Immunosuppressive activity enhances central carbon metabolism and bioenergetics in myeloid-derived suppressor cells in vitro models

Abstract

BackgroundThe tumor microenvironment contains a vast array of pro- and anti-inflammatory cytokines that alter myelopoiesis and lead to the maturation of immunosuppressive cells known as myeloid-derived suppressor cells (MDSCs). Incubating bone marrow (BM) precursors with a combination of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-6 (IL-6) generated a tumor-infiltrating MDSC-like population that impaired anti-tumor specific T-cell functions. This in vitro experimental approach was used to simulate MDSC maturation, and the cellular metabolic response was then monitored. A complementary experimental model that inhibited L-arginine (L-Arg) metabolizing enzymes in MSC-1 cells, an immortalized cell line derived from primary MDSCs, was used to study the metabolic events related to immunosuppression.ResultsExposure of BM cells to GM-CSF and IL-6 activated, within 24 h, L-Arg metabolizing enzymes which are responsible for the MDSCs immunosuppressive potential. This was accompanied by an increased uptake of L-glutamine (L-Gln) and glucose, the latter being metabolized by anaerobic glycolysis. The up-regulation of nutrient uptake lead to the accumulation of TCA cycle intermediates and lactate as well as the endogenous synthesis of L-Arg and the production of energy-rich nucleotides. Moreover, inhibition of L-Arg metabolism in MSC-1 cells down-regulated central carbon metabolism activity, including glycolysis, glutaminolysis and TCA cycle activity, and led to a deterioration of cell bioenergetic status. The simultaneous increase of cell specific concentrations of ATP and a decrease in ATP-to-ADP ratio in BM-derived MDSCs suggested cells were metabolically active during maturation. Moreover, AMP-activated protein kinase (AMPK) was activated during MDSC maturation in GM-CSF and IL-6–treated cultures, as revealed by the continuous increase of AMP-to-ATP ratios and the phosphorylation of AMPK. Likewise, AMPK activity was decreased in MSC-1 cells when L-Arg metabolizing enzymes were inhibited. Finally, inhibition of AMPK activity by the specific inhibitor Compound C (Comp-C) resulted in the inhibition of L-Arg metabolizing enzyme activity and abolished MDSCs immunosuppressive activity.ConclusionsWe anticipate that the inhibition of AMPK and the control of metabolic fluxes may be considered as a novel therapeutic target for the recovery of the immunosurveillance process in cancer-bearing hosts.

The Nature of Myeloid-Derived Suppressor Cells in the Tumor Microenvironment

“…Hammami, et al have shown that in mice, glycolysis increased concurrently with increased Arg1 activity in MDSC. Interestingly, they also note that AMP-activated protein kinase (AMPK) was activated, which normally drives metabolism towards oxidative phosphorylation [98]. In support of this finding, tumor-infiltrating MDSC increased fatty acid uptake and FAO.…”

Inhibition of Fatty Acid Oxidation Modulates Immunosuppressive Functions of Myeloid-Derived Suppressor Cells and Enhances Cancer Therapies

“…Our data using in vitro -derived BM-MDSCs suggest that G-CSF or GM-CSF may also induce FAO (data not shown). A previous study showed that GM-CSF and IL6, instead, upregulate AMPK and glycolysis in BM-MDSCs (40). Recent reports suggest that lactic acid present in high concentrations in the tumor microenvironment can activate MDSCs (29, 41); however, its role in inducing FAO is unknown.…”

Tumor-induced myeloid deviation: when myeloid-derived suppressor cells meet tumor-associated macrophages

“…HIF-1α may also be stabilized by the lactic acid that is produced by aerobic glycolysis (Warburg effect) in cancer cells (36). Alternatively, lactic acid can be actively produced in immune-regulatory myeloid cells by cytokine-activated, anerobic glycolysis (28,37).…”