Rapamycin-based graft-versus-host disease prophylaxis increases the immunosuppressivity of myeloid-derived suppressor cells without affecting T cells and anti-tumor cytotoxicity

The immunosuppressant rapamycin (RAPA) inhibits mammalian target of rapamycin (mTOR) functions and is applied after allogeneic bone marrow transplantation (BMT) to attenuate the development of graft‐versus‐host disease (GVHD), although the cellular targets of RAPA treatment are not well defined. Allogeneic T cells are the main drivers of GVHD, while immunoregulatory myeloid‐derived suppressor cells (MDSCs) were recently identified as potent disease inhibitors. In this study, we analyzed whether RAPA prevents the deleterious effects of allogeneic T cells or supports the immunosuppressive functions of MDSCs in a BMT model with major histocompatibility complex (MHC) classes I and II disparities. RAPA treatment efficiently attenuated clinical and histological GVHD and strongly decreased disease‐induced mortality. Although splenocyte numbers increased during RAPA treatment, the ratio of effector T cells to MDSCs was unaltered. However, RAPA treatment induced massive changes in the genomic landscape of MDSCs preferentially up‐regulating genes responsible for uptake or signal transduction of lipopeptides and lipoproteins. Most importantly, MDSCs from RAPA‐treated mice exhibited increased immunosuppressive potential, which was primarily inducible nitric oxide synthase (iNOS)‐dependent. Surprisingly, RAPA treatment had no impact on the genomic landscape of T cells, which was reflected by unchanged expression of activation and exhaustion markers and cytokine profiles in T cells from RAPA‐treated and untreated mice. Similarly, T cell cytotoxicity and the graft‐versus‐tumor effect were maintained as co‐transplanted tumor cells were efficiently eradicated, indicating that the immunosuppressant RAPA might be an attractive approach to strengthen the immunosuppressive function of MDSCs without affecting T cell immunity.

Involvement of protein kinase C in the control of tRNA modification with queuine in HeLa cells

The eukaryotic tRNA:guanine transglycosylase (TGT) catalyses the base-for-base exchange of guanine for queuine (the q-base)--a nutrition factor for eukaryotes--at position 34 of the anticodon of tRNAsGUN (where 'N' represents one of the four canonical tRNA nucleosides), yielding the modified tRNA nucleoside queuosine (Q). This unique tRNA modification process was investigated in HeLa cells grown under either aerobic (21% O2) or hypoxic conditions (7% O2) after addition of chemically synthesized q-base to q-deficient cells. While the q-base was always inserted into tRNA under aerobic conditions, HeLa cells lost this ability under hypoxic conditions, however, only when serum factors became depleted from the culture medium. The inability to insert q into tRNA did not result from a lack of substrate, because the q-base accumulated within these cells against the concentration gradient, suggesting the presence of an active transport system for this base in HeLa cells. The activity of the TGT enzyme was restored after treatment of the cells with the protein kinase C activator, TPA, even in the presence of mRNA or protein synthesis inhibitors. The results indicate that the eukaryotic tRNA modifying enzyme, TGT, is a downstream target of activated protein kinase C.

The nutrient factor queuine protects HeLa cells from hypoxic stress and improves metabolic adaptation to oxygen availability

Queuine (q), a cyclopentendiol derivative of 7-aminomethyl-7-deazaguanine, is a nutrient factor for lower and higher eukaryotes, except yeast; it is synthesized in eubacteria partly at the level of tRNA. In eukaryotes q is preferentially inserted into the wobble position of specific tRNAs in differentiated and adult tissues, but occurs mainly free in embryonic and fast proliferating cells. HeLa cells grow to a higher cell density under aerobic than under hypoxic conditions only when supplemented with q. Here we show that in hypoxically grown HeLa cells, sufficiently supplied with q, free q accumulated when serum factors become limiting while the respective tRNAs remained completely q deficient. In these cells the levels of lactate dehydrogenase A (LDH A) mRNA and of LDH A protein were at least twofold higher than in aerobically grown cells, independent of the absence or presence of q. In response to q the LDH A4 isoenzyme was further activated by a post-translational mechanism. In q-deficient HeLa cells the activity of the major anoxic stress protein, LDHk, increased as a result of hypoxia; this increase was suppressed by q. In aerobically grown, q-deficient cells significant activities of LDH A4 and LDHk were present; both activities were markedly lowered by q, while the mitochondrial electron flow was improved. The results show that free q is essential for relieving hypoxic stress in HeLa cells that results from oxygen limitation.

Mitogenic stimulation of HeLa cells increases the activity of the anoxic stress protein, LDH 6/k: suppression by queuine

Mitogenic stimulation of quiescent mammalian cells triggers an array of early events crucial for cell cycle progression. Here we show that the activity of the anoxic stress protein, lactate dehydrogenase 6/k, transiently increased after mitogenic stimulation of serum-starved HeLa cells. Regulation of lactate dehydrogenase 6/k activity in early G1 depended on the activity of a receptor tyrosine kinase and on protein and mRNA synthesis, but did not involve protein kinase C. The guanine analog, queuine, an ubiquitously occurring tRNA base of bacterial origin, suppressed the mitogen-induced protein synthesis and also the transient increase in lactate dehydrogenase 6/k activity. The results suggest that queuine relieves hypoxic stress resulting from mitogenic stimulation by suppressing protein synthesis during G0/G1 transition.

Modulation of mammalian cell proliferation by a modified tRNA base of bacterial origin

Addition of the q-base to q-deficient non-transformed mammalian cells stimulated their proliferation. The q-base also improved proliferation of some cancer-derived cell lines, but inhibited proliferation of others. The proliferation of HeLa-S3 carcinoma cells was stimulated by q under aerobic conditions, but was inhibited when the cells had shifted their energy metabolism towards glycolysis as the result of oxygen limitation. Q-deficient cells could not adapt their proliferation to the respective oxygen tension. The q-base stimulated the proliferation of non-transformed fibroblasts but inhibited proliferation of the same cell line, when aerobic glycolysis was increased after transformation with the ras gene. The results suggest that the q-base permits mammalian cells to adapt their proliferation to their specific metabolic state.

Modulation of epidermal growth factor receptor activity and related responses by the 7-deazaguanine derivative, queuine

Epidermal growth factor (EGF) induces autophosphorylation of its cognate receptor at tyrosine residues. Here we show that queuine (q), a widely distributed modified guanine analogue occurring free or as a tRNA wobble base, modulates this EGF receptor activity in vitro and in intact cells. Autophosphorylation of the immunopurified receptor from human A431 epidermoid carcinoma cells was enhanced three to fourfold in the presence of physiological concentrations of q. Using a membrane fraction of A431 cells, a twofold increase in autophosphorylation activity in the presence of q was observed, however, only when the receptor was activated by the ligand. In intact A431 cells, q enhanced the initial ligand-induced autophosphorylation of the EGF receptor three to fourfold. However, upon longer treatment of the cells with EGF in the presence of q, significantly less autophosphorylated receptor was detectable compared with stimulation of cells in the absence of q. A similar q-dependent modulation of EGF receptor autophosphorylation was observed also in human cervical carcinoma cells HeLa-S3. Treatment of q-deficient HeLa cells with EGF induced the c-fos gene expression, transiently increased the activity of the anoxic stress protein LDH k, and stimulated proliferation. Treatment of HeLa cells with EGF in the presence of q resulted in a delayed c-fos gene expression and an accelerated increase and decrease of LDH k activity. The stimulatory effect of low doses of EGF on HeLa cell proliferation was completely antagonized in the presence of q. The results suggest that the mitogenic signalling initiated by the EGF receptor is modulated by q.

Queuine modulates growth of HeLa cells depending on oxygen availability

HeLa cells can be grown in media supplemented with horse serum that is lacking the nutrient factor queuine. The addition of 1 X 10(-8) M queuine to aerobically grown cells caused a slight, but significant, inhibition of growth, whereas cell proliferation was stimulated increasingly when the concentration of queuine was raised from 3 X 10(-8) M to 3 X 10(-7) M. This was also observed when the cells were transiently starved of serum factors. When the cells were grown under hypoxic stress, but otherwise identical conditions, they responded to queuine in an opposite manner. Under conditions of mitogenic stimulation, characteristic new proteins were found in cytosolic, nuclear and mitochondrial fractions of aerobically grown cells. The effects of queuine on cell proliferation at low concentrations are assumed to be mediated by the free base, whereas the effects at higher concentrations possibly involve both, queuine and Q-tRNAs. The 'Q system' appears to mediate growth control in dependence on oxygen availability.