Electron microscopic results on the ventral prostate of the rat after CdCl2 administration. A contribution towards etiology of the cancer of the prostate

E-Cadherin, beta -Catenin and cadmium carcinogenesis

Cadmium (Cd(2+)) is an important industrial and environmental pollutant that has been classified as a human carcinogen. Studies reported in the literature indicate that cadmium may play a role in both the initiation of cancer, by activating oncogenes, and in the progression of cancer, by increasing the metastatic potential of existing cancer cells. However, the mechanisms underlying these effects have yet to be elucidated. Recent studies from our laboratory have shown that cadmium can disrupt the tight junctions between many types of epithelial cells by interfering with the normal function of E-cadherin, a Ca(2+)-dependent cell adhesion molecule that plays a key role in epithelial cell-cell adhesion. This finding may be especially significant because a large volume of evidence indicates that the disruption of E-cadherin-mediated cell adhesion can trigger the beta-catenin-mediated activation of oncogenes in epithelial cells and increase the invasive potential of existing epithelial-derived cancers. The hypothesis that we are proposing is that the cadmium-induced disruption of E-cadherin dependent cell-cell junctions may represent a pivotal step in both the initiation of cancer by cadmium and in the tumor promoting actions of cadmium.

Cadmium chloride-induced dysplastic changes in the ventral rat prostate: an immunohistochemical and quantitative study

BACKGROUND

Cadmium chloride is an environmental toxic that might be implicated in human prostate carcinogenesis. The study was directed: 1) to evaluate the immunoexpression of markers for cell proliferation, apoptosis, and resistance to apoptosis, and 2) to estimate the size of premalignant cell population in the preneoplastic changes induced in ventral prostates of rats treated with cadmium chloride administered in drinking water.

METHODS

The following parameters were calculated in the ventral prostatic lobe of normal rats and rats that received cadmium in drinking water during 18 months: total volume, epithelial volume, total number of epithelial cells, numerical density of epithelial cells, percentage of cells that immunostained to the proliferating cell nuclear antigen (PCNA), percentage of apoptotic cells (evaluated by a DNA fragmentation method), and absolute volume and volume fraction of immunostaining to bcl-2.

RESULTS

The percentage of PCNA immunoreactive nuclei, the bcl-2 expression, and the numerical density of epithelial cells were significantly (P < 0.05) increased in the dysplastic prostatic acini of treated rats in comparison with the normal acini of treated rats and control animals. The percentage of apoptotic nuclei from ventral dysplastic acini was significantly (P < 0.05) decreased in comparison with that of normal acini. A negative correlation between proliferation and apoptosis was found in dysplastic lesions.

CONCLUSIONS

Prostate epithelial dysplasia induced in rats by cadmium presents an increased proliferative activity and high expression of bcl-2 protein, as was described in human prostate intraepithelial neoplasia. However, the rate of apoptosis in rat dysplasia was importantly decreased, in contrast to that observed in some human preneoplastic changes. This decrease might be related to the increase of bcl-2 expression.

Cadmium and prostate cancer

Prostatic cancer is a common and frequently lethal malignant disease. In the United States and other countries the incidence and mortality rate of prostate cancer continue to rise. Cancer of the prostate has an extremely complex etiology and appears dependent on a variety of factors, making linkage to a single factor very difficult to detect. Cadmium is a metallic toxin of great environmental and occupational concern. Cadmium exposure has been associated with human prostatic cancer in some, but not all, epidemiologic studies. Some studies indicate that tissue levels of cadmium in the human prostate correlate with malignant disease. Any association between cadmium and prostatic cancer has been controversial, in large part because of a previous lack of relevant animal models. However, several chronic studies in rats revealing a correlation between cadmium exposure and prostatic tumors have been published over the last several years. These include a study of oral cadmium exposure, a route extremely relevant to human exposure. Several of these chronic studies indicate a hormonal dependence of cadmium-induced prostate cancer. Other supportive work continues to accumulate, such as studies showing in vitro malignant transformation of prostatic epithelial cells with cadmium exposure. In addition, there are indications that the primary biologic tolerance system for cadmium (i.e., the metallothionein gene) may be only poorly active in the specific lobes of the rat prostate in which cadmium induces tumors. The induction in rats of prostate cancer by cadmium treatment clearly supports, but does not definitively establish, a possible role for cadmium as an etiological agent in human prostate cancer. Further research, however, will be required to establish the precise role of cadmium in this important human malignancy.

Toxicological principles of metal carcinogenesis with special emphasis on cadmium

Metals are an important and emerging class of carcinogens. At least three metals, specifically nickel, chromium, and arsenic, are confirmed human carcinogens, and several more are suspected to have carcinogenic potential in man. Considering that the list of known human carcinogens of any type is very small, it becomes clear that metals make up a substantial portion of the list. Furthermore, many metals are very potent carcinogens in laboratory animals. Despite this, relatively little attention has been given to the topic of metal carcinogenesis. The reasons for this relative lack of attention are not clear but perhaps are fostered by a perception that, because metals are the simplest of molecules, their mechanism of action must also be simple. This could not be farther from the truth and, although no clear mechanisms have emerged in the area of metal carcinogenesis, it has become apparent that they are anything but simple. Metal carcinogens possess several unique characteristics including a remarkable target site specificity. Detection of the mechanism, or mechanisms, of metal carcinogenesis has, however, proven elusive, in part because of a wide diversity of metallic carcinogenic agents and the intricate nature of metal interactions in biologic systems. The following review explores this broad topic, with special emphasis on toxicological principles including dose-response relationships and potential mechanisms, using cadmium as an example.