RING1, a novel E3 ubiquitin ligase for CIP2A, negatively regulates smoking-induced lung tumorigenesis

Lung cancer is the second incidence rate of all new cancer diagnoses and the leading cause of cancer-related deaths globally. To develop novel diagnostic and therapeutic targets for lung cancer, there are emerging needs for molecular biological research on the occurrence and progression. Among the targets, CIP2A has been studied for promoting various cancer types, but detailed underlying mechanisms are needed to be further investigated. In this study, it was verified that CIP2A is overexpressed in lung cancer tissues, particularly amongst those with a smoking history, compared to adjacent normal tissues. CIP2A knockdown in lung cancer cells efficiently reduced cell proliferation and migration capacity. Through the mass spectrometry analysis on A549 cells overexpressing CIP2A, RING1 was identified as a candidate for E3 ligase of CIP2A. The interaction of the two proteins and ubiquitin chain regulation by RING1 was verified by immunoprecipitation. RING1 knockdown enhanced proliferation and migration of lung cancer cells, and these effects result from the upregulation of CIP2A and its downstream molecules, c-Myc and cyclin B1. To further study a trigger for RING1 expression changes in lung tumorigenesis, cigarette smoke extract (CSE) was applied for the following experiments. CSE treatment depleted RING1 mRNA, including protein expression. Decreased RING1 level led to upregulation of CIP2A and c-MYC. Collectively, these results reveal novel roles of the RING1 and CIP2A in lung cancer progression caused by smoking and propose a potential biomarker candidate for the therapeutic and diagnostic targets.