Regulation of apolipoprotein B secretion by biliary lipids in CaCo-2 cells

Lipin 2/3 phosphatidic acid phosphatases maintain phospholipid homeostasis to regulate chylomicron synthesis

The lipin phosphatidic acid phosphatase (PAP) enzymes are required for triacylglycerol (TAG) synthesis from glycerol 3-phosphate in most mammalian tissues. The 3 lipin proteins (lipin 1, lipin 2, and lipin 3) each have PAP activity, but have distinct tissue distributions, with lipin 1 being the predominant PAP enzyme in many metabolic tissues. One exception is the small intestine, which is unique in expressing exclusively lipin 2 and lipin 3. TAG synthesis in small intestinal enterocytes utilizes 2-monoacylglycerol and does not require the PAP reaction, making the role of lipin proteins in enterocytes unclear. Enterocyte TAGs are stored transiently as cytosolic lipid droplets or incorporated into lipoproteins (chylomicrons) for secretion. We determined that lipin enzymes are critical for chylomicron biogenesis, through regulation of membrane phospholipid composition and association of apolipoprotein B48 with nascent chylomicron particles. Lipin 2/3 deficiency caused phosphatidic acid accumulation and mammalian target of rapamycin complex 1 (mTORC1) activation, which were associated with enhanced protein levels of a key phospholipid biosynthetic enzyme (CTP:phosphocholine cytidylyltransferase α) and altered membrane phospholipid composition. Impaired chylomicron synthesis in lipin 2/3 deficiency could be rescued by normalizing phospholipid synthesis levels. These data implicate lipin 2/3 as a control point for enterocyte phospholipid homeostasis and chylomicron biogenesis.

Lysophosphatidylcholine for efficient intestinal lipid absorption and lipoprotein secretion in caco-2 cells

Phosphatidylcholine (PC) and its hydrolysates are considered to stimulate intestinal lipid absorption, however, their exact effects on lipoproteins and apolipoprotein (apo) metabolism remain ambiguous. This study aimed to further differentiate the effects of them using fully differentiated enterocyte-like Caco-2 cells. Lipid micelles (oleic acid 0.6, cholesterol 0.05, monooleylglycerol 0.2, taurocholate 2 in mmol/l) with or without choline, PC, and lysoPC (0.2 mmol/l each) were applied apically to Caco-2 cells. ³H-oleic acid and ¹⁴C-cholesterol were added to the micelles when necessary. Secreted lipoproteins were analyzed by a HPLC method. LysoPC had the most potent promoting effect on lipid uptake, and lipoprotein and apolipoprotein B-48 secretion among the molecules tested. LysoPC doubled the output of cholesterol and triglyceride as the lipoprotein component, but PC did not. On the other hand, PC only increased the secretion of apoA-IV in the presence of lipid micelles. These findings confirm that the alteration of PC by PLA₂ hydrolysis is intrinsically involved in the intestinal lipid absorption process and suggest that PC and its hydrolysis are coordinately associated with not only lipid absorption efficiency but also lipoprotein output and metabolism.

Fibrillar collagen type I stimulation of apolipoprotein B secretion in Caco-2 cells is mediated by beta1 integrin

Caco-2 cells spontaneously differentiate into enterocyte-like cells and secrete apolipoprotein B (apoB) lipoproteins. We evaluated the effect of different extracellular matrix proteins on lipoprotein secretion by these cells. Caco-2 cells grown on human amnion connective tissue (HACT) secreted twice as much apoB as control cells on Transwells, but secreted similar amounts of apoA1. Cells cultured on fibrillar collagen type I secreted increased amounts of apoB similar to the cells cultured on HACT, but cells cultured on non-fibrillar collagen type I, type IV collagen or laminin-1 did not. The increased secretion was nullified by a function inhibiting anti-integrin beta1 monoclonal antibody. Therefore, interactions between type I collagen and beta1 integrins augment apoB secretion by Caco-2 cells. Cells on HACT formed a more uniform columnar epithelium with lipid droplets polarized to the basolateral membrane. We also studied the effect of extracellular matrix proteins on transepithelial resistance (TER) of differentiated Caco-2 cells. TER in cells cultured on HACT was similar to that on Transwells, but cells on laminin-1 and collagen IV exhibited higher TER. Thus, various extracellular matrix proteins regulate apoB secretion and TER differently. This new observation that extracellular matrix proteins can enhance apoB secretion in Caco-2 cells could be useful to explore the modulation of lipid transport by these proteins.

Digestion of maize sphingolipids in rats and uptake of sphingadienine by Caco-2 cells

We investigated the digestion of cerebrosides of plant origin prepared from maize, focusing especially on the digestive fates of trans-4, cis-8- and trans-4, trans-8-sphingadienine, which are common in higher plants. In the small intestinal mucosa and cecal contents of rats, the cerebrosidase activity at pH 5.2 toward the glucosyl linkage in maize cerebrosides (glucosylceramides) was similar to that in cerebrosides of mammalian origin. Similarly, the ceramidase activity toward the amide linkage in ceramides prepared from maize cerebrosides at pH 7.0 was the same as that toward ceramides of mammalian origin. In addition, maize cerebrosides were hydrolyzed to ceramide and free sphingoid bases in the digestive tract of rats after oral administration. To further evaluate the uptake by enterocytes of 4,8-sphingadienine, we used differentiated Caco-2 cells, derived from human colonic carcinoma, as a model of intestinal epithelial cells. The accumulation of sphingoid bases in Caco-2 cells incubated with each isomer of sphingadienine was lower than that after incubation with sphingosine (P < 0.05). Verapamil, a P-glycoprotein inhibitor, increased the accumulation of each sphingadienine but not of sphingosine, suggesting that the efflux of sphingadienine of plant origin, but not sphingosine of mammalian origin, was affected by P-glycoprotein. The digestibility of maize cerebrosides appears similar to that of cerebrosides of mammalian origin, but the metabolic fate of sphingoid bases of plant origin within enterocytes differs from that of sphingosine. Isomers of 4,8-sphingadienine degraded from dietary plant cerebrosides appear to be poorly absorbed from the digestive tract.

Enteral infusion of phosphatidylcholine increases the lymphatic absorption of fat, but lowers alpha-tocopherol absorption in rats fed a low zinc diet*

Our previous study has shown that the lymphatic absorption of both fat and alpha-tocopherol (alphaTP) is lowered markedly in rats fed a low zinc diet, with a parallel decrease in lymphatic phospholipid (PL) output. This study was conducted to determine if enteral infusion of phosphatidylcholine (PC) could restore lymphatic absorption of fat and alphaTP in zinc-deficient rats. One group of rats was fed an AIN-93G diet containing 3 mg Zn/kg (low zinc; LZ) and the other was fed the same diet but containing 30 mg Zn/kg (adequate zinc; AZ). Rats were trained to consume two meals daily of equal amounts of food. At 6 wk, each rat with lymph fistula was infused at 3 mL/h with a lipid emulsion containing 3.6 &mgr;mol alphaTP and 565 &mgr;mol [carboxyl-14C]-triolein (14C-OA), with or without 40 &mgr;mol 1,2-dilinoleoyl-PC in 24 mL PBS at pH 6.4. The lymphatic absorptions of fat and alphaTP were determined by measuring 14C-radioactivity and alphaTP appearing in the mesenteric lymph collected hourly for 8 h. When the emulsion devoid of PC was infused, the absorptions of both 14C-OA (41 +/- 4% dose) and alphaTP (431 +/- 55 nmol) in LZ rats were significantly lower than in AZ rats (48 +/- 2% 14C-OA dose and 581 +/- 70 nmol alphaTP). When the emulsion containing PC was infused, the absorption of 14C-OA was restored rapidly to normal in LZ rats, along with a parallel increase in lymphatic PL output. However, PC infusion further lowered the absorption of alphaTP to 311 +/- 20 nmol/8 h in LZ rats and also lowered the absorption of alphaTP in AZ rats (347 +/- 48 nmol/8 h). The results demonstrate that low zinc intake results in impaired intestinal absorption of both alphaTP and fat. The findings also indicate that PC significantly improves the intestinal absorption of fat, but inhibits alphaTP absorption, suggesting that PC affects the intestinal absorption of alphaTP and fat via distinctly different mechanisms.

Apolipoprotein B, a paradigm for proteins regulated by intracellular degradation, does not undergo intracellular degradation in CaCo2 cells

Studies in different liver-derived cells in culture indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the ubiquitin-proteasome pathway is a major mechanism for the degradation. The proteasomal degradation of apoB-100 was postulated to be an intrinsic property of the protein that occurs even in the presence of optimal amounts of lipids supplied to the cell. We examined apoB-100 and apoB-48 biogenesis in CaCo2, a human colon carcinoma cell line. To our surprise, apoB-100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially none of the newly synthesized apoB was degraded before secretion in a 2-h period whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellular apoB isolated from HepG2 cells, little or no ubiquitin was detectable in the intracellular apoB from CaCo2 cells. The amounts of free ubiquitin and total and non-apoB ubiquitinated proteins were comparable in HepG2 and CaCo2 cells, indicating that CaCo2 cells have the necessary machinery for tagging ubiquitin chains onto cellular proteins for proteasomal degradation. Incubation in lipoprotein-deficient serum did not induce apoB degradation, but the addition of a microsomal triglyceride transfer protein inhibitor led to apoB degradation in CaCo2 cells. Finally, similar proportions of apoB polypeptide in isolated microsomes from CaCo2 and HepG2 cells were accessible to exogenously added trypsin, indicating that the mere exposure of apoB nascent chains to the cytosolic compartment is insufficient to cause the proteasomal degradation. Therefore, the intracellular degradation of apoB is not an intrinsic property of the protein, and the phenomenon is neither universal nor inevitable. The unconditional use of apoB as a paradigm for intracellular protein degradation is not warranted.

Postprandial chylomicron formation and fat absorption in multidrug resistance gene 2 P-glycoprotein-deficient mice

BACKGROUND & AIMS

It has been proposed that biliary phospholipids fulfill specific functions in the absorption of dietary fat from the intestine, but the physiological significance has not been established. The aim of this study was to evaluate the role of biliary phospholipids in dietary fat absorption in vivo by using mice homozygous or heterozygous for disruption of the Mdr2 gene (Mdr2((-/-)), Mdr2((+/-))) and control (Mdr2((+/+))) mice. Mdr2((-/-)) mice do not secrete phospholipids and cholesterol into bile, and bile salt secretion is not impaired. Mdr2((+/-)) mice show only impaired (-40%) phospholipid secretion.

METHODS

Methods included an analysis of time dependency of intestinal uptake and plasma appearance of intragastrically administered (radiolabeled) triglycerides and measurement of 3-day fecal fat balance with low- and high-fat diets.

RESULTS

Intragastric administration of olive oil resulted in a rapid increase in plasma triglycerides in Mdr2((+/+)) and Mdr2((+/-)) but not in Mdr2((-/-)) mice. The "postprandial response" of plasma triglycerides could be partially restored in Mdr2((-/-)) mice by intraduodenal infusion of whole rat bile. After intragastric [(3)H]triolein administration in Triton WR1339-pretreated animals, the appearance of (3)H-triglycerides in plasma was reduced by 70% in Mdr2((-/-)) compared with Mdr2((+/+)) mice, excluding accelerated lipolysis as the cause of defective triglyceride response in Mdr2((-/-)) mice. (3)H-triglycerides accumulated in enterocytes in Mdr2((-/-)) mice. Surprisingly, the efficacy of fat absorption as derived from balance studies was not affected and was only minimally affected in Mdr2((-/-)) mice fed low (14 energy percent)- and high (35 energy percent)-fat diets, respectively (all >95%).

CONCLUSIONS

The results show that biliary lipid secretion is necessary for postprandial appearance in plasma of chylomicrons in vivo but not for quantitative absorption of dietary lipids.

Assembly and secretion of chylomicrons by differentiated Caco-2 cells. Nascent triglycerides and preformed phospholipids are preferentially used for lipoprotein assembly

To develop a cell culture model for chyclomicron (CM) assembly, the apical media of differentiated Caco-2 cells were supplemented with oleic acid (OA) together with either albumin or taurocholate (TC). The basolateral media were subjected to sequential density gradient ultracentrifugations to obtain large CM, small CM, and very low density lipoproteins (VLDL), and the distribution of apoB in these fractions was quantified. In the absence of OA, apoB was secreted as VLDL/LDL size particles. Addition of OA (>/=0.8 mM) with TC, but not with albumin, resulted in the secretion of one-third of apoB as CM. Lipid analysis revealed that half of the secreted phospholipids (PL) and triglycerides (TG) were associated with CM. In CM, TG were 7-11-fold higher than PL indicating that CM were TG-rich particles. Secreted CM contained apoB100, apoB48, and other apolipoproteins. Secretion of large CM was specifically inhibited by Pluronic L81, a detergent known to inhibit CM secretion in animals. These studies demonstrate that differentiated Caco-2 cells assemble and secrete CM in a manner similar to enterocytes in vivo. Next, experiments were performed to identify the sources of lipids used for lipoprotein assembly. Cells were labeled with [3H]glycerol for 12 h, washed, and supplemented with OA, TC, and [14C] glycerol for various times to induce CM assembly and to radiolabel nascent lipids. TG and PL were extracted from cells and media and the association of preformed and nascent lipids with lipoproteins was determined. All the lipoproteins contained higher amounts of preformed PL compared with nascent PL. VLDL contained equal amounts of nascent and preformed TG, whereas CM contained higher amounts of nascent TG even when nascent TG constituted a small fraction of the total cellular pool. These studies indicate that nascent TG and preformed PL are preferentially used for CM assembly and provide a molecular explanation for the in vivo observations that the fatty acid composition of TG, but not PL, of secreted CM reflects the composition of dietary fat. It is proposed that in the intestinal cells the preformed PL from the endoplasmic reticulum bud off with apoB as primordial particles and the assembly of larger lipoproteins is dependent on the synthesis and delivery of nascent TG to these particles.

Bile diversion in rats leads to a decreased plasma concentration of linoleic acid which is not due to decreased net intestinal absorption of dietary linoleic acid

Decreased bile secretion into the intestine has been associated with low plasma concentrations of essential fatty acids (EFA) in humans. We studied the mechanism behind this relationship by determining the status and absorption of the major dietary EFA, linoleic acid (LA), in control and 1-week bile-diverted rats. The absorption of LA was quantified by a balance method and by measuring plasma concentrations of [13C]LA after its intraduodenal administration. Absolute and relative concentrations of LA in plasma were decreased in bile-diverted rats (P<0.01 and P<0.001, respectively). Fecal excretion of LA was increased at least 20-fold in bile-diverted rats (0.72+/-0.11 vs. 0.03+/-0.00 mmol/day; P<0.0001). Due to increased chow ingestion by bile-diverted rats, net intestinal absorption of LA was similar between bile-diverted and control rats (1.96+/-0.14 vs. 1.91+/-0.07 mmol/day, respectively; P>0.05). After intraduodenal administration of [13C]LA, plasma concentrations were approximately 3-4-fold lower in bile-diverted rats for at least 6 h (P<0.001). Plasma concentrations of both [12C]arachidonic acid and [13C]arachidonic acid were increased in bile-diverted rats (P<0.05). We conclude that decreased plasma concentrations of LA in 1-week bile-diverted rats are not due to decreased net intestinal absorption of LA, but may be related to increased metabolism of LA.

Regulation of apolipoprotein secretion by biliary lipids in newborn swine intestinal epithelial cells

Biliary lipids, composed of bile acids, cholesterol, and phosphatidylcholine, are a major source of luminal lipid in the small intestine. In the present study in a newborn swine intestinal epithelial cell line (IPEC-1), taurocholate and phosphatidylcholine were found to have no effect on apolipoprotein B (apo B) secretion but did significantly increase the basolateral secretion of apo A-I. This regulation of apo A-I secretion occurred at the pretranslational level for taurocholate and at the posttranslational level for phosphatidylcholine. The regulation of apo A-I secretion by phosphatidylcholine did not involve changes in apo A-I degradation and may involve mobilization of a preformed pool of apo A-I. Cholesterol, whether solubilized with taurocholate or phosphatidylcholine, had no effect on the secretion of either apo B or apo A-I. However, when taurocholate, phosphatidylcholine, and cholesterol were combined, apo B secretion was decreased, and the increase in apo A-I secretion noted with taurocholate and phosphatidylcholine alone was ablated. Another primary bile acid, taurochenodeoxycholate, was found to decrease apo B secretion but had no effect on apo A-I secretion. However, the significance of this effect is uncertain, since this bile acid caused significant cellular membrane injury, as evidenced by increased apical medium lactate dehydrogenase activity. Phosphatidylcholine, but not taurocholate, dramatically increased the basolateral secretion of radiolabeled phospholipid with a modest increase in cellular triglyceride radiolabeling. Furthermore, this effect of phosphatidylcholine on lipid synthesis did not require significant hydrolysis or uptake of the phosphatidylcholine molecule. Studies using radiolabeled taurocholate did not demonstrate active transport of taurocholate by these cells.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Regulation of apolipoprotein B biogenesis in human hepatocytes: posttranscriptional control mechanisms that determine the hepatic production of apolipoprotein B-containing lipoproteins

OBJECTIVES

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins appears to be a common cause of hyperlipoproteinemia in humans. Patients with overproduction states secrete denser cholesterol ester-rich lipoprotein particles which are highly atherogenic. The formation of apoB particles involves a very complex process that requires the coordinated synthesis and assembly of apoB, triglycerides, cholesterol esters, phospholipids, and other components. ApoB expression is an important prerequisite for the assembly and secretion of apoB particles. Evidence to date appears to suggest that apoB expression is regulated posttranscriptionally. ApoB secretion rate is determined at the levels of apoB translocation into the endoplasmic reticulum (ER) as well as degradation within the ER.

RESULTS AND HYPOTHESIS

Based on available data, we postulate that the rate of apoB particle secretion is determined at the critical point where newly-synthesized apoB interacts with core lipids, particularly triglycerides. The supply of these lipids determines the rate of translocation of the apoB molecule across the ER membrane and into the ER lumen. Lipidation of apoB facilitates its proper folding, its assembly into a lipoprotein particle, and its extracellular secretion. In the absence of lipids, apoB is misfolded resulting in the abortion of ER translocation and subsequent degradation by an apoB specific protease.

CONCLUSIONS

The balance between intracellular degradation and extracellular secretion determines the rate at which the human liver secretes apoB particles.