Oxidative phosphorylation induces de novo expression of the MHC class I in tumor cells through the ERK5 pathway.

Most cancer cells use anaerobic-like glycolysis to generate energy instead of oxidative phosphorylation. They also avoid recognition by CTLs, which occurs primarily through decreasing the level of MHC class I (MHC-I) at the cell surface. We find that the two phenomena are linked; culture conditions that force respiration in leukemia cells upregulate MHC-I transcription and protein levels at the cell surface, whereas these decrease in cells forced to perform fermentation as well as in leukemia cells lacking a functional mitochondrial respiratory chain. Forced respiration leads to increased expression of the MAPK ERK5, which activates MHC-I gene promoters, and ERK5 accumulation in mitochondria. Respiration-induced MHC-I upregulation is reversed upon short hairpin RNA-mediated ERK5 downregulation and by inactive mutants of ERK5. Moreover, short hairpin RNA for ERK5 leukemia cells do not tolerate forced respiration. Thus, the expression of ERK5 and MHC-I is linked to cell metabolism and notably diminished by the metabolic adaptations found in tumor cells.

ERK5 knockdown generates mouse leukemia cells with low MHC class I levels that activate NK cells and block tumorigenesis.

Tumor cell-based vaccines are currently used in clinical trails, but they are in general poorly immunogenic because they are composed of cell extracts or apoptotic cells. Live tumor cells should be much better Ags provided that they are properly processed by the host immune system. We show herein that stable expression of a small hairpin RNA for ERK5 (shERK5) decreases ERK5 levels in human and mouse leukemic cells and leads to their elimination by NK cells in vivo. The shERK5 cells show down-regulation of MHC class I expression at the plasma membrane. Accordingly, ectopic activation of the ERK5 pathway induces MHC class I gene expression. Coinjection of shERK5-expressing cells into the peritoneum diminishes survival of engrafted wild-type tumor cells. Moreover, s.c. injection of shERK5-expressing cells strongly diminishes tumor development by wild-type cells. Our results show that shERK5 expression in leukemia cells effectively attenuates their tumor activity and allows their use as a tumor cell-based vaccine.

Impaired anti-leukemic immune response in PKCtheta-deficient mice.

The cancer immunosurveillance hypothesis has found strong experimental support in recent years. It is believed that cytotoxic lymphocytes are important effectors in this process. PKCtheta plays an essential role in proliferation, activation and survival of these cells, but also proliferation and survival of leukemic T cells. In light of this, we tested the role of PKCtheta in T cell leukemia progression by inducing this disease in wild-type (wt) and PKCtheta-deficient mice with moloney-murine leukemia virus (M-MuLV). Leukemic PKCtheta(-/-) and wild-type (wt) mice showed the same profile of leukemic cell types, similar spleen and thymus sizes and comparable hematocrits. In contrast, disease incidence was higher and disease onset more rapid in PKCtheta(-/-) mice. Transfer of leukemic T cells from wt donors into PKCtheta-deficient and wt recipients induced leukemia in 100% and 40% of the mice, respectively. Interestingly, leukemic cells from PKCtheta(-/-) donors induced the disease in only 50% of the PKCtheta-deficient and 10% of the wt recipients. Intravenous injection of low numbers of EL4 cells induced tumors earlier in PKCtheta(-/-) mice. Taken together, our results show that PKCtheta is essential for the immune response to leukemia in mice and raise questions about the chronic treatment of humans with PKCtheta inhibitors.

MHC-I modulation due to changes in tumor cell metabolism regulates tumor sensitivity to CTL and NK cells.

Tumor cells have a tendency to use glucose fermentation to obtain energy instead of mitochondrial oxidative phosphorylation (OXPHOS). We demonstrated that this phenotype correlated with loss of ERK5 expression and with reduced MHC class I expression. Consequently, tumor cells could evade cytotoxic T lymphocyte (CTL)-mediated immune surveillance, but also increase their sensitivity to natural killer (NK) cells. These outcomes were evaluated using two cellular models: leukemic EL4 cells and L929 transformed fibroblasts and their derived ρ° cell lines, which lack mitochondrial DNA. We have also used a L929 cell sub-line that spontaneously lost matrix attachment (L929dt), reminiscent of metastasis generation, that also downregulated MHC-I and ERK5 expression. MHC-I expression is lower in ρ° cells than in the parental cell lines, but they were equally sensitive to CTL. On the contrary, ρ° cells were more sensitive to activated NK cells than parental cells. On the other hand, L929dt cells were resistant to CTL and NK cells, showed reduced viability when forced to perform OXPHOS, and surviving cells increased MHC-I expression and became sensitive to CTL. The present results suggest that when the reduction in MHC-I levels in tumor cells due to glycolytic metabolism is partial, the increase in sensitivity to NK cells seems to predominate. However, when tumor cells completely lose MHC-I expression, the combination of treatments that increase OXPHOS with CTL-mediated immunotherapy could be a promising therapeutic approach.

SUMOylation regulates the transcriptional activity of JunB in T lymphocytes.

The AP-1 family member JunB is a critical regulator of T cell function. JunB is a transcriptional activator of various cytokine genes, such as IL-2, IL-4, and IL-10; however, the post-translational modifications that regulate JunB activity in T cells are poorly characterized. We show here that JunB is conjugated with small ubiquitin-like modifier (SUMO) on lysine 237 in resting and activated primary T cells and T cell lines. Sumoylated JunB associated with the chromatin-containing insoluble fraction of cells, whereas nonsumoylated JunB was also in the soluble fraction. Blocking JunB sumoylation by mutation or use of a dominant-negative form of the SUMO-E2 Ubc-9 diminished its ability to transactivate IL-2 and IL-4 reporter genes. In contrast, nonsumoylable JunB mutants showed unimpaired activity with reporter genes controlled by either synthetic 12-O-tetradecanoylphorbol-13-acetate response elements or NF-AT/AP-1 and CD28RE sites derived from the IL-2 promoter. Ectopic expression of JunB in activated human primary CD4(+) T cells induced activation of the endogenous IL-2 promoter, whereas the nonsumoylable JunB mutant did not. Thus, our work demonstrates that sumoylation of JunB regulates its ability to induce cytokine gene transcription and likely plays a critical role in T cell activation.