LC3-positive extracellular vesicles released from tumor cells promote lung metastasis of breast cancer by inducing vascular permeability.

Tight junctions (TJs) between pulmonary vascular endothelial cells (ECs) constitute the physical barrier that impedes the metastasis of tumor cells. We previously reported that circulating microtubule-associated proteins 1A/1B light chain 3B (LC3)-positive extracellular vesicles (LC3+ EVs) derived from primary breast tumors were essential for establishing the premetastatic niche. However, the roles of LC3+ EVs in inducing vascular permeability and promoting tumor metastasis are unclear. In this study, we revealed that the expression of occludin and tight junction protein 1 [also known as zona occludens protein 1 (ZO-1)], two major TJ proteins, could be reduced by circulating LC3+ EVs, which subsequently increased vascular permeability, facilitated the invasion of circulating tumor cells, and eventually resulted in increased lung metastasis. Heat shock protein 60 (HSP60) was identified as the key molecule on LC3+ EVs that induced the reduction of occludin and ZO-1 through the Toll-like receptor 2 (TLR2)-myeloid differentiation primary response protein MyD88 (MYD88)-Snail Family Transcriptional Repressor 1 (Snai1) signal cascade. Combined with our previous findings, these results demonstrate that removing circulating LC3+ EVs or targeting HSP60 on LC3+ EVs might be a promising way to prevent breast cancer lung metastasis.