalonate metabolism and T cell function

Abstract

The mevalonate pathway is an essential metabolic pathway in T cells regulating development, proliferation, survival, differentiation, and effector functions. The mevalonate pathway is a complex, branched pathway composed of many enzymes that ultimately generate cholesterol and nonsterol isoprenoids. T cells must tightly control metabolic flux through the branches of the mevalonate pathway to ensure sufficient isoprenoids and cholesterol are available to meet cellular demands. Unbalanced metabolite flux through the sterol or the nonsterol isoprenoid branch is metabolically inefficient and can have deleterious consequences for T cell fate and function. Accordingly, there is tight regulatory control over metabolic flux through the branches of this essential lipid synthetic pathway. In this review we provide an overview of how the branches of the mevalonate pathway are regulated in T cells and discuss our current understanding of the relationship between mevalonate metabolism, cholesterol homeostasis and T cell function.

Keywords: T cells; cholesterol; isoprenoids; mevalonate pathway; statins.

Figure 1.. Overview of the mevalonate pathway.

The mevalonate pathway is an essential metabolic pathway that uses acetyl-coA to generate nonsterol isoprenoids (yellow box) and sterols (blue box). FPP sits at a major branch point. FPP can be converted to squalene, which is the first committed step to sterol synthesis. Alternatively, FPP can combine with another molecule of IPP to form GGPP. Both FPP and GGPP are used for protein prenylation. FPP can also be used to synthesize longer chain isoprenoids, such as ubiquinone and heme. HMGCR and SM represent two rate-limiting steps in the mevalonate pathway. Inhibition by physiological (red lines) or pharmacological (brown lines) compounds are depicted. HMGCR is inhibited by oxysterols, methylated sterols, GGPP and statins. SM is inhibited by cholesterol. Dashed arrows represent more than one enzymatic reaction (for simplicity and clarity not all metabolites or enzymes are depicted). Enzymes and reactions that are transcriptionally regulated by SREBP2 are in purple. Abbreviations: acetyl-CoA, acetyl-coenzyme A; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA; HMGCS1, 3-hydroxy-3-methylglutaryl-CoA synthase 1; HMGCR, 3-hydroxy-3-methylglutaryl-coA reductase; IPP, isopentenyl-pyrophosphate; FPP, farnesyl-PP; GGPP, geranylgeranyl-PP; FDFT1, farnesyl-diphosphate farnesyltransferase 1; SM, squalene monooxygenase; GGTase, geranylgeranyltransferase; FTase, farnesyltransferase; ZA, Zaragozic acid A

Figure 2.. Feedback regulation of the mevalonate pathway.

A) SREBP2 is synthesized on the ER as an inactive precursor, complexed to the chaperone protein, SCAP. SREBP2 activation requires transport to the Golgi for further processing. When sterol levels are high, cholesterol binds to SCAP and recruits INSIG which binds to and retains the SCAP-SREBP2 complex in the ER. Alternatively, oxysterols such as 25HC can bind to INSIG which similarly retains the SCAP-SREBP2 complex in the ER, preventing the activation of SREBP2. B) During cholesterol excess, a conformational change in the amphipathic loop of SM permits MARCH6 binding which leads to the ubiquitination and proteasomal degradation of SM. C) Oxysterols and methylated sterols, such as lanosterol, induce the binding of the INSIG-E3 ligase complex to HMGCR, which subsequently leads to its ubiquitination and accelerated ER-associated degradation (ERAD). Oxysterols also induce the binding of UBIAD1 to HMGCR in an INSIG-dependent manner which retains some molecules of HMGCR in the ER. When GGPP accumulates, GGPP binds to UBIAD1 which triggers the release of HMGCR and further augments HMGCR ERAD. Abbreviations: SREBP2, sterol regulatory binding element 2; SCAP, SREBP cleavage-activating protein; HMGCR, 3-hydroxy-3-methylglutaryl-coA reductase; INSIG, insulin induced gene; UBIAD1, UbiA prenyltransferase domain-containing protein 1; GGPP, geranylgeranyl-PP; 25HC, 25-hydroxycholesterol; Ub, ubiquitin

Figure 3.. Antigen-independent regulation of the TCR by PM cholesterol.

A) Cholesterol complexed with sphingomyelin (red ovals) binds to the TM domain of the TCR to promoter nanoclusters and increase antigen sensitivity. Cholesterol and sphingomyelin organize into microdomains that favor the congregation of activating kinases suck as LCK. The TCR is spatially segregated in discrete protein islands. Direct interactions between cholesterol and the TCR keep the TCR in an inactive conformation in which ITAMs on the TCR/CD3 complex are inaccessible to phosphorylation by LCK. Cholesterol sulfate can also hold the TCR in an inactive conformation. B) When cholesterol is depleted from the membrane, some TCRs acquire an active confirmation in which the CD3 cytoplasmic tails are accessible to LCK. Cholesterol depletion also disrupts cholesterol microdomains which allows LCK to interact with the TCR. For simplicity, we have depicted a CD3/TCR model in which the CD3ε and CD3ζ cytosolic domains are bound to the inner leaflet of the PM in the inactive conformation, shielding their phosphorylation by LCK. This likely does not capture TCR/CD3 dynamics. For a more complete discussion of cholesterol-dependent conformational changes in the TCR/CD3 complex, we refer the readers to a review by Schamel and colleagues. Abbreviations: TM, transmembrane domain; pY, phosphorylated tyrosine; TCR, T cell receptor; LCK, lymphocyte-specific tyrosine kinase; ITAM, immunoreceptor tyrosine-based activation motifs

Figure 4.. T cell signaling pathways regulated by prenylated proteins.

Following antigen stimulation, early phosphorylation events on the TCR signal to the RAS and RHO family of GTPases. These adaptor proteins link immunoreceptor signaling to other biochemical and transcriptional networks. RAS is farnesylated and localized to the plasma membrane (PM), where it is activated by the guanidine nucleotide exchange factor (GEF), SOS. Activated RAS initiates the RAF/MEK/ERK pathway. RHEB is also farnesylated and localizes to the lysosome. The RAG GTPases (forming the regulator complex) sense cholesterol in the lysosome and recruit cytosolic mTORC1 to the lysosome, where it is activated by RHEB. Members of the RHO family of GTPases are geranylgeranylated and localized to the PM where they interact with the GEF, VAV1 to regulate processes related to motility and cytoskeleton formation.