Apelin cells in the rat stomach

Peripheral apelin mediates visceral hypersensitivity and impaired gut barrier in a rat irritable bowel syndrome model

Growing evidence indicates that visceral hypersensitivity and impaired gut barrier play an important role in the pathophysiology of irritable bowel syndrome (IBS). In animal models, these changes are known to be mediated via corticotropin-releasing factor (CRF)-Toll like receptor 4 (TLR4)-proinflammatory cytokine signaling. Apelin, an endogenous ligand of APJ, was reported to modulate CRF-induced enhanced colonic motility. In this context, we hypothesized that apelin also modulates visceral sensation and gut barrier, and tested this hypothesis. We measured visceral pain threshold in response to colonic balloon distention by abdominal muscle contractions assessed by electromyogram in rats. Colonic permeability was estimated by quantifying the absorbed Evans blue in colonic tissue. Intraperitoneal (ip) administration of [Ala13]-apelin-13, an APJ antagonist, blocked lipopolysaccharide (LPS)- or CRF-induced visceral hypersensitivity and colonic hyperpermeability (IBS model) in a dose-response manner. These inhibitory effects were blocked by compound C, an AMPK inhibitor, N^G-nitro-L-arginine methyl ester, a nitric oxide (NO) synthesis inhibitor or naloxone in the LPS model. On the other hand, ip [Pyr1]-apelin-13, an APJ agonist, caused visceral hypersensitivity and colonic hyperpermeability, and these effects were reversed by astressin, a CRF receptor antagonist, TAK-242, a TLR4 antagonist or anakinra, an interleukin-1 receptor antagonist. APJ system modulated CRF-TLR4-proinflammatory cytokine signaling to cause visceral hypersensitivity and colonic hyperpermeability. APJ antagonist blocked these GI changes in IBS models, which were mediated via AMPK, NO and opioid signaling. Apelin may contribute to the IBS pathophysiology, and the inhibition of apelinergic signaling may be a promising therapeutic option for IBS.

Serum Elabela level is related to endoscopic activity index in patients with active ulcerative colitis

BACKGROUND

In ulcerative colitis patients, Elabela levels and the relation of Elabela with laboratory parameters is unknown.

AIM

The purpose of this study was to investigate the serum Elabela levels in UC patients and its relationship with other clinical and laboratory findings.

METHODS

Forty-three patients with UC and 40 healthy controls (group I) similar in age and gender were included in the study. Routine patient history, physical examination, and laboratory tests were followed by analysis of serum Elabela levels. Endoscopic activity index (EAI) of patients with UC was calculated. There were two groups of patients: those in remission (group II) and with active disease (group III).

RESULTS

Groups I, II, and III had 40, 22, and 21 participants, respectively. Serum Elabela levels were found to be 3.32 ± 1.25 ng/mL in group I, 3.38 ± 0.88 ng/mL in group II, and 5.48 ± 1.61 ng/mL in group III. Comparing the serum Elabela levels, a statistically significant difference was found between three groups (p < 0.001). Serum Elabela level showed a significant and positive correlation with EAI, leukocyte count, and hs-CRP, while a negative correlation was found with hemoglobin levels in univariate analysis (p < 0.001, for each). In linear regression analysis, these parameters were found to be associated with EAI and hs-CRP (p = 0.049, β = 0.337, and p = 0.015, β = 0.396, respectively).

CONCLUSION

Elabela concentrations in patients with active UC was significantly higher and was associated with EAI and hs-CRP. Blood Elabela concentrations can be useful in the diagnosis and follow-up of patients with active UC.

The Apelinergic System Immuno-Detection in the Abomasum and Duodenum of Sheep Grazing on Semi-Natural Pasture

Simple Summary

The semi-natural pastures in the Apennines represent the feed source for ovine, whose grazing activity helps to preserve the grassland’s biodiversity. Summer drought stress decreases the grassland pastoral value and affects the morpho-functional features of sheep’s digestive systems. A better knowledge of the gastrointestinal system of sheep may contribute to guaranteeing their welfare, a prerequisite for the sustainability of livestock production. This study aimed to immune-localize the apelinergic system in the abomasum and duodenum of sheep grazing on semi-natural pasture during the spring–summer season and to compare its behavior among animal groups fed with or without supplementation. The apelinergic system, composed of apelin and its receptor, is involved in foodintake and the secretion and absorption activities of the digestive apparatus. Apelinergic system molecules were localized at the abomasum lining epithelium and fundic glands level and at the duodenum lining and crypt epithelium, in addition to the neuroendocrine cells. Variations in reactivity were observed in the different feed groups; feed supplementation seemed to maintain the functionality of the apelinergic system in the organs near the status related to the better pasture phase, suggesting that it may be a suitable solution able to counteract the harmful effects of summer drought stress.

Abstract

Apelin (APLN) is an adipokine mainly produced by adipose tissue and related to an individual’s nutritional status as well as digestive apparatus functions. In this work, APLN and its receptor (APLNR) were investigated, by immunohistochemistry, in the abomasum and duodenum of 15 Comisana × Appenninica adult sheep reared in a semi-natural pasture. Organ samples were collected after maximum pasture flowering (M × F group) and after maximum pasture dryness (M × D group); the experimental group (E × p group) received a feed supplementation of 600 grams/day/head of barley and corn in addition to M × D group feeding. APLN and APLNR were identified in the lining epithelium and the fundic gland chief cells of the abomasum. APLNR was observed in the lining epithelium, in the crypts and the serotonin secreting cells of the duodenum. Similar reactivity was observed between the M × F and E × p groups, while the M × D group showed a lower intensity of immunostaining for both APLN and APLNR in all positive structures but the duodenal serotonin neuroendocrine cells. Hence, our findings show that the E × p group presents a picture quite overlapped with M × F and suggest that food supplementation has a maintaining effect on the apelinergic system expression in the investigated digestive tracts of the sheep.

Molecular mechanism of apelin-13 regulation of colonic motility in rats

Apelin is a novel neuropeptide identified as the endogenous ligand for the apelin receptor. Apelin and its receptor are widely distributed in the gastrointestinal tract. Studies have reported that apelin-13 is involved in modulating gastrointestinal motility; however, the evidence is insufficient and the relevant mechanism is still not fully clear. Consequently, our study designed to explore the effect induced by exogenous apelin-13, to analyze the mechanism of action in isolated rat colons and colonic smooth muscle cells. The spontaneous contractions of colonic smooth muscle strips from rat were measured in an organ bath system. L-type calcium currents and large conductance Ca2+-activated K+ (BKCa) currents in rat colonic smooth muscle cells were investigated using the electrophysiological patch-clamp technique. Apelin-13 decreased the spontaneous contractile activity of colonic smooth muscle strips in a dose-dependent manner, and the inhibitory effect was not abolished by tetrodotoxin. The electrophysiological recordings revealed that apelin-13 reduced the crest currents of L-type calcium in a concentration-dependent manner in colonic smooth muscle cells at the test potential of 0 mV. Moreover, apelin-13 moved the current-voltage (I-V) curves of L-type calcium channels upward, but did not change their contour. Furthermore, the characteristics of L-type calcium channels with steady-state activation and steady-state inactivation were not significantly changed. Similarly, application of apelin-13 also significantly decreased BKCa currents in a concentration-dependent manner. In conclusion, apelin-13 inhibited the spontaneous contractile activity of isolated rat colons via the suppression of L-type calcium channels and BKCa channels in colonic smooth muscle cells.

Enteric apelin enhances the stress-induced stimulation of colonic motor functions

In response to stress, apelin and corticotropin-releasing factor (CRF) are upregulated in the gastrointestinal (GI) tract. This study was designed to investigate the effect of stress on endogenous apelin in colon and its regulatory role on colonic motor functions. Colon transit (CT) was measured in rats exposed to acute restraint stress (ARS). APJ and CRF receptor antagonists F13A and astressin were administered intraperitoneally 30 min before ARS loading. Colonic muscle contractions were evaluated by in-vivo motility recording and in-vitro organ bath studies. Detection of apelin or CRF was performed using immunohistochemistry in proximal and distal colon of non-stressed (NS) and ARS-loaded rats. Immunoreactivity of CRF1 with apelin or APJ receptor was detected with double-labeled immunofluorescence in colonic myenteric neurons. Compared with NS rats, ARS accelerated the CT which was attenuated significantly by F13A or astressin. Following ARS, the expression of CRF was increased remarkably in distal colon, while the stress-induced change was not prominent in proximal colon. Apelin-positive cells were detected in myenteric ganglia of distal colon, while no apelin immunoreactivity observed in myenteric neurons of proximal colon. Both apelin and APJ receptor are colocalized with CRF1 in myenteric neurons of distal colon. In the in-vivo colonic motility experiments, apelin-13 exhibited a rapid stimulatory effect. CRF administration increased the motility which was abolished by F13A. Apelin-induced contractions in muscle strips were no longer observed with preadministration of F13A. These results suggest that enteric apelin contributes to the action of CRF on colonic motor functions under stressed conditions.LAY SUMMARYIt has been suggested in rodents that acute stress increases the expression of apelin in gastrointestinal tissues. We have found that under stressed conditions, enteric apelin contributes to the CRF-induced alterations in colonic motor functions through APJ receptor.

The effect of apelin-13 on gastric ischemia/reperfusion injury: the roles of sensory nerves and vagus nerve

Apelin is a peptide that plays a role in physiological processes such as angiogenesis, apoptosis, and proliferation. The aim of this study was to investigate the role of capsaicin-sensitive afferent neurons and vagus in the effect of apelin against ischemia/reperfusion (I/R) injury. The experimental groups were (1) control, (2) I/R, (3) apelin + I/R, (4) vagotomy + I/R, (5) vagotomy + apelin + I/R, (6) capsaicin + I/R, (7) capsaicin + apelin + I/R, (8) lorglumide + I/R, and (9) lorglumide + apelin + I/R. To test the potential gastroprotective effect of apelin-13, apelin-13 (2 mg/kg i.v.) was administered just before both ischemia and reperfusion. A vagotomy was performed 1 week before I/R in the vagotomized groups; capsaicin (125 mg/kg s.c.) was administrated 2 weeks before I/R in the capsaicin-treated groups and lorglumide (5 mg/kg i.p.) was administered 30 min before I/R in the lorglumide-treated groups. After I/R, a variety parameters in gastric tissue were analyzed. cfos expression was determined in brainstem samples. In the I/R group, the lesion index, myeloperoxidase activity, lipid peroxidation, nitric oxide, and tumor necrosis factor-α increased, and mucosal blood flow, prostaglandin-E₂, and calcitonin gene related peptide decreased. Apelin prevented the damaging effects of I/R and increased cfos expression in brainstem areas. Vagotomy, capsaicin, and lorglumide largely eliminated the gastroprotective effects of apelin-13. This study showed that sensory nerves and the vagus play regulatory roles in apelin-induced gastroprotection. Cholecystokinin may play a role in the effect of apelin through sensory neurons.

Apelin system detection in the reproductive apparatus of ewes grazing on semi-natural pasture

Apelin (APLN) is an adipokine with pleiotropic effects involved in the regulation of metabolic, cardiovascular, immune, and electrolyte balance function. Recent studies demonstrated a pivotal role in the regulation of male and female reproduction. APLN and its receptor (APLNR) were found in the hypothalamic-pituitary-gonad axis tissues, regulating gonadotropin release and steroidogenesis. However, to date, there are no studies that describe APLN system in the reproductive apparatus of the sheep. The study was performed on 10 Comisana x Appenninica adult dry ewes reared in a semi-natural pasture. Organ samples were collected from five animals in the two pasture functional phases: after maximum pasture flowering (Group 1) and after maximum pasture dryness (Group 2). Experiments were devised to characterize the gene expression and protein localization of the APLN/APLNR system in ewe reproductive apparatus; in addition, the concentration of plasma APLN was evaluated during the trial. Through immunohistochemical analysis, a positive staining for APLN was observed in the large luteal cells, in the epithelial cell coat of the ampulla, in the uterus epithelial lining and in the uterine glands. APLNR was observed in the granulosa cells, in the large luteal cells, in the secreting cells of the ampulla, in the uterus epithelial lining and uterine glands. The transcripts for APLN and APLNR were evidenced in all organ tissues examined. The highest level of APLN mRNA was detected in the Group 2 ewes in the luteal phase of the ovarian cycle compared to Group 1 ewes in the anestrous one. The relative content of APLN transcript was respectively twofold higher in the ovary (P < 0.05) and uterus (P < 0.05) and threefold higher in the ampulla (P < 0.05) in the Group 2 vs Group 1. The same trend of APLN transcript was evaluated for APLNR mRNA in uterus (P < 0.05) and ovary (P < 0.05). No difference was evidenced between Group 1 and Group 2 for APLNR mRNA levels. The plasma APLN level was fairly constant during the trial period. In conclusion, the present data suggest that the apelinergic system is involved in the reproduction function of ewes, being differentially distributed and expressed in the organs of the reproductive apparatus of ewes; these variations could be related to the sexual cycle and to the cyclic activity of the reproductive apparatus.

Effects of different loading exercises on apelin levels and physical and hematologic parameters of swimmers

Background

The purpose of this study was to investigate the effects of different exercise loads (short, medium and long swimming distances) on apelin levels and some physical and hematologic parameters of male professional swimmers.

Materials and methods

Apelin levels, hematologic parameters, whole blood values and physical measurements, including body mass index (BMI), aerobic power values and anaerobic power values, were also obtained.

Results

It was determined that the thrombocyte, erythrocyte and leukocyte values from the hematologic parameters increased after exercise (p < 0.05). According to the results, there were significant differences (p < 0.05) between the pretest apelin level (2090.75 pg/mg) and the apelin levels taken after swimming M 200 m (4260.43 pg/mg) and after swimming L 400 m (3694.4 pg/mg).

Conclusions

The different exercise loads had significant effects on the hematologic parameters and apelin values in the swimmers. The study also determined the relationships between swimming exercises and aerobic and anaerobic capacity and BMI.

Acute restraint stress induces cholecystokinin release via enteric apelin

Stress increases the apelin content in gut, while exogenous peripheral apelin has been shown to induce cholecystokinin (CCK) release. The present study was designed to elucidate (i) the effect of acute stress on enteric production of apelin and CCK, (ii) the role of APJ receptors in apelin-induced CCK release depending on the nutritional status. CCK levels were assayed in portal vein blood samples obtained from stressed (ARS) and non-stressed (NS) rats previously injected with APJ receptor antagonist F13A or vehicle. Duodenal expressions of apelin, CCK and APJ receptor were detected by immunohistochemistry. ARS increased the CCK release which was abolished by selective APJ receptor antagonist F13A. The stimulatory effect of ARS on CCK production was only observed in rats fed ad-libitum. Apelin and CCK expressions were upregulated by ARS. In addition to the duodenal I cells, APJ receptor was also detected in CCK-producing myenteric neurons. Enteric apelin appears to regulate the stress-induced changes in GI functions through CCK. Therefore, apelin/APJ receptor systems seem to be a therapeutic target for the treatment of stress-related gastrointestinal disorders.

Apelin System in Mammary Gland of Sheep Reared in Semi-Natural Pastures of the Central Apennines

Sheep are the most bred species in the Central Italy Apennine using the natural pastures as a trophic resource and grazing activity is fundamental to maintain the grassland biodiversity: this goal can be reached only ensuring an economical sustainability to the farmers. This study aimed to investigate the apelin/apelin receptor system in ovine mammary gland and to evaluate the differences induced by food supplementation, in order to shed light on this system function. A flock of 15 Comisana x Appenninica adult dry ewes were free to graze from June until pasture maximum flowering (MxF). From this period to pasture maximum dryness (MxD), in addition to grazing, the experimental group (Exp) was supplemented with 600 g/day/head of cereals. Apelin and apelin receptor were assessed by Real-Time PCR and immunohistochemistry on the mammary glands of subjects pertaining to MxF, MxD and Exp groups. They were detected in alveolar and ductal epithelial cells. The pasture maximum flowering group showed significant differences in apelin expression compared with experimental and MxD groups. Apelin receptor expression significantly differed among the three groups. The reduced apelin receptor expression and immunoreactivity levels during parenchyma involution enables us to hypothesize that apelin receptor plays a modulating role in the system control.

Apelin‑13 attenuates ER stress‑associated apoptosis induced by MPP+ in SH‑SY5Y cells

Apelin‑13, a neuropeptide that acts as a ligand for a putative receptor related to the angiotensin II type receptor, elicits neuroprotective effects in numerous neurological conditions, such as Huntington's disease and cerebral ischemia. Parkinson's disease (PD), one of the most prevalent neurodegenerative diseases, is caused by damage to neurons in the brain; however, the underlying mechanism remains unclear. The present study explored the effects of apelin‑13 on SH‑SY5Y human neuroblastoma cells treated with 1‑methyl‑4‑phenylpyridine (MPP+). Cell growth, cell viability, and apoptosis were measured by real‑time cell analysis, the Cell Counting Kit‑8 assay, and flow cytometry, respectively. In addition, the expression levels of extracellular signal‑regulated kinase (ERK) 1/2, p38 mitogen‑activated protein kinase (MAPK), glucose‑regulated protein 78 (GRP78), C/EBP homologous protein (CHOP), and cleaved caspase‑12 were assessed by western blotting. MPP+ treatment decreased the viability of SH‑SY5Y cells and increased their apoptosis; however, these changes were attenuated by pretreatment with apelin‑13. Treatment with MPP+ for 24 h significantly increased the expression levels of phospho‑ERK1/2, phospho‑p38, GRP78, CHOP, and cleaved caspase‑12 in SH‑SY5Y cells. Pretreatment with apelin‑13 significantly attenuated the upregulation of GRP78, CHOP and cleaved caspase‑12 in MPP+‑treated SH‑SY5Y cells, and significantly enhanced the expression levels of phospho‑ERK1/2. Taken together, the present results support a model in which apelin‑13 inhibits MPP+‑induced apoptosis of SH‑SY5Y cells by decreasing the expression of GRP78, CHOP, and cleaved caspase‑12, and by increasing the expression of phospho‑ERK1/2. The present findings suggest that apelin‑13 may be useful for the treatment of PD.

Review: Apelin in disease

Apelin, a regulatory peptide, is a ligand of the APJ receptor that belongs to the G protein-coupled receptor family. Apelin and APJ are widely distributed in the body and play potential physiological roles in the cytoprotection of many internal organs. This review article summarizes information about the roles of the apelin/APJ system in neurological, metabolic, hypertension, respiratory, gastrointestinal, hepatic, kidney and cancerous diseases. It is suggested that apelin positively affects the treatment of non-cancerous diseases and may be considered as a therapeutic drug in many illnesses. However, in cancers, apelin appears as a tumour growth stimulator, and its suggested role is as a marker in the diagnosis of tumour cancers in tissues. In summary, apelin has certain therapeutic abilities and can be useful in the treatment of, e.g., insulin resistance, hypertension, etc., but it also can sometimes serve as a negative factor.

Apelin-13 ameliorates cognitive impairments in 6-hydroxydopamine-induced substantia nigra lesion in rats

Although Parkinson's disease (PD) is well known with its motor deficits, the patients often suffer from cognitive dysfunction. Apelin, as the endogenous ligand of the APJ receptor, is found in several brain regions such as substantia nigra and mesolimbic pathway. However, the role of apelin in cognition and cognitive disorders has not been fully clarified. In this study the effects of apelin-13 were investigated on cognitive disorders in rat Parkinsonism experimental model. 6-hydroxydopamine (6-OHDA) was administrated into the substantia nigra. Apelin-13 (1, 2 and 3μg/rat) was administered into the substantia nigra one week after the 6-OHDA injection. Morris water maze (MWM), object location and novel object recognition tests were performed one month after the apelin injection. 6-OHDA-treated animals showed a significant impairment in cognitive functions which was revealed by the increased in the escape latency and traveled distance in MWM test and decreased in the exploration index in novel object recognition and object location tasks. Apelin-13 (3μg/rat) significantly attenuates the mentioned cognitive impairments in 6-OHDA-treated animals. In conclusion, the data support the pro-cognitive property of apelin-13 in 6-OHDA-induced cognitive deficit and provided a new pharmacological aspect of the neuropeptide apelin.

Pancreatic Exocrine Insufficiency after Bariatric Surgery

Morbid obesity is a lifelong disease, and all patients require complementary follow-up including nutritional surveillance by a multidisciplinary team after bariatric procedures. Pancreatic exocrine insufficiency (PEI) refers to an insufficient secretion of pancreatic enzymes and/or sodium bicarbonate. PEI is a known multifactorial complication after upper gastrointestinal surgery, and might constitute an important clinical problem due to the large number of bariatric surgical procedures in the world. Symptoms of PEI often overlap with sequelae of gastric bypass, making the diagnosis difficult. Steatorrhea, weight loss, maldigestion and malabsorption are pathognomonic for both clinical conditions. Altered anatomy after bypass surgery can make the diagnostic process even more difficult. Fecal elastase-1 (FE1) is a useful diagnostic test. PEI should be considered in all patients after bariatric surgery with prolonged gastrointestinal complaints that are suggestive of maldigestion and/or malabsorption. Appropriate pancreatic enzyme replacement therapy should be part of the treatment algorithm in patients with confirmed PEI or symptoms suggestive of this complication.

Peripheral apelin-13 administration inhibits gastrointestinal motor functions in rats: The role of cholecystokinin through CCK1 receptor-mediated pathway

Apelin is the endogenous ligand of the G protein-coupled receptor APJ. The APJ receptor is widely expressed in gastrointestinal (GI) tissues including stomach and small intestine. Apelin administration was shown to induce the release of cholecystokinin (CCK) which is a well-known alimentary hormone with its inhibitory actions on GI motor functions through CCK₁ receptors on vagal afferent fibers. We investigated whether; (i) peripherally injected apelin-13 alters GI motor functions, (ii) apelin-induced changes are mediated by APJ receptor or CCK₁ receptor and (iii) vagal afferents are involved in inhibitory effects of apelin. Solid gastric emptying (GE) and colon transit (CT) were measured, whereas duodenal phase III-like contractions were recorded in rats administered with apelin-13 (300μg/kg, ip). CCK₁ receptor antagonist lorglumide (10mg/kg, ip) or APJ receptor antagonist F13A (300μg/kg, ip) was administered 30min prior to the apelin-13 injections. Vagal afferent denervation was achieved by systemic administration of vanilloid receptor agonist capsaicin (125mg/kg, sc). Apelin-13 administration significantly (p<0.01) increased the CCK level in portal venous plasma samples. Compared with vehicle-treated rats, apelin-13 significantly delayed both GE (p<0.001) and CT (p<0.01). Pretreatment of lorglumide or F13A completely abolished the apelin-13-induced inhibitory effects on GE and CT, moreover, apelin-13 was found ineffective in rats underwent afferent denervation. F13A administration alone significantly accelerated the basal CT. Apelin-13 noticeably disturbed the duodenal fasting motor pattern by impairing phase III-like contractions while increasing the amplitudes of phase II contractions which were prevented by pretreatment of lorglumide and capsaicin. Compared with vehicle-treated rats, lorglumide and capsaicin significantly (p<0.05) reduced the apelin-13-induced increases in phase II motility index. Peripherally administered apelin-13 inhibits GI motor functions through CCK-dependent pathway which appears to be mediated by CCK₁ receptors on vagal afferents. Peripheral apelin might contribute to the motility changes occurred in postprandial period.

Apelin-13 protects against apoptosis by activating AMP-activated protein kinase pathway in ischemia stroke

Apelin has been proved to be protective against apoptosis induced by ischemic reperfusion. However, mechanisms whereby apelin produces neuroprotection remain to be elucidated. AMP-activated protein kinase (AMPK) is a master energy sensor that monitors levels of key energy metabolites. It is activated via AMPKαThr172 phosphorylation during cerebral ischemia and appears to be neuroprotective. In this study, we investigated the effect of apelin on AMPKα and tested whether apelin protecting against apoptosis was associated with AMPK signals. Focal transient cerebral ischemia/reperfusion (I/R) model in male ICR mice was induced by 60 min of ischemia followed by reperfusion. Apelin-13 was injected intracerebroventricularly 15 min before reperfusion. AMPK inhibitor, compound C, was injected to mice intraperitoneally at the onset of ischemia. In experiment 1, the effect of apelin-13 on AMPKα was measured. In experiment 2, the relevance of AMPKα and apelin-13' effect on apoptosis was measured. Data showed that apelin-13 significantly increased AMPKα phosphorylation level after cerebral I/R. Apelin-13, with the co-administration of saline, reduced apoptosis cells, down-regulated Bax and cleaved-caspase3 and up-regulated Bcl2. However, with the co-administration of compound C, apelin-13 was inefficient in affecting apoptosis and Bax, Bcl2 and cleaved-caspase3. The study provided the evidence that apelin-13 up-regulated AMPKα phosphorylation level in cerebral ischemia insults and AMPK signals participated in the mechanism of apelin-mediated neuroprotection.

High fat diet induced changes in gastric vagal afferent response to adiponectin

Food intake is regulated by vagal afferent signals from the stomach. Adiponectin, secreted primarily from adipocytes, also has a role in regulating food intake. However, the involvement of vagal afferents in this effect remains to be established. We aimed to determine if adiponectin can modulate gastric vagal afferent (GVA) satiety signals and further whether this is altered in high fat diet (HFD)-induced obesity. Female C57BL/6J mice were fed either a standard laboratory diet (SLD) or a HFD for 12weeks. Plasma adiponectin levels were assayed, and the expression of adiponectin in the gastric mucosa was assessed using real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR). The location of adiponectin protein within the gastric mucosa was determined by immunohistochemistry. To evaluate the direct effect of adiponectin on vagal afferent endings we determined adiponectin receptor expression in whole nodose ganglia (NDG) and also specifically in GVA neurons using retrograde tracing and qRT-PCR. An in vitro preparation was used to determine the effect of adiponectin on GVA response to mechanical stimulation. HFD mice exhibited an increased body weight and adiposity and showed delayed gastric emptying relative to SLD mice. Plasma adiponectin levels were not significantly different in HFD compared to SLD mice. Adiponectin mRNA was detected in the gastric mucosa of both SLD and HFD mice and presence of protein was confirmed immunohistochemically by the detection of adiponectin immunoreactive cells in the mucosal layer of the stomach. Adiponectin receptor 1 (ADIPOR1) and 2 (ADIPOR2) mRNA was present in both the SLD and HFD whole NDG and also specifically traced gastric mucosal and muscular neurons. There was a reduction in ADIPOR1 mRNA in the mucosal afferents of the HFD mice relative to the SLD mice. In HFD mice adiponectin potentiated gastric mucosal afferent responses to mucosal stroking, an effect not observed in SLD mice. Adiponectin reduced the responses of tension receptors to circular stretch to a similar extent in both SLD and HFD mice. In conclusion, adiponectin modulates GVA satiety signals. This modulatory effect is altered in HFD-induced obesity. It remains to be conclusively determined whether this modulation is involved in the regulation of food intake and what the whole animal phenotypic consequence is.

Apelin-13 attenuates traumatic brain injury-induced damage by suppressing autophagy

The adipocytokine apelin is a peptide, Apelin and its receptor are abundantly expressed in the nervous and cardiovascular systems. Previous studies had found apelin-13 reduces brain injuries and postischemic cerebral edema through blocking programmed cell death, Apelin-13 is also able to inhibit glucose deprivation induced cardiomyocyte autophagy in a concentration dependent fashion. To observe the effect of Apelin-13 on the brain injury induced by traumatic brain injury (TBI), and explore the effect of Apelin-13 on autophagy in TBI, We performed The neurological test, and the numbers of TBI-induced neural cell death were also counted by propidium iodide labeling. At last, the autophagy associated proteins LC3, Beclin-1, Bcl-2, p62 were also assessed with western-blotting. Compared with saline vehicle groups, the neural cell death, lesion volume, and neural dysfunction were attenuated by apelin-13 after TBI. In additionally, Apelin-13 also reversed TBI induced downregulation of LC3, Beclin-1, Bcl-2, p62 expression, compared with saline vehicle groups, at 24 and 48 h post TBI. Apelin-13 attenuates TBI induced brain damage by suppressing autophagy. All these results revealed that Apelin-13 suppressed autophagy. The autophagy may be involved in the mechanism of Apelin-13 rescue the subsequent damaged neuron in TBI.

The protective effect of apelin against water-immersion and restraint stress-induced gastric damage

The aim of the present study was to investigate the gastroprotective effect of apelin on water-immersion and restraint stress (WIRS)-induced gastric lesions. Male Wistar rats were divided into four groups: control, WIRS, F13A + WIRS and F13A. APJ receptor antagonist F13A was administered to rats to determine the influence of apelin on stress-induced gastric injury. WIRS administered for 6 h resulted in the development of gastric mucosal lesions accompanied by a significant increase in plasma corticosterone. WIRS increased the concentration of 4-hydroxynonenol (4-HNE) + malondialdehyde (MDA) and the expression of apelin and hypoxia inducible factor-1α (HIF-1α) in gastric mucosa. In addition, WIRS reduced the mucosal blood flow and gastric prostaglandin E(2) (PGE(2)) concentration. Plasma corticosterone, which was increased due to stress, was significantly decreased in the F13A + WIRS group. Gastric lesions and the 4-HNE + MDA concentration were also higher in the F13A + WIRS compared to the WIRS group. We conclude that apelin has a gastroprotective effect against stress-induced lesions possibly by reducing lipid peroxidation in gastric mucosa.

Apelin-13 protects the brain against ischemia/reperfusion injury through activating PI3K/Akt and ERK1/2 signaling pathways

Apelin has been proved to protect the heart against ischemia/reperfusion (I/R) injury via the activation of phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal-regulated kinase (ERK) signaling pathways. Whether this protective effect applies to brain I/R injury needed to be explored. We therefore investigated the potential neuroprotective role of Apelin-13 and the underlying mechanisms. Focal transient cerebral I/R model in male ICR mice was induced by 60min of ischemia followed by reperfusion. Apelin-13 intracerebroventricular injection was performed 15 min before reperfusion. Neurological function, infarct volume, brain edema and apoptosis were measured at 24h after stroke. To further test the mechanism of Apelin-13, PI3K inhibitor LY294002 and ERK1/2 inhibitor PD98059 were injected into the lateral cerebral ventricle 15min before ischemia. Compared with the Vehicle group, Apelin-13 significantly ameliorated neurological deficit, infarct volume, brain edema and reduced TUNEL-positive cells. Bax, caspase-3 and cleaved caspase-3 were down-regulated and Bcl-2 up-regulated. While, the effect of Apelin-13 on Bax, Bcl-2, caspase-3 and cleaved caspase-3 was attenuated by LY294002 and PD98059. Apelin protected the brain against I/R insult injury, and this effect may be through activation of PI3K/Akt and ERK1/2 signaling pathways.

Characterization of the endocrine, digestive and morphological adjustments of the intestine in response to food deprivation and torpor in cunner, Tautogolabrus adspersus

The cunner, Tautogolabrus adspersus, is a marine teleost endemic to the cold waters of the Northwest Atlantic Ocean. The cunner is non-migratory and is known for its remarkable ability to endure the freezing winter months with little to no food by entering a torpid/dormant state. To evaluate the physiological strategies employed by the cunner's intestinal tract to withstand food deprivation, fish were sampled for their gut after a four-week period of acute food deprivation during their summer (active/feeding) state, as well as after 4months of overwinter fasting. Digestive capacity was evaluated by measuring digestive enzyme activity and related mRNA transcript expression for trypsin, alkaline phosphatase, alanine aminopeptidase and lipase. In order to assess how gut hormones affect/are affected by acute fasting and torpor, we examined the intestinal mRNA expression of several putative appetite regulators, i.e. CCK, apelin, orexin and mTOR. Short-term summer fasting induced a reduction in the activity, but not the transcript expression, of all digestive enzymes examined as well as a reduction in gut apelin mRNA. Torpor induced a reduction in the activity of all enzymes with the exception of alanine aminopeptidase, and a decrease in mRNA levels of alanine aminopeptidase, orexin, CCK and mTOR. Our results suggest that both acute fasting and long-term fasting induce a reduction in the intestinal function of cunner, as evidenced by an overall decrease in the activities of digestive enzymes and mRNA expression of several factors involved in feeding and digestion.

Tumour co-expression of apelin and its receptor is the basis of an autocrine loop involved in the growth of colon adenocarcinomas

Using a cancer profiling array, our laboratory has shown that apelin gene is up-regulated in half of colon adenocarcinomas. We have therefore postulated that apelin signalling might play a prominent role in the growth of colon tumours. We first confirmed by immunohistochemistry that apelin peptide is overexpressed in human colon adenomas and adenocarcinomas. We also observed a significant overexpression of apelin receptor (APJ) in adjacent sections. We then demonstrated that several colorectal cancer cell lines also expressed apelin and its receptor, the highest gene and peptide expression being detected in LoVo cells. In this cell line, the expression and functionality of apelin receptor were revealed by apelin-induced adenylyl cyclase inhibition and Akt phosphorylation. In addition, apelin clearly protected LoVo cells from apoptosis by inactivating a caspase-dependent pathway and decreasing the degradation of poly ADP ribose polymerase protein (PARP). Finally, treatment of these tumour cells by the (F13A)apelin13 receptor antagonist significantly reduced their proliferation rate. Altogether, these data suggest the existence of an autocrine loop by which constitutive activation of apelin signalling should participate in the growth of colon adenocarcinomas. Accordingly, apelin signalling is a promising pharmacological target for the treatment of human colon adenomas and adenocarcinomas.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

The apelin receptor (APJ; gene symbol APLNR) is a member of the G protein-coupled receptor gene family. Neural gene expression patterns of APJ, and its cognate ligand apelin, in the brain implicate the apelinergic system in the regulation of a number of physiological processes. APJ and apelin are highly expressed in the hypothalamo-neurohypophysial system, which regulates fluid homeostasis, in the hypothalamic-pituitary-adrenal axis, which controls the neuroendocrine response to stress, and in the forebrain and lower brainstem regions, which are involved in cardiovascular function. Recently, apelin, synthesised and secreted by adipocytes, has been described as a beneficial adipokine related to obesity, and there is growing awareness of a potential role for apelin and APJ in glucose and energy metabolism. In this review we provide a comprehensive overview of the structure, expression pattern and regulation of apelin and its receptor, as well as the main second messengers and signalling proteins activated by apelin. We also highlight the physiological and pathological roles that support this system as a novel therapeutic target for pharmacological intervention in treating conditions related to altered water balance, stress-induced disorders such as anxiety and depression, and cardiovascular and metabolic disorders.

Regulation of feeding behavior, gastrointestinal function and fluid homeostasis by apelin

Apelin was first identified and characterized from bovine stomach extracts as an endogenous ligand for the APJ receptor. Apelin/APJ system is abundantly present in peripheral tissues and central nervous system. Apelin plays a broad role in regulating physiological and pathological functions. Recently, many reports have showed the effects of apelin on feeding behavior, however the results are inconsistent, due to different administration routes, animal species, forms of apelin, etc. Apelin has been involved in stimulating gastric cell proliferation, cholecystokinin (CCK) secretion, histamine release, gastric acid and bicarbonate secretion, and regulation of gastrointestinal motility. In addition, apelin produced regulatory effects on drinking behavior, diuresis, arginine vasopressin (AVP) release and glucocorticoids secretion. This article reviews the role of apelin on feeding behavior, gastrointestinal function and fluid homeostasis.

Discovery of 4-oxo-6-((pyrimidin-2-ylthio)methyl)-4H-pyran-3-yl 4-nitrobenzoate (ML221) as a functional antagonist of the apelin (APJ) receptor

The recently discovered apelin/APJ system has emerged as a critical mediator of cardiovascular homeostasis and is associated with the pathogenesis of cardiovascular disease. A role for apelin/APJ in energy metabolism and gastrointestinal function has also recently emerged. We disclose the discovery and characterization of 4-oxo-6-((pyrimidin-2-ylthio)methyl)-4H-pyran-3-yl 4-nitrobenzoate (ML221), a potent APJ functional antagonist in cell-based assays that is >37-fold selective over the closely related angiotensin II type 1 (AT1) receptor. ML221 was derived from an HTS of the ~330,600 compound MLSMR collection. This antagonist showed no significant binding activity against 29 other GPCRs, except to the κ-opioid and benzodiazepinone receptors (<50/<70%I at 10 μM). The synthetic methodology, development of structure-activity relationship (SAR), and initial in vitro pharmacologic characterization are also presented.

Hypoxia-induced up-regulation of apelin is associated with a poor prognosis in oral squamous cell carcinoma patients

Recently, apelin has been shown to be a novel angiogenic factor in various cancers including lung, breast and brain cancer. However, there is limited information regarding the expression and role of apelin in oral cavity cancer. In this study, we determined that apelin expression was localized in the cytoplasm of oral squamous cell carcinoma at various intensities. Strong apelin expression significantly correlated with tumor recurrence and disease-free survival. Using a multivariate analysis, we demonstrated that apelin was an independent prognostic factor for on disease-free survival, age, lymph node metastasis and CA9 expression. Moreover, apelin expression was up-regulated under hypoxic conditions, and exogenous apelin enhanced the proliferation and migration of oral cancer cells. Based on these results, we propose that the presence of hypoxia-induced apelin is a new prognostic factor and potential therapeutic target for oral squamous cell carcinoma.

Central apelin-13 inhibits food intake via the CRF receptor in mice

Apelin, the novel identified peptide, is the endogenous ligand for the APJ. Previous studies have reported the effect of apelin on food intake, however the action of acute central injected apelin on food intake in mice remains unknown. The present study was designed to investigate the mechanism as well as the effect of central apelin-13 on food intake in mice. During the dark period, the cumulative food intake was significantly decreased at 4h after the intracerebroventricular (i.c.v.) injection of 1 and 3μg/mouse apelin-13 and the period food intake was significantly reduced during 2-4h after treatment. In the fasted mice, the cumulative food intake was significantly decreased at 2 and 4h after injection of 3μg/mouse apelin-13. The cumulative water intake was significantly reduced by apelin-13 (3μg/mouse) at 4h after injection in freely feeding and fasted mice. However, during light period, apelin-13 had no influence on food and water intake in freely feeding mice. The APJ receptor antagonist apelin-13(F13A) (6μg/mouse) and the corticotrophin-releasing factor (CRF) receptor antagonist α-helical CRF(9-41) (3μg/mouse) could reverse the inhibitory effect on cumulative food intake/0-4h induced by apelin-13 (3μg/mouse) in freely feeding mice during the dark period, whereas the anorexic effect could not be antagonized by the arginie vasopressin (AVP) receptor antagonist deamino(CH(2))(5)Tyr(Me)AVP (0.5μg/mouse). Taken together, these results suggest that central apelin-13 inhibits food intake in mice and it seems that APJ receptor and CRF receptor, but not AVP receptor, might be involved in this process.

VGF: an inducible gene product, precursor of a diverse array of neuro-endocrine peptides and tissue-specific disease biomarkers

The vgf gene (non-acronymic) is induced in vivo by neurotrophins including Nerve Growth Factor (NGF), Brain Derived Growth Factor (BDNF) and Glial Derived Growth Factor (GDNF), by synaptic activity and by homeostatic and other stimuli. Post-translational processing of a single VGF precursor gives raise to a varied multiplicity of neuro-endocrine peptides, some of which are secreted upon stimulation both in vitro and in vivo. Several VGF peptides, accounting for ∼20% of the VGF precursor sequence, have shown biological roles including regulation of food intake, energy balance, reproductive and homeostatic mechanisms, synaptic strengthening, long-term potentiation (LTP) and anti-depressant activity. From a further ∼50% of VGF derive multiple "fragments", largely identified in the human cerebro-spinal fluid by proteomic studies searching for disease biomarkers. These represent an important starting point for discovery of further VGF products relevant to neuronal brain functions, as well as to neurodegenerative and psychiatric disease conditions. A distinct feature of VGF peptides is their cell type specific diversity in all neuroendocrine organs studied so far. Selective differential profiles are found across the cell populations of pituitary, adrenal medulla and pancreatic islets, and in gastric neuroendocrine as well as some further mucosal cells, and are yet to be investigated in neuronal systems. At the same time, specific VGF peptide/s undergo selective modulation in response to organ or cell population relevant stimuli. Such pattern argues for a multiplicity of roles for VGF peptides, including endocrine functions, local intercellular communication, as well as the possible mediation of intracellular mechanisms.

Effect of centrally administered apelin-13 on gastric emptying and gastrointestinal transit in mice

Apelin, as the endogenous ligand for the APJ, regulates many biological functions, including blood pressure, neuroendocrine, drinking behavior, food intake and colonic motility. The present study was designed to investigate the effect of central apelin-13 on gastric emptying and gastrointestinal transit in mice. Intracerebroventricular (i.c.v.) injection of apelin-13 (3 and 10 μg/mouse) decreased gastric emptying rate by 10.9% and 17.1%. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A) and the opioid receptor antagonist naloxone, respectively. However, intraperitoneal (i.p.) injection of apelin-13 (10-100 μg/mouse) did not affect gastric emptying. Apelin-13 (0.3, 1 and 3 μg/mouse, i.c.v.) inhibited gastrointestinal transit by 16.8%, 23.4% and 19.2%. Apelin-13(F13A) and naloxone could also reverse this antitransit effect induced by apelin-13. Taken together, these results suggest that i.c.v. injected apelin-13 inhibits gastric emptying and gastrointestinal transit and it seems that APJ receptor and opioid receptor might be involved in these processes.

Apelin stimulation of duodenal bicarbonate secretion: feeding-dependent and mediated via apelin-induced release of enteric cholecystokinin

AIMS

Apelin peptides are the endogenous ligand of the G protein-coupled receptor APJ. Proposed actions include involvement in control of cardiovascular functions, appetite and body metabolism. We have investigated the effects of apelin peptides on duodenal bicarbonate secretion in vivo and the release of cholecystokinin (CCK) from acutely isolated mucosal cells and the neuroendocrine cell line STC-1.

METHODS

Lewis × Dark Agouti rats had free access to water and, unless fasted overnight, free access to food. A segment of proximal duodenum was cannulated in situ in anaesthetized animals. Mucosal bicarbonate secretion was titrated (pH stat) and apelin was administered to the duodenum by close intra-arterial infusion. Total RNA was extracted from mucosal specimens, reverse transcripted to cDNA and the expression of the APJ receptor measured by quantitative real-time PCR. Apelin-induced release of CCK was measured using (1) cells prepared from proximal small intestine and (2) STC-1 cells.

RESULTS

Even the lowest dose of apelin-13 (6 pmol kg⁻¹ h⁻¹) caused a significant rise in bicarbonate secretion. Stimulation occurred only in continuously fed animals and even a 100-fold greater dose (600 pmol kg⁻¹ h⁻¹) of apelin was without effect in overnight food-deprived animals. Fasting also induced an eightfold decrease in the expression of APJ receptor mRNA. Apelin induced significant release of CCK from both mucosal and STC-1 cells, and the CCK(A) receptor antagonist devazepide abolished bicarbonate secretory responses to apelin.

CONCLUSION

Apelin-induced stimulation of duodenal electrolyte secretion is feeding-dependent and mediated by local mucosal release of CCK.

Apelin expression in human non-small cell lung cancer: role in angiogenesis and prognosis

INTRODUCTION

The recently discovered bioactive peptide, apelin, has been demonstrated to stimulate angiogenesis in various experimental systems. However, its clinical significance and role in tumor vascularization have not yet been investigated in a human malignancy. Therefore, our aim was to study whether apelin expression is associated with angiogenesis and/or tumor growth/behavior in human non-small cell lung cancer (NSCLC).

METHODS

A total of 94 patients with stage I-IIIA NSCLC and complete follow-up information were included. Apelin expression in human NSCLC samples and cell lines was measured by quantitative reverse-transcriptase polymerase chain reaction, enzyme-linked immunosorbent assay, and immunohistochemistry. Effects of exogenous apelin and apelin transfection were studied on NSCLC cell lines in vitro. In vivo growth of tumors expressing apelin or control vectors were also assessed. Morphometric variables of human and mouse tumor capillaries were determined by anti-CD31 labeling.

RESULTS

Apelin was expressed in all of the six investigated NSCLC cell lines both at the mRNA and protein levels. Although apelin overexpression or apelin treatments did not increase NSCLC cell proliferation in vitro, increasing apelin levels by gene transfer to NSCLC cells significantly stimulated tumor growth and microvessel densities and perimeters in vivo. Apelin mRNA levels were significantly increased in human NSCLC samples compared with normal lung tissue, and high apelin protein levels were associated with elevated microvessel densities and poor overall survival.

CONCLUSIONS

This study reveals apelin as a novel angiogenic factor in human NSCLC. Moreover, it also provides the first evidence for a direct association of apelin expression with clinical outcome in a human cancer.

Apelin is a novel islet peptide

Apelin, a recently discovered peptide with wide tissue distribution, regulates feeding behavior, improves glucose utilization, and inhibits insulin secretion. We examined whether apelin is expressed in human islets, as well as in normal and type 2 diabetic (T2D) animal islets. Further, we studied islet apelin regulation and the effect of apelin on insulin secretion. Apelin expression and regulation was examined in human and animal specimens using immunocytochemistry, in situ hybridization, and real-time PCR. Insulin secretion was studied in INS-1 (832/13) clonal beta cells. APJ-receptor expression was studied using real-time PCR. In human and murine islets apelin was predominantly expressed in beta cells and alpha cells; a subpopulation of the PP cells in human islets also harbored apelin. In porcine and feline islets apelin was mainly expressed in beta cells. APJ-receptor expression was detected in INS-1 (832/13) cells, and in human and mouse islets. A high dose (1microM) of apelin-36 caused a moderate increase in glucose-stimulated insulin secretion (30%; p<0.001), while lower concentrations (10-100nM) of apelin robustly reduced insulin secretion by 50% (p<0.001). Apelin was upregulated in beta cells of T2D db/db mice (47% vs. controls; p<0.02) and GK-rats (74% vs. controls; p<0.002), but human islet apelin expression was unaffected by glucose. On the other hand, human islet apelin expression was diminished after culture in glucocorticoids (16% vs. controls; p<0.01). We conclude that apelin is a novel insulin-regulating islet peptide in humans and several laboratory animals. Islet apelin expression is negatively regulated by glucocorticoids, and upregulated in T2D animals. The presence of apelin receptors in islets suggests a role for apelin as a paracrine or autocrine messenger within the islets.

Ontogeny of apelin and its receptor in the rodent gastrointestinal tract

Apelin is the endogenous ligand for the APJ receptor and both apelin and APJ are expressed in the gastrointestinal (GI) tract. The aim of this study was to define ontogeny of apelin and APJ in the developing rodent GI tract by measuring expression levels and characterizing abundance and cellular localization at an embryonic stage (E18.5 or E21), two postnatal stages (P4, P16) and in the adult. Apelin and APJ mRNA levels were measured by real time RT-PCR, apelin and APJ-containing cells were identified by immunohistochemical (IHC) staining. Gastric, duodenal and colonic apelin and APJ mRNA levels were highest at birth and declined postnatally. In the postnatal rat stomach, few apelin peptide-containing cells were identified, the density of gastric apelin-containing cells increased progressively after weaning and into adulthood. A robust APJ immunostaining was observed postnatally in the epithelium, intestinal goblet cells and in smooth muscle cells. In the adult rat, APJ immunostaining in the surface epithelium and goblet cells decreased markedly. During the early postnatal period, in an apelin-deficient mouse, APJ expression and immunostaining in the gut were reduced suggesting that apelin regulates APJ. Together, our data support a role for the apelin-APJ system in the regulation of smooth muscle, epithelial and goblet cell function in the GI tract.

Involvement of a Stat3 binding site in inflammation-induced enteric apelin expression

Apelin is the endogenous ligand for the APJ receptor; both are expressed in the gastrointestinal tract. Experimental colitis in rodents and inflammatory bowel disease in humans are associated with increased intestinal apelin production. Our aim was to use LPS and proinflammatory cytokine-treated (IL-6 and IFN-gamma) rodents or enteric cells to identify signaling mechanisms underlying inflammation-induced enteric apelin expression. LPS, IL-6, or IFN-gamma treatment of rodents increased enteric apelin expression. Pharmacological blockade of Jak/Stat signaling or IL-6 antibody administration inhibited elevations in enteric apelin expression. Transient transfection experiments showed that LPS, IL-6, or IFN-gamma increased apelin expression by stimulation of apelin promoter activity, and blockade of Jak/Stat signaling abolished elevations in apelin promoter activity. A chromatin immunoprecipitation assay showed that IL-6 induced binding of phospho-Stat3 to a putative Stat3 site in the apelin promoter; mutation of this site abrogated the LPS-induced elevation in apelin promoter activity. Together, our findings indicate that binding of phospho-Stat3 to the apelin promoter is the final step underlying proinflammatory cytokine-induced enteric apelin expression during intestinal inflammation.

A possible role for hypoxia-induced apelin expression in enteric cell proliferation

Apelin is the endogenous ligand for the APJ receptor, and apelin and APJ are expressed in the gastrointestinal (GI) tract. Intestinal inflammation increases intestinal hypoxia-inducible factor (HIF) and apelin expression. Hypoxia and inflammation are closely linked cellular insults. The purpose of these studies was to investigate the influence of hypoxia on enteric apelin expression. Exposure of rat pups to acute hypoxia increased hepatic, stomach-duodenal, and colonic apelin mRNA levels 10-, 2-, and 2-fold, respectively (P < 0.05 vs. controls). Hypoxia also increased colonic APJ mRNA levels, and apelin treatment during hypoxia exposure enhanced colonic APJ mRNA levels further. In vitro hypoxia also increased apelin and APJ mRNA levels. The hypoxia-induced elevation in apelin expression is most likely mediated by HIF, since HIF-activated apelin transcriptional activity is dependent on an intact, putative HIF binding site in the rat apelin promoter. Acute exposure of rat pups to hypoxia lowered gastric and colonic epithelial cell proliferation; hypoxia in combination with apelin treatment increased epithelial proliferation by 50%. In vitro apelin treatment of enteric cells exposed to hypoxia increased cell proliferation. Apelin treatment during normoxia was ineffective. Our studies imply that the elevation in apelin expression during hypoxia and inflammation in the GI tract functions in part to stimulate epithelial cell proliferation.

Expanding role for the apelin/APJ system in physiopathology

Apelin is a bioactive peptide known as the ligand of the G protein-coupled receptor APJ. Diverse active apelin peptides exist under the form of 13, 17 or 36 amino acids, originated from a common 77-amino-acid precursor. Both apelin and APJ mRNA are widely expressed in several rodent and human tissues and have functional effects in both the central nervous system and peripheral tissues. Apelin has been shown to be involved in the regulation of cardiovascular functions, fluid homeostasis, vessel formation and cell proliferation. More recently, apelin has been described as an adipocyte-secreted factor (adipokine), up-regulated in obesity. By acting as circulating hormone or paracrine factor, adipokines are involved in physiological regulations (fat depot development, energy storage, metabolism or eating behavior) or in the promotion of obesity-associated disorders (type 2 diabetes and cardiovascular dysfunctions). In this regard, expression of apelin gene in adipose tissue is increased by insulin and TNFα. This review will consider the main roles of apelin in physiopathology with particular attention on its role in energy balance regulation and in obesity-associated disorders.

Immunohistochemical localization of apelin in human normal breast and breast carcinoma

The peptide apelin is a high-affinity ligand for the G-protein coupled receptor APJ. Apelin/APJ signaling plays important roles in blood pressure regulation, body fluid homeostasis, and cardiovascular development. More recently, it has been recognized that apelin/APJ signaling may also be involved in tumor angiogenesis. Studies in experimental animals have shown that apelin is abundantly secreted in the milk, and the mammary gland contains high level of pre-proapelin mRNAs and apelin protein. High level of apelin mRNA is expressed in cultured human breast carcinoma cell line (Hs 578T). However, the status of apelin expression and localization in human breast carcinoma has not been studied. In the present study immunohistochemistry was performed to investigate the expression and localization of apelin in normal human breast tissue and breast carcinoma. Cytoplasmic apelin immunoreactivity was detected in the ductal and lobular epithelial cells and vascular endothelial cells of the normal breast tissue. The myoepithelial cells were negative. The malignant tumor cells of invasive ductal or lobular carcinoma also expressed similar level of immunoreactive apelin. The fuctional significance of apelin expression in normal nonlactating breast and breast carcinoma warrants further investigation.

Increased colonic apelin production in rodents with experimental colitis and in humans with IBD

Apelin and its receptor, the APJ receptor, are expressed in the gastrointestinal tract. The aims of this study were to examine the effects of sodium dextran sulfate (DSS)-induced experimental colitis in rats and mice and inflammatory bowel disease (IBD) in humans on intestinal apelin production, and the influence of exogenous apelin on colonic epithelial cell proliferation in mice. In rodents with experimental colitis, colonic apelin mRNA levels were elevated during the inflammatory reaction as well as during the tissue repair phase that ensues after DSS withdrawal. Fluctuations in colonic apelin expression were paralleled by similar changes in apelin immunostaining. Apelin immunostaining was increased in the surface epithelium, in epithelial cells along the length of the tubular gland and in the stem cell region at the gland base. In ulcerative colitis (UC) and Crohn's disease patients, apelin immunostaining revealed a pattern of increased intestinal apelin content similar to that observed in rodents with experimental colitis. Administration of synthetic apelin to mice during the recovery phase of DSS-induced colitis stimulated colonic epithelial cell proliferation significantly. Our observations that colonic apelin production is increased during and after DSS exposure indicate that apelin plays multiple roles during the different stages of colitis. Additionally, the stimulatory action of exogenous apelin on colonic epithelial proliferation suggests that the increased apelin production during intestinal recovery stage may contribute to the repair of the intestinal epithelium in experimental rodent models of colitis and in IBD patients.

Therapeutic potential of interfering with apelin signalling

The apelin receptor is a G protein-coupled receptor activated by several apelin fragments. Its tissue distribution suggests that apelin signalling is involved in a broad range of physiological functions. Endothelial cells, which express high levels of apelin receptors, respond to apelin through the phosphorylation of key intracellular effectors associated with cell proliferation and migration. In addition, apelin is a mitogen for endothelial cells and exhibits angiogenic properties in matrigel experiments. This review focuses on the therapeutic potential of apelin signalling, which is associated with pathologies that result from decreased vascularisation (ischemias) or neovascularisation (retinopathies and solid tumors).

Unravelling the roles of the apelin system: prospective therapeutic applications in heart failure and obesity

The apelin receptor was initially classed as an orphan G-protein-coupled receptor, and little was known about its physiological functions until apelin, the endogenous ligand, was identified. Similarities between the structure and anatomical distribution of apelin and its receptor and that of angiotensin II and the angiotensin AT1 receptor provide clues about the physiological functions of this novel signal-transduction system. Now, roles have been established for the apelin system in lowering blood pressure, as a potent cardiac inotrope, in modulating pituitary hormone release and food and water intake, in stress activation, and as a novel adipokine that is excreted from fat cells and regulates insulin. Given its broad array of physiological roles, apelin has attracted much interest as a target for novel therapeutic research and drug design.

Transcriptomes of purified gastric ECL and parietal cells: identification of a novel pathway regulating acid secretion

The gastric entero-chromaffin-like (ECL) cell plays a key regulatory role in peripheral regulation of acid secretion due to the release of histamine that stimulates acid secretion by the parietal cell. Studies in intact animals, gastric glands, and isolated cells after short-term culture have shown expression of stimulatory CCK2 and PAC1 and inhibitory SST2 and Gal1 receptors as well as histidine decarboxylase. However, the pattern of its gene expression as a neuroendocrine cell has not been explored. Comparison of gene expression by 95% pure ECL cells obtained by density gradient, elutriation, and fluorescence-assisted cell sorting with isolates of the intact fundic gastric epithelium (i.e., "subtractive hybridization") identified a variety of additional expressed gene families characteristic of this neuroendocrine cell. These include genes 1) involved in neuropeptide synthesis and secretory vesicle exocytosis, 2) involved in control of inflammation, 3) implicated in healing of the epithelium, 4) encoding inhibitory Gi protein-coupled receptors, 5) playing a role in neuroendocrine regulation of food intake, and 6) encoding proteins likely involved in maintenance of circadian rhythm, in addition to the ECL cell-specific genes histidine decarboxylase and monoamine transporter. Particularly, the inhibitory apelin receptor gene, APJ, was highly expressed in the ECL cell preparation. Because parietal cells express apelin, immunohistochemical and functional studies showed that there is an inhibitory feed back loop between the parietal and ECL cell during gastrin stimulation, providing evidence for a novel pathway of downregulation of acid secretion due to interaction between these two cell types.