title: The proteasome load versus capacity balance determines apoptotic sensitivity of multiple myeloma cells to proteasome inhibition.
abstract: Proteasome inhibitors (PIs) are effective against multiple myeloma (MM), but the mechanisms of action and bases of individual susceptibility remain unclear. Recent work linked PI sensitivity to protein synthesis and proteasome activity, raising the question whether different levels of proteasome expression and workload underlie PI sensitivity in MM cells (MMCs). Exploiting human MM lines characterized by differential PI sensitivity, we report that highly sensitive MMCs express lower proteasome levels and higher proteasomal workload than relatively PI-resistant MMCs, resulting in the accumulation of polyubiquitinated proteins at the expense of free ubiquitin (proteasome stress). Manipulating proteasome expression or workload alters apoptotic sensitivity to PI, demonstrating a cause-effect relationship between proteasome stress and apoptotic responses in MMCs. Intracellular immunostaining in primary, patient-derived MMCs reveals that polyubiquitinated proteins hallmark neoplastic plasma cells, in positive correlation with immunoglobulin (Ig) content, both intra- and interpatient. Moreover, overall proteasome activity of primary MMCs inversely correlates with apoptotic sensitivity to PI. Altogether, our data indicate that the balance between proteasome workload and degradative capacity represents a critical determinant of apoptotic sensitivity of MMCs to PI, potentially providing a framework for identifying indicators of responsiveness and designing novel combination therapies.
authors.affiliation: Division of Genetics and Cell Biology, San Raffaele Scientific Institute, Milano, Italy.
authors.forename: Giada
authors.initials: G
authors.lastname: Bianchi
authors.forename: Laura
authors.initials: L
authors.lastname: Oliva
authors.forename: Paolo
authors.initials: P
authors.lastname: Cascio
authors.forename: Niccolò
authors.initials: N
authors.lastname: Pengo
authors.forename: Francesca
authors.initials: F
authors.lastname: Fontana
authors.forename: Fulvia
authors.initials: F
authors.lastname: Cerruti
authors.forename: Andrea
authors.initials: A
authors.lastname: Orsi
authors.forename: Elena
authors.initials: E
authors.lastname: Pasqualetto
authors.forename: Alexandre
authors.initials: A
authors.lastname: Mezghrani
authors.forename: Valeria
authors.initials: V
authors.lastname: Calbi
authors.forename: Giovanni
authors.initials: G
authors.lastname: Palladini
authors.forename: Nicola
authors.initials: N
authors.lastname: Giuliani
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Anderson
authors.forename: Roberto
authors.initials: R
authors.lastname: Sitia
authors.forename: Simone
authors.initials: S
authors.lastname: Cenci
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D001402:B-Lymphocytes / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D002454:Cell Differentiation / Q000187:drug effects; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology / Q000627:therapeutic use*; D019008:Drug Resistance, Neoplasm / Q000187:drug effects / Q000502:physiology*; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D006367:HeLa Cells; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D051379:Mice; D009101:Multiple Myeloma / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism* / Q000502:physiology; D061988:Proteasome Inhibitors; D013312:Stress, Physiological / Q000187:drug effects / Q000502:physiology; D014407:Tumor Cells, Cultured
chemical_list: D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D061988:Proteasome Inhibitors; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde