Inhibition of somatostatin receptor 5-signaling by mammalian regulators of G-protein signaling (RGS) in yeast

The Role of Receptor-Ligand Interaction in Somatostatin Signaling Pathways: Implications for Neuroendocrine Tumors

Neuroendocrine tumors (NETs) arise from neuroendocrine cells and manifest in diverse organs. Key players in their regulation are somatostatin and its receptors (SSTR1-SSTR5). Understanding receptor-ligand interactions and signaling pathways is vital for elucidating their role in tumor development and therapeutic potential. This review highlights SSTR characteristics, localization, and expression in tissues, impacting physiological functions. Mechanisms of somatostatin and synthetic analogue binding to SSTRs, their selectivity, and their affinity were analyzed. Upon activation, somatostatin initiates intricate intracellular signaling, involving cAMP, PLC, and MAP kinases and influencing growth, differentiation, survival, and hormone secretion in NETs. This review explores SSTR expression in different tumor types, examining receptor activation effects on cancer cells. SSTRs' significance as therapeutic targets is discussed. Additionally, somatostatin and analogues' role in hormone secretion regulation, tumor growth, and survival is emphasized, presenting relevant therapeutic examples. In conclusion, this review advances the knowledge of receptor-ligand interactions and signaling pathways in somatostatin receptors, with potential for improved neuroendocrine tumor treatments.

Untangling the Roles of Anti-Apoptosis in Regulating Programmed Cell Death using Humanized Yeast Cells

Genetically programmed cell death (PCD) mechanisms, including apoptosis, are important for the survival of metazoans since it allows, among things, the removal of damaged cells that interfere with normal function. Cell death due to PCD is observed in normal processes such as aging and in a number of pathophysiologies including hypoxia (common causes of heart attacks and strokes) and subsequent tissue reperfusion. Conversely, the loss of normal apoptotic responses is associated with the development of tumors. So far, limited success in preventing unwanted PCD has been reported with current therapeutic approaches despite the fact that inhibitors of key apoptotic inducers such as caspases have been developed. Alternative approaches have focused on mimicking anti-apoptotic processes observed in cells displaying increased resistance to apoptotic stimuli. Hormesis and pre-conditioning are commonly observed cellular strategies where sub-lethal levels of pro-apoptotic stimuli lead to increased resistance to higher or lethal levels of stress. Increased expression of anti-apoptotic sequences is a common mechanism mediating these protective effects. The relevance of the latter observation is exemplified by the observation that transgenic mice overexpressing anti-apoptotic genes show significant reductions in tissue damage following ischemia. Thus strategies aimed at increasing the levels of anti-apoptotic proteins, using gene therapy or cell penetrating recombinant proteins are being evaluated as novel therapeutics to decrease cell death following acute periods of cell death inducing stress. In spite of its functional and therapeutic importance, more is known regarding the processes involved in apoptosis than anti-apoptosis. The genetically tractable yeast Saccharomyces cerevisiae has emerged as an exceptional model to study multiple aspects of PCD including the mitochondrial mediated apoptosis observed in metazoans. To increase our knowledge of the process of anti-apoptosis, we screened a human heart cDNA expression library in yeast cells undergoing PCD due to the conditional expression of a mammalian pro-apoptotic Bax cDNA. Analysis of the multiple Bax suppressors identified revealed several previously known as well as a large number of clones representing potential novel anti-apoptotic sequences. The focus of this review is to report on recent achievements in the use of humanized yeast in genetic screens to identify novel stress-induced PCD suppressors, supporting the use of yeast as a unicellular model organism to elucidate anti-apoptotic and cell survival mechanisms.

Expression, purification and characterization of yellow grouper Epinephelus awoara regulator of G protein signaling 16 protein

Regulators of G-protein signaling (RGS) proteins are a family of proteins, which accelerate GTPase-activity intrinsic to the alpha subunits of heterotrimeric G-proteins and play crucial roles in the physiological control of G-protein signaling. Here, yellow grouper RGS16 protein was expressed in Escherichia coli and purified by Ni-NTA affinity chromatography. The expression level of the fusion protein was up to 30% of the total cellular protein.Western blotting analysis showed that a band with the molecular mass of about 21 Kda was detected. The purified recombinant protein was used to prepare polyclonal antibody, and antiserum obtained was highly specific with the titer of over 1:32,000. Additionally, RGS16 protein was expressed in the Tn-5B1-4 insect cells. Western blotting analysis revealed that the expressed protein had immunoreactivity.

Somatostatin receptor biology in neuroendocrine and pituitary tumours: part 1--molecular pathways

Neuroendocrine tumours (NETs) may occur at many sites in the body although the majority occur within the gastroenteropancreatic axis. Non-gastroenteropancreatic NETs encompass phaeochromocytomas and paragangliomas, medullary thyroid carcinoma, anterior pituitary tumour, broncho-pulmonary NETs and parathyroid tumours. Like most endocrine tumours, NETs also express somatostatin (SST) receptors (subtypes 1–5) whose ligand SST is known to inhibit endocrine and exocrine secretions and have anti-tumour effects. In the light of this knowledge, the idea of using SST analogues in the treatment of NETs has become increasingly popular and new studies have centred upon the development of new SST analogues. We attempt to review SST receptor (SSTR) biology primarily in neuroendocrine tissues, focusing on pituitary tumours. A full data search was performed through PubMed over the years 2000–2009 with keywords ‘somatostatin, molecular biology, somatostatin receptors, somatostatin signalling, NET, pituitary’ and all relevant publications have been included, together with selected publications prior to that date. SSTR signalling in non-neuroendocrine solid tumours is beyond the scope of this review. SST is a potent anti-proliferative and anti-secretory agent for some NETs. The successful therapeutic use of SST analogues in the treatment of these tumours depends on a thorough understanding of the diverse effects of SSTR subtypes in different tissues and cell types. Further studies will focus on critical points of SSTR biology such as homo- and heterodimerization of SSTRs and the differences between post-receptor signalling pathways of SSTR subtypes.

Modulation of the neuronal response to ischaemia by somatostatin analogues in wild-type and knock-out mouse retinas

Somatostatin acts at five G protein-coupled receptors, sst(1)-sst(5). In mouse ischaemic retinas, the over-expression of sst(2) (as in sst(1) knock-out mice) results in the reduction of cell death and glutamate release. In this study, we reported that, in wild-type retinas, somatostatin, the multireceptor ligand pasireotide and the sst(2) agonist octreotide decreased ischaemia-induced cell death and that octreotide also decreased glutamate release. In contrast, cell death was increased by blocking sst(2) with cyanamide. In sst(2) over-expressing ischaemic retinas, somatostatin analogues increased cell death, and octreotide also increased glutamate release. To explain this reversal of the anti-ischaemic effect of somatostatin agonists in the presence of sst(2) over-expression, we tested sst(2) desensitisation because of internalisation or altered receptor function. We observed that (i) sst(2) was not internalised, (ii) among G protein-coupled receptor kinases (GRKs) and regulators of G protein signalling (RGSs), GRK1 and RGS1 expression increased following ischaemia, (iii) both GRK1 and RGS1 were down-regulated by octreotide in wild-type ischaemic retinas, (iv) octreotide down-regulated GRK1 but not RGS1 in sst(2) over-expressing ischaemic retinas. These results demonstrate that sst(2) activation protects against retinal ischaemia. However, in the presence of sst(2) over-expression sst(2) is functionally desensitised by agonists, possibly because of sustained RGS1 levels.

Nck-1 selectively modulates eIF2alphaSer51 phosphorylation by a subset of eIF2alpha-kinases

Phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2 (eIF2) on Ser51 is an early event associated with the down-regulation of protein synthesis at the level of translation and initiation of a transcriptional program. This constitutes a potent mechanism to overcome various stress conditions. In mammals, four eIF2alpha-kinases [PKR-like endoplasmic reticulum kinase (PERK), dsRNA-activated protein kinase (PKR), heme regulated inhibitor (HRI) and general control nonderepressible-2 (GCN2)], activated following specific stresses, have been shown to be involved in this process. In this article, we report that the ubiquitously expressed adaptor protein Nck, composed only of Src homology domains and classically implicated in cell signaling by activated plasma membrane receptor tyrosine kinases, modulates eIF2alpha-kinase-mediated eIF2alphaSer51 phosphorylation in a specific manner. Our results show that Nck not only prevents eIF2alpha phosphorylation upon PERK activation, as reported previously, but also reduces eIF2alpha phosphorylation in conditions leading to PKR and HRI activation. By contrast, the overexpression of Nck in mammalian cells fails to attenuate eIF2alphaSer51 phosphorylation in response to amino acid starvation, a stress well known to activate GCN2. This observation is further confirmed by showing that Nck fails to alter eIF2alphaSer51 phosphorylation in Saccharomyces cerevisiae, for which the sole eIF2alpha-kinase is Gcn2p. Our results suggest the existence of a novel mechanism that specifically modulates the phosphorylation of eIF2alpha on Ser51 under various stress conditions.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Characterization of a novel alternatively spliced human transcript encoding an N-terminally truncated Vps24 protein that suppresses the effects of Bax in an ESCRT independent manner in yeast

Elucidating novel anti-apoptotic regulatory pathways is central to further understanding the molecular basis of several pathologies, including cancer. We have previously reported the identification of several mammalian cDNAs effective in preventing the lethal effects of heterologous expression of a pro-apoptotic BAX cDNA in yeast [Yang, Z., Khoury, C., Jean-Baptiste, G., Greenwood, M.T., 2006. Identification of mouse sphingomyelin synthase 1 (SMS1) as a suppressor of Bax mediated cell death in yeast. FEMS Yeast Res. 6, 751-762]. Here we report that one of the Bax suppressors encodes a novel 156 amino acid variant of the human Vps24 protein, Vps24beta, that lacks the N-terminal lipid binding domain of the well characterized 222 residue Vps24 (Vps24alpha). We demonstrate that the VPS24beta cDNA represents an expressed transcript that is likely produced by alternative splicing of the human VPS24 gene. Vps24alpha, but not Vps24beta, prevented the temperature and salt sensitive growth defects observed in a yeast mutant lacking a functional VPS24 gene. In contrast, Vps24beta, but not Vps24alpha, suppressed the inhibitory effects of Bax on yeast growth. Vps24beta protein also suppressed the effects of Bax in mutants lacking other VPS genes suggesting that a functional ESCRT pathway, of which the yeast Vps24p is an essential component, is not required for Vps24beta function. Taken together, we demonstrate that the human VPS24 gene gives rise to two functionally distinct proteins, one of which is involved in the ESCRT pathway and another novel protein that serves an anti-apoptotic role.

Investigating RGS proteins in yeast

Regulator of G protein signalling (RGS) proteins are vital in the adaptation of cells to stimulation via G protein-coupled receptors. Yeast have been integral in elucidating the important role that RGS proteins play within cellular processes. In addition to extensive characterisation of the endogenous RGS proteins, these organisms have enabled the identification and analysis of numerous mammalian homologues. The simplicity and plasticity of the yeast pheromone-response pathway has facilitated studies which would have been impossible in mammalian systems and it is certain that yeast will continue to have a great impact on this field of research in the future.

The mouse sphingomyelin synthase 1 (SMS1) gene is alternatively spliced to yield multiple transcripts and proteins

Sphingomyelin synthase 1 (SMS1) is a recently identified 413-residue protein that plays a critical role in sphingolipid metabolism by catalyzing the conversion of ceramide and phosphatidylcholine to sphingomyelin and diacylglycerol (DAG). We have previously reported the isolation of a mouse SMS1 encoding cDNA that contains a unique 5' UTR sequence. Three other mouse SMS1 cDNAs that differed in their 5' and 3' non-coding sequences were present in GenBank. In order to ascertain the origin of the unique 5' and 3' UTR sequences, we analyzed the structure of the mouse SMS1 gene. Analysis of the four different SMS1 cDNA sequences and of the corresponding mouse genomic fragment revealed that the SMS1 gene consists of 16 exons that are alternatively spliced to produce 4 different mRNAs (SMS1alpha1, SMS1alpha2, SMS1beta and SMS1gamma) and 3 different proteins (SMS1alpha, SMS1beta and SMS1gamma). RT-PCR was used to demonstrate that all four SMS1 cDNAs represent expressed transcripts that show distinctly different tissue distributions. Transcripts for SMS1alpha1, SMS1alpha2 and SMS1beta were found to increase in response to the pro-apoptotic effects of TNF-alpha. Finally, using a yeast-based assay, we confirmed that SMS1alpha prevents the growth inhibitory effects of Bax but SMS1beta neither prevents nor enhances the effects of Bax or of SMS1alpha. Taken together these results demonstrate the complexity of SMS1 gene structure, expression and function.

Functional analysis of heterologous GPCR signalling pathways in yeast

G protein-coupled receptors (GPCRs) regulate diverse biological processes in eukaryotes and such conservation allows an almost unrestricted interchange of signalling components between different cell types. Yeasts are attractive hosts in which to study GPCRs--they are amenable to both genetic and biochemical manipulation and their robustness, low cost and our ability to create strains that lack endogenous GPCRs make them ideal starting points for the development of assays suitable for high-throughput screening. Here we introduce readers to the possibilities of using yeast to analyse GPCRs describing the endogenous signalling pathways, the development of assays for heterologous GPCRs and the technology to elucidate GPCR structure and activity, focusing on the budding yeast Saccharomyces cerevisiae and recent developments using the fission yeast Schizosaccharomyces pombe.

Ocular albinism 1 protein: trafficking and function when expressed in Saccharomyces cerevisiae

The ocular albinism 1 (Oa1) protein is believed to be involved in the biogenesis of melanosomes, but its cellular localization is controversial and its function is unknown. Based upon sequence homology, it has been predicted that Oa1 belongs to the G protein coupled receptor (GPCR) superfamily. We used the yeast Saccharomyces cerevisiae as a genetically amenable system to study the localization and function of Oa1. Sucrose gradient and immunofluorescence studies revealed that when expressed in yeast, Oa1 localizes to the prevacuolar compartment, the functional equivalent of the mammalian late endosome. Oa1 behaved as G protein coupled receptor in a yeast-based GPCR signalling assay. Extracts of cultured melanocytes, and, in particular, a particulate fraction from cultured melanocytes, stimulated Oa1-mediated GPCR signalling.

Peptide and non-peptide G-protein coupled receptors (GPCRs) in skeletal muscle

G-protein coupled receptors (GPCRs) represent a large class of cell surface receptors that mediate a multitude of functions. Over the years, a number of GPCRs and ancillary proteins have been shown to be expressed in skeletal muscle. Unlike the case with other muscle tissues like cardiac and vascular smooth muscle cells, there has been little attempt at systematically analyzing GPCRs in skeletal muscle. Here we have compiled all the GPCRs that are expressed in skeletal muscle. In addition, we review the known function of these receptors in both skeletal muscle tissue and in cultured skeletal muscle cells.

Beta adrenergic receptor-mediated atrial specific up-regulation of RGS5

Previous investigations had suggested that signaling from the overexpressed beta(2) adrenergic in the heart of transgenic TG4 mice was dampened in the atria. Using an RT-PCR based strategy, we have identified Regulator of G-protein Signaling 5 (RGS5) as being up-regulated in the atria of TG4 mice. Northern blot analysis demonstrated that RGS5 levels were 3 fold higher in the atria of TG4 mice. Western blot analysis of a panel of rat tissues demonstrated that basal expression of RGS5 protein was confined to the heart and skeletal muscle. Furthermore, RGS5 protein was detected in skeletal muscle C2C12 and cardiomyocyte HL-1 cultured cell lines. As observed for RGS5 mRNA levels in TG4 mice, RGS5 protein levels were increased in the atria of rats that were administered the beta adrenergic agonist isoproterenol during a 14 day period. Taken together, these results indicate that RGS5 is a housekeeping RGS in the heart and in skeletal muscle while its beta adrenergic-mediated induction in the atrium suggests that it also has a highly specialized function.

Knock-down of RGS4 and beta tubulin in CHO cells expressing the human MT1 melatonin receptor prevents melatonin-induced receptor desensitization

Previously, it has been shown that chronic melatonin exposure in MT1-CHO cells results in receptor desensitization while at the same time producing drastic morphological changes. The addition of a depolymerizing agent during the melatonin pretreatment period prevents MT1 receptor desensitization and the changes in cellular morphology. The lack of morphological change in the presence of a depolymerizing agent is easily explained by the inability of the microtubules to polymerize, however, the prevention of receptor desensitization is a little more complex and may involve G-protein activation. The goal of this study was to determine whether melatonin-induced MT1 receptor desensitization is regulated by proteins known to regulate G-protein activation states, beta-tubulin and RGS4,using anti sense knockdown approaches. The expression of RGS4 mRNA in CHO cells was confirmed using RT PCR and successful knockdown of each was confirmed by western blot analysis or quantitative PCR. Pretreatment of MT1-CHO cells, transfected with the nonsense probes and exposed to melatonin, resulted in a desensitization of the receptor, an increase in forskolin-induced cAMP accumulation, an increase in 2-[125I]-iodomelatonin binding and no change in the affinity of melatonin for the MT1 receptor. However, knockdown of either beta-tubulin or RGS4 in MT1-CHO cells followed by pretreatment with melatonin attenuated the desensitization of melatonin receptors, decreased total 2-[125I]-iodomelatonin binding, and did not affect neither the forskolin response nor the affinity of melatonin for the MT1 receptor. Perhaps RGS4 and beta-tubulin modulate Galpha-GDP and Galpha-GTP states thus modulating MT1 melatonin receptor function.

Gα protein dependent and independent effects of human RGS1 expression in yeast

Regulators of G-protein Signalling (RGS) regulate the functional lifetime of G-Protein Coupled Receptor (GPCR)-activated heterotrimeric G-protein by serving as GTPase Accelerating Proteins (GAPs) for the G(alpha) subunit. A number of mammalian RGSs can functionally replace the yeast RGS containing SST2 gene and inhibit GPCR signalling. Using yeast strains harbouring a G(betagamma)-responsive FUS1-LacZ reporter gene, we demonstrate that heterologously expressed mammalian RGS1 also serves to decrease basal signalling in the absence of agonist. Although this effect was dependent on the expression of a GPA1-encoded functional G(alpha) protein, the GPCR itself was nevertheless not required. Using the GAL1 inducible promoter to express RGS1, we further demonstrate that in addition to serving as a GAP for Gpa1p in yeast, RGS1 is a dosage-dependent inhibitor of growth. This effect is specific to RGS1 since growth is not altered in cells expressing either mammalian RGS2 or RGS5. We further demonstrate that neither of the two yeast G(alpha) proteins is responsible for mediating this growth inhibitory effect of RGS1. Taken together, our results indicate that RGS1 can function in both G-protein-dependent and -independent manners in yeast.

The N-terminal non-RGS domain of human regulator of G-protein signalling 1 contributes to its ability to inhibit pheromone receptor signalling in yeast

Regulators of G-protein signalling (RGS) are a family of proteins that interact with G-proteins to regulate negatively G-protein coupled receptor (GPCR) signalling. In addition to a conserved core domain that is necessary and sufficient for their GTPase activating protein (GAP) like activity, RGSs possess N- and C-terminal motifs that confer distinct functional differences. In order to identify the role of the non-RGS region of human RGS1, we have characterized a series of fusions between RGS1 and GFP in a yeast mutant lacking the RGS containing SST2 gene. Using both halo assays as well as a GPCR responsive FUS1-LacZ reporter gene, we demonstrate that a RGS1-GFP fusion inhibits GPCR signalling in yeast while GFP fusions containing either the N-terminus non RGS sequence of RGS1(1-68) or the sequence containing the RGS box of RGS1(68-197) produce proteins that retain RGS1 activity. These results suggest that both the N-terminal and the RGS box of RGS1 function to inhibit signalling. Analysis of a series of mutants spanning the entire N-terminal non-RGS region of RGS1 produced by conservative segment exchange (CSE) mutagenesis showed little loss of function in yeast. This suggests that the overall structure of the N-terminal region of RGS1 rather than specific motifs or residues is required for its function.