Exosomes as a diagnostic and prognostic tool for prostate cancer
Diederick Duijvesz a, Theo Luider b, Chris H. Bangma a, Guido Jenster a .
Accepted 20 December 2010, Published online 29 December 2010, pages 823 - 831 European urology.
In terms of PCa, the reports on exosomes are very few. One of the firstExosomes as Biomarker Treasure Chests for Prostate Cancer
studies reported no apparent differences between exosomes of benign origin and malignant origin regarding their synthesis, storage, and release [63]. Most likely, these vesicles differ in biochemical properties. Unfortunately, to date, no high-throughput techniques such as mass spectrometry and microarray have been used to evaluate the differences between exosomes of benign origin and malignant origin to identify new biomarkers. Four studies used mass spectrometry to profile exosomes derived from PCa cell lines, xenografts, and metastases. To search for PCa-secreted proteins, serum from PCa-xenografted mice was analysed by mass spectrometry [25], and [26]. All of the identified proteins were screened for human-specific sequences by extensive database searching. The proteins containing human-specific sequences were of PCa origin. Interestingly, the subcellular localisation of most of these proteins is cytoplasmic, supporting the idea that these proteins are secreted in mouse blood through exosomes. Indeed, proteomic profiling of exosomes derived from human PCa cell lines confirmed the presence of almost all of the previously identified serum proteins [26]. Two other studies analysed vesicles from prostate cell lines and vertebral PCa metastases by mass spectrometry and identified proteins related to angiogenesis, signal transduction pathways, and cancer progression [51], and [64], including caveolin-1, Akt, pyruvate kinase M2, programmed cell death 6 interacting protein, and poly(A)-binding protein 1. Subsequent in vitro functional assays (eg, migration and proliferation assays) demonstrated that these vesicles can influence cancer microenvironment and promote cancer progression. Although these findings are promising, further investigations are needed to fully elucidate the role of PCa exosomes in cancer development.
Aside from these biological studies, exosomes and exosomal content from patient samples have also been evaluated for their potential as biomarkers. Urinary exosomes from 10 organ-confined PCa patients undergoing hormonal therapy prior to radical radiotherapy were analysed [53]. Other than a considerable variation in the quantity of total exosomal proteins, no difference was observed between healthy men and PCa patients. Although these results do not specify which proteins are present in exosomes, it emphasises the technical feasibility of assessing exosomal proteins to evaluate the clinical status of PCa. However, better sample preparation, such as immunoaffinity isolation, and more robust technical approaches are needed to define significant differences with such huge variation.
RNA expression analysis of urine-derived and PCa cell line–derived exosomes revealed that the known RNA markers for PCa, such as the TMPRSS2:ERG fusion gene and prostate cancer antigen 3 (PCA3), can be detected in exosomes by reverse transcriptase–polymerase chain reaction [10]. The TMPRSS2:ERG fusion transcripts were detected in urinary exosomes from two patients with high Gleason scores but not in those from two patients with low Gleason scores [45]. PCA3 mRNA was detected in exosomes derived from all patients. Interestingly, none of the hormone-treated patients showed detectable levels of TMPRSS2:ERG or PCA3 RNAs, suggesting that the response to treatment might reduce the size of PCa tissue and thereby decrease the expression levels of these androgen-responsive genes.
