In a study published in Journal of Biological Chemistry on July 21, a research team led by Prof. WANG Likun from the Institute of Biophysics of the Chinese Academy of Sciences revealed that endoplasmic reticulum (ER) stress-induced activation of the unfolded protein response (UPR) - specifically through the PERK and IRE1α pathways - modulates lysosomal function to promote exosome release in mouse melanoma cells.

Exosomes, small extracellular vesicles released when multivesicular bodies (MVBs) fuse with the plasma membrane, play a crucial role in intercellular communication by transferring proteins, lipids, and nucleic acids between cells. Their secretion is tightly regulated and often influenced by cellular stress conditions. A key question in the field has been how cells under ER stress engage this non-conventional secretory pathway.

In this study, the researchers showed that activation of PERK and IRE1α under ER stress reduces lysosomal acidity and suppresses the fusion of MVBs with lysosomes. This diversion prevents the degradation of MVBs, allowing them instead to fuse with the plasma membrane and release exosomes.

Using thapsigargin (Tg) to induce ER stress, they found that knocking out IRE1α markedly reduced extracellular vesicle (EV) secretion. In contrast, PERK knockout had little direct effect on EV release. However, PERK deletion led to a notable increase in XBP1s protein, a downstream effector of IRE1α, suggesting that IRE1α may compensate for the loss of PERK during ER stress. These findings indicate that both PERK and IRE1α contribute to exosome secretion in a cooperative manner under stress conditions.

Further experiments revealed that PERK deficiency increased lysosomal acidity and enzymatic activity, promoting MVB-lysosome fusion and accelerating MVB degradation - which in turn suppressed exosome release. To probe whether PERK activation alone could drive exosome secretion, the researchers treated cells with ionomycin, which elevates cytosolic calcium levels and selectively activates PERK. The results revealed that PERK activation, even in the absence of ER stress, was sufficient to enhance exosome production.

Similarly, deletion of IRE1α also led to increased lysosomal acidity and degradation activity, thereby reducing exosome output - although the underlying molecular mechanisms remain to be clarified.

These findings uncover a novel mechanism by which the UPR pathways PERK and IRE1α regulate exosome secretion, showing that ER-derived stress signals can reshape the fate of intracellular vesicles by tipping the balance between degradation and secretion. These insights deepen our understanding of stress adaptation and intercellular communication.