KRas plays a negative role in regulating IDO1 expression.

Ras proteins are integral to the mediation of signaling cascades to downstream effectors, regulating a multitude of cellular processes. Mutations within Ras and its associated signaling pathways are implicated in various human pathologies, including inflammatory disorders and malignancies. The immune checkpoint proteins, programmed cell death protein 1 (PD-1) and its ligands PD-L1, along with Indoleamine 2,3-dioxygenase-1 (IDO1), are pivotal in facilitating tumor immune escape. While the influence of oncogenic Ras on PD-L1 expression is extensively documented, the regulatory role of KRas in IDO1 expression remains inadequately understood. In the current study, we demonstrate that IDO1 and PD-L1 expressions are differentially regulated in KRas-mutant cancers. Treatment with the KRas(G12C)-specific inhibitor, ARS-1620, significantly increased IDO1 expression, which inversely correlated with PD-L1 expression in the KRas(G12C)-mutant H358 cell line. Notably, IDO1 expression was slightly diminished in KRas-mutant patients with lung and pancreatic ductal adenocarcinomas. Experimental data revealed that IFN-γ induces IDO1 expression; however, this induction is attenuated in the presence of constitutively active KRas. These findings suggest that KRas signaling negatively regulates IDO1 expression while enhancing PD-L1 expression. Moreover, the induction of IDO1 expression following KRas inhibition appears to operate independently of the MAPK pathway. Our results propose that concurrent targeting of KRas and IDO1 could potentiate therapeutic efficacy in KRas-mutant cancers, overcoming resistance to immune checkpoint blockade.