Differences between the regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and low density lipoprotein receptor in human hepatoma cells and fibroblasts reside primarily at the translational and post-translational levels.

We have carried out parallel analyses of the regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGr) and low density lipoprotein receptor (LDLr) in two highly differentiated human hepatoma cell lines, HepG2 and Hep3B, and primary cultures of human fibroblasts. Analyses of the levels of HMGr and LDLr mRNAs under a variety of culture conditions that perturb intracellular sterol metabolism, or which differ in the levels of extracellular sterols, indicated that the hepatoma cells and fibroblasts responded similarly in terms of the repression or induction ratios of both mRNAs. However, the absolute levels of the mRNAs were severalfold higher in the hepatoma cells. The major difference between the responses of the hepatoma cells and fibroblasts involved the increase in expression of LDLr which occurred upon shifting the cells from complete to lipoprotein-depleted serum. Under these conditions, the 3-fold increase in rate of synthesis of LDLr in the hepatomas was closely matched by increases in the level of its mRNA. In the case of fibroblasts, a 10-fold increase in translational efficiency was required to explain the 30-fold change in rate of synthesis of LDLr. Polysome profiles from both hepatoma cells and fibroblasts suggest that the rate of elongation or termination on LDLr mRNA is relatively low in the presence of reconstituted complete serum, and that it increases in fibroblasts upon lipoprotein depletion, but not in the hepatoma cells. These data indicate that hepatic expression of LDLr may be relatively refractory to induction by decreased circulating levels of lipoprotein when compared with peripheral tissues.