Notch Signaling and PD-1/PD-L1 Interaction in Hepatocellular Carcinoma: Potentialities of Combined Therapies

Abstract

Immunotherapy has shown significant improvement in the survival of patients with hepatocellular carcinoma (HCC) compared to TKIs as first-line treatment. Unfortunately, approximately 30% of HCC exhibits intrinsic resistance to ICIs, making new therapeutic combinations urgently needed. The dysregulation of the Notch signaling pathway observed in HCC can affect immune cell response, reducing the efficacy of cancer immunotherapy. Here, we provide an overview of how Notch signaling regulates immune responses and present the therapeutic rationale for combining Notch signaling inhibition with ICIs to improve HCC treatment. Moreover, we propose using exosomes as non-invasive tools to assess Notch signaling activation in hepatic cancer cells, enabling accurate stratification of patients who can benefit from combined strategies.

Keywords: Notch; PD-1; PD-L1; combination therapy; hepatocellular carcinoma (HCC); immunotherapy.

Figure 1

Schematic representation of canonical Notch signaling pathway. (a) Schematic representation of the structure of Notch receptors and ligands. (b) Each Notch receptor is synthesized as a single precursor protein, cleaved into a heterodimer within the Golgi apparatus, and transported on the cell surface. Upon ligand binding on the neighboring cell, the receptor undergoes consecutive cleavages by TACE and γ-secretase, which release the Notch intracellular domain (NICD). NICD translocates into the nucleus, where it interacts with the transcription factor CSL, displaces co-repressors (CoRs), and recruits co-activators (CoAs), such as MAML. This process results in the activation of downstream target genes belonging to the HERP, HES, and HEY families. Figure was created with BioRender.com.

Figure 2

PD-1/PD-L1 signaling pathway. (a) PD-1 and PD-L1 structures consist of an extracellular domain, a transmembrane domain, and an intracytoplasmic region (signal motifs). (b) Antigens presented by Antigen-Presenting Cells (APCs) are recognized by the T-cell receptor (TCR; first signal). The second signal is delivered when CD80 and CD86 on the APCs engage CD28 on the T cells (red arrows indicate the activation signals, black and dotted arrows represent the suppression of the activation process). (c) The first signal for T-cell activation is provided by the binding of the T-cell receptor (TCR) with the antigen (Ag) presented by the APC in the context of the Major Histocompatibility Complex (MHC). A cascade of events follows, triggering the proliferation of effector T cells. Upon antigen-mediated activation (red arrows), the PD-1 receptor is transiently expressed on T cells and interacts with Programmed death-ligand 1 (PD-L1) overexpressed on the surface of APCs or cancer cells, suppressing the immune response (black arrows). Following antigen clearance, the PD-1 levels on responding T cells decrease. Persistent antigen exposure and/or inflammation, such as in chronic infections and cancer, elicits a constitutive high and sustained PD-1 expression, which can progressively lead to “exhausted” T lymphocytes with impaired effector functions. Figure created with BioRender.com.

Figure 3

Schematic representation of the known functional interactions between Notch and PD-1/PD-L1 pathways. Notch signaling regulates the expression of PD-1 in CD8+ T lymphocytes, promoting PDCD1 transcription (depicted by the red arrow) by the direct interaction of the NICD-CSL complex with the PDCD1 promoter. In tumor cells, the Notch pathway determines the upregulation of PD-L1 ligand expression (red arrow). However, the exact mechanism of regulation needs to be elucidated further. Figure created with BioRender.com.