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Natural killer (NK) cells are key players in the immune system. They use receptors on their cell surface to identify target cells. However, to escape being killed by the immune system, cancer cells such as thyroid cancer cells, use various methods to suppress the function of NK cells. Thus, this study aims to elucidate how thyroid cancer cells downregulate NK cell function in a co-culture system. We found that thyroid cancer cells suppress NK cell cytotoxicity and inhibit the expression of activating receptors, such as NKG2D and NKp46, by regulating indoleamine 2,3-dioxygenase (IDO). Also, thyroid cancer cells produce kynurenine using IDO, which causes NK cell dysfunction. Kynurenine enters NK cells via the aryl hydrocarbon receptor (AhR) on the surfaces of the NK cells, which decreases NK cell function and NK receptor expression via the signal transducer and activator of transcription (STAT) 1 and STAT3 pathways. In addition, STAT1 and STAT3 directly regulated the expression of NKG2D and NKp46 receptors by binding to the promoter region. Conclusively, NK cell function may be impaired in thyroid cancer patients by IDO-induced kynurenine production. This implies that IDO can be used as a target for thyroid cancer therapeutics aiming at improving NK cell function. [ABSTRACT FROM AUTHOR]

• Ginsenoside Rg3 increasd NK cell cytolytic activity. • Ginsenoside Rg3 activated NK cells through the MAPK/ERK signaling pathway. • Ginsenoside Rg3 enhanced the functional maturation of CB-derived NK cells. • Ginsenoside 20(R)-Rg3 was effective at activating NK cells and was activated through the MAPK/ERK signaling pathway. Ginseng is one of the most widely used herbal remedies for various diseases worldwide. Ginsenoside Rg3 (G-Rg3), the main component of ginseng, possesses several pharmacological properties, including anti-inflammatory, anti-tumor, antioxidant, anti-obesity, and immunomodulatory activities. However, the effect of G-Rg3 on natural killer (NK) cells in humans is not fully understood. Here, we investigated the effect of G-Rg3 on NK cell function and differentiation and elucidated the underlying mechanism. G-Rg3 increased NK cell cytotoxicity and simultaneously increased the expression of NK-activating receptors, NKp44, NKp46, and NKp30. Additionally, G-Rg3 increased the mRNA expression of NK cytolytic molecules, granzyme B and perforin. The expression of CD107a, a marker of NK cell degranulation, also increased in G-Rg3–treated NK cells. We therefore proceeded to identify which MAPK signaling pathway was involved in G-Rg3–mediated cytolytic activity. Treatment with G-Rg3 increased the phosphorylation levels of extracellular signal–regulated kinase (ERK), whereas ERK inhibition eliminated G-Rg3–induced NK cell cytotoxicity, suggesting the involvement of the ERK pathway. G-Rg3 did not affect the rate of differentiation of human cord-blood–derived NK cells; however, it increased the functional maturation of differentiated NK cells and promoted their cytotoxicity. The G-Rg3 isomer, 20(R)-Rg3, effectively activated NK cells via the extracellular signal-regulated kinase (ERK) signaling pathway, whereas 20(S)-Rg3 had no effect on NK cell activity. Altogether, the results demonstrated that 20(R)-Rg3 promoted NK cell activity via activation of the MAPK/ERK pathway, suggesting that 20(R)-Rg3 may be used as an activator of NK cell cytotoxicity for the treatment of diverse types of cancers. [ABSTRACT FROM AUTHOR]