V-set immunoregulatory receptor as a novel molecular target for genitourinary cancers.

515 Background: Immune checkpoint inhibitors (ICI) target immune receptors on the cell surface of T lymphocytes to enhance the immune response to cancer, and have been reported to be less toxic and more long-lasting than conventional chemotherapy. Representative immune checkpoint molecules include PD-1 and CTLA-4, and therapeutic drugs targeting these receptors have been effectively used for patients with metastatic melanoma, renal cell carcinoma, and non-small cell lung cancer. Recent studies have identified novel immune checkpoint molecules, and VSIR is one of them. VSIR (V-set immunoregulatory receptor) is a single-pass transmembrane protein known to be expressed in immune cells such as T cells and macrophages, and is supposed to reduce the activity of immune cells by binding to VSIG3 expressed by tumor cells. There have been several reports of VSIR in tumor tissues, including renal cancer, but the expression and function of VSIR in renal cancer cells themselves have not been examined in detail. Methods: We employed molecular and cellular biological technologies to obtain protein expression signature of siVSIR-treated genital and renal cancer cells. Results: Suppression of VSIR expression significantly reduced cell viability, and Western blot revealed that suppression of VSIR expression attenuated pAKT, pFoxO1, pERK, and pMEK signals. These findings suggest that VSIR is involved in the survival function of tumor cells, and that this function may be maintained by participating in the PI3K-AKT pathway and the Raf-MEK-ERK pathway. We further investigated the role of VSIR in regulation of AKT pathway revealing that suppression of VSIR expression reduced phosphorylation of FoxO1 and induced its nuclear localization. While nuclear FoxO1 induced the expression of p21 and p27 and G1 arrest in siVSIR-treated genital and renal cancer cells. Conclusions: Our results suggested that VSIR was involved in growth signal and cell cycle regulation in genital and renal cancer cells. It is expected that elucidating the detailed molecular mechanisms of immune checkpoint molecules will lead to further development of cancer immunotherapy, including immune checkpoint inhibitors in the future.