RNF6 Inhibits Lung Adenocarcinoma Cell Proliferation by Promoting Cyclin D2 Degradation

The E3 ubiquitin ligase RING finger protein 6 (RNF6) has been widely recognized for its role in promoting tumorigenesis in multiple cancers. However, we found that it is downregulated in lung adenocarcinoma (LUAD), and the molecular rationale for this discrepancy remains unclear. In the present study, we find that RNF6, but not its ΔRING inactive form, inhibits LUAD cell proliferation and migration and sensitizes LUAD to chemotherapy. To understand the molecular mechanism, we utilize affinity purification/tandem mass spectrometry (MS-MS) to analyze RNF6-interacting proteins and find that cyclin D2 (CCND2), a key regulator of the G1-S transition in the cell cycle. RNF6 physically binds to CCND2 and mediates its K48-linked polyubiquitination and subsequent degradation. However, ΔRING RNF6 fails to mediate CCND2 for ubiquitination and degradation. Moreover, Thr280 is critically important for CCND2 stability. When Thr280 is mutated, CCND2 becomes more stable and less ubiquitinated by RNF6. Furthermore, RNF6 arrests LUAD cell cycle at the G1 phase by inhibiting the CCND2/phospho-Rb signaling pathway, which is consistent with decreased cell proliferation. Lastly, RNF6 curtails the growth of LUAD xenografts in vivo, associated with decreased CCND2 expression. Therefore, RNF6 is a novel E3 ligase of CCND2 and suppresses LUAD cell proliferation. Implications: This study reveals a novel regulation on cell-cycle transition in LUAD and suggests the RNF6/CCND2 axis may represent an alternative therapeutic target for the treatment of LUAD.