Intestinal apolipoprotein A-I and B-48 metabolism: effects of sustained alterations in dietary triglyceride and mucosal cholesterol flux.

In recent studies (1985. J. Lipid Res. 26:368-379 and 1986. J. Lipid Res. 27:30-39) we characterized aspects of synthesis of rat intestinal apolipoproteins (apo) A-I and B-48 in vivo, and their short term regulation by dietary and biliary lipid flux. We now report studies extending these observations to the effects on intestinal apoA-I and apoB-48 metabolism of sustained (3 or 6 weeks) isocaloric intake of diets containing 0-30% (by weight) triglyceride, the latter as either butter fat (saturated) or corn oil (polyunsaturated). Additional studies were conducted to determine, separately, the effects of perturbations of intestinal mucosal cholesterol flux and hypothyroidism on intestinal apoA-I and apoB-48 metabolism. Intestinal synthesis (% total protein) of apoA-I and apoB-48 was not influenced by either dietary triglyceride quantity or quality (saturated vs. polyunsaturated fat); the values that were obtained were strictly comparable to those of both chow-fed animals and animals maintained for 3 weeks on fat-free chow. Intestinal apoA-I synthesis was not influenced by either acute or chronic perturbations of mucosal cholesterol flux. Hypothyroid rats demonstrated a 50% suppression of jejunal apoA-I synthesis. Intestinal synthesis of apoB-48, by contrast, appeared to undergo regulation by chronic (but not acute) perturbations of mucosal cholesterol flux. Maneuvers that augmented intestinal cholesterol uptake (particularly hypothyroidism) appeared to suppress intestinal apoB-48 synthesis by over 40%, while Surfomer (AOMA) administration reduced cholesterol absorption (control, 54 +/- 7%; AOMA, 26 +/- 8%; P less than 0.0005) and resulted in a 24% increase in apoB-48 synthesis by jejunal enterocytes. Intracellular intestinal lipoproteins demonstrated marked cholesteryl ester enrichment of the triglyceriderich lipoprotein fractions in hypercholesterolemic, hypothyroid rats. When all the groups were compared, cholesterol absorption (used as an index of mucosal cholesterol uptake) was negatively correlated with jejunal apoB-48 synthesis (r = -0.92, P less than 0.05). The data suggest that regulation of rat intestinal apoA-I and apoB-48 metabolism is independent of triglyceride flux. It is further concluded that an important regulatory effect of mucosal cholesterol flux can be demonstrated on enterocyte apoB-48 synthesis. Finally, the data suggest the additional possibility that circulating levels of thyroid hormone may exert an independent effect on the expression of rat intestinal apolipoproteins A-I and B-48.