A plastic SQSTM1/p62-dependent autophagic reserve maintains proteostasis and determines proteasome inhibitor susceptibility in multiple myeloma cells

Abstract

Multiple myeloma (MM) is the paradigmatic proteasome inhibitor (PI) responsive cancer, but many patients fail to respond. An attractive target to enhance sensitivity is (macro)autophagy, recently found essential to bone marrow plasma cells, the normal counterpart of MM. Here, integrating proteomics with hypothesis-driven strategies, we identified the autophagic cargo receptor and adapter protein, SQSTM1/p62 as an essential component of an autophagic reserve that not only synergizes with the proteasome to maintain proteostasis, but also mediates a plastic adaptive response to PIs, and faithfully reports on inherent PI sensitivity. Lentiviral engineering revealed that SQSTM1 is essential for MM cell survival and affords specific PI protection. Under basal conditions, SQSTM1-dependent autophagy alleviates the degradative burden on the proteasome by constitutively disposing of substantial amounts of ubiquitinated proteins. Indeed, its inhibition or stimulation greatly sensitized to, or protected from, PI-induced protein aggregation and cell death. Moreover, under proteasome stress, myeloma cells selectively enhanced SQSTM1 de novo expression and reset its vast endogenous interactome, diverting SQSTM1 from signaling partners to maximize its association with ubiquitinated proteins. Saturation of such autophagic reserve, as indicated by intracellular accumulation of undigested SQSTM1-positive aggregates, specifically discriminated patient-derived myelomas inherently susceptible to PIs from primarily resistant ones. These aggregates correlated with accumulation of the endoplasmic reticulum, which comparative proteomics identified as the main cell compartment targeted by autophagy in MM. Altogether, the data integrate autophagy into our previously established proteasome load-versus-capacity model, and reveal SQSTM1 aggregation as a faithful marker of defective proteostasis, defining a novel prognostic and therapeutic framework for MM.

Lycorine Downregulates HMGB1 to Inhibit Autophagy and Enhances Bortezomib Activity in Multiple Myeloma

“…Furthermore, combination of additional concentrations of lycorine and BTZ were tested and calculations of combination index (CI) values using CompuSyn software indicated synergistic effects between lycorine and BTZ in vitro (Figure 5B). Recent studies suggested that BTZ induced autophagy is responsible for acquired resistance against this drug 38-41. Next, we aimed to investigate the effect of lycorine-BTZ combination on autophagy.…”

Evidence for the Role of BAG3 in Mitochondrial Quality Control in Cardiomyocytes

“…SQSTM1 has been reported to play a protective role under conditions of proteasome inhibition (Milan et al, 2015). P62 functions as a cargo receptor and binds to both ubiquitinated and non-ubiquitinated substrates and triggers their autophagic clearance (Pankiv et al, 2007; Watanabe et al, 2011; Behl et al, 2011).…”

Endoplasmic reticulum stress in the development of multiple myeloma and drug resistance

“…Autophagy has begun to emerge as an additional factor that may confer resistance to proteasome inhibition. Within resistant cells, Sequestosome‐1 (SQSTM1)‐dependent autophagy provides a protection mechanism to proteasome inhibitor‐induced protein aggregation and cell death . Inhibition of SQSTM1 within resistant cells sensitised cells to proteasome inhibition .…”