Abstract
Background: Proteasome inhibitors (PIs) are a cornerstone of multiple myeloma (MM) therapy, exploiting the high rate of immunoglobulin production and protein turnover in malignant plasma cells. However, intrinsic and acquired resistance to PIs remains a significant barrier to durable clinical responses. Hypoalbuminemia is a frequent and clinically relevant symptom in MM, often associated with poor prognosis, but its underlying causes are not fully understood. To date, it has not been explored whether MM plasma cells actively consume albumin and whether this contributes to hypoalbuminemia or interferes with current therapeutics.Although albumin catabolism has been described in endothelial cells and some solid tumors, its role in hematologic malignancies has not been investigated. Here, we report for the first time that MM cells internalize and degrade extracellular albumin, predominantly via lysosomal pathways, and that this mechanism contributes directly to resistance against proteasome inhibition.
Methods: Human MM cell lines (RPMI-8226, JJN3, MM1.S, H929, and KMS11), representing a range of PI sensitivities, were cultured in media with defined amino acid composition and varying concentrations of human serum albumin. Cell viability in response to PIs (bortezomib and carfilzomib) was measured using MTT and Cell Titer-Glo assays. To determine whether albumin affected drug sensitivity, PI dose-response assays were conducted under low and high albumin conditions, with or without lysosomal inhibitors (ammonium chloride and leupeptin). Intracellular albumin accumulation and degradation pathway activation were assessed via Western blotting for proteasomal and lysosomal markers.
Results: MM cells exhibited active uptake of extracellular albumin through endocytosis. Once internalized, albumin was metabolized to provide both essential and nonessential amino acids, supporting cell proliferation and enhancing antioxidant defenses. A strong correlation was observed between the extent of albumin consumption and resistance to PIs. Notably, under high-albumin conditions, MM cells became significantly more resistant to PI-induced cytotoxicity. Even in the absence of other metabolic stressors, high concentrations of albumin alone were sufficient to trigger PI resistance.
Mechanistically, although small quantities of albumin could be degraded by the proteasome, elevated albumin levels induced robust lysosomal activation, diverting protein degradation away from the proteasome and effectively bypassing PI-induced stress. Under low-albumin conditions, PI treatment led to intracellular albumin accumulation in sensitive cells. In contrast, high-albumin conditions enhanced lysosomal degradation, mitigating the effects of proteasome blockade. Protein profiling revealed the upregulation of lysosomal enzymes and reduced proteasome dependence in albumin-treated cells. Importantly, inhibition of lysosomal function with ammonium chloride (partial) or leupeptin (complete) restored PI sensitivity, even under high-albumin conditions.
Conclusion: This study identifies albumin-driven lysosomal activation as a novel and clinically relevant mechanism of PI resistance in MM. For the first time, we demonstrate that MM cells consume albumin as a nutrient source, offering a plausible explanation for the hypoalbuminemia commonly observed in patients. Our findings suggest that lysosomal degradation serves as a compensatory protein catabolism pathway during elevated extracellular albumin uptake. This has important therapeutic implications: reliance on lysosomal degradation renders MM cells less susceptible to standard PIs, and dual inhibition of lysosomal and proteasomal pathways may overcome resistance. These findings lay the groundwork for designing next-generation therapeutics that simultaneously target proteasomal and lysosomal degradation pathways to overcome drug resistance in multiple myeloma and related hematological malignancies.
