Modulation of LDL receptor mRNA stability by phorbol esters in human liver cell culture models

In the human hepatocarcinoma cell lines HepG2 and Hep3B, low density lipoprotein receptor (LDLR) mRNA levels were rapidly and transiently induced after treatment with phorbol-12-myristate-13-acetate (PMA), increasing by approximately 50-fold and 8-fold, respectively, within 4 h before returning to near basal levels by 24 h. The difference in magnitude of mRNA accumulation between these cell lines is at least partly due to a rapid 2- to 2.5-fold stabilization of LDLR mRNA in HepG2 cells after PMA treatment. Stabilization of LDLR mRNA in response to PMA was also observed in HH01 cells, a human hepatocyte coculture system derived from normal human liver. In both HepG2 and HH01 cells, PMA treatment induced a rapid morphological change with characteristics of cytoskeletal reorganization. The changes in morphology and stabilization of LDLR mRNA by PMA were coincident in the cell lines tested and were independent of de novo gene expression. Subcellular fractionation studies indicated that LDLR polysomes may be associated with the cytoskeleton in HepG2 cells. Disruption of the action cytoskeleton but not microtubules abrogated stabilization of LDLR mRNA by PMA. These data suggest that components of the actin cytoskeleton are involved in the regulated decay of LDLR mRNA in some human liver cell culture systems.