INDUCTION OF METALLOTHIONEIN SYNTHESIS BY CADMIUM AND ZINC IN CULTURED RABBIT KIDNEY-CELLS (RK-13)

The effects of increasing concentrations of Zn(II) and Cd(II) on the e xpression of the four isometallothioneins (isoMTs), namely MT-1a, MT-2 a, MT-2d and MT-2e, in rabbit kidney cells (RK-13) and the development of cellular tolerance to these metal ions were studied. The results s howed that, whereas in parental cells MT concentration was low and com posed nearly exclusively of MT-2a and MT-1a. all four isoMTs increased massively in abundance when the cells were exposed to toxic concentra tions of Zn(II) or Cd(II), the relative increase being largest in the two minor isoforms MT-2d and MT-2e. While the response of the four iso MTs to the challenge by Zn(II) or Cd(II) was qualitatively comparable, there were differences in sensitivity and delay time, Cd(II) being th e more efficient inducer and much faster in eliciting the onset of iso MT synthesis. An even larger production of isoMTs resulted when RK-13 cells were cultured in the presence of a series of metal concentration s yielding sub-lines of increased metal tolerance. In this instance to o, there were marked differences in the response to Cd(II) and Zn(II). Thus, in cells of sub-lines selected for tolerance to moderate concen trations of Cd(II) the kinetic analysis of isoMT accretion gave indica tions of a saturable induction process while no such evidence was fort hcoming for Zn(II). In sub-line cells selected for tolerance to the hi ghest concentrations of Cd(II) or Zn(II) isoMT formation was increased by another order of magnitude, reaching for some isoforms a 100- to 1 000-fold augmentation over the amounts measured in cells of the unexpo sed parental cells. A potentiation of this magnitude goes beyond the r ange of ordinary regulation of gene expression. It is to be viewed ins tead as an enlargement of the capacity of isoMT synthesis acquired by a variety of mechanisms in the surviving cells.