Colloidal gold ultraimmunocytochemical localization of DNA and RNA adducts in rat hepatocytes

RNA adduction derived from electrophilic species in vitro and in vivo

RNA molecules are essential for cell function by not only serving as genetic materials, but also providing cells with structural support and catalytic functions. Due to nucleophilicity of nucleobases, RNA molecules can react with electrophilic species thus to be "adducted". The electron-deficient agents potentially inducing adduction exist in a variety of natural sources including metabolic products of biomolecules. Although evident and readily detected in human tissue, RNA adduction remains poorly understood for their physiological and pathological function. In this article, we review a collection of exogenous and endogenous molecular species that participate in RNA adduction and elaborates on the chemical nature of their RNA adduction sites. Furthermore, we provide perspectives on the potential of RNA adducts as biomarkers of environmental insults. Finally, we project future investigations that are necessary for understanding the mechanisms of cellular toxicity of RNA adduction.

Quality control of chemically damaged RNA

The "central dogma" of molecular biology describes how information contained in DNA is transformed into RNA and finally into proteins. In order for proteins to maintain their functionality in both the parent cell and subsequent generations, it is essential that the information encoded in DNA and RNA remains unaltered. DNA and RNA are constantly exposed to damaging agents, which can modify nucleic acids and change the information they encode. While much is known about how cells respond to damaged DNA, the importance of protecting RNA has only become appreciated over the past decade. Modification of the nucleobase through oxidation and alkylation has long been known to affect its base-pairing properties during DNA replication. Similarly, recent studies have begun to highlight some of the unwanted consequences of chemical damage on mRNA decoding during translation. Oxidation and alkylation of mRNA appear to have drastic effects on the speed and fidelity of protein synthesis. As some mRNAs can persist for days in certain tissues, it is not surprising that it has recently emerged that mRNA-surveillance and RNA-repair pathways have evolved to clear or correct damaged mRNA.

Decreased interleukin-2 receptor and cell cycle changes in murine lymphocytes exposed in vitro to low doses of cadmium chloride

Relationships between in vitro cadmium-related cell cytotoxicity, ultrastructural changes and altered cell cycle were determined at 21-72 h after mitogenic stimulation of C57BL/6 mouse spleen lymphocytes with concanavalin A (Con A). Relatively low doses, 0.6-10 microM cadmium (Cd), added at 4 h after the mitogen activation, induced a significant cell cytotoxicity and reduced the lymphoblastic activity of the cells. Cytometric analysis of the lymphoid cell cycle at 72 h revealed that at concentrations > or = 0.6 microM Cd, the number of cells arrested in G0 + G1 phase increased, whereas the proportions of cells of the S and G2 + M phases were substantially reduced. Staining of cells with fluorescent anti-CD25 monoclonal antibody showed a cadmium-related decreased number and relative mean fluorescence of CD25+ cells, demonstrating a decreased level of interleukin-2 receptor (IL-2R). Furthermore, immunogold ultramicroscopic assay was developed for determination of intracellular interleukin-2 (IL-2) in cadmium-treated lymphocytes. The level of cytoplasmic and nuclear IL-2, localized in situ by colloidal gold ultraimmunocytochemical technique, has been estimated as markedly decreased in cells treated with > or = 1.2 microM Cd, as compared with the untreated controls. Disorganization/fragmentation of mitochondrion cristae and dilatation of cisternae of the Golgi apparatus appeared as the major ultrastructural change in 1.2 microM Cd-treated lymphocytes. Interestingly, addition of cadmium in the incubation medium, up to 4 h after mitogen activation, also interacted with lymphoproliferative mechanisms of cells in G0 + G1, S and G2 + M phase. Overall, multiple ultrastructural changes of Cd-treated lymphoid cells were clearly related with the reduced cell viability and reduced number of activated lymphoblasts.

Immunocytochemical evidence for a nuclear and a cytoplasmic O6-methylguanine repair mechanism in cultured rat hepatocytes

The localization of DNA and RNA adducts was studied at the ultrastructural level using antibodies directed against O6-metG and the protein A-gold technique. Primary rat hepatocyte cultures were exposed for 2-24 h to 5 mM N-nitrosodimethylamine (NDMA) or 0.1 mM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In both cases, the O6-metG immunoreactive sites were concentrated in the nucleus and in the rough endoplasmic reticulum (RER) rich cytoplasmic regions. The highest gold labeling density measured was observed at 2 h of NDMA or MNNG treatment. However, after a 24-h exposure, very little labeling was observed in both the nuclear and the cytoplasmic compartments. The rate of disappearance of immunoreactive sites was faster in the cytoplasm than in the nucleus, Untreated control preparations showed no specific immunogold labeling. Furthermore, when cells were exposed first to NDMA and MNNG for a few hours and then to culture medium containing no genotoxin, and subsequently were reexposed to NDMA or MNNG for a few hours, very little labeling of both the nuclear and cytoplasmic compartments was observed. Control preparations without a second genotoxin exposure showed a normal labeling pattern. Control preparations without genotoxin showed no gold labeling. Our results provide evidence for the existence of a cytoplasmic O6-metG repair mechanism that behaves like its nuclear counterpart.

O6-methylguanine-DNA adducts in rat lymphocytes after in vivo exposure to N-nitrosodimethylamine (NDMA)

A non-invasive approach in immunopathological risk assessment was applied for analysis of the in vivo formation of DNA adducts. DNA methylation was studied in peripheral blood lymphocytes (PBLs) collected from Sprague-Dawley rats exposed to a single dose (75 mg/kg b.w.) of N-nitrosodimethylamine (NDMA). Three different techniques were applied for characterization and quantification of DNA adducts: (i) colloidal gold ultraimmunocytochemical localization of O6-methylguanosine (O6-meG)-DNA adducts, using affinity-purified, polyclonal antibody directed against O6-meG, (ii) quantitative assay using enzyme-linked immunosorbent assay (ELISA), amplified by the avidin-biotin (AB) system, and (iii) high-performance liquid chromatography (HPLC). The O6-meG-immunoreactive sites in PBLs seem to be concentrated in the nucleus. However, significant immunolabelling was also noted in the cytoplasm of the in vivo NDMA-exposed PBLs. Control preparations showed no specific gold immunolabelling. The O6-meG-DNA adduct formation in PBLs and hepatocytes, at 2-24 h following the exposure to NDMA, was analogous for both types of cells. The data showed high correlation for the ELISA and HPLC analytical methods. The data suggest an efficient O6-metG-DNA repair mechanism in lymphocytes, possibly analogous to the enzymatic repair of DNA adducts in liver cells.

Mercuric chloride affects protein secretion in rat primary hepatocyte cultures: a biochemical ultrastructural, and gold immunocytochemical study

The toxicity of mercury on hepatocytes was studied at the ultrastructural, biochemical, and immunocytochemical levels. Albumin metabolism was examined because it is a representative liver-specific function. A novel cytochemical method using the protein A-gold technique for the in situ localization of albumin in hepatocyte cultures was applied. Primary rat hepatocyte cultures were exposed to increasing HgCl2 concentrations. Cytotoxicity was assessed by measuring the release of lactic dehydrogenase from the cells. At the highest exposure concentration tested (50 microM), Hg was found to be significantly cytotoxic in contrast to what occurred at 5.0 and 0.5 microM. The level of albumin secreted, as measured by ELISA, was decreased by approximately 38% at 5.0 microM HgCl2 and was found not to be different from that of controls at lower concentrations. The ultrastructural analysis showed that hepatocytes treated with 5.0 microM HgCl2 undergo drastic morphological changes such as a decreased number of ribosomes associated with the rough endoplasmic reticulum, and the disappearance of the latter organelle, proliferation of the smooth endoplasmic reticulum, and dilatation of both the Golgi apparatus and the biliary canaliculus-like structures. Immunocytochemical detection of albumin-immunoreactive sites using protein A-gold labeling further revealed that these were less abundant in hepatocytes treated with 5.0 microM HgCl2 (-64%) as compared to control preparations. These results suggest that one of the effects of mercury on hepatocytes is to affect liver-specific functions such as albumin production, possibly through interference with ribosomal function. This study also demonstrates for the first time the applicability of the high-resolution protein A-gold technique for toxicological investigations on hepatocytes in vitro.