Sphingomyelinase Regulates the Release of Protumoral Mitochondria Containing-Vesicles from AML Cells

Mitochondrial transfer from bone marrow stromal cells to acute myeloid leukaemia (AML) blasts is protumoral (Marlein et al., 2017; Moschoi et al., 2016) . We have previously shown that this was through a NADPH oxidase-2 (Nox2) dependent mechanism which increases AML fitness both in vitro and in vivo. Furthermore, we recently have shown that AML cells export mitochondria in vesicles which are subsequently phagocytosed by bone marrow macrophages (Moore et al., 2022) . Next, we have investigated how the release of mitochondria-containing vesicles is regulated and how this process contributes to the fitness of AML blasts.

Sphingolipids are a component of the cell membrane which impact the stability and curvature of the membrane. They have been shown to play a role in the biogenesis of vesicles and a number of the enzymes involved in sphingolipid metabolism are dysregulated in AML, therefore we investigated the role of sphingolipids in mitochondrial exocytosis via vesicle formation.

To understand the role of vesicles in regulating AML fitness, we used primary human AML (collected with approval on the East of England - Cambridgeshire and Hertfordshire Research Ethics Committee and the UK Health Research Authority, IRAS Project ID - 33753) and mouse syngeneic AML models (MN1 and HOXA9/Meis). We find that the myosin inhibitor blebbistatin inhibits vesicle release and results in an increased mitochondrial mass and increased apoptosis. In order to investigate the involvement of one of the major sphingolipids, ceramide, we targeted neutral sphingomyelinase (GW4869) which is responsible for ceramide synthesis. We treated the AML cells in vitro with GW4869 for 24 h which induced a large increase in vesicle release. Using the mitochondrial stain Mitotracker Green (MTG) we confirm that these vesicles contain mitochondria. The vesicles were isolated, DNA was extracted, and RT-qPCR performed which confirms increased mitochondrial DNA content. Others have previously shown that low levels of cellular reactive oxygen species (ROS) are associated with leukaemic stem cells (LSCs). DCF staining of AML treated with GW4869 shows low levels of cellular ROS. To assess how this process might impact the stemness of AML, we performed in vivo experiments. Pretreatment of MN1 cells with GW4869 accelerated engraftment in C57Bl/6 animals which corresponded with a decrease in cellular ROS.

In summary, we have demonstrated the mechanism of mitochondrial exocytosis is important for AML fitness and that by disrupting sphingolipid metabolism, dysregulation of this process leads to increased tumour burden.