Uptake and tumor-suppressive pathways of exosome-associated GKN1 protein in gastric epithelial cells

Abstract

BACKGROUND: Gastrokine 1 (GKN1) is a stomach-specific tumor suppressor that is secreted into extracellular space as an exosomal cargo protein. The objective of this study was to investigate the uptake and tumor-suppressive pathways of exosome-associated GKN1 protein in gastric epithelial cells. <br>METHODS: Immunofluorescent and Western blot analysis were used to investigate gastric-specific uptake of HFE-145-derived exosomes. Binding affinity of HFE-145 derived exosomes with integrin proteins was examined using protein microarray chip. Tumor suppressor activities of exosome-carrying GKN1 protein were analyzed using transwell co-culture, MTT assay, BrdU incorporation, immunoprecipitation, and Western blot analysis. <br>RESULTS: HFE-145-derived exosomes were internalized only into HFE-145 gastric epithelial cells and gastric cancer cells. Gastric-specific uptake of stomach-derived exosomes required integrin α6 and αX proteins. Clathrin and macropinocytosis increased the uptake of exosomes into gastric epithelial cells, whereas caveolin inhibited the uptake of exosomes. Transwell co-culture of AGS cells with HFE-145 cells markedly inhibited viability and proliferation of AGS cells. Following uptake of HFE-145-derived exosomes in recipient cells, GKN1 protein bound to HRas and inhibited the binding of HRas to b-Raf and c-Raf which subsequently downregulated HRas/Raf/MEK/ERK signaling pathways in AGS, MKN1 cells, and MKN1-derived xenograft tumor tissues. In addition, exosomal GKN1 protein suppressed both migration and invasion of gastric cancer cells by inhibiting epithelial-mesenchymal transition. <br>CONCLUSIONS: Gastric-specific uptake of exosomes derived from gastric epithelial cells requires integrin α6 and αX proteins in both gastric epithelial cells and exosomes. Exosomal GKN1 protein inhibits gastric carcinogenesis by downregulating HRas/Raf/MEK/ERK signaling pathways.