Abstract
<div>Abstract<p><b>Purpose:</b> Recent studies have shown that the process of protein neddylation was abnormally activated in several human cancers. However, it is unknown whether and how UBE2F, a less characterized neddylation E2, regulates lung cancer cell survival, and whether and how NOXA, a proapoptotic protein, is ubiquitylated and degraded by which E3 and via which ubiquitin linkage.</p><p><b>Experimental Design:</b> Methods of immunohistochemistry and immunoblotting were utilized to examine UBE2F protein expression. The biological functions of UBE2F were evaluated by <i>in vitro</i> cell culture and <i>in vivo</i> xenograft models. The <i>in vivo</i> complex formation among UBE2F-SAG-CUL5-NOXA was measured by a pulldown assay. Polyubiquitylation of NOXA was evaluated by <i>in vivo</i> and <i>in vitro</i> ubiquitylation assays.</p><p><b>Results:</b> UBE2F is overexpressed in non–small cell lung cancer (NSCLC) and predicts poor patient survival. While UBE2F overexpression promotes lung cancer growth both <i>in vitro</i> and <i>in vivo</i>, UBE2F knockdown selectively inhibits tumor growth. By promoting CUL5 neddylation, UBE2F/SAG/CUL5 tri-complex activates CRL5 (Cullin-RING-ligase-5) to ubiquitylate NOXA via a novel K11, but not K48, linkage for targeted proteasomal degradation. CRL5 inactivation or forced expression of K11R ubiquitin mutant caused NOXA accumulation to induce apoptosis, which is rescued by NOXA knockdown. Notably, NOXA knockdown rescues the UBE2F silencing effect, indicating a causal role of NOXA in this process. In lung cancer tissues, high levels of UBE2F and CUL5 correlate with a low level of NOXA and poor patient survival.</p><p><b>Conclusions:</b> By ubiquitylating and degrading NOXA through activating CRL5, UBE2F selectively promotes lung cancer cell survival and could, therefore, serve as a novel cancer target. <i>Clin Cancer Res; 23(4); 1104–16. ©2016 AACR</i>.</p></div>
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