In the claudin-3 stained specimens, the average staining scores were highest in PCa and Mets. PIN had a lower absolute staining score than PCa and Mets, although the differences were not significant. Both BPH and NAC had significantly less staining than PCa and Mets (Figure 1). These differences support the notion that claudin-3 is upregulated in many cases of prostatic adenocarcinoma. It does not appear that stronger expression exists in all cases, however, as there were 42 cases of PCa and 22 cases of Mets that had staining scores below 100.
Previous studies have shown that Gleason scores of 7 or higher were more frequently associated with lower expression of claudin-1. Additionally, Landers, et al., demonstrated that claudin-4 was upregulated in primary and metastatic prostate cancer, although they state that it tended to be expressed more in primary tumors with a Gleason score of 6 than those with a score of 7 or higher. In this current study, no significant differences were seen when the cases of PCa were compared by Gleason score (Figure 2).
Sheehan, et al., have also reported that claudin-3 expression correlates with advanced stage and tumor recurrence in prostate cancer , which coincides with another study of urothelial carcinoma, where claudin-3 has also been shown to correlate with advanced stage and poor survival. In this current study, however, the highest average staining appeared to be in stage 2 PCa, which was significantly higher than stage 3 PCa staining, but not significantly higher than that noted in stage 4 PCa (Figure 3). Although statistically different, it is worth noting that these absolute staining scores are similar, reflecting that the difference in stage staining may not be of enough magnitude to use clinically. In a study of gastric cancer, claudin-3 has also been shown to be less expressed in advanced stage cases, which may suggest that claudin family expression patterns vary by cancer type.
As claudin-3 is a tight junction protein, it is interesting to note that in addition to membranous staining, cytoplasmic staining was also seen in select cores, most prominently in cases of PCa and Mets (Figure 4). Rangel, et al., have reported positive cytoplasmic immunostaining for claudin-3 in ovarian tumors and have suggested that this mislocation may be the result of abnormal pathway activation in cancer. This finding may indicate that a similar occurrence takes place in prostate cancer and may warrant further investigation.
Based on this study, it appears that claudin-3 is moderately to strongly expressed in the majority of cases of prostate cancer and may serve as an important biomarker for prostate cancer diagnosis, both primary and metastatic, however the discrimination between primary and metastatic cases in this study is not great enough to merit its usage as a marker to predict the risk of metastasis.
Some groups have speculated on the specific mechanism whereby claudin-3 may be involved in carcinogenesis. D'Souza, et al., have shown that tight junction strength decreased when claudin-3, when designed to contain a T192D mutation mimicking a phosphorylated state, was overexpressed in ovarian cancer cell line OVCA433, a mechanism possibly enabling invasion. Additionally, Agarwal, et al, have noted that ovarian epithelial cells specifically designed to constitutively express claudin-3 were found to have higher activity of matrix metalloproteinase-2, which may also contribute to invasion, although this level did not decrease following siRNA knockdown of claudin-3, possibly indicating that other pathways also promote its expression. This could be one explanation for the fact that strong claudin-3 staining is not noted in all cases of PCa.
Finally, in addition to its potential role as a predictive biomarker for cancer, claudin-3 has also been shown to bind clostridium perfringens enterotoxin, subsequently leading to toxin-mediated cytolysis, prompting some to suggest that this may indicate a future therapy in select cases of prostate cancer[31, 32]. Should such a therapy be developed, the immunohistochemical profiles of patients for this marker may become even more important, as it may serve to guide therapy selection.