Inhibition of Autophagy Promotes Bortezomib-Mediated Cell Death in Myeloma Cells

The 26S proteasome is a key regulator of proteins controlling many important cellular functions, including cell cycle progression, differentiation, gene transcription and apoptosis. Proteasome inhibition is a new therapeutic strategy that has shown promise in the treatment of B cell malignancies, primarily multiple myeloma. We and others have demonstrated that proteasome inhibitors induce endoplasmic reticulum (ER) stress and activate an unfolded protein response (UPR) in transformed cells. Our previous work demonstrated that bortezomib induces an endoplasmic reticulum (ER) stress response ultimately leading to calcium-dependent apoptotic cell death. Co-treatment of myeloma cells with the mitochondrial uniporter inhibitor ruthenium red (RuR) abrogated bortezomib mediated cell death, indicating that the cytotoxic effects of proteasomal inhibition requires dysregulation of intracellular Ca²⁺. Intracellular Ca²⁺ has also been implicated in the cellular stress response known as autophagy or “self-eating”. Macroautophagy (hereafter referred to as autophagy) is induced by various cellular stresses including nutrient deprivation, metabolic insufficiency, interruption of growth factor signaling, elevated ROS, accumulation of intracellular Ca²⁺, and the UPR. The biochemical events linking the cellular stress response with the induction of autophagy, and the relationship between autophagy and apoptosis is not well understood.

In this study, we investigate the role of the Ca²⁺ dependent serine protease, calpain, as a mediator of the conversion from autophagic cell survival to accelerated cell death in the ER stress response. We demonstrate that the proteasome inhibitor, bortezomib, initiates autophagy in myeloma cells, and protection from bortezomib-mediated cell death by mitochondrial Ca²⁺ inhibitors is associated with a promotion and stabilization of the autophagosome. This response can be reversed, and indeed, accelerated, leading to enhanced cell death, by blockade of calpain activity. Inhibition of calpain activity with the tri-peptide zLLY-FMK (Calpain Inhibitor IV, (CiIV) or the non-peptide inhibitor, PD150606, demonstrated a significant increase in the cytotoxic activity of bortezomib. Similarly, elimination of the small catalytic subunit, CAPNS1, using siRNA, enhanced bortezomib-mediated cell death, and prevented autophagosome-lysosomal progression. Furthermore, inhibition of calpain by clinically approved HIV protease inhibitors including Nelfinavir, Ritonavir, Saquinavir, and Indinavir sulfate, significantly increased the cytotoxic activity of bortezomib in vitro. We suggest that disregulation of Ca²⁺ by bortezomib-mediated ER stress activates the autophagic survival response. Inhibition of mitochondrial Ca²⁺ uptake by the uniporter inhibitor RuR promotes autophagy, and confers resistance to bortezomib. Conversely, inhibition of the Ca²⁺-dependent serine protease, calpain, prevents autophagolysosome maturation, and subverts the survival response to cell death. These data are likely to have important clinical implications for the treatment of refractory myeloma and other B cell malignancies.