STRESS PROTEIN-SYNTHESIS INDUCED BY CADMIUM-CYSTEINE IN RAT-KIDNEY

Biomarkers are important tools which enable toxicologists to reliably predict and detect ex- posures to xenobiotics and resultant cell injur y, ultimately improving risk assessments. Since the de novo synthesis of stress proteins can be detected early after exposure to some agents , analysis of toxicant-induced changes in gene expression, i.e. altera tions in patterns of protein synthesis, may be useful to develop as bi omarkers of exposure and toxicity. We are utilizing various xenobiotic s as tools to study stress protein synthesis in target organs in order to evaluate the target tissue-specificity of this response. Previous data from this laboratory have demonstrated that induction of stress p roteins in rat liver, but not kidney, after acute exposure to CdCl2 pr ecedes hepatotoxicity. Since kidney is a target tissue after chronic C d exposure, it was of interest to examine stress protein synthesis in this tissue. However, dose-limiting hepatotoxicity precluded this eval uation. Cd complexed with molecules such as cysteine (cys) or metallot hionein has been used in acute dosing regimens as a tool in order to s tudy the nephrotoxicity of Cd. Therefore, this study was undertaken in order to evaluate Cd-induced stress protein synthesis in an important tissue known to be injured after chronic exposure, i.e. kidney. Speci fic objectives included comparing stress protein synthesis in rat kidn ey and liver after acute exposure to Cd-cys and CdCl2, determining the Cd threshold concentration for renal stress protein synthesis and ass essing the relationship between stress protein synthesis and nephropat hy. Male rats were exposed to equivalent doses of Cd as CdCl2 or Cd-cy steine (molar ratio Cd:cys=1:15). Kidney Cd concentrations increased 5 -fold after i.v. injection of Cd-cys compared to CdCl2, mimicking Cd d istribution following chronic exposure. After exposure to Cd, tissue s lices were incubated with S-35-methionine. Slices were subsequently ho mogenized and centrifuged, and the 16000 g supernatants were subjected to SDS-polyacrylamide gel electrophoresis. Proteins which had incorpo rated S-35-methionine were detected by autoradiography. De novo synthe sis of 70, 90 and 110 kDa proteins was enhanced in liver, but not in k idney, 4 h after injection of 2 mg Cd/kg as CdCl2. In contrast, dose-r elated increases in synthesis of these proteins were observed in kidne y 4 h after injection of 1 and 2 mg Cd/kg as Cd-cys, but not at lower dosages. In addition, synthesis of a 68 kDa kidney protein was inhibit ed at 2 mg Cd/kg as Cd-cys. The threshold for Cd-induced stress protei n synthesis was shown to be between 4 and 8 mu g Cd/g tissue. These al terations in protein synthesis in kidney occurred at tissue Cd concent rations lower than those which resulted in renal injury, as assessed b y histopathology and PAH uptake into renal slices. Thus, altered patte rns of protein synthesis may serve as markers of cell injury or indica tors of cellular stress in target organs.