[Show abstract][Hide abstract] ABSTRACT: The rate-limiting step in the transit of absorbed dietary fat across the enterocyte is the generation of the pre-chylomicron transport vesicle (PCTV) from the endoplasmic reticulum (ER). This vesicle does not require coatomer-II (COPII) proteins for budding from the ER membrane and contains vesicle-associated membrane protein 7, found in intestinal ER, which is a unique intracellular location for this SNARE protein. We wished to identify the protein(s) responsible for budding this vesicle from ER membranes in the absence of the requirement for COPII proteins. We chromatographed rat intestinal cytosol on Sephacryl S-100 and found that PCTV budding activity appeared in the low molecular weight fractions. Additional chromatographic steps produced a single major and several minor bands on SDS-PAGE. By tandem mass spectroscopy, the bands contained both liver and intestinal fatty acid-binding proteins (L- and I-FABP) as well as four other proteins. Recombinant proteins for each of the six proteins identified were tested for PCTV budding activity; only L-FABP and I-FABP (23% the activity of L-FABP) were active. The vesicles generated by L-FABP were sealed, contained apolipoproteins B48 and AIV, were of the same size as PCTV on Sepharose CL-6B, and by electron microscopy, excluded calnexin and calreticulin but did not fuse with cis-Golgi nor did L-FABP generate COPII-dependent vesicles. Gene-disrupted L-FABP mouse cytosol had 60% the activity of wild type mouse cytosol. We conclude that L-FABP can select cargo for and bud PCTV from intestinal ER membranes.
[Show abstract][Hide abstract] ABSTRACT: Dietary long chain fatty acids are absorbed in the intestine, esterified to triacylglycerol, and packaged in the unique lipoprotein of the intestine, the chylomicron. The rate-limiting step in the transit of chylomicrons through the enterocyte is the exit of chylomicrons from the endoplasmic reticulum in prechylomicron transport vesicles (PCTV) that transport chylomicrons to the cis-Golgi. Because chylomicrons are 250 nm in average diameter and lipid absorption is intermittent, we postulated that a unique SNARE pairing would be utilized to fuse PCTV with their target membrane, cis-Golgi. PCTV loaded with [(3)H]triacylglycerol were incubated with cis-Golgi and were separated from the Golgi by a sucrose step gradient. PCTV-chylomicrons acquire apolipoprotein-AI (apoAI) only after fusion with the Golgi. PCTV became isodense with Golgi upon incubation and were considered fused when their cargo chylomicrons acquired apoAI but docked when they did not. PCTV, docked with cis-Golgi, were solubilized in 2% Triton X-100, and proteins were immunoprecipitated using VAMP7 or rBet1 antibodies. In both cases, a 112-kDa complex was identified in nonboiled samples that dissociated upon boiling. The constituents of the complex were VAMP7, syntaxin 5, vti1a, and rBet1. Antibodies to each SNARE component significantly inhibited fusion of PCTV with cis-Golgi. Membrin, Sec22b, and Ykt6 were not found in the 112-kDa complex. We conclude that the PCTV-cis-Golgi SNARE complex is composed of VAMP7, syntaxin 5, Bet1, and vti1a.
[Show abstract][Hide abstract] ABSTRACT: Intestinal dietary triacylglycerol absorption is a multi-step process. Triacylglycerol exit from the endoplasmic reticulum (ER) is the rate-limiting step in the progress of the lipid from its apical absorption to its basolateral membrane export. Triacylglycerol is transported from the ER to the cis Golgi in a specialized vesicle, the pre-chylomicron transport vesicle (PCTV). The vesicle-associated membrane protein 7 (VAMP7) was found to be more concentrated on PCTVs compared with ER membranes. VAMP7 has been previously identified associated with post-Golgi sites in eukaryotes. To examine the potential role of VAMP7 in PCTV trafficking, antibodies were generated that identified a 25 kDa band consistent with VAMP7 but did not crossreact with VAMP1,2. VAMP7 was concentrated on intestinal ER by immunofluorescence microscopy. Immunoelectron microscopy showed that the ER proteins Sar1 and rBet1 were present on PCTVs and colocalized with VAMP7. Iodixanol gradient centrifugation showed VAMP7 to be isodense with ER and endosomes. Although VAMP7 localized to intestinal ER, it was not present in the ER of liver and kidney. Anti-VAMP7 antibodies reduced the transfer of triacylglycerol, but not newly synthesized proteins, from the ER to the Golgi by 85%. We conclude that VAMP7 is enriched in intestinal ER and that it plays a functional role in the delivery of triacylglycerol from the ER to the Golgi.
[Show abstract][Hide abstract] ABSTRACT: Intestinal apolipoprotein A-IV expression is highly regulated by dietary lipid in newborn swine, suggesting a role in lipid absorption. Constitutive overexpression of apoA-IV in newborn swine enterocytes enhances basolateral secretion of triacylglycerol (TG) in TG-rich lipoproteins 4.9-fold (Lu, S., Yao, Y., Meng, S., Cheng, X., and Black, D. D. (2002) J. Biol. Chem. 277, 31929-31937). To investigate the mechanism of this enhancement, IPEC-1 cells were transfected with a tetracycline-regulatable expression system (Tet-On). In cells incubated with oleic acid, a dose response relationship was observed between medium doxycycline concentration and basolateral apoA-IV and TG secretion. Similarly regulated expression of apoA-I did not enhance lipid secretion. The mean diameter of TG-rich lipoproteins secreted from doxycycline-treated cells was larger than from untreated cells (87.0 nm versus 53.4 nm). Basolateral apoB secretion decreased. Using the same expression system, full-length human apoA-IV (376 amino acids); a "pig-like" human apoA-IV, lacking the C-terminal EQQQ repeats (361 amino acids); and a "chicken-like" apoA-IV, further truncated to 343 amino acids, were expressed in IPEC-1 cells. With increasing protein secretion, cells expressing the full-length human apoA-IV displayed a 2-fold increase in TG secretion; in sharp contrast, cells expressing the pig-like human apoA-IV displayed a 25-fold increase in TG secretion and a 27-fold increase in lipoprotein diameter. When human apoA-IV was further truncated to yield a chicken-like protein, TG secretion was inhibited. We conclude that overexpression of swine apoA-IV enhances basolateral TG secretion in a dose-dependent manner by increasing the size of secreted lipoproteins. These data suggest that the region in the human apoA-IV protein from residues 344 to 354 is critical to its ability to enhance lipid secretion, perhaps by enabling the packaging of additional core TG into chylomicron particles. The EQQQ-rich region may play an inhibitory or modulatory role in chylomicron packaging in humans.
[Show abstract][Hide abstract] ABSTRACT: The budding of vesicles from endoplasmic reticulum (ER) that contains nascent proteins is regulated by COPII proteins. The mechanisms that regulate lipid-carrying pre-chylomicron transport vesicles (PCTVs) budding from the ER are unknown. To study the dependence of PCTV-ER budding on COPII proteins we examined protein and PCTV budding by using ER prepared from rat small intestinal mucosal cells prelabeled with (3)H-oleate or (14)C-oleate and (3)H-leucine. Budded (3)H-oleate-containing PCTVs were separated by sucrose density centrifugation and were revealed by electron microscopy as 142-500 nm vesicles. Our results showed the following: (1) Proteinase K treatment did not degrade the PCTV cargo protein, apolipoprotein B-48, unless Triton X-100 was added. (2) PCTV budding was dependent on cytosol and ATP. (3) The COPII proteins Sar1, Sec24 and Sec13/31 and the membrane proteins syntaxin 5 and rBet1 were associated with PCTVs. (4) Isolated PCTVs were able to fuse with intestinal Golgi. (5) Antibodies to Sar1 completely inhibited protein vesicle budding but increased the generation of PCTV; these changes were reversed by the addition of recombinant Sar1. (6) PCTVs formed in the absence of Sar1 did not contain the COPII proteins Sar1, Sec24 or Sec31 and did not fuse with the Golgi complex. Together, these findings suggest that COPII proteins may not be required for the exit of membrane-bound chylomicrons from the ER but that they or other proteins may be necessary for PCTV fusion with the Golgi.
[Show abstract][Hide abstract] ABSTRACT: We identified the enzyme responsible for alkaline lipolysis in mucosa of rat small intestine. RT-PCR was used to amplify a transcript that, by cloning and sequencing, is identical to pancreatic triacylglycerol lipase. In rats fed normal laboratory chow, pancreatic triacylglycerol lipase mRNA was detected in all four quarters of the small intestine, with the first quarter expressing about three times as much of this transcript as was found in the more distal three-quarters combined. Both acutely and chronically administered dietary fat were shown to regulate pancreatic triacylglycerol lipase mRNA expression and lipase activity. The synthesis of pancreatic triacylglycerol lipase protein by the small intestine was demonstrated by in vivo radiolabeling experiments using [(35)S]methionine/cysteine followed by immunoprecipitation with an anti-pancreatic triacylglycerol lipase antibody. Immunohistochemical studies suggest that pancreatic triacylglycerol lipase protein expression is restricted to enterocytes throughout the small intestine. To our knowledge, this is the first report identifying rat small intestinal mucosa as a site of pancreatic triacylglycerol lipase synthesis and the first demonstration of its modulation in the mucosa by dietary fat. We propose that pancreatic triacylglycerol lipase is used by the intestine to hydrolyze the mucosal triacylglycerol that is not transported in chylomicrons.