Progesterone pretreatment enhances cellular sensitivity to cadmium despite a marked activation of the metallothionein gene.

Previously, we found that in vivo pretreatment with progesterone markedly increased cadmium lethality in rats, apparently by enhancing cadmium-induced hepatonecrosis. Therefore, the present study was designed to investigate this phenomenon at the molecular level in an in vitro system. TRL-1215 rat liver cells were exposed to various concentrations of progesterone (0, 1, 10, and 100 microM) for 24 hr and subsequently exposed to cadmium (0, 1, 5, 10, and 50 microM; as CdCl2) for an additional 24 hr. Although the levels of progesterone used were essentially nontoxic, progesterone pretreatment resulted in a concentration-dependent increase in sensitivity to cadmium as assessed by loss of mitochondrial enzyme activity (tetrazolium-based dye assay) and loss of cytosolic enzyme activity (glutamic oxaloacetic transaminase). The effects of progesterone treatment on intracellular levels of metallothionein (MT), an inducible metal-binding protein generally associated with cadmium tolerance, were also measured. Progesterone (100 microM) alone increased MT levels 2.4-fold, while cadmium (10 microM) alone resulted in a 7-fold increase over control. Progesterone pretreatment followed by cadmium exposure caused a marked, 16-fold induction in MT synthesis, a level of activity that has been associated with acquired tolerance to cadmium. In addition, progesterone pretreatment clearly induced transcription of the MT gene as evidenced by enhanced cadmium-induced accumulation of cellular MT mRNA. Progesterone pretreatment had no effect on the level of glutathione, a cellular thiol thought to be important in detoxication of cadmium prior to MT gene activation and MT protein accumulation, or on cellular accumulation of cadmium during the initial 3 hr of exposure to the metal. The proportion of total cellular cadmium bound to MT in cells pretreated with progesterone was greater than that in the cells treated with cadmium alone, indicating an enhanced sequestration of the metal by MT after pretreatment. These results indicate that progesterone, at nontoxic levels, markedly exacerbates cadmium toxicity at the cellular level in liver cells. This is in accord with the observed progesterone-induced enhancement of the hepatotoxic effects of cadmium in vivo. The observed facilitation of cytotoxicity is not based in altered toxicokinetics of cadmium and occurs despite a pronounced activation of the MT gene resulting in an enhanced sequestration of cadmium by MT. The mechanism by which progesterone enhances cadmium toxicity deserves further study.