Synthesis of a gene encoding parathyroid hormone-like protein-(1-141): purification and biological characterization of the expressed protein

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

Production and characterisation of gilthead sea bream (Sparus auratus) recombinant parathyroid hormone related protein

The production and purification of gilthead sea bream recombinant parathyroid hormone related protein [sbPTHrP(1-125)] using an Escherichia coli system and one step purification process with continuous elution gel electrophoresis is reported. The cDNA encoding sbPTHrP(1-125) was cloned into a prokaryotic expression vector pET-11a. The recombinant plasmid was used to transfect E. coli BL21(DE3) pLysS and sbPTHrP(1-125) synthesis was induced by addition of 1mM isopropyl-beta-d-thiogalactopyranoside. The rapid one step isolation method gave pure sbPTHrP(1-125) as judged by SDS-PAGE and yielded up to 40mg/L of culture medium (3.3mg protein/g of bacteria). The bioactivity of recombinant sbPTHrP(1-125) assessed using an in vitro scale bioassay was found to be equipotent to PTHrP(1-34) in stimulating cAMP accumulation. Assessment of the immunological reactivity of the isolated protein by Western blot revealed it cross-reacts with antisera specific for the N-terminal and C-terminal region of PTHrP. In a radioimmunoassay specific for piscine N-terminal (1-34aa) PTHrP, the recombinant sbPTHrP(1-125) was equipotent with PTHrP(1-34) in displacing labelled (125)I-PTHrP(1-36) PTHrP from the antisera. The availability of recombinant sbPTHrP will allow the development of region specific assays and studies aimed at defining post-secretory processing of this protein and its biological activity in fish.

Expression and characterization of recombinant rat parathyroid hormone-related peptide (1-141) and an amino-terminally-truncated analogue (38-141)

We have synthesized and purified recombinant parathyroid hormone related peptide (PTHrP (1-141)) and PTHrP (38-141) using an E. coli system that requires minimal purification. The cDNAs encoding PTHrP (1-141) and PTHrP (35-141) respectively were inserted into the multiple cloning site of the pTrcHis-B bacterial expression plasmid. The PTHrP encoded sequences were thereby fused at their NH2-termini to six histidine residues within the fusion protein. The recombinant plasmids were transfected into E. coli cells and PTHrP synthesis was induced by addition of 1 mM isopropyl-beta-D-thiogalactopyranoside (IPTG) at 37 degrees C. The recombinant fusion proteins were purified by binding of the histidine residues to a nickel column followed by gradient elusion and dialysis. PTHrP (1-141) was released from its fusion protein by cyanogen bromide cleavage, whereas PTHrP (38-141) was released by enzymatic digestion with enterokinase. This rapid isolation method resulted in pure PTHrP (1-141) and (38-141) as judged by SDS-polyacrylamide gel electrophoresis and NH2-terminal sequence analysis. PTHrP (1-141) stimulated cAMP accumulation and mobilized intracellular calcium ([Ca2+]i) in UMR106 osteoblast-like cells, and stimulated phosphate transport in OK/E renal cells, whereas PTHrP (38-141) was inert in these bioassays. Availability of PTHrP and its NH2-terminally truncated analogue, which lacks the sequence necessary for its hypercalcemic actions, will enable their biological activities to be examined in greater detail.

Nucleolar localization of parathyroid hormone-related peptide enhances survival of chondrocytes under conditions that promote apoptotic cell death

Parathyroid hormone-related peptide (PTHrP) is a mediator of cellular growth and differentiation as well as a cause of malignancy-induced hypercalcemia. Most of the actions of PTHrP have been attributed to its interaction with a specific cell surface receptor that binds the N-terminal domain of the protein. Here we present evidence that PTHrP promotes some of its cellular effects by translocating to the nucleolus. Localization of transiently expressed PTHrP to the nucleolus was dependent on the presence of a highly basic region at the carboxyl terminus of the molecule that bears homology to nucleolar targeting sequences identified within human retroviral (human immunodeficiency virus type 1 and human T-cell leukemia virus type 1) regulatory proteins. Endogenous PTHrP also localized to the nucleolus in osseous cells in vitro and in vivo. Moreover, expression of PTHrP in chondrocytic cells (CFK2) delayed apoptosis induced by serum deprivation, and this effect depended on the presence of an intact nucleolar targeting signal. The present findings demonstrate a unique intracellular mode of PTHrP action and a novel mechanism by which this peptide growth factor may modulate programmed cell death.

Parathyroid hormone-related protein

We review the current state of knowledge of the molecular properties and actions of parathyroid hormone-related protein (PTHrP) both in cancer patients and in normal physiology. PTHrP is a common product of squamous cancers and is the major mediator of the syndrome of humoral hypercalcemia of malignancy (HHM) by its actions through parathyroid hormone receptors in bone and kidney. Recently developed radioimmunoassays and tissue localization techniques indicate that PTHrP is produced by many more cancers than was originally indicated by clinical studies and that it contributes significantly to malignancy-related hypercalcemia associated with other etiologies, for example, cancers metastatic to bone and hematological malignancies. The gene encoding PTHrP is complex, with multiple exons coding for up to 12 alternate transcripts and three different length proteins, potentially in a tissue-specific manner, by the use of three promoters. Its expression is regulated by hormones and growth factors, and the untranslated exons display features in common with many cytokine genes. Although potential endocrine actions of PTHrP are evident in fetal development, further evidence suggesting that the normal physiological role of PTHrP is predominantly as a locally produced regulator/cytokine comes from localization studies and investigations of its actions in a variety of tissues. Such studies indicate that in addition to its parathyroid hormone-like actions, PTHrP has multiple activities, including those in fetal development, placental calcium transfer, lactation, smooth muscle relaxation, and on epithelial cell growth. Although PTHrP was discovered because of its production by cancers, evidence for its actions as a local regulator highlights the importance of understanding its roles not only in the etiology of HHM in cancer patients but also in normal tissues.

Tartrate resistant acid phosphatase activity in rat cultured osteoclasts is inhibited by a carboxyl terminal peptide (osteostatin) from parathyroid hormone-related protein

A carboxyl-terminal peptide sequence ("osteostatin") from parathyroid hormone related protein has been shown to have an inhibitory effect on osteoclastic bone resorption--an action opposite to its amino-terminal sequence. In this study, we proposed that inhibition of osteoclastic bone resorption by osteostatin was associated with reduction of tartrate resistant acid phosphatase (TRAcP) activity in osteoclasts. Our results have indicated that osteostatin reduced TRAcP activity in a dose dependent manner. This effect of osteostatin was both sensitive (half maximal effect approximately 5 x 10(-13) M) and potent (maximum inhibition approximately 50% of control). In the first 90 min of treatment, however, reduction of TRAcP activity was erratic but became persistent and progressive when the time course was extended. Moreover, throughout the experimental period the levels of TRAcP activity in the culture medium had fallen significantly. It appears that osteostatin has a biphasic effect on TRAcP activity, inhibiting its secretion and either suppressing its synthesis or increasing its degradation. In addition, osteostatin induced rapid cellular retraction of both human and rat cultured osteoclasts, which was morphologically distinct from that produced by calcitonin.

Comparison of the effects of various lengths of synthetic human parathyroid hormone-related peptide (hPTHrP) of malignancy on bone resorption and formation in organ culture

Parathyroid hormone-related peptide (PTHrP) has been shown to be the pathogenic agent in humoral hypercalcemia of malignancy (HHM), but the molecular forms that are secreted have not been fully characterized. PTHrP 1-34 has effects similar to parathyroid hormone (PTH), but C-terminal regions of the peptide, such as the 107-139 fragment found to inhibit resorption in a study by Fenton et al (1991), may have other biological activities not shared with PTH. We have compared the effects of the longer forms of recombinant human PTHrP (hPTHrP 1-84, 1-108, and 1-141) with hPTHrP 1-34 and synthetic bovine PTH (bPTH) 1-34 on bone resorption and formation in cultured neonatal mouse calvariae and fetal rat long bones. hPTHrP 1-84, 1-108, and 1-141 were qualitatively similar to hPTHrP 1-34 and PTH 1-34 in stimulating 45Ca release from both neonatal mouse calvariae and fetal rat long bones and in inhibiting the incorporation of [3H]-proline into collagenase digestible protein (CDP) and stimulating the incorporation of [3H]-thymidine (3H-TdR) in neonatal mouse calvariae. However, hPTHrP 1-108 and 1-141 were less potent at stimulating 45Ca release and inhibiting CDP labeling than hPTHrP 1-34, while hPTHrP 1-84 showed an intermediate potency. Since hPTHrP 1-108 and 1-141 were quite similar in potency, the difference cannot be attributed to an inhibitory effect of the 107-139 fragment. All the peptide lengths tested showed similar potency in stimulating [3H]-TdR incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of carboxy-terminal fragments of parathyroid hormone-related protein in renal failure

We have recently demonstrated elevations of separate amino- and carboxy-terminal parathyroid hormone-related protein (PTHrP) fragments in patients with humoral hypercalcemia of malignancy (HHM) using both a two-site immunoradiometric assay (IRMA) with amino-terminal specificity for PTHrP, and with a carboxy-terminal radioimmunoassay (RIA) for PTHrP(109-138). PTHrP(109-138) immunoactivity from plasma of patients with HHM could not be extracted using an amino-terminal PTHrP immunoaffinity column, indicating that the carboxy-terminal region circulates as a discrete peptide. Carboxy-terminal immunoreactive (i) PTHrP levels were also elevated in normocalcemic patients with chronic renal failure (without cancer), whereas amino-terminal iPTHrP levels were normal in patients with renal failure. In order to further define the renal handling of carboxy-terminal PTHrP peptides, we have evaluated circulating iPTHrP(109-138) concentrations in patients with a wide range of renal function. We studied 25 patients with abnormal renal function of diverse etiologies whose creatinine clearances ranged from 66 ml/min to less than 5 ml/min. All patients had undetectable or low (< or = 2 pmol/liter) concentrations of iPTHrP(1-74). iPTHrP(109-138) concentrations were undetectable in patients with creatinine clearances > or = 20 ml/min, but became elevated in patients with creatinine clearances < 20 ml/min. The log of iPTHrP(109-138) correlated negatively with the log of creatinine clearance (r = 0.88, P = 0.0001). Mean iPTHrP(109-138) levels were slightly higher for patients on hemodialysis (32.7 +/- 3.1 pM) than for those on chronic ambulatory peritoneal dialysis (22.1 +/- 3.4 pM; P < 0.05), suggesting that some carboxy-terminal PTHrP fragments may be cleared to a greater extent by the peritoneal membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of human parathyroid-hormone-related protein(1-141) in Saccharomyces cerevisiae. A correct amino-terminal processing vital for the hormone's biological activity is obtained by an ubiquitin fusion protein approach

Gene fusions have been widely used in heterologous expression systems as a technique to stabilize the recombinant product against proteolysis, increase the translational initiation efficiency or to serve as an affinity handle for the purification of the protein. A further advantage is the potential to generate an authentic amino terminus of the foreign protein when this is vital for its biological activity, such as for the ability of human parathyroid-hormone-related protein (hPTHrP) to mediate activation of adenylate cyclase. We report here the construction and utility of a ubiquitin fusion protein system for production of the otherwise short-lived hPTHrP(1-141) as a carboxyl extension to ubiquitin in yeast. A hybrid gene containing the hPTHrP(1-141) cDNA coding region fused in-frame to the 3' end of the yeast ubiquitin cDNA was constructed and expressed under the control of the regulatable yeast metallothionein promoter. The recombinant protein was purified to homogeneity and finally characterized by N-terminal amino acid sequencing and amino acid composition analysis, demonstrating that the fusion protein was cleaved correctly and quantitatively in vivo by an ubiquitin-specific yeast endoprotease to generate authentic hPTHrP(1-141). hPTHrP(1-141) stimulated adenylate cyclase in rat osteosarcoma cell membranes to the same extent as equimolar amounts of recombinant human parathyroid hormone(1-84) and [Tyr34]hPTHrP(1-34)amide. Thus, this expression cloning strategy permits the production of authentic, biologically active recombinant hPTHrP(1-141), and the procedure can easily be adapted to make PTHrP analogues for further studies of its domain-specific activities and biological roles.

Parathyroid hormone-related protein stimulates prostaglandin E2 release from human osteoblast-like cells: modulating effect of peptide length

Parathyroid hormone-related protein (PTHrP) is a potent bone-resorbing protein that frequently mediates the humoral hypercalcemia of malignancy syndrome. Since prostaglandins may mediate the bone-resorptive action of certain hormones, we examined the effect of PTHrP on prostaglandin E2 (PGE2) secretion by human osteoblast-like cells. There was low-level basal secretion of PGE2 by Saos-2 cells (8.1 +/- 0.6 pg/ml). Using four different preparations of PTHrP, it was observed that with increasing peptide length, from 36 to 141 amino acids, a significant increase in efficacy for PGE2 release was seen in these cells. All forms of PTHrP were agonists for PGE2 release, with effects seen at concentrations as low as 10(-12) M in 48 h conditioned media. The amino terminus of the molecule appeared critical for this effect since the truncated derivative PTHrP-(7-34) did not induce significant PGE2 secretion. However, the influence of peptide length could not be explained by differential activation of adenylate cyclase since [Tyr36]PTHrP-(1-36)amide was equipotent to the longest peptide preparation, PTHrP-(1-141), in stimulating cyclic AMP accumulation in the Saos-2 cells. In contrast, PTHrP-(1-141) was significantly more effective than [Tyr35]PTHrP-(1-36)-amide in inducing a rise in cytosolic calcium. Further, this effect was noted at concentrations lower than those that caused significant cyclic AMP accumulation in the Saos-2 cells. PTHrP-(1-141) induced the release of PGE2 from primary human bone cell cultures to levels entirely comparable to those seen in the Saos-2 cells. PTHrP-(1-141) also induced PGE2 release by cultured fetal rat long bones at 72 h. We conclude that the carboxy-terminal region of PTHrP has important effects on cellular signal transduction pathways and on the release of a potent bone-active cytokine, PGE2.

Synthetic parathyroid hormone-like protein (1-74) is anabolic for bone in vivo

Parathyroid hormone-related protein (PTHRP) has recently been purified from human tumors associated with the syndrome of humoral hypercalcemia of malignancy. The gene encoding PTHRP has been cloned, and based on predicted amino acid sequence, polypeptides comprising the first 36 [36Tyr(1-36) PTHRP amide] and 74 [(1-74)PTHRP] amino acids have been synthesized. Human (h) PTHRP (1-36) and (1-74) are potent bone-resorbing agents, and are catabolic for bone in vivo when given continuously at high doses. Bovine parathyroid hormone (bPTH) (1-34) is also catabolic for bone at high dose levels, but when given in low doses for weeks to months, it is anabolic. Although PTHRP possess several PTH-like properties in bone, hPTHRP (1-34) is reported to be only weakly anabolic in vivo. As polypeptide length influences PTHRP action, we evaluated hPTHRP(1-74) as an anabolic agent for bone in vivo. Twenty-four 4-week-old male Sprague-Dawley rats were given daily subcutaneous injections of hPTHRP(1-74) (1 and 2 nmol/100 g body weight, bw), bPTH(1-34) (4 nmol/100 g bw) or vehicle. Rats were sacrificed on day 12, and serum calcium, phosphorus, and 1,25 dihydroxyvitamin D and femoral bone dry weight, calcium content, and hydroxyproline content were measured. Serum calcium and phosphorus were equivalent in all groups. A significant increase in dry bone weight was observed in both PTHRP-treated groups compared with controls. PTHRP also caused a significant, dose-dependent increase in bone calcium and hydroxyproline content. Results of these studies indicate that PTHRP (1-74) is anabolic for bone in vivo when administered at low-dosage levels for a prolonged period.

Parathyroid hormone-related peptide in human milk measured by a mid-molecule radioimmunoassay

Several peptide hormones and growth factors have been found in human milk, and we present here the results of measurements of parathyroid hormone-related peptide (PTHrP). A radioimmunoassay (RIA) using a polyclonal antiserum against the mid-region of the molecule has been developed. In milk collected during the first 6 days after parturition, the PTHrP concentrations showed large interindividual variations ranging from 0.3 to 13.7 nmol-Eq/L (0.5 to 24.4 ng-Eq/mL) (n = 67) and increased between days 3 and five postpartum. PTHrP also increased during the first 4 collecting days when measured in milk from the same mother during a prolonged period. On fast-protein liquid chromatography (FPLC), the bulk of PTHrP eluted with a molecular weight of approximately 10 to 12 kd after treatment with urea. After mid-molecule immunoaffinity extraction of PTHrP from milk, higher levels were obtained by the mid-molecule RIA than by an aminoterminal assay, indicating that all fragments did not contain the aminoterminal. Parts of immunoextracted milk PTHrP stimulated cyclic adenosine monophosphate (cAMP) production in rat osteosarcoma cell line, UMR-106. In conclusion, we have found PTHrP-like immunoreactivity in human milk using a mid-region RIA. Parts of the immunoextracts also contained the aminoterminal and possessed PTH-like bioactivity. Whether PTHrP in human milk plays a physiological role in the maternal breast or in the newborn gastrointestinal tract is unknown, but the present observations demonstrate that a portion of the PTHrP is at least potentially biologically active.

Stimulation by parathyroid hormone-related protein and transforming growth factor-alpha of phosphate transport in osteoblast-like cells

Parathyroid hormone (1-34) [PTH-(1-34)] has been shown to stimulate sodium-dependent phosphate transport (NaPiT) in UMR-106 osteoblast-like cells through a cAMP-dependent mechanism. Whether a synthetic amino-terminal fragment of parathyroid hormone-related protein (PTHrP) or the full-length molecule, which are recognized to interact with the same receptor as PTH, affect NaPiT in the same way is not known. We investigated and compared the effects of bPTH-(1-34), PTHrP-(1-34), and PTHrP-(1-141) on NaPiT and cAMP production in the osteoblastic cell line UMR-106. Each of the three peptides increased cAMP production and exerted a concentration-dependent stimulation of NaPiT after incubation for 4-6 h. We also studied the effect of transforming growth factor-alpha (TGF-alpha), which is another tumoral product secreted by certain hypercalcemia-associated tumors, on NaPiT and the TGF-alpha-induced modulation of the response to PTHrP or PTH. TGF-alpha caused a 30% stimulation of NaPiT, which remained stable from 6 to 24 h, by a cAMP-independent mechanism. In contrast, TGF-alpha attenuated cAMP production stimulated by PTH, PTHrP-(1-34), or PTHrP-(1-141). PTHrP or PTH did not further increase NaPiT in TGF-alpha-treated cells. These results indicate that NaPiT, a possibly important function of osteoblastic cells, was similarly affected by PTH and PTHrP. TGF-alpha increased NaPiT and modulated in a similar way the effects of both PTH and PTHrP.

Effects of transforming growth factor-alpha on parathyroid hormone- and parathyroid hormone-related protein-mediated bone resorption and adenylate cyclase stimulation in vitro

The effects of transforming growth factor-alpha (TGF alpha) were determined on the ability of parathyroid hormone (PTH) or parathyroid hormone-related protein (PTHrP) to stimulate bone resorption and adenylate cyclase in vitro. Bovine PTH-(1-34) and human PTHrP-(1-34) were equipotent in their ability to stimulate bone resorption in neonatal mouse calvaria with maximal stimulation (2.9 and 2.8-fold increases in 45Ca release, respectively) at a concentration of 10 nM. Combinations of TGF alpha with bPTH-(1-34) or hPTHrP-(1-34) had additive effects on their ability to stimulate bone resorption when submaximal concentrations of the agonists were used. There was no evidence of synergism between TGF alpha bPTH-(1-34) or hPTHrP-(1-34) in their ability to stimulate bone resorption in vitro, nor was TGF alpha able to increase bone resorption induced by maximal concentrations of bPTH-(1-34) or hPTHrP-(1-34). TGF alpha potentiated the effects of either bPTH-(1-34) or hPTHrP-(1-34) on the stimulation of adenylate cyclase in osteoblast-like ROS 17/2.8 cells. These data indicate that TGF alpha has additive effects with submaximal concentrations of PTH or PTHrP on their ability to stimulate bone resorption which may be important in the pathogenesis of humoral hypercalcemia of malignancy.

Structural requirements for the action of parathyroid hormone-related protein (PTHrP) on bone resorption by isolated osteoclasts

Parathyroid hormone-related protein (PTHrP) plays a major role in the syndrome of humoral hypercalcemia of malignancy (HHM) by its actions on bone and kidney. In this study an isolated osteoclast bone resorption assay was used to investigate the actions of this peptide and the structure-activity relationships for its resorption effect. As with PTH, neither synthetic nor recombinant PTHrP preparations stimulated resorption within highly purified osteoclast populations. Resorption was stimulated only in the presence of contaminating osteoblasts or in cocultures with the osteoblast-like cell line UMR-106. In the presence of osteoblasts PTHrP-(1-34) and PTHrP-(1-84) stimulated bone resorption in a dose-dependent manner with a potency comparable to that of PTH-(1-34) on a molar basis. The biologic activity of the PTHrP was shown to reside in the first 34 amino acids, and within that region the structural requirements for promotion of osteoclastic resorption resembled closely those for promotion of cyclic AMP formation in osteoblast-like cells. Using emulsion autoradiography with iodinated PTHrP-(1-34) and PTHrP-(1-84) on mixed bone cell preparations from neonatal rats, specific binding was demonstrated only to osteoblasts, not to osteoclasts. These results clearly demonstrate that PTHrP is a potent stimulator of bone resorption and that these effects are, like those of PTH, mediated by initial actions upon cells of the osteoblast lineage.

Parathyroid hormone-related protein: biochemistry and molecular biology

This article critically reviews the current state of knowledge regarding the recently identified and cloned novel hormone parathyroid hormone-related protein (PTHrP). PTHrP is produced by tumors associated with the syndrome of humoral hypercalcemia of malignancy giving rise to the parathyroid hormone (PTH)-like symptoms characteristic of the syndrome. Areas that will be reviewed include identification, purification and cloning, localization, actions, and significance of PTHrP in cancers and normal physiology. The structure and regulation of the PTHrP gene that may be ancestrally related to the PTH gene will also be discussed. Studies in vivo and in vitro with synthetic and recombinant PTHrP sequences and antibodies developed against them have established that the PTH-like actions of PTHrP are mediated via the N-terminal sequences, which show some limited sequence homology with PTH. Evidence for PTH and non-PTH-like actions of PTHrP in normal physiology, which implicate a role for PTHrP in fetal and neonatal development, is also presented.

Urinary excretion of parathyroid hormone-related protein fragments in patients with humoral hypercalcemia of malignancy and hypercalcemic tumor-bearing nude mice

To investigate whether parathyroid hormone-related protein (PTHrP), a hypercalcemia-inducing factor responsible for malignancy-associated hypercalcemia (MAH), is excreted into urine of these patients, radioimmunoassay was established using antiserum specific for the C-terminal region of PTHrP-(127-141). Immunoreactive PTHrP (iPTHrP) was detected in the urine of all patients with MAH (n = 6) in whom nephrogenous cyclic AMP excretion was elevated. However, iPTHrP was not detected in the urine of normal subjects (n = 25) or hypercalcemic patients with primary hyperparathyroidism (n = 8). In normocalcemic patients with malignant disorders iPTHrP was not detected in the urine in most cases (24 of 25 patients) but was detectable in 1 of 25 patients. iPTHrP was also detected in the urine of hypercalcemic nude mice transplanted with PTHrP-producing tumors, but not in the urine of control and normocalcemic nude mice transplanted with PTHrP-nonproducing tumor. Furthermore, size-exclusion high-performance liquid chromatography revealed that the molecular weight of iPTHrP is about 2000-6000 daltons in the urine of patients as well as tumor-bearing nude mice. These data indicate that the fragments of the C-terminal region of PTHrP are excreted into the urine of patients with MAH and in a few normocalcemic patients with malignancies, suggesting that the measurement of iPTHrP in the urine is potentially useful in the differential diagnosis of hypercalcemia, particularly in differentiating humoral hypercalcemia of malignancy and primary hyperparathyroidism.

Treatment of humoral hypercalcemia of malignancy in rats with inhibitors of carbonic anhydrase

The enzyme carbonic anhydrase has been suggested as a critical participant in osteoclast-mediated bone resorption. In humoral hypercalcemia of malignancy (HHM) intense osteoclastic bone resorption is principally responsible for the observed hypercalcemia. We therefore undertook to examine the effect of the carbonic anhydrase inhibitor acetazolamide on the hypercalcemia induced by the H500 Leydig cell tumor in Fisher rats, a well-described model of HHM. Acetazolamide treatment for 10 h at 10 mg/h resulted in a significant fall in serum calcium in the five drug-treated animals (14.2 +/- 0.9 to 11.5 +/- 0.1 mg/dl, p less than 0.05). Conversely, the six animals infused with vehicle alone showed a significant rise in serum calcium (12.5 +/- 0.5 to 13.8 +/- 0.1 mg/dl, p less than 0.05). At the end of the infusion, the acetazolamide-treated animals had a significantly lower mean serum calcium than those receiving vehicle alone (11.5 +/- 0.1 versus 13.8 +/- 0.1, p less than 0.05). There was no significant change in serum phosphorus, urine calcium, urine phosphorus, or nephrogenous cyclic AMP excretion between the two groups. Acetazaolamide and HTS 5-(3-hydroxybenzoyl)-2-thiophenesulfonamide, another carbonic anhydrase inhibitor, both significantly inhibited in vitro bone resorption induced by 5 X 10(-9) M 36Tyr(1-36)-PTHrP-amide (PTHrP, parathyroid hormone-related protein). Acetazolamide also inhibited the resorption induced by 10(-8) M (1-141)-PTHrP and 2.5 X 10(-9) M (1-74)-PTHrP. We conclude that acetazolamide is effective in lowering the serum calcium in animals with humoral hypercalcemia of malignancy. The data are consistent with the hypothesis that the mechanism of action for this effect is direct inhibition of osteoclast-mediated bone resorption.

Quantitative evaluation of anticalciuretic effects of synthetic parathyroid hormonelike peptides

The PTH-like peptide (PTHLP) responsible for hypercalcemia in many patients with humoral hypercalcemia of malignancy (HHM) acts on PTH receptors in bone and kidney, and large doses of PTHLPs have been shown to reduce urinary calcium excretion. However, PTHLPs have not been assessed quantitatively for effects on renal calcium excretion at concentrations (5-100 pM) now known to be found in the serum of patients with HHM. We perfused isolated rat kidneys with synthetic [tyr-36] PTHLP-(1-36)amide [PTHLP-(1-36)], PTHLP-(1-74), and synthetic bovine PTH-(1-34). The ratio of calcium to sodium clearances (CCa/CNa), a measure of distal tubular calcium transport, was reduced to the same extent by PTH, PTHLP-(1-36), and PTHLP-(1-74) (54.3 +/- 3.9, 52.9 +/- 3.9, and 52.7 +/- 1.3% reductions from control), respectively) at maximal doses (35-50 pM and higher), with half-maximal effects at 10, 18, and 32 pM, respectively. PTH, PTHLP-(1-36), and PTHLP-(1-74) all increased fractional phosphate excretion over control (p less than 0.05 each). All three peptides were natriuretic, at least doubling fractional Na excretion (p less than 0.05 or less). Urinary cAMP excretion was increased by all three. None had any effect on GFR or renal vascular resistance. These results indicate that clinically relevant concentrations of PTHLPs are anticalciuretic and natriuretic, with maximal effects similar to those of PTH. Differences in anticalciuretic potencies are small but may explain differences among patients, depending on the size(s) and concentrations of the native circulating form(s) of the peptide.

Inhibition of renal membrane adenylate cyclase by extract of pancreatic cancer associated with humoral hypercalcemia of malignancy

The parathyroid hormone (PTH)-like activity, defined by the stimulation of cAMP production in MC3T3E1 cells, in the extract of a pancreatic cancer associated with humoral hypercalcemia of malignancy (HHM) was eluted in two peaks (I and II) by reverse phase HPLC. Both peaks dose-dependently inhibited the binding of human (h) PTH(1-34) to canine renal membrane in an essentially similar fashion to hPTH(1-34) or PTH-related protein (rP). In the renal membrane, neither of these peaks stimulated adenylate cyclase (AC) but rather dose-dependently inhibited AC activity stimulated by hPTH(1-34), PTH-rP(1-34) or forskolin. In rat renal cortical slices, however, both peaks could exhibit their own stimulatory effect and did not inhibit PTH or forskolin-stimulated cAMP production. It has been concluded that a factor which inhibits AC activity, probably reflecting the direct action at catalytic site, can occasionally be produced with PTH-like factor. Although PTH-like and AC-inhibiting activities were very close on reverse phase HPLC, currently the interrelation between these two activities is not clear. It may be important to be aware of the presence of such a factor in the evaluation of the bioassay data employing a broken cell preparation, which is often used to assess the PTH-like activity of tumor products.

Parathyroid hormone-like peptide: molecular characterization and biological properties

Parathyroid hormone-like peptide (PLP) is a newly discovered novel peptide that interacts with renal and osseous parathyroid hormone (PTH) receptors through its amino-terminal sequence, which is homologous with PTH. Human PLP is the product of a complex single-copy gene, existing in several isoforms which result from alternative exon usage. The peptide was implicated as a causal agent in the hypercalcemia associated with a variety of malignancies, but it may also modulate calcium homeostasis in some normal physiological states and mediate cellular growth or differentiation.