The signaling function of IDO1 incites the malignant progression of mouse B16 melanoma

ABSTRACT

Indoleamine 2,3 dioxygenase 1 (IDO1), a leader tryptophan-degrading enzyme, represents a recognized immune checkpoint molecule. In neoplasia, IDO1 is often highly expressed in dendritic cells infiltrating the tumor and/or in tumor cells themselves, particularly in human melanoma. In dendritic cells, IDO1 does not merely metabolize tryptophan into kynurenine but, after phosphorylation of critical tyrosine residues in the non-catalytic small domain, it triggers a signaling pathway prolonging its immunoregulatory effects by a feed-forward mechanism. We here investigated whether the non-enzymatic function of IDO1 could also play a role in tumor cells by using B16-F10 mouse melanoma cells transfected with either the wild-type Ido1 gene (Ido1^WT) or a mutated variant lacking the catalytic, but not signaling activity (Ido1^H350A). As compared to the Ido1^WT-transfected counterpart (B16^WT), B16-F10 cells expressing Ido1^H350A (B16^H350A) were characterized by an in vitro accelerated growth mediated by increased Ras and Erk activities. Faster growth and malignant progression of B16^H350A cells, also detectable in vivo, were found to be accompanied by a reduction in tumor-infiltrating CD8⁺ T cells and an increase in Foxp3⁺ regulatory T cells. Our data, therefore, suggest that the IDO1 signaling function can also occur in tumor cells and that alternative therapeutic approach strategies should be undertaken to effectively tackle this important immune checkpoint molecule.

KEYWORDS:

• Indoleamine 2
• 3-dioxygenase 1 (IDO1)
• immune checkpoint
• immunoreceptor Tyrosine-based inhibitory motif (ITIM)
• IDO1 signaling function
• B16 melanoma
• tryptophan metabolism

Acknowledgments

The article is dedicated to the memory of Ursula Grohmann (1961–2022) who spent her life in studying the pleiotropy of the IDO1 protein.

We thank Dr. Serena Camerini from Istituto Superiore di Sanità (Rome, Italy) for the technical support in the proteomic analysis.

Author Contributions

C.O. and G.M. designed the study and interpreted the data; E.O. generated melanoma cell lines and collected the data; E.O., MTP, E.P., S.R. performed the in vitro experiments; C.V. and C.S. performed the in vivo experiments; M.G. performed flow cytometry analysis; S.B. performed the computational analysis of the proteomic data; A.M. performed the statistical analysis; M.L.B. and G.M. drafted the manuscript; C.O. edited and revised the manuscript.

All authors have read and agreed to the published version of the manuscript.

Disclosure statement

The authors report no conflict of interest.

Data availability statement

Raw data were generated at the Dept. Medicine and Surgery – University of Perugia (Italy). Derived data supporting the findings of this study are available from the corresponding author [C.O.] on request. Proteomic raw data are available under the following DOI: 10.6084/m9.figshare.20518329.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/2162402X.2023.2170095

Additional information

Funding

This work was supported by the Italian Ministry of Education, University, and Research (PRIN2017-2017BA9LM5 to CO, PRIN2020-2020L45ZW4 to CO, PRIN2017WJZ9W9 to MTP), by Associazione Italiana per la Ricerca sul Cancro (AIRC 2019-23084 to CV).