Exosomal proteins as prostate cancer biomarkers in urine: From mass spectrometry discovery to immunoassay-based validation

Exosomes have recently appeared as a novel source of non-invasive cancer biomarkers since tumor-specific molecules can be found in exosomes isolated from biological fluids. We have previously analyzed the proteome of urinary exosomes by mass spectrometry, and identified proteins differentially expressed in prostate cancer patients compared to healthy males. Since mass spectrometry is so far not commonly used in clinical laboratories, we have here investigated whether antibody-based methods such as Western blot or ELISA can be used to validate the use of the identified proteins as prostate cancer biomarkers. Western blot experiments designed to detect flotillin 2, TMEM256, Rab3B and LAMTOR1 showed that the level of these proteins was higher in urinary exosomes from prostate cancer patients compared to healthy males. Furthermore, a receiver operating characteristic curve of flotillin 2 in samples from 16 controls and 16 patients showed an area under the curve of 0.91, and 88% sensitivity at a threshold set to give 94% specificity. In addition, ELISA-based detection of flotillin 2 and PARK7 showed that the combination of these proteins was able to distinguish prostate cancer patients and healthy controls with 68% sensitivity and 93% specificity. Several promising biomarkers identified by mass spectrometry could not be evaluated by Western blot or ELISA due to their low exosomal amount and/or lack of good antibodies. In conclusion, our results show that several urinary exosomal proteins identified as prostate cancer biomarkers by mass spectrometry have a high diagnostic value also when analyzed by immunology-based methods, thus bringing these biomarkers closer to a potential clinical use.

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