Cancer cells communicate with each other via exosomes in the tumor microenvironment. However, measuring trace amounts of
proteins in exosomes is difficult, and thus the
cancer stemness-promoting mechanisms of exosomal
proteins have not been elucidated. In the present study, we attempted to quantify trace amounts of 78-kDa
glucose-regulated
protein (
GRP78), which is involved in
cancer progression, in exosomes released from cultured
gastric cancer cells using an ultrasensitive ELISA combined with
thio-NAD cycling. We also evaluated the
cancer stemness-promoting effects by the application of high-GRP78-containing exosomes to cultured
gastric cancer cells. The ultrasensitive ELISA enabled the detection of
GRP78 at a limit of detection of 0.16 pg/mL. The stemness of
cancer cultured cells incubated with high-GRP78-containing exosomes obtained from GRP78-overexpressed cells was increased on the basis of both an MTT assay and a wound healing assay. Our results demonstrated that the ultrasensitive ELISA has strong potential to measure trace amounts of
proteins in exosomes. Further, exosomes with a high concentration of
GRP78 promote the
cancer stemness of surrounding cells. The technique for quantifying
proteins in exosomes described here will advance our understanding of
cancer stemness progression via exosomes.