Internalization and trafficking of the human and rat growth hormone-releasing hormone receptor

Signaling mechanism of growth hormone-releasing hormone receptor

The hypothalamic peptide growth hormone-releasing hormone (GHRH) stimulates the secretion of growth hormone (GH) from the pituitary through binding and activation of the pituitary type of GHRH receptor (GHRH-R), which belongs to the family of G protein-coupled receptors with seven potential membrane-spanning domains. Splice variants of GHRH-Rs (SV) in human tumors and other extra pituitary tissues were identified and their cDNA was sequenced. Among the SVs, splice variant 1 (SV1) possesses the greatest similarity to the full-length GHRH-R and remains functional by eliciting cAMP signaling and mitogenic activity upon GHRH stimulation. A large body of work have evaluated potential clinical applications of agonists and antagonists of GHRH in diverse fields, including endocrinology, oncology, cardiology, diabetes, obesity, metabolic dysfunctions, Alzheimer's disease, ophthalmology, wound healing and other applications. In this chapter, we briefly review the expression and potential function of GHRH-Rs and their SVs in various tissues and also elucidate and summarize the activation, molecular mechanism and signalization pathways of these receptors. Therapeutic applications of GHRH analogs are also discussed.

Synthesis and structure-activity studies on novel analogs of human growth hormone releasing hormone (GHRH) with enhanced inhibitory activities on tumor growth

The syntheses and biological evaluations of new GHRH analogs of Miami (MIA) series with greatly increased anticancer activity are described. In the design and synthesis of these analogs, the following previous substitutions were conserved: D-Arg², Har⁹, Abu¹⁵, and Nle²⁷. Most new analogs had Ala at position 8. Since replacements of both Lys¹² and Lys²¹ with Orn increased resistance against enzymatic degradation, these modifications were kept. The substitutions of Arg at both positions 11 and 20 by His were also conserved. We kept D-Arg²⁸, Har²⁹ -NH₂ at the C-terminus or inserted Agm or 12-amino dodecanoic acid amide at position 30. We incorporated pentafluoro-Phe (Fpa₅), instead of Cpa, at position 6 and Tyr(Me) at position 10 and ω-amino acids at N-terminus of some analogs. These GHRH analogs were prepared by solid-phase methodology and purified by HPLC. The evaluation of the activity of the analogs on GH release was carried out in vitro on rat pituitaries and in vivo in male rats. Receptor binding affinities were measured in vitro by the competitive binding analysis. The inhibitory activity of the analogs on tumor proliferation in vitro was tested in several human cancer cell lines such as HEC-1A endometrial adenocarcinoma, HCT-15 colorectal adenocarcinoma, and LNCaP prostatic carcinoma. For in vivo tests, various cell lines including PC-3 prostate cancer, HEC-1A endometrial adenocarcinoma, HT diffuse mixed β cell lymphoma, and ACHN renal cell carcinoma cell lines were xenografted into nude mice and treated subcutaneously with GHRH antagonists at doses of 1-5μg/day. Analogs MIA-602, MIA-604, MIA-610, and MIA-640 showed the highest binding affinities, 30, 58, 48, and 73 times higher respectively, than GHRH (1-29) NH₂. Treatment of LNCaP and HCT-15 cells with 5μM MIA-602 or MIA-690 decreased proliferation by 40%-80%. In accord with previous tests in various human cancer lines, analog MIA-602 showed high inhibitory activity in vivo on growth of PC-3 prostate cancer, HT-mixed β cell lymphoma, HEC-1A endometrial adenocarcinoma and ACHN renal cell carcinoma. Thus, GHRH analogs of the Miami series powerfully suppress tumor growth, but have only a weak endocrine GH inhibitory activity. The suppression of tumor growth could be induced in part by the downregulation of GHRH receptors levels.

Antagonists of growth hormone-releasing hormone receptor induce apoptosis specifically in retinoblastoma cells

Retinoblastoma (RB) is the most common intraocular cancer in children worldwide. Current treatments mainly involve combinations of chemotherapies, cryotherapies, and laser-based therapies. Severe or late-stage disease may require enucleation or lead to fatality. Recently, RB has been shown to arise from cone precursor cells, which have high MDM2 levels to suppress p53-mediated apoptosis. This finding leads to the hypothesis that restoring apoptosis mechanisms in RBs could specifically kill the cancer cells without affecting other retinal cells. We have previously reported involvement of an extrapituitary signaling pathway of the growth hormone-releasing hormone (GHRH) in the retina. Here we show that the GHRH receptor (GHRH-R) is highly expressed in RB cells but not in other retinal cells. We induced specific apoptosis with two different GHRH-R antagonists, MIA-602 and MIA-690. Importantly, these GHRH-R antagonists do not trigger apoptosis in other retinal cells such as retinal pigmented epithelial cells. We delineated the gene expression profiles regulated by GHRH-R antagonists and found that cell proliferation genes and apoptotic genes are down- and up-regulated, respectively. Our results reveal the involvement of GHRH-R in survival and proliferation of RB and demonstrate that GHRH-R antagonists can specifically kill the RB cells.

Interaction of PICK1 with C-terminus of growth hormone-releasing hormone receptor (GHRHR) modulates trafficking and signal transduction of human GHRHR

Release of growth hormone (GH) from the somatotroph is regulated by binding GH-releasing hormone (GHRH) to its cognate receptor (GHRHR), one of the members of the G protein-coupled receptor (GPCR) superfamily. Proteins bound to the carboxy (C)-terminus of GPCR have been reported to regulate intracellular trafficking and function of the receptor; however, no functionally significant protein associated with GHRHR has been reported. We have identified a protein interacting with C-kinase 1 (PICK1) as a binding partner of GHRHR. In vitro binding assay revealed the PDZ-domain of PICK1 and the last four amino acid residues of GHRHR were prerequisite for the interaction. Further, in vivo association of these proteins was confirmed. Immunostaining data of a stable cell line expressing GHRHR with or without PICK1 suggested the C-terminus of GHRHR promoted cell surface expression of GHRHR and PICK1 affected the kinetics of the cell surface expression of GHRHR. Furthermore, cAMP production assay showed the C-terminus of GHRHR is involved in the regulation of receptor activation, and the interaction of GHRHR with PICK1 may influence intensities of the signal response after ligand stimulation. Thus, the interaction of the C-terminus of GHRHR with PICK1 has a profound role in regulating the trafficking and the signaling of GHRHR. [Supplementary Figure: available only at http://dx.doi.org/10.1254/jphs.12287FP].

Analyzing the role of receptor internalization in the regulation of melanin-concentrating hormone signaling

The regulation of appetite is complex, though our understanding of the process is improving. The potential role for the melanin-concentrating hormone (MCH) signaling pathway in the treatment of obesity is being explored by many. It was hypothesized that internalization of MCH receptors would act to potently desensitize cells to MCH. Despite potent desensitization of ERK signaling by MCH in BHK-570 cells, we were unable to observe MCH-mediated internalization of MCH receptor 1 (MCHR1) by fluorescence microscopy. A more quantitative approach using a cell-based ELISA indicated only 15% of receptors internalized, which is much lower than that reported in the literature. When β-arrestins were overexpressed in our system, removal of receptors from the cell surface was facilitated and signaling to a leptin promoter was diminished, suggesting that internalization of MCHR1 is sensitive to cellular β-arrestin levels. A dominant-negative GRK construct completely inhibited loss of receptors from the cell surface in response to MCH, suggesting that the internalization observed is phosphorylation-dependent. Since desensitization of MCH-mediated ERK signaling did not correlate with significant loss of MCHR1 from the cell surface, we hypothesize that in this model system regulation of MCH signaling may be the result of segregation of receptors from signaling components at the plasma membrane.

Altered vitamin E status in Niemann-Pick type C disease

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in many species. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in the NPC1 or NPC2 gene, which regulates lipid transport through the endocytic pathway. NPC disease is characterized by massive intracellular accumulation of unesterified cholesterol and other lipids in lysosomal vesicles. We examined the roles that NPC1/2 proteins play in the intracellular trafficking of tocopherol. Reduction of NPC1 or NPC2 expression or function in cultured cells caused a marked lysosomal accumulation of vitamin E in cultured cells. In vivo, tocopherol significantly accumulated in murine Npc1-null and Npc2-null livers, Npc2-null cerebella, and Npc1-null cerebral cortices. Plasma tocopherol levels were within the normal range in Npc1-null and Npc2-null mice, and in plasma samples from human NPC patients. The binding affinity of tocopherol to the purified sterol-binding domain of NPC1 and to purified NPC2 was significantly weaker than that of cholesterol (measurements kindly performed by R. Infante, University of Texas Southwestern Medical Center, Dallas, TX). Taken together, our observations indicate that functionality of NPC1/2 proteins is necessary for proper bioavailability of vitamin E and that the NPC pathology might involve tissue-specific perturbations of vitamin E status.

A straightforward route to the synthesis of a surface-enhanced Raman scattering probe for targeting transferrin receptor-overexpressed cells

A tumor cell targeting surface-enhanced Raman scattering (SERS) probe has been successfully synthesized by using p-mercaptobenzoic acid (pMBA) as both the SERS reporter and the conjugation agent for attaching transferrin molecules, which shows experimentally the targeting ability for transferrin receptor-overexpressed HeLa cells and exhibits strong SERS signals when being incubated inside cells. To prove that the uptake of such a SERS probe is through a Tf-receptor-mediated endocytosis process, two control experiments: (1) HeLa cells being incubated with the probe at 4 degrees C and (2) HeLa cells being pre-blocked with free transferrin at 37 degrees C, were employed. The difference of SERS intensity between the transferrin-overexpressed HeLa cells and transferrin-pre-blocked HeLa cells indicates that the probe has the potential to selectively target tumor cells.

Localized suppression of cortical growth hormone-releasing hormone receptors state-specifically attenuates electroencephalographic delta waves

Growth hormone-releasing hormone (GHRH) promotes non-rapid eye movement sleep (NREMS), in part via a well characterized hypothalamic sleep-promoting site. However, GHRH may also act in the cortex to influence sleep. Application of GHRH to the surface of the cortex changes electroencephalographic (EEG) delta power. GHRH and the GHRH receptor (GHRHR) mRNAs are detectable in the rat cortex, and the expression of cortical GHRHR is activity dependent. Here, we microinjected a GHRH antagonist or GHRHR small interfering RNA (siGHRHR) onto the somatosensory cortex surface in rats. The unilateral application of the GHRH antagonist ipsilaterally decreased EEG delta wave power during NREMS, but not wakefulness, during the initial 40 min after injection. Similarly, the injection of siGHRHR reduced cortical expression of GHRHR and suppressed NREMS EEG delta wave power during 20-24 h after injection. Using the fura-2 calcium imaging technique, cultured cortical cells responded to GHRH by increasing intracellular calcium. Approximately 18% of the GHRH-responsive cells were GABAergic as illustrated by glutamic acid decarboxylase-67 (GAD67) immunostaining. Double labeling for GAD67 and GHRHR in vitro and in vivo indicated that only a minority of cortical GHRHR-containing cells were GABAergic. Our data suggest that endogenous cortical GHRH activates local cortical cells to affect EEG delta wave power state-specifically. Results are also consistent with the hypothesis that GHRH contributes to local network state regulation.

Cellular localization of kinin B1 receptor in the spinal cord of streptozotocin-diabetic rats with a fluorescent [Nalpha-Bodipy]-des-Arg9-bradykinin

Background

The kinin B₁ receptor (B₁R) is upregulated by pro-inflammatory cytokines, bacterial endotoxins and hyperglycaemia-induced oxidative stress. In animal models of diabetes, it contributes to pain polyneuropathy. This study aims at defining the cellular localization of B₁R in thoracic spinal cord of type 1 diabetic rats by confocal microscopy with the use of a fluorescent agonist, [Nα-Bodipy]-des-Arg⁹-BK (BdABK) and selective antibodies.

Methods

Diabetes was induced by streptozotocin (STZ; 65 mg/kg, i.p.). Four days post-STZ treatment, B₁R expression was confirmed by quantitative real-time PCR and autoradiography. The B₁R selectivity of BdABK was determined by assessing its ability to displace B₁R [¹²⁵I]-HPP-desArg¹⁰-Hoe140 and B₂R [¹²⁵I]-HPP-Hoe 140 radioligands. The in vivo activity of BdABK was also evaluated on thermal hyperalgesia.

Results

B₁R was increased by 18-fold (mRNA) and 2.7-fold (binding sites) in the thoracic spinal cord of STZ-treated rats when compared to control. BdABK failed to displace the B₂R radioligand but displaced the B₁R radioligand (IC₅₀ = 5.3 nM). In comparison, IC₅₀ values of B₁R selective antagonist R-715 and B₁R agonist des-Arg⁹-BK were 4.3 nM and 19 nM, respectively. Intraperitoneal BdABK and des-Arg⁹-BK elicited dose-dependent thermal hyperalgesia in STZ-treated rats but not in control rats. The B₁R fluorescent agonist was co-localized with immunomarkers of microglia, astrocytes and sensory C fibers in the spinal cord of STZ-treated rats.

Conclusion

The induction and up-regulation of B₁R in glial and sensory cells of the spinal cord in STZ-diabetic rats reinforce the idea that kinin B₁R is an important target for drug development in pain processes.

Densovirus infectious pathway requires clathrin-mediated endocytosis followed by trafficking to the nucleus

Junonia coenia densovirus (JcDNV) is an ambisense insect parvovirus highly pathogenic for lepidopteran pests at larval stages. The potential use of DNVs as biological control agents prompted us to reinvestigate the host range and cellular mechanisms of infection. In order to understand the early events of infection, we set up a functional infection assay in a cell line of the pest Lymantria dispar to determine the intracellular pathway undertaken by JcDNV to infect a permissive lepidopteran cell line. Our results show that JcDNV particles are rapidly internalized into clathrin-coated vesicles and slowly traffic within early and late endocytic compartments. Blocking late-endocytic trafficking or neutralizing the pH with drugs inhibited infection. During internalization, disruption of the cytoskeleton, and inhibition of phosphatidylinositol 3-kinase blocked the movement of vesicles containing the virus to the nucleus and impaired infection. In summary, our results define for the first time the early endocytic steps required for a productive DNV infection.

Rapid resensitization of purinergic receptor function in human platelets

BACKGROUND

Adenosine diphosphate (ADP) is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), the P2Y(1) and P2Y(12) purinergic receptors. Recently, we demonstrated that both receptors desensitize and internalize in human platelets by differential kinase-dependent mechanisms.

OBJECTIVES

To demonstrate whether responses to P2Y(1) and P2Y(12) purinergic receptors resensitize in human platelets and determine the role of receptor traffic in this process.

METHODS

These studies were undertaken either in human platelets or in cells stably expressing epitope-tagged P2Y(1) and P2Y(12) purinergic receptor constructs.

RESULTS

In this study we show for the first time that responses to both of these receptors can rapidly resensitize following agonist-dependent desensitization in human platelets. Further, we show that in human platelets or in 1321N1 cells stably expressing receptor constructs, the disruption of receptor internalization, dephosphorylation or subsequent receptor recycling is sufficient to block resensitization of purinergic receptor responses. We also show that, in platelets, internalization of both these receptors is dependent upon dynamin, and that this process is required for resensitization of responses.

CONCLUSIONS

This study is therefore the first to show that both P2Y(1) and P2Y(12) receptor activities are rapidly and reversibly modulated in human platelets, and it reveals that the underlying mechanism requires receptor trafficking as an essential part of this process.

Effects of a high-glucose environment on the pituitary growth hormone-releasing hormone receptor: type 1 diabetes compared with in vitro glucotoxicity

The present study investigated the effects of diabetes and high glucose on GHRH receptor (GHRH-R) mRNA and protein levels in the pituitary of diabetic rats 2, 21, and 60 days post-streptozotocin (post-STZ) administration. Two days post-STZ, the 2.5-kb GHRH-R mRNA transcript was increased. Twenty-one days post-STZ, both the 2.5- and 4-kb transcripts and a 72-kDa (125)I-GHRH-GHRH-R complex were elevated. Sixty days post-STZ, the 4-kb transcript remained increased and the 45-kDa (125)I-GHRH-GHRH-R complex (functional receptor) was decreased. Hypothalamic GHRH mRNA and serum total IGF-I levels were reduced at all three time points. To better understand the role of high glucose on GHRH-R regulation, time-course effects of 33 compared with 6 mM d-glucose (DG) were examined in cultured anterior pituitary cells from 2-mo-old healthy rats. Membrane lipoperoxidation was present in 33 mM DG, and GHRH-R mRNA levels were diminished after 24 h, Fluo-GHRH internalization was marginal after 16-24 h, and GHRH-induced cAMP levels were decreased after 24 and 48 h. Altogether, these results indicate that the increase of the 2.5-kb GHRH-R mRNA transcript in vivo could be a consequence of a decrease of hypothalamic GHRH mRNA levels in STZ rats. Since it does not affect primarily functional GHRH-R levels, the initial diminution of circulating IGF-I levels could result from a decreased GHRH-R stimulation by GHRH. Thus, the effect of glucotoxicity would be related to a decrease of functional GHRH-R protein, as observed in rats 60 days post-STZ and in cultured pituitary cells from healthy rats exposed to a high-glucose environment.

Ganglioside partitioning and aggregation in phase-separated monolayers characterized by bodipy GM1 monomer/dimer emission

The distribution of Bodipy GM1 in monolayers of binary and ternary lipid mixtures with coexisting fluid and ordered phases has been examined using a combination of atomic force microscopy and near-field scanning optical microscopy. Monolayers deposited at high (30 mN/m) and low (5 or 10 mN/m) surface pressures were examined and compared to those containing the same concentration of unlabeled ganglioside. Measurements of monomer and dimer Bodipy emission were used to distinguish aggregated from dilute ganglioside levels. For binary DPPC/DOPC monolayers, Bodipy GM1 is distributed throughout both the fluid and ordered phases at low surface pressures, and both labeled and unlabeled gangliosides result in a reduction in the size of ordered DPPC domains at 0.4% and the appearance of small aligned ganglioside-rich domains at 4%. In agreement with earlier studies, GM1 is heterogeneously distributed in small islands in the condensed DPPC domains at high surface pressure. By contrast, Bodipy GM1 causes the disappearance of large DPPC domains at 0.4% and the formation of a new GM1-rich phase at 4%. The addition of both gangliosides leads to a comparable loss of large ordered domains at low surface pressure and the appearance of a new GM1-rich phase at 30 mN/m for ternary lipid mixtures containing cholesterol. The results demonstrate the complexity of GM1 partitioning and illustrate the utility of complementary AFM and high spatial resolution two-color fluorescence experiments for understanding Bodipy GM1 aggregation and distribution.

The inhibitors of protein acylation, cerulenin and tunicamycin, increase voltage-dependent Ca(2+) currents in the insulin-secreting INS 832/13 cell

As it has been suggested that protein acylation plays a role in nutrient stimulus-secretion coupling in the pancreatic beta-cell, we examined the insulin-secreting INS 832/13 beta-cell line for evidence that protein acylation was involved. The perforated whole-cell configuration was employed to voltage-clamp INS 832/13 cells. Voltage pulses were applied and Ca(2+) currents measured in the presence and absence of the protein acylation inhibitors cerulenin and tunicamycin. Both inhibitors enhanced the peak amplitude of I(Ca,L). Both increased the peak inward current in the range between -40 and +30mV and shifted the apparent maximum current by 10mV in the hyperpolarizing direction without affecting the activation threshold of -40mV. The two drugs had qualitatively and quantitatively similar effects. Steady-state activation curves revealed that cerulenin and tunicamycin shifted the activation curves in the hyperpolarization direction. Activation time constants were significantly reduced in the presence of both drugs. The Ca(2+) charge influx was increased by the drugs at all potentials tested. In contrast to these effects on the L-type Ca(2+) channel, the two inhibitors of protein acylation had no effect on the ATP-sensitive K(+) channel. The results suggest that protein acylation exerts a tonic inhibitory effect on L-type Ca(2+) channel function in the insulin-secreting beta-cell.

Inhibition of insulin secretion by cerulenin might be due to impaired glucose metabolism

BACKGROUND

Cerulenin, an inhibitor of protein acylation, has been used as a tool to study the potential role of protein acylation in a variety of activities in different cells, and in stimulus-secretion coupling in pancreatic islets and clonal beta-cells.

METHODS

In the present study we investigated its effects on stimulated insulin secretion, glucose metabolism and utilization, oxygen consumption and ATP levels.

RESULTS

In isolated rat pancreatic islets, cerulenin pre-treatment (100 microM) inhibited insulin secretion in response to glucose, and to the non-hydrolysable analogue of leucine, aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH). These data are in accord with the hypothesis that protein acylation could be involved in the stimulation of insulin secretion. However, we also found that cerulenin profoundly decreased glucose oxidation, glucose utilization, oxygen consumption and ATP levels. Consequently, decreased metabolism provides an alternative mechanism to inhibition of protein acylation that could explain the inhibition of insulin secretion by cerulenin.

CONCLUSIONS

Inhibition of insulin secretion by cerulenin can no longer be taken as evidence in favour of a role for protein acylation in the control of insulin release. As protein acylation is known to be involved in the normal functioning of proteins in stimulus-secretion coupling and exocytosis, more direct approaches to understand its role(s) are required.

AT1 receptor-mediated accumulation of extracellular angiotensin II in proximal tubule cells: role of cytoskeleton microtubules and tyrosine phosphatases

Long-term angiotensin II (ANG II) administration is associated with increased ANG II accumulation in the kidney, but intrarenal compartment(s) involved in this response remains to be determined. We tested the hypothesis that 1) extracellular ANG II is taken up by proximal tubule cells (PTCs) through AT(1) receptor-mediated endocytosis, 2) this process is regulated by cytoskeleton microtubule- and tyrosine phosphatase-dependent mechanisms, and 3) AT(1) receptor-mediated endocytosis of ANG II has a functional relevance by modulating intracellular cAMP signaling. In cultured PTCs, [(125)I]Tyr-labeled ANG II and fluorescein labeled-ANG II were internalized in a time-dependent manner and colocalized with the endosome marker Alexa Fluor 594-transferrin. Endocytosis of extracellular ANG II was inhibited by the AT(1) receptor blocker losartan (16.5 +/- 4.6%, P < 0.01 vs. ANG II, 78.3 +/- 6.2%) and by the tyrosine phosphatase inhibitor phenylarsine oxide (PAO; 30.0 +/- 3.5%, P < 0.05 vs. ANG II). Intracellular ANG II levels were increased by approximately 58% (basal, 229.8 +/- 11.4 vs. ANG II, 361.3 +/- 11.8 pg ANG II/mg protein, P < 0.01), and the responses were blocked by losartan (P < 0.01), the cytoskeleton microtubule inhibitor colchicine (P < 0.05), and PAO (P < 0.01), whereas depletion of clathrin-coated pits with hyperosmotic sucrose had no effect (356.1 +/- 25.5 pg ANG II/mg protein, not significant). ANG II accumulation was associated with significant inhibition of both basal (control, 15.5 +/- 2.8 vs. ANG II, 9.1 +/- 2.4 pmol/mg protein, P < 0.05) and forskolin-stimulated cAMP signaling (forskolin, 68.7 +/- 8.6 vs. forskolin + ANG II, 42.8 +/- 13.8 pmol/mg protein, P < 0.01). These effects were blocked by losartan and PAO. We conclude that extracellular ANG II is internalized in PTCs through AT(1) receptor-mediated endocytosis and that internalized ANG II may play a functional role in proximal tubule cells by inhibiting intracellular cAMP signaling.

BODIPY-conjugated neuropeptide Y ligands: new fluorescent tools to tag Y1, Y2, Y4 and Y5 receptor subtypes

N-terminal labelled fluorescent BODIPY-NPY peptide analogues were tested in Y1, Y2, Y4 and Y5 receptor-binding assays performed in rat brain membrane preparations and HEK293 cells expressing the rat Y1, Y2, Y4 and Y5 receptors. BODIPY TMR/FL-[Leu31, Pro34]NPY/PYY were able to compete for specific [125][Leu31, Pro34]PYY-binding sites with an affinity similar to that observed for the native peptide at the Y1 (Ki=1-6 nM), Y2 (Ki>1000 nM), Y4 (Ki=10 nM) and Y5 (Ki=1-4 nM) receptor subtypes. BODIPY FL-PYY(3-36) was able to compete for specific Y2 (Ki=10 nM) and Y5 (Ki=30 nM) binding sites, but had almost no affinity in Y1 and Y4 assays. BODIPY FL-hPP was able to compete with high affinity (Ki; 1 and 15 nM) only in Y4 and Y5 receptor-binding assays. BODIPY TMR-[cPP(1-7), NPY(19-23), Ala31, Aib32, Gln34]hPP and BODIPY TMR-[hPP(1-17), Ala31, Aib32]NPY were potent competitors only on specific Y5-binding sites (Ki=0.1-0.6 nM). As expected, these fluorescent peptides inhibited forskolin-induced cAMP accumulation, demonstrating that they retained their agonist properties. When tested in confocal microscopy imaging, fluorescent Y1 and Y5 agonists internalized in a time-dependent manner in Y1 and Y5 transfected cells, respectively. These results demonstrate that BODIPY-conjugated NPY analogues retain their selectivity, affinity and agonist properties for the Y1, Y2, Y4 and Y5 receptor subtypes, respectively. Thus, they represent novel tools to study and visualize NPY receptors in living cells.

Regulation of the pituitary growth hormone-releasing hormone receptor in ageing male and female LOU rats: new insights into healthy ageing

Ageing is characterised by a decrease of somatotroph functionality, involving growth hormone-releasing hormone receptor (GHRH-R). The present study was conducted in LOU/C/jall (LOU) rats, a strain described as a model of healthy ageing, which is characterised by a low adiposity and long life expectancy without developing severe pathologies. Effects of age and diet (chow versus self-selection), on levels of anterior pituitary GHRH-R mRNA transcripts, were assessed in male and female LOU rats. The effect of age on pituitary GHRH-R functionality was examined in the anterior pituitary of both males and females fed chow diet. Moreover, serum insulin-like growth factor-I (IGF-I), T4 and leptin were measured because changes in their concentration could affect GHRH-R expression. In the pituitary of 18-month-old male and female LOU/C/jall rats fed standard chow, the level of 2.5-kb GHRH-R mRNA transcript, coding for functional GHRH-R, was significantly decreased. In 24- to 34-month-old males and females, it progressively returned to the level of younger animals, suggesting an enrichment of the group with survivors maintaining functional GHRH-R. In males and females repeatedly submitted to self-selection, this phenomenon was not observed. Studies with the GHRH-R agonist, Fluo-GHRH, revealed that 73% of 16-18-month-old male and female rats studied did not show an increase of fluorescence density characteristic of receptor-mediated internalisation upon incubation at 37 degrees C. In the other 27%, the increase of fluorescence was identical to that observed in pituitaries of young rats, suggesting the presence of an optimal level of functional GHRH-R. Serum levels of leptin, free T4 and total IGF-I decreased more drastically in ageing males and in rats fed a self-selection diet. A positive correlation was demonstrated between leptin and IGF-I levels in ageing males and females fed standard chow and ageing females submitted to a self-selection regimen. In conclusion, healthy ageing in LOU rats fed chow diet appears to be associated with a maintenance of functional pituitary GHRH-R levels found in younger rats but not necessarily with those of serum leptin, T4 and IGF-I.