Abstract Background: Advance stages of metastatic melanoma (MM) transitions reversibly between melanocytic and mesenchymal (MES)-like cell states, enabling invasion, therapeutic resistance, and immune evasion. While transcriptional regulators of this plasticity are well described, the role of post-translational control remains underexplored. The ubiquitin E3 ligase KPC1 (RNF123) regulates NF-κB signaling through p105 processing, but its contribution to melanoma cell-state transitions is unknown. We investigated whether KPC1 regulates MES reprogramming through ubiquitin-dependent modulation of ZEB1, a central MES transcription factor. Methods: We integrated bulk RNA-seq, single-cell RNA-seq, and proteomic datasets from TCGA-SKCM, GSE115978, and PXD006003. Deconvolution (BayesPrism, CODEFACS) was used to derive melanoma-cell-specific expression profiles, and Monocle3 was applied to reconstruct pseudospatial trajectories. Functional validation included siRNA knockdown, qRT-PCR, Western blotting, cycloheximide chase, MG132 treatment, ubiquitination assays, and wound-healing migration assays. Multiplex immunofluorescence (mIF) on a clinically annotated stage III/IV melanoma FFPE tissue microarray (n = 110) assessed protein-level associations and clinical relevance. Results: Across TCGA and scRNA-seq datasets, KPC1 expression showed a strong inverse association with MES gene signatures and was lowest in melanoma cells enriched for MES pathways. Trajectory analysis demonstrated that MES progression accompanied a continuous decline in KPC1. KPC1 knockdown did not change ZEB1 mRNA but markedly increased ZEB1 protein, indicating post-transcriptional regulation. Loss of KPC1 delayed ZEB1 degradation in CHX-chase assays, reduced ZEB1 ubiquitination, enhanced CDH2 expression, suppressed CDH1, and increased cell migration. Proteasome inhibition further confirmed that ZEB1 turnover is KPC1-dependent. In melanoma patient samples, low KPC1 protein co-localized with high ZEB1 and CDH2 and predicted significantly shorter overall survival. Combined evaluation of KPC1, ZEB1, and CDH2 improved prognostic resolution beyond individual markers. Conclusions: KPC1 functions as a post-translational suppressor of MES-like reprogramming in metastatic melanoma by promoting ubiquitin-dependent degradation of ZEB1. Loss of KPC1 stabilizes ZEB1, activates MES transcriptional and phenotypic programs, and correlates with poor clinical outcomes. These findings identify KPC1 as a key regulator of melanoma cell-state plasticity and supports its potential utility as a theragnostic target. Citation Format: Yusuke Nakano, Matias Bustos, Kelly Chong, Yoshinori Hayashi, Aaron Ciechanover, Dave Hoon. KPC1 regulates mesenchymal reprogramming in metastatic melanoma through ubiquitin-dependent degradation of ZEB1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4091.

PURPOSE:Regulator of chromosome condensation 1 (RCC1) and RCC2 have been shown to play important roles in the regulation of cell cycle, DNA damage response, and nucleocytoplasmic transport. MATERIALS AND METHODS:DNA (592-gene or whole exome) and RNA (whole transcriptome) sequencing was performed at Caris Life Sciences (Phoenix, AZ). Samples were stratified by RCC1 expression quartile thresholds (Q1: low, Q4: high) for small cell lung cancer (SCLC; n = 876), non-small cell lung cancer (NSCLC; n = 21,603), gastric cancer (GC; n = 1,908), pancreatic cancer (PC; n = 5,071), and colorectal cancer (CRC; n = 14,892). Statistical significance was determined using chi-square and Wilcoxon rank-sum tests and adjusted for multiple comparisons (*P < .05). Corresponding analyses were run for RCC2. RESULTS:Median RCC1 mRNA expression was highest in SCLC (14.3 transcript per million [TPM]), followed by GC (9.9), NSCLC (9.9), CRC (9.8), and PC (6.9). Similar to RCC1, the median RCC2 expressions were highest in SCLC (36.2 TPM). Tumor mutational burden-high rates were positively associated with increasing RCC1 expression quartiles (Q1-4) in NSCLC (31%-41%), GC (7%-22%), and CRC (5%-17%) and with increasing RCC2 expression in NSCLC and CRC only. Higher expression with RCC1 and RCC2 was associated with worse overall survival in NSCLC (hazard ratio [HR] for RCC1 and RCC2 were 1.3 and 1.3, respectively), PC (HR for RCC1 and RCC2 were 1.5 and 1.12, respectively), and CRC (HR for RCC1 and RCC2 were 1.3 and 1.03, respectively). CONCLUSION:RCC1 and RCC2 expression is a negative prognostic marker in NSCLC, PC, and CRC. Further studies to investigate RCC1 and RCC2 function at the molecular level may provide opportunities for novel targeted drug development.