PS1361 BIDIRECTIONAL MITOCHONDRIAL TRANSFER BETWEEN MYELOMA CELLS AND BONE MARROW STROMAL CELLS CONTRIBUTES TO THE CHEMORESISTANCE OF MYELOMA CELLS

Background: Neoplastic cells and bone marrow (BM) stromal cells interact intimately in the marrow and communicate with each other in numerous ways such as paracrine signaling via cytokines and chemokines, cell‐to‐cell interactions through tunneling nanotubes (TNTs) or gap junctions, and the secretion and incorporation of microvesicles (MVs). As the acquisition of cancer drug resistance was associated with mitochondrial transfer in several malignancies, we investigated the possibility of a similar phenomenon in multiple myeloma (MM) using an in vitro stromal cell–malignant plasma cell co‐culture system. Aims: Our primary goal was to establish whether mitochondrial transfer occurs between BM stromal cells and primary myeloma cells, and if so, exactly by what mechanism. We also wanted to clarify if this phenomenon could contribute to the drug resistance of myeloma cells. Methods: Mitochondrial exchange between BM stromal cells and myeloma cells was quantified by flow cytometry and visualized by confocal laser scanning microscopy. Cellular toxicity of different drugs was assessed by high throughput screening. Results: We found a relatively fast, bidirectional mitochondrial transfer between BM stromal cells and malignant plasma cells. As mitochondria can be transferred through TNTs, gap junctions, or via MVs, we selectively inhibited these pathways to dissect the exact mechanisms involved. According to our results, only cytochalasin D, a potential blocker of TNT formation, was able to reduce significantly the amount of transferred mitochondria in a dose‐dependent manner; however, the transmission of mitochondria has also been slightly reduced by inhibiting the MV uptake or gap junctions, suggesting a dominant but non‐exclusive role of TNTs in this process. In further experiments we demonstrated that stromal cells accumulate substantial amounts of mitochondria from myeloma cells delivered by MVs, whereas myeloma cells incorporate much less MVs and thus less mitochondria from stromal cells. Confocal microscopic analysis also confirmed bidirectional mitochondrial transport between stromal and myeloma cells through TNTs. Finally, we evaluated the possible contribution of stromal cell‐derived and myeloma cell‐derived mitochondria to the drug resistance of malignant plasma cells. Several drugs including apoptosis inducers, proteasome inhibitors, and epigenetic modifiers were tested, and it was clearly demonstrated that drug treatment always enhances the transfer of mitochondria from stromal cells to myeloma cells in a dose‐dependent manner (see figure for data on carfilzomib). Summary/Conclusion: Our cytotoxic assays demonstrated that myeloma cells are more resistant to drug treatment in co‐cultures than alone, and it is most likely caused by bi‐directional mitochondrial transfer. Our results provide additional clues for understanding the biological processes that underlie cell adhesion‐mediated drug resistance upon myeloma therapy.This work was supported by grant NVKP_16–1–2016–0005 from the NKFIA, Hungary