To investigate the role of interferon-induced protein 6 (IFI6) in renal cell carcinoma (RCC) progression and the molecular mechanism in sunitinib resistance.

The differences of IFI6 expression between tumor and normal tissues were analyzed using TCGA data and clinical samples. The human RCC cells (786-O and ACHN) was transfected with either non-targeting siRNA control (scramble) or IFI6 siRNA (siIFI6). Cell proliferation, migration, and invasion were comparatively assessed between control and knockdown groups using CCK-8 and Transwell assays. Apoptosis-related proteins (caspase3、cleaved caspase3、Bcl2、BAX) were detected via Western blot. IFI6 expression was measured in sunitinib-resistant cells (786-O-SuR), and the changes in the half maximal inhibitory concentration (IC₅₀) of sunitinib were evaluated after IFI6 knockdown. Stable OSRC-2 cell lines with IFI6 knockdown (shIFI6) were established, and a subcutaneous tumor xenograft model in nude mice was performed with sunitinib treatment to evaluate its in vivo efficacy. The experiment was divided into four groups: scramble + DMSO, shIFI6 + DMSO, scramble + sunitinib, and shIFI6 + sunitinib. Independent samples t-test was used for comparisons between two groups, and paired samples t-test was used for paired comparisons. For comparisons among three or more groups, one-way ANOVA was employed. Dunnett's t-test was used for pairwise comparison between groups.

IFI6 was significantly upregulated in RCC tissues versus normal tissues (P < 0.05), with high expression correlating with poor prognosis (P < 0.05). IFI6 knockdown markedly suppressed the abilities of proliferation [786-O: (1.33 ± 0.11), (1.24 ± 0.11) vs (2.15 ± 0.15) ; ACHN: (2.03 ± 0.14), (1.59 ± 0.14) vs (3.05 ± 0.18) ; P < 0.05], migration [786-O: (169.67 ± 31.01), (140.67 ± 19.40) vs (371.67 ± 58.05) cells; ACHN: (245.67 ± 33.25), (177.33 ± 18.45) vs (558.00 ± 83.83) cells; P < 0.05], and invasion [786-O: (55.33 ± 9.07), (46.67 ± 7.77) vs (102.00 ± 8.54) cells; ACHN: (43.33 ± 12.66), (37.33 ± 11.93) vs (118.00 ± 12.49) cells; P < 0.05]. Knockdown of IFI6 led to an increased expression of cleaved caspase 3 and BAX, while the expression of total caspase 3 and Bcl2 was decreased. Compared with 786-O, the expression levels of IF16 mRNA and protein were elevated in 786-O-SuR (P < 0.05 for both). The IC₅₀ concentration of sunitinib in the siIFI6-1 and siIFI6-2 groups was significantly lower than that in the Scramble group in 786-O, ACHN and 786-O-SuR cells [ 786-O: (5.02 ± 0.15), (4.40 ± 0.47) vs (7.28 ± 0.28) μmol/L; ACHN: (4.56 ± 0.17), (4.20 ± 0.19) vs (6.68 ± 0.31) μmol/L; 786-O-SuR: (8.02 ± 0.45), (7.50 ± 0.49) vs (16.61 ± 1.16) μmol/L; P < 0.05 for all ]. In vivo tumor formation experiments in nude mice revealed that, the tumor mass and volume in the Scramble + Sunitinib group and shIFI6 + DMSO group were significantly smaller than those in the Scramble + DMSO group [mass: (318.36 ± 40.81), (308.54 ± 36.97) vs (532.76 ± 79.59) mg, volume: (362.68 ± 58.76) and (349.96 ± 59.38) vs (681.20 ± 150.85) mm³; P < 0.05 for all], and the tumor mass and volume in the shIFI6 + Sunitinib group were further reduced compared with the Scramble + Sunitinib group [mass: (109.28 ± 34.43) vs (318.36 ± 40.81) mg, volume: (112.04 ± 40.62) vs (362.68 ± 58.76) mm³; P < 0.05 for all ].

This study reveals that IFI6 promotes RCC progression and confers sunitinib resistance by suppressing apoptosis. Targeting IFI6 sensitizes RCC to therapeutic agents, demonstrating potential clinical significance.