Am J Physiol Endocrinol Metab. 2022 Nov 23.
This article briefly reviews cancer immunity and the role of gut microbiota in carcinogenesis, followed by an understanding of mechanisms by which inosine is involved in cancer immunometabolism. Constitutional cells of immune systems play a paradoxical role in cancer treatment. Anti-tumor immunity depends on the T-cell priming against tumor antigens, while the pro-tumor signaling is triggered by inflammatory mediators present in the tumor microenvironment. Studies link the microbiome with metabolism and immunity-two main factors implicated in carcinogenesis. Gut microbiota has been shown to affect both anti-tumor immunity and pro-tumor immune signaling. Additionally, there is mounting evidence that the human microbiome can play a role in the immunotherapeutic effects, both response and resistance. Inosine-5'-monophosphate dehydrogenase (IMPDH) is a highly conservative enzyme widely expressed in mammals. Cell signaling pathways utilize molecular inosine, which is a crucial secondary metabolite in purine metabolism and a molecular messenger in the process. Recent research has identified inosine as a critical regulator of immune checkpoint inhibition (ICB) therapeutic response in various tumor types. Three bacterial species, Bifidobacterium pseudopodium, Lactobacillus, and Olsenella, were found to produce inosine or its metabolite hypoxanthine and induce Th1 differentiation and effector functions via the inosine-A2AR-cAMP-PKA pathway upon ICB initiation, after being injected into the systemic circulation. Moreover, inosine acts as a substitute carbon source for T cell metabolism in glucose-restricted environments, i.e., the tumor microenvironment, assisting T cell proliferation and differentiation while enhancing sensitivity to ICB, reinforcing the notion that inosine metabolism may contribute to anti-tumor immunity. Also, inosine is a strong agonist of the adenosine receptor, A2AR, and A2AR signaling can affect T cell responses and anti-tumor immunity, making the inosine-A2AR pathway blockage a candidate for cancer treatment. Further research is required to investigate inosine as a cancer immunometabolism therapy.
Keywords: Cancer; Immunity; Immunometabolism; Immunotherapy; Inosine