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Diabetes Volume 64, July 2015
Changting Xiao,1 Satya Dash,1 Cecilia Morgantini,1 Khosrow Adeli,2 and Gary F. Lewis1
Gut Peptides Are Novel Regulators of Intestinal Lipoprotein Secretion: Experimental and Pharmacological Manipulation of Lipoprotein Metabolism
PERSPECTIVES IN DIABETES
Diabetes 2015;64:2310–2318 | DOI: 10.2337/db14-1706
Individuals with metabolic syndrome and frank type 2 diabetes are at increased risk of atherosclerotic cardiovascular disease, partially due to the presence of lipid and lipoprotein abnormalities. In these conditions, the liver and intestine overproduce lipoprotein particles, exacerbating the hyperlipidemia of fasting and postprandial states. Incretin-based, antidiabetes therapies (i.e., glucagon-like peptide [GLP]-1 receptor agonists and dipeptidyl peptidase-4 inhibitors) have proven efficacy for the treatment of hyperglycemia. Evidence is accumulating that these agents also improve fasting and postprandial lipemia, the latter more significantly than the former. In contrast, the gut-derived peptide GLP-2, cosecreted from intestinal L cells with GLP-1, has recently been demonstrated to enhance intestinal lipoprotein release. Understanding the roles of these emerging regulators of intestinal lipoprotein secretion may offer new insights into the regulation of intestinal lipoprotein assembly and secretion and provide new opportunities for devising novel strategies to attenuate hyperlipidemia, with the potential for cardiovascular disease reduction.
Individuals with metabolic syndrome or frank type 2 diabetes (T2D) are at increased risk for cardiovascular disease (CVD) contributed to by the atherogenic dyslipidemia that frequently accompanies these conditions (1,2). The typical lipid abnormalities in these conditions include elevated plasma triglycerides (TG) carried
predominantly in triglyceride-rich lipoproteins (TRLs) (e.g., VLDL and chylomicrons), low HDL cholesterol (HDL-C) and HDL particle numbers, and qualitative changes in lipoproteins (e.g., predominance of small, dense LDL and glycation and oxidation of lipoprotein particles) (3). Elevated TRL plays a pathophysiological role in HDL lowering and the generation of small, dense LDL via lipid exchange and modification of the composition of these denser lipoprotein classes (4–6). In addition to the well-documented, impaired clearance of TRL particles from the circulation, both intestinal (apolipoprotein [apoB]-48–containing) and hepatic (apoB-100–containing) TRL production are increased in insulin-resistant states and T2D (7). Understanding the molecular regulation of lipoprotein production by the liver and intestine may provide clues for the development of therapeutic strategies to attenuate the atherogenic dyslipidemia. Intestinal lipoprotein (chylomicron) production, in particular, is no longer viewed as being regulated only by fat ingestion and luminal fat content, although ingested fat is still unarguably the dominant determinant of intestinal lipoprotein production. Instead, it is now appreciated that chylomicron production, even in the fasted state, is subject to a variety of hormonal and nutritional effectors and is dysregulated in insulin-resistant and diabetic states (8). More recently, studies of the role of gut hormones, glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2), and pharmacological regulators of their plasma concentration have provided new insights
1Departments of Medicine and Physiology and Banting & Best Diabetes Centre, University of Toronto, Toronto, Ontario, Canada 2Program in Molecular Structure & Function, Research Institute, The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada
Received 6 November 2014 and accepted 22 February 2015.
Corresponding author: Gary F. Lewis,
[email protected].
See accompanying article, p. 2338.
© 2015 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered.
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into the regulation of chylomicron production, which will be discussed in this Perspective. INCRETIN-BASED THERAPIES IMPROVE FASTING AND POSTPRANDIAL LIPID PROFILES IN PATIENTS WITH T2D
Incretin-based therapies (i.e., GLP-1 receptor [GLP-1R] agonists and dipeptidyl peptidase-4 [DPP-4] inhibitors) are antidiabetes agents with proven efficacy in glucose lowering (9,10). The mechanism of action of these agents centers on modulation of pancreatic hormone secretion by GLP-1. GLP-1 is secreted by the intestinal L cells in response to meal ingestion. It enhances glucose-dependent insulin secretion and inhibits glucagon secretion, among other biological functions. GLP-1R agonists activate the GLP-1R, thus providing an exogenous source of GLP-1 activity. DPP-4 inhibitors preserve the endogenously secreted GLP-1 by inhibiting the enzyme DPP-4 that rapidly degrades GLP-1 and many other peptides (11). These compounds offer several additional benefits compared with other classes of antidiabetes medications, including a low risk of hypoglycemia. With long-term use, incretinbased therapies also improve insulin sensitivity and pancreatic function and are weight neutral (DPP-4 inhibitors) or induce weight loss (GLP-1R agonists), which contribute to effective glycemic control. Besides glucose lowering, evidence is accumulating that incretin-based therapies provide pleiotropic benefits, one of which is improvement in lipid parameters. GLP-1R Agonists Improve Fasting and Postprandial Lipid Profiles in Patients With T2D
Improvement in both fasting and postprandial lipid profiles has been documented in clinical trials examining the glucose-lowering efficacy of several GLP-1R agonists and DPP-4 inhibitors in patients with T2D. Several recent meta-analyses indicated modest reductions in fasting LDL-C, total cholesterol, and TG with small or no significant improvement in HDL-C following chronic treatments with GLP-1R agonists (12–14) or DPP-4 inhibitors (13,15). For example, treatment with exenatide or liraglutide for several weeks to 3 years reduced fasting levels of TG, total cholesterol and LDL-C, increased HDL-C (16–19); and reduced fasting apoB (19–22), apoB-48 (a surrogate measure of intestinal lipoprotein particle numbers) (23), and free fatty acid (FFA) (24– 26). Compared with GLP-1R agonists, DPP-4 inhibitors are less effective in lowering fasting lipids. In some but not all studies, sitagliptin decreased fasting TG (21) and decreased both fasting TG and apoB-48 (27). Alogliptin decreased fasting TG, apoB-48, and remnant-like particles (28), and anagliptin reduced fasting TG, apoB, and LDL-C (29). These effects may be related to the efficacy of these therapies in improving glycemic control with both GLP-1R agonists and DPP-4 inhibitors (12,30,31) and in reducing weight that is more notable with GLP-1R agonists than DPP-4 inhibitors (12,32,33). Future studies are required to determine the contribution of weight loss and improved
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glycemic control to fasting and postprandial lipids and to delineate the effects of these compounds on lipids independent of improved glycemic control and weight loss. GLP-1R agonists are consistently associated with improvements in postprandial lipids (Table 1). Postprandial TG was lowered with exenatide (16,26,34,35) or liraglutide treatments (23) of varied treatment length, ranging from several weeks to 1 year. Besides postprandial TG, 51 weeks of treatment with exenatide also decreased apoB-48, VLDL cholesterol, and FFA during mixed-meal tests (16). Similarly, liraglutide reduced postprandial apoB-48 concentrations, along with reductions in total cholesterol and LDL-C (23). In healthy humans, a 390-min infusion of native GLP-1 markedly reduced postprandial TG excursion (36). In individuals with impaired glucose tolerance or recent-onset T2D, a single subcutaneous dose of exenatide also attenuated postprandial TG excursion (37). GLP-1R agonists are also associated with qualitative changes in lipoprotein particles and atherosclerosisrelated markers. For instance, exenatide treatment decreased oxidative stress markers (e.g., malondialdehyde and oxidized LDL-to-LDL-C ratio [16]), while liraglutide shifted the composition of LDL particles away from small, dense LDL particles and decreased the ratio of apoB/ apoA-I (19). DPP-4 Inhibitors Decrease Postprandial TG and apoB-48
Improvements in postprandial lipids by DPP-4 inhibitors were more notable compared with fasting lipids (Table 1). For instance, vildagliptin treatment for 4 weeks lowered postprandial chylomicron TG (CM-TG) and apoB-48 (38), and sitagliptin treatment for 2 weeks decreased postprandial TG (26). Sitagliptin treatment for 6 weeks decreased postprandial TG and apoB-48, total apoB, VLDL cholesterol, and FFA during an oral lipid tolerance test (39). Beneficial effects on postprandial lipids were also observed with alogliptin, thus 16 weeks of alogliptin treatment decreased postprandial TG in plasma and TRL, along with decreased chylomicron apoB-48 and cholesterol (28), and 1-week treatment decreased postprandial TG and apoB in subjects without diabetes (40). Postprandial TG, apoB, and LDL-C were reduced with anagliptin (29). Vildagliptin therapy for 4 weeks decreased postprandial remnant-like particles and increased LDL size (41), qualitative changes in lipoprotein size and composition that have been associated with reduced risk of atherosclerosis. In addition, sitagliptin was found to reduce plasma markers of low-grade inflammation and cell adhesion molecules (42), which are also potentially antiatherosclerotic changes. No clear contribution of fasting TG to postprandial TG excursion can be established from the chronic studies that examined both. Decreased postprandial TG response to 3week treatment with liraglutide was not accompanied by decreased fasting TG (23). With regard to DPP-4 inhibitors, both postprandial TG excursion and fasting TG were
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Table 1—Postprandial lipid responses to GLP-1R agonists and DPP-4 inhibitors Ref.
Subjects
Treatments
Changes in postprandial lipid parameters
GLP-1R agonists 36
Healthy
GLP-1, 390 min (1.2 pmol/kg/min infusion)
Abolished postprandial TG excursion, ↓ FFA (231%)
37
IGT, T2D
Exenatide, single dose (10 mg)
↓ TG, apoB-48, RLP-C, RLP-TG, apoCIII
26
T2D
Exenatide, 2 weeks (5 mg b.i.d. 1 week, then 10 mg b.i.d. 1 week)
↓ TG
34
T2D
Exenatide, 2 weeks (5 mg b.i.d. 1 week, then 10 mg b.i.d. 1 week)
↓ TG
35
T2D
Exenatide, 4 weeks (0.08 mg/kg/injection b.i.d., t.i.d.)
Trend toward ↓ TG
16
T2D
Exenatide, 1 year (5 mg b.i.d. 4 weeks, then increased to 10 mg b.i.d.)
↓ iAUC: TG, FFA, VLDL-C, apoB-48
23
T2D
Liraglutide, 3 weeks (weekly dose escalation, from 0.6 mg/day to 1.8 mg/day with 0.6 mg increment)
↓ AUC: TG (228%), apoB-48 (233%); ↓ iAUC: TG (257%), apoB-48 (257%)
DPP-4 inhibitors 26
T2D
Sitagliptin, 2 weeks (100 mg q.a.m.)
↓ TG
39
T2D
Sitagliptin, 6 weeks (100 mg/day)
↓ AUC: TG (29.4%), apoB-48 (27.8%), apoB (25.1%), VLDL-C (9.3%), FFA (27.6%)
38
T2D
Vildagliptin, 4 weeks (50 mg b.i.d.)
↓ iAUC: TG (222%), CM-TG (291%), CM-apoB-48, CM-C
41
T2D
Vildagliptin, 4 weeks (50 mg b.i.d.)
↓ AUC: TG (232%), RLP-TG (238%); ↑ LDL size
40
Healthy
Alogliptin, 1 week (25 mg/day)
↓ iAUC: TG, apoB-48, RLP-C; ↓ TG, apoB-48, RLP-C
28
T2D
Alogliptin, 16 weeks (25 mg/day)
↓ iAUC: TG, CM-TG, CM-apoB, VLDL1-TG, VLDL-apoB
29
T2D
Anagliptin, 12 weeks (200 mg/day)
↓ TG (258.9%), non–HDL-C (213%), LDL-C (27.9%), RLP-C (23.66%), apoB (25.7%), apoB-48 (28.37%)
Effects are postprandial concentrations, AUC, or iAUC, and wherever available, percentage difference from baseline or placebo in parentheses. AUC, area under the curve; C, cholesterol; iAUC, incremental AUC; IGT, impaired glucose tolerance; RLP, remnant-like lipoprotein particle.
reduced with alogliptin (28) or anagliptin (29). However, postprandial TG excursion was attenuated by sitagliptin (39) and vildagliptin (38) in T2D or alogliptin in healthy individuals (40) without significantly lower fasting TG. Future studies are needed to specifically examine to what extent changes in fasting TG contribute to the attenuation of postprandial TG excursion by GLP-1R agonists and DPP-4 inhibitors. The difference in fasting lipids with GLP-1R agonists and DPP-4 inhibitors may be attributed to the level of biological GLP-1 activity achieved with these two therapeutic strategies. Compared with subcutaneous injection of GLP-1R agonists, which raise GLP-1 plasma concentration to pharmacological levels, oral DPP-4 inhibitors only modestly elevate circulating GLP1 within the physiological range (43). GLP-1R agonists induce weight loss and delay gastric emptying, whereas DPP-4 inhibitors do not. It is also possible that, due to the broad spectrum of substrates of DPP-4, DPP-4 inhibitors affect lipid metabolism through mechanisms beyond GLP-1 activity. Mechanisms Whereby GLP-1R Agonists and DPP-4 Inhibitors Ameliorate Postprandial Lipemia
Improvement in postprandial lipemia by GLP-1R agonists and DPP-4 inhibitors may be related to less effective
dietary fat handling, decreased secretion, and/or increased clearance of intestinal lipoproteins. GLP-1 is well known to slow gastric emptying (44,45), which is expected to slow the passage of dietary fat to the small intestine. GLP-1 was also shown to inhibit gastric lipase secretion (46) and intestinal motility (47). These effects may translate into diminished efficiency with regard to processing of dietary fat. Exendin-4 treatment lowered jejunal microsomal triglyceride transfer protein activity and diminished jejunal TG availability in hamsters (48). This was associated with an increase in TG levels in the luminal content of the intestine 2 h post fat load, suggesting impairment in lipid absorption. Alternatively, exendin-4 slows gastric emptying, which could limit the amount of lipids available for absorption and chylomicron assembly in the small intestine. Whether GLP-1R agonist treatments are accompanied by increased fecal lipid excretion or compensatory absorption in the distal gut remains unknown. As neither GLP-1R agonists nor DPP4 inhibitors cause clinical steatorrhea, these compounds may be associated with slowed digestion and absorption rate without clinically apparent fat malabsorption. Reduced postprandial lipemia may be the result of decreased secretion and/or increased clearance of
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chylomicrons. In the very few studies so far that have examined intestinal lipoprotein kinetics in humans in response to GLP-1 agonists (exenatide, single dose in healthy subjects) (49) or DPP-4 inhibitors (sitagliptin, single dose in healthy subjects or 6-week treatment in T2D patients) (27,50), fractional catabolic rates were not significantly affected. The current available evidence thus supports the view that the reduced postprandial chylomicron concentration is primarily due to reduced production rather than an increase in particle clearance. With more chronic treatment, clearance of chylomicrons may be increased, but this has not yet been examined. Chronic use of GLP-1R agonists induces weight loss and both GLP-1R agonists and DPP-4 inhibitors attenuate insulin resistance and augment b-cell function. As both hepatic and intestinal lipoprotein production are stimulated by elevated circulation FFA in humans (51), greater antilipolytic effects of insulin on adipose tissue in this setting result in less FFA substrate delivery to the liver and intestine, which could potentially lead to attenuated VLDL-TG and/or CM-TG synthesis and secretion. Insulin resistance, in the whole body and at the intestinal level, is associated with overproduction of intestinal lipoproteins (52–54). Several lines of evidence also suggest that GLP-1 may modulate intestinal lipoprotein synthesis and secretion beyond gastric emptying, lipid digestion, and gut motility. In rats, when lipids were infused directly into the duodenum, GLP-1 infusion decreased lipid absorption, along with reduced lymph flow and apoB output (55). This indicates that the GLP-1 effect on intestinal lipid handling may occur between the intestinal lumen and lymph output. In hamsters, administration of sitagliptin or exendin-4 (a GLP-1R agonist) prior to oral fat load decreased TRL TG and apoB-48 accumulation in the circulation following triton injection (56). However, when exendin-4 was given 1 h after oral gavage of olive oil to allow lipids to pass through the stomach into the duodenum (i.e., to circumvent the slowing of gastric emptying by GLP-1), the suppression in TG and apoB-48 was not abolished (56). Our recent studies in healthy humans support an acute, direct inhibitory effect of incretin-based therapies on intestinal lipoprotein production (49,50). Intestinal lipoprotein synthesis and secretion are complex processes (8), with GLP-1 directly or indirectly modifying a number of the regulatory factors. For example, GLP-1 stimulates glucose-dependent insulin secretion and insulin acutely inhibits intestinal lipoprotein production (57). In addition, intestinal lipoprotein production is stimulated by elevated circulating FFA (51). Short-term GLP-1 infusion or exenatide injection suppressed circulating levels of FFA in humans (36,37), and sitagliptin treatment for 4 weeks also decreased postprandial levels of FFA (39), which is expected to contribute to the reduced apoB-48 production. The experimental design of our mechanistic study in humans allowed us to isolate the intestine-specific effect of the incretin-based therapies from gastric emptying,
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changes in pancreatic hormones or FFA, glycemic control, or weight reduction. To overcome the expected inhibition of gastric emptying with exenatide, nutrients were infused directly into the duodenum. TRL kinetics was studied under the conditions of a pancreatic clamp to minimize the changes in pancreatic hormone secretion. As a result, insulin levels were only modestly and transiently increased following exenatide injection. Under such conditions, intestinal lipoprotein production was significantly decreased by exenatide (49), suggesting a direct inhibitory effect, possibly at the intestinal enterocyte level. This notion was further supported by a subsequent study in humans in which a single dose of sitagliptin decreased intestinal lipoprotein production while insulin levels were matched in the treatment and placebo arms of the study (50). Plasma FFA levels were similar between exenatide or sitagliptin and placebo in both studies; therefore, the reduction in apoB-48 production was not due to changes in FFA levels in these experimental settings. Neither exenatide nor sitagliptin affected TRL apoB-100 production in acute studies in healthy humans (49,50). This might be due to the lack of GLP-1R expression in the liver (58). More recently, longer-term (6 weeks) treatment with sitagliptin suppressed both TRL apoB-48 and apoB-100 production rates in patients with T2D (27). The mechanism of improvement in hepatic lipoprotein production in this study remains to be further elucidated, although it is likely explained by indirect factors, such as improvement in glycemia with chronic use, rather than direct inhibition of hepatic lipoprotein synthesis and secretion. In contrast, a direct action of GLP-1 on intestinal lipoprotein production is supported by ex vivo studies in isolated hamster intestinal enterocytes, where exendin-4 inhibited apoB-48 secretion into the medium (56). Consistent with this, GLP-1R expression has been identified in the small intestine of humans (59). The molecular mechanisms where GLP-1 regulates chylomicron synthesis, assembly, and secretion remain to be elucidated. Interestingly, exendin4 administered directly into the central nervous system also suppressed intestinal lipoprotein secretion in hamsters (60). GLP-1R is expressed in certain regions in the brain, which underlies the mechanism whereby GLP-1 induces weight loss (61). The relative contribution of the central nervous system to the regulation of intestinal lipoprotein production has not been fully established, has not yet been demonstrated in humans, and warrants further study. Published data provide some clues but are not conclusive regarding particle size change with incretinbased therapy. GLP-1R agonists have been shown to lower postprandial TG concentration in healthy and insulinresistant humans following ingestion of a high-fat meal (36,37), as well as in rats, hamsters, and mice following oral lipid administration (56,62). A reduction in TRL TG concentration could be explained by fewer particles of unchanged size, smaller particles (carrying less TG per particle) but unchanged number, or the combination in
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which there are fewer particles that are also reduced in size. Given that GLP-1R agonists have been shown to reduce TRL apoB-48 concentration and secretion rate, this suggests that fewer particles are present that are either normal in size or are smaller, with reduced lipidation. We have observed that acute intraperitoneal exendin-4 treatment of healthy hamsters reduced the number of large TRL particles, suggesting a possible shift toward the formation of smaller chylomicrons (48). Taken together, these data suggest that in addition to fewer chylomicrons being secreted, it is possible that these particles are also smaller in size. In contrast, it should be noted that when lipid was infused intraduodenally (i.e., the effect of the GLP-1R agonist in slowing gastric emptying was experimentally “eliminated”) intraperitoneal exendin-4 lowered TRL apoB-48 concentration without affecting TRL TG concentration in healthy hamsters. These results mirror those from healthy humans receiving intraduodenal lipid infusion (49) and were found to result from a shift toward the secretion of larger, more highly lipidated particles during exendin-4 treatment (as assessed by fast protein liquid chromatography in the hamster; particle size not directly assessed in humans) (S. Farr, K. Adeli, unpublished data). This may suggest that GLP-1 has direct effects on intestinal apoB-48 output, while its ability to lower postprandial TG concentration and particle lipidation (size) may rely on proximal processes within the stomach or upper gastrointestinal tract. Further studies are necessary to confirm these findings and offer a mechanistic explanation. Native GLP-1, GLP-1R agonists, and DPP-4 inhibitors have been suggested to provide cardiovascular risk benefits (63–65). In addition, as DPP-4 cleaves several other peptides, many of which directly affect the heart and blood vessels, DPP-4 inhibitors may offer additional cardiovascular benefits beyond elevating GLP-1 (65). A less appreciated potential for these compounds to provide cardiovascular benefits is their capacity to improve lipid profiles, as discussed above. Prospective, controlled, hard CVD outcomes trials are required to investigate the true CVD risk benefits of incretin-based therapies, if they indeed exist. Two outcomes trials examined the DPP-4 inhibitors alogliptin in patients with T2D after acute coronary syndrome (66) and saxagliptin in patients with T2D who had a history or were at increased risk of CVD events (67). These trials demonstrated noninferiority over placebo but did not provide evidence of CVD risk reduction. It remains to be determined in ongoing and future clinical trials, some of which are soon to be released, whether GLP-1R agonists and/or DPP-4 inhibitors elicit CVD benefits. GLP-2 STIMULATES “PREFORMED” INTESTINAL LIPOPROTEIN PARTICLE RELEASE
GLP-2, a gut peptide that is also derived from posttranslational processing of the proglucagon gene in intestinal L cells (68), as is GLP-1, has also been implicated in intestinal lipid handling. It is cosecreted with GLP-1 in a 1:1 molar ratio in response to nutrient ingestion.
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GLP-2 promotes intestinal growth and nutrient absorption. It increases bowel mass by promoting proliferation and decreasing apoptosis of the enterocytes. GLP-2 also provides cytoprotection and plays a critical role in the adaptive response to intestinal injury and stress (69). This biological effect of GLP-2 is currently being used for treatment of short bowel syndrome and related gastrointestinal abnormalities, such as malabsorption, inflammation, and mucosal damage. Not surprising, with its enhancement in intestinal growth and function, GLP-2 is known to enhance intestinal absorption of several macronutrients, including carbohydrates and amino acids. This is associated with enhanced mucosal hexose transport, increased expression of genes encoding nutrient transporters, and increased expression of digestive enzymes along the gastrointestinal tract (69). GLP-2 regulation of intestinal lipid handling has recently been investigated. In animal models, GLP-2 enhanced intestinal luminal lipid absorption and increased secretion of intestinal lipoproteins in vivo, as well as TRL apoB-48 secretion ex vivo, in cultured jejunal fragments derived from hamsters (70). In healthy humans receiving an intravenous infusion of GLP-2 for 6.5 h, postprandial plasma TG and FFA concentrations were increased (71). The mechanism whereby GLP-2 exacerbates postprandial lipemia is not fully understood. In hamsters, GLP-2 administration did not stimulate apoB-48 synthesis or increase mRNA expression of Mttp that encodes microsomal triglyceride transfer protein. GLP-2 increased the expression of fully glycosylated CD36 in the jejunum, and its enhancement of chylomicron secretion was lost in CD362/2 mice (70). The role of CD36 in GLP-2 accentuation of postprandial lipemia in humans requires further elucidation as chylomicron remnants are increased in the postprandial state in humans with CD36 deficiency (72). GLP-2 infusion stimulated glucagon secretion and inhibited the secretion of gastric acid (71) and ghrelin (73) and modestly inhibited gastric emptying (74). The contribution of these factors to intestinal lipid uptake and secretion is not clear, but experimental hyperglucagonemia did not promote intestinal lipoprotein production in humans (75). In our recent study in healthy humans receiving intraduodenal infusion of mixed macronutrients, GLP-2 significantly increased TRL TG and apoB-48 concentrations within 30 min of a single subcutaneous injection during a pancreatic clamp (76). Such an effect was rapid and transient, peaking at approximately 1 h following GLP-2 administration and returned to baseline at around 3 h. This effect was not likely due to increased TRL particle production or decreased clearance, as suggested by mathematical modeling. Instead, it was most plausible that GLP-2 resulted in the release of stored, “preformed” particles from the enterocytes or lymph vessels. Indeed, 7 h after ingestion of a meal containing retinyl palmitate, which labels chylomicrons, GLP-2 injection resulted in a rapid rise in plasma and TRL retinyl palmitate as well as TRL apoB-48, indicating that GLP-2
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Figure 1—Mechanisms of GLP-1 and GLP-2 regulation of intestinal lipoprotein secretion. Gut peptides GLP-1 and GLP-2 affect intestinal lipid uptake and lipoprotein secretion via effects on multiple target organs. GLP-1 stimulates glucose-dependent insulin secretion, decelerates gastric emptying, and inhibits gut motility, which may indirectly contribute to decreased absorption of dietary fat and chylomicron secretion. GLP-1 may also directly inhibit chylomicron synthesis and secretion. Chronic enhancement of GLP-1 action, such as with administration of GLP-1R agonists or DPP-4 inhibitors, improves weight control, glycemia, and insulin sensitivity, which may indirectly reduce both intestinal and hepatic lipoprotein production. Collectively, GLP-1 reduces TRL production and lowers plasma TG. GLP-2 acutely stimulates the release of stored “preformed” chylomicrons from the enterocytes or lymphatic vessels into the circulation, thereby increasing plasma TG. The net physiological effect of these opposing and apparently paradoxical actions of two gut peptides, secreted on a 1:1 molar ratio from intestinal L cells, on dietary lipid absorption, chylomicron secretion, and whole-body lipid homeostasis remains to be elucidated.
promoted the release of intestinally stored lipids and lipoproteins. The effect of GLP-2 on chylomicron particle size has not been directly examined in humans. In healthy individuals receiving intraduodenal lipid infusion, GLP-2 increased TRL apoB-48 more robustly than TRL TG under pancreatic clamp conditions (76), suggesting release of smaller, less lipidated particles. Acute GLP-2 treatment of healthy hamsters raised both postprandial TG and apoB-48 concentrations, in part via enhanced CD36-mediated fatty acid uptake (56). An increase in TG accumulation in the chylomicron fractions of the plasma was observed with GLP-2 treatment, as assessed by fast-protein liquid chromatography (S. Farr, K. Adeli, unpublished data); however, further studies are needed to assess whether this can be attributed to increased particle number, size, or both. While the rapid mobilization of stored apoB-48 by GLP2 is a distinct feature, it is surprising considering the location of GLP-2 receptor (GLP-2R) expression. The GLP-2 receptor is expressed in various tissues, including the stomach, small and large bowel, brain, and lung, with the intestine having the highest expression of GLP-2Rs. Although the L cells that secrete GLP-2 are adjacent to the enterocytes, the GLP-2R is not expressed on the cell surface of enterocytes, the cells responsible for chylomicron synthesis,
assembly, and secretion (77). Instead, GLP-2Rs are expressed in the enteroendocrine cells of the proximal small intestine (jejunum). In porcine intestine, in addition to expression in enteroendocrine cells, GLP-2Rs are also expressed in endothelial nitric oxide synthase–expressing neurons and vasoactive intestinal polypeptide-positive enteric neurons (78). GLP-2 infusion upregulated endothelial nitric oxide synthase expression and activity and stimulated intestinal blood flow in pigs (78). In humans, GLP-2 increased blood flow primarily in the superior mesenteric artery (79). It is possible that GLP-2 stimulates chylomicron release by increasing local blood flow associated with enhanced nitric oxide production. This hypothesis remains to be tested in future studies. The different actions of GLP-1 and GLP-2 on intestinal lipoprotein secretion may be related to their difference in site of action (i.e., neural stimulation, indirect endocrine effect, or paracrine effect), which requires further elucidation. The net effect of these gut peptides on chylomicron secretion is further complicated by their different rates of clearance from the circulation. GLP-1 and GLP-2 are cosecreted in a 1:1 molar ratio from the L cells and both are substrates of DPP-4. However, their degradation by DPP-4 occurs at different rates. GLP-1 degradation is rapid, with a half-life of ;1.5 min, whereas GLP-2 is more
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stable with a half-life of ;7 min (80,81). This results in more sustained circulating levels of GLP-2 compared with GLP-1. In hamsters, short-term, simultaneous infusion of GLP-1 and GLP-2 at a 1:1 molar ratio increased intestinal lipid absorption and elevated TRL TG and apoB-48. With more prolonged (120 min) infusion or with sitagliptin inhibition of DPP-4 activity, lipid response to oral fat load was reduced (62). In humans, as discussed above, sitagliptin treatment inhibited intestinal lipoprotein production in a pattern similar to exenatide treatment (49,50). Although the effects described above are pharmacological, it is likely that GLP-2 facilitates dietary fat absorption under physiological conditions. With sustained GLP-1 activity, induced by treatments with long-lasting GLP-1R agonists or DPP-4 inhibitors, postprandial lipemia is attenuated by GLP-1. CONCLUSIONS AND FUTURE DIRECTIONS
Recent advances in the study of gut peptides GLP-1 and GLP-2 have provided new insight into the regulation of intestinal lipoprotein secretion (Fig. 1). Besides glycemic control, incretin-based therapies improve fasting lipid profiles in clinical trials, more so for GLP-1R agonists than DPP-4 inhibitors, and ameliorate postprandial lipemia during meal tests. This is achieved via multiple pathways, including GLP-1 action on pancreatic hormone secretion, gastric emptying, gut motility, weight control, and improvement in glycemic control and metabolic status. In addition, GLP-1R agonists and DPP-4 inhibitors have been shown to directly inhibit intestinal lipoprotein production. The related gut peptide GLP-2, on the other hand, enhances intestinal lipid absorption and the release of intestinally derived lipoprotein particles. With the increased use of incretin-based therapies in glycemic control and the GLP-2 analogs in the treatment of intestinal disorders, their actions in relation to the modulation of postprandial lipemia should be recognized and examined further. Several critical questions remain, such as the mechanisms of GLP-1R agonists and DPP-4 inhibitors on intestinal lipoprotein production, the long-term CVD outcomes of incretin-based therapies, the mechanisms of GLP-2 regulating intestinal lipoprotein release, the contribution of central GLP-1R signaling to intestinal lipoprotein metabolism, and long-term consequences of GLP-2 analog use on lipids and atherosclerosis. The coordinated secretion yet distinct physiological roles of GLP-1 and GLP-2 require more in-depth study in health and disease. Pharmacological manipulation of gut hormone actions may provide beneficial effects on atherogenic lipoproteins. A more complete understanding of the regulation of intestinal lipoprotein secretion by gut hormones may provide new opportunities for developing novel strategies to reduce CVD risk.
Funding. S.D. and C.M. are recipients of postdoctoral fellowship awards from the Banting & Best Diabetes Centre, University of Toronto, and S.D. is the
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recipient of a Focus on Stroke 12 Fellowship Award from the Heart and Stroke Foundation of Canada. The authors acknowledge funding from the Heart and Stroke Foundation of Ontario (to K.A.) and the Canadian Institutes of Health Research and the Heart and Stroke Foundation of Ontario (to G.F.L.). G.F.L. holds the Sun Life Financial Chair in Diabetes and the Drucker Family Chair in Diabetes Research. Duality of Interest. No potential conflicts of interest relevant to this article were reported.
References 1. Bornfeldt KE, Tabas I. Insulin resistance, hyperglycemia, and atherosclerosis. Cell Metab 2011;14:575–585 2. Ginsberg HN, MacCallum PR. The obesity, metabolic syndrome, and type 2 diabetes mellitus pandemic: part I. Increased cardiovascular disease risk and the importance of atherogenic dyslipidemia in persons with the metabolic syndrome and type 2 diabetes mellitus. J Cardiometab Syndr 2009;4:113–119 3. Ginsberg HN. Insulin resistance and cardiovascular disease. J Clin Invest 2000;106:453–458 4. Chapman MJ, Ginsberg HN, Amarenco P, et al.; European Atherosclerosis Society Consensus Panel. Triglyceride-rich lipoproteins and high-density lipoprotein cholesterol in patients at high risk of cardiovascular disease: evidence and guidance for management. Eur Heart J 2011;32:1345–1361 5. Lewis GF, Rader DJ. New insights into the regulation of HDL metabolism and reverse cholesterol transport. Circ Res 2005;96:1221–1232 6. Packard CJ. Triacylglycerol-rich lipoproteins and the generation of small, dense low-density lipoprotein. Biochem Soc Trans 2003;31:1066–1069 7. Adeli K, Lewis GF. Intestinal lipoprotein overproduction in insulin-resistant states. Curr Opin Lipidol 2008;19:221–228 8. Xiao C, Lewis GF. Regulation of chylomicron production in humans. Biochim Biophys Acta 2012;1821:736–746 9. Lovshin JA, Drucker DJ. Incretin-based therapies for type 2 diabetes mellitus. Nat Rev Endocrinol 2009;5:262–269 10. Amori RE, Lau J, Pittas AG. Efficacy and safety of incretin therapy in type 2 diabetes: systematic review and meta-analysis. JAMA 2007;298:194–206 11. Mulvihill EE, Drucker DJ. Pharmacology, physiology, and mechanisms of action of dipeptidyl peptidase-4 inhibitors. Endocr Rev 2014;35:992–1019 12. Monami M, Dicembrini I, Nardini C, Fiordelli I, Mannucci E. Effects of glucagonlike peptide-1 receptor agonists on cardiovascular risk: a meta-analysis of randomized clinical trials. Diabetes Obes Metab 2014;16:38–47 13. Dai X, Wang H, Jing Z, Fu P. The effect of a dual combination of noninsulin antidiabetic drugs on lipids: a systematic review and network meta-analysis. Curr Med Res Opin 2014;30:1777–1786 14. Sun F, Wu S, Wang J, et al. Effect of glucagon-like peptide-1 receptor agonists on lipid profiles among type 2 diabetes: a systematic review and network meta-analysis. Clin Ther 2015;37:225–241.e8 15. Monami M, Vitale V, Ambrosio ML, et al. Effects on lipid profile of dipeptidyl peptidase 4 inhibitors, pioglitazone, acarbose, and sulfonylureas: meta-analysis of placebo-controlled trials. Adv Ther 2012;29:736–746 16. Bunck MC, Cornér A, Eliasson B, et al. One-year treatment with exenatide vs. insulin glargine: effects on postprandial glycemia, lipid profiles, and oxidative stress. Atherosclerosis 2010;212:223–229 17. Buse JB, Drucker DJ, Taylor KL, et al.; DURATION-1 Study Group. DURATION-1: exenatide once weekly produces sustained glycemic control and weight loss over 52 weeks. Diabetes Care 2010;33:1255–1261 18. Klonoff DC, Buse JB, Nielsen LL, et al. Exenatide effects on diabetes, obesity, cardiovascular risk factors and hepatic biomarkers in patients with type 2 diabetes treated for at least 3 years. Curr Med Res Opin 2008;24: 275–286 19. Ariel D, Kim SH, Abbasi F, Lamendola CA, Liu A, Reaven GM. Effect of liraglutide administration and a calorie-restricted diet on lipoprotein profile in overweight/obese persons with prediabetes. Nutr Metab Cardiovasc Dis 2014;24: 1317–1322
diabetes.diabetesjournals.org
20. Buse JB, Henry RR, Han J, Kim DD, Fineman MS, Baron AD; Exenatide-113 Clinical Study Group. Effects of exenatide (exendin-4) on glycemic control over 30 weeks in sulfonylurea-treated patients with type 2 diabetes. Diabetes Care 2004; 27:2628–2635 21. Pratley RE, Nauck M, Bailey T, et al.; 1860-LIRA-DPP-4 Study Group. Liraglutide versus sitagliptin for patients with type 2 diabetes who did not have adequate glycaemic control with metformin: a 26-week, randomised, parallelgroup, open-label trial. Lancet 2010;375:1447–1456 22. Zinman B, Gerich J, Buse JB, et al.; LEAD-4 Study Investigators. Efficacy and safety of the human glucagon-like peptide-1 analog liraglutide in combination with metformin and thiazolidinedione in patients with type 2 diabetes (LEAD-4 Met+TZD). Diabetes Care 2009;32:1224–1230 23. Hermansen K, Bækdal TA, Düring M, et al. Liraglutide suppresses postprandial triglyceride and apolipoprotein B48 elevations after a fat-rich meal in patients with type 2 diabetes: a randomized, double-blind, placebo-controlled, cross-over trial. Diabetes Obes Metab 2013;15:1040–1048 24. Zander M, Madsbad S, Madsen JL, Holst JJ. Effect of 6-week course of glucagon-like peptide 1 on glycaemic control, insulin sensitivity, and beta-cell function in type 2 diabetes: a parallel-group study. Lancet 2002;359:824–830 25. Toft-Nielsen MB, Madsbad S, Holst JJ. Continuous subcutaneous infusion of glucagon-like peptide 1 lowers plasma glucose and reduces appetite in type 2 diabetic patients. Diabetes Care 1999;22:1137–1143 26. DeFronzo RA, Okerson T, Viswanathan P, Guan X, Holcombe JH, MacConell L. Effects of exenatide versus sitagliptin on postprandial glucose, insulin and glucagon secretion, gastric emptying, and caloric intake: a randomized, crossover study. Curr Med Res Opin 2008;24:2943–2952 27. Tremblay AJ, Lamarche B, Kelly I, et al. Effect of sitagliptin therapy on triglyceride-rich lipoprotein kinetics in patients with type 2 diabetes. Diabetes Obes Metab 2014;16:1223–1229 28. Eliasson B, Möller-Goede D, Eeg-Olofsson K, et al. Lowering of postprandial lipids in individuals with type 2 diabetes treated with alogliptin and/or pioglitazone: a randomised double-blind placebo-controlled study. Diabetologia 2012;55: 915–925 29. Kakuda H, Kobayashi J, Kakuda M, Yamakawa J, Takekoshi N. The effect of anagliptin treatment on glucose metabolism and lipid metabolism, and oxidative stress in fasting and postprandial states using a test meal in Japanese men with type 2 diabetes. Endocrine 2015;48:1005–1009 30. Aroda VR, Henry RR, Han J, et al. Efficacy of GLP-1 receptor agonists and DPP-4 inhibitors: meta-analysis and systematic review. Clin Ther 2012;34:1247– 1258.e22 31. Esposito K, Chiodini P, Maiorino MI, Bellastella G, Capuano A, Giugliano D. Glycaemic durability with dipeptidyl peptidase-4 inhibitors in type 2 diabetes: a systematic review and meta-analysis of long-term randomised controlled trials. BMJ Open 2014;4:e005442 32. Craddy P, Palin HJ, Johnson KI. Comparative effectiveness of dipeptidylpeptidase-4 inhibitors in type 2 diabetes: a systematic review and mixed treatment comparison. Diabetes Ther 2014;5:1–41 33. Sun F, Wu S, Guo S, et al. Effect of GLP-1 receptor agonists on waist circumference among type 2 diabetes patients: a systematic review and network meta-analysis. Endocrine 2015;48:794–803 34. Schwartz SL, Ratner RE, Kim DD, et al. Effect of exenatide on 24-hour blood glucose profile compared with placebo in patients with type 2 diabetes: a randomized, double-blind, two-arm, parallel-group, placebo-controlled, 2-week study. Clin Ther 2008;30:858–867 35. Fineman MS, Bicsak TA, Shen LZ, et al. Effect on glycemic control of exenatide (synthetic exendin-4) additive to existing metformin and/or sulfonylurea treatment in patients with type 2 diabetes. Diabetes Care 2003;26:2370–2377 36. Meier JJ, Gethmann A, Götze O, et al. Glucagon-like peptide 1 abolishes the postprandial rise in triglyceride concentrations and lowers levels of non-esterified fatty acids in humans. Diabetologia 2006;49:452–458 37. Schwartz EA, Koska J, Mullin MP, Syoufi I, Schwenke DC, Reaven PD. Exenatide suppresses postprandial elevations in lipids and lipoproteins in
Xiao and Associates
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individuals with impaired glucose tolerance and recent onset type 2 diabetes mellitus. Atherosclerosis 2010;212:217–222 38. Matikainen N, Mänttäri S, Schweizer A, et al. Vildagliptin therapy reduces postprandial intestinal triglyceride-rich lipoprotein particles in patients with type 2 diabetes. Diabetologia 2006;49:2049–2057 39. Tremblay AJ, Lamarche B, Deacon CF, Weisnagel SJ, Couture P. Effect of sitagliptin therapy on postprandial lipoprotein levels in patients with type 2 diabetes. Diabetes Obes Metab 2011;13:366–373 40. Noda Y, Miyoshi T, Oe H, et al. Alogliptin ameliorates postprandial lipemia and postprandial endothelial dysfunction in non-diabetic subjects: a preliminary report. Cardiovasc Diabetol 2013;12:8–15 41. Matikainen N, Taskinen MR. The effect of vildagliptin therapy on atherogenic postprandial remnant particles and LDL particle size in subjects with type 2 diabetes. Diabet Med 2013;30:756–757 42. Tremblay AJ, Lamarche B, Deacon CF, Weisnagel SJ, Couture P. Effects of sitagliptin therapy on markers of low-grade inflammation and cell adhesion molecules in patients with type 2 diabetes. Metabolism 2014;63:1141–1148 43. Drucker DJ, Nauck MA. The incretin system: glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors in type 2 diabetes. Lancet 2006;368:1696–1705 44. Meier JJ, Gallwitz B, Salmen S, et al. Normalization of glucose concentrations and deceleration of gastric emptying after solid meals during intravenous glucagon-like peptide 1 in patients with type 2 diabetes. J Clin Endocrinol Metab 2003;88:2719–2725 45. Nauck MA, Kemmeries G, Holst JJ, Meier JJ. Rapid tachyphylaxis of the glucagon-like peptide 1-induced deceleration of gastric emptying in humans. Diabetes 2011;60:1561–1565 46. Wøjdemann M, Wettergren A, Sternby B, et al. Inhibition of human gastric lipase secretion by glucagon-like peptide-1. Dig Dis Sci 1998;43:799–805 47. Hellström PM. GLP-1: broadening the incretin concept to involve gut motility. Regul Pept 2009;156:9–12 48. Farr S, Baker C, Naples M, et al. Central nervous system regulation of intestinal lipoprotein metabolism by glucagon-like peptide-1 via a brain-gut axis. Arterioscler Thromb Vasc Biol 2015;35:1092–1100 49. Xiao C, Bandsma RH, Dash S, Szeto L, Lewis GF. Exenatide, a glucagon-like peptide-1 receptor agonist, acutely inhibits intestinal lipoprotein production in healthy humans. Arterioscler Thromb Vasc Biol 2012;32:1513–1519 50. Xiao C, Dash S, Morgantini C, Patterson BW, Lewis GF. Sitagliptin, a DPP-4 inhibitor, acutely inhibits intestinal lipoprotein particle secretion in healthy humans. Diabetes 2014;63:2394–2401 51. Duez H, Lamarche B, Valéro R, et al. Both intestinal and hepatic lipoprotein production are stimulated by an acute elevation of plasma free fatty acids in humans. Circulation 2008;117:2369–2376 52. Duez H, Lamarche B, Uffelman KD, Valero R, Cohn JS, Lewis GF. Hyperinsulinemia is associated with increased production rate of intestinal apolipoprotein B-48-containing lipoproteins in humans. Arterioscler Thromb Vasc Biol 2006;26:1357–1363 53. Federico LM, Naples M, Taylor D, Adeli K. Intestinal insulin resistance and aberrant production of apolipoprotein B48 lipoproteins in an animal model of insulin resistance and metabolic dyslipidemia: evidence for activation of protein tyrosine phosphatase-1B, extracellular signal-related kinase, and sterol regulatory element-binding protein-1c in the fructose-fed hamster intestine. Diabetes 2006;55:1316–1326 54. Veilleux A, Grenier E, Marceau P, Carpentier AC, Richard D, Levy E. Intestinal lipid handling: evidence and implication of insulin signaling abnormalities in human obese subjects. Arterioscler Thromb Vasc Biol 2014;34:644–653 55. Qin X, Shen H, Liu M, et al. GLP-1 reduces intestinal lymph flow, triglyceride absorption, and apolipoprotein production in rats. Am J Physiol Gastrointest Liver Physiol 2005;288:G943–G949 56. Hsieh J, Longuet C, Baker CL, et al. The glucagon-like peptide 1 receptor is essential for postprandial lipoprotein synthesis and secretion in hamsters and mice. Diabetologia 2010;53:552–561
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57. Pavlic M, Xiao C, Szeto L, Patterson BW, Lewis GF. Insulin acutely inhibits intestinal lipoprotein secretion in humans in part by suppressing plasma free fatty acids. Diabetes 2010;59:580–587 58. Pyke C, Heller RS, Kirk RK, et al. GLP-1 receptor localization in monkey and human tissue: novel distribution revealed with extensively validated monoclonal antibody. Endocrinology 2014;155:1280–1290 59. Körner M, Stöckli M, Waser B, Reubi JC. GLP-1 receptor expression in human tumors and human normal tissues: potential for in vivo targeting. J Nucl Med 2007;48:736–743 60. Farr S, Naples M, Baker C, Adeli K. Glucagon-like peptide-1 reduces intestinal lipid availability and lipoprotein production via a gut-brain axis [abstract]. Diabetes 2013;62(Suppl. 1):A163 61. Baggio LL, Drucker DJ. Glucagon-like peptide-1 receptors in the brain: controlling food intake and body weight. J Clin Invest 2014;124:4223–4226 62. Hein GJ, Baker C, Hsieh J, Farr S, Adeli K. GLP-1 and GLP-2 as yin and yang of intestinal lipoprotein production: evidence for predominance of GLP-2-stimulated postprandial lipemia in normal and insulin-resistant states. Diabetes 2013;62: 373–381 63. Scheen AJ. Cardiovascular effects of gliptins. Nat Rev Cardiol 2013;10:73–84 64. Sivertsen J, Rosenmeier J, Holst JJ, Vilsbøll T. The effect of glucagon-like peptide 1 on cardiovascular risk. Nat Rev Cardiol 2012;9:209–222 65. Ussher JR, Drucker DJ. Cardiovascular actions of incretin-based therapies. Circ Res 2014;114:1788–1803 66. White WB, Cannon CP, Heller SR, et al.; EXAMINE Investigators. Alogliptin after acute coronary syndrome in patients with type 2 diabetes. N Engl J Med 2013;369:1327–1335 67. Scirica BM, Bhatt DL, Braunwald E, et al.; SAVOR-TIMI 53 Steering Committee and Investigators. Saxagliptin and cardiovascular outcomes in patients with type 2 diabetes mellitus. N Engl J Med 2013;369:1317–1326 68. Baggio LL, Drucker DJ. Biology of incretins: GLP-1 and GIP. Gastroenterology 2007;132:2131–2157 69. Drucker DJ, Yusta B. Physiology and pharmacology of the enteroendocrine hormone glucagon-like peptide-2. Annu Rev Physiol 2014;76:561–583
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70. Hsieh J, Longuet C, Maida A, et al. Glucagon-like peptide-2 increases intestinal lipid absorption and chylomicron production via CD36. Gastroenterology 2009;137:997–1005.e1–e4 71. Meier JJ, Nauck MA, Pott A, et al. Glucagon-like peptide 2 stimulates glucagon secretion, enhances lipid absorption, and inhibits gastric acid secretion in humans. Gastroenterology 2006;130:44–54 72. Masuda D, Hirano K, Oku H, et al. Chylomicron remnants are increased in the postprandial state in CD36 deficiency. J Lipid Res 2009;50:999–1011 73. Banasch M, Bulut K, Hagemann D, et al. Glucagon-like peptide 2 inhibits ghrelin secretion in humans. Regul Pept 2006;137:173–178 74. Nagell CF, Wettergren A, Pedersen JF, Mortensen D, Holst JJ. Glucagon-like peptide-2 inhibits antral emptying in man, but is not as potent as glucagon-like peptide-1. Scand J Gastroenterol 2004;39:353–358 75. Xiao C, Pavlic M, Szeto L, Patterson BW, Lewis GF. Effects of acute hyperglucagonemia on hepatic and intestinal lipoprotein production and clearance in healthy humans. Diabetes 2011;60:383–390 76. Dash S, Xiao C, Morgantini C, Connelly PW, Patterson BW, Lewis GF. Glucagon-like peptide-2 regulates release of chylomicrons from the intestine. Gastroenterology 2014;147:1275–1284, e4 77. Yusta B, Huang L, Munroe D, et al. Enteroendocrine localization of GLP-2 receptor expression in humans and rodents. Gastroenterology 2000;119:744–755 78. Guan X, Karpen HE, Stephens J, et al. GLP-2 receptor localizes to enteric neurons and endocrine cells expressing vasoactive peptides and mediates increased blood flow. Gastroenterology 2006;130:150–164 79. Bremholm L, Hornum M, Andersen UB, Holst JJ. The effect of glucagon-like peptide-2 on arterial blood flow and cardiac parameters. Regul Pept 2010;159: 67–71 80. Tavares W, Drucker DJ, Brubaker PL. Enzymatic- and renal-dependent catabolism of the intestinotropic hormone glucagon-like peptide-2 in rats. Am J Physiol Endocrinol Metab 2000;278:E134–E139 81. Kieffer TJ, McIntosh CH, Pederson RA. Degradation of glucose-dependent insulinotropic polypeptide and truncated glucagon-like peptide 1 in vitro and in vivo by dipeptidyl peptidase IV. Endocrinology 1995;136:3585–3596
