TISSUE-SPECIFIC CHANGES IN ACYL-COA-CHOLESTEROL ACYLTRANSFERASE (ACAT) MESSENGER-RNA LEVELS IN RABBITS

A human cDNA clone (K1) was recently isolated that encodes functional acyl-CoA:cholesterol acyltransferase (ACAT) protein (Chang et al. J. B iol. Chem. 1993. 268: 20747-20755). We used the K1 clone to screen a r abbit liver cDNA library and isolated a 919 base pair partial rabbit c DNA (ACAT(14b)) that was greater than 90% homologous with the human nu cleotide sequence. Northern blotting using the rabbit ACAT cDNA(14b) r evealed the existence of at least six related mRNA species (ranging fr om 6.2 to 1.7 kb) in various rabbit tissues. Using an RNAse protection assay, ACAT mRNA(14b) was detected in twelve separate rabbit organs. Adrenal gland contained the highest concentrations of ACAT mRNA(14b) ( per mu g of total RNA) being 20-, 30-, and 50-fold higher than small i ntestine, aorta, and liver, respectively. Additional studies with isol ated liver cell populations revealed that rabbit hepatic nonparenchyma l cells contained 30-fold more ACAT mRNA(14b) (per mu g of total RNA) than parenchymal cells. To determine whether ACAT mRNA(14b) levels are regulated in vivo, rabbits were fed for 4 weeks a high fat/high chole sterol diet (HFHC; 0.5% cholesterol, 3% coconut oil, 3% peanut oil) at which point they were either kept for an additional 4 weeks on the HF HC-diet or switched to the HFHC-diet plus CI-976 (50 mg/kg), a potent and specific ACAT inhibitor; another group of rabbits was fed a chow d iet for the entire 8 weeks. The HFHC-diet caused a 2- and 3-fold incre ase in hepatic and aortic ACAT mRNA(14b) levels, respectively, in comp arison to chow-fed animals; there was no change in adrenal or small in testine levels. CI-976 treatment lowered ACAT mRNA(14b) levels by 60% and 40% in liver and aorta, respectively, in comparison to the HFHC co ntrols; again there was no change in adrenal or small intestine levels . These data indicate that ACAT mRNA(14b) levels increase in a tissue specific manner in response to dietary fat and cholesterol.