The prostaglandine E2 transporting organic anion transporting polypeptide OATP4A1: A potential prognostic marker in colorectal cancer?

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Background: Organic anions transporters (OATPs) are important for tumor progression and therapeutic response by regulating cellular levels of hormones, second messenger proteins and drugs. OATP4A1 is a transporter of pro-inflammatory prostaglandin E2 and may contribute to cancer progression. Data on the expression of OATP4A1 and its clinical impact in primary colorectal cancer (CRC) is rare. Our study was designed to proof the overexpression of OATP4A1 in primary CRC. Methods: Frozen samples from 20 unselected CRC patients (pat) and five CRC cell lines were analyzed for OATP4A1 mRNA expression by real time PCR (mean level normalized to the calibrator, MNE). Immunohistochemistry was performed on paraffin- embedded tumor sections from 50 CRC pat., UICC 0-II (25/50 with subsequent relapse). An automatic quantitative image analysis program was applied to quantify OATP4A1 expression. Expression and intensity was correlated with clinical parameters and relapses. Results: Significant (p>0.05) higher levels of OATP4A1 mRNA were observed in 20 cancer samples as compared to adjacent non-cancerous tissue (2.44 vs. 0.46 MNE). The highest expression (9.85 MNE) was observed in a well-differentiated tumor sample. Similar high levels were observed in the COGA1A cell line, expression in the other cell lines ranged between 1.83 and 0.28 MNE. Immunoreactive staining for OATP4A1 was located in the membrane and occasionally in the cytosol of tumor cells, it was exclusively membrane located in the adjacent non-cancerous epithelial cells. The staining intensity was significantly higher in cancer cells compared to non-cancerous areas (1528±326 vs.376±218) while staining of stroma cells was only occasionally detectable. Surprisingly, the highest OATP4A1 levels were observed in immune cells (2839±381 vs.298±56). Data on the clinical impact of OATP4A1 in the early stage CRC pat. will be presented at the meeting. Conclusions: The profound expression of OATP4A1 in CRC cells and in the inflammatory infiltrates supports its implication on cancer progression. Suitability of OATP4A1 as a potential prognostic marker has to be established on a larger patient collective.