Intestinal triacylglycerol storage pool size changes under differing physiological conditions

Efficient Intestinal Digestion and On Site Tumor-Bioactivation are the Two Important Determinants for Chylomicron-Mediated Lymph-Targeting Triglyceride-Mimetic Docetaxel Oral Prodrugs

The oral absorption of chemotherapeutical drugs is restricted by poor solubility and permeability, high first-pass metabolism, and gastrointestinal toxicity. Intestinal lymphatic transport of lipophilic prodrugs is a promising strategy to improve the oral delivery efficiency of anticancer drugs via entrapment into a lipid formulation and to avoid first-pass metabolism. However, several basic principles have still not been clarified, such as intestinal digestibility and stability and on-site tumor bioactivation. Herein, triglyceride-mimetic prodrugs of docetaxel (DTX) are designed by conjugating them to the sn-2 position of triglyceride (TG) through different linkage bonds. The role of intestinal digestion in oral absorption of TG-like prodrugs is then investigated by introducing significant steric-hindrance α-substituents into the prodrugs. It is surprisingly found that poor intestinal digestion leads to an unsatisfactory bioavailability but efficient intestinal digestion of TG-like prodrugs with a less steric-hindrance linkage (DTX-S-S-TG) facilitating oral absorption. Moreover, it is found that the TG-like reduction-sensitive prodrug (DTX-S-S-TG) has good stability during intestinal transport and blood circulation, and on-demand release of docetaxel at the tumor site, leading to a significantly improved antitumor efficiency with negligible gastrointestinal toxicity. In summary, the chylomicron-mediated lymph-targeting triglyceride-mimetic oral prodrug approach provides a good foundation for the development of oral chemotherapeutical formulations.

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 triacylglycerol synthesis in fat absorption and systemic energy metabolism

The intestine plays a prominent role in the biosynthesis of triacylglycerol (triglyceride; TAG). Digested dietary TAG is repackaged in the intestine to form the hydrophobic core of chylomicrons, which deliver metabolic fuels, essential fatty acids, and other lipid-soluble nutrients to the peripheral tissues. By controlling the flux of dietary fat into the circulation, intestinal TAG synthesis can greatly impact systemic metabolism. Genes encoding many of the enzymes involved in TAG synthesis have been identified. Among TAG synthesis enzymes, acyl-CoA:monoacylglycerol acyltransferase 2 and acyl-CoA:diacylglycerol acyltransferase (DGAT)1 are highly expressed in the intestine. Their physiological functions have been examined in the context of whole organisms using genetically engineered mice and, in the case of DGAT1, specific inhibitors. An emerging theme from recent findings is that limiting the rate of TAG synthesis in the intestine can modulate gut hormone secretion, lipid metabolism, and systemic energy balance. The underlying mechanisms and their implications for humans are yet to be explored. Pharmacological inhibition of TAG hydrolysis in the intestinal lumen has been employed to combat obesity and associated disorders with modest efficacy and unwanted side effects. The therapeutic potential of inhibiting specific enzymes involved in intestinal TAG synthesis warrants further investigation.

Lipidomic and spatio-temporal imaging of fat by mass spectrometry in mice duodenum during lipid digestion

Intestinal absorption of dietary fat is a complex process mediated by enterocytes leading to lipid assembly and secretion of circulating lipoproteins as chylomicrons, vLDL and intestinal HDL (iHDL). Understanding lipid digestion is of importance knowing the correlation between excessive fat absorption and atherosclerosis. By using time-of-flight secondary ion mass spectrometry (TOF-SIMS), we illustrated a spatio-temporal localization of fat in mice duodenum, at different times of digestion after a lipid gavage, for the first time. Fatty acids progressively increased in enterocytes as well as taurocholic acid, secreted by bile and engaged in the entero-hepatic re-absorption cycle. Cytosolic lipid droplets (CLD) from enterocytes were originally purified separating chylomicron-like, intermediate droplets and smaller HDL-like. A lipidomic quantification revealed their contents in triglycerides, free and esterified cholesterol, phosphatidylcholine, sphingomyelin and ceramides but also in free fatty acids, mono- and di-acylglycerols. An acyl-transferase activity was identified and the enzyme monoacylglycerol acyl transferase 2 (MGAT2) was immunodetected in all CLD. The largest droplets was also shown to contain the microsomal triglyceride transfer protein (MTTP), the acyl-coenzyme A-cholesterol acyltransferases (ACAT) 1 and 2, hormone sensitive lipase (HSL) and adipose triglyceride lipase (ATGL). This highlights the fact that during the digestion of fats, enterocyte CLD contain some enzymes involved in the different stages of the metabolism of diet fatty acids and cholesterol, in anticipation of the crucial work of endoplasmic reticulum in the process. The data further underlines the dual role of chylomicrons and iHDL in fat digestion which should help to efficiently complement lipid-lowering therapy.

Nocturnin regulates circadian trafficking of dietary lipid in intestinal enterocytes

BACKGROUND

Efficient metabolic function in mammals depends on the circadian clock, which drives temporal regulation of metabolic processes. Nocturnin is a clock-regulated deadenylase that controls its target mRNA expression posttranscriptionally through poly(A) tail removal. Mice lacking nocturnin (Noc(-/-) mice) are resistant to diet-induced obesity and hepatic steatosis yet are not hyperactive or hypophagic.

RESULTS

Here we show that nocturnin is expressed rhythmically in the small intestine and is induced by olive oil gavage and that the Noc(-/-) mice have reduced chylomicron transit into the plasma following the ingestion of dietary lipids. Genes involved in triglyceride synthesis and storage and chylomicron formation have altered expression, and large cytoplasmic lipid droplets accumulate in the apical domains of the Noc(-/-) enterocytes. The physiological significance of this deficit in absorption is clear because maintenance of Noc(-/-) mice on diets that challenge the chylomicron synthesis pathway result in significant reductions in body weight, whereas diets that bypass this pathway do not.

CONCLUSIONS

Therefore, we propose that nocturnin plays an important role in the trafficking of dietary lipid in the intestinal enterocytes by optimizing efficient absorption of lipids.

Jejunal wall triglyceride concentration of morbidly obese persons is lower in those with type 2 diabetes mellitus

The overproduction of intestinal lipoproteins may contribute to the dyslipidemia found in diabetes. We studied the influence of diabetes on the fasting jejunal lipid content and its association with plasma lipids and the expression of genes involved in the synthesis and secretion of these lipoproteins. The study was undertaken in 27 morbidly obese persons, 12 of whom had type 2 diabetes mellitus (T2DM). The morbidly obese persons with diabetes had higher levels of chylomicron (CM) triglycerides (P < 0.001) and apolipoprotein (apo)B48 (P = 0.012). The jejunum samples obtained from the subjects with diabetes had a lower jejunal triglyceride content (P = 0.012) and angiopoietin-like protein 4 (ANGPTL4) mRNA expression (P = 0.043). However, the apoA-IV mRNA expression was significantly greater (P = 0.036). The jejunal triglyceride content correlated negatively with apoA-IV mRNA expression (r = -0.587, P = 0.027). The variables that explained the jejunal triglyceride content in a multiple linear regression model were the insulin resistance state and the apoA-IV mRNA expression. Our results show that the morbidly obese subjects with diabetes had lower jejunal lipid content and that this correlated negatively with apoA-IV mRNA expression. These findings show that the jejunum appears to play an active role in lipid homeostasis in the fasting state.

Effect of rhGH on the synthesis and secretion of VLDL to lymph and plasma from the intestine of the female rat

The intravenous administration of rhGH (recombinant human Growth Hormone) to fasting female rats causes an increase in the rate of synthesis and secretion of VLDL (very low density lipoproteins). This phenomenon has three striking characteristics: (1) the demonstration of an unexpected lipogenic effect of rhGH, (2) its rapid occurrence after intravenous injection of the hormone and (3) the apparent dependence on the levels of circulating estrogens, as deduced by the lack of effect of rhGH on males and castrated females. The target tissue for the lipogenic effect was traced to the intestine by means of perfusion experiments of isolated duodenal loops. Impairment of liver blood supply discarded this tissue as the source of VLDL induced by rhGH. After a single dose of rhGH (T(1/2)=16min), the increase in plasma TAG (triacylglycerides) levels followed a positive exponential course that lasted ca. 3h. The same phenomenon (with no significant differences in kinetic parameters) was observed in three other experimental circumstances: fasting intact virgin female rats with impaired hepatic circulation, perfusion of isolated duodenum and sampling of mesenteric lymph. It is assumed that rhGH stimulates the synthesis of TAG and VLDL by the physiological mechanisms already present in enterocytes. Because increased plasma levels of VLDL and GH have been demonstrated in the last week of rat pregnancy, we believe that the reported phenomenon has physiological implications, hypothetically associated with fetal lung maturation. As an hypothesis, we suggest that the effect of growth hormone (of pituitary or placental origin) on the synthesis and secretion of VLDL by enterocytes uses a nongenomic pathway.

Lipid-based delivery systems and intestinal lymphatic drug transport: a mechanistic update

After oral administration, the majority of drug molecules are absorbed across the small intestine and enter the systemic circulation via the portal vein and the liver. For some highly lipophilic drugs (typically log P > 5, lipid solubility > 50 mg/g), however, association with lymph lipoproteins in the enterocyte leads to transport to the systemic circulation via the intestinal lymph. The attendant delivery benefits associated with lymphatic drug transport include a reduction in first-pass metabolism and lymphatic exposure to drug concentrations orders of magnitude higher than that attained in systemic blood. In the current review we briefly describe the mechanisms by which drug molecules access the lymph and the formulation strategies that may be utilised to enhance lymphatic drug transport. Specific focus is directed toward recent advances in understanding regarding the impact of lipid source (both endogenous and exogenous) and intracellular lipid trafficking pathways on lymphatic drug transport and enterocyte-based first-pass metabolism.

Lipids and lipid-based formulations: optimizing the oral delivery of lipophilic drugs

Highly potent, but poorly water-soluble, drug candidates are common outcomes of contemporary drug discovery programmes and present a number of challenges to drug development - most notably, the issue of reduced systemic exposure after oral administration. However, it is increasingly apparent that formulations containing natural and/or synthetic lipids present a viable means for enhancing the oral bioavailability of some poorly water-soluble, highly lipophilic drugs. This Review details the mechanisms by which lipids and lipidic excipients affect the oral absorption of lipophilic drugs and provides a perspective on the possible future applications of lipid-based delivery systems. Particular emphasis has been placed on the capacity of lipids to enhance drug solubilization in the intestinal milieu, recruit intestinal lymphatic drug transport (and thereby reduce first-pass drug metabolism) and alter enterocyte-based drug transport and disposition.

Sn-2-monoacylglycerol, not glycerol, is preferentially utilised for triacylglycerol and phosphatidylcholine biosynthesis in Atlantic salmon (Salmo salar L.) intestine

Pathways of lipid resynthesis in the intestine of fish are relatively unknown. Various reports have suggested the existence of both sn-1,3-specific (pancreatic) and non-specific (bile salt-activated) lipase activity operating on dietary triacylglycerol (TAG) in the intestinal lumen of fish during digestion. Thus, sn-2-monoacylglycerol (2-MAG) and glycerol, respective hydrolytic products of each lipase, are absorbed and utilised for glycerolipid synthesis in enterocytes via two alternative routes: monoacylglycerol (MAG) and glycerol-3-phosphate (G3P) pathways. Despite different precursors, both pathways converge at the production of sn-1,2-diacylglycerol (1,2-DAG) where TAG or phosphatidylcholine (PC) synthesis can occur. To elucidate the relative activities of MAG and G3P pathways in Atlantic salmon enterocytes, intestinal segments were mounted in Ussing chambers where equimolar mixtures of sn-2-oleoyl-[1,2,3-(3)H]glycerol (2-MAG) and [(14)C(U)]glycerol, plus unlabelled 16:0 and 18:2n-6 as exogenous fatty acid sources, were delivered in bile salt-containing Ringer solution to the mucosa. The MAG pathway predominated, over the G3P pathway, synthesizing ca. 95% of total TAG and ca. 80% of total PC after a 3 h incubation period at 10 degrees C. Further, the 1,2-DAG branch point into TAG or PC was polarised towards TAG synthesis (6:1) via the MAG pathway but more evenly distributed between TAG and PC (1:1) via the G3P pathway. Effect of long-chain saturated, monounsaturated and polyunsaturated fatty acids on the synthesized TAG/PC ratio was assessed by individually exchanging 16:0, 18:1n-9 or 18:2n-6, for 16:0+18:2n-6, in mucosal solutions. TAG synthesis was influenced considerably more than PC synthesis, via either pathway, by exogenous fatty acids utilised. 18:1n-9 significantly stimulated TAG synthesis via the MAG pathway yielding a TAG/PC ratio of 12:1. Alternatively, 18:2n-6 stimulated TAG synthesis the most via the G3P pathway (TAG/PC=4:1). 16:0 significantly attenuated TAG synthesis via either pathway. Micellar fatty acid species also significantly affected intestinal active transport mechanisms as shown by decreasing transepithelial potential (TEP) and short-circuit current (SSC) with increasing fatty acid unsaturation. The epithelial integrity was, however, not compromised after 3 h of exposure to any of the fatty acids. The implications of these findings on dietary fatty acid composition and enterocytic lipid droplet accumulation are discussed.

AN EXAMINATION OF THE INTERPLAY BETWEEN ENTEROCYTE-BASED METABOLISM AND LYMPHATIC DRUG TRANSPORT IN THE RAT

The current study has examined whether drugs that are transported to the systemic circulation via the intestinal lymph (and therefore associate with lipoproteins within the enterocyte) are accessible to enterocyte-based metabolic processes. The impact of changes to the mass of lipid present within the enterocyte-based lymph lipid precursor pool (LLPP) on the extent of enterocyte-based drug metabolism has also been addressed. Low (5 mg oleic acid/h) or high [20 mg oleic acid/5.2 mg lyso-phosphatidylcholine/h] lipid dose formulations containing halofantrine (which is lymphatically transported and metabolized) or dichlorodiphenyltrichloroethane (DDT) (which is lymphatically transported and relatively metabolically inert) and radiolabeled oleic acid were infused into the duodenum of lymph duct-cannulated rats. After 5 h, drug and radiolabeled oleic acid were removed from the infusions, allowing calculation of the first order turnover rate constants describing drug and oleic acid transport from the LLPP into lymph from the washout profiles. In one group of animals, bolus doses of ketoconazole were also administered to inhibit cytochrome P450-based metabolism. The rate constant describing halofantrine transport from the LLPP into the lymph was lower than that of oleic acid, whereas these differences were abolished in the presence of ketoconazole. DDT and oleic acid exhibited similar turnover rate constants. The data therefore suggest that enterocyte-based metabolism removes halofantrine from the LLPP before transport into the lymph. Furthermore, enhancing the lymphatic transport of halofantrine by coadministration of larger quantities of lipid reduced the difference between the turnover rate constant for halofantrine and oleic acid and seemed to reduce the extent of enterocyte-based metabolism.

The Lymph Lipid Precursor Pool Is a Key Determinant of Intestinal Lymphatic Drug Transport

The influence of the size and turnover kinetics of the enterocyte-based lymph lipid precursor pool (LLPP) on intestinal lymphatic drug transport has been examined. Mesenteric lymph duct-cannulated rats were infused intraduodenally with low (2-5 mg/h) or high (20 mg/h) lipid-dose formulations containing 100 microg/h halofantrine (Hf, a model drug) and 1 microCi/h (14)C-oleic acid (OA) (as a marker for lipid transport) until steady-state rates of lipid(dX(L)/dt)(ss) and drug (dD(L)/dt)(ss) transport in lymph were obtained. After 5 h, the infusion was changed to formulations of the same composition but excluding (14)C-OA and Hf, allowing calculation of the first order rate constants describing turnover of lipid (K(X)) and drug (K(D)) from the LLPP into the lymph from the washout kinetics. The mass of lipid (X(LP)) and drug (D(LP)) in the LLPP was also determined. Biliary-lipid output was determined in a separate group of rats that had been infused with the same formulations. The results indicate that after administration of high lipid doses, lymphatic drug transport is dependent on the mass of exogenous lipid available in the LLPP and the rate of lipid pool turnover into the lymph. In contrast, after administration of low lipid doses, biliary-derived endogenous lipids are most likely to be the primary drivers of drug incorporation into the LLPP and lymph. Therefore, the LLPP size and composition seem to be major determinants of lymphatic drug transport, and formulation components, which increase lipid pool size, may therefore enhance lymphatic drug transport.

Bile Increases Intestinal Lymphatic Drug Transport in the Fasted Rat

PURPOSE

This study was conducted to determine the influence of (1) the source of recruited endogenous fatty acid (FA), and (2) bile on intestinal lymphatic transport of halofantrine (Hf) in fasted rats.

METHODS

Endogenous FA output in bile, and exogenous ((14)C radiolabeled) FA, endogenous FA, and Hf transport in mesenteric lymph were determined following administration of low dose lipid formulations containing either 4 or 40 mg of exogenous FA [oleic acid (OA)] and different amounts of bile salt (BS) and lysophosphatidylcholine (LPC) to fasted rats.

RESULTS

Administration of 40 mg of OA recruited endogenous FA and Hf transport into intestinal lymph, whereas 4 mg OA did not. However, addition of BS to the 4-mg OA dose led to stimulation of endogenous FA recruitment into lymph and an increase in lymphatic transport of Hf and endogenous FA output in bile. Addition of LPC to the 4-mg OA dose (dispersed in BS) caused a substantial increase in endogenous FA transport in lymph; however, no coincident increase in either lymphatic transport of Hf or endogenous FA output in bile was observed.

CONCLUSION

Biliary-derived endogenous FA has a higher propensity to support lymphatic transport of Hf compared to other sources of endogenous FA. The results suggest that this is related to the disparate trafficking of these alternate sources of endogenous FA within the enterocyte.