An aspartate residue at the extracellular boundary of TMII and an arginine residue in TMVII of the gastrin-releasing peptide receptor interact to facilitate heterotrimeric G protein coupling

The mammalian bombesin receptor subfamily of G protein-coupled receptors currently consists of the gastrin-releasing peptide receptor (GRP-R), neuromedin B receptor, and bombesin receptor subtype 3. All three receptors contain a conserved aspartate residue (D98) at the extracellular boundary of transmembrane domain II and a conserved arginine residue (R309) near the extracellular boundary of transmembrane domain VII. To evaluate the functional role of these residues, site-directed GRP-R mutants were expressed in fibroblasts and assayed for their ability to both bind agonist and catalyze exchange of guanine nucleotides. Alanine substitution at GRP-R position 98 or 309 reduced agonist binding affinity by 24- and 56-fold, respectively, compared to wild-type GRP-R. Single swap GRP-R mutations either resulted in no receptor expression in the membrane (D98R) or the protein was not able to bind agonist (R309D). In contrast, the double swap mutation (D98R/R309D) had high-affinity agonist binding, reduced from wild-type GRP-R by only 6-fold. In situ reconstitution of urea-extracted membranes expressing either wild-type or mutant (D98A or R309A) GRP-R with G(q) indicated that alanine substitution greatly reduced G protein catalytic exchange compared to wild-type GRP-R. The D98R/R309D GRP-R had both a higher intrinsic basal activity and a higher overall catalytic exchange activity compared to wild-type; however, the wild-type GRP-R produced a larger agonist-stimulated response relative to the double swap mutant. Taken together, these data show that GRP-R residues D98 and R309 are critical for efficient coupling of GRP-R to G(q). Furthermore, our findings are consistent with a salt bridge interaction between these two polar and oppositely charged amino acids that maintains the proper receptor conformation necessary to interact with G proteins.

Pharmacology and cell biology of the bombesin receptor subtype 4 (BB4-R)

Recently, a fourth member of the bombesin (Bn) receptor family (fBB4-R) was isolated from a cDNA library from the brain of the frog, Bombina orientalis. Its pharmacology and cell biology are largely unknown, and no known natural cell lines or tissues possess sufficient numbers of fBB4-R's to allow either of these to be determined. To address these issues, we have used three different strategies. fBB4-R expression in cells widely used for other Bn receptor subtypes was unsuccessful as was expression in two frog cell lines. However, stable fBB4-R cell lines were obtained in CHO-K1 cells which were shown to faithfully demonstrate the correct pharmacology of the related Bn receptor, the GRP receptor, when expressed in these cells. [DPhe6,betaAla11,Phe13,Nle14]Bn(6-14) was found to have high affinity (Ki = 0.4 nM) for the fBB4 receptor and 125I-[DTyr6,betaala11,Phe13,Nle14]Bn(6-14) to be an excellent ligand for this receptor. The fBB4-R had a unique pharmacology for naturally occurring Bn-related agonists, with the presence of a penultimate phenylalanine being critical for high-affinity interaction. It also had a unique profile for six classes of Bn antagonists. The fBB4-R was coupled to phospholipase C with activation increasing [3H]inositol phosphates and mobilizing Ca2+ almost entirely from cellular sources. There was a close correlation between agonist the receptor occupation and the receptor activation. Three of the five classes of Bn receptor antagonists that interacted with higher affinity with the fBB4-R functioned as fBB4-R antagonists and two as partial agonists. fBB4-R activation stimulated increases in phospholipase D (PLD) over the same range of concentrations at which it activated phospholipase C. These results demonstrate that the fBB4 receptor has a unique pharmacology for agonists and antagonists and is coupled to phospholipase C and D. The availability of these cell lines, this novel ligand, and the identification of three classes of antagonists that can be used as lead compounds should facilitate the further investigation of the pharmacology and cell biology of the BB4 receptor.

Identification of a unique ligand which has high affinity for all four bombesin receptor subtypes

Four subtypes of bombesin receptors are identified (gastrin-releasing peptide receptor, neuromedin B receptor, the orphan receptor bombesin receptor subtype 3 (BB3 or BRS-3) and bombesin receptor subtype 4 (BB4)), however, only the pharmacology of the gastrin-releasing peptide receptor has been well studied. This lack of data is due in part to the absence of a general ligand. Recently we have discovered a ligand, 125I-[D-Tyr6,betaAla11,Phe13,Nle14]bombesin-(6-1 4) that binds to BRS-3 receptors. In this study we investigate its ability to interact with all four bombesin receptor subtypes. In rat pancreatic acini containing only gastrin-releasing peptide receptor and in BB4 transfected BALB cells, this ligand and 125I-[Tyr4]bombesin, the conventional gastrin-releasing peptide receptor ligand, gave similar results for receptor number, affinity for bombesin and affinity for the unlabeled ligand. In neuromedin B receptor transfected BALB cells, this ligand and 125I-[D-Tyr0]neuromedin B, the generally used neuromedin B receptor ligand, gave similar results for receptor number, neuromedin B affinity or the unlabeled ligand affinity. Lastly, in BRS-3 transfected BALB cells, only this ligand had high affinity. For all four bombesin receptors this ligand had an affinity of 1-8 nM and was equal or greater in affinity than any other specific ligands for any receptor. The unlabeled ligand is specific for gastrin-releasing peptide receptors on rat pancreatic acini and did not inhibit binding of 125I-cholecystokinin octapeptide (125I-CCK-8), 125I-vasoactive intestinal peptide (125I-VIP) or 125I-endothelin to their receptors. The unlabeled ligand was an agonist only at the gastrin-releasing peptide receptor in rat acini and did not interact with CCK(A) receptors or muscarinic M3 acetylcholine receptors to increase [3H]inositol phosphates. These results demonstrate 125I-[D-Tyr6,betaAla11,Phe13,Nle14]bombesin-(6-1 4) is a unique ligand with high affinity for all subtypes of bombesin receptors. Because of the specificity for bombesin receptors, this ligand will be a valuable addition for such pharmacological studies as screening for bombesin receptor agonists or antagonists and, in particular, for investigating BRS-3 cell biology, a receptor for which no ligand currently exists.

A human gene encodes a putative G protein-coupled receptor highly expressed in the central nervous system

The mammalian bombesin (Bn)-like neuropeptide receptors gastrin-releasing peptide receptor (GRP-R) and neuromedin B receptor (NMB-R) transduce a variety of physiological signals that regulate secretion, growth, muscle contraction, chemotaxis and neuromodulation. We have used reverse transcription-polymerase chain reaction (PCR) to isolate a cDNA from human brain mRNA, GPCR/CNS, that encodes a putative G protein-coupled receptor (GPCR) based upon the presence of the paradigmatic seven heptahelical transmembrane domains in its predicted amino acid sequence. Analysis of the deduced protein sequence of GPCR/CNS reveals this putative receptor to be 98% identical to the deduced amino acid sequence of a recently reported gene product and minimally identical (approximately 23%) to both murine GRP-R and human endothelin-B (ET-B) receptor. Our deduced protein sequence differs at 12 positions, scattered throughout the open reading frame, relative to the original sequence. A 3.7 kb GPCR/CNS mRNA species is expressed in vivo in a tissue-specific manner, with highest levels detected in brain and spinal cord, lower levels found in testis, placenta and liver, but no detectable expression observed in any other tissue. Analysis of GPCR/CNS genomic clones reveals that the human gene contains one intron that is about 21 kb in length that divides the coding region into two exons and maps to human chromosome 7q31. No specific binding is observed with either a newly identified ligand (DTyr6, beta Ala11, Phe13, Nle14]Bn-(6-14)) having high affinity for all Bn receptor subtypes or Bn after GPCR/CNS is stably expressed in fibroblasts. No elevation in inositol trisphosphate is observed after the application of micromolar levels of either DPhe6, beta Ala11, Phe13, Nle14]Bn-(6-14) or Bn, a concentration of agonist known to activate all four known Bn receptor subtypes. When GPCR/CNS is expressed in Xenopus oocytes, no activation of the calcium-dependent chloride channel is detected despite the addition of micromolar levels of Bn peptide agonists. We conclude that the natural ligand for this receptor is none of the known naturally occurring Bn-like peptides and the true agonist for GPCR/CNS remains to be elucidated.

Fibroblast growth factor-1 induction of delayed-early mRNA expression in NIH 3T3 cells is prolonged by heparin addition

Fibroblast growth factor (FGF)-1, also known as acidic FGF, is a multifunctional heparin-binding protein that is mitogenic for a wide variety of cell types cultured in vitro and a potent angiogenic agent in vivo. These cellular responses are mediated via high-affinity binding to a family of four membrane-spanning tyrosine kinase receptors. FGF-1-stimulated mitogenesis is potentiated by heparin, a sulfated glycosaminoglycan. In this study, we examined the effect of exogenous heparin on FGF-1-inducible gene expression in murine NIH 3T3 cells using both wild-type FGF-1 and FGF-1/glu132, an FGF-1 mutant with a reduced apparent affinity for heparin. The induction levels and temporal expression kinetics of two immediate-early response mRNAs (early growth response gene-1, thrombospondin-1) as well as two delayed-early response mRNAs (proliferin, ornithine decarboxylase) were monitored by Northern blot hybridization analysis. We found that although FGF-1 alone can promote the initial induction of these four mRNAs, heparin coaddition is necessary for prolonged delayed-early mRNA expression. This heparin effect occurs when cells are stimulated with wild-type FGF-1 but not with FGF-1/glu132. Furthermore, FGF-1 and heparin must be added together at the initial time of mitogen stimulation and they must remain present in the cell culture medium for a minimum period of 8 h to promote sustained delayed-early mRNA expression. These findings are consistent with the proposal that heparin promotes a long-term FGF-1:FGFR interaction which is required for sustained delayed-early gene expression and a full mitogenic response.

Fibroblast growth factor-1 stimulation of quiescent NIH 3T3 cells increases G/T mismatch-binding protein expression

Polypeptide growth factors promote cell-cycle progression in part by the transcriptional activation of a diverse group of specific genes. We have used an mRNA differential-display approach to identify several fibroblast growth factor (FGF)-1 (acidic FGF)-inducible genes in NIH 3T3 cells. Here we report that one of these genes, called FGF-regulated (FR)-3, is predicted to encode G/T mismatch-binding protein (GTBP), a component of the mammalian DNA mismatch correction system. The murine GTBP gene is transiently expressed after FGF-1 or calf serum treatment, with maximal mRNA levels detected at 12 and 18 h post-stimulation. FGF-1-stimulated NIH 3T3 cells also express an increased amount of GTBP as determined by immunoblot analysis. These results indicate that elevated levels of GTBP may be required during the DNA synthesis phase of the cell cycle for efficient G/T mismatch recognition and repair.

Identification by targeted differential display of an immediate early gene encoding a putative serine/threonine kinase

Fibroblast growth factor (FGF)-1 mitogenic signal transduction is mediated in part by gene products that are specifically expressed in response to cell surface receptor binding and activation. We have used a targeted differential display method to identify FGF-1-inducible genes in murine NIH 3T3 fibroblasts. Here we report that one of these genes is predicted to encode a novel serine/threonine-specific protein kinase. This putative kinase has been named Fnk, for FGF-inducible kinase. The deduced Fnk amino acid sequence has 49, 36, 33, 32, and 22% overall identity to mouse serum-inducible kinase (Snk), mouse polo-like kinase (Plk), Drosophila polo, Saccharomyces Cdc5, and mouse Snk/Plk-akin kinase (Sak), respectively. These proteins are all members of the polo subfamily of structurally related serine/threonine kinases. The Plk, polo, Cdc5, and Sak kinases are required for cell division. FGF-1 induction of Fnk mRNA expression is first detected at 30 min after mitogen addition, reflects transcriptional activation, and does not require de novo protein synthesis. FGF-2, platelet-derived growth factor-BB, calf serum, or phorbol myristate acetate treatment of quiescent cells also induces fnk gene expression. Fnk mRNA is expressed in vivo in a tissue-specific manner, with relatively high levels detected in newborn and adult mouse skin. These results indicate that Fnk may be a transiently expressed protein kinase involved in the early signaling events required for growth factor-stimulated cell cycle progression.

A delayed-early gene activated by fibroblast growth factor-1 encodes a protein related to aldose reductase

The addition of polypeptide mitogens to quiescent cell lines induces the expression of various gene products, some of which are likely to perform functions critical for cell cycle progression, DNA synthesis, and mitosis. We have used a differential display approach to identify fibroblast growth factor (FGF)-1-inducible genes in NIH-3T3 cells. One of these genes, termed FGF-regulated (FR)-1, encodes a 316-amino acid protein with approximately 82% amino acid sequence identity to an abundant protein expressed in mouse vas deferens and approximately 70% identity to human aldose reductase. The function of the vas deferens protein is unknown; however, aldose reductase is an NADPH-dependent monomeric oxidoreductase implicated in the pathogenesis of diabetic complications. FGF-1 induction of FR-1 mRNA expression is first detectable at 4 h after mitogen addition and is dependent on de novo RNA and protein synthesis. FGF-2 or phorbol ester treatment can also increase FR-1 mRNA levels; in contrast, whole blood serum or individual growth factors present in serum have only minimal effects on FR-1 mRNA expression. FR-1 mRNA is detectable in a number of mouse tissues but is most abundant in newborn liver and in adult intestine, ovary, and testis. These results raise the possibility that aldose reductase-related proteins may play a role in FGF-1- and FGF-2-stimulated mitogenesis.

Fibroblast growth factor-1 induces phosphofructokinase, fatty acid synthase and Ca(2+)-ATPase mRNA expression in NIH 3T3 cells

Polypeptide growth factors act in part by inducing the expression of specific proteins that perform functions critical to cell cycle progression. We have used a differential display technique to identify genes that are expressed at higher levels following fibroblast growth factor (FGF)-1 (acidic FGF) stimulation of quiescent murine NIH 3T3 fibroblasts. Three such genes--liver (B-type) phosphofructokinase (PFK), fatty acid synthase (FAS) and sarco(endo)plasmic reticulum Ca(2+)-ATPase type 2 (SERCA2)--are described in this report. The level of FAS and SERCA2 mRNA expression is increased rapidly after FGF-1 addition; in contrast, PFK mRNA is induced with kinetics more typical of delayed-early genes. These results indicate that enhanced expression of the PFK, FAS and SERCA2 proteins may be important for FGF-1-stimulated cell proliferation.

Structure-function studies of heparin-binding (acidic fibroblast) growth factor-1 using site-directed mutagenesis

The heparin-binding or fibroblast growth factors (HBGFs) modulate cell growth and migration, angiogenesis, wound repair, neurite extension, and mesoderm induction. Relatively little is known regarding the precise mechanism of action of these growth factors or the structural basis for their action. A better understanding of the structural basis for the different activities of these proteins should lead to the development of agonists and antagonists of specific HBGF activities. In this report, we summarize evidence that indicates that the heparin-binding and mitogenic activities of HBGF-1 can be dissociated from the receptor-binding activities of the growth factor by site-directed mutagenesis of a single lysine residue. Thus, the mutant HBGF-1 has normal receptor-binding activity and is capable of stimulating tyrosine kinase activity and proto-oncogene expression but is not able to elicit a mitogenic response. A similar dissociation of early events such as proto-oncogene expression from the mitogenic response is observed when the human wild-tupe HBGF-1 is used in the absence of added heparin. These results indicate that intracellular sites of action by the growth factor may be required to complete the mitogenic response. Further evidence for this idea is provided by transfection experiments where NIH 3T3 cells are engineered to produce large quantities of wild-type or mutant HBGF-1. Production of wild-type induces a transformed phenotype, whereas over-production of the mutant does not. The majority of both forms of the protein is found in the nuclear fraction of the transfected cells. Additional site-directed mutagenesis of putative nuclear translocation sequences in the wild-type protein do not affect mitogenic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible dissociation of the heparin-binding and mitogenic activities of heparin-binding (acidic fibroblast) growth factor-1 from its receptor-binding activities by site-directed mutagenesis of a single lysine residue

The fibroblast or heparin-binding growth factors (HBGFs) are thought to be modulators of cell growth and migration, angiogenesis, wound repair, neurite extension, and mesoderm induction. A better understanding of the structural basis for the different activities of these proteins should facilitate the development of agonists and antagonists of specific HBGF activities and identification of the signal transduction pathways involved in the mechanisms of action of these growth factors. Chemical modification studies of Harper and Lobb (Harper, J. W., and R. R. Lobb. 1988. Biochemistry. 27:671-678) implicated lysine 132 in HBGF-1 (acidic fibroblast growth factor) as being important to the heparin-binding, receptor-binding, and mitogenic activities of the protein. We changed lysine 132 to a glutamic acid residue by site-directed mutagenesis of the human cDNA and expressed the mutant protein in Escherichia coli to obtain sufficient quantities for functional studies. Replacement of this lysine with glutamic acid reduces the apparent affinity of HBGF-1 for immobilized heparin (elutes at 0.45 M NaCl vs. 1.1 M NaCl for wild-type). Mitogenic assays established two points: (a) human recombinant HBGF-1 is highly dependent on the presence of heparin for optimal mitogenic activity, and (b) the change of lysine 132 to glutamic acid drastically reduces the specific mitogenic activity of HBGF-1. The poor mitogenic activity of the mutant protein does not appear to be due to a reduced affinity for the HBGF receptor. Similarly, the mutant HBGF-1 can stimulate tyrosine kinase activity and induce protooncogene expression. Differences in the biological properties of the wild-type and mutant proteins were observed in transfection studies. Mutant HBGF-1 expression in transfected NIH 3T3 cells did not induce the same transformed phenotype characteristic of cells expressing wild-type HBGF-1. Together these data indicate that different functional properties of HBGF-1 may be dissociated at the structural level.

Characterization of link protein(s) from human intervertebral-disc tissues

Proteoglycan aggregates (A1) were prepared from the anulus fibrosus, nucleus pulposus and cartilage-endplate tissues of postnatal (0-6-month-old)-and young-adult (20-30-year-old)-human intervertebral discs. The A1 fractions from young-adult disc contained a greater proportion of non-aggregating proteoglycans than did postnatal tissues. After dissociative CsCl-density-gradient fractionation of the A1, more than 90% of the uronic acid was found in the postnatal A1D1, whereas only 60-80% of the hexuronate was present in the A1D1 isolated from young-adult disc tissues. These results indicated that more lower-buoyant-density proteoglycans occur in the young-adult disc. Link-protein-rich fractions (A1D3) were subjected to SDS/polyacrylamide-gel electrophoresis and immunolocation analyses using monoclonal antibodies specific for epitopes on link protein or proteoglycan. Under non-reducing conditions, the major link protein present in postnatal disc tissues was link protein 1. By contrast, all three link proteins (1, 2 and 3) were detected in young-adult tissues, with the smaller link protein 3 predominating. Analyses of the A1D3 fractions under reducing conditions also indicated the presence of link-protein-degradation peptides (Mr approx. 26,000) from young-adult disc tissues, but not from postnatal tissues. Sequential Sepharose CL-6B and Sephacryl S-300 chromatography in 4 M-guanidinium chloride was employed to separate the link proteins of the A1D3 fraction from protein-rich proteoglycan. Immunolocation analyses indicated that postnatal samples contained no detectable contaminating proteoglycan fragments. However, young-adult link-protein preparations could not be separated from hyaluronic acid-binding region and other proteoglycan fragments by means of these chromatographic procedures. The studies indicate that, compared with hyaline articular cartilage, degraded link protein and proteoglycan accumulate at an early age in young-adult disc tissues. These partially degraded proteoglycan aggregate components may significantly alter the biomechanical properties of disc tissues.