Epithelial ovarian cancer (EOC) consists of four major subtypes: clear cell
carcinoma (CCC),
endometrioid adenocarcinoma (EA),
mucinous adenocarcinoma (MA) and serous
adenocarcinoma (SA). Relative to the other subtypes, the prognosis of CCC is poor due to a high recurrence rate and
chemotherapy resistance, but CCC-specific
biomarkers have yet to be identified. With the aim of identifying diagnostic and treatment
biomarkers for CCC, we analyzed 96 cases of EOC (32 CCC, 13 EA, 19 MA, 32 SA) using liquid chromatography/mass spectrometry (LC/MS) followed by immunohistochemistry (IHC) and quantitative reverse transcription PCR (RT-qPCR). Semi-quantification of
protein differences between subtypes showed upregulation of 150
proteins and downregulation of 30
proteins in CCC relative to the other subtypes. Based on hierarchical clustering that revealed a marked distinction in the expression levels of
cystatin B (CYTB) and
Annexin A4 (ANXA4) in CCC relative to the other subtypes, we focused the study on CYTB and ANXA4 expression in EOCs by IHC, RT-qPCR and western blot analyses using tissue specimens and cultured cells. As a result, compared to the other subtypes, CCC showed significantly high expression levels of CYTB and ANXA4 in the analyses. To examine the possibility of CYTB and ANXA4 as serum diagnostic
biomarkers of CCC, we checked the
protein levels in
conditioned media and cell lysates using culture cells. Compared with the other subtypes, CCC cell lines showed a significantly higher level of expression of CYTB in both
conditioned media and cell lysates, while ANXA4 showed a higher level of expression in cell lysates only. Our results demonstrate that CYTB and ANXA4 overexpression may be related to
carcinogenesis and histopathological differentiation of CCC. CYTB may be a secreted
protein, and may serve as a potential serum diagnostic
biomarker of CCC, while ANXA4 may be useful as an intracellular marker.