E3 ligase TRIM22 promotes melanoma proliferation by regulating cell cycle progression through K63-linked ubiquitination of p21.

Melanoma, a highly aggressive skin cancer with limited therapeutic options, demonstrates poor prognosis in advanced stages. Tripartite motif-containing 22 (TRIM22), an E3 ubiquitin ligase of the tripartite motif (TRIM) family, is implicated in tumorigenesis, but its working mechanism remains poorly understood in melanoma. In this study, we found that expression of TRIM22 was abnormally upregulated in melanoma tissues, correlating with tumor stages. Functional analysis demonstrated that TRIM22 promoted melanoma cell proliferation in vitro. Furthermore, we found that in malignant melanoma, TRIM22 expression is negatively correlated to the level of p21, an inhibitor of cell cycle. With quantitative real-time PCR (qRT-PCR) assay and cycloheximide (CHX) treatment, we confirmed that TRIM22 suppressed p21 expression at protein level. Via S-Protein pull-down assay, we found that p21 could interact with TRIM22 at the SPRY domain. A ubiquitination assay proved that TRIM22 promoted the K63-linked ubiquitination of p21, and thereby induced p21 degradation through the proteasome pathway to accelerate cell cycle progression. Moreover, we discovered that overexpression of TRIM22 could not bring further boost of cell proliferation in p21 knockdown melanoma cells, indicating an epistatic role of p21 to TRIM22. Overall, our findings elucidated that TRIM22 acted as an E3 ligase targeting p21 for degradation to promote melanoma progression, which improved the understanding of TRIM22 function and provided more clues for developing TRIM22 as a potential target for malignant melanoma treatment.