Clarithromycin overcomes stromal cell-mediated drug resistance against proteasome inhibitors in myeloma cells via autophagy flux blockage leading to high NOXA expression.

We previously reported that macrolide antibiotics, such as clarithromycin (CAM), blocked autophagy flux, and simultaneous proteasome and autophagy inhibition by bortezomib (BTZ) plus CAM resulted in enhanced apoptosis induction in multiple myeloma (MM) cells via increased endoplasmic reticulum (ER) stress loading. However, in actual therapeutic settings, cell adhesion-mediated drug resistance between bone marrow stromal cells (BMSC) and MM cells has been known to be a barrier to treatment. To investigate whether CAM could enhance BTZ-induced cytotoxicity in MM cells under direct cell adhesion with BMSC, we established a co-culture system of EGFP-labeled MM cells with BMSC. The cytotoxic effect of BTZ on MM cells was diminished by its interaction with BMSC; however, the attenuated cytotoxicity was recovered by the co-administration of CAM, which upregulates ER stress loading and NOXA expression. Knockout of NOXA in MM cells canceled the enhanced cell death by CAM, indicating that NOXA is a key molecule for cell death induction by the co-administration of CAM. Since NOXA is degraded by autophagy as well as proteasomes, blocking autophagy with CAM resulted in the sustained upregulation of NOXA in MM cells co-cultured with BMSC in the presence of BTZ. Our data suggest that BMSC-associated BTZ resistance is mediated by the attenuation of ER stress loading. However, the addition of CAM overcomes BMSC-associated resistance via upregulation of NOXA by concomitantly blocking autophagy-mediated NOXA degradation and transcriptional activation of NOXA by ER stress loading.