Enzyme Elimination Blocks Exosome Release and Slows Cancer

The reported diameter of exosomes is between 30 and 100 nanometers, which is larger than low-density lipoproteins but much smaller than red blood cells. Exosomes, which contain RNA, proteins, lipids, and metabolites that are reflective of the cell type of origin, are either released from the cell when multivesicular bodies (MVBs) fuse with the plasma membrane, or they are released directly from the plasma membrane. Exosomes have specialized functions and play a key role in coagulation, intercellular signaling, and waste management.

Cancer cells secrete copious amounts of exosomes, and elevated intracellular calcium ions (Ca2+) are critical for tumor progression and metastasis, but the underlying cellular mechanisms are unknown. Munc13-4 is a member of the UNC13 family of proteins, which appears to play a role in vesicle maturation during exocytosis and is involved in regulation of cytolytic granules secretion. Munc13-4 is a Ca2+-dependent receptor and Rab-binding protein that is required for Ca2+-dependent membrane fusion. Rab GTPases regulate many steps of membrane trafficking, including vesicle formation, vesicle movement along actin and tubulin networks, and membrane fusion.

Investigators at the University of Wisconsin (Madison, USA) reported in the June 21, 2018, issue of the Journal of Cell Biology that acute elevation of Ca2+ in cancer cells stimulated a fivefold increase in exosome release that was eliminated by Munc13-4 knockdown and was not restored by Ca2+ binding–deficient Munc13-4 mutants.

Depletion of Munc13-4 in highly aggressive breast carcinoma MDA-MB-231 cells reduced the size of multivesicular bodies, indicating a role for Munc13-4 in MVB maturation. Munc13-4 used a Rab11-dependent trafficking pathway to generate MVBs competent for exosome release. Furthermore, depletion of Munc13-4 reduced the release of MT1-MMP-containing exosomes from breast cancer cells and slowed metastasis by inhibiting the cells' ability to degrade the extracellular matrix.

"Overall, we think that increased expression of Munc13-4, combined with elevated calcium levels, drives enhanced exosome release by highly aggressive cancer cells, and that Munc13-4 is a potential target for therapeutic intervention," said senior author Dr. Thomas F.J. Martin, professor of biochemistry at the University of Wisconsin.

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