Parathyroid hormone-related peptide in the human fetal uro-genital tract

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

PTH and PTH-related peptide enhance steroid secretion from human adrenocortical cells

Parathyroid hormone (PTH) and PTH-related peptide (PTH-RP) are two hypercalcemic hormones that share a common receptor subtype, the PTH/PTH-RP receptor. PTH and PTH-RP concentration dependently enhanced basal aldosterone and cortisol secretion from dispersed human adrenocortical cells, with a maximal effective concentration (approximately 2-fold increase) of 10(-8) M. The secretagogue effect of 10(-8) M PTH or PTH-RP was abolished by the PTH/PTH-RP receptor antagonist [Leu11,D-Trp12]-PTH-RP-(7-34)-amide (10(-6) M). PTH and PTH-RP (10(-8) M) raised cAMP and inositol-triphosphate release by dispersed adrenocortical cells, and these effects were blocked by the adenylate cyclase inhibitor SQ-22536 (10(-4) M) and the phospholipase C (PLC) inhibitor U-73122 (10(-5) M), respectively. SQ-22536 (10(-4) M) and U-73122 (10(-5) M) partially inhibited aldosterone and cortisol response to 10(-8) M PTH and PTH-RP; when added together, they abolished it. Similar results were obtained by using the protein kinase (PK)A and PKC inhibitors H-89 and calphostin C (10(-5) M). It is concluded that PTH and PTH-RP exert a sizeable secretagogue action on the human adrenal cortex, probably acting through the PTH/PTH-RP receptor coupled with both adenylate cyclase/PKA- and PLC/PKC-dependent signaling cascades.

Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hypothalamus- pituitary-adrenal axis

Secretin, glucagon, gastric inhibitory polypeptide (GIP), and parathyroid hormone (PTH) belong, together with vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase (AC)-activating polypeptide, to a family of peptides (the VIP-secretin-glucagon family), which also includes growth hormone-releasing hormone and exendins. All the members of this peptide family possess a remarkable amino-acid sequence homology, and bind to G-protein-coupled receptors, whose signaling mechanism primarily involves AC/protein kinase A and phospholipase C/protein kinase C cascades. VIP and pituitary AC-activating polypeptide play a role in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, and in this review we survey findings that also other members of the VIP-secretin-glucagon family may have the same function. Secretin and secretin receptors are expressed in the hypothalamus and pituitary gland, and secretin inhibits adrenocorticotropic hormone (ACTH) release. No evidence is available for the presence of secretin receptors in adrenal glands, but secretin selectively depresses the glucocorticoid response to ACTH of dispersed zona fasciculata-reticularis (ZF/R) cells. Glucagon and glucagon-like peptide-1 are contained in the hypothalamus, and all the components of the HPA axis are provided with glucagon and glucagons-like-1 receptors. These peptides exert a short-term inhibitory effect on stress-induced pituitary ACTH release and depress the ZF/R cell response to ACTH by inhibiting the AC/protein kinase A cascade; they also stimulate hypothalamic arginine-vasopressin release. GIP receptors are present in the ZF/R of the normal adrenals, and are particularly abundant in some types of adrenocortical adenomas and hyperplasias. GIP, through the activation of the AC/protein kinase A cascade, evokes a sizeable glucocorticoid secretagogue effect, leading to the identification of a food/GIP-dependent Cushing's syndrome. PTH and PTH-related protein are expressed in the hypothalamus and pituitary gland, and PTH and PTH-related protein receptors in all the components of the HPA axis. Both peptides enhance ACTH and arginine-vasopressin release, as well as stimulate aldosterone and glucocorticoid secretion of dispersed zona glomerulosa and ZF/R cells, respectively. The involvement of growth hormone-releasing hormone and exendins in the functional regulation of the HPA axis has not yet been extensively investigated.

Immunohistochemical detection of parathyroid hormone-related protein in a cutaneous squamous cell carcinoma causing humoral hypercalcemia of malignancy

Humoral hypercalcemia of malignancy is a cancer-related hypercalcemia caused by production of humoral factors by malignant cells in patients without bone metastases. Squamous cell carcinomas are the tumors most frequently associated with humoral hypercalcemia of malignancy, and parathyroid hormone-related protein is the main humoral factor implicated. In spite of the fact that normal keratinocytes produce parathyroid hormone-related protein, it is highly unusual for patients with squamous cell carcinomas of the skin to present with humoral hypercalcemia of malignancy. We present a well-documented case of cutaneous squamous cell carcinoma complicated by hypercalcemia in a patient with high levels of plasma parathyroid hormone-related protein and immunohistochemical evidence of high parathyroid hormone-related protein production by the tumoral cells.

Immunocytochemical expression of parathyroid hormone related protein (PTHrP) in odontogenic jaw cysts

OBJECTIVE

To investigate the immunocytochemical expression of parathyroid-hormone-related protein (PTHrP) in odontogenic jaw cysts.

DESIGN

Retrospective study of archival tissue.

SETTING

University department, UK.

MATERIAL

Odontogenic keratocysts (n=27), and dentigerous and radicular cysts (n=10 each).

INTERVENTION

Immunocytochemistry by biotin streptavidin technique.

MAIN OUTCOME MEASURE

Intensity of staining of PTHrP determined by TV image analysis.

RESULTS

The epithelial linings of all the odontogenic keratocysts, 9/10 dentigerous, and 8/10 radicular cysts showed reactivity for PTHrP mainly localised to the basal and suprabasal layers. Odontogenic keratocyst linings expressed significantly higher levels of PTHrP than those of dentigerous and radicular cysts (P<0.003 in each case). There were no differences in epithelial expression of PTHrP between solitary, recurrent and naevoid basal cell carcinoma syndrome-associated odontogenic keratocysts. The fibrous tissue walls of all types of cyst reacted strongly for PTHrP with a trend towards decreasing intensity from odontogenic keratocysts, to dentigerous and then radicular cysts.

CONCLUSION

It is possible that PTHrP modulates growth and bone resorption in odontogenic cysts. PTHrP may act synergistically with interleukin-1 to increase bone resorption or stimulate osteoblasts and inhibit osteoclasts (resulting in reduced resorption) via its transforming growth factor beta-like activity.

Parathyroid hormone--related protein (PTHrP) is an epidermal growth factor-regulated secretory product of human prostatic epithelial cells

Parathyroid hormone-related protein (PTHrP) has previously been shown to be expressed in human prostatic tissue and in prostatic cancer cell lines. In the present study, PTHrP immunoreactivity was detected in the glandular epithelium of normal prostate and benign prostatic hyperplasia (BPH), as well as in prostatic adenocarcinoma (CaP). Epithelial cell cultures derived from normal, BPH, and CaP tissues were also stained by antibodies against PTHrP, and northern analysis revealed multiple transcripts of PTHrP in the cellular RNA. PTHrP (1-34) was measurable by radioimmunoassay (RIA) in media conditioned by the prostatic epithelial cell cultures, and PTHrP accumulated in conditioned media during a 72 hr time course. Addition of complete growth medium to starved cells resulted in increased PTHrP mRNA levels by 1 hr, with maximal stimulation at 8-24 hr. Several individual factors contained in the complete growth medium were tested for their ability to regulate PTHrP expression. Epidermal growth factor (EGF) was the major inducer of PTHrP expression, while cholera toxin, bovine pituitary extract, hydrocortisone, and insulin had minimal or no effect on PTHrP transcript levels. Since each of these factors is growth stimulatory, the unique ability of EGF to induce PTHrP is apparently unrelated to mitogenicity. 1,25-Dihydroxyvitamin D3[1,25(OH)2D3], an inhibitor of PTHrP expression in several other cell types, had no effect on steady-state levels of PTHrP mRNA expressed by epithelial cells in complete growth medium, although prostate cells have vitamin D receptors and are responsive to 1,25(OH)2D3 in other ways. Our results indicate that PTHrP expression is not confined to the neuroendocrine cells of the human prostate and that our culture system can be used as a model to investigate the role of PTHrP in the prostate.

Parathyroid hormone-related protein gene expression in invasive cervical tumors

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is the main tumor-derived factor responsible for the hypercalcemia of malignancy.

METHODS

Using a polyclonal serum to the 37-67 region of PTHrP, and 35S-labeled riboprobes, the authors investigated the cellular expression of PTHrP mRNA and peptide in formalin fixed, paraffin embedded sections from 16 invasive cervical tumors. In addition, the relationship among the histologic cell type, degree of differentiation, pattern of invasion, and tumor expression of PTHrP were examined.

RESULTS

PTHrP mRNA and peptide were identified in 10 of 10 and 16 of 16 tumors examined, respectively. Overall strong mRNA expression with moderate to intense intracellular staining for peptide was associated, with adenosquamous carcinoma displaying a spray pattern of invasion.

CONCLUSION

PTHrP mRNA and peptide were observed in all cervical tumors studied. Despite their high frequency of expression of PTHrP, cervical tumors seldom give rise to humoral hypercalcemia of malignancy, but the autocrine/paracrine effects of PTHrP may be important in the growth and dedifferentiation of the malignant cell population.

Effects of epidermal growth factor on parathyroid hormone-related protein production by mammary epithelial cells

Parathyroid hormone-related protein (PTHrP) plays a prominent role in the pathogenesis of humoral hypercalcemia of malignancy. However, it is also expressed in various nonmalignant tissues, particularly during fetal organogenesis and tissue differentiation. Thus, PTHrP is synthesized in skin, placenta, and mammary gland during lactation. Little is known, however, about the regulation of PTHrP synthesis and release in nontumoral cells. We investigated the regulation of PTHrP production by epidermal growth factor (EGF), a factor of major importance in the development of lactating breast, in primary cultures of rat mammary epithelial cells. EGF stimulated the production of immunoreactive and bioactive PTHrP in a time- and concentration-dependent manner. A 12 h incubation with 10 ng/ml of EGF increased PTHrP production by 36.0 +/- 7.1% (n = 7 experiments, p < 0.01). This was accompanied by an increase in PTHrP mRNA steady-state levels. The production of PTHrP was stimulated by the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA) by 82.9 +/- 9.7% (n = 4 experiments, p < 0.01). The effects of PMA and EGF were additive. The EGF-induced stimulation appeared to be independent of extracellular calcium concentration, prostaglandin, or cAMP synthesis, but may have involved tyrosine kinase-mediated mechanisms. These results indicate that EGF was capable of increasing the production of PTHrP by cultured mammary epithelial cells. They also suggest that factors activating the PKC pathway are involved in the upregulation of PTHrP expression in mammary epithelial cells.

Parathyroid hormone-related protein and calcium phosphate metabolism

There is marked homology between the parathyroid hormone (PTH) and PTH-related protein (PTHrP) molecules at the amino terminal but the rest of the molecules are quite different, providing immunologically distinct peptides. However, they interact with the same receptor. Thus, PTHrP mediates biological actions reminiscent of PTH. PTHrP gene is a single copy gene, producing one to three mRNA transcripts through alternative splicing of the carboxy terminal, encoding peptides of 139, 141 or 173 amino acids. Having been recently isolated from malignant tumours, PTHrP is now considered to be the major mediator of humoral hypercalcaemia of malignancy (HHM). The PTH-like effects of PTHrP on the kidney and bone have been well characterized. The increase in renal tubular calcium reabsorption and the reduction in tubular phosphate reabsorption with a concomitant rise in nephrogenous cyclic AMP constitute the pathophysiological changes in the renal handling of calcium and phosphate in HHM. The osteotropic contribution to the malignant hypercalcaemia has been validated by enhanced osteoclastic bone resorption--an indirect effect of the amino terminal portion of the PTHrP molecule on osteoblasts. However, PTHrP has also been detected in a large number of normal adult tissues/organs as well as in human and animal fetuses. Fetal plasma calcium is higher than maternal and this is achieved by active transport of calcium across the placenta. Using ovine placental perfusion models, PTHrP, which is believed to originate from fetal parathyroid glands and the placenta itself, has been demonstrated to sustain this calcium gradient. Active placental transport of magnesium, but not phosphate, was also shown to be enhanced by PTHrP.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-related peptide can regulate the growth of human lung cancer cells, and may form part of an autocrine TGF-alpha loop

Parathyroid hormone-related peptide (PTHrP) and transforming growth factor-alpha (TGF-alpha) were found to stimulate proliferation of human lung cancer cells (BEN-57). TGF-alpha stimulated PTHrP secretion from these cells. The polyclonal antisera raised against PTHrP significantly inhibited the growth of BEN-57 cells, and also the proliferation induced by TGF-alpha. Treatment of cells for up to 10 days with either a PTHrP receptor antagonist (PTHrP(7-34)) or PTHrP antiserum significantly inhibited the subsequent growth of these cells. We suggest that PTHrP may be a component of a complex autocrine loop involving TGF-alpha.

Parathyroid hormone-related peptide: expression in fetal and neonatal development

Hypercalcaemia frequently complicates the clinical management of cancer. Many factors have been implicated in the pathogenesis of this humoral hypercalcaemia of malignancy, the most recent candidate being parathyroid hormone-related peptide (PTHrP). Until now, this peptide has been detected only in some normal and transformed adult tissues. In recent years, it has become apparent that tumours are capable of expressing and secreting factors previously elaborated only during fetal life. Many of these factors act to stimulate the growth of both tumour and fetal cells in an autocrine manner. The data presented here demonstrate that PTHrP is expressed in the human and rat fetus throughout gestation. Immunocytochemistry reveals a gestationally related, changing pattern of expression which is paralleled by changes in mRNA transcription. These data support the hypothesis that PTHrP may function as a fetal growth factor.

Adenylate cyclase stimulating activity immunologically similar to parathyroid hormone-related peptide can be extracted from fetal rat long bones

We observed that culture medium conditioned with fetal rat long bones contained peptides immunologically related to the parathyroid hormone-related peptide of malignancy (PTHrP) and stimulated cyclic AMP production in canine renal cortical membranes. Because the adenylate cyclase stimulating activity (CSA) of the medium increased when bone resorption was stimulated, it was suspected that these peptides were stored in the matrix and released during the resorption process. In this work, we extracted the noncollagenous proteins of fetal rat long bones and found that the extract contained significant amounts of CSA. The biologic activity of the extract was abolished after trypsin digestion and eluted at 24 and 37 kD on filtration HPLC. The CSA of bone extract and of both HPLC peaks could be inhibited by 3-34 and 7-34 parathyroid hormone analogs. It was not blocked by an antiserum directed against the N-terminal region of parathyroid hormone, but it was significantly inhibited after an overnight preincubation with an antiserum directed against the 1-11 fragment of PTHrP. One band migrating at 18 kD could be visualized after SDS-PAGE electrophoresis of bone extract and immunoblotting with the anti-PTHrP antiserum. We conclude that an adenylate cyclase stimulator immunologically similar to PTHrP is present in the matrix of fetal rat long bones. Adenylate cyclase stimulating peptides of lower molecular weight found in bone-conditioned medium could be active fragments formed by proteolysis during the resorption process.