Objective: CD24 is an established marker for poor prognosis in ovarian and other carcinomas. Acquisition of cytoplasmic CD24, as opposed to membranous expression, has been correlated with a higher invasiveness of tumor cells. Exosomes are small vesicles of endosomal origin that are often secreted by tumor cells. Given the emerging role of exosomes in tumor progression, we investigated whether cytoplasmic CD24 expression is correlated with the secretion of CD24 in exosomes.
Methods: We used CD24 transfected carcinoma cell lines, ovarian carcinoma cell lines and malignant ascites fluid of ovarian carcinoma patients. Exosomes were isolated via ultracentrifugation and sucrose density fractionation and subsequently examined by Western blot analysis and gelatine zymography. In tissue sections CD24 was detected by immunohistochemical staining.
Results: We show that CD24 is released by transfected as well as endogenously expressing cells in vesicles that represent exosomes. CD24 was also identified in exosomes isolated from ascites fluid of ovarian carcinoma patients. In 16 ovarian carcinomas analyzed no correlation between CD24 in tumor tissue sections and the appearance of CD24 in exosomes was detected. Furthermore, we provide evidence that the epithelial cell adhesion molecule (EpCAM), known to be overexpressed in ovarian carcinomas, is secreted in exosomes. The ascites exosomes contain gelatinolytic enzymes.
Conclusions: Our study identifies CD24 and EpCAM as cargo proteins of exosomes of cultured cell lines and malignant ascites. The exosome-associated proteolytic activity in the tumor vicinity might augment tumor invasion into the stroma.