Inactivation of Proprotein Convertases in T Cells Inhibits PD-1 Expression and Creates a Favorable Immune Microenvironment in Colorectal Cancer

Abstract

Proprotein convertases (PC) activate precursor proteins that play crucial roles in various cancers. In this study, we investigated whether PC enzyme activity is required for expression of the checkpoint protein programmed cell death protein 1 (PD-1) on cytotoxic T lymphocytes (CTL) in colon cancer. Although altered expression of the PC secretory pathway was observed in human colon cancers, only furin showed highly diffuse expression throughout the tumors. Inhibition of PCs in T cells using the general protein-based inhibitor a1-PDX or the pharmacologic inhibitor Decanoyl-Arg-Val-Lys-Arg-chloromethylketone repressed PD-1 and exhausted CTLs via induction of T-cell proliferation and apoptosis inhibition, which improved CTL efficacy against microsatellite instable and microsatellite stable colon cancer cells. In vivo, inhibition of PCs enhanced CTL infiltration in colorectal tumors and increased tumor clearance in syngeneic mice compared with immunodeficient mice. Inhibition of PCs repressed PD-1 expression by blocking proteolytic maturation of the Notch precursor, inhibiting calcium/NFAT and NF-kB signaling, and enhancing ERK activation. These findings define a key role for PCs in regulating PD-1 expression and suggest targeting PCs as an adjunct approach to colorectal tumor immunotherapy.

Proprotein convertases: Key players in inflammation-related malignancies and metastasis

“…4). Interestingly, the expression of several PCs, particularly Furin, was found to be altered in these patients [37]. Therapeutic strategies for the treatment or prevention of IBD aim to reduce the endogenous levels of TNF-α, which is a key pathophysiologic element of the disease [46] and involves the PCs in its full activity like other inflammatory mediators.…”

“…Overexpression of Furin was found to enhance tumor growth and aggressiveness of tumor cells [31]. On the other hand, while inhibition of PCs activity in several tumor cells using the general PC inhibitor (α1-PDX) was found to abrogate their malignant phenotype and reduce their ability to induce tumor progression, angiogenesis and metastases formation in mice [19,[32][33][34][35][36][37], the inhibition of the PCs in other cancer cells exacerbated their malignant phenotype [30,38]. These effects were associated with the inhibition of the processing of several molecules involved in the malignant phenotypes of tumor cells and tumorigenesis such as metalloproteases (MT1-MMP, MMP11/Str3), adhesion molecules (integrins, cadherins), cytokines (TGF family), growth factors (PDGFs, VEGF-C, VEGF-D, IGF-1), and growth factor receptors (IGF-1 receptor) [30,[37][38][39] (Fig.…”

“…4). The expression of several PCs was examined in a variety of cancers [19,37]. In some cases, expression of a number of PCs correlated with metastatic potential of cancer cells and aggressiveness.…”

“…TME-secreted VEGFD was shown to promote downstream TGFβ and E-cadherin signaling in cancer cells, resulting in increased cancer cell invasion into the lymphovascular system. The activity and/or the expression of the majority of these mediators involved in tumor and the TME communication are regulated by the proprotein convertases [19,[32][33][34][35][36][37]. These include cytokines, angiogenic factors (VEGF-C, VEGF-D), matrix metalloproteinase and adhesion molecules (Integrins, cadherins, selectins, immunoglobulins) ( Fig.…”

“…4). The cleavage of these substrates by Furin-like enzymes was found to be crucial for the mediation of their functions in vitro and during tumor angiogenesis [19,[32][33][34][35][36][37]. Inhibition of Furin-like enzymatic activity in tumor cells expressing the general PCs inhibitor α1-PDX, reduces the ability of these tumor cells to form vascularized tumors in mice [32][33][34].…”