Nutritional and hormonal regulation of expression of the gene for malic enzyme.

We have provided a historical and personal description of the analysis of physiological and molecular mechanisms by which diet and hormones regulate the activity of hepatic malic enzyme. For the most part, our analyses have been reductionist in approach, striving for increasingly simpler systems in which we can ask more direct questions about the molecular nature of the signaling pathways that regulate the activity of malic enzyme. The reductionist approaches that were so successful at analyzing molecular mechanisms in cells in culture may now provide the means to analyze more definitively questions about the physiological mechanisms involved in nutritional regulation of gene expression. In addition to physiological questions, however, there are still many aspects of the molecular mechanisms that have not been elucidated. Despite considerable effort from many laboratories, the molecular mechanisms by which T3 regulates transcription are not clear. Similarly, the molecular details for the mechanisms by which glucagon, insulin, glucocorticoids, and fatty acids regulate gene expression remain to be determined. The role of fatty acids is particularly interesting because it may provide a model for mechanisms by which genes are regulated by metabolic intermediates; this is a form of transcriptional regulation widely used by prokaryotic organisms and extensively analyzed in prokaryotic systems, but poorly understood in higher eukaryotes. At any specific time, there is, of course, only one rate of transcription for each copy of the malic-enzyme gene in a cell. Our long-term objective is to understand how signals from all of the relevant regulatory pathways are integrated to bring about that rate.