Cadmium-induced expression of metallothionein and suppression of RNA to DNA ratios during molluscan development.
One-week-old larvae of the mollusc Crassostrea virginica, an oyster, were exposed to Cd concentrations ranging from the control with no added Cd to 0.2 microM CdCl2 for 24 hr. Concentration-dependent increases in total cadmium accumulation, cytosolic and metallothionein-bound Cd concentrations, and levels of the MT mRNA were detectable over this range of exposure concentrations. Increases in these measures were apparent at the lowest exposure concentration of 0.005 microM, with dramatic increases observed between 0.04 and 0.2 microM Cd. Although the concentrations of MT-bound Cd increased, the fraction of total Cd that was bound to MTs declined in larvae treated with 0.04 and 0.2 microM Cd, presumably due to the inability of MT biosynthesis to keep up with the higher rates of Cd uptake at the higher concentrations. This decreased effectiveness of MTs in sequestering Cd in relation to the total Cd load and the associated increase in the nonthionein Cd fraction at these higher treatment conditions coincided with a decline in the RNA to DNA ratio and an increase in a nonacetylated variant of the MT. The former is indicative of an inhibition of larval metabolism and growth. The latter is an unexpected form for this MT and is believed to signify disruption of normal cotranslational modification of the N-terminal amino acid sequence.
Roesijadi
G
G
University of Maryland, Chesapeake Biological Laboratory, Solomons, Maryland 20688, USA.
Hansen
K M
KM
Fuentes
M E
ME
D000818:Animals; D002104:Cadmium / Q000633:toxicity*; D004247:DNA / Q000032:analysis*; D004305:Dose-Response Relationship, Drug; D008668:Metallothionein / Q000096:biosynthesis* / Q000235:genetics; D008974:Mollusca / Q000187:drug effects* / Q000502:physiology; D012313:RNA / Q000032:analysis*; D012333:RNA, Messenger / Q000032:analysis
D012333:RNA, Messenger; D002104:Cadmium; D012313:RNA; D004247:DNA; D008668:Metallothionein