Capsaicin as new adjuvant in anti-cancer immunotherapy

Red chili (Capsicum frutescens) is widely used as a spice for flavoring foods worldwide. Accumulating evidence has shown multiple pharmacological effects of Capsicum on a variety of physiological systems [1,2]. Pungent capsaicinoids (capsaicin, dihydrocapsaincin), antioxidant vitamins (ascorbic acid, vitamin E), carotenoids (β-carotene, β-cryptoxanthine) and several organic acids and minerals are the major active chemical substance found in Capsicum frutescens [2]. Capsaicin (CPS) (8-methyl-N-vanillyl-6-nonenamide) is an irritant for mammals, including humans. The burning and painful sensations associated with CPS result from its chemical interaction with sensory neurons. CPS, is a derivative of vanillyl amide (8-methyl-N-vanillyl-6-nonenamide). It binds to a receptor called the vanilloid receptor subtype 1 (VR1 or TRPV1) belonging to the Transient Receptor Potential Vanillod subfamily [3]. CPS has been used medicinally for centuries because it can reduce cholesterol, blood lipid content, blood sugar content and it also has properties of anti-oxidative, anti-inflammatory, anti-obesity and analgesic [4]. At present, it has been demonstrated that CPS exerts anti-cancer activity and immunomodulatory functions [5]. Indeed, the engagement of TRPV1 on dendritic cells by CPS modulates their function by upregulating antigen-presenting and costimulatory molecules, in addition to initiating their migration to draining lymph nodes [6]. Moreover, CPS induces damage-associated molecular patterns of immunogenic cell death in human bladder cancer cells [7]. Bladder cancer has been characterized as a tumor group in which the immunological response is relatively well preserved [8]. A number of immune checkpoint inhibitors have been approved as first-line therapy in case of cisplatin-ineligible patients or as second-line therapy for patients with metastatic urothelial carcinoma of the bladder. However, only 30% of patients with metastatic urothelial carcinoma will respond to this type of immunotherapy [9]. Among the immunocheckpoints, programmed death-ligand 1 (PD-L1) seems to predict response to immune checkpoint inhibitors in such patients. PD-L1 is the transmembrane protein ligand of PD-1, expressed on the cell membrane in T and B lymphocytes, antigen presenting cells and can be induced in tumor cells within the tumor microenvironment. The PD-1/PD-L1 pathway negatively regulates T cell activation, thus it plays an important role in controlling anti-tumor immunity response [10]. Recently, several clinical trials targeting PD-1/PD-L1 pathways using anti-PD-1 or anti PD-L1 antibodies demonstrated the clinical benefit for the patients with bladder cancer and one of this antibodies, atezolizumab, was approved by Food and Drug Administration in USA [11]. Therefore, compounds able to affect the expression of PD-L1 deserve further study. Herein, we evaluated the ability of CPS, at not cytotoxic dose, to influence PD-L1 expression, both at mRNA and protein levels, in 5637 and T24 bladder cancer cell lines. We found that the exposure of both cell lines to CPS at 50 μM for 12 h and 24 h can increase the expression of PD-L1 evaluated by RT-PCR, western blot and immunohystochemistry analyses. Preliminary results showed that the mechanism involved in CPS-mediated upregulation of PD-L1 expression is ROS-independent and TRPV1-mediated. Moreover, the involvement of interferon 1/1, through the activation of the NME/NM23 nucleoside diphosphate kinase 4 (NME4)/STATs signaling pathway has been evaluated. Further studies in vivo and in vitro are definitely required to completely address the CPS immunomodulatory capability in bladder cancer and to elucidate the related anti-tumor effects.