Tumor exosomal RNPEP promotes lung metastasis of liver cancer via inducing cancer-associated fibroblast activation

Cancer-associated fibroblasts (CAFs) are essential players in the tumor microenvironment (TME) due to their roles in facilitating tumor progression and metastasis. It is worth noting that the high-metastatic hepatocellular carcinoma (HCC) cell-derived exosomes have exhibited the ability to transform normal fibroblasts into CAFs, which further fosters the lung metastasis of low-metastatic HCC cells. Yet, the mechanisms underlying this tumor exosome-induced metastatic niche formation are poorly explored. In this study, the secreted protein arginyl aminopeptidase (RNPEP) was highly expressed in the plasma of patients with HCC. In addition, high-metastatic HCC cells showed augmented RNPEP expression levels in their exosomes. These exosomes induced obvious CAF-like properties in the human fibroblast cell line MRC-5, as evidenced by the increased CAF marker expression, and enhanced migratory ability. More strikingly, the secretions from high-metastatic tumor exosome-educated MRC-5 cells increased tumor stemness and promoted epithelial–mesenchymal transition (EMT) in MHCC-97L cells, a low-metastatic HCC cell line. However, the knockdown of RNPEP in exosomes from high-metastatic HCC cells abated the changes described above. Animal studies in vivo highlighted the pro-tumor and pro-metastatic effects of exosomal RNPEP on MHCC-97L cells by inducing CAF activation. Furthermore, tumor-derived exosomal RNPEP induced the activation of NF-κB signaling in MRC-5 cells, a critical pathway associated with CAF activation. Collectively, these results provide novel insight into tumor-derived exosomal RNPEP for its crosstalk with CAFs during HCC lung metastasis.