Mechanisms in Protein - Induced Satiety - Food Proteins or Their Hydrolysates as Regulators o

Food Proteins or Their Hydrolysates as Regulators of Satiety

3. Mechanisms in Protein - Induced Satiety

Secretion of CCK occurs in response to food intake by enteric endocrine I - cells that are scattered throughout the mucosa of the small intestine (Cummings and Overduin 2007 ; Little et al. 2005 ).

Basal plasma CCK concentrations are generally in the low picomolar range in most species and increase approximately 5 - to 10 - fold following meal ingestion, with dietary fat and protein shown as the most potent stimulators of CCK release (Douglas et al. 1988 ). It is believed that CCK release by dietary protein is regulated by endogenous, trypsin - sensitive CCK - releasing peptides (Liddle 1995 ), but some studies, both in vivo and in vitro, have found that dietary protein and its peptide react on CCK - producing cells directly to stimulate CCK release (Beucher et al. 1994 ; Cordier - Bussat et al. 1997 ; Nemoz - Gaillard et al. 1998 ). The presence of protein hydrolysate in the duodenum effectively stimulates the release of CCK from endocrine cells, resulting proteins. However, such an effect could not be

reproduced so far in a human intervention study.

Diepvens and co - workers (2008) obtained only subtle effects of pea protein hydrolysate on satiety and hunger perceptions compared to the nonhydro- lyzed protein.

Taken together, these studies demonstrated that proteins from different sources and proteins vs.

protein hydrolysates differ with respect to their satiety hormone - releasing properties. However, considering the total scientifi c evidence, it is not conclusive so far whether the protein source as well as the integrity in humans has a clinical relevant effect on a behavioral response, that is, reduction in food intake and/or perception of satiety.

3. Mechanisms in

together, these fi ndings support the hypothesis that entry of di - and tripeptides into cells in the intestinal mucosa via PepT1 is involved in the sensory trans- duction process. The protein hydrolysate content of the intestinal lumen is signalled to vagal afferent nerve terminals to induce activation of a neural pathway and a vagal refl ex inhibition of gastric motility. Recently Foltz et al. (2008) have demonstrated that the source of protein hydrolysates could impact stimulation of CCK 1 R in a dose - dependent manner. Indeed, during protein hydrolysis, short - chain peptides are formed, which possibly act as bioactive peptides at CCK 1 R.

In addition, one should not exclude a direct action of CCK on the central nervous system (at the level of the area postrema or hypothalamus). Indeed, some works of Bowen et al. (2006a, 2006b) showed that in normal and overweight subjects, a preload of 1.1 MJ containing 50 g of whey, soy, and gluten proteins increases the duration of the postprandial blood elevation of CCK compared to a 1.1 MJ glucose preload. The same result was obtained with 1 MJ preloads containing 55 g of whey and casein with a correlation between a concentration of postprandial CCK and appetite ratings, but not with energy intake that remains unchanged after a high - protein preload or a high - carbohydrate (glucose) preload. Finally, the involvement of CCK in acute protein - induced satiety seems to be dose - dependent.

Indeed, the replacement of 25 g of whey proteins by 25 g of fructose leads to a lower elevation of CCK concentration compared to a 50 g whey protein preload in obese people (Bowen et al. 2007 ).

However, as pointed out earlier, despite the fact that high loads of proteins induced changes in plasma levels of satiety hormones, this is not accompanied in all studies by changes in subsequent food intake (Bowen et al. 2006a ).

GLP - 1 is a gastrointestinal hormone that is released in response to food intake from the distal small intestine. Kieffer and Habener (1999) have demonstrated that it is derived from proglucagon and produced by cleavage of glucagon in the pan- creas, small intestine, and central nervous system.

Its peripheral release during meal intake reduces food intake as shown by Chelikani et al. (2005) in in the activation of CCK 1 receptor (CCK 1 R) on

vagal afferent nerve terminals (Raybould 1998 ).

Studies in animals and humans using different protein sources suggest that in order to stimulate CCK secretion effi ciently, the protein needs to be hydrolyzed to short - chain peptides and amino acids (Darcel et al. 2005 ). In addition, selected proteins and hydrolysates have been suggested to increase CCK secretion from proteolysis due to competitive peptidase inhibition (Liddle 1995 ). Recently a receptor, GPR93, was identifi ed in the apical membrane of enterocytes and described to be activated by protein hydrolysates (Choi et al. 2007a ). When overexpressed in the enteroendocrine cell line STC - 1, GPR93 activation by protein hydrolysates induced CCK secretion from STC - 1 cells (Choi et al. 2007b ). Several works have shown that inhibition of gastric motility in response to protein hydrolysates is mediated by a CCK 1 R and vagal afferent refl ex pathway (White et al. 2000 ; Raybould and Lloyd 1994 ). Indeed, stimulation of vagal afferent activity in response to protein digests is blocked by specifi c antagonists of CCK 1 R; and activation of the marker of neuronal activation, fos, in the brainstem is increased by intestinal perfusion of protein hydrolysates via a CCK 1 R mechanism (Zittel et al. 1994 ; Glatzle et al. 2001 ). The precise mechanism and the cascade of events by which a protein hydrolysate is detected by intestinal epithelial cells and causes the release of CCK from endocrine cells remain unclear.

A main process for the uptake of protein hydrolysates in the intestine is the proton - coupled oligopeptide transporter PepT1, a member of the family of peptide transporters found in all species from bac- teria to humans. This transporter, localized to the apical membrane of enterocytes, is the exclusive oligopeptide transporter of the intestinal mucosa and has a requirement for the di - and tripeptide structure (Rubio - Aliaga and Daniel 2002 ). Darcel et al. (2005) have demonstrated that the ability of luminal perfusion of protein hydrolysates to stimulate CCK - responsive vagal afferent fi ber discharge and inhibit gastric motility was blocked in the presence of a competitive inhibitor of PepT1. Moreover, Cefaclor, a specifi c substrate for PepT1, increased CCK - responsive vagal afferent fi ber activity. Taken

GLP - 1 was found to be higher with the high - protein diet than with the normal - protein diet after dinner, which confi rms similar results of Johnson and Vickers (1993) in men. However, in acute conditions, Smeets et al. (2008) did not obtain differences in concentrations of satiety - related hormones including GLP - 1 and PYY after a single normal or high - protein diet (10% or 25% of energy brought by proteins, respectively). As suggested before, the source of proteins could also infl uence the postprandial profi le of GLP - 1. Hall et al. (2003) have shown that a large load of whey proteins leads to a greater increase in plasma GLP - 1 concentrations than casein. Moreover, the degree of protein fractionation seems to affect GLP - 1 secretion. In a study by Aziz and Anderson (2003) , the GLP - 1 receptor agonist, Exendin - 4, was given intraperitoneally to rats. Food intake was measured when Exendin - 4 was given alone or with preloads of intact whey and casein proteins, their hydrolysates, and amino acid mixtures. Both Exendin - 4 and the preloads sup- pressed food intake. Since the effect of Exendin - 4 was reduced by the protein hydrolysates and by the amino acid preloads, the results support a role for the end products of protein digestion and GLP - 1 release in the suppression of food intake in response to protein ingestion. However, some contradiction exists. In a human study, Calbet and Holst (2004) observed similar GLP - 1 and PYY plasma responses after intragastric administration of complete cow milk protein and three different hydrolysates thereof varying in degree of hydrolysis. Thus, in this study plasma GLP - 1 responses seem to be independent of the degree of protein fractionation.

Ghrelin is produced by the stomach and is, thus far, the only orexigenic hormone identifi ed.

Peripheral administration of ghrelin increases food intake in rodents and humans. Ghrelin mediates its orexigenic effect via stimulation of NPY/AgRP arcuate neurons. Bowen et al. (2006a) have recently demonstrated that the postprandial suppression of ghrelin seems to be prolonged after a high - protein liquid preload containing 50 g of whey, soy, or gluten compared to a high - carbohydrate preload in lean men. The same result appears in overweight men after consumption of 55 g of either whey or rats (Figure 10.2 ) and in humans as shown by

Verdich et al. (2001) . Turton et al. (1996) have also shown that central injection of GLP - 1 leads to a reduction of food intake.

The anorexigenic actions of GLP - 1 include a glucose - dependent insulinotropic effect on the pan- creatic B cells as shown by Kreymann et al. (1987) and Mojsov et al. (1987) ; and an inhibition of gastric emptying (Nauck et al. 1997 ).

Recently, studies of Bowen et al. (2006a) have showed in humans that the ingestion of a high - protein preload leads to an increase in plasma concentration of GLP - 1 relative to a high - carbohydrate preload. GLP - 1 plasma concentrations peak later after the high - protein preload, which results in maintaining a high concentration. The long - term action of GLP - 1 has been studied by Lejeune et al.

(2006) . Lean women were fed in energy balance with a normal or high - protein diet that contained 10/30/60% or 30/40/30% of energy from protein/

carbohydrate/fat during 4 days (4 - day total consumption of about 60 and 180 g of protein in normal and high - protein diet). On the fourth day, GLP - 1 concentration was measured throughout the day.

00 2 4 6 8 10 12 14 16 18

Time (h)

Cumulative Food Intake (g)

infusion

GLP-1 pmol·kg^–1·min^–1 0

170

† ‡ ‡ ‡ ‡ n=14

‡

‡ ‡

‡ ‡ ‡ ‡ ‡ ‡ ‡

Figure 10.2. Effect of 3 - hour intravenous infusions of GLP - 1 on cumulative food intake in rats. Rats that were normally fed received 3 - hour jugular vein infusions of GLP - 1 at 0 and 170 pmol kg - 1 min - 1 beginning 15 minutes before dark onset. Food intake was determined from continuous com- puter recordings of changes in food bowl weight. Values are means ± SE; † p < 0.01; ‡ p < 0.001 (from Chelikani et al.

2005 ) .

(G6Pase), which controls the last step of gluconeogenesis, is upregulated in the fasted state and down- regulated in the fed state. The existence of intestinal gluconeogenesis and glucose portal sensing through portal vagus afferent fi bers has also been hypothesized as an alternative mechanism for the elevated satiety related to a high - protein diet (Mithieux et al.

2005 ). Supporting evidence that protein - induced satiety is partly mediated via nonvagal pathways, of which portal glucose sensing could be one possibil- ity, is coming from vagal deafferiation studies.

L ’ Heureux - Bouron et al. (2003) have shown that the depressing effect of a high - protein diet on food intake is not abolished after vagotomy in the rat.

However, the opposite, that is, signifi cant reduction of the protein - induced food intake inhibitory effect, was observed in a similar study by Froetschel et al.

(2001) . Taken together, these observations do not support intestinal gluconeogenesis and its role through glucose portal sensing in the effect of high - protein feeding on food intake. The relevance and physiological signifi cance of intestinal gluconeogenesis stays thus as a subject of debate and needs to be confi rmed (Azzout - Marniche et al. 2007 ; Habold et al. 2005 ; Martin et al. 2007 ).

3.3. Plasma Amino Acids as Central Satiety Signals

A rise in plasma amino acid concentration in response to either ingestion of protein, protein hydrolysates and amino acids or by infusion of an amino acid mixture, appears to be accompanied by a waning of appetite. Indeed, it has been suggested that an elevated concentration of plasma amino acids, which cannot be channeled into protein syn- thesis, may serve as a satiety signal for a food intake - regulating mechanism and thereby result in depressed food intake. The variations in plasma hormones and free amino acid concentrations can be directly recorded by the central nervous system mainly through the concomitant variations in their intracerebral level. The role of histidine and tyrosine as precursors of histamine and dopamine has been questioned. Indeed, histidine has been reported to decrease food intake possibly through the activation casein (Bowen et al. 2006b ). However, Lejeune et al.

(2006) did not show a difference in ghrelin concentration after 4 days of high - protein vs. normal - protein diet, which supports fi ndings from Moran et al.

(2005) showing that the satiating effect of protein is not related to postprandial ghrelin secretion.

3.2. Energy Expenditure and Glucose as Metabolic Signals in Protein - Induced Satiety Metabolic signals, including an increase in energy expenditure and the production of glucose through gluconeogenesis, have also been hypothesized as signals in protein - induced satiety. It is currently accepted that protein stimulates diet - induced thermogenesis to a greater extent than other macronu- trients (Raben et al. 2003 ). The Atwater factor for protein (17 kJ/g) gives a refl ection of the net metabo- lizable energy value; the factor accounts for diges- tive and urinary losses. Diet - induced thermogenic losses induced by protein are above this value and are higher than those for either carbohydrate or fat.

Lejeune et al. (2006) reported that the higher satiety effect of a protein - rich diet was related to an increase in total and diet - induced thermogenesis in women (diet - induced thermogenesis of 0.91 ± 0.25 and 0.69 ± 0.24 MJ/day for a high and a normal - protein diet, respectively). These differences of about 2%

change in daily energy intake are not huge but could be important in long - term high - protein diets.

Several hypotheses were proposed to explain these observations, including an increase in liver amino acid oxidation and metabolism. The main gluconeogenic organ is the liver and de novo syn- thesis of glucose in the liver from gluconeogenic precursors, including amino acids, is stimulated by a high - protein diet in the fed state. This process could be involved in the satiating effect of protein through a modulation of glucose homeostasis and glucose signalling to the brain. Azzout - Marniche et al. (2007) have demonstrated that when the protein content of the diet is increased, phosphoenolpyru- vate carboxylase kinase (PEPCK), which controls the initial conversion of oxaloacetate to phospho- enolpyruvate, is upregulated in the fasted and in the fed state. In contrast, glucose 6 - phosphatase

ICV - applied leucine alone has the same effect on food intake as a mixture of ICV - applied amino acids. However, it remains questionable whether under physiological conditions cerebal leucine levels can be suffi ciently increased after oral appli- cation of a leucine - rich protein source.

Ropelle et al. (2008) showed also that the AMP - Activated Protein Kinase (AMPK) and the mam- malian Target of Rapamycin (mTOR) are involved in the satiety induced by high - protein diets. AMPK is the downstream component of a kinase cascade that acts as a sensor of cellular energy charge and is activated by an increase in the AMP - to - ATP ratio.

Once activated, AMPK phosphorylates acetyl - CoA carboxylase (ACC) and switches on energy - producing pathways at the expense of energy - depleting processes (Ropelle et al. 2008 ). A high - protein diet and ICV leucine administration suppress AMPK and ACC phosphorylation in the hypothalamus of rats, which is accompanied by a decreased AMP - to - ATP ratio. In this respect, mTOR has been discussed to function as a molecular sensor and switch in depen- dence of the metabolic status of the cell. It has been demonstrated that a high - protein diet as well as ICV administration of free amino acids or leucine alone led to an activation of mTOR in the hypothalamus (Morrison et al. 2007 ; Ropelle et al. 2008 ). Moreover, high - protein diets modulate AMPK and mTOR in the same specifi c neuronal subset, the arcuate and paraventricular nuclei of the hypothalamus (ARC and PVN, respectively). AMPK and mTOR may have overlapping and reciprocal functions (Cota et al. 2006 ; Kimura et al. 2003 ) (Figure 10.4 ).

Finally, the activation of mTOR and the suppression of AMPK phosphorylation activity seem to modulate hypothalamic neuropeptides: decrease of concentration of Neuropeptide Y (NPY) and Agouti related peptide (Agrp), both orexigenic neuropeptides, and increased Pro - opiomelanocortin (POMC) expression, which is anorexigenic.

3.4. Protein - Induced Satiety and Central Neuronal Pathways

At the brain level, two afferent pathways are involved in protein and amino acid monitoring: the of histamine neurons. In rats, diet supplementation

with histidine has been reported to be negatively correlated with food intake, weight gain, and weight of fat pads by Kasaoka et al. (2004, 2005) . Moreover, meal pattern has been reported to be affected by central factors related to the interaction between dopamine and serotonin within the lateral hypothalamic area (LHA) and ventromedial nucleus (VMN) (Meguid et al. 2000 ), which is known to have an appetite - suppressing effect. Indeed, Blundell and Latham (1978) have demonstrated a serotonin - induced dose - related reduction in both meal size and feeding rate, whereas analysis of dopaminergic mechanisms show that dopamine D2 receptor block- ade rate is associated with an enhancement of meal size and a decrease of meal frequency. However, Bassil et al. (2007) failed to show an effect of a 5% - supplemented diet of either histidine or tyrosine on food intake in Sprague - Dawley rats. In contrast, Cota et al. (2006) have shown that an intracerebro- ventricular (ICV) administration of leucine reduces food intake and body weight and improves glucose and cholesterol metabolism in rats and mice (Figure 10.3 ) to the same extent as an increase in dietary leucine (Zhang et al. 2007 ).

Moreover, Morrison et al. (2007) have demonstrated that this effect is specifi c to leucine, since

Figure 10.3. Intracerebroventicular administration of L - leucine in fasted rats decreases 4 hours and 24 hours food intake. The mean ± SE of 6 – 7 rats used for each treatment group is shown. * p < 0.05 versus PBS - treated rats; #p < 0.05 versus rats treated with 0.2 μ g of L - leucine in 2 μ l of PBS (leu 0.2) (from Cota et al. 2006 ) .

(1998) have shown that, when infusing glucose (4.5 kcal), linoleic acid, or an amino acid mixture (both 1.5 kcal) into the duodenum of unanaesthe- tized rats, all compounds suppress sham feeding, but the amino acid mixture triggers relatively little c - fos in the NTS and area postrema compared to glucose and linoleic acid. This suggests that amino acids, although potent inducers of satiety, affect ingestion by processes different from those subserving lipids and carbohydrates. This result is in agreement with the work of Zittel et al. (1994) showing that expression of c - fos - like immunoreactivity was seen in the NTS after infusions of lipid emulsion (2.7 kcal), D - glucose (2.9 kcal), but not after infusions of glucose at 1.1 kcal, mannitol, hydrochloric acid, and casein hydrolysate (1.2 kcal).

Peripheral information is further centralized in the hypothalamus, which is critical in the regulation of food intake and energy homeostasis. Energy homeostasis - regulating circuits are found within and connecting these nuclei. In the ARC, the activation of POMC neurons induces a reduction in food intake. These neurons are responsible for the ano- rectic response induced by circulating leptin (Cowley et al. 2001 ) through the activation of neurons in the PVN (Balthasar et al. 2005 ). Activation of POMC neurons is also inseparable from the behavior of another population in the ARC, NPY/Agrp neurons whose activation is potent in increasing food intake and inhibiting POMC neuron activation. Faipoux et al. (2008) showed that, in the case of ingestion of a high - protein meal, the number of double - labeled Fos (marker of the activation of POMC neurons) and α - MSH (characterizing POMC neurons) increased. There is also a decrease in the activation of non - POMC neurons. This result is less pro- nounced when a high - protein diet is consumed for several days (21 days) rather than briefl y. Moreover, because arcuate neurons are mainly POMC or NPY, it could be hypothesized that NPY neurons are less activated after high - protein meals.

Dalam dokumen Peptides in Food and Health (Halaman 140-145)