Transcriptional Profiling of Rat Hepatic Response to Pb2+ Exposure via Drinking Water

Lead contamination of soil and drinking water represent a significant risk lo human health, especially for developing infants. Various methods have been employed in determining lead intoxication effects on humans using rats as model organisms, including histopathological analysis, elemental tissue analysis, biochemical analyses such as glycogen levels and aminolevulinic acid (ALA) content. In recent studies, toxico­genomics and proteomics analyses (study of gene expression and protein synthesis activity responses to toxicants) are being used. In this study, Affymetrix Microarray analyses were used lo assess hepatic gene expression patterns in six week post-weaning male Fisher 344 rats exposed ad libitum to 0, 50 or 500 ppm Pb2 (as acetate) through drinking water for 30 and 90 days, respectively. Two independent experiments were carried out for each time point. Following each exposure period, rats were euthanized with CO2 and blood was collected by cardiac puncture for serum analysis. Livers were excised and snap frozen in liquid nitrogen. Total RNA was isolated from rat livers using a Qiagen RNA Isolation kit. Then cDNA was synthesized for use as a template for T7 polymerase in the synthesis of cRNA molecules. These cRNA molecules were purified, fragmented and hybridized to Rat Expression Array Set 230 (RAE 230A) and subsequently scanned as described in Affymetrix GeneChipTM one­cycle eukaryotic target labeling assay. A total of over 3600 genes were then used in differential gene expression analysis by cluster analysis and also for Gene Ontology (GO) analysis. Effects associated with lead exposure were found to be both dose- and time-dependent. Normalization to quantitative reference (control) gene expression reduced the numbers of affected genes. Clustering patterns appear to be similar for both time points and dose levels. However, the short-term exposure group (30d) showed far fewer genes being affected by ± 2-fold than in the sub-chronic treatment group (90d). This was confirmed by multi-dimensional scaling plot that shows majority of arrays congregate around the origin. Gene ontology analysis revealed 15 GO categories were affected by chronic (90 day) lead exposure, while only three GO categories were observed lo be significantly affected for short exposure (30 day) periods.