The ability of acetaldehyde, a respiratory carcinogen present in tobacco smoke and automotive emissions, to affect cell viability, thiol status and intracellular Ca2+ levels and to cause DNA damage and mutations has been studied using cultured human cells. Within a concentration range of 3-100 mM, a 1 h exposure to acetaldehyde decreases colony survival and inhibits uptake of the vital dye neutral red in bronchial epithelial cells. Acetaldehyde also causes both DNA interstrand cross-links and DNA protein cross-links whereas no DNA single strand breaks are detected. The cellular content of glutathione is also decreased by acetaldehyde, albeit, without concomitant changes in the glutathione redox status or in the content of protein thiols. Transient or sustained increases in cytosolic Ca2+ occur within minutes following exposure of cells to acetaldehyde. Moreover, acetaldehyde significantly decreases the activity of the DNA repair enzyme O6-methylguanine-DNA methyltransferase. Finally, a 5 h exposure to acetaldehyde causes significant levels of 6-thioguanine resistance mutations in an established mutagenesis model involving skin fibroblasts. The results indicate that mM concentrations of acetaldehyde cause a wide range of cytopathic effects associated with multistep carcinogenesis. The fact that acetaldehyde, in relation to its cytotoxicity, causes comparatively higher genotoxicity and inhibits DNA repair more readily than other major aldehydes in tobacco smoke and automotive emissions is discussed.

The initiation of protein synthesis on mRNAs within eukaryotic cells is achieved either by a 59 cap-dependent mechanism or through internal initiation directed by an internal ribosome entry site (IRES). Picornavirus IRES elements, located in the 59 untranslated region (59UTR), contain extensive secondary structure and multiple upstream AUG codons. These features can be expected to inhibit cap-dependent initiation of translation. However, we have now shown that certain mutant hepatitis C virus-like picornavirus IRES elements (from porcine teschovirus-1 and avian encephalomyelitis virus), which are unable to direct internal initiation, are not significant barriers to efficient translation of capped monocistronic mRNAs that contain these defective elements within their 59UTRs. Moreover, the translation of these mRNAs is highly sensitive to the expression of an enterovirus 2A protease (which induces cleavage of eIF4G) and is also inhibited by hippuristanol, a specific inhibitor of eIF4A function, in contrast to their parental wild-type IRES elements. These results provide a possible basis for the evolution of viral IRES elements within the context of functional mRNAs that are translated by a cap-dependent mechanism.

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