Regulation of surfactant-like particle secretion by Caco-2 cells

Surfactant-like particle (SLP) is a phosphatidylcholine (PC)-rich membrane produced in the small intestine, and its secretion is increased by fat feed ing. In Caco-2 cells known to produce SLP, preincubation with [H-3]palmitat e labelled the SLP and was used as a marker for newly secreted membrane. SL P-associated PC and protein (d = 1.07-1.08 g/ml in a linear non-equilibrium NaBr gradient) were secreted in parallel with triacylglycerols (TG) and at a rate about twice the control rate in response to feeding cells with an o leate/egg PC mixture. Cholesterol and apolipoprotein A-I identified only a small peak corresponding to high-density lipoprotein (HDL), but the largest peak corresponded with SLP (d = 1.07-1.08). Palmitate incorporation into P C showed a similar small peak migrating at the density of HDL, but most lab elled PC secreted from the cells was due to SLP. PC secretion, alkaline pho sphatase activity, and newly synthesized immunoprecipitated SLP proteins fr om conditioned serum-free media migrated together at a density of greater t han or equal to1.21 g/ml in a lipoprotein NaBr step gradient, and represent ed SLP. Glycerol incorporated into TG migrated at a peak density of 1.12 g/ ml, consistent with HDL secretion from cells incubated in serum-free media. These data confirm that the secreted PC in SLP is distinct from lipoprotei n particles. Incorporation of [H-3]palmitate into the PC fraction of either whole cell homogenate or isolated brush border membranes was not affected by oleate/egg PC feeding. Both Pluronic L-81, an inhibitor of chylomicron s ecretion, and BMS-197636-02, a microsomal triglyceride transfer protein inh ibitor, blocked the secretion of both TG and PC. Elevation of intracellular cAMP levels that stimulate surfactant secretion from type II pneumocytes c aused a 50% reduction in SLP and TG secretion from Caco-2 cells. These resu lts confirm the SLP response to fat feeding found in vivo, further supporti ng a role for SLP in TG secretion from the enterocyte, and show that the re gulation of SLP secretion differs from that of pulmonary surfactant. (C) 20 01 Elsevier Science B.V. All rights reserved.