Features of the renal parathyroid hormone-parathyroid hormone-related protein receptor derived from structural studies of receptor fragments

Characterisation of ligand binding to the parathyroid hormone/parathyroid hormone-related peptide receptor in MCF7 breast cancer cells and SaOS-2 osteosarcoma cells

Parathyroid hormone-related peptide (PTHrP) and parathyroid hormone (PTH)/PTHrP-receptor, PTH/PTHrP-R, are frequently expressed in mammary carcinomas as well as in bone cells. In this study we compared the ligand binding characteristics of the PTH/PTHrP-R in SaOS-2 human osteosarcoma cells with those in MCF7 breast cancer cells. We used both Scatchard analysis of saturation kinetics for iodinated ligand and the level of expressed receptor protein by visualising the single radio-labelled receptor-ligand complex from isolated membrane preparations from the two cell lines. In MCF7 cells, ligand binding, (receptor number) was increased by prior exposure of the cultured cells to epidermal growth factor (EGF), estradiol (E2), or dexamethasone (DEX), and decreased following calcitriol (1,25 DHCC). In contrast in the SaOS-2 cells, PTH/PTHrP-R number was increased by exposure to E2 and 1,25DHCC and decreased by DEX while EGF had no effect. These data were confirmed when the PTH/PTHrP-R was cross linked with (125)I-PTHrP-1-34(Tyr), and extended by visualising the intensity of the isolated radiolabelled receptor complex by autoradiography following SDS-PAGE at several time points during the treatment.

In vitro folding, functional characterization, and disulfide pattern of the extracellular domain of human GLP-1 receptor

The N-terminal, extracellular domain of the receptor for glucagon-like peptide 1 (GLP-1 receptor) was expressed at a high level in E. coli and isolated as inclusion bodies. Renaturation with concomitant disulfide bond formation was achieved from guanidinium-solubilized material. A soluble and active fraction of the protein was isolated by ion exchange chromatography and gel filtration. Complex formation with GLP-1 was shown by cross-linking experiments, surface plasmon resonance measurements, and isothermal titration calorimetry. The existence of disulfide bridges in the N-terminal receptor fragment was proven after digestion of the protein with pepsin. Further analysis revealed a disulfide-binding pattern with links between cysteines 46 and 71, 62 and 104, and between 85 and 126.

Cysteine residues are involved in structure and function of melanocortin 1 receptor: Substitution of a cysteine residue in transmembrane segment two converts an agonist to antagonist

Reduction of disulfide bonds in human melanocortin 1 receptor (hMC1R) with increasing concentrations of DTT (dithiothreitol) resulted in a decrease in the binding of [125I]-ACTH (adrenocorticotropic hormone, L-isomer) in an uniphasic manner and a decrease in [125I]-NDP-MSH ([Nle(4),D-Phe(7)]-alpha-melanocyte stimulating hormone; D-isomer) binding in a biphasic manner. Pretreatment of hMC1R with 10 mM DTT resulted in a 36-fold loss of affinity for alpha-MSH (L-isomer) without affecting the affinity of NDP-MSH (D-isomer). To characterize the role of individual cysteine residues, we employed site-directed mutagenesis to substitute cysteine by glycine at all fourteen positions in hMC1R and analysed wild-type and mutant receptors for ligand binding and cAMP signalling. Single point mutation of four cysteine residues in extracellular loops to glycine (C35G, C267G, C273G, and C275G) resulted in a complete loss of binding for [125I]-NDP-MSH. Moreover, mutants with normal ligand binding, at positions C191G (transmembrane segment 5), C215G (third intracellular loop), and C315G (C-terminal loop) failed to generate cAMP signal in response to both agonists alpha-MSH and NDP-MSH. Mutant at position C78G (with wild-type binding to alpha-MSH as well as NDP-MSH) generated a cAMP signal in response to alpha-MSH (identical to wild-type hMC1R) but interestingly could not be stimulated by NDP-MSH. Moreover, this single amino acid substitution converted NDP-MSH from being an agonist to antagonist at the C78G mutant receptor. These findings demonstrate that (i) alpha-MSH and ACTH (L-isomers) are different from D-isomer NDP-MSH in their sensitivity to DTT for receptor binding, (ii) cysteine residues in N-terminus and extracellular loop three make disulfide bridges and are needed for structural integrity of hMC1R, (iii) cysteine residues in transmembrane segments and intracellular loops are required for receptor-G-protein coupling, (iv) C78 in transmembrane segment two is required for generating a functional response by D-isomer agonist (NDP-MSH) but not by L-isomer agonist (alpha-MSH), and (v) wild-type receptor agonist NDP-MSH is an antagonist at the mutant C78G receptor.

Serpentine receptors for parathyroid hormone, calcitonin and extracellular calcium ions

The cloning of the receptors for PTH, CT and extracellular calcium ions represents a significant advance in the elucidation of the mechanisms through which extracellular calcium ions are regulated. All are members of the superfamily of GPCR, and the inclusion of the Ca2+o-sensing receptor in this superfamily documents that extracellular calcium ions can serve as an extracellular first messenger, in addition to subserving their better known role as a key intracellular second messenger. Furthermore, it has proved possible to identify several human diseases that result from inactivating or activating mutations in the PTH or Ca2+o-sensing receptor. Finally, the availability of these cloned receptors will enable many more studies on structure-function relationships for these receptors as well as clarifying their tissue distribution, regulation and roles in health and disease. It may also be possible to design novel therapeutic agents that permit manipulation of the receptors when their function is abnormal.

Functional properties of a synthetic chicken parathyroid hormone-related protein 1-36 fragment

The biologic activities of human parathyroid hormone-related protein [hPTHrP(1-34] and bovine PTH [bPTH(1-34)] are remarkably similar despite marked sequence divergence in their primary binding domain, residues 25-34. Chicken PTHrP (cPTHrP) is identical to hPTHrP through residue 21. However, in the 25-34 region, cPTHrP displays three fewer basic residues than hPTHrP and contains five residues not present in any other member of the PTH/PTHrP family. To assess the biologic consequences of these structural differences, we compared the activities of synthetic [36Tyr]cPTHrP(1-36)NH2 and hPTHrP(1-34)NH2 with those of bPTH(1-34) in avian systems (chicken renal plasma membranes and 19 day chick embryonic bone cells) and mammalian systems [canine renal plasma membranes and rat osteosarcoma cells (UMR-106-H5)]. In both avian and mammalian systems the binding affinity of [36Tyr]cPTHrP(1-36)NH2 (0.8-3.4 nM) was approximately one-half that of hPTHrP(1-34)NH2 (0.4-1.1 nM). The potencies of [36Tyr]cPTHrP(1-36)NH2 and hPTHrP(1-34)NH2 for activation of adenylate cyclase were similar in canine renal membranes (5.2 and 6.7 nM) and chick bone cells (1.0 nM). In UMR-106 cells and chicken renal membranes the potency of [36Tyr[cPTHrP(1-36)NH2 for activation of adenylate cyclase was about one-half that of [36Tyr]hPTHrP(1-36)NH2. Binding of 125I-[36Tyr]cPTHrP(1-36)NH2 to chick bone cells and chicken renal membranes was completely displaced by bPTH(1-34) and hPTHrP(1-34)NH2: thus there was no evidence for a distinct chicken PTHrP receptor. In general, [36Tyr]cPTHrP(1-36)NH2 and hPTHrP(1-34)NH2 activated adenylate cyclase similarly despite their sequence differences in the 25-32 region.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of parathyroid hormone-related peptide on chick growth plate chondrocytes

Parathyroid hormone-related peptide (PTHrP) and parathyroid hormone (PTH) have similar biological effects in vitro that are mediated through the PTH receptor. PTH receptors have been demonstrated in the zone of provisional calcification and the hypertrophic zone of the cartilaginous growth plate. The current study examined the biological effects of PTHrP on chick growth plate chondrocytes. Chondrocytes were exposed to varying doses of PTHrP for 24 h, and the incorporation of radioactive thymidine into DNA was used as an index of proliferation. A dose-dependent stimulation of proliferation was seen, with a maximal 27-fold increase at 50 nM PTHrP. A dose-dependent stimulation of cAMP was seen, with a maximal effect at a dose of 50 nM. Proteoglycan synthesis, measured by incorporation of radioactive sulfate, was stimulated, with a maximal effect of 65% at 1 nM. Collagen synthesis and alkaline phosphatase activity from both cellular and matrix vesicle sources decreased in a dose-dependent fashion, with a maximal inhibition of approximately 50% of the control value. The physiologic significance of the PTH and PTHrP-responsiveness of growth plate chondrocytes is uncertain at the present time. It is possible that PTH or PTHrP, or both, act as a systemic, developmental modulator of cellular proliferation and differentiation in the growth plate.

Dissociation between parathyroid hormone-stimulated cAMP and calcium increase in UMR-106-01 cells

We used the osteogenic sarcoma cell line, UMR-106-01, to determine whether the rise in free cytosolic Ca2+ concentration ([Ca2+]i) and cellular cAMP following PTH stimulation are able to be regulated independently. For this purpose, we compared the effect of a PTH antagonist, stimulation of protein kinase C, augmentation by prostaglandins, and the time course of desensitization of the two cellular responses. Two x 10(-7) M of the PTH antagonist 8,18Nle 34Tyr-bPTH(3-34) amide ([Nle,Tyr]bPTH(3-34)A) was required to inhibit 10(-9) M bPTH(1-34)-stimulated cAMP generation by 50%. 10(-7) M bPTH(1-34) completely overcame the inhibition induced by 10(-6) M [Nle,Tyr]bPTH(3-34)A. Only 7 x 10(-8) M and 2.7 x 10(-7) M [Nle,Tyr]bPTH(3-34)A were required to half maximally inhibit the [Ca2+]i increase evoked by 3 x 10(-8) and 10(-7) M bPTH(1-34), respectively. In addition, dissociation between [Ca2+]i and cAMP signals was observed when modulation by protein kinase C and prostaglandins was tested. Preincubation of the cells with 10 nM TPA for 5 minutes markedly inhibited the PTH-evoked [Ca2+]i increase. Short incubation with PGF2 alpha augmented the PTH-evoked [Ca2+]i increase. Similar pretreatments had no effect on the PTH-stimulated cAMP increase. Finally, preincubation with 1.5 x 10(-9) M bPTH(1-34) for 20 minutes almost completely blocked the effect of 10(-7) M bPTH(1-34) on [Ca2+]i, while preincubation with 5 x 10(-9) M bPTH(1-34) for 4 hours was required to inhibit the effect of 10(-8) M bPTH(1-34) on cAMP production by 50%. The differences in the regulation of the two PTH-stimulated cellular signaling systems, in particular, the response to antagonists and the time course of desensitization, could be at the level of the PTH receptor(s) or at a postreceptor domain.