Dual Pathways of UBE4B Inhibit Apoptosis in p53-Positive Tumor Cells via CCAR2 Degradation

Apoptosis, or programmed cell death, is a fundamental process essential for tissue homeostasis, development, and the elimination of damaged or potentially cancerous cells. Here, we identify the E3/E4 ubiquitin ligase UBE4B as a critical suppressor of apoptosis in p53-proficient tumor cells, functioning through a previously uncharacterized dual mechanism. Initially, an orthogonal ubiquitin transfer screening approach identified CCAR2 as a UBE4B substrate. We demonstrate that UBE4B interacts with and ubiquitinates CCAR2, promoting its proteasomal degradation. Furthermore, we found that UBE4B concurrently targets p53 itself for ubiquitin-dependent degradation. Functionally, UBE4B overexpression suppresses apoptosis, whereas rescue experiments indicate that restoring p53 expression reverses this suppression more effectively than restoring CCAR2, highlighting the dominance of the direct p53 degradation pathway. Mechanistically, UBE4B deficiency leads to CCAR2 accumulation, which inhibits SIRT1 activity, thereby enhancing p53 acetylation and stability; this effect is reversed upon CCAR2 co-depletion. Consistently, transcriptional profiling confirms that UBE4B downregulates key p53 target genes (e.g., BAX, PUMA) through this dual-pathway regulation. In summary, our study establishes that UBE4B acts as a key apoptosis suppressor by coordinately degrading both p53 and its positive regulator CCAR2, revealing a targetable vulnerability in p53-wild-type tumors.