Cell-specific expression of cytosolic phosphoenolpyruvate carboxykinase in transgenic mice.

The gene encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK) is expressed in multiple cell types in diverse tissues including liver, kidney, intestine, and white and brown adipose tissues. It can thus be considered a model system for examining the regulation of cell-specific transcription. The PEPCK gene is transcribed from a single start site, but studies of transgenic mice have revealed that distinct cis-acting elements (and thus different trans-acting factors) regulate PEPCK expression in hepatocytes, renal proximal tubule epithelial cells, and adipocytes. Hepatocytes require elements between -457 and +69 bp; renal proximal tubule epithelia require elements between -363 and +69 bp; and adipocytes require elements between -2086 and -888 bp. An additional element downstream of +69 bp is required to either attenuate PEPCK mRNA levels in liver and fat or increase renal PEPCK mRNA. We hypothesize that the transcription factors C/EBP and DBP are the principal tissue-specific regulators in liver, and that HNF-1 and perhaps C/EBP are important for kidney-specific PEPCK expression. We propose that the putative downstream element is involved in regulating PEPCK mRNA turnover in liver and fat. Finally, we suggest that the fat-specific element is an enhancer that requires a novel adipogenic regulatory factor, ARF6, to function. The long-term objective will be to fine map the cis-acting elements and identify the cognate trans-acting factors that regulate PEPCK in liver, kidney and fat. This information will help elucidate the combinatorial mechanisms that control the cell-specific expression of this complex gene.