Regulation of post-translational modification of PD-L1 and associated opportunities for novel small-molecule therapeutics

Abstract

PD-L1 is overexpressed on the surface of tumor cells and binds to PD-1, resulting in tumor immune escape. Therapeutic strategies to target the PD-1/PD-L1 pathway involve blocking the binding. Immune checkpoint inhibitors have limited efficacy against tumors because PD-L1 is also present in the cytoplasm. PD-L1 of post-translational modifications (PTMs) have uncovered numerous mechanisms contributing to carcinogenesis and have identified potential therapeutic targets. Therefore, small molecule inhibitors can block crucial carcinogenic signaling pathways, making them a potential therapeutic option. To better develop small molecule inhibitors, we have summarized the PTMs of PD-L1. This review discusses the regulatory mechanisms of small molecule inhibitors in carcinogenesis and explore their potential applications, proposing a novel approach for tumor immunotherapy based on PD-L1 PTM.

Keywords: N-linked glycosylation; PD-L1; PTMs; acetylation; methylation; palmitoylation; phosphorylation; ubiquitination.

Graphical abstract

Figure 1.

The schematic diagram for two signals of T-cell activation. When a T-cell encounters an Ag, it initially binds to the MHC using its TCR. Subsequently, a second signal is generated when the costimulatory molecule on APCs interacts with the corresponding receptor or ligand on the surface of T-cells such as CD28 binding to CD80. Both signals collaborate to initiate T-cell function. Ag: Antigen; APC: Antigen-presenting cell; MHC: Major histocompatibility complex; TCR: T-cell receptor.

Figure 2.

The schematic diagram for tumor immune escape and principles of immune checkpoint inhibitors therapy. T-cells serve as ‘sentinels’ capable of identifying tumor cells within the body and executing cytotoxic attacks. Tumor cells will produce the PD-L1 protein in response to the presence of the PD-1 protein on T-cells, which binds to the PD-1 protein and transmits inhibitory signals, thereby impeding normal T-cell function, inducing T-cell apoptosis and facilitating immune evasion. ICIs block the binding of PD-1 to PD-L1, thereby enabling T-cell functionality. ICI: Immune checkpoint inhibitor.

Figure 3.

Chemical structure of PD-L1 inhibitor, PD-L1 glycosylation inhibitors and agonist.

Figure 4.

Chemical structures of small molecule PD-L1 polyubiquitination, deubiquitination and phosphorylation inhibitors.

Figure 5.

Regulations of PD-L1 by proteolysis targeting chimera. The AbTAC of PD-L1 is a specific antibody that binds to both PD-L1 and the E3 ubiquitin ligase complex RNF-43, thereby inducing the lysosomal degradation pathway for PD-L1. Peptide PROTAC targets palmitoacyltransferase DHHC3, which stabilizes PD-L1, leading to its degradation through the lysosomal pathway. Another peptide-PROTAC, named FOXM1-Protac, degrades PD-L1 by inhibiting the degradation of FOXM1. PROTAC: Proteolysis targeting chimera.

Figure 6.

Regulations of PD-L1 though polyubiquitination, deubiquitination. PD-L1 can be degraded through binding to E3 ubiquitin ligase complexes, such as cullin3 SPOP, STUB1, β-Trcp, HUWEI, HRD1, ARIH1, MARCH8 (blue circle), followed by proteasomal degradation. The ubiquitination of PD-L1 can also be inhibited by deubiquitinating enzymes such as CSN5, USP22, OTUB1, USP9X (orange circle). Currently, some small molecule inhibitors like curcumin, C-15 and WP1130 have been discovered to inhibit the PD-L1 deubiquitination pathway and degrade PD-L1. Several upstream factors and enzymes including CDK4, CMTM4/6, GSK3 and ERAD are involved in regulating the ubiquitination of PD-L1.

Figure 7.

Chemical structure of palmitoacyltransferase inhibitors.

Figure 8.

Regulations of PD-L1 by post-translational modifications. The schematic illustrates the molecular functional domain of the PD-L1 protein and its various PTMs. PD-L1 consists of two extracellular Ig-like domains, a TM region and an ICD. The activity and stability of PD-L1 are regulated by a range of PTMs, including N-glycosylation (G, red circles), phosphorylation (P, yellow circles), methylation (M, purple squares), palmitoylation (PM, five-pointed yellow stars) and acetylation (Ac, green circles). Glycosylation and palmitoylation are crucial for maintaining the stability of PD-L1. Ubiquitination can facilitate the degradation of PD-L1. Phosphorylation can modulate levels of PD-L1 by indirectly influencing glycosylation and ubiquitination. ICD: Intracellular domain; PTM: Post-translational modification; TM: Transmembrane.