Down-regulation of mRNA for the low density lipoprotein receptor in transgenic mice containing the gene for human cholesteryl ester transfer protein. Mechanism to explain accumulation of lipoprotein B particles.
To evaluate the effects of cholesteryl ester transfer protein (CETP) on apoB-containing lipoproteins, we analyzed plasma lipoproteins from three different lines of human CETP transgenic mice, with plasma CETP concentration ranging from low (1.5 microgram/ml) to high levels (8.5 micrograms/ml). With increasing CETP concentration, very low density lipoprotein and low density lipoprotein (LDL) cholesteryl ester (CE) and apoB were progressively increased, and high density lipoprotein CE was decreased. To investigate the mechanism of accumulation of lipoproteins containing apoB (lipoprotein B), the abundance of hepatic LDL receptor mRNA was determined. LDL receptor mRNA was reduced as a result of CETP expression, with maximum repression to about 48% of the level of non-transgenic mice. Among the different lines of CETP transgenic mice there was an inverse relationship between plasma CETP concentration and hepatic LDL receptor mRNA abundance (r = -0.94, p < 0.01). CETP expression also led to increased cholesterol and cholesteryl ester content in liver and to decreased abundance of mRNAs encoding 3-hydroxy-3-methylglutaryl-coenzyme A reductase and 7-alpha-hydroxylase. Thus, CETP expression results in increased cholesteryl ester concentration in very low density lipoprotein and LDL, probably reflecting both CE transfer from high density lipoprotein and accumulation of lipoprotein B particles. The accumulation of lipoprotein B particles results from CETP-mediated down-regulation of liver LDL receptors, possibly due to enhanced return of cholesterol to the liver.