Invited lecture: Fighting tumoral immune resistance

A number of cancer vaccine trials have now been completed, and regressions of metastatic melanomas were observed in 10-20% of the patients, in the absence of any toxicity. It appears that an important factor limiting the efficacy of immunotherapy is the development of local mechanisms allowing tumors to resist immune rejection. The challenge is to identify such mechanisms and design therapeutic approaches to inactivate them so as to boost the efficacy of cancer immunotherapy. Two such mechanisms will be described. The first is based on the expression by tumor cells of Indoleamine 2,3-dioxygenase (IDO), a tryptophan-degrading enzyme inducing a local tryptophan depletion that severely affects T lymphocyte proliferation. Our data in a preclinical model indicate that the efficacy of therapeutic vaccination of cancer patients could be improved by concomitant administration of an inhibitor of IDO. The second mechanism is based on the expression by tumor cells of galectin-3, which induces T cell anergy by trapping TCR molecules in a lattice preventing their association with CD8 molecules. Anergy can be reversed with sugars that bind galectin-3. This work was supported in part by grants from the European Community under the Sixth Programme (convention “Cancerimmunotherapy”), and by belgian grants financed by the Région Wallonne under the programme BIOWIN (Convention “CANTOL”), the Fonds National de la Recherche Scientifique, and the Fondation contre le Cancer

ECHDC1 prevents the synthesis of branched fatty acids

Fatty acid synthesis proceeds by the consecutive addition of two-carbon units from malonyl-CoA to acetyl-CoA. This leads to linear structures. Yet, in rare cases fatty acids can contain branches. Most commonly, methyl-branches occur on the extremity of the carbon chain, when breakdown products of branched amino acids are used to start fatty acid synthesis. ECHDC1 can decarboxylate (m)ethyl-malonyl-CoA. We hypothesized that the inactivation of ECHDC1 would lead to the formation of fatty acids with (m)ethyl- branches in the middle of the carbon chain, when fatty acid synthase erroneously uses methyl- or ethyl-malonyl-CoA instead of malonyl-CoA to extend the carbon chain. To test this hypothesis, we genetically inactivated ECHDC1 in cell lines and in mice. We discovered accumulation of methyl-branched fatty acids in two different ECHDC1-deficient adipocyte cell models and in different tissues of ECHDC1 knockout mice. No ethyl-branched fatty acids were detected in these samples, although ethyl-malonyl-CoA concentrations were strongly increased. Surprisingly, ECHDC1 knockout mice excreted short ethyl-branched fatty acids and their -keto, -hydroxy or monounsaturated equivalents as glycine or taurine conjugates in urine. Preliminary evidence suggests that these adducts are abortive intermediates of mitochondrial fatty acid synthesis. Thus, our studies demonstrate that the enzyme ECHDC1 serves to prevent the formation of branched-chain fatty acids in both cytoplasm and mitochondria

A mitochondrial switch promotes tumor metastasis

Cancers evolve a subpopulation of tumor cells that metabolically rely on glycolysis uncoupled from oxidative phosphorylation irrespectively of oxygen availability (aerobic glycolysis). Given that most metastases are abnormally avid for glucose (which is the rationale for their clinical detection using FDG-PET) and because clinical data show a positive correlation between lactate production and tumor metastasis, we reasoned that cells performing aerobic glycolysis could constitute a population of metastatic progenitor cells that would remain glycolytic in the blood stream. We found a different metabolic phenotype, though. Indeed, using serial rounds of in vitro selection of highly invasive tumor cells (starting from wild-type SiHa human cervix adenocarcinoma cells) and in vivo selection of supermetastatic tumor cells (starting from B16-F10 mouse melanoma cells), we identified a mitochondrial switch corresponding to an overload of the TCA cycle with preserved mitochondrial functions (including ATP production) but increased mitochondrial superoxide production. The switch provided a metastatic advantage which was phenocopied by moderate OXPHOS inhibition associated with mild mitochondrial superoxide increase. Thus, two different events, OXPHOS overload or moderate OXPHOS inhibition, promote superoxide-dependent tumor cell migration, invasion, clonogenicity, and metastasis; demonstrating the central role of mitochondrial superoxide generation in the pathogenesis of metastasis. Consequently, we report that mitochondria-specific superoxide scavenging (using mitoTEMPO or mitoQ) inhibits metastatic dissemination from primary mouse and human tumors, which opens a new avenue for the therapeutic prevention of tumor metastasis. Study supported by ERC starting grant #243188-TUMETAB

Metabolic adaptation of tumor cells under chronic acidosis: a shift towards reductive glutamine metabolism driven by the SIRT1/HIF2 axis

Cancer progression is strongly influenced by the physico-chemical properties of the tumor microenvironment. In particular, tumor cells must adapt to survive under low pO2 and low pH. Although the impact of hypoxia on tumor metabolism is well described, little is known on how tumor cells adapt their metabolism to acidosis. Here, we exposed tumor cells derived from various tissues to low pH conditions (pH 6.5) for several weeks until they ended up proliferating at the same rate as parent cells maintained at pH 7.4. This low pH acclimation triggered the reprogramming of tumor cells from a mainly glycolytic metabolism towards the preferred use of glutamine as documented by tracking the fate of [U-13C] glucose and [U-13C] glutamine by GC-MS analysis of metabolites. The metabolic switch was mediated by SIRT1, a protein deacetylase activated by the increased pool of NAD+ in low pH-adapted cells, through two distinct mechanisms. First, free acetate acted as a counteranion to export excess protons out of the cells via MCT1, maintaining the intracellular pH in a physiological range. Second, SIRT1 stimulated the activity of HIF2 thereby supporting the glutamine metabolism via the upregulation of the glutamine transporter SLC1A5 and enzymes supporting the reductive glutamine metabolism including IDH1. Finally, pharmacological inhibition of either glutamine metabolism with the glutaminase inhibitor BPTES or SIRT1 deacetylase activity preferentially killed low pH-adapted cancer cells in vitro (vs. parent cells) and delayed the growth of corresponding tumor xenografts in vivo. Altogether, these data indicate that a major metabolic shift from glucose to glutamine metabolism is induced in tumor cells chronically exposed to an acidic environment and importantly makes them particularly suited for dedicated pharmacological treatments

Keynote Lecture: From the identification of tumor antigens recognized by T cells to immunotherapy

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Membrane protein GARP is a receptor for latent TGF-b on the surface of activated human Treg

Human regulatory T cell (Treg) and T helper (Th) clones secrete the latent form of transforming growth factor beta (TGF-beta), in which the mature TGF-beta protein is bound to the latency-associated peptide (LAP), and is thereby prevented from binding to the TGF-beta receptor. We previously showed that upon T cell receptor (TCR) stimulation, human Treg clones but not Th clones produce active TGF-beta and bear LAP on their surface. Here, we show that latent TGF-beta, i.e. both LAP and mature TGF-beta, binds to GARP, a transmembrane protein containing leucine rich repeats which is present on the surface of stimulated Treg clones but not on Th clones. Membrane localization of latent TGF-beta mediated by binding to GARP may be necessary for the ability of Treg to activate TGF-beta upon TCR stimulation. However, it is not sufficient as lentiviral mediated expression of GARP in human Th cells induces binding of latent TGF-beta to the cell surface, but does not result in the production of active TGF-beta upon stimulation of these Th cells