Selective PRMT5 Inhibitors Suppress Human CD8+ T Cells by Upregulation of p53 and Impairment of the AKT Pathway Similar to the Tumor Metabolite MTA

Abstract

Genetic alterations in tumor cells provide promising targets for antitumor therapy. Recently, loss of methylthioadenosine phosphorylase (MTAP), a deletion frequently occurring in cancer, has been shown to create vulnerability to the inhibition of the protein arginine methyltransferase 5 (PRMT5). MTAP deficiency leads to accumulation of methylthioadenosine (MTA), which reduces PRMT5 activity, and thus, sensitizes the tumor cells to selective PRMT5 inhibitors (PRMT5i). PRMT5i are investigated as a new strategy to selectively kill MTAP-deficient tumor cells by blocking residual PRMT5 activity, but also to treat PRMT5-overexpressing tumors. Although many studies investigated the role of PRMT5 in cancer, only little data exist about the effect of PRMT5 inhibition on immune cells. As we could show that the tumor metabolite MTA suppresses T cells, we asked whether selective PRMT5 inhibition is detrimental for T-cell immune responses. Therefore, we examined the effect of the synthetic PRMT5 inhibitor EPZ015666 on human CD8 þ T cells in direct comparison with the naturally occurring PRMT5-inhibiting molecule MTA. Both compounds reduced T-cell proliferation, viability, and functionality. In addition, T-cell metabolism was impaired upon PRMT5 inhibition. These effects coincided with the induction of p53 expression and reduced AKT/mTOR signaling. Our data clearly demonstrate that PRMT5 activity is involved in various cellular processes of human CD8 þ T cells associated with essential T-cell functions. Therefore, not only tumor cells, but also antitumor immune responses, are compromised by PRMT5 inhibitors. This emphasizes the importance of considering side effects on the immune system when developing new strategies to specifically target not only MTAP-deficient tumors.

The Oncometabolite 5′-Deoxy-5′-Methylthioadenosine Blocks Multiple Signaling Pathways of NK Cell Activation

“…In other immune cell populations, it has been shown that MTA exerts its suppressive effect by different mechanisms. One supposed mechanism is the interaction with adenosine receptors due to the adenosine residue in MTA and another one due to its intracellular inhibition of the protein arginine methyltransferase 5 (PRMT5) ( 8 , 21 , 22 ).…”

“…Our group discovered that MTA blocks T cell metabolism and proliferation, thereby suppressing the induction of antigen-specific CD8 T cells and cytokine production by CD4 T cells. Interestingly, the MTA effect was reversible since T cells regained their proliferative capacity after removal of MTA from the culture medium ( 7 , 8 ). Moreover, TNF production in macrophages upon toll-like receptor (TLR) stimulation, e.g., with LPS, is sufficiently suppressed by MTA in an adenosine A2 receptor-dependent manner ( 9 , 10 ).…”

“…MTA selectively inhibits PRMT5 at low concentrations (≤ 10 μM), whereas other PRMTs were inhibited only at higher concentrations. MTAP-deficient tumor cells accumulate high levels of MTA leading to a reduced PRMT5 activity, which renders these cells more susceptible toward additional PRMT5 inhibition compared to MTAP-competent cells ( 8 , 22 , 41 , 42 ). Cell signaling events are mediated by different protein–protein interactions, which are regulated through post-translational modification (PTM) of which phosphorylation is the best studied one ( 43 ).…”

“…Proper removal of MTA by MTAP is essential to guarantee an effective performance of the polyamine synthesis pathway and of methylation processes ( 6 ). We have previously demonstrated that accumulation of MTA due to MTAP deficiency is able to suppress proliferation, activation, and differentiation of human T cells ( 7 , 8 ). In addition, an immune-suppressive effect of MTA has been demonstrated as well within cells of the innate immune system including macrophages ( 9 , 10 ) and NK cells ( 11 ).…”

Synthesis and Anticancer Cytotoxicity of Azaaurones Overcoming Multidrug Resistance

“…Toxicity was screened in 2 concentrations (10 µM and 100 µM) for 96 h in a final volume of 200 µL. For positive control (representing toxicity), we used a toxic concentration (25 µM) of NSC-57969 [28]; and for negative control, we used 0.4% DMSO-containing medium (referring to 100% growth). Fluorescent intensity of mCherry was detected by Perkin Elmer EnSpire Multimode Plate Reader (fluorescent cells were characterized in our earlier publications) [15,29].…”