Expression of hepatocyte low-density lipoprotein receptor-related protein is post-transcriptionally regulated by extracellular matrix

The low-density lipoprotein receptor-related protein (LRP) is a multifunctional member of the low-density lipoprotein receptor family that has been implicated in a variety of physiologic and pathologic processes. However, little is known about LRP regulation at the molecular level, and the factors that might mediate LRP have not yet been characterized. This is particularly true of hepatocytes, an important site of LRP expression. Hepatocyte gene expression is known to be dependent on extracellular matrix composition, although the effect of extracellular matrix on lipoprotein receptor expression has not yet been investigated. Also, the mechanisms by which the extracellular matrix affects hepatocyte gene expression are not well understood. In this study, we show that hepatocyte LRP expression decreases rapidly at the mRNA, protein, and functional levels on collagen type I, but remains high on an Engelbreth-Holm-Swarm sarcoma matrix-preparation, Matrigel. LRP function was assessed with ligand binding studies and a novel cytotoxicity assay, using Pseudomonas exotoxin A. Investigation of the mechanism of LRP down-regulation revealed a two-fold longer LRP mRNA half-life in hepatocytes cultured on Matrigel relative to collagen. Taken together, these studies reveal that LRP expression in primary hepatocytes is dependent on the extracellular matrix, and that matrix-dependent differences in hepatocyte LRP mRNA expression are primarily due to changes in mRNA stability, indicating for the first time that the expression of LRP is subject to post-transcriptional regulation.