LACK OF CORRELATION BETWEEN ACAT MESSENGER-RNA EXPRESSION AND CHOLESTEROL ESTERIFICATION IN PRIMARY LIVER-CELLS

A partial rabbit cDNA clone (14b) for ACAT has been characterized and used to demonstrate that hepatic and aortic ACAT mRNA(14b) abundance i ncreased 2-3-fold in rabbits receiving a high fat/high cholesterol-die t compared to chow fed animals (Pape et al, (1995) J. Lipid Res. 36, 8 23-838). Because of those data we hypothesized that increased hepatic cholesteryl ester mass and synthesis rates in rabbit liver cells are a ssociated with an increase in ACAT mRNA(14b) levels. To test this hypo thesis we altered cellular cholesteryl ester mass and synthesis rates in primary parenchymal and nonparenchymal cells using various extracel lular agents and measured the accumulated mass of ACAT mRNA(14b). Pare nchymal cells incubated with rabbit beta VLDL or mevalonolactone displ ayed a 6-10-fold increase in cellular cholesteryl ester mass over a th ree day treatment with no significant changes in cellular free cholest erol, triacylglycerols, or ACAT mRNA(14b) levels; HMG CoA reductase an d LDL receptor mRNA mass decreased initially as a result of cholestery l ester loading. Treatment of parenchymal cells with CI-976, an ACAT i nhibitor, showed a marked reduction in cholesteryl ester synthetic rat e compared to beta VLDL controls but displayed no change in ACAT mRNA( 14b) levels. A mixed population of rabbit hepatic nonparenchymal cells was incubated with beta VLDL for 24 h in culture which resulted in a 6-fold increase in cellular cholesteryl ester mass; there was no chang e in ACAT mRNA(14b) levels. In an in vivo study, rabbits consuming a h igh fat/high cholesterol-diet for three weeks showed a 10-fold increas e in hepatic cholesteryl ester with no significant changes in ACAT mRN A(14b) levels. Together these data indicate that rabbit liver cellular cholesteryl ester mass increases of up to 10-fold are not correlated with ACAT mRNA(14b) changes. Thus, hepatic ACAT mRNA(14b) expression a nd cellular cholesterol esterification do not appear to be coordinatel y regulated at this level of cholesteryl ester loading.