Metabolic adaptations to dietary fat malabsorption in chylomicron-deficient mice.

A mouse model of chylomicron deficiency was recently developed; these mice express a human apolipoprotein (apo) B transgene in the liver but do not synthesize any apoB in the intestine. Despite severe intestinal fat malabsorption, the mice maintain normal concentrations of plasma lipids and liver-derived apoB 100-containing lipoproteins. We investigated the metabolic mechanisms by which plasma lipid levels are kept normal. De no v o lipogenesis (DNL) and cholesterogenesis were measured by mass isotopomer distribution analysis (MIDA). Plasma non-esterified fatty acid (NEFA) fluxes and hepatic re-esterification of labelled plasma NEFA were also measured. Hepatic and plasma triacylglycerol (TG) concentrations and plasma NEFA fluxes were not different between chylomicron-deficient mice and controls. The contribution from DNL to the hepatic TG pool was only modestly higher in chylomicron-deficient mice [12±2.1% ( n = 7) compared with 3.7±1.0% ( n = 9); means±S.E.M.], whereas cholesterogenesis was markedly elevated. The fractional contribution from plasma NEFA to hepatic TG was greatly elevated in the chylomicron-deficient animals (62% compared with 23%). Accordingly, 73% of hepatic TG was neither from DNL nor from plasma NEFA in controls, presumably reflecting prior contribution from chylomicron remnants, compared with only 26% in the chylomicron-deficient group. The long-term contribution from DNL to adipose fat stores reached approximately the same steady-state values (≈ 30%) in the two groups. Body fat accumulation was much lower in chylomicron-deficient animals; thus, whole-body absolute DNL was significantly lower. We conclude that plasma and hepatic TG pools and hepatic secretion of apoB-containing particles are maintained at normal levels in chylomicron-deficient mice, not by de no v o fatty acid synthesis, but by more avid re-esterification of plasma NEFA, replacing the normally predominant contribution from chylomicrons, and that some dietary fat can be absorbed by apoB-independent mechanisms.