The scaffold protein DLG4 facilitates RNF63-mediated ubiquitination and degradation of STAT3 in non-small cell lung cancer.

BACKGROUND: The Disks-large homolog (DLG) family has been found to govern multiple key processes in human cancers. However, their role in non-small cell lung cancer (NSCLC) remains unknown. METHODS: The expression of DLG4 was determined by immunoblotting and q-PCR. The interacting proteins of DLG4 were identified by affinity purification mass spectrometry. The ubiquitination level of STAT3 was verified by denaturation-IP. The protein interactions were determined by co-IP. The clinical significance of DLG4, RNF63, and STAT3 was evaluated by immunohistochemical staining. RESULTS: In this study, by evaluating the expression levels of human DLG protein (DLG1-DLG5), we found that DLG4 is significantly downregulated in lung adenocarcinoma (LUAD) and lung squamous cell carcinoma (LUSC), two major types of NSCLC. DLG4 overexpression impairs cell proliferation and epithelial-mesenchymal transition migration (EMT) of NSCLC cells. The xenograft model also verifies the inhibitory effects of DLG4 on tumor growth in vivo. Moreover, we determined that DLG4 functions as a novel regulator of STAT3. Mechanistically, DLG4 directly interacts with STAT3 and recruits E3 ubiquitin ligase RNF63 (MKRN3) to STAT3, which promotes STAT3 K48-linked polyubiquitination and proteasome-mediated degradation. Importantly, in human NSCLC specimens, endogenous DLG4 and RNF63 expression levels are inversely correlated with that of STAT3. Moreover, low DLG4 and RNF63 expression correlates with poor patient survival in NSCLC. CONCLUSION: our findings define the role of DLG4 that can diminish NSCLC cell proliferation and tumorigenesis through degrading STAT3 in an RNF63-dependent manner. This work suggests a new treatment strategy against NSCLC caused by aberrant activation of STAT3.