Intestinal contribution to secretion of very low density lipoproteins into plasma

Immunohistochemical and histoplanimetrical study on the endothelial receptor involved in transportation of minute chylomicrons into subepithelial portal blood in intestinal villi of the rat jejunum

A portion of the minute chylomicrons less than 75 nm in diameter are transcytosed from the extravascular tissue into the subepithelial blood capillaries (sBC) in the villous apices of the rat jejunum. However, the details of the transportation mechanism have not been clarified. In this study, the endothelial receptor involved in the transportation of minute chylomicrons into the sBC’s lumina was immunohistochemically and histoplanimetrically examined in intestinal villi of the rat jejunum. Immunopositivity for very low density lipoprotein (VLDL) receptor was detected on the luminal and basal surfaces of the endothelial cells of sBC in approximately 68% of those apices of jejunal villi that possessed numerous chylomicrons in the lamina propria, while VLDL receptor was detected on the endothelial cells of sBC in only approximately 8% of intestinal villi that possessed few or no chylomicrons in the lamina propria. No immunopositivity for LDL receptor was detected in the sBC of all intestinal villi. These findings suggest that VLDL receptor is expressed by the endothelial cells of the sBC in conjunction with the filling of the lamina propria of jejunal villi with many chylomicrons produced by the villous columnar epithelial cells and that the VLDL receptor mediates the transportation of minute chylomicrons, maybe VLDL, into the subepithelial portal blood from the extravascular tissue of the rat jejunal villi.

The effect of administered dose of lipid-based formulations on the in vitro and in vivo performance of cinnarizine as a model poorly water-soluble drug

The influence of varying the amount of lipid co-administered with the drug on drug solubilisation and absorption is poorly understood. In the current study, the effect of lipid dose on the in vitro drug distribution is compared with the in vivo absorption of cinnarizine (CZ) when formulated using long-chain triacylglyceride (LCT) and medium-chain triacylglycerides (MCT). At a fixed drug-lipid ratio, in the closed in vitro model, the drug concentrations in the aqueous phase increased and decreased for MCT and LCT, respectively, with increasing lipid dose. However, in vivo, the oral bioavailability (F%) of CZ was independent of the quantity of lipid administered for both MCT and LCT, but was higher for LCT (32.1 ± 2.3%) than for MCT (16.6 ± 2.3%). Increasing the quantity of lipid relative to the dose of CZ resulted in an increase in the oral F% when the lipid mass was increased from 125 to 250 mg, but was no greater at 500 mg lipid dose. The results confirm the limitations of the in vitro model but positively indicate that the use of the rat as a pre-clinical model for studying the bioavailability of poorly water-soluble drugs is not compromised by the mass of formulation administered.

Intestinal lymphatic transport for drug delivery

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.

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.

Assembly and secretion of hepatic very-low-density lipoprotein

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of very-low-density-lipoprotein lipid secretion in hepatocyte cultures derived from diabetic animals

Hepatocytes were derived from 2-3-day streptozotocin-diabetic rats and maintained in culture for up to 3 days. Compared with similar cultures from normal animals, these hepatocytes secreted less very-low-density-lipoprotein (VLDL) triacylglycerol, but the decrease in the secretion of VLDL non-esterified and esterified cholesterol was not so pronounced. This resulted in the secretion of relatively cholesterol-rich VLDL particles by the diabetic hepatocytes. Addition of insulin for a relatively short period (24 h) further decreased the low rates of VLDL triacylglycerol secretion from the diabetic hepatocytes. The secretion of VLDL esterified and non-esterified cholesterol also declined. These changes occurred irrespective of whether or not exogenous fatty acids were present in the culture medium. Little or no inhibitory effect of insulin was observed after longer-term (24-48 h) exposure to the hormone. Both dexamethasone and a mixture of lipogenic precursors (lactate plus pyruvate) stimulated VLDL triacylglycerol and cholesterol secretion, but not to the levels observed in hepatocytes from normal animals. The low rate of hepatic VLDL secretion in diabetes contrasts with the increase in whole-body VLDL production rate. This suggests that the intestine is a major source of plasma VLDL in insulin-deficient diabetes.

Expression of rat apolipoprotein A-IV and A-I genes: mRNA induction during development and in response to glucocorticoids and insulin

Rat apolipoproteins (apo-) A-IV and A-I share many structural similarities, the most notable of which is a domain of repeated docosapeptides with amphipathic helical potential. Although the genes for apo-A-IV and apo-A-I probably diverged from a common ancestor, these proteins seem to have developed different functions in their evolution. In the present study, cloned cDNAs were used to characterize the expression of apo-A-IV and apo-A-I mRNAs in a wide variety of adult rat tissues, as well as in small intestine and liver obtained from fetal, suckling, and weanling animals; comparisons were made to the expression of apo-E mRNA. The apo-A-IV and apo-A-I mRNAs were most abundant in adult small intestine and liver, with trace amounts detected in other tissues. Substantial amounts of these mRNAs were detected in the yolk sac, suggesting that this fetal tissue plays an important role in lipid metabolism during gestation. Noncoordinate accumulation of apo-A-IV and apo-A-I mRNAs was observed within and between the liver and small intestine during neonatal development. The apo-A-IV mRNA levels in the developing small intestine and liver appeared to correlate with their triglyceride secretion rates, suggesting that this protein plays an important role in the metabolism of triglyceride-rich lipoproteins. When dexamethasone (0.1 microM), insulin (0.01 microM), or insulin and dexamethasone together were incubated with primary cultures of nonproliferating adult rat hepatocytes, apo-A-IV mRNA levels were 4-, 7-, and 11-fold higher, respectively, than in non-hormone-treated control hepatocytes. Hormone administration resulted in a 2-fold greater amount of apo-A-I mRNA in each case, with no significant change in the level of apo-E mRNA. The overall results suggest that these structurally related apolipoproteins are regulated in substantially different ways.

Regulation of serum apolipoprotein E metabolism: role of chylomicron metabolism

The mechanism by which high-fat, high-cholesterol diets lower serum apolipoprotein E (apoE) concentration was studied in rats by feeding a single high-fat, high-cholesterol meal and by intravenously infusing chylomicrons containing low and high amounts of cholesterol. Serum apoE concentrations were unchanged 9 hours after an olive oil meal but were decreased by 35% after an olive oil/cholesterol-enriched meal. The decrease in serum apoE concentrations with the olive oil/cholesterol meal was accompanied by a decrease in apoE concentration in the high density lipoprotein fraction. Three hours after the intravenous infusion of cholesterol-enriched chylomicrons, serum apoE concentrations decreased 40%, whereas serum apoE concentrations decreased by only 10% when chylomicrons with low cholesterol concentrations or saline were infused. It is concluded that the metabolism of cholesterol-enriched chylomicrons results in an increased removal of serum apoE and that the cholesterol content of chylomicrons is a determinant of serum apoE removal.

The prognostic significance of hypocholesterolemia in hospitalized patients

Hypocholesterolemia (less than 120 mg/dl) was found in 200 out of 3,700 patients admitted consecutively to a University Department of Internal Medicine. In 91, hypocholesterolemia as defined above could be confirmed in a second serum sample on the following day. Mortality during hospitalization in this group was 32%. When diagnoses were grouped into 5 categories, prognosis was particularly poor in heart (36%) and liver disease (31%) and in malignancies (33%), while patients with malabsorption syndromes had a considerably better prognosis (10% mortality). Hypocholesterolemia was associated with a low cholesterol/phospholipid ratio and may be among the first signs of further deterioration of disease.

Apolipoprotein B is structurally and metabolically heterogeneous in the rat

Electrophoresis of rat apolipoprotein B (apoB) on 5% polyacrylamide gels in the presence of NaDodSO4 separates three major components: PI, which comigrates with human low density lipoprotein (LDL) apoB; PII, a slightly faster-moving satellite band; and PIII, which migrates somewhat more slowly than myosin heavy chain. The proportion of PIII decreases with increasing density of the parent rat lipoprotein, from 90% an 70%, respectively, in chylomicrons and very low density lipoproteins (VLDL), to 7% in the major LDL2 (density 1.038-1.063 g/ml) fraction. A major component that comigrates with rat PIII is a marker for human chylomicron apoB, being absent from human VLDL, intermediate density lipoprotein (IDL), and LDL. Preliminary immunological and peptide mapping data show that rat apoB PI and PIII are closely related structurally, with the latter possibly being a large fragment of the former. Both peptides are synthesized in rat liver and found in Golgi secretory vesicles. Kinetic tracer experiments show that rat PI and PIII are present on separate VLDL particles, both of which are extensively removed from the circulation at the remnant stage, and that the declining PIII-to-PI/II ratios in IDL and LDL may be attributed to the more rapid turnover of PIII-containing lipoproteins at all levels, particularly within the LDL density range.

Effect of the type of diet on the distribution of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in rat small intestine

The jejunoileal gradient for the HMG-CoA reductase activity in the microsomal fraction of the epithelial cells of the small intestine of rats given commercial pellets was reserved within a few days after changing the ration to a semipurified diet. The response of the reductase was essentially the same in villus and crypt cells.

Mechanism of hypertriglyceridaemia in diabetic patients with fasting hyperglycaemia

Several aspects of lipid metabolism were studied to define the mechanism of hypertriglyceridaemia in insulin-independent diabetic patients with fasting hyperglycaemia. Patients with insulin-independent diabetes were more obese (p < 0.001) and had a significantly (p < 0.001) higher mean (+/- SEM) fasting plasma triglyceride concentration (387 +/- 66 mg/dl) than did either insulin-dependent diabetics (133 +/- 11 mg/dl) or normal (73 +/- 1 mg/dl) subjects. Very low density lipoprotein secretion rate was also significantly (p < 0.01 - < 0.001) higher in patients with insulin-independent diabetes (14.65 +/- 1.37 mg/kg/h) as compared to 7.64 +/- 0.60 mg x kg/h and 9.86 +/- 0.75 mg/kg/h in normal subjects and patients with insulin-dependent diabetes, respectively. However, the relationship between plasma triglyceride concentration and very low density lipoprotein-triglyceride secretion was similar in diabetics and in normals. The diabetic groups had equivalent degrees of fasting and postprandial hyperglycaemia, and comparable elevations of fasting plasma nonesterified free fatty acid levels (insulin-independent = 0.72 +/- 0.07 mmol/L, insulin-dependent = 0.63 +/- 0.08 mmol/L). Postprandial plasma insulin concentrations, however, reached normal levels in insulin-independent diabetics and were higher (p < 0.001) than in insulin-dependent diabetics. Thus, hypertriglyceridaemia in insulin-independent diabetics with fasting hyperglycaemia was associated with increased hepatic very low density lipoprotein-triglyceride secretion, and normal plasma insulin levels. The lower triglyeride levels in the insulin-dependent diabetics is assumed to be due to their relative hypoinsulinaemia.