Acute modulation of the extent of apoB mRNA editing and the relative ratesof syntheses of apoB48 and apoB100 in cultured rat hepatocytes by osmotic and other stress stimuli

The mRNA for apolipoprotein B (apoB) is edited by the enzyme APOBEC-1, whic h acts as part of a multiprotein complex or editosome. In cultured rat hepa tocytes obtained from fed animals this results in the presence of edited an d unedited apoB mRNA in a ratio of approximately 3:2 in the basal state. In this study we show that hyper-osmotic media, which induce cell shrinkage, resulted in an acute increase in the degree of editing of apoB mRNA (hypo-o smotic conditions had no effect). This increase was accompanied by a parall el and highly positively correlated change in the ratio of the rate of synt hesis of apoB48 relative to that of apoB100. These changes occurred in the absence of any changes in the overall APOBEC-1 mRNA levels, indicating that the activation of editing occurred at a post-transcriptional level. Levels of total apoB mRNA were also unaffected by hyper-osmotic exposure of the c ells indicating that changes in the relative rates of synthesis of apoB48 a nd apoB100 were due to post/translational events. Exposure of cells to anis omycin at concentrations (50 mu g/ml) that inhibit protein synthesis or to the transcriptional inhibitor actinomycin D produced changes in the degree of apoB mRNA editing that were similar to those given by hyper-osmotic shoc k indicating that editing is able to respond acutely to transcriptional or translational inhibition. Anisomycin, at concentrations (50 ng/ml) that act ivate SAPK/JNK but do not inhibit protein synthesis, gave only a fraction o f the effect of hyper-osmotic shock. SB203580, an inhibitor of p38 kinase, did not attenuate the effects of hyper-osmotic conditions on APOBEC-1 editi ng. These observations suggest that these MAPkinase pathways play a relativ ely minor part in the transduction of the osmotic stimulus to the editing m echanism. The hyper-osmotically-induced increase in apoB mRNA editing was a lso insensitive to PD98059 and wortmannin (inhibitors of MEK and PI3 kinase , respectively). These data provide evidence that apoB mRNA editing is capa ble of acute modulation independently of transcriptional or translational m echanisms and suggest that one or more components of the editosome may unde rgo post-translational activation.