The protein kinase C-related kinases PRK1, 2, and 3 are regulated by PI-3 kinase signaling and by Rho binding [10
]. PRK1 and PRK3 have been found in prostate adenocarcinomas, in which the former has been shown to enhance AR activity, even in the presence of the antagonist cyproterone acetate [9
]. PRK3 function in the prostate cancer cell line PC-3 appears to be important for the invasive properties of these cells, both in 3D culture and following orthotopic injection. PRK1 has not been comprehensively analyzed by tissue microarray in normal adult tissues or cancers, including those of the ovary, which like prostate cancers, have AR signaling as an important pathway. AR has been shown to be present in normal ovarian epithelium, ovarian cancer cell lines, and human ovarian carcinomas. [19
] Furthermore, in ovarian cancer cell culture, androgens promote cell growth, while flutamide, which antagonizes AR, abolishes androgen-stimulated growth. [19
We observed PRK1 immunopositivity in cells of the hematolymphoid system, mesothelium, and in a wide variety of epithelial cell types. PRK1 was present in epithelial cells of the gynecologic tract including those of the endometrium, fallopian tube, and ovary. Elsewhere, PRK1 was observed in ducts of the breast and pancreas, and in bronchial epithelium. Notably, it was absent in normal epithelium of the prostate, bladder, stomach, colon, and liver. Although PRK1 was often present in carcinomas (such as tumors of the ovary, breast, and pancreas) derived from normal epithelium that expressed this protein, it was also often seen in neoplasms from sites whose normal epithelium lacked PRK1. The finding of PRK1 in carcinomas of the prostate, colon, and stomach, for instance, indicates deregulation of this kinase, and suggests the importance of the PI-3 kinase signaling pathway in these neoplasms.
Our survey of the ten most common types of fatal carcinomas in humans detected the highest average level of PRK1
mRNA in ovarian serous carcinomas, significantly higher than that for all of the others (p=0.05). Using the Oncomine database for tumor expression (www.oncomine.org
), we searched for PRK1
in 1,911 neoplasms of diverse types surveyed by the International Genomics Consortium Expression Project for Oncology (Phoenix, AZ) (https://expo.intgen.org/expo/public/2006/01/01). Using Affymetrix U133 plus 2.0 arrays for neoplasms in which more than five of a specific type were examined, the highest median normalized expression was found for carcinomas of the ovary (N=241), fallopian tube (N=6), and peritoneum (N=14). At the protein level, we found a high level of positivity for PRK1 in nearly 90% of serous ovarian carcinomas. Endometrial endometrioid adenocarcinomas and mesotheliomas were not included in our gene expression array, but also demonstrated frequent strong positivity for PRK1 by immunohistochemistry. Staining of mesotheliomas and serous ovarian carcinomas is of interest given the similarities between these tumors, and the fact that both had weaker protein expression in the normal tissues compared with that of the corresponding tumors.
Using the Oncomine database for ovarian tumors from the study of Jazaeri et al., the highest median normalized expression for PRK1
was found for serous carcinomas (N=35) compared with endometrioid (N=5), clear cell (N=3), and adenocarcinomas not otherwise specified (N=16) [22
]. As we also found relatively high PRK1 mRNA and protein in ovarian serous carcinomas, we explored the levels of activated PRK1, total and activated levels of its paralog PRK2, and the immediate upstream regulator of PRK1, PDK, in examples of this type of ovarian tumor and normal ovarian epithelium. Previously, Lu et al. showed that PRK1
mRNA levels were higher in serous carcinomas compared to that for normal ovarian surface epithelium (Oncomine database) [23
]. Although normal ovarian surface epithelium contained PRK1, the phosphorylation states of both PRK1 and PDK were increased in ovarian carcinomas. While this could suggest a role for PRK1 in ovarian cancer development or progression, the mechanism responsible for overexpression is unclear. As PRK1 is downstream of PI-3 kinase, it is possible that in some cases amplification of PIK3CA leads to PRK1 overexpression. In one study, amplification of PIK3CA occurred in 13% of high-grade serous carcinomas [5
]. More recently, PIK3R3
mRNA was found to be higher in ovarian cancers compared with that for normal ovary [24
]. Chronic PI-3 kinase signaling in PC-3 cells due to loss of PTEN is partly responsible for PRK3 levels in these cells [10
]. Thus, it is possible that tumors expressing higher levels of PI-3 kinase might promote PRK1
expression or protein stabilization as part of a feed forward mechanism.
In summary, our results show that PRK1 is present in normal cells of the hematolymphoid system, mesothelium and a variety of epithelia including those from the gynecologic tract. It is also variably expressed by a wide variety of carcinomas, and most consistently present in malignant mesotheliomas, endometrial adenocarcinomas, and serous ovarian carcinomas. Of the common fatal carcinomas, PRK1
mRNA is most highly expressed in serous ovarian cancers, where it and its upstream regulator, PDK1, are readily detectable in a phospho-activated state. While information regarding the direct substrates of PRK phosphorylation is currently limited to histone H3 [8
], factors involved in transcriptional regulation and function of the actin cytoskeleton are clearly anticipated. Our analysis of PRK1 in ovarian carcinoma should provide a setting for defining downstream targets and developing insight into a relatively unexplored branch of the PI-3 kinase pathway. Finally, the interaction between PRK1 and AR and the subsequent modulation of the expression of androgen-regulated genes in ovarian carcinomas remains to be examined.