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 injury, 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. alterations in patterns of protein synthesis, may be useful to develop as biomarkers of exposure and toxicity. We are utilizing various xenobiotics 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 proteins in rat liver, but not kidney, after acute exposure to CdCl2 precedes hepatotoxicity. Since kidney is a target tissue after chronic Cd exposure, it was of interest to examine stress protein synthesis in this tissue. However, dose-limiting hepatotoxicity precluded this evaluation. Cd complexed with molecules such as cysteine (cys) or metallothionein has been used in acute dosing regimens as a tool in order to study 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. Specific objectives included comparing stress protein synthesis in rat kidney and liver after acute exposure to Cd-cys and CdCl2, determining the Cd threshold concentration for renal stress protein synthesis and assessing the relationship between stress protein synthesis and nephropathy. Male rats were exposed to equivalent doses of Cd as CdCl2 or Cd-cysteine (molar ratio Cd:cys=1:15). Kidney Cd concentrations increased 5-fold after i.v. injection of Cd-cys compared to CdCl2, mimicking Cd distribution following chronic exposure. After exposure to Cd, tissue slices were incubated with S-35-methionine. Slices were subsequently homogenized and centrifuged, and the 16000 g supernatants were subjected to SDS-polyacrylamide gel electrophoresis. Proteins which had incorporated S-35-methionine were detected by autoradiography. De novo synthesis of 70, 90 and 110 kDa proteins was enhanced in liver, but not in kidney, 4 h after injection of 2 mg Cd/kg as CdCl2. In contrast, dose-related increases in synthesis of these proteins were observed in kidney 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 inhibited at 2 mg Cd/kg as Cd-cys. The threshold for Cd-induced stress protein synthesis was shown to be between 4 and 8 mu g Cd/g tissue. These alterations in protein synthesis in kidney occurred at tissue Cd concentrations lower than those which resulted in renal injury, as assessed by histopathology and PAH uptake into renal slices. Thus, altered patterns of protein synthesis may serve as markers of cell injury or indicators of cellular stress in target organs.