Synthetic parathyroid hormone-like protein-(1-74): biochemical and physiological characterization

A Hospital Survey of Hypocalcemia and Hypophosphatemia in Malignancy

We evaluated the incidence of hypo- versus hypercalcemia and hypo- versus hyperphosphatemia in a survey of 158 patients with malignancy; 55/158 had bone metastases. When serum calcium levels were corrected for albuminemia, the incidence of hypo- and hypercalcemia was respectively 10.8% and 10.1 %. Hypophosphatemia was found in 29.7% patients, hyperphosphatemia in 2.5 %. The incidence was slightly different in presence of bone metastases. Hypocalcemia and hypophosphatemia prevailed In osteoblastic metastases and hypercalcemia in osteolytic metastases. The incidence of hypocalcemia and hypophosphatemia in malignancy was therefore surprisingly high, even apart from the presence of bone metastases. Both hypo- and hypercalcemia were associated with elevated serum alkaline phosphatase levels. Moreover, a calcium-phosphorus product reduction was observed in osteoblastic metastases, suggesting a condition of secondary hyperparathyroidism.

Synovial fluids from patients with osteoarthritis and rheumatoid arthritis contain high levels of parathyroid hormone-related peptide

High levels of immunoreactive and biologically active parathyroid hormone-related peptide (PTHrP) were detected in synovial fluids from patients with osteoarthritis (OA) and rheumatoid arthritis (RA). The levels of PTHrP immunoreactivity in synovial fluids, measured by a two-site immunoradiometric assay (IRMA) which detects hPTHrP(1-72) or longer peptides and a radioimmunoassay (RIA) specific to the carboxy-terminal portion of hPTHrP, were 3.2 +/- 0.3 pmol of hPTHrP(1-86)/l and 61 +/- 7.0 pmol of hPTHrP(109-141)/l in OA patients (mean +/- SE, n = 23), and 4.8 +/- 0.8 pmol of hPTHrP(1-86)/l and 164 +/- 30 pmol of hPTHrP(109-141)/l in RA patients (n = 26). Synovial fluid PTHrP levels distributed above the normal plasma reference ranges in each assay (0.7-2.6 pmol of hPTHrP(1-86)/l; 16-60.6 pmol of hPTHrP(109-141)/l). After concentration using sequential cation-exchange and reverse-phase chromatography, synovial fluid exhibited the activity that stimulated cyclic adenosine monophosphate (cAMP) accumulation in osteoblastic ROS 17/2.8 cells expressing PTH/PTHrP receptors. The cAMP accumulation activity in synovial fluid was sensitive to coincubation with excess hPTHrP(3-40), a PTH/PTHrP receptor antagonist, and was completely neutralized by preincubation with a monoclonal antibody specific to hPTHrP but not PTH. Immunohistochemical analysis of RA synovium revealed that PTHrP was localized in fibroblast-like cells in the synovial pannus invading articular cartilage. Our data show that PTHrP is produced locally by the diseased synovial tissue and released into synovial fluid at high concentrations, allowing us to hypothesize that PTHrP plays a novel role as a paracrine/autocrine factor in the pathology of OA and RA.

Structural and physiologic characterization of the mid-region secretory species of parathyroid hormone-related protein

Parathyroid hormone-related protein (PTHrP) is initially translated as a preprohormone which is posttranslationally processed to yield a family of mature secretory forms. Most attention has focused on the amino-terminal portion of the molecule which is homologous to parathyroid hormone. It is clear, however, that a mid-region species of PTHrP is posttranslationally cleaved from the highly conserved mid-region of PTHrP, and that the amino terminus of this peptide is Ala38. The purposes of the current study were three: 1) to confirm that Arg37 immediately preceding Ala38 serves as a posttranslational processing site in the PTHrP precursor, 2) to determine the carboxyl terminus of the mid-region secretory species of PTHrP, and 3) to synthesize this authentic mid-region secretory form of PTHrP and determine whether it is biologically active. The results indicate that: 1) Arg37 is indeed a processing site in the PTHrP precursor; 2) three distinct mid-region PTHrP species are generated by posttranslational processing, PTHrP(38-94)amide, PTHrP(38-95), and most likely, PTHrP(38-101); and 3) synthetic mid-region PTHrP(38-94)amide is active in four different biological systems. These studies confirm the finding that PTHrP is a prohormone. More importantly, they define a novel, biologically active highly conserved mid-region secretory form of PTHrP.

Signaling by N- and C-terminal sequences of parathyroid hormone-related protein in hippocampal neurons

Parathyroid hormone-related protein (PTHrP) is synthesized in the brain, and a single type of cloned receptor for the N-terminal portion of PTHrP and PTH is present in the central nervous system. Nothing is known about the physiological actions or signaling pathways used by PTHrP in the brain. Using cultured rat hippocampal neurons, we demonstrate that N-terminal PTHrP[1-34] and PTH[1-34] signal via cAMP and cytosolic calcium transients. The cAMP response showed strong acute (< or = 6 h) homologous and heterologous desensitization after preincubation with PTHrP or PTH. In contrast, the acute calcium response did not desensitize after preincubation with PTHrP; in fact, preincubation dramatically recruited additional responsive neurons. Unexpectedly, C-terminal PTHrP[107-139], which does not bind or activate the cloned PTH/PTHrP receptor, signaled in neurons via cytosolic calcium but not cAMP. Although some neurons responded to both PTHrP[1-34] and PTHrP[107-139], others responded only to PTHrP[1-34]. We conclude that certain hippocampal neurons exhibit dual signaling in response to PTHrP[1-34] and that some neurons have a receptor for C-terminal PTHrP that signals only via cytosolic calcium.

Involvement of alkaline phosphatase in the modulation of receptor signaling in osteoblasts: evidence for a difference between human parathyroid hormone-related protein and human parathyroid hormone

We have previously reported that alkaline phosphatase (ALPase) is functionally involved in calcium uptake by several osteoblast-like cell lines. We have extended these studies to investigate the actions of ALPase on the cAMP response to and the receptor binding of human parathyroid hormone (hPTH) and human parathyroid hormone-related protein (hPTHrP). Pretreatment of human osteoblast-like SaOS-2 cells with human placental ALPase (hpALPase) inhibited the cAMP response to hPTH(1-34) but had no effect on the actions of hPTHrP(1-34) or vasoactive intestinal peptide. The inhibitory effect was reversed by L-Phe-Gly-Gly, an inhibitor of hpALPase. Treatment of SaOS-2 cells with hpALPase modestly reduced the binding of hPTH to 70% of control values, with little or no effect on the binding of hPTHrP. Bovine kidney and calf intestine ALPases were without effect on either the cAMP response or binding of hPTH or hPTHrP in SaOS-2 cells. In rat osteoblast-like ROS 17/2.8 cells, hpALPase had no effect on cAMP production stimulated by hPTH(1-34) or hPTHrP(1-34), arguing against a nonspecific effect of hpALPase. We suggest that, in SaOS-2 cells, the common PTH/PTHrP receptor can differentiate between the agonist activities of hPTH and hPTHrP by a mechanism that is sensitive to hpALPase.

Macrophage colony-stimulating factor release and receptor expression in bone cells

Colony-stimulating factors (CSF) may play a role in bone resorption. To examine whether osteoblasts secrete colony-stimulating activity (CSA) in response to parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP), conditioned medium (CM) from ROS 17/2.8 cells and primary rat osteoblasts were examined for induction of clonal growth of cultured rat bone marrow cells. Untreated cells constitutively secreted CSA, which increased with PTH and PTHrP treatment. The colonies formed were principally comprised of macrophages, and preincubation of CM with antiserum to murine macrophage colony-stimulating factor (M-CSF) neutralized most of the CSA, suggesting that the osteoblast-derived CSA was predominantly due to M-CSF. PTHrP treatment upregulated steady-state M-CSF mRNA levels. To investigate a paracrine role for M-CSF in bone we examined bone tissue and cells for the M-CSF receptor c-fms using immunohistochemical techniques and demonstrated staining of mature osteoclasts both in situ and after isolation. We conclude that M-CSF is responsible for the majority of the CSA released by PTH- and PTHrP-treated rat osteoblasts. In addition we identified CSF-1 receptor expression in mature osteoclasts. These data suggest that M-CSF is a mediator of osteoblast-osteoclast interaction in PTH- and PTHrP-induced bone resorption.

A case of squamous cell lung carcinoma with high concentration of parathyroid hormone-related peptide in serum and pleural effusion presenting hypercalcemia

A 57-year-old man with lung squamous cell carcinoma revealed hypercalcemia, hypophosphoremia, elevation of nephrogenous cAMP and metabolic alkalosis. Serum parathyroid hormone (PTH) and 1,25(OH)2D3 concentrations were not elevated. These findings were consistent with those in humoral hypercalcemia of malignancy (HHM). PTH-related peptide (PTHrP) concentrations were determined using N- and C-terminal specific radioimmunoassays (PTHrP-N, PTHrP-C), and elevation of both PTHrP-N and PTHrP-C concentrations in the serum was noted (PTHrP-N, 27 pmol/liter (normal < 5); PTHrP-C, 1408 pmol/liter (normal < 50)). High concentration of PTHrP (946 pmol/liter for PTHrP-N and 5983 pmol/liter for PTHrP-C) was also found in the pleural fluid obtained at autopsy. Immunohistochemical study, using paraffin-embedded sections of the tumor tissue obtained at autopsy, revealed numerous PTHrP-positive cells and expression of PTHrP gene was confirmed by Northern blot analysis. These findings indicate that PTHrP, produced in the tumor tissue, was secreted into the blood stream, which caused HHM in the patient. Gel permeation chromatography of the serum and pleural fluid revealed several peaks of both PTHrP-N and PTHrP-C. Molecular forms of PTHrP-N were larger than those of PTHrP-C in the serum as well as pleural fluid. These findings indicate that multiple forms of PTHrP molecules are present in the serum and pleural fluid. Granulocytosis was also noted in the patient. However, granulocyte- and granulocyte macrophage-colony stimulating factor were not detected in the serum, and the mechanism of the granulocytosis in the patient was unclear.

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.

Effects of guanine nucleotides and parathyroid hormone on inositol 1,4,5-trisphosphate metabolism in canine renal cortical tubular cell membranes

Parathyroid hormone (PTH) and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma S) increase levels of the second messenger inositol 1,4,5-triphosphate (IP3) and other inositol phosphates (IP) in several membrane preparations of PTH-responsive cells. We present evidence here indicating that in a membrane preparation of canine renal cortical tubular cells bPTH-(1-84), bPTH-(1-34), [N-Leu8,18Tyr34]bPTH-(3-34)NH2, and the human PTH related peptide fragment hPTHrP-(1-34)NH2 all increase levels of inositol phosphate (IP) but [Tyr34]-bPTH-(7-34)NH2 and hPTHrP-(7-34)NH2 have no significant effects on IP accumulation. Increases in IPs are generally attributed to increased formation of IPs and appear to be mediated by a G protein. However, increased levels of IPs may also result from inhibition of the phosphatases are responsible for their metabolism. We investigated the effect of PTH and GTP-gamma S on the metabolism of IP3 in canine renal cortical tubular membranes. These membranes rapidly metabolize [3H]IP3 (47% at 15 s). Decreases in [3H]IP3 at all time points are accounted for quantitatively by increases in the sum of its breakdown products: [3H]IP2, [3H]IP1, and [3H]inositol. After 5 minutes of exposure to membranes, the vast majority of [3H]IP3 (84%) is converted to its terminal metabolite, [3H]inositol. GTP-gamma S (100 microM) inhibits the amount of [3H]IP3 metabolized in 15 s by 70% and reduces the amount of [3H]inositol ultimately formed in 5 minutes by 64%. ATP-gamma S, ATP, and 2,3-bisphosphoglycerate (100 microM) also inhibit [3H]IP3 hydrolysis in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoaffinity purification of parathyroid hormone-related protein from bovine milk and human keratinocyte-conditioned medium

Parathyroid hormone-related proteins (PRHrP) are a novel family of proteins that appear to be responsible for humoral hypercalcemia of malignancy. Although PTHrP derived from human tumors have been purified and their N-terminal amino acid sequence determined, and although the structure of the PTHrP gene and its alternatively spliced mRNA transcripts have been defined, the secretory and circulating form(s) of the protein are unknown. Purification of PTHrP in the past has been difficult, requiring multiple chromatographic steps and months or years to complete. To define naturally occurring PTHrP species we have developed a rapid and efficient immunoaffinity purification method. Bovine milk (250 ml) and human keratinocyte-conditioned medium (3000 ml) were affinity purified using a 300 microliters affinity-purified polyclonal anti-PTHrP-(1-36) antibody column and a single RP-HPLC step. Purification required only 7-10 days and yielded a 3-4% recovery. Quantities of PTHrP sufficient for silver-stained SDS-PAGE, Western analysis, and N-terminal amino acid sequence were obtained. In contrast to conventional purification schemes, affinity purification of PTHrP is rapid and efficient and can be applied to biologic samples that contain PTHrP in low abundance. These methods can be applied to the purification and characterization of the as yet undefined secretory and circulating forms of PTHrP.

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.

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.

Intrauterine occupancy controls expression of the parathyroid hormone-related peptide gene in preterm rat myometrium

The parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHRP) genes are members of a gene family. Whereas PTH is a classical peptide hormone, mounting evidence suggests that the PTHRP may have predominately local actions. We report here that the PTHRP gene is expressed in rat myometrium, with a major peak in PTHRP mRNA expression occurring in the 48 hr immediately preceding parturition. A similar peak in peptide content was found in tissue extracts by biological and immunological assays, but the PTHRP could not be detected in the peripheral circulation or in uterine vein plasma during late gestation. By in situ hybridization histochemistry, PTHRP mRNA was demonstrated in both the longitudinal and circular layers of smooth muscle but was absent in the endometrium. The rise in myometrial PTHRP mRNA in late gestation was dependent upon intrauterine occupancy; it was greatly reduced or absent in nongravid uterine horns. These findings indicate that the expression of the PTHRP gene in preterm myometrium is under the control of a local stimulus and suggest that the PTHRP may play a paracrine or autocrine role in the uterus during the antepartum period, possibly involving myometrial contractility.

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.

Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer

Tumors from patients with humoral hypercalcemia of cancer produce a parathyroid hormone-related protein (PTHRP). We have developed two region-specific immunoassays capable of measuring PTHRP in plasma: an immunoradiometric assay directed toward PTHRP amino acid sequence 1 to 74 and a radioimmunoassay directed toward PTHRP amino acid sequence 109 to 138. Sixty normal subjects had low or undetectable plasma PTHRP (1 to 74) concentrations (mean, 1.9 pmol per liter) and undetectable PTHRP (109 to 138) concentrations (less than 2.0 pmol per liter). Patients with humoral hypercalcemia of cancer (n = 30) had elevated levels of both PTHRP (1 to 74) (mean, 20.9 pmol per liter) and PTHRP (109 to 138) (mean, 23.9 pmol per liter). The plasma concentrations of immunoreactive PTHRP correlated with the levels of urinary cyclic AMP excreted; in some patients, the concentrations decreased after the tumors were resected. Patients with chronic renal failure (n = 15) had plasma PTHRP (1 to 74) concentrations similar to those in the normal subjects, but their plasma PTHRP (109 to 138) concentrations were elevated (mean, 29.6 pmol per liter). The levels of both peptides were normal in patients with hyperparathyroidism and those with hypercalcemia due to various other causes. Breast milk contained high concentrations of PTHRP. An anti-PTHRP (1 to 36) immunoaffinity column failed to extract PTHRP (109 to 138) immunoactivity from plasma, suggesting that the C-terminal region circulates as a separate peptide. We conclude that plasma PTHRP concentrations are high in the majority of patients with cancer-associated hypercalcemia and that the circulating forms of PTHRP in such patients include both a large N-terminal (1 to 74) peptide and a C-terminal (109 to 138) peptide. Measuring the concentrations of PTHRPs may be useful in the differential diagnosis of hypercalcemia.

Effect of parathyroid hormone-like peptides on 25-hydroxyvitamin D-1 alpha-hydroxylase activity in rodents

The role of vitamin D metabolism in the humoral hypercalcemia of malignancy syndrome (HHM) is unclear. We studied in vivo and in vitro effects of synthetic parathyroid hormone-like peptides (PTH-LPs) on rodent renal 25-OHD-1 alpha-hydroxylase activity. Infusion of mice with PTH-LP-(1-36) at 10 pmol/h for 12 and 24 h showed significant (429 +/- 139% and 937 +/- 413%, respectively) stimulation of control enzyme activity. Infusion for 36 h demonstrated diminution of activity to levels nearer to the unstimulated state (228 +/- 36% of control). In that maximal activity was observed after 24 h of infusion, we examined 1 alpha-hydroxylase activity after variable dosages of PTH-LP-(1-36) at this time point. Animals infused with PTH-LP-(1-36) at dosages of 2.5, 10, and 30 pmol/h for 24 h demonstrated 1 alpha-hydroxylase activities of 0.71 +/- 0.12, 4.74 +/- 2.09, and 9.91 +/- 1.01 ng.mg protein-1.20 min-1 (means +/- SD), respectively, all significantly greater than control activity (0.51 +/- 0.20 ng.mg protein-1.20 min-1). PTH-LP-(1-36) and PTH-LP-(1-74) were comparable in potency to bovine (b)PTH-(1-34) in stimulating 1 alpha-hydroxylase. Direct in vitro incubation of PTH-LP-(1-36) with renal slices resulted in stimulation of 1 alpha-hydroxylase activity up to 200% of control levels, comparable to that seen with equimolar concentrations of bPTH-(1-34).(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-related peptide gene is expressed in the mammalian central nervous system

A parathyroid hormone-related peptide (PTHRP) has been identified in human tumors associated with the syndrome of humoral hypercalcemia of malignancy. While parathyroid hormone (PTH) gene expression appears to be limited to the parathyroid glands, PTHRP mRNA has been identified in a variety of normal tissues. To investigate the apparent expression of the PTHRP in the central nervous system, we examined extracts of whole rat brain for PTHRP bioactivity by measuring adenylate cyclase-stimulating activity (ACSA) in a PTH-sensitive assay. Extracts consistently contained ACSA and this activity was completely inhibited by a PTHRP antiserum but was unaffected by a PTH antiserum. ACSA was found in a number of anatomic subregions of rat brain, being greatest in the cortex and telencephalon. RNase protection analysis revealed PTHRP transcripts in total RNA prepared from whole rat brain and from the same anatomic subregions. By in situ hybridization histochemistry, we found that the highest levels of PTHRP gene expression occurred in neurons of the cerebral cortex, hippocampus, and cerebellar cortex. These studies demonstrate that both PTHRP mRNA and biological activity are present in a number of regions of rat brain. The widespread expression of this peptide by multiple types of neurons suggests that the PTHRP may play a general role in neuronal physiology.

Development of endocrine pathways in the regulation of calcium homeostasis

The mammalian fetus is maintained hypercalcaemic relative to its mother by the action of a calcium pump believed to be located at the basement membranes of the epithelial cells of the fetal chorion. It has recently been demonstrated that the activity of this putative pump is stimulated by a new fetal hormone, parathyroid hormone-related protein, described originally as the product of a human BEN cell line which was derived from a lung tumour associated with hypercalcaemia of malignancy. Whereas the circulating level of immunoreactive parathyroid hormone in the fetus is very low, in keeping with the hypercalcaemia, the plasma concentrations of bioactive parathyroid hormone and parathyroid hormone-related protein can be measured using a sensitive cytochemical bioassay and the separate concentrations assessed by pre-incubation with appropriate antisera. The total plasma concentration of both hormones is inversely related to the prevailing calcium ion concentration but the set point of parathyroid hormone-related protein is probably higher than that for parathyroid hormone. Probably as a result of the hypercalcaemia, the circulating concentration of calcitonin is also higher than in maternal plasma and may serve to limit bone resorption to favour net bone accretion as part of the overall growth of the fetus. Vitamin D and its most active metabolite, 1,25(OH)2D, can pass across the placenta in either direction, in contrast to most peptide hormones. In addition to the supply of some 1,25(OH)2D by the mother to her fetus, the fetal placenta and fetal kidneys can all synthesize 1,25(OH)2D. The relative concentrations circulating in maternal and fetal plasma pools vary with the species, presumably as a result of differing importance of the three sources of supply to the fetus and the relative concentrations of vitamin D-binding protein circulating in mother and fetus. The importance of parathyroid hormone-related protein derived from fetal parathyroid glands has been clearly demonstrated in the fetal sheep. Such animals develop rickets following the removal of their parathyroid glands, despite the demonstration of this substance in fetal placental membranes. However, the relative importance of the parathyroid glands versus the placenta and its membranes as the principal source of parathyroid hormone-related protein remains to be elucidated and may vary with species.

Clonal rat parathyroid cell line expresses a parathyroid hormone-related peptide but not parathyroid hormone itself

A novel parathyroid hormone-related peptide has been identified in tumors associated with the syndrome of humoral hypercalcemia of malignancy. Subsequently, mRNAs encoding this peptide have been found to be expressed in a number of normal tissues, including the parathyroids. Using Northern blotting, RNase protection, and immunochemical techniques, we examined a clonal rat parathyroid cell line originally developed as a model system for studying parathyroid cell physiology. We found that this line expresses the parathyroid hormone-related peptide but not parathyroid hormone itself. Secretion of the parathyroid hormone-related peptide varied inversely with extracellular calcium concentration, but neither calcium nor 1,25-dihydroxyvitamin D3 appeared to influence steady-state parathyroid hormone-related peptide mRNA levels. This clonal line may prove to be an interesting system for studying the factors responsible for tissue-specific parathyroid hormone and parathyroid hormone-related peptide gene expression.