AUTHOR=Perrotta Cristiana , Cervia Davide , Di Renzo Ilaria , Moscheni Claudia , Bassi Maria Teresa , Campana Lara , Martelli Cristina , Catalani Elisabetta , Giovarelli Matteo , Zecchini Silvia , Coazzoli Marco , Capobianco Annalisa , Ottobrini Luisa , Lucignani Giovanni , Rosa Patrizia , Rovere-Querini Patrizia , De Palma Clara , Clementi Emilio 

TITLE=Nitric Oxide Generated by Tumor-Associated Macrophages Is Responsible for Cancer Resistance to Cisplatin and Correlated With Syntaxin 4 and Acid Sphingomyelinase Inhibition

JOURNAL=Frontiers in Immunology

VOLUME=Volume 9 - 2018

YEAR=2018

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2018.01186

DOI=10.3389/fimmu.2018.01186

ISSN=1664-3224

ABSTRACT=Tumor microenvironment is fundamental for cancer progression and chemoresistance. Among stromal cells tumor-associated macrophages (TAMs) represent the largest population of infiltrating inflammatory cells in malignant tumors, promoting their growth, invasion and immune evasion. M2-polarized TAMs are endowed with the NO generating enzyme iNOS. NO has divergent effects on tumors, since it can either stimulate tumor cells growth or promote their death depending on the source of it; likewise the role of iNOS in cancer differs depending on the cell type. The role of NO generated by TAM has not been investigated. Using different tumor models in vitro and in vivo we found that nitric oxide (NO) generated by inducible nitric oxide synthase (iNOS) of M2-polarized TAMs is able to protect tumor cells from apoptosis induced by the chemotherapeutic agent cisplatin (CDDP). Here, we demonstrate that the protective effect of NO depends on the inhibition of acid sphingomyelinase (A-SMase), which is activated by CDDP in a pathway involving the death receptor CD95. Mechanistic insights indicate that NO actions occur via generation of cyclic GMP and activation of protein kinase G (PKG), inducing phosphorylation of syntaxin 4 (synt4), a SNARE protein responsible for A-SMase trafficking and activation. Noteworthy, phosphorylation of synt4 at serine 78 by PKG is responsible for the proteasome-dependent degradation of synt4, which limits the CDDP-induced exposure of A-SMase to the plasma membrane of tumor cells. This inhibits the cytotoxic mechanism of CDDP reducing A-SMase-triggered apoptosis. 
This is the first demonstration that endogenous NO system is a key mechanism through which TAMs protect tumor cells from chemotherapeutic drug-induced apoptosis. The identification of the pathway responsible of A-SMase activity downregulation in tumors leading to chemoresistance warrants further investigations as a means to identify new anti-cancer molecules capable to specifically inhibit synt4 degradation.