Peripheral administration of palmitoylated prolactin-releasing peptide induces Fos expression in hypothalamic neurons involved in energy homeostasis in NMRI male mice

Lipidized Prolactin-Releasing Peptide as a New Potential Tool to Treat Obesity and Type 2 Diabetes Mellitus: Preclinical Studies in Rodent Models

Obesity and type 2 diabetes mellitus (T2DM) are preconditions for the development of metabolic syndrome, which is reaching pandemic levels worldwide, but there are still only a few anti-obesity drugs available. One of the promising tools for the treatment of obesity and related metabolic complications is anorexigenic peptides, such as prolactin-releasing peptide (PrRP). PrRP is a centrally acting neuropeptide involved in food intake and body weight (BW) regulation. In its natural form, it has limitations for peripheral administration; thus, we designed analogs of PrRP lipidized at the N-terminal region that showed high binding affinities, increased stability and central anorexigenic effects after peripheral administration. In this review, we summarize the preclinical results of our chronic studies on the pharmacological role of the two most potent palmitoylated PrRP31 analogs in various mouse and rat models of obesity, glucose intolerance, and insulin resistance. We used mice and rats with diet-induced obesity fed a high-fat diet, which is considered to simulate the most common form of human obesity, or rodent models with leptin deficiency or disrupted leptin signaling in which long-term food intake regulation by leptin is distorted. The rodent models described in this review are models of metabolic syndrome with different severities, such as obesity or morbid obesity, prediabetes or diabetes and hypertension. We found that the effects of palmitoylated PrRP31 on food intake and BW but not on glucose intolerance require intact leptin signaling. Thus, palmitoylated PrRP31 analogs have potential as therapeutics for obesity and related metabolic complications.

Cholecystokinin system is involved in the anorexigenic effect of peripherally applied palmitoylated prolactin-releasing peptide in fasted mice

Prolactin-releasing peptide (PrRP) has been proposed to mediate the central satiating effects of cholecystokinin (CCK) through the vagal CCK1 receptor. PrRP acts as an endogenous ligand of G protein-coupled receptor 10 (GPR10), which is expressed at the highest levels in brain areas related to food intake regulation, e.g., the paraventricular hypothalamic nucleus (PVN) and nucleus of the solitary tract (NTS). The NTS and PVN are also significantly activated after peripheral CCK administration. The aim of this study was to determine whether the endogenous PrRP neuronal system in the brain is involved in the central anorexigenic effect of the peripherally administered CCK agonist JMV236 or the CCK1 antagonist devazepide and whether the CCK system is involved in the central anorexigenic effect of the peripherally applied lipidized PrRP analog palm-PrRP31 in fasted lean mice. The effect of devazepide and JMV236 on the anorexigenic effects of palm-PrRP31 as well as devazepide combined with JMV236 and palm-PrRP31 on food intake and Fos cell activation in the PVN and caudal NTS was examined. Our results suggest that the anorexigenic effect of JMV236 is accompanied by activation of PrRP neurons of the NTS in a CCK1 receptor-dependent manner. Moreover, while the anorexigenic effect of palm-PrRP31 was not affected by JMV236, it was partially attenuated by devazepide in fasted mice. The present findings indicate that the exogenously influenced CCK system may be involved in the central anorexigenic effect of peripherally applied palm-PrRP31, which possibly indicates some interaction between the CCK and PrRP neuronal systems.

Palmitoylation of Prolactin-Releasing Peptide Increased Affinity for and Activation of the GPR10, NPFF-R2 and NPFF-R1 Receptors: In Vitro Study

The anorexigenic neuropeptide prolactin-releasing peptide (PrRP) is involved in the regulation of food intake and energy expenditure. Lipidization of PrRP stabilizes the peptide, facilitates central effect after peripheral administration and increases its affinity for its receptor, GPR10, and for the neuropeptide FF (NPFF) receptor NPFF-R2. The two most potent palmitoylated analogs with anorectic effects in mice, palm¹¹-PrRP31 and palm-PrRP31, were studied in vitro to determine their agonist/antagonist properties and mechanism of action on GPR10, NPFF-R2 and other potential off-target receptors related to energy homeostasis. Palmitoylation of both PrRP31 analogs increased the binding properties of PrRP31 to anorexigenic receptors GPR10 and NPFF-R2 and resulted in a high affinity for another NPFF receptor, NPFF-R1. Moreover, in CHO-K1 cells expressing GPR10, NPFF-R2 or NPFF-R1, palm¹¹-PrRP and palm-PrRP significantly increased the phosphorylation of extracellular signal-regulated kinase (ERK), protein kinase B (Akt) and cAMP-responsive element-binding protein (CREB). Palm¹¹-PrRP31, unlike palm-PrRP31, did not activate either c-Jun N-terminal kinase (JNK), p38, c-Jun, c-Fos or CREB pathways in cells expressing NPFF-1R. Palm-PrRP31 also has higher binding affinities for off-target receptors, namely, the ghrelin, opioid (KOR, MOR, DOR and OPR-L1) and neuropeptide Y (Y₁, Y₂ and Y₅) receptors. Palm¹¹-PrRP31 exhibited fewer off-target activities; therefore, it has a higher potential to be used as an anti-obesity drug with anorectic effects.

Prolactin-Releasing Peptide: Physiological and Pharmacological Properties

Prolactin-releasing peptide (PrRP) belongs to the large RF-amide neuropeptide family with a conserved Arg-Phe-amide motif at the C-terminus. PrRP plays a main role in the regulation of food intake and energy expenditure. This review focuses not only on the physiological functions of PrRP, but also on its pharmacological properties and the actions of its G-protein coupled receptor, GPR10. Special attention is paid to structure-activity relationship studies on PrRP and its analogs as well as to their effect on different physiological functions, mainly their anorexigenic and neuroprotective features and the regulation of the cardiovascular system, pain, and stress. Additionally, the therapeutic potential of this peptide and its analogs is explored.

Design of a Long-Acting and Selective MEG-Fatty Acid Stapled Prolactin-Releasing Peptide Analog

Anorexigenic peptides offer promise as potential therapies targeting the escalating global obesity epidemic. Prolactin-releasing peptide (PrRP), a novel member of the RFamide family secreted by the hypothalamus, shows therapeutic potential by decreasing food intake and body weight in rodent models via GPR10 activation. Here we describe the design of a long-acting PrRP using our recently developed novel multiple ethylene glycol-fatty acid (MEG-FA) stapling platform. By incorporating serum albumin binding fatty acids onto a covalent side chain staple, we have generated a series of MEG-FA stapled PrRP analogs with enhanced serum stability and in vivo half-life. Our lead compound 18-S4 exhibits good in vitro potency and selectivity against GPR10, improved serum stability, and extended in vivo half-life (7.8 h) in mouse. Furthermore, 18-S4 demonstrates a potent body weight reduction effect in a diet-induced obesity (DIO) mouse model, representing a promising long-acting PrRP analog for further evaluation in the chronic obesity setting.

Prolactin-releasing peptide improved leptin hypothalamic signaling in obese mice

The situation following anti-obesity drug termination is rarely investigated, eventhough a decrease in body weight needs to be sustained. Therefore, this study examined the impact of twice-daily peripheral administration of 5 mg/kg [N-palm-γGlu-Lys¹¹] prolactin-releasing peptide 31 (palm¹¹-PrRP31) in mice with diet-induced obesity (DIO from consuming a high-fat diet) after 28 days of treatment (palm¹¹-PrRP31 group) and after 14 days of peptide treatment followed by 14 days of discontinuation (palm¹¹-PrRP31 + saline group). At the end of the treatment, cumulative food intake, body weight and subcutaneous fat weight/body weight ratio and leptin plasma level were reduced significantly in both the palm¹¹-PrRP31 group and the palm¹¹-PrRP31 + saline group compared to the saline control group. This reduction correlated with significantly increased FOSB, a marker of long-term neuronal potentiation, in the nucleus arcuatus and nucleus tractus solitarii, areas known to be affected by the anorexigenic effect of palm¹¹-PrRP31. Moreover, activation of leptin-related hypothalamic signaling was registered through an increase in phosphoinositide-3-kinase, increased phosphorylation of protein kinase B (PKB, AKT) and enhanced extracellular signal-regulated kinase 1/2 phosphorylation. Besides, lowered apoptotic markers c-JUN N-terminal kinase and c-JUN phosphorylation were registered in the hypothalami of both palm¹¹-PrRP31-treated groups. This study demonstrates that palm¹¹-PrRP31 positively affects feeding and leptin-related hypothalamic signaling, not only after 28 days of treatment but even 14 days after the termination of a 14-day long treatment without the yo-yo effect.

LC-MS/MS analysis of lipidized analogs of prolactin-releasing peptide utilizing a monolithic column and simple sample preparation

AIM

Novel compounds for obesity treatment are currently being studied employing lipidized analogs of anorexigenic neuropeptides. Various analogs of prolactin-releasing peptide have demonstrated their ability to decrease food intake. Adequate analytical tools are required to support corresponding research. Methodology & results: An analytical method was developed that includes simple dilution of plasma samples prior to liquid chromatography-mass spectrometry and employs a monolithic column for the determination of lipidized analogs of prolactin-releasing peptide in complex biological samples. A multiple reaction monitoring approach was applied that included matrix calibration and an internal standard and produced a linear calibration range 20-200 ng ml^-1 in rat and macaque plasma samples.

CONCLUSION

A straightforward, simple and reliable analytical method was developed satisfying major validation criteria.

Impact of novel palmitoylated prolactin-releasing peptide analogs on metabolic changes in mice with diet-induced obesity

Analogs of anorexigenic neuropeptides, such as prolactin-releasing peptide (PrRP), have a potential as new anti-obesity drugs. In our previous study, palmitic acid attached to the N-terminus of PrRP enabled its central anorexigenic effects after peripheral administration. In this study, two linkers, γ-glutamic acid at Lys¹¹ and a short, modified polyethylene glycol at the N-terminal Ser and/or Lys¹¹, were applied for the palmitoylation of PrRP31 to improve its bioavailability. These analogs had a high affinity and activation ability to the PrRP receptor GPR10 and the neuropeptide FF2 receptor, as well as short-term anorexigenic effect similar to PrRP palmitoylated at the N-terminus. Two-week treatment with analogs that were palmitoylated through linkers to Lys¹¹ (analogs 1 and 2), but not with analog modified both at the N-terminus and Lys¹¹ (analog 3) decreased body and liver weights, insulin, leptin, triglyceride, cholesterol and free fatty acid plasma levels in a mouse model of diet-induced obesity. Moreover, the expression of uncoupling protein-1 was increased in brown fat suggesting an increase in energy expenditure. In addition, treatment with analogs 1 and 2 but not analog 3 significantly decreased urinary concentrations of 1-methylnicotinamide and its oxidation products N-methyl-2-pyridone-5-carboxamide and N-methyl-4-pyridone-3-carboxamide, as shown by NMR-based metabolomics. This observation confirmed the previously reported increase in nicotinamide derivatives in obesity and type 2 diabetes mellitus and the effectiveness of analogs 1 and 2 in the treatment of these disorders.

Prolactin-releasing peptide: a new tool for obesity treatment

Obesity is an escalating epidemic, but an effective noninvasive therapy is still scarce. For obesity treatment, anorexigenic neuropeptides are promising tools, but their delivery from the periphery to the brain is complicated because peptides have a low stability and limited ability to cross the blood-brain barrier. In this review, we summarize results of several studies with our newly designed lipidized analogs of prolactin-releasing peptide (PrRP). PrRP is involved in feeding and energy balance regulation as demonstrated by obesity phenotypes of both PrRP- and PrRP-receptor-knockout mice. Lipidized PrRP analogs showed binding affinity and signaling in PrRP receptor-expressing cells similar to natural PrRP. Moreover, these analogs showed high binding affinity also to anorexigenic neuropeptide FF (NPFF)-2 receptor. Acute peripheral administration of myristoylated and palmitoylated PrRP analogs to mice and rats induced strong and long-lasting anorexigenic effects and neuronal activation in the brain areas involved in food intake regulation. Two-week-long subcutaneous administration of palmitoylated PrRP31 and myristoylated PrRP20 lowered food intake, body weight, improved metabolic parameters and attenuated lipogenesis in mice with diet-induced obesity. A strong anorexigenic, body weight-reducing and glucose tolerance-improving effect of palmitoylated-PrRP31 was shown also in diet-induced obese rats after its repeated 2-week-long peripheral administration. Thus, the strong anorexigenic and body weight-reducing effects of palmitoylated PrRP31 and myristoylated PrRP20 make these analogs attractive candidates for antiobesity treatment. Moreover, PrRP receptor might be a new target for obesity therapy.

Effect of palmitoylated prolactin-releasing peptide on food intake and neural activation after different routes of peripheral administration in rats

Obesity is an escalating epidemic, but an effective non-invasive therapy is still scarce. For obesity treatment, anorexigenic neuropeptides are promising tools, but their delivery from the periphery to the brain is complicated by their peptide character. In order to overcome this unfavorable fact, we have applied the lipidization of neuropeptide prolactin-releasing peptide (PrRP), whose strong anorexigenic effect was demonstrated. A palmitoylated analog of human PrRP (h palm-PrRP31) was injected in free-fed Wistar rats by three routes: subcutaneous (s.c.), intraperitoneal (i.p) (both 5 mg/kg) and intravenous (i.v.) (from 0.01 to 0.5 mg/kg). We found a circulating compound in the blood after all three applications with the highest concentration after i.v. administration. This corresponds to the effect on food intake, which was also strongest after i.v. injection. Moreover, this is in agreement with the fact that the expression of c-Fos in specific brain regions involved in food intake regulation was also highest after intravenous application. Pharmacokinetic data are further supported by results obtained from dynamic light scattering and CD spectroscopy. Human palm-PrRP31 analog showed a strong tendency to micellize, and formation of aggregates suggested lower availability after i.p. or s.c. application. We have demonstrated that palm-PrRP influenced food intake even in free fed rats. Not surprisingly, the maximal effect was achieved after the intravenous application even though two orders of magnitude lower dose was used compared to both two other applications. We believe that palm-PrRP could have a potential as an antiobesity drug when its s.c. application would be improved.