Investigations on the estrogenic activity of the metallohormone cadmium in the rat intestine

Cadmium (Cd), a toxic heavy metal and an important environmental pollutant, is now also regarded as potential endocrine disruptor. Its estrogenic effects have been examined so far just in classical target tissues, e.g. uterus, and mostly upon intraperitoneal (i.p.) injection of CdCl2. Yet, estrogen receptors are also expressed in the gut, and food is the main source of cadmium intake in the general population. Therefore, possible estrogenic effects were now investigated in the intestine of ovariectomized Wistar rats after oral short- and long-term administration of CdCl2 (0.05-4 mg/kg bw on 3 days by gavage and 0.4-9 mg/kg bw for 4 weeks in drinking water) or upon i.p. injection (0.00005-2 mg CdCl2/kg bw), and compared to steroid estrogen (estradiol or ethinylestradiol) treated groups. Analysis of Cd in kidneys and small intestine by atomic absorption spectrometry showed dose-dependent increases in tissue levels with rather high Cd concentrations in the gut, both after oral and i.p. administration. Expression of metallothionein (MT1a), a typical metal response parameter, was clearly induced in kidney and small intestine of several CdCl2 treated groups, but also notably increased by steroid estrogens. Levels of estrogen-regulated genes, i.e. pS2/TFF1, vitamin D receptor (VDR), and estrogen receptor alpha and beta (ER alpha/beta) were studied as parameters of hormonal activity: The intestinal mRNA expression of pS2/TFF1 was significantly decreased in the estrogen reference groups, but also after single i.p. injection and oral long-term administration of CdCl2. In contrast, the mRNA and protein expression of the VDR were unaffected by long-term administration of Cd via drinking water. We detected expression of ER beta, but not ER alpha in the small intestine of OVX rats. ER beta mRNA and protein expression were significantly down-regulated by Cd, similar to the ethinylestradiol reference group. The mRNA expression and immunostaining of proliferating cell nuclear antigen (PCNA), as an index for cell proliferation, revealed decreases after long-term administration of Cd and ethinylestradiol. In summary, cadmium exposure was shown to modulate molecular and functional parameters of estrogenicity in the intestinal tract of OVX rats. As the intestine is known to express predominantly ER beta, and is an important site of interaction with dietary contaminants, it is indicated to further investigate specific molecular mechanisms of cadmium and estrogen receptor interactions in more detail.