Ring finger protein 6 enhances chemo-resistance by transcriptionally activating proliferating cell nuclear antigen expression and attenuating DNA damage in lung adenocarcinoma

The E3 ubiquitin ligase RING finger protein 6 (RNF6) is elevated in several cancers, including prostate and colorectal cancers. Here, we extended the finding of elevated RNF6 expression levels and its association with poor prognosis in patients with lung adenocarcinoma (LUAD). Genome-wide RNA sequencing in H3255 cells with RNF6 knockdown, followed by analysis of differentially expressed genes using Clusters of Orthologous Groups and gene set enrichment analysis revealed aberrations in genes related to DNA repair, especially double-strand break (DSB) repair. RNF6 knockdown increased γH2AX foci, a biomarker for DSBs in H3255 and A549 LUAD cells, and enhanced DNA damage induced by chemotherapy in cisplatin-resistant A549/CDDP cells. In a series of experiments in cultured cells, as well as in nude mice carrying xenografts, RNF6 knockdown restored the sensitivity of A549/CDDP cells to cisplatin treatment. Mechanistically, RNA sequencing in RNF6-knockdown cells revealed the significant downregulation of proliferating cell nuclear antigen (PCNA), an oncogene that promotes DNA repair. Re-chromatin immunoprecipitation assay results suggested the formation of a RNF6-TCF4 complex that binds to the PCNA promoter to activate its transcription. Downregulation of RNF6 reduced TCF4 recruitment to PCNA promoters in H3255 and A549 cells, indicating that RNF6 regulates PCNA transcription to a certain extent by regulating TCF4 binding to PCNA promoters. The collective results implicate RNF6 overexpression as a molecular target in the management of cisplatin-resistant LUAD.