Cadmium-bound metallothionein induces apoptosis in rat kidneys, but not in cultured kidney LLC-PK1 cells

The ability of cadmium-bound metallothionein (Cd-MT) to induce apoptosis was investigated in vivo and in vitro. Administration of purified Cd-MT (0.15 mg MT bound Cd per kg body weight) to the rat induces DNA fragmentation, a biochemical characteristic of apoptosis in the kidney at 16 h, which was detectable by ethidium bromide staining on an agarose gel. It was still detected 24 h after administration. Induction of apoptosis by Cd-MT was specific to kidney; it was not observed in cerebrum, cerebellum, heart, lung, liver, testis, dorsolateral prostate, and ventral prostate. In contrast, addition of Cd-MT (0.01-100 mu M) to the cultured porcine kidney LLC-PK1 cells failed to induce apoptosis under the condition where cadmium chloride (10 mu M) did. There was no additivity of induction of apoptosis by CdCl2 (10 mu M) in the presence of Cd-MT (0.01-100 mu M). To examine the effect of intracellular MT on cadmium - induced apoptosis in cultured cells, new cell lines were established, which constitutively produce MT, being termed as Cd-r-LLC-PK1 cells since Cd-MT exogenously added had much less permeability to the cultured cells. Followed by exposure of wild-type LLC-PK1 cells to 50 mu M CdCl2 for 24 h, the surviving cells (Cd-r-LLC-PK1 cells) induce MT at the level of 1.9 mu g/2 x 10(6) cells. In Cd-r-LLC-PK1 cells, 10 mu M CdCl2 failed to induce apoptosis, but 60 mu M CdCl2 could exert the apoptotic response, indicating that intracellular MT which was induced by CdCl2 did not facilitate CdCl2-elicited apoptosis. Furthermore, chromatin in rat kidneys was condensed by Cd-MT, but not that in LLC-PK1 cells. Thus, Cd-MT induces apoptosis in rat kidneys, but not in the cultured renal cells, suggesting that the ionic form of cadmium was required for programmed cell death.