Alternative promoter usage and tissue specific expression of the mouse somatostatin receptor 2 gene

The influence of elevated hormone levels on physiologic accumulation of 68Ga-DOTATOC

OBJECTIVE

PET/CT imaging with ⁶⁸Ga-1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid-D-Phe¹-Tyr³-octreotide (DOTATOC) is useful in patients with neuroendocrine tumors (NETs). Functioning NETs by definition secrete abnormal levels of hormones, causing clinical symptoms. It is known that physiologic accumulation can be seen in some organs, but it remains unknown whether elevated hormone levels can affect the physiologic accumulation pattern of ⁶⁸Ga-DOTATOC. We aimed to investigate the influence of higher hormone levels on physiologic accumulation of ⁶⁸Ga-DOTATOC.

METHODS

A total of 167 patients with known or suspected NET lesions were enrolled in this study. The numbers of patients with elevations of ACTH, gastrin, insulin, and no elevation were 10, 25, 7, and 125, respectively. We compared the maximum standardized uptake value (SUVmax) in various organs of each group.

RESULTS

In the group with elevated ACTH levels, SUVmax in the pituitary gland, the uncinate process of the pancreas and adrenal glands was lower than those in the group with no elevation (5.7 ± 1.9 vs. 8.4 ± 3.1, P = 0.015; 4.7 ± 3.5 vs. 6.4 ± 2.8, P = 0.037; 10.8 ± 4.8 vs. 13.9 ± 4.7, P = 0.020, respectively). There were no differences in physiologic uptake of ⁶⁸Ga-DOTATOC in the thyroid gland, the pancreatic body, the liver, the spleen, the bowel, or the kidney.

CONCLUSIONS

In NET patients with elevated ACTH levels, physiologic uptake of ⁶⁸Ga-DOTATOC in the pituitary gland, the uncinate process of the pancreas and adrenal glands was significantly decreased. Other organs were unaffected.

Multiple non-coding exons and alternative splicing in the mouse Mas protooncogene

The Mas protooncogene encodes a G protein-coupled receptor with the common seven transmembrane domains, expressed mainly in the testis and brain. We provided evidence that Mas is a functional angiotensin-(1-7) receptor and can interact with the angiotensin II type 1 (AT1) receptor. The gene is transcriptionally regulated during development in the brain and testis, but its structure was unresolved. In this study we used 5'- and 3'-RACE, RT-PCR, and RNase-protection assays to elucidate the complete Mas gene structure and organization. We identified 12 exons in the mouse Mas gene with 11 in the 5' untranslated mRNA, which can be alternatively spliced. We also showed that Mas transcription can start from 4 tissue-specific promoters, whereby testis-specific Mas mRNA is transcribed from two upstream promoters, and the expression of Mas in the brain starts from two downstream promoters. Alternative splicing and multiple promoter usage result in at least 12 Mas transcripts in which different 5' untranslated regions are fused to a common coding sequence. Moreover, termination of Mas mRNA is regulated by two different polyadenylation signals. The gene spans approximately 27 kb, and the longest detected mRNA contains 2,451 bp. Thus, our results characterize the Mas protooncogene as the gene with the most complex gene structure of all described members of the gene family coding for G protein-coupled receptors.

Twins in spirit - episode I: comparative preclinical evaluation of [(68)Ga]DOTATATE and [(68)Ga]HA-DOTATATE

Background

Recently, an intra-patient comparison demonstrated that the somatostatin (sst) ligand [⁶⁸Ga]HA-DOTATATE ([⁶⁸Ga]DOTA-3-iodo-Tyr³-octreotate) provides PET images comparable to or superior to those obtained with [⁶⁸Ga]DOTATATE. To provide a comprehensive basis for nevertheless observed slight differences in tracer biodistribution and dosimetry, the characteristics of [⁶⁸Ga]HA-DOTATATE were investigated in a detailed preclinical study.

Methods

Affinities of ^natGa-HA-DOTATATE and ^natGa-DOTATATE to sst_1–5 were determined using membrane preparations and [¹²⁵I]SST-28 as radioligand. Internalization into AR42J cells was studied in dual-tracer studies with [¹²⁵I]TOC as internal reference. Biodistribution was investigated using AR42J tumor-bearing CD1 mice, and specificity of tracer uptake was confirmed in competition studies by coinjection of 0.8 mg TOC/kg.

Results

Sst₂ affinities (IC₅₀) of [^natGa]HA-DOTATATE (1.4 ± 0.8 nM, logP: −3.16) and [^natGa]DOTATATE (1.2 ± 0.6 nM, logP: −3.69) were nearly identical. Both compounds displayed IC₅₀ > 1 μM for sst_1,3,4, while sst₅ affinity was markedly increased for ^natGa-HA-DOTATATE (102 ± 65 nM vs >1 μM for ^natGa-DOTATATE). [^natLu]HA-DOTATATE and [^natLu]DOTATATE showed slightly lower, identical sst₂ affinities (2.0 ± 1.6 and 2.0 ± 0.8 nM, respectively) and sst₃ affinities of 93 ± 1 and 162 ± 16 nM. Internalization of [⁶⁸Ga]HA-DOTATATE was tenfold higher than that of [¹²⁵I]TOC but only sixfold higher for [⁶⁸Ga]DOTATATE and [¹⁷⁷Lu]HA-DOTATATE. While [⁶⁸Ga]HA-DOTATATE and [⁶⁸Ga]DOTATATE had shown similar target- and non-target uptake in patients, biodistribution studies in mice at 1 h post injection (n = 5) revealed slightly increased non-specific uptake of [⁶⁸Ga]HA-DOTATATE in the blood, liver, and intestines (0.7 ± 0.3, 1.0 ± 0.2, and 4.0 ± 0.7 %iD/g vs 0.3 ± 0.1, 0.5 ± 0.1, and 2.7 ± 0.8 %iD/g for [⁶⁸Ga]DOTATATE). However, sst-mediated accumulation of [⁶⁸Ga]HA-DOTATATE in the pancreas, adrenals, and tumor was significantly enhanced (36.6 ± 4.3, 10.8 ± 3.2, and 33.6 ± 10.9 %iD/g vs 26.1 ± 5.0, 5.1 ± 1.4, and 24.1 ± 4.9 %iD/g, respectively). Consequently, tumor/background ratios for [⁶⁸Ga]HA-DOTATATE in the AR42J model are comparable or slightly increased compared to [⁶⁸Ga]DOTATATE.

Conclusions

The present preclinical data fully confirm the general biodistribution pattern and excellent in vivo sst-targeting characteristics previously observed for [⁶⁸Ga]HA-DOTATATE in patients. The effect of slightly enhanced lipophilicity on background accumulation and normal organ dose is compensated by the high uptake of [⁶⁸Ga]HA-DOTATATE in tumor. Thus, [⁶⁸Ga]HA-DOTATATE represents a fully adequate, freely available substitute for [⁶⁸Ga]DOTATATE and, given the superb sst-targeting characteristics of [¹⁷⁷Lu]HA-DOTATATE in vitro, potential applicability for sst-targeted PRRT.

Medical suppression of hypercortisolemia in Cushing's syndrome with particular consideration of etomidate

Cushing's syndrome is associated with excessive cortisol secretion by the adrenal gland or ectopic tumours and may result in diabetes, hypertension, and life-threatening infections with high mortality rates especially in the case of surgical resection. Although surgical resection is the treatment of choice, patients may benefit from preceding medical therapy. This may especially be useful as an adjunctive approach in emergency settings, if patients cannot undergo surgery, if surgery or radiotherapy fails, or if the tumour recurs. Medical therapy can be categorized in three different groups-inhibition of steroidogenesis, suppression of adrenocorticotropic hormone, and antagonism of the glucocorticoid receptor. However, the majority of common drugs are not available for parenteral administration, which may evoke a management problem in emergency settings or in patients unable to tolerate oral medication. The carboxylated imidazole etomidate is a well known parenteral induction agent for general anaesthesia. Besides its hypnotic properties, etomidate also has α-adrenergic characteristics and inhibits the enzyme 11-deoxycortisol ß-hydroxylase, which catalyzes the final step of the conversion of cholesterol to cortisol. Adverse outcomes have been reported when used for sedation in septic or trauma patients probably by its interference with steroid homeostasis. However, its capability of inhibition of the 11-deoxycortisol ß-hydroxylase leads to suppression of cortisol secretion which has been demonstrated to be a useful tool in severe and complicated hypercortisolemia. Within this article, we review the data concerning different pharmacological approaches with particular consideration of etomidate in order to suppress steroidogenesis in patients with Cushing's syndrome.

Pituitary-directed medical treatment of Cushing's disease

The treatment of Cushing's disease is very complex and represents a challenge for clinicians. Transphenoidal surgical excision of adrenocorticotropic hormone (ACTH)-secreting pituitary adenoma remains the treatment of choice but, unfortunately, the rate of cure at long-term follow-up is suboptimal and recurrences are high, even in the hands of skilled neurosurgeons. Other treatment options, such as bilateral adrenalectomy and pituitary radiotherapy, are currently in use but no treatment has proven fully satisfactory during the lengthy progress of this chronic and devastating disease. Nelson's syndrome and hypopituitarism are of particular concern as affected patients need lifelong hormone-replacement therapy and have notably increased mortality. Although medical treatment represents a second-line treatment option in patients with Cushing's disease, so far pharmacological therapy has been considered a transient and palliative treatment. Many drugs have been employed: they may act at the hypothalamic-pituitary level, decreasing ACTH secretion; at the adrenal level, inhibiting cortisol synthesis (steroidogenesis inhibitors); or at the peripheral level by competing with cortisol (glucocorticoid receptor antagonists). Recently, there has been renewed interest in the medical therapy of Cushing's disease and pituitary-directed drugs include old compounds commercially available for other diseases, such as cabergoline, and new promising compounds, such as pasireotide (SOM230) or retinoic acid. This review focuses on the tumor-directed pharmacological approaches for the management of Cushing's disease based on the recent identification of possibile targets at a pituitary level.

Identification of an upstream promoter of the human somatostatin receptor, hSSTR2, which is controlled by epigenetic modifications

Somatostatin is a neuropeptide that inhibits exocrine and endocrine secretions of several hormones and negatively regulates cell proliferation. These events are mediated through somatostatin engagement on one of five G protein-coupled receptors named SSTR1 to STTR5. Somatostatin binding to SSTR2 mediates predominantly antisecretory and antiproliferative effects; two important biological activities in the gastroenteropancreatic endocrine and exocrine system. Herein we demonstrate novel regulatory sequences for human (h) SSTR2 transcription. By genomic DNA sequence analysis, we reveal two CpG islands located 3.8 kb upstream from the transcription start site. We identify a novel transcription start site and a promoter region within one of these CpG islands. We demonstrate that two epigenetic modifications, DNA methylation and histone acetylation, regulate the activation of hSSTR2 upstream promoter. Furthermore, we show that the transcription from this upstream promoter region directly correlates to hSSTR2 mRNA expression in various human cell lines. A combined treatment of a demethylating agent, 5-aza-2-deoxycytidine and a histone deacetylase inhibitor, trichostatin A, leads to increased expression of hSSTR2 mRNA in cell lines in which the CpG island is methylated. The epigenetic regulation of this promoter region results in differential expression of hSSTR2 mRNA in human cell lines. This study reveals the existence of a novel upstream promoter for the hSSTR2 gene that is regulated by epigenetic modifications, suggesting for complex control of the hSSTR2 transcription.

Pharmacological management of Cushing's syndrome: an update

The treatment of choice for Cushing's syndrome remains surgical. The role for medical therapy is twofold. Firstly it is used to control hypercortisolaemia prior to surgery to optimize patient's preoperative state and secondly, it is used where surgery has failed and radiotherapy has not taken effect. The main drugs used inhibit steroidogenesis and include metyrapone, ketoconazole, and mitotane. Drugs targeting the hypothalamic-pituitary axis have been investigated but their roles in clinical practice remain limited although PPAR-gamma agonist and somatostatin analogue som-230 (pasireotide) need further investigation. The only drug acting at the periphery targeting the glucocorticoid receptor remains Mifepristone (RU486). The management of Cushing syndrome may well involve combination therapy acting at different pathways of hypercortisolaemia but monitoring of therapy will remain a challenge.

A vitamin A-free diet results in impairment of the rat hippocampal somatostatinergic system

Previous studies have revealed the presence of retinoid specific receptors in the hippocampus and have demonstrated that vitamin A deficiency produces a severe deficit in spatial learning and memory which are linked to a proper hippocampal functioning. It is also well known that the tetradecapeptide somatostatin binds to specific receptors in the hippocampus and, when injected into this brain area, facilitates the acquisition of spatial tasks. In addition, depletion of somatostatin by cysteamine impairs acquisition of these tasks. Taken together, these studies support the idea that the hippocampal somatostatinergic system might be regulated by vitamin A. Hence, we evaluated the effects of vitamin A deprivation and subsequent administration of vitamin A on the rat hippocampal somatostatinergic system. Rats fed a vitamin A-free diet exhibited a significant reduction of somatostatin-like immunoreactivity content in the hippocampus whereas the somatostatin mRNA levels were unaltered. Vitamin A deficiency increased the somatostatin receptor density and its dissociation constant. Functional Gi activity as well as the capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity was decreased in vitamin A deficiency rats as compared with the control animals. All these parameters were fully restored when vitamin A was replaced in the diet. Furthermore, we found that the Gialpha1, Gialpha2 and Gialpha3 protein levels were unaltered in hippocampal membranes from rats fed a vitamin A-free diet whereas subsequent vitamin A administration to these rats caused a significant increase in the levels of Gialpha1 and Gialpha2. Altogether, the present findings suggest that dietary vitamin A levels modulate the somatostatinergic system in the rat hippocampus.

Distinct functional properties of native somatostatin receptor subtype 5 compared with subtype 2 in the regulation of ACTH release by corticotroph tumor cells

In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst5). After 72 h, the multiligand SS analog SOM230, which has a very high sst5 binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst5 in the regulation of ACTH release, we conducted additional studies with mouse AtT-20 cells. SOM230 showed a 7-fold higher ligand binding affinity and a 19-fold higher potency in stimulating guanosine 5'-O-(3-thiotriphosphate) binding in AtT-20 cell membranes compared with OCT. SOM230 potently suppressed CRH-induced ACTH release, which was not affected by 48-h dexamethasone (DEX) pretreatment. However, DEX attenuated the inhibitory effects of OCT on ACTH release, whereas it increased the inhibitory potency of BIM-23268, an sst5-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst5 mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B(max)) by 72% when 125I-Tyr3-labeled OCT was used as radioligand, whereas B(max) declined only by 17% when AtT-20 cells were treated with [125I-Tyr11]SS-14. These data suggest that the sst5 protein, compared with sst2, is more resistant to glucocorticoids. Finally, after SS analog preincubation, compared with OCT both SOM230 and BIM-23268 showed a significantly higher inhibitory effect on CRH-induced ACTH release. In conclusion, our data support the concept that the sst5 receptor might be a target for new therapeutic agents to treat Cushing's disease.

First (18)F-labeled tracer suitable for routine clinical imaging of sst receptor-expressing tumors using positron emission tomography

PURPOSE

Despite excellent radionuclide characteristics, no (18)F-labeled peptides are available for quantitative peptide receptor mapping using positron emission tomography (PET) so far, mainly due to time-consuming multistep radiosyntheses with limited overall yields. A newly developed two-step chemoselective conjugation method allows rapid and high-yield [(18)F]fluorination of peptides via oxime formation and was applied for the synthesis of new (18)F-labeled carbohydrated Tyr(3)-octreotate (TOCA) analogs with optimized pharmacokinetics suitable for clinical routine somatostatin-receptor (sst) imaging.

EXPERIMENTAL DESIGN

(18)F-labeled glucose (Gluc-S-) and cellobiose (Cel-S-) derivatives of aminooxy-functionalized TOCA were synthesized via oxime formation with 4-[(18)F]fluorobenzaldehyde ([(18)F]FBOA-peptides). Both the in vitro internalization profile of Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)TOCA in hsst(2)-expressing Chinese hamster ovary cells (dual tracer protocol) and their biodistribution in AR42J tumor-bearing mice were investigated and compared with two [(18)F]fluoropropionylated ([(18)F]FP) analogs, Gluc-Lys([(18)F]FP)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA.

RESULTS

In contrast to [(18)F]FP-labeling (3 h), chemo-selective [(18)F]FBOA-formation (50 min) afforded the respective radiopeptides in high yields (65-85%). In vitro, Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)-TOCA showed high internalization (139 +/- 2 and 163 +/- 8 of the reference [(125)I]Tyr(3)-octreotide, respectively), which was reflected by high tumor accumulation in vivo [21.8 +/- 1.4 and 24.0 +/- 2.5% of injected dose/g (1 h), respectively]. How-ever, only Cel-S-Dpr([(18)F]FBOA)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA (tumor: 15.1 +/- 1.5% of injected dose/g) with its very low accumulation in all of the nontarget organs showed improved tumor:organ ratios compared with Gluc-Lys([(18)F]FP)TOCA. For Cel-S-Dpr([(18)F]FBOA)TOCA,tumor:organ ratios (1 h) were 42:1, 27:1, 15:1, 3:1, and 208:1 for blood, liver, intestine, kidney, and muscle, respectively.

CONCLUSION

Due to the fast and high-yield chemoselective radiofluorination strategy and to its excellent pharmacokinetics, Cel-S-Dpr([(18)F]FBOA)TOCA represents the first tracer suitable for routine clinical application in PET somatostatin receptor imaging.

The role of somatostatin analogs in Cushing's disease

Somatostatin (SRIF) has been proposed to be of therapeutic interest in the medical treatment of Cushing's disease. While in vitro data demonstrate the presence of SRIF-receptor subtype (sst) expression in corticotroph adenomas, the current clinically available SRIF-analog Octreotide, predominantly targeting sst(2), is ineffective in lowering ACTH levels in Cushing's disease and only appears to inhibit the release of ACTH in Nelson's syndrome. In the present review, current knowledge on the physiological role of SRIF in the regulation of ACTH secretion by the anterior pituitary gland, as well as by corticotroph tumor cells is summarized. In addition, the role of glucocorticoids in regulating sst-mediated inhibition of ACTH release by corticotroph adenoma cells is discussed. Recently, it was reported that the novel multiligand SRIF-analog SOM230 might have the potential to be of therapeutic interest for Cushing's disease. On the basis of the potent suppressive effects on ACTH release by SRIF-analogs with high binding affinity to sst(5) and the observation that sst(5) expression and action is relatively resistant to glucocorticoid treatment, including the recent observation that sst(5) is the predominant sst expressed in human corticotroph adenomas, it is hypothesized that sst(5) may be a new therapeutic target for the control of ACTH and cortisol hypersecretion in patients with pituitary dependent Cushing's disease.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Role of glucocorticoids in the regulation of pituitary somatostatin receptor subtype (sst1-sst5) mRNA levels: evidence for direct and somatostatin-mediated effects

Glucocorticoids can differentially regulate somatostatin (SRIH) receptor subtype expression depending on the duration of treatment, dose used and tissue type examined. In order to determine if glucocorticoids are critical regulators of pituitary SRIH receptor synthesis in vivo, we examined the effect of adrenalectomy (ADX), with and without dexamethasone (DEX; 200 microg/day for 8 days) treatment, on the relative expression levels of the SRIH receptor subtypes, sst1-sst5, by multiplex RT-PCR. ADX increased pituitary sst2 mRNA levels, but did not significantly alter mRNA levels of the other SRIH receptor subtypes. These findings indicate that pituitary sst2 synthesis is normally under inhibitory control of endogenous glucocorticoids. High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA, independent of adrenal status. DEX also decreased sst2, sst3 and sst4 mRNA levels and increased sst5 mRNA levels by short-term in vitro application (10 nM, 4 h) in primary rat pituitary cell cultures, indicating DEX regulation of sst2-sst5 in vivo is at least in part due to a direct action at the level of the pituitary. However, the inhibitory actions of DEX on sst1 mRNA levels observed in vivo were not consistently replicated in vitro. In order to determine if the somatotrope population of the pituitary would display a similar response to DEX, fluorescent-activated cell sorting was used to obtain somatotrope-enriched cultures (>95% growth hormone immunopositive cells). DEX treatment (10 nM, 4 h) of somatotropes decreased sst2 and sst3, but did not alter sst5 mRNA levels. These results indicate that the effects of DEX on sst5 mRNA levels observed in unsorted pituitary cell cultures might be due to changes in sst5 expression in pituitary cell types other than somatotropes. Since excess glucocorticoids are thought to enhance SRIH tone, we also tested if ligand activation of SRIH receptor subtypes in vitro could mimic any of the actions of DEX on SRIH receptor mRNA levels observed in vivo. To this end, unsorted pituitary cell cultures and somatotrope-enriched cultures were treated with SRIH (1 and 100 nM) for 4 h. SRIH increased sst3 and sst5 mRNA levels, in both culture systems. These results suggest that the DEX-induced increase in endogenous SRIH tone may contribute to enhanced sst5 mRNA levels observed in vivo. However, the stimulatory actions of SRIH on sst3 mRNA levels observed in vitro might be overridden by direct inhibitory actions of DEX.

Gene structure of the human metabotropic glutamate receptor 5 and functional analysis of its multiple promoters in neuroblastoma and astroglioma cells

The metabotropic glutamate receptor 5 (mGluR5) has a discrete tissue expression mainly limited to neural cells. Expression of mGluR5 is developmentally regulated and undergoes dramatic changes in association with neuropathological disorders. We report the complete genomic structure of the mGluR5 gene, which is composed of 11 exons and encompasses approximately 563 kbp. Three clusters of multiple transcription initiation sites located on three distinct exons (IA, IB, and II), which undergo alternative splicing, have been identified. The 5'-flanking regions of these exons were isolated and, using a luciferase reporter gene assay, shown to possess active promoter elements in SKN-MC neuroblastoma and U178-MG astroglioma cells. Promoter IA was characterized by a CpG island; promoter IB contained a TATA box, and promoter II possessed three active Oct-1-binding sites. Preferential luciferase activity was observed in SKN-MC concomitant with differential DNA binding activity to several responsive elements, including CREB, Oct-1, C/EBP, and Brn-2. Exposure to growth factors produced enhanced expression of promoters IB and II in astroglioma cells and activation of NF-kappa B. These results suggest that alternative 5'-splicing and usage of multiple promoters may contribute regulatory mechanisms for tissue- and context-specific expression of the mGluR5 gene.

Identification of a novel isoform of Cdk9

Positive transcription factor b (P-TEFb) is required for RNA polymerase II to make the transition from abortive to productive elongation. This important factor is a heterodimer of a cyclin-dependent kinase, cyclin-dependent kinase 9 (Cdk9), and one of four cyclin partners, cyclin T1, T2a, T2b or K. We demonstrate here that there exists in cells a second form of Cdk9 that is 13 kDa larger than the protein originally identified. Both of these forms, which we name Cdk9(42) and Cdk9(55), are present in HeLa and NIH/3T3 cells. Cdk9(55) is generated from an mRNA that originates from a second promoter located upstream of the startpoint of transcription used to generate mRNAs encoding Cdk9(42). Antibodies specific for Cdk9(55) immunoprecipitate Cdk(55) and cyclin T1, but not Cdk9(42). Cdk9(55) in the immunoprecipitates is active as judged by its ability to phosphorylate the carboxyl-terminal domain of the largest subunit of RNA polymerase II. Recently it has been shown that the activity of P-TEFb is negatively regulated in cells by reversible association with a small cellular RNA called 7SK. We show here that P-TEFb molecules containing either form of Cdk9 are found in association with 7SK and both complexes are disrupted by treatment with 600 mM KCl. The relative abundance of Cdk9(55) and Cdk9(42) changes in different cell types, including HeLa, NIH/3T3, human macrophages and mouse lung tissue. Additionally, treatment of macrophages with lipopolysaccharides or infection with human immunodeficiency virus alters the relative abundance of the two forms of Cdk9.

Transcription of different exons 1 of the human neuronal nitric oxide synthase gene is dynamically regulated in a cell- and stimulus-specific manner

An extensive screening of the human neuronal nitric oxide synthase (nNOS) mRNAs in various human tissues and cell lines unraveled an extreme complexity in the transcription of this gene. Using 5' rapid amplification of cDNA ends (5'-RACE), ten different exons 1 (named 1a-1l) were identified. They were spliced in a cell-specific manner to a common exon 2, which bears the translational start site. Three first exons (1d, 1g and 1f) were used predominantly for the transcription of the nNOS gene (146 out of 197 5'-RACE clones contained these exons). Exon 1k was found alone, but in many instances was interposed between exons 1b, 1d, 1g, 1i or 1j and the common exon 2. In addition to the cell-specific heterogeneity of human nNOS transcripts, nNOS is highly regulated at the transcriptional level. In resting A673 neuroepithelioma cells, the prevalent nNOS transcript was the exon 1g mRNA (with minor expression of exons 1d+1k and exon 1f mRNAs). When the cells were treated with dibutyryl-cAMP, nNOS mRNA was markedly upregulated. This upregulation was solely due to an increase in exon 1f mRNA, while the expression of the other mRNA species remained unchanged. Human HaCat keratinocyte-like cells expressed the exon 1i+1k and 1i nNOS transcripts under basal conditions. When stimulated with epidermal growth factor, only the exon 1i+1k transcript was upregulated. Although these nNOS transcripts do not differ in their translated region, the various mRNAs may trigger post-transcriptional effects such as changes in mRNA stability and translation efficiency.

Identification and characterization of a novel promoter of the mouse mu opioid receptor gene (Oprm) that generates eight splice variants

Our previous studies isolated and characterized eight splicing variants containing exon 11 as the first coding exon. The location of exon 11 about 10 kb upstream from exon 1 dual promoters implied another promoter to drive expression of the exon 11 variants. I now identify the second promoter in the 5'-flanking region of exon 11. One major transcriptional start point was determined. Sequence analysis indicated that the 5'-flanking region of the exon 11 contained a putative TATA box, several CAAT boxes and a number of cis-acting elements. Functional analysis suggested that exon 11 promoter activity was most evident in neuronal-like cells. A basal core region containing the TATA box, a negative region and a positive region were identified. Electrophoretic mobility shift assays with the nuclear extracts from NIE-115 cells revealed several protein complexes that likely contained TATA box-associated factors, NF-1-like and cMyc-Max-like proteins, respectively. It also showed that a TATA-binding protein specifically bound to the TATA box fragment. Mutation analysis suggested that the TATA box in the basal core region played a fundamental role in the exon 11 promoter, whereas the NF-1 site acted as a positive element.

Identification of an upstream pituitary-active promoter of human somatostatin receptor subtype 5

Somatostatin receptor subtype 5 (sst5) has been linked to inhibition of PRL and insulin secretion. We characterized the genomic structure of the human sst5. The transcription start site was located 94 nucleotides upstream of the initiator ATG codon. Sequence analysis of 5'-inverse PCR products revealed the presence of a 6.1-kb intron in the 5'-untranslated region. RT-PCR analysis indicated tissue-specific activation of the newly identified upstream promoter in pituitary, but not in small intestine, lung, or placenta. A -1741 promoter directed significant levels of luciferase expression in GH(4) rat pituitary cells, Skut-1B endometrium cells, and JEG3 chorion carcinoma cells, which was absent in COS-7 monkey kidney cells. A minimal -101 promoter was sufficient to allow tissue-specific expression. Its activity in COS-7 cells was not enhanced by cotransfection of the pituitary-specific transcription factor Pit-1. Analysis of deletion constructs revealed a GC-rich region immediately upstream of the transcription start site, which is necessary for promoter activity. Somatostatin led to a significant inhibition, and forskolin and thyroid hormone to a significant stimulation of pituitary-specific promoter activity. Further mapping suggested a cAMP-responsive element located between -101 and the transcription start site, and thyroid hormone-responsive elements between -1741 and -1269 and between -317 and -101. These studies identified an upstream promoter of the sst5 gene with tissue-specific activity.

The p66Shc longevity gene is silenced through epigenetic modifications of an alternative promoter

The mammal Shc locus encodes three overlapping isoforms (46, 52, and 66 kDa) that differ in the length of their N-terminal regions. p46/p52Shc and p66Shc have been implicated, respectively, in the cytoplasmic propagation of growth and apoptogenic signals. Levels of p66Shc expression correlate with life span duration in mice. p46Shc and p52Shc are ubiquitously expressed, whereas p66Shc is expressed in a cell lineage-specific fashion. However, the mechanisms underlying the regulation of Shc protein expression are unknown. Here we report the identification of two alternative promoters, driving the transcription of two mRNAs coding for p46/p52Shc and p66Shc. We show that treatment with an inhibitor of histone deacetylases or with a demethylating agent results in induction of p66Shc expression in cells that normally do not express this isoform but leaves the levels of the two other isoforms unchanged. Moreover, analysis of the methylation pattern of the p66Shc promoter in a panel of primary and immortalized human cells showed inverse correlation between p66Shc expression and methylation density of its promoter. These results identify histone deacetylation and cytosine methylation as the mechanisms underlying p66Shc silencing in nonexpressing cells.

Mutational analysis of the mouse somatostatin receptor type 5 gene promoter

We have previously characterized the structure of the murine somatostatin receptor type 5 gene (sst5). Initial transient transfection studies in pituitary somatolactotropes (GH(3)) mapped the promoter activity of this gene to a region 290 bp upstream of the transcription start site. The current study identifies the sst5 promoter region critical for basal activity. A series of deletions was generated, and promoter activity was localized to a region between -83 and -19. Similar promoter deletion patterns were evident in five pituitary cell types. Seven 10-bp transversion mutations encompassing the region between -83 and -19 were generated, and functional activity was assessed. Promoter activity was reduced by the mutations spanning -67 to -47 compared with the wild-type construct. Another mutation between -26 and -17 resulted in promoter activity reduction in GH(3) cells, but not TtT-97 thyrotropes. Deoxyribonuclease I protection analysis of the sst5 promoter region between -208/+47 was performed using GH(3) and TtT-97 nuclear extracts. The most striking protected regions, located between -61 and -41 and -25 and -3, correlated with functionally important regions identified by transfection studies. In summary, the mouse sst5 gene promoter has been characterized, and functional activity and nuclear factor interactions were mapped to two specific promoter regions. The region between -67 and -47 appears to contain a nucleotide sequence critical for basal transcriptional regulation of the mouse sst5 gene in pituitary cells.

Characterization of the human somatostatin receptor type 4 promoter

Somatostatin (SRIF) exerts inhibitory effects on virtually all endocrine and exocrine secretions. Five distinct SRIF receptor subtypes (sst 1-5) have been identified. In contrast to the other subtypes, very little is known about specific functions of sst4. We investigated structure and regulation of the human sst4 gene. A genomic clone containing the 5' region of the sst4 gene was isolated. 1.5 kb of the promoter was sequenced and putative transcription factor binding sites were identified. The transcription start site was located 88 nucleotides upstream of the translation start site. A -984 sst4 promoter directed significant levels of luciferase expression in GH4 rat pituitary cells, Skut-1B endometrium cells, and BEAS-2B human bronchial epithelial cells, whereas only low activity was detected in JEG3 chorion carcinoma cells or COS-7 monkey kidney cells. A minimal -209 promoter allowed cell specific expression, its activity in COS-7 cells is not enhanced by co-transfection of the pituitary-specific transcription factor Pit-1. An enhancer element was localized between nt -459 and -984. We did not find any regulation of the sst4 promoter region analyzed by SRIF, forskolin, TPA, IGF-1, EGF, T3, glucocorticoids or 17beta-estradiol. These studies identify the 5' region of the sst4 gene. Furthermore, specific activity of the promoter in various cell lines is demonstrated.

Alternative promoters drive the expression of the gene encoding the mouse axonal glycoprotein F3/contactin

F3/Contactin is a neuronal glycoprotein which mediates axonal growth control via complex interactions with a number of cell surface or matrix components. As part of this developmental role, its expression undergoes differential regulation during the maturation of definite neuronal populations within the central and peripheral nervous tissue. To elucidate the underlying molecular mechanisms we study here the organization of the regulatory region of the mouse F3/Contactin gene. We show that this region displays peculiar features in that it spans more than 80 kb, bears very large introns and includes four untranslated exons which undergo complex splicing events leading to 11 potential arrangements of the F3/Contactin mRNA 5' end. Within this region we identify three alternative neurospecific promoters which, as deduced from the developmental profile of the associated 5' exons (A1,C1,0), drive two different patterns of F3/Contactin gene expression. The activity of the A1 exon-associated promoter displays only minor developmental changes and is likely to contribute to the basal level of the F3/Contactin gene expression; by contrast, the activities of the exon C1- and exon 0-associated promoters are significantly upregulated at the end of the first postnatal week. The data indicate that differential regulation of the F3/Contactin expression during development may depend upon alternative utilization of distinct promoter elements and may involve complex splicing events of the 5' untranslated exons. Several consensuses for homeogene transcription factors are scattered within the identified regulatory region, in agreement with the general assumption of homeotic gene regulation of neural morphoregulatory molecules.

Transcriptional activation of mouse sst2 somatostatin receptor promoter by transforming growth factor-beta. Involvement of Smad4

The sst2 somatostatin receptor is an inhibitory G protein-coupled receptor, which exhibits anti-tumor properties. Expression of sst2 is lost in most human pancreatic cancers. We have cloned 2090 base pairs corresponding to the genomic DNA region upstream of the mouse sst2 (msst2) translation initiation codon (ATG). Deletion reporter analyses in mouse pituitary AtT-20 and human pancreatic cancer PANC-1, BxPC-3, and Capan-1 cells identify a region from nucleotide -260 to the ATG codon (325 base pairs) showing maximal activity, and a region between nucleotides -2025 and -260 likely to comprise silencer or transcriptional suppressor elements. In PANC-1 and AtT-20 cells, transforming growth factor (TGF)-beta up-regulates msst2 transcription. Transactivation is mediated by Smad4 and Smad3. The cis-acting region responsible for such regulation is comprised between nucleotides -1115 and -972 and includes Sp1 and CAGA-box sequences. Expression of Smad4 in Smad4-deficient Capan-1 and BxPC-3 cells restores TGF-beta-dependent and -independent msst2 transactivation. Expression of Smad4 in BxPC-3 cells reestablishes both endogenous sst2 expression and somatostatin-mediated inhibition of cell growth. These findings demonstrate that msst2 is a new target gene for TGF-beta transcription regulation and underlie the possibility that loss of Smad4 contributes to the lack of sst2 expression in human pancreatic cancer, which in turn may contribute to a stimulation of tumor growth.

Multiple regulation of adenylyl cyclase activity by G-protein coupled receptors in human foetal lung fibroblasts

The pharmacological profile of adenylyl cyclase activity was analysed in WI-38 human foetal lung fibroblasts. Among various agents that act through G-protein coupled receptors, only the beta-adrenergic agonist isoproterenol stimulated and the tetradecapeptide somatostatin (SRIF, sst) inhibited the enzyme activity. The use of the reverse transcription-polymerase chain reaction (RT-PCR) methodology with appropriate cDNAs allowed us to identify the expression of four subtypes of SRIF transmembrane receptors (sst1-4 but not sst5 receptors) in this cell line. By RT-PCR and immunochemistry techniques, we also demonstrated the expression of stimulatory (alpha(s)) and inhibitory (alpha(i1), alpha(i2) and alpha(i3)) G-protein subunits. The known role of the adenylyl cyclase system in cell proliferation and differentiation mechanisms together with the present analysis of the corresponding regulatory network in fibroblasts of human foetal lung add knowledge on the cell line WI-38 that is widely used as a model system in studying cell growth. The importance of this cell class in normal and abnormal lung function and development reinforces the significance of these results.

Modulation of pituitary somatostatin receptor subtype (sst1-5) messenger ribonucleic acid levels by changes in the growth hormone axis

The role of individual components of the hypothalamic-pituitary-GH axis in the modulation of pituitary somatostatin (SRIF) receptor subtype (sst1-5) synthesis was assessed using multiplex RT-PCR to measure receptor messenger RNA (mRNA) levels in normal rats and spontaneous dwarf rats (SDRs). In SDRs, a strain with no immunodetectable GH, pituitary sst1 and sst2 mRNA levels were elevated, sst5 mRNA levels were reduced, and sst3 and sst4 mRNA levels did not significantly differ from those in normal controls. Treatment of SDRs with GH (72 h), but not insulin-like growth factor I, significantly decreased sst2 mRNA levels and increased sst4 and sst5 mRNA levels above vehicle-treated control levels. To test whether more rapid changes in circulating GH levels could alter SRIF receptor subtype expression, normal rats were infused (iv) with GH-releasing hormone (GHRH) for 4 h in the presence or absence of SRIF antiserum. GHRH infusion increased pituitary sst1 and sst2 and decreased sst5, but had no effect on sst3 and sst4 mRNA levels. Immunoneutralization of SRIF, which produced a rise in circulating GH levels, did not alter basal or GHRH-mediated SRIF receptor subtype expression. These observations indicate that acute suppression of SRIF tone does not regulate pituitary SRIF receptor subtype mRNA levels in vivo. The possibility that elevated circulating GH concentrations induced by GHRH infusion were responsible for the observed changes in SRIF receptor subtype mRNA levels was examined by infusing SDRs with GHRH for 4 h. GHRH did not increase sst1 mRNA levels in SDRs above their already elevated value. However, GHRH infusion produced an increase in sst2 and a decrease in sst5 mRNA levels similar to those observed in normal rats, indicating that the acute effects of GHRH on SRIF receptor subtype expression are independent of circulating GH levels. Primary rat pituitary cell cultures were incubated with GHRH (10 nM) or forskolin (10 microM) for 4 h to determine whether GHRH could directly mediate SRIF receptor subtype mRNA. GHRH treatment increased sst1 and sst2 mRNA levels and decreased sst5 mRNA levels, but had no effect on sst3 and sst4, similar to the results in vivo. The effect of forskolin mimicked that of GHRH on sst1, sst2, and sst5 mRNA, suggesting that GHRH acts through cAMP to directly mediate gene transcription or mRNA stability of these SRIF receptor subtypes. In addition, forskolin reduced sst3 and sst4 expression. These results strongly suggest that rat pituitary sst1, sst2, and sst5 mRNA levels are regulated both in vivo and in vitro by GHRH. The stimulatory action of GHRH on sst1 and sst2 and the inhibitory action on sst5 indicate that these receptor subtypes have independent and unique roles in the modulation of pituitary GH release.

Identification of functional somatostatin receptors and G-proteins in a new line of human foetal lung fibroblasts

A new line (FP) of human foetal lung fibroblasts was analysed for the expression of functional, G-protein coupled somatostatin receptors (SSTR). By means of RT-PCR, we identified the expression of SSTR1, SSTR2, SSTR3 and SSTR4, but not SSTR5, subtypes. The same technical approach evidenced the expression of stimulatory (alphas) and inhibitory (alphai1, alphai2 and alphai3) G-protein subunits. The functionality of SSTR was established from the observation of a dose-dependent inhibitory role of SST upon isoproterenol-stimulated adenylyl cyclase activity, an effect that involves G-protein action. Moreover, the functionality of G-proteins was assessed by means of experiments with forskolin and a nonhydrolysable GTP analogue that showed either Gi or Gs activation in the regulation of adenylyl cyclase. Present results represent a first pharmacological characterization of this new line of human foetal lung fibroblasts. The selective presence of some SSTR subtypes and G-protein subunits in addition to the regulatory network of the adenylyl cyclase pathway are features of recognized involvement in cell growth mechanisms. It is of interest for a cell class widely used to study this topic but also important in lung physiology and pathophysiology.

Gene structure and regulation of the somatostatin receptor type 2

The diverse biological effects of the hormone somatostatin are mediated by five genetic different receptor subtypes (sst1-sst5), which belong to the superfamily of G-protein coupled receptors with seven transmembrane domains. The sst2 subtype is unique among the somatostatin receptors in its structure, since it is expressed in two protein variants which differ within their carboxy-terminal ends, generated by alternative splicing. Within the 5' untranslated region of the gene two introns separate three transcriptional units with distinct promoters. Due to the latter feature, the sst2 gene is also unique among all somatostatin receptor genes regarding its transcriptional regulation. The three alternative promoters are tissue specifically active and show alternative responsiveness to extracellular signals. The second sst2 promoter is important for expression of the gene in tissues where somatostatin has essential physiological functions, such as brain, pituitary and gastrointestinal tissues. Furthermore, it contains cis-acting regulatory elements involved in the transcriptional response to elevated cyclic AMP levels and glucocorticoids.

Cloning of the mouse somatostatin receptor subtype 5 gene: promoter structure and function

Somatostatin is a peptide hormone whose actions are mediated by five somatostatin receptor subtypes (sstl-5). In the pituitary, somatostatin inhibits TSH release from thyrotropes and GH release from somatotropes. We have shown that sst5 transcripts and protein are induced by thyroid hormone in TtT-97 thyrotropic tumors. To map sequences responsible for promoter activity in pituitary cells, we cloned the mouse sst5 coding region of 362 amino acids and 12 kb of upstream DNA. Initial transfection studies in TtT-97 or GH3 cells mapped high levels of basal promoter activity to a 5.6-kb fragment upstream of the translational start, whereas shorter genomic fragments had low activity. To identify the transcriptional start site we used 5' RACE with TtT-97 poly A+ RNA and a sst5 antisense coding region primer. Sequence comparison between the complementary DNA and the gene revealed that the mouse sst5 gene contains 3 exons and 2 introns. The entire coding region was contained in exon 3. Two differently sized RACE products demonstrated alternate exon splicing of two untranslated exons in TtT-97 cells. A promoter fragment from -290/+48 linked to a luciferase reporter demonstrated 600- and 900-fold higher activity over a promoterless control in GH3 mammosomatotropes and TtT-97 thyrotropes, respectively, whereas a larger fragment extending to -6400 exhibited no additional promoter activity. Cloning of the sst5 gene will facilitate the mapping of basal and regulated responses at the transcriptional level.

Genomic structure and transcriptional regulation of the human somatostatin receptor type 2

Somatostatin exerts inhibitory effects on virtually all endocrine and exocrine secretions. The somatostatin receptor subtype 2 (sst2) acts as a critical molecule for growth hormone regulation and cell proliferation. We investigated the structure and regulation of the human sst2 gene. A genomic clone including the sst2 gene was isolated, 1.5 kb of the promoter was sequenced and putative transcription factor binding sites were identified. The transcription start site was located 93 nucleotides upstream of the translation start site. The nucleotide sequences of the complete gene and 0.5 kb of 3' region were determined. A possible polyadenylation signal was identified. Transcriptional regulation was investigated by transient transfections using various promoter fragments. A -1100 sst2 promoter directed significant levels of luciferase expression in GH4 rat pituitary cells and Skut1-B endometrium cells whereas only low activity was detected in JEG3 chorion carcinoma cells or COS-7 monkey kidney cells. A minimal -252 promoter allowed cell specific expression. We did not find any regulation of the sst2 promoter by somatostatin, forskolin, TRH, TPA, T3, and 17beta-estradiol. Glucocorticoids lead to a significant inhibition of sst2 promoter activity. Further mapping suggest a glucocorticoid-responsive element between -905 and -707 and between -252 and -163. These studies demonstrate the nature of the human sst2 gene and identify its 5' and 3' flanking regions. Furthermore, specific activity of the promoter and regulation by various hormones is demonstrated.

Regulation of mouse somatostatin receptor type 2 gene expression by glucocorticoids

Somatostatin is a regulatory peptide with important inhibitory functions. Its effects are mediated by five receptors, sst1-sst5. Previous studies revealed that sst2 contains three alternative, tissue specific promoters. Here, molecular mechanisms for the regulation of sst2 transcription by glucocorticoids were investigated. Reporter gene assays in NG 108-15 cells and electrophoretic mobility shift assays revealed that a glucocorticoid responsive element at position -1044 on the second promoter mediates dexamethasone induction. These findings, the existence of a cAMP response element and the tissue dependent activity (brain, pituitary and gastrointestinal tissues) indicate the importance of the second sst2 gene promoter.

Somatostatin and its receptor family

Somatostatin (SST), a regulatory peptide, is produced by neuroendocrine, inflammatory, and immune cells in response to ions, nutrients, neuropeptides, neurotransmitters, thyroid and steroid hormones, growth factors, and cytokines. The peptide is released in large amounts from storage pools of secretory cells, or in small amounts from activated immune and inflammatory cells, and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells that are widely distributed in the brain and periphery. These actions are mediated by a family of seven transmembrane (TM) domain G-protein-coupled receptors that comprise five distinct subtypes (termed SSTR1-5) that are endoded by separate genes segregated on different chromosomes. The five receptor subtypes bind the natural SST peptides, SST-14 and SST-28, with low nanomolar affinity. Short synthetic octapeptide and hexapeptide analogs bind well to only three of the subtypes, 2, 3, and 5. Selective nonpeptide agonists with nanomolar affinity have been developed for four of the subtypes (SSTR1, 2, 3, and 4) and putative peptide antagonists for SSTR2 and SSTR5 have been identified. The ligand binding domain for SST ligands is made up of residues in TMs III-VII with a potential contribution by the second extracellular loop. SSTRs are widely expressed in many tissues, frequently as multiple subtypes that coexist in the same cell. The five receptors share common signaling pathways such as the inhibition of adenylyl cyclase, activation of phosphotyrosine phosphatase (PTP), and modulation of mitogen-activated protein kinase (MAPK) through G-protein-dependent mechanisms. Some of the subtypes are also coupled to inward rectifying K(+) channels (SSTR2, 3, 4, 5), to voltage-dependent Ca(2+) channels (SSTR1, 2), a Na(+)/H(+) exchanger (SSTR1), AMPA/kainate glutamate channels (SSTR1, 2), phospholipase C (SSTR2, 5), and phospholipase A(2) (SSTR4). SSTRs block cell secretion by inhibiting intracellular cAMP and Ca(2+) and by a receptor-linked distal effect on exocytosis. Four of the receptors (SSTR1, 2, 4, and 5) induce cell cycle arrest via PTP-dependent modulation of MAPK, associated with induction of the retinoblastoma tumor suppressor protein and p21. In contrast, SSTR3 uniquely triggers PTP-dependent apoptosis accompanied by activation of p53 and the pro-apoptotic protein Bax. SSTR1, 2, 3, and 5 display acute desensitization of adenylyl cyclase coupling. Four of the subtypes (SSTR2, 3, 4, and 5) undergo rapid agonist-dependent endocytosis. SSTR1 fails to be internalized but is instead upregulated at the membrane in response to continued agonist exposure. Among the wide spectrum of SST effects, several biological responses have been identified that display absolute or relative subtype selectivity. These include GH secretion (SSTR2 and 5), insulin secretion (SSTR5), glucagon secretion (SSTR2), and immune responses (SSTR2).

Expression of somatostatin receptor subtypes on guinea pig gastric and colonic smooth muscle cells

In vivo and in vitro studies have demonstrated that somatostatin can influence motility and smooth muscle contractility of the stomach and colon. Recent studies have proposed that some of these effects may be mediated by somatostatin receptors (sst) directly on the smooth muscle cells. If this is correct, the sst receptor subtypes that are present are unknown. This study aimed to resolve these points. Because nucleotide sequences of guinea pig sst genes are unknown, we used sst subtype-specific primers based on comparisons of human and rat sst subtypes and performed RT-PCR of DNase I-treated total RNA from guinea pig total brain. PCR products were cloned in pCR II and sequenced and showed 87% (sst(1)), 90% (sst(2)), 90% (sst(3)), 99% (sst(4)), and 80% (sst(5)), respectively, nucleotide homology to the same region (transmembrane 4-6) of the human sst genes. Homology to rat sequences were lower. PCR products were obtained from first-strand cDNA derived from DNase I-treated RNA from dispersed guinea pig gastric and colonic smooth muscle cells. In gastric and colonic smooth muscle cells, we detected sst(1)-sst(3) and sst(5), and all were confirmed by sequencing. The presence of sst(4) was shown by Southern blot analysis and hybridization with a guinea pig sst(4)-specific primer. RT-PCR from cultured colonic and gastric smooth muscle cells devoid of any neural elements gave identical results. These results demonstrate that in the guinea pig all five sst subtypes are present directly on gastric and colonic smooth muscle cells. Previous studies have suggested that a predominant sst(3) subtype on gastric and a sst(5) subtype on colonic muscle cells mediated somatostatin's contractile effects, but the finding here that all five sst subtypes exist on both of these cells suggests that other sst subtypes have only a small or no contractile effect, sst subtypes in guinea pig have a different pharmacological profile from rat or human sst, or these other sst subtypes have some yet undescribed physiological function in muscle cells.

Characterization of the 5'-flanking promoter region of the rat somatostatin receptor subtype 3 gene

We investigated the 5'-flanking promoter region of the rat somatostatin receptor subtype 3 (rSSTR3). Using a cDNA probe, genomic clones containing the 5'-flanking promoter region of the rSSTR3 gene were isolated. A sequence of 5.4 kb directly upstream from the start codon was analyzed and two introns were found in the 5' untranslated region (UTR) of the cDNA sequence. The transcriptional initiation site was determined by 5' rapid amplification of cDNA ends (RACE), primer extension and RNase protection analysis with cerebellar RNA. Two major transcriptional initiation sites were found at position 1040 (tsp1) and -856 (tsp2) relative to the translational initiation site. Like a number of other promoters of G-protein-coupled receptors, the rSSTR3 gene lacks TATA and CAAT motifs and includes G+C-rich regions. Functional analysis of the promoter region by transfecting rSSTR3 luciferase-reporter gene constructs into rat pituitary GH3 cells and HEK 293 cells indicated that a 107-bp region upstream of tsp2 was sufficient to drive transcription. Furthermore a 562-bp region at position -1304 to -1865 upstream of the ATG start codon exerted a negative regulatory effect on transcriptional activity.

Identification of a functional 3',5'-cyclic adenosine monophosphate response element within the second promoter of the mouse somatostatin receptor type 2 gene

Important physiological actions of somatostatin are mediated by the somatostatin receptor type 2. Its transcription is regulated by three tissue specific, alternative promoters. It is known that the mRNA of the somatostatin receptor type 2 gene is induced by cAMP, but little is known about the mechanisms underlying this regulation. We have identified and characterized a cAMP responsive element located at nucleotide -162 on the second promoter of the gene consisting of the classical palindromic octameric sequence 5'-TGACGTCA-3'. Using transient expression of reporter gene deletion constructs in NGC108-15 cells the necessity of the intact element for forskolin-induced reporter gene activity was demonstrated. The first and the third promoter are not responsive to forskolin, nor did any promoter respond to the phorbol ester PMA. Electrophoretic mobility shift assays in combination with competition experiments suggest the interaction of the promoter element with the cAMP responsive element binding protein.