Structural requirements of parathyroid hormone/parathyroid hormone-related peptide receptors for phospholipase C activation and regulation of phosphate uptake.
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Animals↗
Biomedical subjects
G V Segre
Publications and source records attributed to G V Segre.
The rat, mouse, and human genes encoding the receptor for parathyroid hormone and parathyroid hormone-related peptide are highly homologous.
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Animals↗
The rat, mouse and human genes encoding the receptor for parathyroid hormone and parathyroid hormone-related peptide are highly homologous.
The organization of the PTH/PTHrP receptor gene is highly homologous in three mammalian species, rat, human and mouse. This gene extends over 22 kb and contains at least 15 exons and 14 introns. The most 5' exon we have identified (exon U) is followed by an approximately 1kb intron. The second exon (exon S) encodes the initiator methionine and the putative signal peptide and is followed by the largest intron of this gene (about 11 kb). The amino-terminal extracellular region is encoded by 4 exons (E1, E2, E3 and G); exon G contains all 4 potential glycosylation sites. Membrane-spanning domains 1-4 and portions of their connecting intracellular and extracellular loops are encoded by 4 exons (M1, M2, M3 and M4). The second extracellular loop and portions of 4th and 5th membrane-spanning domains are encoded by one exon, EL2. The 5th membrane-spanning domain and portion of the 3rd intracellular loop are encoded by one exon, M5. The 6th membrane-spanning domain, the 3rd extracellular loop and the proximal part of the 7th membrane-spanning domain are encoded by one single exon (M6/7); the remaining sequence of the 7th membrane-spanning domain is encoded by a short exon, M7. The carboxy-terminal tail of the receptor and the 3' untranslated region are encoded by one single exon, exon T. The 3' untranslated region does not contain the classical polyadenylation signal, AATAAA. Expression in COS-7 cells of a minigene constructed of a 5' rat cDNA fragment (1.3 Kb) ligated in-frame to a 3' genomic fragment at the NsiI site, which is located in exon M6/7 resulted in a transcript that was translated into a functional receptor; it bound PTH and showed PTH-stimulated accumulation of intracellular cAMP. Therefore, the PTH/PTHrP receptor gene contains alternative 3' sequences that allow cleavage and polyadenylation of its transcript.
Animals↗
Differences in binding affinities of human PTH(1-84) do not alter biological potency: a comparison between chemically synthesized hormone, natural and mutant forms.
The purpose of this study was to evaluate receptor binding affinities and biological properties in vitro and in vivo of various recombinant hPTH(1-84) forms representing the natural hormone and a mutagenized hPTH form, [Gln26]hPTH(1-84) (QPTH), after expression in E. coli and Saccharomyces cerevisiae. In LLC-PK1 cells stably transformed with the rat PTH/PTHrP receptor, chemically synthesized hPTH(1-84) and QPTH showed a reduced binding affinity (apparent Kd 18 and 23 nM, respectively) than the recombinant, hPTH(1-84) (apparent Kd 9.5 nM). All recombinant hPTH forms showed a similar potency to stimulate cellular cAMP production (EC50 1.5 nM) and significantly better than chemically synthesized hPTH (EC50 5.7 nM). All hormone forms showed an about equipotent activity in causing elevation in serum calcium, increased excretion of urine phosphate, and cAMP. Thus, the natural recombinant PTH forms showed higher binding affinities and adenylate cyclase activation potencies in LLC-PK1 cells, but the reduced receptor binding affinity exerted by QPTH did not transcend differences in cAMP generation and in vivo biological activities.
Animals↗
Transforming growth factor-beta1 regulates steady-state PTH/PTHrP receptor mRNA levels and PTHrP binding in ROS 17/2.8 osteosarcoma cells.
The effect of transforming growth factor beta1 (TGF-beta1) on the expression of mRNA for the parathyroid hormone receptor and binding of iodinated parathyroid hormone-related protein in ROS 17/2.8 osteosarcoma cells was evaluated. TGF-beta1 stimulated a 2-7-fold increase in steady state mRNA levels for the parathyroid hormone receptor at a maximal dose of 5 ng/ml, with increased levels of expression at 6 h of TGF-beta1-incubation, and peak levels at 8-24 h. Receptor binding studies revealed a significant increase in PTHrP-specific binding with TGF-beta1 doses as low as 0.5 ng/ml and a 55% increase in numbers of receptors with no alteration in binding affinity with 5.0 ng/ml TGF-beta1. Time course studies indicated that receptor binding was increased at 24 h with peak levels reached at 48 h of treatment. PTH-stimulated cAMP levels were significantly increased in ROS 17/2.8 cells treated with TGF-beta1 (0.5 ng/ml) for 48 h. These data indicate that TGF-beta1 upregulates steady-state mRNA, ligand binding and PTH/PTHrP receptor signaling in rat osteosarcoma cells. The effects of TGF-beta1 on bone may be attributed in part to regulation of the PTH/PTHrP receptor at the molecular level.
Animals↗
Expression of parathyroid hormone-related protein and its receptor in human umbilical cord: evidence for a paracrine system involving umbilical vessels.
OBJECTIVE: Our purpose was to study the expression and localization of parathyroid hormone-related protein and expression of the parathyroid hormone/parathyroid hormone-related protein receptor in human umbilical cord. STUDY DESIGN: The expression and localization of parathyroid hormone-related protein and expression of the parathyroid hormone/parathyroid hormone-related protein receptor were studied in isolated tissues from the human umbilical cord by Northern analysis, reverse-transcriptase polymerase chain reaction, Southern gel analysis, and immunolocalization procedures at the light and electron microscopic levels. RESULTS: Parathyroid hormone-related protein was abundantly expressed in the umbilical cord. Immunohistochemical and immunocytochemical techniques confirmed hormone localization in the amnion epithelial layer and in vascular smooth muscle and endothelial cells in vessels from the umbilical cord and placental chorionic plate. Multiplex reverse-transcriptase polymerase chain reaction identified expression of receptor messenger ribonucleic acid in vessels of the umbilical cord; this finding was verified by means of Southern gel analysis of the products of the reverse-transcriptase polymerase chain reaction. CONCLUSION: A parathyroid hormone-related protein paracrine system appears to exist in human umbilical cord. We suggest that it may be involved in the control of fetal placental circulation.
Endothelium, Vascular↗
Biochemical markers of renal osteodystrophy in pediatric patients undergoing CAPD/CCPD.
Serum intact PTH [1-84] levels were evaluated as a potential non-invasive method for the diagnosis of renal osteodystrophy in children treated with CAPD/CCPD. Sixty-eight bone biopsy samples were obtained from 55 patients, aged 13 +/- 5 (X +/- SD) years, undergoing CAPD/CCPD for 29 +/- 13 months; osteitis fibrosa was present in 34 cases, mild lesions of secondary hyperparathyroidism in six, 15 had adynamic lesions, and 13 were classified as normal histology. Serum calcium levels were higher in patients with adynamic bone or normal bone histology than in those with secondary hyperparathyroidism, whereas serum phosphorus, alkaline phosphatase and PTH levels were greater in patients with osteitis fibrosa. The combination of a serum PTH level > 200 pg/ml and a serum calcium value < 10 mg/dl was 85% sensitive and 100% specific for identifying patients with high-turnover lesions of bone. Serum PTH values < 200 pg/ml were 100% sensitive but only 79% specific for patients with adynamic bone; specificity increased to 92%, however, using the combined criteria of a PTH level < 150 pg/ml and a serum calcium level > 10 mg/dl. Higher serum calcium levels and serum PTH values within or below the normal range characterize patients with the adynamic lesion of renal osteodystrophy. Serum PTH levels of approximately 200 pg/ml are useful for distinguishing patients with low-turnover lesions of renal osteodystrophy from those with secondary hyperparathyroidism.
Adolescent↗
Development of adynamic bone in patients with secondary hyperparathyroidism after intermittent calcitriol therapy.
Intermittent calcitriol therapy is commonly used to treat secondary hyperparathyroidism in patients undergoing regular dialysis, but there is little available information about the histologic response of bone to this form of therapy. Accordingly, 14 children and adolescents with biopsy-proven secondary hyperparathyroidism were treated with intermittent oral or intraperitoneal doses of calcitriol for 12 months. Biochemical indices of mineral metabolism including serum intact PTH levels were measured monthly throughout the study, and bone biopsies were repeated at the end of treatment. Before treatment, 11 patients had osteitis fibrosa and three had mild lesions of secondary hyperparathyroidism. Histologic improvement was seen in 12 of 14 patients, and osteitis fibrosa resolved in 10 of 11 cases. Bone formation decreased in all patients during intermittent calcitriol therapy, falling from 861 +/- 380 to 150 +/- 170 microns2/mm2/day, P < 0.001. Bone formation decreased to normal in six patients, but six patients developed adynamic lesions of bone with subnormal bone formation rates. Serum PTH and alkaline phosphatase levels declined in those who developed adynamic bone, but values remained elevated in patients with normal rates of bone formation at follow-up evaluation. Neither the mean dose of calcitriol nor the average dose per kilogram body weight differed in patients with adynamic lesions. Thus, adynamic renal osteodystrophy develops in a substantial number of patients during intermittent calcitriol therapy. Although declining serum PTH and alkaline phosphatase levels suggest the development of the adynamic lesion, bone formation decreases in some patients despite persistently high serum PTH levels. Calcitriol may directly suppress osteoblastic activity in patients with secondary hyperparathyroidism when given in large doses to patients undergoing peritoneal dialysis.
Adolescent↗
The renal PTH/PTHrP receptor is down-regulated in rats with chronic renal failure.
Hypocalcemia, hyperphosphatemia, and resistance to the action of PTH are well characterized features in the setting of advanced chronic renal failure (CRF). Although the underlying mechanisms are ill-understood, clinical and experimental evidence points to both PTH receptor down-regulation and post-receptor abnormalities in their pathogenesis. In the present study we have examined the effect of advanced CRF in rats on the renal expression of PTH/PTHrP receptor (PTH-R). CRF was created by a standard two-step operation (5/6 nephrectomy). Four weeks thereafter, 19 uremic rats were compared with 23 sham-operated rats. Uremic rats had higher mean (+/- SD) plasma creatinine levels than control rats, 164 +/- 107 microM versus 43 +/- 5 microM, respectively. They also had higher plasma phosphorus and iPTH levels, 4.70 +/- 1.71 mM versus 2.59 +/- 0.37 mM and 561 +/- 336 versus 27 +/- 18 pg/ml, respectively. Mean plasma total calcium and blood ionized calcium were significantly lower in uremic than in control rats, 2.13 +/- 0.06 mM versus 2.61 +/- 0.10 mM and 1.07 +/- 0.11 versus 1.31 +/- 0.06 mM, respectively. Mean plasma calcitriol concentration was also significantly lower in uremic than in control rats, 39.8 +/- 14.6 and 80.4 +/- 15.2 pg/ml, respectively. Nine out of the 19 rats were examined for renal PTH-R gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)
Adenylyl Cyclases↗
Calcitriol therapy and calcium-regulated PTH secretion in patients with secondary hyperparathyroidism.
Calcitriol lowers serum parathyroid hormone (PTH) levels in patients with secondary hyperparathyroidism, but its effect on calcium-regulated PTH release remains controversial. Thus 11 patients with secondary hyperparathyroidism underwent dynamic tests of parathyroid function before and after 4 mo of intermittent calcitriol therapy. Serum calcitriol levels rose from 8 +/- 1 to 55 +/- 9 (SE) pg/ml, P < 0.01, serum total and ionized calcium levels increased, and serum PTH levels decreased from 584 +/- 154 to 154 +/- 31 pg/ml, P < 0.05. The maximum increase in serum PTH during hypocalcemia did not differ before (248 +/- 78 pg/ml) or after (280 +/- 100 pg/ml) treatment, but the increase in PTH, expressed as a percentage of preinfusion values, was greater after treatment (329 +/- 73 vs. 132 +/- 10%, P < 0.05). The decreases in serum PTH during calcium infusions did not differ before (70 +/- 5%) or after (73 +/- 5%) therapy, and the set point for PTH release did not change (1.20 +/- 0.03 vs. 1.23 +/- 0.01 mmol/l, not significant). Calcitriol modifies PTH secretion during hypocalcemia in secondary hyperparathyroidism without affecting the set point for PTH release; although calcitriol lowers serum PTH levels, it may also restore the secretory reserve of hyperplastic parathyroid tissues during hypocalcemia.
Adolescent↗
Parathyroid hormone induces sequential c-fos expression in bone cells in vivo: in situ localization of its receptor and c-fos messenger ribonucleic acids.
Localization of PTH/PTH-related peptide (PTH/PTHrP) receptor mRNA and PTH induction of c-fos expression were examined in bones of 4-week-old rats by in situ hybridization. Receptor transcripts were most highly expressed by growth plate chondrocytes from lower proliferating to upper hypertrophic cell layers. PTH/PTHrP receptor mRNA also was expressed in osteoblasts as well as in some bone marrow stromal cells. Subcutaneous administration of human PTH-(1-84) (225 micrograms/kg) induced rapid, transient, and sequential expression of the protooncogene c-fos mRNA in cells in bone. Osteoblasts and chondrocytes expressing PTH/PTHrP receptor transcripts as well as some stromal cells expressed c-fos mRNA first (15-60 min), followed by transient expression in the majority of stromal cells and in osteoclasts (1-2 h). Delayed and transient induction of c-fos in cells with few or no detectable receptor transcripts suggests that PTH acts indirectly on stromal cells and osteoclasts by either stimulating osteoblasts to secrete a substance(s) that acts locally and/or inducing changes in cell-cell contacts between osteoblasts and adjacent cells.
Animals↗
Regulation of parathyroid hormone (PTH)/PTH-related peptide receptor messenger ribonucleic acid by glucocorticoids and PTH in ROS 17/2.8 and OK cells.
To study mechanisms controlling the expression of PTH/PTH-related peptide (PTHrP) receptors in ROS 17/2.8 and OK cells, we investigated the regulation of PTH/PTHrP receptor availability and receptor mRNA levels by glucocorticoids and PTH. Treatment of ROS 17/2.8 cells with dexamethasone (1 microM) for 2, 4, and 6 days increased specific binding of PTH to 148 +/- 12%, 203 +/- 10%, and 344 +/- 9% (mean +/- SD), respectively, compared to that in untreated control cells. PTH-stimulated cAMP accumulation also increased with dexamethasone treatment (1 microM) from 230 +/- 15%, 382 +/- 9%, and 820 +/- 9% after 2, 4 and 6 days, respectively, compared to that in untreated cells. Treatment of ROS 17/2.8 cells with [Nle8,Nle18,Tyr34]bovine PTH-(1-34) amide (NlePTH; 100 nM) alone or together with dexamethasone (1 microM), however, markedly decreased PTH binding and PTH-stimulated cAMP accumulation. Northern blot analysis showed that dexamethasone dramatically increased steady state levels of PTH/PTHrP receptor mRNA in a time- and dose-dependent manner, which did not occur when NlePTH (100 nM) was added concomitantly to the cultures. As previously reported, daily NlePTH treatment of ROS 17/2.8 cells reduced PTH/PTHrP receptor availability and PTH-stimulated cAMP accumulation markedly within 2 days, which remained at these low levels during continued PTH treatment. In contrast, the identical treatment reduced steady state levels of PTH/PTHrP receptor mRNA in ROS 17/2.8 transiently and to only a slight extent, which then returned to pretreatment levels. Treatment of OK cells with NlePTH (100 nM) for 1, 2, and 4 days decreased PTH binding to 56 +/- 6%, 44 +/- 4%, and 64 +/- 4% (mean +/- SD) and PTH-stimulated cAMP accumulation to 42 +/- 6%, 19 +/- 4%, and 21 +/- 3% (mean +/- SD), respectively, compared to values in untreated control cells. The same treatment, however, had no significant effect on steady state levels of PTH/PTHrP receptor transcripts. In contrast to its effects in ROS 17/2.8 cells, dexamethasone (1 microM) treatment of OK cells for 1-4 days did not affect PTH binding, nor did it significantly affect steady state levels of PTH/PTHrP receptor mRNA, although the latter was slightly lowered by dexamethasone treatment. PTH-stimulated cAMP accumulation was unchanged after 1-day treatment with dexamethasone and modestly rose to 142 +/- 4% of the control value by day 4 of glucocorticoid exposure.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
The extracellular amino-terminal region of the parathyroid hormone (PTH)/PTH-related peptide receptor determines the binding affinity for carboxyl-terminal fragments of PTH-(1-34).
The recombinant human PTH/PTH-related peptide (PTHrP) receptor, when transiently expressed in COS-7 cells, binds [Nle8,18,Tyr34] bovine PTH-(7-34)amide [PTH-(7-34)], human PTH-(10-34)amide [PTH-(10-34)], and bovine PTH-(15-34)amide [PTH-(15-34)] with at least 50-fold higher affinity than does the rat receptor homolog. In contrast, PTH-(1-34) binding affinities are similar for both receptor homologs. To map those areas of the PTH/PTHrP receptors that determine the binding specificity for carboxyl-terminal fragments of PTH-(1-34), we constructed chimeric rat/human PTH/PTHrP receptors. These bound PTH-(1-34) with normal affinity and, therefore, must have an overall conformation that resembles that of native receptors. Chimeras with the amino-terminal extracellular domain of the human PTH/PTHrP receptor have a considerably higher binding affinity for PTH-(7-34), PTH-(10-34), and PTH-(15-34) than do the reciprocal receptor constructs in which the amino-terminal region is from the rat PTH/PTHrP receptor. The opossum PTH/PTHrP receptor homolog also binds PTH-(7-34) with higher affinity than the rat receptor, and studies of rat/opossum chimeras confirm the importance of the amino-terminal extracellular domain in determining the PTH-(7-34) binding specificity. Mutant rat and human PTH/PTHrP receptors in which either residues 61-105 of the extracellular region or most of the intracellular tail were deleted have PTH-(7-34) binding characteristics indistinguishable from those of either wild-type receptor. These findings indicate that the amino-terminal extracellular region of the PTH/PTHrP receptor contains a domain(s) that largely determines the binding affinity of amino-terminally truncated PTH analogs. This region, therefore, is likely to constitute a site for ligand-receptor interaction.
Animals↗
Parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor and its messenger ribonucleic acid in rat aortic vascular smooth muscle cells and UMR osteoblast-like cells: cell-specific regulation by angiotensin-II and PTHrP.
PTH-related protein (PTHrP) is produced in vascular smooth muscle, where it is believed to act as a local vasorelaxant by activating either the classical PTH or a unique PTHrP receptor. We used a newly cloned complementary DNA encoding the rat PTH/PTHrP receptor to study the expression of its messenger RNA (mRNA) in primary aortic vascular smooth muscle cells (VSMC) and in UMR-106 osteoblast-like cells under basal conditions and in response to treatment with agonists. Both cell types expressed a 2.4-kilobase PTH/PTHrP receptor mRNA transcript and exhibited hormone-induced desensitization of PTHrP-(1-34)NH2-stimulated cAMP. In VSMC, angiotensin-II, which induces PTHrP expression, also rapidly (30 min) desensitized the cAMP response and down-regulated (75-90%) receptor mRNA within 1 h. Treatment of cells with phorbol 12-myristate 13-acetate (0.1 microM) mimicked these effects, whereas neither PTHrP-(1-34)NH2, forskolin, nor (Bu)2cAMP altered receptor mRNA expression. By contrast, in UMR-106 cells, PTHrP-(1-34)NH2 induced time- and dose-dependent decreases in receptor mRNA that were preceded by pronounced desensitization (cAMP and ligand binding) of cell surface receptors. These effects were mimicked by (Bu)2cAMP and forskolin, but not by phorbol 12-myristate 13-acetate, suggesting that both receptor mRNA down-regulation and receptor desensitization in UMR cells were mediated through a protein kinase-A pathway. We suggest that VSMC and UMR cells express a common receptor, which is subject to cell-specific regulation. Such diversity in the PTH/PTHrP receptor regulatory mechanisms provides a means for restricting the length and duration of the cellular response to hormone in a cell/tissue-specific manner.
Angiotensin II↗
Determinants of [Arg2]PTH-(1-34) binding and signaling in the transmembrane region of the parathyroid hormone receptor.
Previously, we reported that [Arg2]PTH-(1-34) bound to the rat osteosarcoma cell line, ROS 17/2.8, with 2-fold higher apparent affinity than it did to the opossum kidney cell line, OK, yet the analog was only a weak partial agonist for cAMP stimulation with ROS 17/2.8 cells, whereas it was a full cAMP agonist with OK cells. These results suggested that the rat and opossum PTH receptors differ in a region recognized by the hormone's amino-terminus. In this report we show that the cloned PTH receptors derived from ROS 17/2.8 and OK cells, expressed in COS-7 cells, also displayed altered responses to [Arg2]PTH-(1-34). Thus, [Arg2]PTH-(1-34) bound to the cloned rat PTH receptor with 7-fold higher affinity than it did to the cloned opossum PTH receptor, and in cAMP stimulation assays, it was a much weaker agonist with the rat receptor than it was with the opossum receptor. Studies with rat/opossum PTH receptor chimeras suggested that the membrane-spanning region of the receptor contributed to the different binding and signaling responses to [Arg2]PTH-(1-34). Point mutation analysis identified three sites in or near the extracellular ends of transmembrane domains V and VI, which specifically affected [Arg2]PTH-(1-34) binding and signaling.
Amino Acid Sequence↗
Role of the extracellular regions of the parathyroid hormone (PTH)/PTH-related peptide receptor in hormone binding.
The PTH/PTH-related peptide receptor is a member of a newly discovered family of G-protein-coupled receptors. Strikingly conserved features among these receptors include the positioning of eight extracellular cysteines and several other residues that are located predominantly within the membrane-embedded region. Deletion mutants or receptors with point mutations of the highly conserved cysteine residues were transiently expressed in COS-7 cells to evaluate PTH binding and PTH-stimulated cAMP production. Deletion of residues 61-105, which are encoded by exon E2 in the PTH/PTH-related peptide receptor gene, did not affect receptor function. An epitope derived from Haemophilus influenza hemagglutinin was, therefore, introduced into this portion of most receptors to allow the independent assessment of cell surface expression. PTH binding capacity was not reduced by the deletion of residues 258-278 in the first extracellular loop. Receptors with deletion of either residues 31-47 in the amino-terminal extension or residues 431-440 in the third extracellular loop failed to bind PTH, although expression of the receptor on the cell surface was only marginally reduced. Most other receptor mutants, including those in which each of the six cysteines in the amino-terminus was replaced by serines, failed to be processed and/or expressed appropriately, whereas the substitution of cysteine-281 or -351 had a less severe effect. The combined replacement of both cysteines concomitantly increased PTH binding and cell surface expression, suggesting the formation of a disulfide bond between these two residues. Our data indicate that residues near the amino-terminus and within the third extracellular loop are necessary for ligand binding, whereas more than 25% of the receptor's extracellular region appears not to be involved.
Amino Acid Sequence↗
Down-regulation of parathyroid (PTH)/PTH-related peptide receptor immunoreactivity and PTH binding in opossum kidney cells by PTH and dexamethasone.
Recent data have shown that PTH down-regulation of its receptor on opossum kidney (OK) cells is not associated with any change in the steady state level of the PTH/PTH-related peptide (PTHrP) receptor messenger RNA. For analysis of down-regulation of the PTH/PTHrP receptor in OK cells, the present work uses a specific receptor anti-serum, SR-2, that is useful for detection and quantification of PTH/PTHrP receptor immunoreactivity on intact cells bearing the opossum PTH/PTHrP receptor. SR-2 specifically binds to COS-7 cells transiently expressing the opossum PTH/PTHrP receptor complementary DNA (OK-O), to LLCPK1 cells stably expressing the recombinant opossum PTH/PTHrP receptor (AOK cells), and to OK cells expressing endogenous PTH/PTHrP receptors, but not to mock-transfected COS-7 cells or untransfected LLCPK1 cells. SR-2 binding was also linearly correlated with PTH binding in COS-7 cells transfected with different amounts of OK-O plasmid DNA. Treatment with PTH (100 nM) for 4 and 6 h did not significantly down-regulate the PTH/PTHrP receptor immunoreactivity, although PTH binding was decreased to 51% and 49% of control, respectively, and PTH-stimulated cAMP accumulation was decreased to 27% and 28% of control, respectively. Treatment with PTH (100 nM) for 24 and 48 h significantly decreased PTH binding to 51% and 60% of control and decreased PTH/PTHrP receptor immunoreactivity to 68% and 58% of control, respectively. Incubation of OK cells with 0.1 nM to 1 microM PTH for 4 h did not down-regulate the PTH/PTHrP receptor immunoreactivity, although PTH binding was decreased dramatically. Scatchard blot analysis revealed that the binding affinity was decreased by 7-fold in OK cells treated with PTH for 4 h without change in receptor number. Conversely, treatment of OK cells with PTH for 24 h resulted in a parallel decrease in both receptor number and receptor immunoreactivity without any change in receptor binding affinity. Treatment of OK cells with dexamethasone (0.1 nM to 1 microM) had no effect on PTH binding or PTH/PTHrP receptor immunoreactivity. Incubation of OK cells with both dexamethasone (1 microM) and PTH (0.1 nM to 1 microM), however, caused a significantly greater down-regulation of both PTH binding and PTH/PTHrP receptor immunoreactivity than in cells treated with PTH alone. These data indicate that during the first 4 h of exposure of OK cells to PTH, PTH/PTHrP receptors remain on the cell surface but have lowered affinity to bind the ligand and that dexamethasone potentiates the effect of PTH on PTH/PTHrP receptor down-regulation in OK cells.
Animals↗