Aside from basal and squamous cell carcinomas of the skin, prostate cancer is the most frequently occurring cancer in males, with more than 217,000 new cases estimated to have occurred in the United States during 2010. In addition, it is the second leading cause of male cancer mortality, responsible for an estimated 32,050 cancer deaths per year [1
]. Despite this, after the implementation of prostate-specific antigen screening, researchers have estimated that clinically insignificant prostate cancer is actually overdiagnosed at a rate of 29% for whites and 44% for blacks [2
]. Consequently, many researchers and clinicians have voiced a need for ways to discern high-risk patients in need of aggressive treatment from those in whom the consequences of overtreatment would outweigh the benefits [3,4
There is currently a limited amount of information in the literature on biomarkers with the potential to discern the clinical behaviors of prostate tumors [5
]. Evaluating the expression of novel prostate cancer biomarkers may yield not only candidates for improved diagnosis but also prognosis prediction and therapy selection.
Slit2 is a secretory glycoprotein that maps to the gene locus 4p15.2 [6
]. A homolog of the Drosophila
protein Slit2, it exists with two other mammalian slit proteins, Slit1 and Slit3, all of which are characterized by four leucine-rich repeat domains at its N-terminus, a series of epidermal growth factor-like domains, a laminin G domain, and a cysteine-rich C-terminus [7
]. As a ligand in the Slit/Robo system, Slit2 is responsible for guiding neural cell migration, where it has been demonstrated to prevent inappropriate midline axonal crossing events [8,9
Functions for Slit2 outside the central nervous system have also been discovered. Among these functions are inhibiting leukocyte chemotaxis and vascular smooth muscle migration [10,11
]. Interestingly, another work has also demonstrated that Slit2 is capable of attracting breast cancer cells [12
The discovery of Slit2 as a guidance cue across multiple tissue types has prompted many studies to examine its potential as a biomarker for cancer, with two contrasting roles for Slit2 having been proposed, with some suggesting that it functions as a tumor suppressor, whereas others propose that it plays a role in oncogenesis.
Supporting the notion that Slit2 confers antitumorigenic properties are studies of cancer cells lines that have demonstrated Slit2
promoter methylation in 59% of the breast, 77% of the non-small cell lung, and 55% of the small cell lung cancer cell lines examined [13
]. Epigenetic silencing of Slit2
has additionally been demonstrated in cases of acute and chronic lymphocytic leukemia [14
], glioma [15
], renal cell carcinoma [16
], cervical cancer [17
], and hepatocellular carcinoma [18
], in which the level of Slit2
messenger RNA (mRNA) has also been shown to decrease with increasing metastatic potential [19
Alternatively, evidence also exists to state that Slit2 possesses an oncogenic function. Avci et al. [20
] have reported that Slit2 and Robo1, a Slit2 receptor, can also be upregulated in hepatocellular carcinomas with advanced stages and poor tumor differentiation. In addition, an increase in Slit2
mRNA has also been noted in canine malignant mammary tumors [21
]. Schmid et al. [12
] have shown through the use of a transwell migration assay that the addition of Slit2 is capable of inducing direct migration of breast cancer cells and, in an appropriate cell line, may induce brain metastasis. Using a Boyden chamber assay, Wang et al. [22
] have demonstrated that Slit2 is capable of attracting endothelial cells. In addition, they have also demonstrated in vitro
that Slit2 is capable of promoting angiogenic activity, increasing tubular network formation in cancer. Moreover, in a study of pancreatic islet tumors in mice, Yang et al. [23
] intercrossed a transgenic mouse overexpressing Slit2 with a nonmetastatic RIP-Tag2 mouse tumor model and noted that the expression of Slit2 enhanced lymphangiogenesis and promoted lymph node metastasis.
To date, only two studies have been conducted to examine the expression of Slit2 in prostate cancer, both of which showed contrasting results. One documented an increase in Slit2
mRNA expression across multiple cases of prostate cancer, especially those with a hormone refractory status [24
]. The other demonstrated promoter hypermethylation of the Slit2
gene, supporting a hypothesis of reduced gene expression [25
To our knowledge, the immunoprofiles of Slit2 have not previously been compared between specimens of benign prostate, primary prostate adenocarcinoma, and metastatic prostate adenocarcinoma. Assessing them will provide further information about the expression of Slit2 in prostate cancer, as well as help elucidate its potential as a diagnostic or prognostic biomarker. Here, we compare the immunohistochemical profiles in a series of 11 cases of normal donor prostate (NDP), 35 cases of normal tissue adjacent to prostatic adenocarcinoma (NAC), 15 cases of benign prostatic hyperplasia (BPH), 35 cases of high-grade prostatic intraepithelial neoplasia (HGPIN), 106 cases of primary prostatic adenocarcinoma (PCa), and 37 cases of metastatic prostatic adenocarcinoma (Mets) to examine if either a tumor suppressor or an oncogenic function for Slit2 can be suggested.
Our findings indicate that the overall expression of Slit2 was higher in HGPIN and PCa than in NDP and NAC (P < .05), although not all cases of HGPIN and PCa featured high expression levels. Although the overall differences between Mets and the normal tissues were not significant, several Mets cases individually featured high Slit2 staining intensities.