(PA-464) APOBEC3B-Driven Metabolic Adaptation Contributes to Bortezomib Resistance in Multiple Myeloma

Multiple myeloma (MM) is characterized by profound genomic instability, with APOBEC3 enzymes serving as major drivers of mutagenesis during disease progression. Recent studies have shown that APOBEC3-driven mutations are enriched in high-risk MM patients and may influence metabolic pathways, particularly oxidative phosphorylation (OXPHOS). However, the functional role of APOBEC3B in MM metabolism remains poorly defined. This study investigates how APOBEC3B expression modulates mitochondrial metabolism and metabolic rewiring under proteasome inhibition in MM cells.

 

Methods:   

We established APOBEC3B knockdown (KD), knockout (KO), and overexpression (OE) models in MM cell lines using CRISPR/Cas9 (NCI-H929) and lentiviral transduction (OPM-2). APOBEC3B expression was validated by qRT-PCR and western blot (WB). DNA double-strand breaks (DSBs) were assessed by γH2AX via flow cytometry and WB. Mitochondrial function was evaluated using Seahorse XF Analyzer (OCR/ECAR). Cell viability was measured with an ATP-based luminescence assay. Cells were treated with bortezomib (BTZ, 3–12 nM) alone or with dexamethasone (Dex), and metabolic and viability changes were assessed.

Results:   

CRISPR-mediated KO of APOBEC3B in NCI-H929 (high endogenous A3B) and OE in OPM-2 (no endogenous A3B) was successfully established and validated. APOBEC3B OE significantly increased γH2AX, while KO reduced DSBs. BTZ treatment induced DNA damage and increased APOBEC3B expression regardless of baseline APOBEC3B levels. Seahorse analysis revealed that APOBEC3B OE in OPM-2 significantly enhanced mitochondrial respiration (basal, coupled, maximal OCR, and respiratory reserve) compared to EV controls under both untreated and BTZ-treated (10–12 nM) conditions. In contrast, APOBEC3B KO in NCI-H929 led to reduced mitochondrial function under both untreated and 3 nM BTZ-treated conditions. Notably, across all treatment conditions, mitochondrial activity decreased compared to untreated controls, reflecting the metabolic suppressive effect of BTZ. Despite APOBEC3B’s role in metabolic rewiring, ATP-based viability assays showed no significant survival difference between OE and KO cells under BTZ, suggesting that elevated mitochondrial activity provides metabolic flexibility but does not directly confer survival advantage under proteasome inhibition.

Conclusions:   

APOBEC3B promotes mitochondrial rewiring and enhanced respiration under proteasome inhibitor stress but does not significantly alter viability in MM cells treated with bortezomib. These findings suggest that APOBEC3B contributes to metabolic adaptation without directly promoting resistance and highlight potential metabolic vulnerabilities that could be exploited for therapeutic targeting in MM.