Melanoma-derived versican reactivates tumor-associated macrophages by upregulating pyruvate carboxylase through TLR2-MyD88-RelB axis under normoxia

Abstract

Relieving hypoxia in the tumor microenvironment (TME) promotes innate and adaptive immunity. Our previous research demonstrated that reoxygenation of the TME promotes the phagocytosis and tumor-killing functions of tumor-associated macrophages (TAMs) by upregulating pyruvate carboxylase (PCB). However, the mechanism remains obscure. In the present study, we find that versican derived from melanoma cells binds to TLR2 and activates the downstream transcription factor RelB, which transcribes PCB under normoxia. Blocking the versican-TLR2-MyD88-RelB axis not only reverses the upregulation of PCB in TAMs but also hinders the clearance of tumor cells by TAMs. Our work suggests a pathway that modulates the functions of TAMs under normoxia, which could be harnessed for strengthening anti-tumor immunity.

1. Introduction

Tumor-associated macrophages (TAMs) account for the largest fraction of the immune infiltrate in most solid malignancies [1]. By promoting tumor growth, angiogenesis, metastasis, and immunosuppression, TAMs have emerged as critical pro-tumor immunocytes in the tumor microenvironment [2]. Therefore, the presence of TAMs in tumors is generally associated with poor prognosis, and the depletion of TAMs has recently been exploited for cancer therapy [3,4]. Even so, the phenotypes and functions of TAMs are plastic [5]. They can also act as effector cells by engulfing tumor cells and recruiting cytotoxic T cells to activate anti-tumor immunity [6]. Consistent with this, accumulating evidence shows that macrophages are required for the efficacy of chemotherapy [7] and programmed death ligand-1 (PD-L1) blockade therapy [7,8]. Thus, reorienting TAMs to antitumor macrophages is superior to depleting TAMs indiscriminately.

Several strategies have been proposed to reorient TAMs, including Toll-like receptor (TLR) activation, CD40 agonism, and PI3Kγ inhibition [6]. However, owing to their side effects on macrophages in normal tissues and organs, these agents have limited clinical impact [9,10]. Our previous study revealed that relieving hypoxia in the tumor microenvironment reverses the inhibition of pyruvate carboxylase (PCB) in TAMs and reactivates the tumor-killing functions of TAMs [11]. As the seminal step in gluconeogenesis, PCB plays a dominant role in controlling the rate of gluconeogenesis [12]. By catalyzing the carboxylation of pyruvate, PCB replenishes the pool of oxaloacetate, which can be used not only for gluconeogenesis but also to supplement the intermediates of the tricarboxylic acid cycle [13]. The emerging role of PCB in regulating the function of immune cells has gained considerable attention in recent years [14,15]. Cytotoxic T cells also rely on PCB to exert anti-tumor function [14]. Therefore, modulating the expression or activity of PCB in immune cells, such as TAMs and CD8+ T cells, is a promising strategy to improve the efficiency of immunotherapy. However, the mechanism by which PCB expression is upregulated in TAMs under normoxia is unknown, which is the basis for the development of immunotherapies involving PCB intervention in TAMs.

In this study, we discovered that the upregulation of PCB in TAMs is induced by melanoma-derived versican under normoxia. Mechanistically, versican promotes PCB expression through the TLR2-MyD88-RelB axis. Blocking the versican-TLR2-MyD88-RelB pathway reversed the increased engulfment of tumor cells by TAMs. Thus, our work provides potential targets for reactivating TAMs and cytotoxic T cells that suffer from the harsh tumor microenvironment and partially rely on PCB to combat tumors.