PROGESTERONE PRETREATMENT ENHANCES CELLULAR-SENSITIVITY TO CADMIUM DESPITE A MARKED ACTIVATION OF THE METALLOTHIONEIN GENE

Previously, we found that in vivo pretreatment with progesterone marke dly increased cadmium lethality in rats, apparently by enhancing cadmi um-induced hepatonecrosis. Therefore, the present study was designed t o investigate this phenomenon at the molecular level in an in vitro sy stem. TRL-1215 rat liver cells were exposed to various concentrations of progesterone (0, 1, 10, and 100 mu M) for 24 hr and subsequently ex posed to cadmium (0, 1, 5, 10, and 50 mu M; as CdCl2) for an additiona l 24 hr. Although the levels of progesterone used were essentially non toxic, progesterone pretreatment resulted in a concentration-dependent increase in sensitivity to cadmium as assessed by loss of mitochondri al enzyme activity (tetrazolium-based dye assay) and loss of cytosolic enzyme activity (glutamic oxaloacetic transaminase). The effects of p rogesterone treatment on intracellular levels of metallothionein (MT), an inducible metal-binding protein generally associated with cadmium tolerance, were also measured. Progesterone (100 mu M) alone increased MT levels 2.4-fold, while cadmium (10 mu M) alone resulted in a 7-fol d 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 cadmiu m. In addition, progesterone pretreatment clearly induced transcriptio n of the MT gene as evidenced by enhanced cadmium-induced accumulation of cellular MT mRNA. Progesterone pretreatment had no effect on the l evel of glutathione, a cellular thiol thought to be important in detox ication of cadmium prior to MT gene activation and MT protein accumula tion, or on cellular accumulation of cadmium during the initial 3 hr o f exposure to the metal. The proportion of total cellular cadmium boun d to MT in cells pretreated with progesterone was greater than that in the cells treated with cadmium alone, indicating an enhanced sequestr ation of the metal by MT after pretreatment. These results indicate th at progesterone, at nontoxic levels, markedly exacerbates cadmium toxi city at the cellular level in liver cells. This is in accord with the observed progesterone-induced enhancement of the hepatotoxic effects o f cadmium in vivo. The observed facilitation of cytotoxicity is not ba sed in altered toxicokinetics of cadmium and occurs despite a pronounc ed activation of the MT gene resulting in an enhanced sequestration of cadmium by MT. The mechanism by which progesterone enhances cadmium t oxicity deserves further study. (C) 1997 Academic Press