Post-translational modifications of cancer immune checkpoints: mechanisms and therapeutic strategies

Abstract

Immunotherapies, particularly immune checkpoint inhibitors (ICIs), have revolutionized cancer clinical management, but low response rates and treatment resistance remain challenging. Protein post-translational modifications (PTMs) are critical for governing protein expression, localization, functions, and interactions with other cellular molecules, which notably build up the diversity and complexity of the proteome. A growing body of evidence supports that PTMs influence immunotherapy efficacy and outcomes by post-translationally modulating the expression and functions of immune checkpoints. Therefore, understanding the PTM mechanisms that govern immune checkpoints is paramount for developing novel treatment strategies to improve immunotherapy efficacy and overcome resistance. This review provides an overview of the current comprehension of the regulatory mechanisms by which PTMs (glycosylation, phosphorylation, ubiquitination, acetylation, succinylation, palmitoylation, lactylation, O-GlcNAcylation, UFMylation, and neddylation) modulate immune checkpoints to unveil potential therapeutic targets. Moreover, this review discusses the potential of therapeutic strategies targeting PTMs of immune checkpoints, providing insights into the combination treatment with ICIs in maximizing the benefits of immunotherapy and overcoming resistance.

Keywords: Combination treatment; Immune checkpoints; Immunotherapy; Post-translational modifications; Treatment resistance.

Fig. 1

Overview of cellular interactions contributing to tumor immunity, including well-studied immune checkpoints (CTLA-4, PD-1, PD-L1), and other immune checkpoints with negative (LAG-3 and MHC-II, TIM-3 and its ligands, TIGIT and its ligands, KIR and MHC-I, NKG2D and its ligands, CD47 and SIRPα, B7-H4, B7-H6) or positive (GITR, OX40, 4-1BB, CD28, ICOS) immune regulation

Fig. 2

Post-translational modifications that regulate cancer immune checkpoints, encompassing well-characterized PTMs (glycosylation, phosphorylation, ubiquitination), protein-acylation modifications (acetylation, succinylation), palmitoylation (a lipid-associated protein modification), lactylation (a metabolism-associated protein modification), ubiquitin-like small-molecular protein modifications (UFMylation, neddylation)

Fig. 3

Roles and mechanisms of glycosylation in regulating expression and functions of immune checkpoints. Glycosylation of PD-1, PD-L1, PD-L2, B7-H4, and B7-H6 mediates their protein stability, localization, and interactions, thus influencing immune suppression. Glycosylation of PD-L1 is mediated by glycosyltransferases (STT3, B3GNT3, MAN2A1, GLT1D1, B4GALT1), regulatory proteins and signaling pathways (AMPK, IL-6-JAK1-STT3A, GMPS, TMUB1, VPS11/18, cancer senescence-induced RPN1), and glycosylation of PD-L2 is mediated by FUT8

Fig. 4

Roles and mechanisms of phosphorylation in regulating expression and functions of immune checkpoints. Phosphorylation of immune checkpoints (PD-L1, TIM-3, and CD47) modulates their stability, localization and function. GSK3β, GSK3α, PKCα, ERK, AMPK, CK2, NEK2, LRRK2, CDK5, and JAK1 are responsible for PD-L1 phosphorylation. Interaction between TIM-3 and its ligand Gal-9 as well as PtdSer engagement induce TIM-3 phosphorylation. EGFR-mediated c-Src induces CD47 phosphorylation

Fig. 5

Roles and mechanisms of ubiquitination in regulating expression and functions of immune checkpoints. Ubiquitination and degradation of PD-L1 is induced by E3 ubiquitin ligases (β-TrCP, MARCH8, SPOP, HRD1, ARIH1, NEDD4, TNFAIP3/A20, TRIM21, Parkin), and is suppressed by deubiquitinases (USP2/7/8/10/14/19/22/9X/51, OTUB1/2/3, UBQLN4, CSN5). Ubiquitination and degradation of PD-1 is mediated by E3 ubiquitin ligases (FBXO38, FBW7, c-Cbl, MDM2, FBW7, MDM2), and its deubiquitination is mediated by deubiquitinase USP24. CTLA-4 ubiquitination and lysosomal degradation is induced by TRAF6, and its deubiquitination is mediated by USP8. Engagement of LAG-3 with its ligands MHC-II and FGL1 induces its non-K48-linked polyubiquitination. E3 ubiquitin ligases (TRIM21, TRAF2) are responsible for CD47 ubiquitylation and degradation. Deubiquitinases (USP10, USP2a) are responsible for deubiquitination of B7-H4. 4-1BB is ubiquitinated by FBXL20 E3 ubiquitin ligase, and deubiquitinated by deubiquitinases A20 and CYLD

Fig. 6

Roles and mechanisms of acetylation in regulating expression and functions of immune checkpoints. CBP/p300-induced acetylation and HDAC2/3- and SIRT1-induced deacetylation of PD-L1 mediate its protein localization and expression. p53 acetylation-induced recruitment of acetyltransferase cofactors (CBP, p300, TIP60) promotes PD-1 acetylation. IFNG-JAK1/2-mediated phosphorylation of STAT1 licenses CBP/p300-induced CTLA-4 acetylation. CBP/p300 induce acetylation of NKG2D ligands (MICA/B and ULBP2) and B7-H6, while HDAC1/2 induce their deacetylation

Fig. 7

Roles and mechanisms of succinylation and O-GlcNAcylation in regulating expression and functions of immune checkpoints. CPT1A succinyltransferase-induced succinylation of PD-L1 and O-GlcNAcylation of PD-L1 separately promote and prevent PD-L1 degradation through the endosomal-lysosomal pathway

Fig. 8

Roles and mechanisms of palmitoylation and lactylation in regulating expression and functions of immune checkpoints. Palmitoyltransferase ZDHHC3 palmitoylates PD-L1 to prevent its degradation, which can be modulated by FASN and H. pylori-induced SQLE upregulation. Palmitoylation of TIM-3, B7-H4, and CD80 is separately induced by ZDHHC9, ZDHHC3, and ZDHHC20, thus preventing their degradation. H3K18la of PD-L1 increases its transcription, which is mediated by CAFs-secreted LOX, PRMT3, STAT5-mediated glycolytic genes

Fig. 9

Roles and mechanisms of UFMylation and neddylation in regulating expression and functions of immune checkpoints. UFMylation of PD-L1 destabilizes PD-L1 through antagonizing its ubiquitination, which is mediated by UFL1 UFMylation E3 ligase and UFSP2. Pharmacological inhibition of neddylation by Pevonedistat upregulates the expression of PD-L1 and NKG2D ligands MICA and MICB. Upon CD47/SIRPα signaling, SHP2 is deneddylated by SENP8