Multiple myeloma cells are exceptionally sensitive to heat shock, which overwhelms their proteostasis network and induces apoptosis

Significance

Proteasome inhibitors are a highly effective treatment for multiple myeloma, a cancer of plasma cells. These studies were undertaken to understand why myeloma cells are exceptionally sensitive to proteasome inhibition. One proposed explanation is that myeloma proteasomes must continually degrade the large amounts of misfolded immunoglobins these cells produce. If so, myeloma cells should also be unusually sensitive to heat shock, which increases substrate load on 26S proteasomes. Unlike other cells studied, myeloma cell lines undergo extensive apoptosis at 43 °C, and, thus, are highly susceptible to proteotoxic stress. Also, shifting to only 39 °C markedly enhanced their sensitivity to proteasome inhibitors. Thus, mild hyperthermia (e.g. 39 °C) may enhance the efficacy of proteasome inhibitors in the treatment of myeloma.

Abstract

Proteasome inhibitors, such as bortezomib (BTZ), are highly effective and widely used treatments for multiple myeloma. One proposed reason for myeloma cells’ exceptional sensitivity to proteasome inhibition is that they produce and continually degrade unusually large amounts of abnormal immunoglobulins. We, therefore, hypothesized that, heat shock may also be especially toxic to myeloma cells by causing protein unfolding, increasing further the substrate load on proteasomes, and, thus, putting further stress on their capacity for protein homeostasis. After a shift from 37 to 43 °C, all four myeloma lines studied underwent extensive apoptosis in 4 h, unlike 13 nonmyeloma cell lines, even though the myeloma cells induced heat-shock proteins and increased protein degradation similar to other cells. Furthermore, two myeloma lines resistant to proteasome inhibitors were also more resistant to 43 °C. Shifting myeloma cells to 43, 41, or 39 °C (which was not cytotoxic) dramatically increased their killing by proteasome inhibitors and inhibitors of ubiquitination or p97/VCP. Combining increased temperature with BTZ increased the accumulation of misfolded proteins and substrate load on the 26S proteasome. The apoptosis seen at 43 °C and at 39 °C with BTZ was mediated by caspase-9 and was linked to an accumulation of the proapoptotic Bcl-2-family member Noxa. Thus, myeloma cells are exceptionally sensitive to increased temperatures, which greatly increase substrate load on the ubiquitin-proteasome system and eventually activate the intrinsic apoptotic pathway. Consequently, for myeloma, mild hyperthermia may be a beneficial approach to enhance the therapeutic efficacy of proteasome inhibitors.