DIETARY-FAT ELEVATES HEPATIC APOA-I PRODUCTION BY INCREASING THE FRACTION OF APOLIPOPROTEIN-A-I MESSENGER-RNA IN THE TRANSLATING POOL

Elevated plasma high density lipoprotein cholesterol (HDL-C) levels are associated with a decreased risk for coronary heart disease, Ironically, diets enriched in saturated fat and cholesterol (HF/HC diets), which tend to accelerate atherosclerotic processes by increasing LDL cholesterol levels, also raise HDL-C, We have recently reported, using a human apoA-I (hapoA-1) transgenic mouse model, that the elevation of HDL-C by a HF/HC diet is attributable, in part, to an increase in the hepatic production of hapoA-1, To further define the hepatocellular processes associated with this induction, we have prepared primary hepatocytes from hapoA-1 transgenic mice. Rates of hapoA-1 secretion were 40% greater from cells prepared from animals fed the HF/HC relative to a low fat-low cholesterol (LF/LC) control diet, The abundance of hapoA-1 mRNA in these cells was similar between hepatocytes prepared from the HF/HC and LF/LC diet fed animals, suggesting a post-transcriptional mechanism that does not involve mRNA stability, Inhibition of secretion using brefeldin A revealed an increase in cellular hapoA-1 accumulation, Thus, the HF/HC diet apparently affects hepatic hapoA-1 production via a mechanism that is manifest prior to the exit of newly synthesized hapoA-1 from the Golgi, Pulse-chase experiments revealed a 39% greater peak hapoA-1 synthesis, with no difference in the degradation of total labeled hapoA-1 protein, as a result of the HF/HC diet feeding, Finally, resolution of liver S10 extracts via sucrose density sedimentation and metrizamide density equilibrium gradient centrifugation analyses both revealed similar increases (31 and 24%, respectively) in the relative percentage of hapoA-1 mRNA associated with the translating polysomal fractions as a result of the HF/HC feeding, Together, these data suggest that the HF/HC diet affects hepatic hapoA-1 production via a specific modulation in the relative amount of hapoA-1 mRNA in the polysomal pool, These observations provide an opportunity to explore a new mechanism regulating apoA-1 production and might lead to the development of novel therapies to elevate plasma HDL-C levels.