Intracellular events in the assembly of chylomicrons in rabbit enterocytes

The aim of this study was to determine the intra- cellular events in chylomicron assembly in adult villus en- terocytes. We have used novel methods for separation of the intracellular components of the secretory compartment (rough and smooth endoplasmic reticulum (RER and SER, respectively) and Golgi), and their membrane and luminal components, from villus enterocytes isolated from rabbit small intestine. The steady state composition of the compo- nents of the secretory compartment and the intracellular pools of newly synthesized apolipoprotein B-48 (apoB-48) and triacylglycerol (TAG) was determined. The observa- tions indicate that the SER is the main site of TAG synthesis and of chylomicron assembly. Newly synthesized apoB-48 and TAG accumulate in the SER membrane and are trans- ferred into the lumen in a microsomal triglyceride transfer protein-dependent step. In enterocytes isolated from chow- fed rabbits, in which fat absorption is relatively slow, trans- fer of apoB-48 and TAG from the SER membrane into the lumen appears to be rate limiting. In enterocytes from fat- fed rabbits, TAG accumulates in the lumen of the SER, sug- gesting that movement out of the SER lumen becomes rate limiting, when chylomicron secretion is markedly stimu- lated. In these cells, the cytosolic TAG also increased to 450 m g/g enterocytes, compared with 12 m g/g enterocytes from chow-fed rabbits, indicating that transfer of TAG from the SER membrane into the secretory pathway can become satu- rated, so that newly synthesized TAG moves into the cytosol. — Cartwright, I. J., D. Plonne, and J. A. Higgins. In- tracellular events in the assembly of chylomicrons in rabbit enterocytes. J. Lipid Res. 2000. 41: 1728-1739.