Abstract
Blocking interaction of the immune checkpoint receptor PD-1 with its ligand PD-L1 is associated with good clinical outcomes in a broad variety of malignancies. High levels of PD-L1 promote tumor growth by restraining CD8+ T-cell responses against tumors. Limiting PD-L1 expression and function is therefore critical for allowing the development of antitumor immune responses and effective tumor clearance. Pyruvate kinase isoform M2 (PKM2) is also a key player in regulating cancer as well as immune responses. PKM2 catalyzes the final rate-limiting step of glycolysis. Furthermore, PKM2 as a dimer translocates to the nucleus, where it stimulates hypoxia-inducible factor 1α (Hif-1α) transactivation domain function and recruitment of p300 to the hypoxia response elements (HRE) of Hif-1α target genes. Here, we provide the first evidence of a role for PKM2 in regulating the expression of PD-L1 on macrophages, dendritic cells (DCs), T cells, and tumor cells. LPS-induced expression of PD-L1 in primary macrophages was inhibited by the PKM2 targeting compound TEPP-46. Furthermore, RNA silencing of PKM2 inhibited LPS-induced PD-L1 expression. This regulation occurs through direct binding of PKM2 and Hif-1α to HRE sites on the PD-L1 promoter. Moreover, TEPP-46 inhibited expression of PD-L1 on macrophages, DCs, and T cells as well as tumor cells in a mouse CT26 cancer model. These findings broaden our understanding of how PKM2 may contribute to tumor progression and may explain the upregulation of PD-L1 in the tumor microenvironment.
Highlights
Immune checkpoint interactions play a crucial role in host maintenance of immune homeostasis
We investigated whether modifying Pyruvate kinase isoform M2 (PKM2) using the small molecule activator TEPP-46 would alter the expression of LPS-induced PD-L1 in immune cells
There are extensive reports pointing to a role for PKM2 in cell proliferation and tumor progression, where PKM2 is essential for aerobic glycolysis, the dominant metabolic pathway utilized by cancer cells
Summary
Immune checkpoint interactions play a crucial role in host maintenance of immune homeostasis. These include CTLA-4:CD80/CD86, PD1:PD-L1/PD-L2, GAL9:TIM3, TCR:LAG3, and HVEM:BTLA, all of which can contribute to the suppression of T cell effector function in the tumor microenvironment. This suppression supports tumor progression, a process that has been termed “immunoediting,” and which has been the focus of extensive efforts to restore host antitumor immunity during immunotherapy. PKM2 Regulates Expression of PD-L1 complex enhances antitumor immunity, with several approved antibodies targeting this interaction in use [4]. A clear understanding of how expression of PD-L1 is regulated is critical in the design of novel combination cancer immunotherapies
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