Abnormal intracellular lipid processing contributes to fat malabsorption in cystic fibrosis patients

Abstract

A common feature of cystic fibrosis (CF) is the functional derangement of the exocrine pancreas, which affects output of pancreatic lipase. This condition results in severe dietary malabsorption due to the poor hydrolysis of triacylglycerol (TG) in the lumen of the small intestine. Despite the benefits of pancreatic enzyme supplements, patients with CF present with persistent intestinal fat malabsorption. The aim of the present investigation was to determine whether defects in the intracellular phase of lipid transport occur in this pathophysiology in addition to the known disturbed digestive processes. Our hypothesis was tested by incubating intestinal biopsies from six CF and six healthy subjects with radiolabeled lipid and protein precursors. Lipid esterification and secretion were markedly decreased by 22-31% and 38-42%, respectively, in CF samples, as noted by the low incorporation of [(14)C]palmitic acid into TGs, phospholipids, and cholesteryl esters in patients' duodenal explants and culture media compared with controls (100%). Accordingly, the output of TG-rich lipoproteins was substantially reduced (P < 0.05), and a similar trend was observed for high-density lipoproteins. Because intestinal lipoprotein assembly/secretion shows an absolute requirement for apolipoprotein (apo) B-48, radioactive labeling experiments were performed; these experiments demonstrated a significantly (P < 0.05) diminished synthesis of apoB-48 (40%) and apoA-I (30%). Given the critical role of microsomal triglyceride transfer protein in the formation of apoB-containing lipoproteins, its activity was determined and not found to be altered in CF intestinal tissue. Together, these results suggest that CF malabsorption may also be caused by defects in mucosal mechanisms leading to abnormal lipoprotein delivery into the blood circulation.