Identification and characterization of parathyroid hormone receptors on dog kidney, human kidney, chick bone and human dermal fibroblast. A comparative study of functional and structural properties

Anca in Dialysis Patients: A Role for Bioincompatibility?

Background

Anti-neutrophil cytoplasmic autoantibodies (ANCA) have been described in patients suffering from systemic vasculitis such as Wegener granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome and other pathological conditions. In this paper we report a greater incidence of ANCA in hemodialysis patients as compared to peritoneal dialysis patients, predialytic uremic patients and non-renal patients; a possible role for dialysis bioincompatibility in ANCA generation was also investigated.

Methods

A total of 335 uremics in substitutive treatment (176 in hemodialytic treatment and 159 in peritoneal dialysis) were examined for ANCA positivity. A total of 189 patients with advanced renal failure in conservative treatment and 100 healthy subjects were used as control. The dialysis techniques were standard hemodialysis (n = 119), low volume hemodiafiltration (n = 26) and hemofiltration (n = 31). ANCA positivity was examined by immunofluorescence (IF): diffuse finely granular staining was considered as classical positive reaction (C-ANCA) and P-ANCA was diagnosed if a perinuclear staining was observed. EIA for proteinase-3 (anti PR-3) and myeloperoxidase-antibodies (anti-MPO) were also performed.

Results

In non-renal patients and in patients with pre-dialytic renal insufficiency none were found ANCA positive. In peritoneal dialysis patients all but one were ANCA negative with IF, with all EIA test resulting negative. In hemodialytic patients, a positive IF test was found in 26 (14.7%) for P-ANCA and in 5 (2.8%) for C-ANCA; using the EIA test 23 (13%) patients were positive for MPO and 12 (6.8%) for PR-3.

Conclusions

No correlation with age, primary renal diseases, dialytic age, dialysis membrane materials was found; regarding the different extracorporeal dialytic techniques a higher incidence (p < 0.02) was detected in patients undergoing HDF. Backfiltration of contaminated dialysate may induce ANCA via an increased cytokine generation. (Int J Artif Organs 2000; 23: 97–103)

Stk11 (Lkb1) deletion in the osteoblast lineage leads to high bone turnover, increased trabecular bone density and cortical porosity

The mTOR pathway couples energy homeostasis to growth, division and survival of the cell. Stk11/Lkb1 is a critical serine-threonine protein kinase in the inhibition of mTOR pathway action. In the mammalian skeleton, Stk11 regulates the transition between immature and hypertrophic chondrocytes. Here, we have focused on the action of Stk11in the osteoblast lineage through osteoblast specific-removal of Stk11 activity. In the mouse model system, specification and primary organization of the neonatal boney skeleton is independent of Stk11. However, histological, molecular and micro-CT analysis revealed a marked perturbation of normal bone development evident in the immediate post-natal period. Cortical bone was unusually porous displaying a high rate of turnover with new trabeculae forming in the endosteal space. Trabecular bone also showed enhanced turnover and marked increase in the density of trabeculae and number of osteoclasts. Though mutants showed an expansion of bone volume and trabecular number, their bone matrix comprised large amounts of osteoid and irregularly deposited woven bone highlighted by diffuse fluorochrome labeling. Additionally, we observed an increase in fibroblast-like cells associated with trabecular bone in Stk11 mutants. Stk11 down-regulates mTORC1 activity through control of upstream modulators of the AMP kinase family: an increase in the levels of the phosphorylated ribosomal protein S6, a target of mTORC1-mediated kinase activity, on osteoblast removal of Stk11 suggests deregulated mTORC1 activity contributes to the osteoblast phenotype. These data demonstrate Stk11 activity within osteoblasts is critical for the development of normally structured bone regulating directly the number and coordinated actions of osteoblasts, and indirectly osteoclast number.

Benign parathyroid adenoma presenting with unusual parathyroid crisis, anemia and myelofibrosis

Although the clinical symptoms of patients with benign parathyroid adenoma are usually nonspecific and benign, a malignant presentation of the benign disease may sometimes occur. Here, we report a case of a 58-year-old woman who presented with aggravated sacrum pain, general malaise, and polydipsia. Initial laboratory findings revealed hypercalcemia, normocytic anemia, and impaired renal function. Acute hypercalcemic crisis manifested and primary hyperparathyroidism was diagnosed together with myelofibrosis on account of the result of bone marrow biopsy. Excision of a parathyroid adenoma was performed, and the anemia and bone marker regressed later. These findings suggested that benign parathyroid adenoma may mimic the clinical presentation of parathyroid carcinoma, releasing excess parathyroid hormone and resulting in hyperparathyroid crisis. In addition, primary hyperparathyroidism can be associated with anemia and myelofibrosis.

Evidence that the cells responsible for marrow fibrosis in a rat model for hyperparathyroidism are preosteoblasts

We examined proliferation of cells associated with PTH-induced peritrabecular bone marrow fibrosis in rats as well as the fate of those cells after withdrawal of PTH. Time-course studies established that severe fibrosis was present 7 d after initiation of a continuous sc PTH infusion (40 microg/kg.d). To ascertain cell proliferation, rats were coinfused for 1 wk with PTH (treated) or vehicle (control) and [3H]thymidine (1.5 mCi/rat). Groups of control and treated rats were killed immediately (d 0) and 1 wk (d 7) later. Few osteoblasts (Obs) and osteocytes in treated and control groups were radiolabeled on d 0. Peritrabecular cells expressing a fibroblastic (Fb) phenotype and surrounded by an extracellular matrix were not present in controls on either d 0 or d 7. Multiple cell layers of Fbs lined most (70%) of the bone surface on d 0 in treated rats and nearly all (85%) of the Fbs were radiolabeled. Fbs had entirely disappeared from bone surfaces on d 7. Eighty-five percent of the Obs on and 73% of the osteocytes within the active remodeling sites were radiolabeled. Immunohistochemistry revealed that Fbs induced by PTH treatment produced osteocalcin, osteonectin, and core binding factor-alpha1. These data provide compelling evidence that Fbs recruited to bone surfaces in response to a continuous PTH infusion undergo extensive proliferation, express osteoblast-specific proteins, and produce an extracellular matrix that is similar to osteoid. After restoration of normal PTH levels, Fbs differentiated to Obs, providing further evidence that Fbs are preosteoblasts.

Stimulation of protein kinase C activity in cells expressing human parathyroid hormone receptors by C- and N-terminally truncated fragments of parathyroid hormone 1-34

The parathyroid hormone (PTH) fragment PTH(1-34) stimulates adenylyl cyclase, phospholipase C (PLC), and protein kinase C's (PKCs) in cells that express human, opossum, or rodent type 1 PTH/PTH-related protein (PTHrP) receptors (PTHR1s). Certain carboxyl (C)-terminally truncated fragments of PTH(1-34), such as human PTH(1-31) [hPTH-(1-31)NH2], stimulate adenylyl cyclase but not PKCs in rat osteoblasts or PLC and PKCs in mouse kidney cells. The hPTH(1-31)NH2 peptide does fully stimulate PLC in HKRK B7 porcine renal epithelial cells that express 950,000 transfected hPTHR1s per cell. Amino (N)-terminally truncated fragments, such as bovine PTH(3-34) [bPTH(3-34)], hPTH(3-34)NH2, and hPTH(13-34), stimulate PKCs in Chinese hamster ovary (CHO) cells expressing transfected rat receptors, opossum kidney cells, and rat osteoblasts, but an intact N terminus is needed to stimulate PLC via human PTHR1s in HKRK B7 cells. We now report that the N-terminally truncated analogs bPTH(3-34)NH2 and hPTH(13-34)OH do activate PKC via human PTHR1s in HKRK B7 cells, although less effectively than hPTH(1-34)NH2 and hPTH(1-31)NH2. Moreover, in a homologous human cell system (normal foreskin fibroblasts), these N-terminally truncated fragments stimulate PKC activity as strongly as hPTH(1-34)NH2 and hPTH(1-31)NH2. Thus, it appears that unlike their opossum and rodent equivalents, hPTHR1s can stimulate both PLC and PKCs when activated by C-terminally truncated fragments of PTH(1-34). Furthermore, hPTHR1s, like the PTHR1s in rat osteoblasts, opossum kidney cells, and rat PTHR1-transfected CHO cells also can stimulate PKC activity by a mechanism that is independent of PLC. The efficiency with which the N-terminally truncated PTH peptides stimulate PKC activity depends on the cellular context in which the PTHR1s are expressed.

Effects of excess PTH on nonclassical target organs

The classical target organs for parathyroid hormone (PTH) are the bone and kidneys. In uremia, however, numerous studies have shown that PTH may also affect the function of a number of nonclassical organs and tissues besides the bone and kidney, including the brain, heart, smooth muscles, lungs, erythrocytes, lymphocytes, pancreas, adrenal glands, and testes. Most of these effects do not apply to the generally accepted actions or normal regulatory mechanisms of PTH. Thus, the potential role of PTH as one of the possibly many toxins in uremia is of current interest. The molecular basis for the actions of elevated PTH levels on various nonrenal and nonskeletal organs or tissues might be mediated via the widespread distribution of the classical PTH/PTH-related peptide (PTHrP) receptors and via the novel PTH2 receptors. The present survey deals with an evaluation of the nonrenal and nonskeletal effects of excess PTH in uremia, taking into consideration the presently available information on the organ-specific expression of the classical and novel PTH receptors, and of the expression and function of PTHrP.

Parathyroid hormone binding to cultured avian osteoclasts

Parathyroid hormone (PTH) increases serum calcium concentration via a controversial cellular mechanism. We investigated whether PTH binds avian osteoclasts. Isolated hypocalcaemic hen osteoclasts were incubated with [125I]--bovine PTH (1-84). Specific binding of the hormone to the cells, which reached the equilibrium within 60 min, was observed. Half maximal binding was reached by 10 min. Binding was competitively inhibited by increasing doses of unlabeled PTH, and was about 55% displaced by adding, at the equilibrium, 10(-6) M unlabeled PTH. Autoradiography demonstrated specific label on the osteoclast. The cellular mechanism activated by the hormone remains to be elucidated.

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

Our earlier results indicated that the binding moiety of the renal PTH receptor is an 85 kD protein that is susceptible to proteolytic cleavage to a 70 kD form that supports high-affinity binding and Gs coupling, and to a 50-55 kD form that contains the ligand binding domain but does not couple to Gs. In the present study we used [125I]hPTHrP-(1-34)amide and a chemical cross-linking technique to discern the structural features of the intact 85 kD PTH/PTHrP receptor that are retained in the proteolyzed forms to "structurally map" the receptor. The results of lectin chromatography and endoglycosidase treatment show that the partially proteolyzed receptor forms retain the complex, N-linked glycans present on the intact receptor. This conclusion is further supported by the finding that wheat germ agglutinin was equally effective at competitively inhibiting specific [125I]hPTHrP-(1-34)A binding to the 70 kD form and the intact 85 kD receptor. Specific binding of [125I]hPTHrP-(1-34)A to the intact 85 kD receptor or to the 70 kD form was completely abolished by treatment with disulfide reducing agents, and both partially proteolyzed receptor forms (70- and 50 kD) were shown to retain the small (less than or equal to 14 kD) labeled fragment that is released from the intact receptor by disulfide reduction. Lectin chromatography and endoglycosidase treatment revealed that the less than or equal to 14 kD receptor component is not glycosylated. The less than or equal to 14 kD fragment does not contain a transmembrane spanning region, as its release from the membrane can be affected without detergent solubilization. Identical partial proteolytic maps of the receptor were obtained whether the receptor was covalently labeled with [125I]hPTHrP-(1-34)amide or [125I]bPTH-(1-34). These results suggest a model of the renal PTH/PTHrP receptor binding moiety as a single-chain protein in which the sites of glycosylation, ligand binding, and the functionally critical disulfide bonds are in extracellular domains near one end of the protein and the sites of proteolysis reside near the other end of the protein. These studies also provide further confirmation that PTH and PTHrP bind to a structurally indistinguishable renal receptor and validate the use of PTHrP as a ligand for studies designed to characterize and purify the PTH receptor.

Desensitization of parathyroid hormone receptors on cultured bone cells

Administration of excessive amounts of parathyroid hormone (PTH) in the treatment of osteoporosis can reverse the beneficial effects of a low-dose, intermittent regime. To investigate the direct actions and the possible cellular mechanisms of PTH in inducing desensitization of PTH receptors, we studied the effects of desensitization on rat osteoblastic UMR-106 cells. When the osteoblasts were preincubated with bPTH-(1-34), complete refractoriness to a subsequent challenge with the hormone developed within 1 h and at hormone concentrations as low as 5 nM. When osteoblasts thus desensitized were incubated in hormone-free medium, recovery of the cAMP responses began within 2 h and reached maximum after 16 h. Cycloheximide did not affect the process of desensitization. [Nle8,Nle18,Tyr34]bPTH-(3-34)amide significantly impaired the desensitization process by PTH-(1-34) but did not have stimulatory effect on cAMP responses. No significant heterologous desensitization was obvious after preincubation with isoprenaline (50 microM), prostaglandin E1 (50 microM), or prostaglandin E2 (50 microM) for 2 h. Binding experiments with [125I]PLP-(1-36)amide after desensitization revealed that there was an approximate twofold decrease in receptor affinities as analyzed by Scatchard analysis, showing that the decrease in affinity was prominent in the process of desensitization. When the cells were treated with monensin during desensitization, PTH challenge after desensitization produced significantly lower cyclic AMP responses. Recovery after desensitization occurred over a period of 16 h. Inclusion of monensin, but not cycloheximide, impaired the recovery. The results show that homologous desensitization of rat osteoblasts to PTH is brought about by the occupancy of receptors by PTH-(1-34) but not by cAMP generation itself.(ABSTRACT TRUNCATED AT 250 WORDS)