Abstract 2903: The exosome marker CD63 and Caveolin-1 in breast cancer patients

The tetraspanin CD63, a proposed exosome marker, has been reported to be expressed in solid tumors. For instance, CD63 status in carcinoma cells was reported to be a significant independent prognostic factor in gastric cancer patients, with its overexpression causing adversity in patients with gastrointestinal stromal tumors. However, the expression of CD63 has remained virtually unknown in breast cancer pathological specimens. In addition, the detection of high levels of exosomes expressing CD63 and Caveoloin-1 in the plasma of melanoma patients was reported as a novel tool for the clinical management of cancer patients. Caveolin-1 has also been reported to negatively regulate the endocytosis of exosomes. Tetraspanin proteins are shown to form complicated interaction networks in cells to exert their functions. In this study, we immunolocalized CD63 and Caveolin-1 in the surgical pathology specimens of 88 breast cancer patients. CD63 was immunolocalized in the cytoplasm of both the cancer and stromal cells. The number of CD63-positive cancer cells was significantly higher in HER2-positive patients and the levels of CD63-positive stromal cells were also significantly higher in those with high pathologic T factor (pT) and proliferation marker protein Ki-67 labeling index and low pathologic N factor (pN). The number of CD63-double-positive cancer cells and stromal cells was associated with high histological grade, high pT, and high Ki67 expressions. CD63-positive cancer cells and Caveolin-1-positive cancer cells were both significantly higher in Estrogen receptor-positive breast cancer patients with high histological grade and pT. To the best of our knowledge, this is the first study demonstrating the significance of CD63 in breast cancer. Our results indicated that CD63-double-positive cancer cells and stromal cells could be involved in clinically and biologically aggressive subtypes of breast cancer. Further studies are required to clarify the clinical significance of the associations between CD63 and Caveolin-1 in patients with breast cancer. Citation Format: Erina Iwabuchi, Yasuhiro Miki, Ayako Kanai, Takanori Ishida, Hironobu Sasano. The exosome marker CD63 and Caveolin-1 in breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2903.