Control of chylomicron export from the intestine

The control of chylomicron output by the intestine is a complex process whose outlines have only recently come into focus. In this review we will cover aspects of chylomicron formation and prechylomicron vesicle generation that elucidate potential control points. Substrate (dietary fatty acids and monoacylglycerols) availability is directly related to the output rate of chylomicrons. These substrates must be converted to triacylglycerol before packaging in prechylomicrons by a series of endoplasmic reticulum (ER)-localized acylating enzymes that rapidly convert fatty acids and monoacylglycerols to triacylglycerol. The packaging of the prechylomicron with triacylglycerol is controlled by the microsomal triglyceride transport protein, another potential limiting step. The prechylomicrons, once loaded with triacylglycerol, are ready to be incorporated into the prechylomicron transport vesicle that transports the prechylomicron from the ER to the Golgi. Control of this exit step from the ER, the rate-limiting step in the transcellular movement of the triacylglycerol, is a multistep process involving the activation of PKCζ, the phosphorylation of Sar1b, releasing the liver fatty acid binding protein from a heteroquatromeric complex, which enables it to bind to the ER and organize the prechylomicron transport vesicle budding complex. We propose that control of PKCζ activation is the major physiological regulator of chylomicron output.

Dietary and biliary phosphatidylcholine activates PKCζ in rat intestine

Chylomicron output by the intestine is proportional to intestinal phosphatidylcholine (PC) delivery. Using five different variations of PC delivery to the intestine, we found that lyso-phosphatidylcholine (lyso-PC), the absorbed form of PC, concentrations in the cytosol (0 to 0.45 nM) were proportional to the input rate. The activity of protein kinase C (PKC)ζ, which controls prechylomicron output rate by the endoplasmic reticulum (ER), correlated with the lyso-PC concentration suggesting that it may be a PKCζ activator. Using recombinant PKCζ, the Km for lyso-PC activation was 1.49 nM and the Vmax 1.12 nM, more than the maximal lyso-PC concentration in cytosol, 0.45 nM. Among the phospholipids and their lyso derivatives, lyso-PC was the most potent activator of PKCζ and the only one whose cytosolic concentration suggested that it could be a physiological activator because other phospholipid concentrations were negligible. PKCζ was on the surface of the dietary fatty acid transport vesicle, the caveolin-1-containing endocytic vesicle. Once activated, PKCζ, eluted off the vesicle. A conformational change in PKCζ on activation was suggested by limited proteolysis. We conclude that PKCζ on activation changes its conformation resulting in elution from its vesicle. The downstream effect of dietary PC is to activate PKCζ, resulting in greater chylomicron output by the ER.

Intestinal caveolin-1 is important for dietary fatty acid absorption

How dietary fatty acids are absorbed into the enterocyte and transported to the ER is not established. We tested the possibility that caveolin-1 containing lipid rafts and endocytic vesicles were involved. Apical brush border membranes took up 15% of albumin bound (3)H-oleate whereas brush border membranes from caveolin-1 KO mice took up only 1%. In brush border membranes, the (3)H-oleate was in the detergent resistant fraction of an OptiPrep gradient. On OptiPrep gradients of intestinal cytosol, we also found the (3)H-oleate in the detergent resistant fraction, separate from OptiPrep gradients spiked with (3)H-oleate or (3)H-triacylglycerol. Caveolin-1 immuno-depletion of cytosol removed 91% of absorbed (3)H-oleate whereas immuno-depletion using IgG, or anti-caveolin-2 or -3 or anti-clathrin antibodies removed 20%. Electron microscopy showed the presence of caveolin-1 containing vesicles in WT mouse cytosol that were 4 fold increased by feeding intestinal sacs 1mM oleate. No vesicles were seen in caveolin-1 KO mouse cytosol. Caveolin-1 KO mice gained less weight on a 23% fat diet and had increased fat in their stool compared to WT mice. We conclude that dietary fatty acids are absorbed by caveolae in enterocyte brush border membranes, are endocytosed, and transported in cytosol in caveolin-1 containing endocytic vesicles.

Phosphorylation of Sar1b protein releases liver fatty acid-binding protein from multiprotein complex in intestinal cytosol enabling it to bind to endoplasmic reticulum (ER) and bud the pre-chylomicron transport vesicle

Native cytosol requires ATP to initiate the budding of the pre-chylomicron transport vesicle from intestinal endoplasmic reticulum (ER). When FABP1 alone is used, no ATP is needed. Here, we test the hypothesis that in native cytosol FABP1 is present in a multiprotein complex that prevents FABP1 binding to the ER unless the complex is phosphorylated. We found on chromatography of native intestinal cytosol over a Sephacryl S-100 HR column that FABP1 (14 kDa) eluted in a volume suggesting a 75-kDa protein complex that contained four proteins on an anti-FABP1 antibody pulldown. The FABP1-containing column fractions were chromatographed over an anti-FABP1 antibody adsorption column. Proteins co-eluted from the column were identified as FABP1, Sar1b, Sec13, and small VCP/p97-interactive protein by immunoblot, LC-MS/MS, and MALDI-TOF. The four proteins of the complex had a total mass of 77 kDa and migrated on native PAGE at 75 kDa. When the complex was incubated with intestinal ER, there was no increase in FABP1-ER binding. However, when the complex member Sar1b was phosphorylated by PKCζ and ATP, the complex completely disassembled into its component proteins that migrated at their monomer molecular weight on native PAGE. FABP1, freed from the complex, was now able to bind to intestinal ER and generate the pre-chylomicron transport vesicle (PCTV). No increase in ER binding or PCTV generation was observed in the absence of PKCζ or ATP. We conclude that phosphorylation of Sar1b disrupts the FABP1-containing four-membered 75-kDa protein complex in cytosol enabling it to bind to the ER and generate PCTV.

The biogenesis of chylomicrons

The absorption of dietary fat is of increasing concern given the rise of obesity not only in the United States but throughout the developed world. This review explores what happens to dietary fat within the enterocyte. Absorbed fatty acids and monoacylglycerols are required to be bound to intracellular proteins and/or to be rapidly converted to triacylglycerols to prevent cellular membrane disruption. The triacylglycerol produced at the level of the endoplasmic reticulum (ER) is either incorporated into prechylomicrons within the ER lumen or shunted to triacylglycerol storage pools. The prechylomicrons exit the ER in a specialized transport vesicle in the rate-limiting step in the intracellular transit of triacylglycerol across the enterocyte. The prechylomicrons are further processed in the Golgi and are transported to the basolateral membrane via a separate vesicular system for exocytosis into the intestinal lamina propria. Fatty acids and monoacylglycerols entering the enterocyte via the basolateral membrane are also incorporated into triacylglycerol, but the basolaterally entering lipid is much more likely to enter the triacylglycerol storage pool than the lipid entering via the apical membrane.

A novel multiprotein complex is required to generate the prechylomicron transport vesicle from intestinal ER

Dietary lipid absorption is dependent on chylomicron production whose rate-limiting step across the intestinal absorptive cell is the exit of chylomicrons from the endoplasmic reticulum (ER) in its ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). This study addresses the composition of the budding complex for PCTV. Immunoprecipitation (IP) studies from rat intestinal ER solubilized in Triton X-100 suggested that vesicle-associated membrane protein 7 (VAMP7), apolipoprotein B48 (apoB48), liver fatty acid-binding protein (L-FABP), CD36, and the COPII proteins were associated on incubation of the ER with cytosol and ATP. This association was confirmed by chromatography of the solubilized ER over Sephacryl S400-HR in which these constituents cochromatographed with an apparent kDa of 630. No multiprotein complex was detected when the ER was chromatographed in the absence of PCTV budding activity (resting ER or PKCzeta depletion of ER and cytosol). Treatment of the ER with anti-apoB48 or anti-VAMP7 antibodies or using gene disrupted L-FABP or CD36 mice all significantly inhibited PCTV generation. A smaller complex (no COPII proteins) was formed when only rL-FABP was used to bud PCTV. The data support the conclusion that the PCTV budding complex in intestinal ER is composed of VAMP7, apoB48, CD36, and L-FABP, plus the COPII proteins.

Sec24C is required for docking the prechylomicron transport vesicle with the Golgi

The rate-limiting step in the transit of dietary fat across the intestinal absorptive cell is its exit from the endoplasmic reticulum (ER) in a specialized ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). PCTV bud off from the ER membranes and have unique features; they are the largest ER-derived vesicles (average diameter 250 nm), do not require GTP and COPII proteins for their formation, and utilize VAMP7 as a v-N-ethylmaleimide sensitive factor attachment protein receptor (SNARE). However, PCTV require COPII proteins for their fusion with the Golgi, suggesting a role for them in Golgi target recognition. In support of this, PCTV contained each of the five COPII proteins when docked with the Golgi. When PCTV were fused with the Golgi, the COPII proteins were present in greatly diminished amounts, indicating they had cycled back to the cytosol. Immuno-depletion of Sec31 from the cytosol did not affect PCTV-Golgi docking, but depletion of Sec23 resulted in a 25% decrease. Immuno-depletion of Sec24C caused a nearly complete cessation of PCTV docking activity, but on the addition of recombinant Sec24C, docking activity was restored. We conclude that the COPII proteins are present at docking of PCTV with the Golgi and that Sec24C is required for this event. Sec23 plays a less important role.

Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding SREBP-1c (sterol regulatory element-binding protein-1c). Nascent SREBP-1c is synthesized and embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi in coatomer protein II (COPII) vesicles where two sequential cleavages generate the transcriptionally active NH(2)-terminal fragment, nSREBP-1c. There is limited indirect evidence to suggest that insulin may also regulate the posttranslational processing of the nascent SREBP-1c protein. Therefore, we designed experiments to directly assess the action of insulin on the post-translational processing of epitope-tagged full-length SREBP-1c and SREBP-2 proteins expressed in cultured hepatocytes. We demonstrate that insulin treatment led to enhanced post-translational processing of SREBP-1c, which was associated with phosphorylation of ER-bound nascent SREBP-1c protein that increased affinity of the SREBP-1c cleavage-activating protein (SCAP)-SREBP-1c complex for the Sec23/24 proteins of the COPII vesicles. Furthermore, chemical and molecular inhibitors of the phosphoinositide 3-kinase pathway and its downstream kinase protein kinase B (PKB)/Akt prevented both insulin-mediated phosphorylation of nascent SREBP-1c protein and its posttranslational processing. Insulin had no effect on the proteolysis of nascent SREBP-2 under identical conditions. We also show that in vitro incubation of an active PKB/Akt enzyme with recombinant full-length SREBP-1c led to its phosphorylation. Thus, insulin selectively stimulates the processing of SREBP-1c in rat hepatocytes by enhancing the association between the SCAP-SREBP-1c complex and COPII proteins and subsequent ER to Golgi transport and proteolytic cleavage. This effect of insulin is tightly linked to phosphoinositide 3-kinase and PKB/Akt-dependent serine phosphorylation of the precursor SREBP-1c protein.

PKC zeta-mediated phosphorylation controls budding of the pre-chylomicron transport vesicle

Dietary triacylglycerols are absorbed by enterocytes and packaged in the endoplasmic reticulum (ER) in the intestinal specific lipoprotein, the chylomicron, for export into mesenteric lymph. Chylomicrons exit the ER in an ER-to-Golgi transport vesicle, the pre-chylomicron transport vesicle (PCTV), which is the rate-limiting step in the transit of chylomicrons across the cell. Here, we focus on potential mechanisms of control of the PCTV-budding step from the intestinal ER. We incubated intestinal ER with intestinal cytosol and ATP to cause PCTV budding. The budding reaction was inhibited by 60 nM of the PKC inhibitor Gö 6983, suggesting the importance of PKCzeta in the generation of PCTV. Immunodepletion of PKCzeta from the cytosol and the use of washed ER greatly inhibited the generation of PCTVs, but was restored following the addition of recombinant PKCzeta. Intestinal ER incubated with intestinal cytosol and [gamma-(32)P]ATP under conditions supporting the generation of PCTVs showed the phosphorylation of a 9-kDa band following autoradiography. The phosphorylation of this protein correlated with the generation of PCTVs but not the formation of protein vesicles and was inhibited by depletion of PKCzeta. Phosphorylation of the 9-kDa protein was restored following the addition of recombinant PKCzeta. The association of the 9-kDa protein with proteins that are important for PCTV budding was phosphorylation dependent. We conclude that PKCzeta activity is required for PCTV budding from intestinal ER, and is associated with phosphorylation of a 9-kDa protein that might regulate PCTV budding.

Development and physiological regulation of intestinal lipid absorption. II. Dietary lipid absorption, complex lipid synthesis, and the intracellular packaging and secretion of chylomicrons

Research in dietary fat absorption has developed urgency because of the widely recognized epidemic of obesity in the United States. Despite its clinical importance, many controversies exist over some of the basic aspects of this process from the mechanisms of fatty acid uptake to the control of triacylglycerol export in chylomicrons. Recent advances have included the identification of a number of fatty acid transporters, the discovery of families of acyl-CoA synthetase long chains and acyltransferases, a physiological function for liver-fatty acid binding protein, and the characterization of the prechylomicron transport vesicle transporting chylomicrons from the endoplasmic reticulum to the Golgi.

Liver fatty acid-binding protein initiates budding of pre-chylomicron transport vesicles from intestinal endoplasmic reticulum

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.

Vesicle-associated membrane protein 7 is expressed in intestinal ER

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.

The identification of a novel endoplasmic reticulum to Golgi SNARE complex used by the prechylomicron transport vesicle

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.

A GPI-anchored co-receptor for tissue factor pathway inhibitor controls its intracellular trafficking and cell surface expression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) lacks a membrane attachment signal but it remains associated with the endothelial surface via its association with an, as yet, unidentified glycosyl phosphatidylinositol (GPI)-anchored co-receptor.

OBJECTIVES/METHODS

Cellular trafficking of TFPI within aerolysin-resistant ECV304 and EA.hy926 cells, which do not express GPI-anchored proteins on their surface, was compared with their wild-type counterparts.

RESULTS AND CONCLUSIONS

Although aerolysin-resistant cells produce normal amounts of TFPI mRNA, TFPI is not expressed on the cell surface and total cellular TFPI is greatly decreased compared with wild-type cells. Additionally, normal, not increased, amounts of TFPI are secreted into conditioned media indicating that TFPI is degraded within the aerolysin-resistant cells. Confocal microscopy and studies using metabolic inhibitors demonstrate that aerolysin-resistant cells produce TFPI and transport it into the Golgi with subsequent degradation in lysosomes. The experimental results provide no evidence that cell surface TFPI originates from secreted TFPI that binds back to a GPI-anchored protein. Instead, the data suggest that TFPI tightly, but reversibly, binds to a GPI anchored co-receptor in the ER/Golgi. The co-receptor then acts as a molecular chaperone for TFPI by trafficking it to the cell surface of wild-type cells or to lysosomes of aerolysin-resistant cells. TFPI that escapes co-receptor binding is secreted through the same pathway in both wild-type and aerolysin-resistant cells. The data provide a framework for understanding how TFPI is expressed on endothelium.

Overexpression of apolipoprotein A-IV enhances lipid secretion in IPEC-1 cells by increasing chylomicron size

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.

Acetaldehyde disrupts tight junctions and adherens junctions in human colonic mucosa: protection by EGF and L-glutamine

Acetaldehyde, a toxic metabolite of ethanol oxidation, is suggested to play a role in the increased risk for gastrointestinal cancers in alcoholics. In the present study, the effect of acetaldehyde on tyrosine phosphorylation, immunofluorescence localization, and detergent-insoluble fractions of the tight junction and the adherens junction proteins was determined in the human colonic mucosa. The role of EGF and L-glutamine in prevention of acetaldehyde-induced effects was also evaluated. Acetaldehyde reduced the protein tyrosine phosphatase activity, thereby increasing the tyrosine phosphorylation of occludin, E-cadherin, and beta-catenin. The levels of occludin, zonula occludens-1, E-cadherin, and beta-catenin in detergent-insoluble fractions were reduced by acetaldehyde, while it increased their levels in detergent-soluble fractions. Pretreatment with EGF or L-glutamine prevented acetaldehyde-induced protein tyrosine phosphorylation, redistribution from intercellular junctions, and reduction in the levels of detergent-insoluble fractions of occludin, zonula occludens-1, E-cadherin, and beta-catenin. These results demonstrate that acetaldehyde induces tyrosine phosphorylation and disrupts tight junction and adherens junction in human colonic mucosa, which can be prevented by EGF and glutamine.

Inhibition of carnitine palmitoyltransferase in the rat small intestine reduces export of triacylglycerol into the lymph

Following digestion of dietary triacylglycerol (TAG), intestinal epithelial cells absorb fatty acids and monoacylglycerols that are resynthesized into TAG by enzymes located on the endoplasmic reticulum (ER). A study in rat liver (Abo-Hashema, K. A., M. H. Cake, G. W. Power, and D. J. Clarke. 1999. Evidence for TAG synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases. J. Biol. Chem. 274: 35577-35582) showed that there is a carnitine-dependent ER lumenal synthesis of TAG. We wanted to test the hypothesis that a similar pathway was present in rat intestine by utilizing etomoxir, a specific inhibitor of carnitine palmitoyltransferase (CPT). Intraduodenal infusion of etomoxir inhibited CPT activity in the ER by 69%. Etomoxir did not affect either the uptake of intraduodenally infused [3H]glyceryltrioleate by the intestinal mucosa or the production of mucosal [3H]TAG, excluding the possibility that etomoxir interfered with TAG absorption or synthesis. Etomoxir did not inhibit protein synthesis, glucose, cholesterol or palmitate absorption or metabolism, or ATP concentrations. Etomoxir substantially (74%) diminished lymph TAG output from intralumenally infused glyceryltrioleate. In conclusion, these data strongly support the hypothesis that an ER CPT system exists and is necessary for processing dietary TAG into chylomicrons. The significant reduction in lymphatic output of chylomicron TAG on etomoxir treatment suggests that the major source of chylomicron TAG is a diacylglyceroltransferase on the lumenal surface of the ER.

COPII proteins are required for Golgi fusion but not for endoplasmic reticulum budding of the pre-chylomicron transport vesicle

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.

Regulation of MTP expression in developing swine

To define the developmental expression of microsomal triglyceride transfer protein (MTP) large subunit mRNA and protein, samples of small intestine and liver were collected from 40-day gestation fetal, 2-day-old newborn, 3-week-old suckling, and 2-month-old weanling swine. In fetal animals, MTP mRNA expression was high in intestine and liver. Postnatally, jejunal expression paralleled the intake of a high-fat breast milk diet and declined after weaning. Ileal expression was comparable with that of jejunum in 2-day-old animals, but declined to low levels afterward. Hepatic expression declined postnatally and remained low. MTP protein expression generally paralleled mRNA expression, except in fetal intestine in which no 97 kDa protein was detected. In 2-day-old piglets, a high-triacylglycerol diet increased jejunal and ileal MTP mRNA levels, as compared to a low-triacylglycerol diet. To test the roles of glucocorticoids and fatty acids in MTP regulation, a newborn swine enterocyte cell line (IPEC-1) was used. Except at day 2 of differentiation, dexamethasone did not influence MTP expression. Fatty acids either up-regulated or down-regulated MTP expression, depending on the specific fatty acid and duration of exposure. Although programmed genetic cues regulate MTP expression during development, clearly the amount and fatty acid composition of dietary lipid also play regulatory roles.

Nutrient absorption

Many advances in the study of nutrient absorption have been made with the use of molecular and genetic techniques; however, standard in vivo studies have provided interesting and important new information. Omega-3 long-chain fatty acids have unexpected effects on lipoprotein formation and secretion in neonatal intestinal cells; this needs to be considered in the modification of infant formulas. Rexinoids affect intestinal cholesterol homeostasis via two receptors: retinoic acid receptor/liver X receptor (cholesterol efflux to lumen) and retinoic acid receptor/farnesoid X receptor (cholesterol catabolism). Absorption of the antioxidant plant polyphenol quercetin involves interaction with the glucose transporter and deglycolsylation and conjugation reactions. Cells of the polarized human colon cancer cell line, CaCo-2, take up phenylalanine by two mechanisms: passive uptake across the basolateral membrane, and temperature-dependent transcellular movement from apical to basolateral media. Absorption of vitamins A and E is markedly enhanced in normal and damaged intestine by the administration of restructured triacylglycerols derived from fish oil and medium-chain fatty acids. Surprisingly, dietary protein and phosphorus apparently have no significant effect on the efficiency of calcium absorption in adult women. Finally, many studies examined a variety of genes that regulate iron absorption and homeostasis.

The intestine expresses pancreatic triacylglycerol lipase: regulation by dietary lipid

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.

Effect of increasing lipid loads on the ability of the endoplasmic reticulum to transport lipid to the Golgi

We have previously shown (Mansbach, C. M. and P. J. Nevin, 1998. J. Lipid Res. 39: 963;-968) that after the development of a mass steady state with respect to triacylglycerol absorption in rats, the introduction of radiolabeled trioleoylglycerol, while maintaining the input rate of trioleoylglycerol constant at 135 micromol/h, was followed by a slow (60 min) achievement of a radiolabel steady state in the intestinal endoplasmic reticulum (ER). We hypothesized that this was due to the large input load and that the time to steady state would be shorter at lower lipid loads. Rats were infused intraduodenally with 22.5, 45, 90, or 135 micromol trioleoylglycerol/h for 6 h to obtain a mass steady state in the intestine. [(3)H]trioleoylglycerol was added to the infusate and the ER and Golgi were isolated from the proximal intestine after 5;-60 min of radiolabel infusion. The time required to reach a radiolabel steady state in the ER lengthened from 10 min at the 22.5 micromol/h infusion rate to 60 min at the 135 micromol/h rate. Similar data were obtained for the Golgi. Incubation of the ER with lipase reduced the ER;-triacylglycerol amount by 43% and increased its specific activity by 73%. The amount of [(3)H]TG-dpm in the ER was not reduced unless taurocholate, 10 mm, and colipase were added. We conclude that as the rate of triacylglycerol infusion is increased, TG movement from the ER to the Golgi progressively lengthens until finally all the triacylglycerol infused cannot be transported. A portion of this triacylglycerol is disposed on the cytoplasmic face of the ER and thus able to be attacked by lipase whereas another fraction is sequestered in the ER lumen and immune to lipase attack unless the ER membrane is solubilized.

Nutrient absorption

Some key advances occurred last year in understanding mechanisms involved in nutrient absorption. A novel "prechylomicron transport vesicle" was identified; its movement to the Golgi is the rate-limiting step for triacylglycerol absorption. A scavenger receptor (type BI) in the brush border membrane appears to facilitate cholesterol uptake. Several studies define mechanisms for gastrointestinal peptide hormone stimulation of glucose uptake. An oligopeptide transporter, PepT1, is transcriptionally upregulated by certain dietary amino acids and dipeptides. Surprisingly, both insulin and fasting double the maximum velocity for dipeptide uptake (via PepT1), but they act by different mechanisms. Three transporters, SMVT (sodium-dependent multivitamin transporter for biotin and pantothenate), SVCT (for vitamin C), and CaT1 (for Ca uptake from the lumen) have been cloned and are active when expressed in various cells. Additional studies provide insights on Ca absorption and vitamin D action in aging, estrogen deficiency, and adaptation to a low Ca diet. Nramp2, also called DMT1 (divalent metal ion transporter), seems to be a major regulator of transferrin-independent, nonheme iron uptake. Finally, the protein HFE associates with the transferrin receptor and is part of an iron-sensing mechanism that regulates iron absorption. It is defective in hereditary hemochromatosis. HFE and Nramp2 (DMT1) genes are reciprocally regulated.

Nutrient absorption

Interesting advances occurred recently in nutrient absorption. Kinetics of triacylglycerol appearance in endoplasmic reticulum, Golgi apparatus, and lymph support the hypothesis that endoplasmic reticulum-to-Golgi transport is rate-limiting for lipid absorption. Apolipoprotein B does not appear necessary for initial formation of chylomicron-sized lipid particles in the endoplasmic reticulum, but rather for their movement out of the endoplasmic reticulum and to the Golgi. If peptides are protected from luminal proteolysis by fatty acylation, or if a nonpeptide drug, acyclovir, is esterified with valine to enhance bioavailability, the peptides nevertheless are absorbed by peptide transporters. Experimental conditions needed to use human ileal mucosa for in vitro absorption studies are described. Intestinal mucosa contains leptin receptors, and leptin inhibits galactose absorption, suggesting a new site for leptin's modulation of body mass. The enhancer element for the apoB gene is located much farther from its structural gene in the intestine than in the liver.

Nutrient absorption

Interesting advances occurred recently in nutrient absorption. Kinetics of triacylglycerol appearance in endoplasmic reticulum, Golgi apparatus, and lymph support the hypothesis that endoplasmic reticulum-to-Golgi transport is rate-limiting for lipid absorption. Apolipoprotein B does not appear necessary for initial formation of chylomicron-sized lipid particles in the endoplasmic reticulum, but rather for their movement out of the endoplasmic reticulum and to the Golgi. If peptides are protected from luminal proteolysis by fatty acylation, or if a nonpeptide drug, acyclovir, is esterified with valine to enhance bioavailability, the peptides nevertheless are absorbed by peptide transporters. Experimental conditions needed to use human ileal mucosa for in vitro absorption studies are described. Intestinal mucosa contains leptin receptors, and leptin inhibits galactose absorption, suggesting a new site for leptin's modulation of body mass. The enhancer element for the apoB gene is located much farther from its structural gene in the intestine than in the liver.

Prechylomicron transport vesicle: isolation and partial characterization

The intestine is able to regulate its output rate of chylomicrons, the major intestinal triacylglycerol (TG) transport vehicle. We have proposed that a vesicle, transporting the developing chylomicron from the endoplasmic reticulum (ER) to the Golgi, is the rate-limiting step in the process of TG transit through the enterocyte [Am. J. Physiol. 273 (Gastrointest. Liver Physiol. 36): G18-G30, 1997]. We wished to isolate and characterize this vesicle. The apical portion of rat intestinal cells were avulsed, and the mucosa was stirred in buffer. The supernatant was centrifuged in two different sucrose gradients, and the top 2.5 ml of the last gradient were collected and concentrated. Electron microscopy showed a 200-nm vesicle. The vesicle contained immunoidentifiable apolipoprotein (apo) B48 and apo A-IV but very little apo A-I, although apo A-I was present in the ER and Golgi. [3H]TG-loaded vesicles delivered [3H]TG to the Golgi but not the ER. Marker enzyme assays also indicate that the isolated fraction is different from the ER and Golgi fractions. We conclude that we have isolated a vesicle that is post-ER but pre-Golgi that vectorially transports TG to the Golgi.

Intracellular movement of triacylglycerols in the intestine

The intestine can vary its triacylglycerol output rate depending on differing physiological conditions. The rate-limiting step in the complex process from fatty acid and monoacylglycerol entry to triacylglycerol export is unknown but suggested to be the transport of triacylglycerol from the endoplasmic reticulum to the Golgi. The present studies were carried out to test this hypothesis. The conversion rate of absorbed fatty acid to mucosal triacylglycerol was studied in rats infused intraduodenally with trioleoylglycerol, 135 micromol/h, for 6 h followed by [3H]oleate. In 30 sec, 79% of the mucosal 3H-labeled fatty acid was esterified to [3H]triacylglycerol. The increase in the 3H specific activity of triacylglycerol in the endoplasmic reticulum and Golgi was studied in similarly prepared rats except that the radio-label was [3H]trioleoylglycerol. The endoplasmic reticulum triacylglycerol specific activity was always less than that of the Golgi with a steady state not reached until 60 min of [3H]trioleoylglycerol infusion. The steady state of [3H]triacylglycerol in the lymph was not reached until 70 min of infusion. We conclude that the data are consistent with the rate-limiting step in intestinal triacylglycerol export being the movement of triacylglycerol from the endoplasmic reticulum to the Golgi as the conversion of absorbed fatty acid to triacylglycerol is rapid and the movement of triacylglycerol from the Golgi to the lymph is rapid as well.

Advances in nutrition and gastroenterology: summary of the 1997 A.S.P.E.N. Research Workshop

BACKGROUND

The 1997 A.S.P.E.N. Research Workshop was held at the annual meeting in San Francisco, on January 26, 1997. The workshop focused on advances in clinical and basic research involving the interface between nutrient and luminal gastroenterology.

METHODS

Presentations on the genetic regulation of gastrointestinal development, the molecular biology of small intestinal adaptation, the effect of nutrition support on intestinal mucosal mass, the relationship between nutrition and gastrointestinal motility, nutrient absorption, and gastrointestinal tract substrate metabolism were made by the preeminent leaders in the field.

RESULTS

The investigators presented an insightful analysis of each topic by reviewing data from their own laboratories and the published literature.

CONCLUSIONS

This workshop underscored the important interactions between nutrition and luminal gastroenterology at the basic science, metabolic/physiologic, and clinical levels. The integration of presentations from the different disciplines provided a unique interaction of information and ideas to advance our understanding of nutrition and gastrointestinal tract.

Determinants of triacylglycerol transport from the endoplasmic reticulum to the Golgi in intestine

The ability of the intestinal cell to export triacylglycerol (TG) is a physiologically regulatable function. The intracellular site where this occurs is unknown, although available evidence suggests that the step between the endoplasmic reticulum (ER) and the Golgi is the most likely. We studied this process in rat enterocytes that were isolated from the proximal intestine. A novel system was developed in which [3H]TG was transported from ER to the Golgi. This process was time, ATP, temperature, and cytosol dependent. The cytosolic factor(s) was heat and trypsin sensitive. TG transport was directly proportional to the amount of added nonradiolabeled acceptor Golgi. The rate of TG transported to the Golgi was the fastest in cells isolated from rats that had been intraduodenally infused in vivo with glyceryltrioleate (TO) plus phosphatidylcholine and slowest in cells isolated from bile-fistulated rats infused with TO in vivo compared with cells from in vivo TO-infused, bile duct intact rats, mimicking the relative transport rates seen in vivo. TG transport in vitro could not be quenched by adding TG emulsions, chylomicrons, liposomes, or guanosine 5'-O-(3-thiotriphosphate). Cytosol from the liver and kidney supported TG transport, but the Golgi from liver or kidney did not accept TG from intestinal ER. We conclude that an intestinally specific, active transport mechanism transports TG from the ER to the Golgi and that this might be a regulatory step in TG export from the intestinal cell.

Chylomicron remnant uptake by enterocytes is receptor dependent

During glyceryl trioleate absorption in the rat, mucosal triacylglycerol (TG) fatty acids have been shown to consist of only 71% exogenous oleate. Chylomicron remnants are enriched with endogenous TG fatty acids, compared with their parent chylomicrons, which consist primarily of exogenous TG fatty acids. Because enterocytes have the apolipoprotein B-100/E receptor, this study was directed at determining whether the cells can take up and metabolize chylomicron remnants and, if so, whether this was receptor mediated. Isolated enterocytes were incubated with purified 3H-labeled chylomicron remnants. The remnants were shown to be taken up by the basolateral membrane, not the apical membrane. Remnant uptake was proportional to time and number of enterocytes, and saturation kinetics were observed. Nonradiolabeled remnants, human low-density lipoprotein (LDL), anti-LDL receptor antibody, and receptor-associated protein, an LDL-related receptor inhibitor, were all shown to compete for or reduce 3H-remnant uptake. Remnants taken up by the enterocytes could not be removed on incubation with excess human LDL. Uptake was shown to be greatest in the villus tips of the proximal intestine. These studies suggest that enterocytes take up chylomicron remnants by a receptor-mediated process from their basolateral membranes and that the remnants could provide a source of endogenous TG fatty acids for the enterocytes.

Intestinal triacylglycerol storage pool size changes under differing physiological conditions

The intestinal mucosal triacylglycerol storage pool consists of triacylglycerol that is predominantly transported from the intestine via the portal vein rather than in chylomicrons (Am. J. Physiol. 1991. 261: G530-G538). Here we examined the size of the storage pool under varying physiological conditions. Four groups of rats were infused intraduodenally for 4 h. Group A was fasted; group B was infused with trioleoylglycerol, 135 mumol/h; group C was infused with trioleoylglycerol, 135 mumol/h plus phosphatidylcholine, 9 mumol/h; and group D was bile-diverted and infused with trioleoylglycerol, 135 mumol/h. The amount of triacylglycerol in the mucosa increased from groups A to D (A > B > C > D) but the storage pool triacylglycerol was least in groups A and C and greatest in groups B and D. The percentage of trioleoylglycerol in mucosal triacylglycerol was greater in groups B and D than in group A and greater in all groups than the percentage of oleate in the total fatty acids. We conclude that the triacylglycerol storage pool size varies inversely with the efficiency of lymphatic lipid output, which is greatest in rats infused with trioleoylglycerol plus phosphatidylcholine (group C) and least in bile-diverted rats infused with trioleoylglycerol (group D).

Role of the intestine in chylomicron remnant clearance

When 810 mumol of [3H]glyceryl trioleate (TO) were infused intraduodenally over 6 h into rats, 29% of the triacylglycerol (TG) acyl groups in the mucosa were not from the infusate. We tested the hypothesis that chylomicron remnants contribute to the mucosal pool of nondietary TG acyl groups, since the acyl group composition of the chylomicron remnants was 58% oleate, compared with 90% in their parent chylomicrons. Purified 3H-labeled remnants were generated from chylomicrons formed in rats receiving TO intraduodenally, with 95% of the remnant disintegrations per minute (dpm) being in TG. The 3H-remnants were infused intravenously into rats receiving either saline or 135 mumol/h TO intraduodenally. In the saline-infused rats, 32% of the infused 3H dpm were in the proximal and 19% in the distal intestine and 32% were in the liver. In the fat-infused rats, 12% of the infused 3H dpm were in the proximal and 5% were in the distal gut and 29% were in the liver. When [3H]cholesterol-labeled remnants were infused intravenously and saline was infused intraduodenally, the percentage uptake into the mucosa was nearly the same as with the TG label, but comparable uptake by the liver increased. We conclude that the intestine competes with the liver for chylomicron remnant TG and cholesterol.

Effect of brefeldin A on lymphatic triacylglycerol transport in the rat

Brefeldin A (BFA) has been shown in in vitro studies to either collapse the Golgi into the endoplasmic reticulum (ER) or the peripheral organelles into the trans-Golgi network. Our goal was to determine the effect of BFA on intestinal lipid transport, since the Golgi is thought to play an important role in this process and simultaneously establish the effectiveness of BFA in an in vivo system. We infused rats intraduodenally with glyceryl tri-[3H]oleate at 135 mumol/h for 15 h and included BFA, 750 micrograms/h, during hours 4-7 of infusion. Mass and lipid disintegrations per minute output into the lymph fell to 9% of input rates at 8 h of infusion and returned to steady-state values at 12 h of infusion. Both chylomicron and very low-density lipoprotein output were severely affected by the BFA. Electron microscopy showed that the Golgi was collapsed into the ER. Mucosal triacylglycerol (TG) mass and disintegrations per minute were increased at 7 h of infusion in BFA infused rats vs. controls in the proximal half of the intestine. Lipid absorption, lipase activity, and mucosal TG synthesis were normal in the BFA-treated rats. We conclude that BFA works in vivo and in the intestine collapses the Golgi into the ER. As a consequence, lymphatic TG transport was severely affected.

Alkaline lipase in rat intestinal mucosa: physiological parameters

The present studies describe an alkaline active lipase in the rat intestinal mucosa. Alkaline lipase activity was determined using a glyceryl tri[14C]oleate emulsion at pH 8.0. Subcellular fractions were prepared from mucosal homogenates by differential centrifugation. Cells from villus to crypt were sequentially released using citrate and EDTA buffer. The enzyme was found to be most active in the proximal quarter of the intestine and in villus tips. Most of the activity was found in the cytosolic fraction. Bile salts stimulated the enzyme activity threefold. The presence of both Ca2+ and taurocholate was essential for optimal activity. Mucosal activity was greatly reduced on intraduodenal glyceryl trioleate infusion. Activity was restored when phosphatidylcholine was added to the glyceryl trioleate infusion. The fact that mucosal lipase has its greatest activity in the villus tips of the proximal intestine and in cytosol suggests that it may play an important role in mucosal glyceryltrioleate metabolism.

Portal transport of long acyl chain lipids: effect of phosphatidylcholine and low infusion rates

The transport of absorbed long acyl chain lipids in the portal vein of rats has been shown to be 39% when the duodenal input rate is 135 mumol/h glyceryl trioleate (TO) [C. M. Mansbach II, R. F. Dowell, and D. Pritchett, Am. J. Physiol. 255 (Gastrointest. Liver Physiol. 18): G530-G539, 1991]. These calculations were based on a new experimental model in which portal flux is calculated from the knowledge of portal flow and the concentration of the lipids in excess in the portal vein vs. the carotid artery. To test this model, rats were infused for 6 h with a low rate of [3H]TO (27 mumol/h) with or without phosphatidylcholine (9 mumol/h) or with [3H]TO (135 mumol/h) plus phosphatidylcholine (9 mumol/h) or with [3H]TO (135 mumol/h) plus phosphatidylcholine (9 mumol/h). In all three cases, portal flux was expected to be less. Portal transport was 16.5% of the input rate in the low-dose group, 1.4% in the high-dose group given phosphatidylcholine, and 0.5% in the low-dose plus phosphatidylcholine group. There was no net transport of fatty acid in the portal vein in any of the three cases. These data show that portal lipid transport is dependent on the lipid load and that it is greatly reduced at high loads by including phosphatidylcholine in the lipid infusion.

Uptake and metabolism of circulating fatty acids by rat intestine

The present study was designed to investigate the uptake and metabolism of circulating fatty acids by the intestinal mucosa in rats actively absorbing glyceryl trioleate given intraduodenally to determine the plasma fatty acid contribution to mucosal triacylglycerol. Rats with duodenal, femoral vein, carotid artery, and mesenteric lymph duct cannulas were used. [3H]oleate was constantly infused into the femoral vein while glyceryl trioleate was infused into the duodenum (135 mumol/h). After 5 h of infusion, a mass and radioactive steady state existed in the plasma and mucosa. At 6 h of infusion, the plasma oleate specific activity was sixfold greater than mucosal oleate and 50 times greater than mucosal triacylglycerol oleate; 86% of the mucosal oleate disintegrations/minute were in triacylglycerol. Chylomicron triacylglycerol oleate specific activity was less than that of the mucosa. Furthermore, the percentage of mucosal triacylglycerol acyl groups composed of oleate was greater than the percentage of oleate in mucosal free fatty acids. The data indicate that fatty acids are taken up by the mucosa during active fat absorption and metabolized primarily to triacylglycerols by the mucosa. The triacylglycerols in the mucosa synthesized from circulating fatty acids are selected against as a precursor of chylomicron triacylglycerol. The results support our previous hypothesis suggesting that the mucosa has at least two pools of neutral lipid (J. Lipid Res. 23: 1009-1019, 1982) and that steady-state conditions as performed here yield different results from previous work using bolus tracer injection techniques.

Topical application of WR-2721 to prevent radiation-induced proctosigmoiditis. A phase I/II trial

Patients undergoing x-ray therapy to the pelvis have intestinal symptoms proportional to the volume treated and the dose delivered. WR-2721, S-2 (3-aminopropylaminoethyl) phosphorothioic acid, is an organic thiophosphate compound that selectively protects normal tissues against radiation effects. A Phase I/II study was done to test the ability of topical application of WR-2721 to protect the mucosa of the rectosigmoid from radiation damage. Thirty-one patients were enrolled in this study, of which, seven were control subjects. Twenty-four patients received WR-2721 daily, in enema form, 45 minutes before treatment. The patients were assigned by groups of three to receive increasing doses of WR-2721 beginning with 100 mg/enema to 450 mg/enema. Rectal mucosal biopsies were obtained within the treated field before, during, and at the end of therapy. The degree of damage to the rectal mucosa was scored on the basis of a 0 to 4 scale (with 0, least damage to 4, most damage) as determined by the percentage of damaged mucosal crypt glands. The patients' symptoms were recorded once a week during the entire course of therapy. The biopsy scores of the control group were slightly higher than those of the treatment groups; however, this difference did not appear to be significant. In the treated groups, there was a slight decrease in the biopsy scores with increasing doses of WR-2721, but this trend was not sustained. There were no differences among any of the groups in the symptoms experienced during the course of therapy. This study showed that WR-2721 could be administered safely in enema form in doses ranging from 100 to 450 mg/enema, but this drug did not protect the rectosigmoid mucosa from radiation damage at the doses administered.

Effect of fat pre-feeding on bile flow and composition in the rat

Studies were conducted in rats to determine if the increase in lymph triacylglycerol output on pre-feeding a 20% glyceryltrioleate diet (Mansbach, C.M., II and Arnold, A. (1986) Am. J. Physiol. 251, G263-269) was due to an increase in phosphatidylcholine output into bile. Rats who were fed chow or pre-fed the 20% fat diet were equipped with biliary and duodenal cannulas and infused with glucose-saline while bile was collected hourly. The next day a taurocholate-glyceryltrioleate infusion was given and bile collected for 5 h. Bile flow, bile acid, phosphatidylcholine and cholesterol output were greater in the chow fed group than controls during the 6 h of the glucose saline period. Outputs were low overnight. During the taurocholate-glyceryltrioleate infusion, bile flow, bile acid, phosphatidylcholine and cholesterol output were all greater in the fat pre-fed group than the chow fed controls. We conclude that fat pre-feeding profoundly influences biliary composition and flow. The 2-fold increase in biliary phosphatidylcholine output during duodenal lipid infusion offers a potential explanation for the increased delivery of triacylglycerol into the lymph in rats on a similar fat pre-feeding program.

Purification and partial characterization of intestinal acid lipase

Intestinal acid lipase is an enzyme whose greatest specific activity is localized to the villus tips of the proximal intestine (Rao, R.H. and Mansbach, C.M. (1990) Biochim. Biophys. Acta 1043, 273-280). This suggests that it plays a role in the processing of dietary lipids. We purified the enzyme in order to better characterize it. Acid lipase was isolated from intestinal mucosa of rats by a combination of ammonium sulfate precipitation, butanol extraction and chromatography on DEAE Bio-Gel, CM Bio-Gel and Sephadex G-75. This resulted in a single protein of Mr 53,700 on SDS-polyacrylamide gel electrophoresis. The isolation scheme produced a 3344-fold purification resulting in an enzyme whose specific activity was 801 mumols/min per mg protein. The yield was 50%. The purified enzyme was stimulated (20-fold) by the addition of tauro- or glycocholate but no other conjugated bile acid. A sharp peak in activity occurred at pH 5.6. The pI of the enzyme was 6.2. The reaction products produced under prolonged incubation suggested that monoacylglycerol was not hydrolyzed since an overabundance of monoacylglycerol was found with respect to the amount of fatty acid produced. These results suggested that intestinal acid lipase is potentially important in the metabolism of dietary lipids. Its proportionate role awaits further documentation.

Phospholipases: old enzymes with new meaning

Phospholipases are enzymes that hydrolyze specific portions of phospholipid molecules. Their role in the digestion of exogenous phospholipids and as the active principle in snake and bee venoms has long been appreciated. Interest has increased in phospholipases recently because of new data implicating them in the inflammatory response. The ability of phospholipases to hydrolyze bacterial phospholipids has also received considerable attention. These new data have brought pertinence to studies of the physicochemical nature of potential substrates that greatly influence enzyme activity. Interest in the regulation of enzyme activity, both by physiological and pharmacological means, has increased as the importance of the phospholipases in response to various stimuli has become better appreciated. Finally, considerable interest has focused on the role of the phospholipases in response to hormones in a variety of cell systems. Data pertinent to all of these areas of interest will be discussed in this review with a view toward stimulating those with an interest in gastrointestinal physiology to apply them to their own areas of research in the gastrointestinal tract or liver.

Acid lipase in rat intestinal mucosa: physiological parameters

Previous studies have shown that up to a half of infused triacylglycerol does not exit the intestine via lymphatics. This suggests the presence of a mucosal lipase which could provide fatty acids for potential transport via the portal vein. The present study describes an acid-active lipase in rat intestinal mucosa. Acid lipase was assayed using a glyceryl tri[14C]oleate emulsion (pH 5.8). Mucosal homogenates were differentially centrifuged to yield cellular organelles and cytosol. Cells were sequentially released from villi using citrate and EDTA. The enzyme was found to be most active in the proximal quarter intestine and in the upper third of villi. Its greatest activity was in the lysosomal fraction. Esophageal diversion demonstrated that lingual lipase was not the precursor of the mucosal acid lipase. Bile salts stimulated activity 3- to 5-fold, but other neutral or anionic detergents were inhibitory. Of the detergents tested, taurocholate at super critical micellar concentrations could restore activity only with SDS. Sepharose 6B chromatography suggested that the enzyme partitioned into an SDS and taurocholate mixed micelle. We conclude that mucosal acid lipase is a distinct, intrinsic enzyme of the intestinal mucosa. It is predominantly lysosomal in origin. The location of its greatest activity in the villus tips of the proximal intestine suggests that it is potentially involved in mucosal triacylglycerol disposal.

Isolation and characterization of a mucosal triacylglycerol pool undergoing hydrolysis

Absorbed and processed mucosal neutral lipid has been shown to be composed of at least two pools of triacylglycerol. One is likely to subserve chylomicron formation, and the other appears to be transported from the intestine via a nonlymphatic route. In the present study, 50 +/- 5% of the mucosal lipid pellets was centrifuged at 75,000 g.min [low-speed pellet (LSP)]. Discontinuous sucrose density gradient centrifugation of LSP showed that 61 +/- 7% of the lipid banded at the 0.25-0.86 M sucrose interface. Neutral lipid analysis showed that this subfraction was only 58% triacylglycerol, suggesting it was undergoing hydrolysis. Active lipolytic activity in vitro was found on incubation. The lipase had an alkaline pH optimum (pH 8.5) and persisted despite pancreatic ductular diversion. Lipolysis in vivo in a LSP fraction was shown by infusing [14C]glyceryltrioleate for 3.5 h followed by [3H]glyceryltrioleate for 30 min. Discontinuous sucrose density centrifugation of the LSP followed by an analysis of the lipids at the 0.25-0.86 M sucrose interface showed that 14C-neutral lipids were only 70 +/- 6% triacylglycerol, whereas 3H-neutral lipids were 88 +/- 2% triacylglycerol. 3H entered LSP slowly compared with the floating lipid in the same centrifuge tube. These studies suggest both in vivo and in vitro mucosal lipolysis by a specific, alkaline-active lipase. The turnover rate of LSP is likely to be slow by comparison with neutral lipid floating to the top of the centrifuge tube.

Intraenterocyte distribution of absorbed lipid and effects of phosphatidylcholine

The present studies were designed to investigate the distribution of absorbed lipid in intestinal mucosal cells and to identify the chylomicron precursor pool. Rats were infused intraduodenally with glyceryl tri[9,10(n)-3H]oleate (135 mumol/h); other rats were in addition infused with 9 mumol/h of phosphatidylcholine. After 5-h infusion the proximal one-half of intestine was removed and the mucosa obtained. It was found that 50% of the radioactivity in the whole homogenate pelleted on centrifugation at 75,000 g.min. The supernatant was further fractionated by high-speed centrifugation resulting in a floating lipid layer, a supernatant, and a microsomal pellet. The results showed that these subcellular fractions had a triacylglycerol specific activity 46-52% of the infusate's specific activity. Including phosphatidylcholine in the duodenal lipid infusion increased the triacylglycerol specific activity of all subcellular fractions (70%) resulting in a specific activity approaching that of the infusate, which would be expected of chylomicron triacylglycerol. These studies demonstrate 1) that considerable mucosal lipid is distributed into a low-speed pellet, 2) that mucosal triacylglycerol specific activity can be greatly increased by including phosphatidylcholine in a lipid infusion, and 3) that despite obtaining multiple subcellular fractions, the chylomicron precursor pool could not be clearly identified in the mucosa of control rats.

Isolation of the early phase of chylomicron formation in intestinal epithelial cells of rats

Lipid is first observed electron microscopically in the rough endoplasmic reticulum of intestinal epithelial cells during active lipid absorption. We have been able to isolate this subcellular fraction by using discontinuous sucrose gradients of 0.25/0.86/1.11 M sucrose. A preliminary low speed centrifugation of mucosal homogenate removed the heavier subcellular organelles. The resulting supernatant was centrifuged at 5.25 x 10(6) x g.min. The pellet from this centrifugation was placed on top of the gradient and the fractions isolated at the density interfaces after centrifugation at 25.5 x 10(6) x g.min. The isolated fractions were characterized enzymatically and electron microscopically. Electron microscopically, the fractions were predominantly composed of rounded vesicles decorated with ribosomes. Most contained lipid droplets whose diameters were 453 nm in the lighter membranes and 245 nm in the membranes isolated from the heavier density region. The vesicles contained NADPH cytochrome c reductase and glucose-6-phosphatase activity indicative of the presence of microsomes. Contamination with other subcellular organelles was minimal. These studies demonstrate a method which enables the isolation of vesicles containing chylomicron-sized particles which are from the earliest phase of chylomicron formation. Isolation of chylomicrons from these vesicles will enable a better understanding of the maturation process of chylomicrons as they traverse the intestinal epithelial cell.