ROLES OF SYNTHESIS AND DEGRADATION IN THE REGULATION OF METALLOTHIONEIN ACCRETION IN A CHICKEN MACROPHAGE-CELL LINE

Metallothionein (MT) is a metal-binding protein rapidly accreted in many tissues in response to trace elements or hormones. To gain an understanding of the regulation of MT accretion, rates of MT synthesis and degradation were determined by using a decay-kinetics technique. A chicken macrophage-cell line (HD 11) that rapidly accretes incremental amounts of MT when stimulated with increasing concentrations of Zn2+ or Cd2+ was studied. The maximum rate of MT accretion occurred at 50 μm-Zn2+ or 20 μM-Cd2+. The absolute rate of MT accretion was less in macrophages incubated with 25 μM- as compared with 50 μM-Zn2+, owing to decreased and increased rates of MT synthesis and degradation respectively. The absolute rate of MT accretion was less in macrophages incubated with 10 μM- as compared with 20 μM-Cd2+, owing to a decreased rate of MT synthesis with no change in degradation. Compared with macrophages continually incubated with 50 μM-Zn2+, removal of Zn2+ from medium previously containing 50 μM-Zn2+ decreased the absolute rate of MT accretion, owing to decreased and increased rates of MT synthesis and degradation respectively. Removal of Cd2+ from medium previously containing 20 μM-Cd2+ also decreased the absolute rate of MT accretion in macrophages. Unlike Zn2+ removal, the decrease in MT accretion was due to a decreased rate of MT synthesis with no change in degradation. When macrophages incubated with 50 μM-Zn2+ were subsequently incubated with 20 μM-Cd2+, rates of MT synthesis and accretion were decreased as compared with cells continually incubated with 50 μM-Zn2+ or 20 μM-Cd2+. When macrophages incubated with 20 μM-Cd+2 were subsequently incubated with 50 μM-Zn+2, rates of MT synthesis and accretion were increased as compared with cells continually incubated with 50 μM-Zn+2 or 20 μM-Cd+2. Switching the metal in the incubation medium did not influence the rate of MT degradation. Our results indicate that the rate of MT accretion is determined by variations in the rates of MT synthesis and degradation, depending upon the inducing metal and the concentration of the metal.