Proximal fluids are found adjacent to a given tissue or organ and contain a repertoire of secreted proteins and shed cells reflective of the physiological state of that tissue 1
. Hence, proximal fluids are rapidly emerging as discovery sources of proteins, metabolites, and genetic biomarkers for cancers. Seminal plasma and expressed prostatic secretion (EPS) are prostate-proximal fluids that can be collected in the clinic. Seminal plasma consists of fluids originating not only from the prostate, but also from several other male accessory glands such as the seminal vesicles, epididymis, and Cowper’s gland. In aging men with prostatic disease, obtaining seminal fluids for diagnostic assays and screening purposes is highly dependent on patient age and disease status, and thus not an attractive assay medium. In contrast, EPS can be readily obtained in the clinic by forced (or expressed
) ejection of prostatic fluids into the urethra following prostate massage. For evaluation of some types of prostatitis, a vigorous massage is done to force fluid through the urethra to collect a few drops from the tip of the penis for subsequent use in microorganism cultures. If a more gentle massage is performed in the context of a digital rectal exam (DRE) of the prostate, the EPS is then immediately collected in voided urine post-exam (termed post-DRE urine, or post-DRE EPS urine
). Currently, a commercial genetic assay for prostate cancer detection, based on the presence of a non-coding RNA (PCA3
), uses post-DRE EPS urines as a source of prostate-derived genetic material 2–4
In the context of prostate cancer, undiluted EPS can be collected while the patient is under anesthesia, just prior to prostatectomy. In comparison to the clinical DRE massages, a more vigorous digital rectal massage can be done to force prostatic fluids into the urethra with subsequent collection from the penis. For prostate cancer biomarker discovery purposes, direct EPS has several ideal clinical features beyond its nature as a proximal fluid. Since EPS are collected just prior to prostatectomy, clinical information that led to the decision to perform a prostatectomy is readily available 1
. Post-prostatectomy pathology staging and grading can also be obtained for comparison with the pre-surgical assessments. When linked with clinical outcome data, these direct EPS fluids thus have the potential to be used for discovery of both diagnostic and prognostic biomarkers for management of prostate cancer. A large number of pre-prostatectomy EPS fluids have been collected by our translational research group 1
, which we term ‘direct EPS
’ to distinguish them from other EPS designations.
In this study, we applied a MudPIT-based proteomics approach to obtain a detailed inventory of proteins present in pre-prostatectomy direct EPS samples. By using a high mass accuracy Orbitrap MS and rigorous identification criteria, we provide a high quality accounting of prostatic secretions to a degree not previously reported. To further increase the utility of the current proteomic resource, we have systematically integrated our data with a multitude of publicly available databases (i.e. human tissue mRNA microarray, the Human Protein Atlas
, alternative proteomics data). Additionally, the entire proteomics data was deposited to the Tranche server (www.proteomecommons.com
) for others to utilize. We hypothesize that this comprehensive proteomic analysis of direct EPS will provide an important resource to study prostate biology and/or facilitate the discovery of novel prostate cancer biomarkers.