Cell-cell interactions in the regulation of the expression of hepatic enzymes

The expression of enzymes and other proteins in liver parenchyma is heterogeneous and shows distinct features depending on whether all or only a subset of hepatocytes are involved. Recent advances in the elucidation of the factors that control zonated expression have provided new insights into the regulatory mechanisms that might underlie the different patterns of heterogeneity. While humoral factors seem to play a predominant role in the dynamic adaptation of zonated expression, newly identified intrahepatic (zonation) factors might be responsible for a static dissection of the parenchyma into different expression compartments. These different principles for determining heterogeneous enzyme expression are presented and discussed for cholesterol 7 alpha-hydroxylase and glutamine synthetase.