Parathyroid hormone-related protein in the cardiovascular system

Parathyroid hormone/parathyroid hormone-related peptide regulate osteosarcoma cell functions: Focus on the extracellular matrix (Review)

Osteosarcoma (OS) is a primary bone tumor of mesenchymal origin mostly affecting children and adolescents. The OS extracellular matrix (ECM) is extensively altered as compared to physiological bone tissue. Indeed, the main characteristic of the most common osteoblastic subtype of OS is non-mineralized osteoid production. Parathyroid hormone (PTH) is a polypeptide hormone secreted by the chief cells of the parathyroid glands. The PTH-related peptide (PTHrP) may be comprised of 139, 141 or 173 amino acids and exhibits considerate N-terminal amino acid sequence homology with PTH. The function of PTH/PTHrP is executed through the activation of the PTH receptor 1 (PTHR1) and respective downstream intracellular pathways which regulate skeletal development, bone turnover and mineral ion homeostasis. Both PTHR1 and its PTH/PTHrP ligands have been shown to be expressed in OS and to affect the functions of these tumor cells. This review aims to highlight the less well known aspects of PTH/PTHrP functions in the progression of OS by focusing on ECM-dependent signaling.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Effects of parathyroid hormone-related peptide on the large conductance calcium-activated potassium channel and calcium homeostasis in vascular smooth muscle cells

AIM

To demonstrate the impact of the parathyroid hormone-related peptide (PTHrP) on the large conductance calcium-activated potassium (BKCa) channels in vascular smooth muscle cells (VSMC) and hyperpolarization of the cell membrane and its dependence on calcium.

MATERIALS AND METHODS

VSMC were isolated from rat aorta and further subcultured. Four experiments were conducted in calcium-release measurements and each of them consisted of a control group, PTHrP, chemical substance, and PTHrP + chemical substance. Chemical substances used were: iberiotoxin, xestospongin C, xestospongin D, and thapsigargin, respectively. Fura-2 imaging was used to determine changes in calcium release of VSMC. In membrane-potential experiments, groups were designed similarly to the Fura-2 imaging experiments: iberiotoxin, BAPTA, and xestospongin D were added, in respective order. Changes in the membrane potential were examined using the fluorescence dye (DiBAC).

RESULTS

Given in a dose between 0.01 and 1.0 μmol/L, PTHrP caused a concentration-dependent decrease in fluorescence intensity, with a maximum effect at 0.5 μmol/L. The decrease, therefore, demonstrated a PTHrP-induced hyperpolarization of the VSMC. The effect was blocked by use of iberiotoxin (100 nmol/L), a highly selective inhibitor of BKCa. Furthermore, when the calcium chelator BAPTA (10 μmol/L) was added, there was a significant reduction in PTHrP-induced hyperpolarization. Use of PTHrP (0.5 μmol/L) also decreased the fluorescence intensity of the indicator for intracellular calcium, Fura-2AM (a membrane-permeable derivative of Fura 2). This effect was re-blocked by use of iberiotoxin. Xestospongin C (3 μmol/L) and xestospongin D (6 μmol/L), both inhibitors of the inositol 1,4,5 trisphosphate-triggered calcium release, inhibited the effects of PTHrP. Additionally, thapsigargin (1 μmol/L), a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase inhibitor, inhibited the effect of PTHrP.

CONCLUSION

The results of our study show that PTHrP induces hyperpolarization and activates BKCa in VSMC. The activation of BKCa channels is calcium dependent; activation is linked to the inositol 1,4,5 trisphosphate-triggered calcium release and is also dependent on the endo/sarcoplasmic reticulum calcium pump.

Parathyroid hormone related-protein promotes epithelial-to-mesenchymal transition in prostate cancer

Parathyroid hormone-related protein (PTHrP) possesses a variety of physiological and developmental functions and is also known to facilitate the progression of many common cancers, notably their skeletal invasion, primarily by increasing bone resorption. The purpose of this study was to determine whether PTHrP could promote epithelial-to-mesenchymal transition (EMT), a process implicated in cancer stem cells that is critically involved in cancer invasion and metastasis. EMT was observed in DU 145 prostate cancer cells stably overexpressing either the 1-141 or 1-173 isoform of PTHrP, where there was upregulation of Snail and vimentin and downregulation of E-cadherin relative to parental DU 145. By contrast, the opposite effect was observed in PC-3 prostate cancer cells where high levels of PTHrP were knocked-down via lentiviral siRNA transduction. Increased tumor progression was observed in PTHrP-overexpressing DU 145 cells while decreased progression was observed in PTHrP-knockdown PC-3 cells. PTHrP-overexpressing DU 145 formed larger tumors when implanted orthoptopically into nude mice and in one case resulted in spinal metastasis, an effect not observed among mice injected with parental DU 145 cells. PTHrP-overexpressing DU 145 cells also caused significant bone destruction when injected into the tibiae of nude mice, while parental DU 145 cells caused little to no destruction of bone. Together, these results suggest that PTHrP may work through EMT to promote an aggressive and metastatic phenotype in prostate cancer, a pathway of importance in cancer stem cells. Thus, continued efforts to elucidate the pathways involved in PTHrP-induced EMT as well as to develop ways to specifically target PTHrP signaling may lead to more effective therapies for prostate cancer.

Knockdown of parathyroid hormone related protein in smooth muscle cells alters renal hemodynamics but not blood pressure

Parathyroid hormone-related protein (PTHrP) belongs to vasoactive factors that regulate blood pressure and renal hemodynamics both by reducing vascular tone and raising renin release. PTHrP is expressed in systemic and renal vasculature. Here, we wanted to assess the contribution of vascular smooth muscle cell endogenous PTHrP to the regulation of cardiovascular and renal functions. We generated a mouse strain (SMA-CreERT2/PTHrPL2/L2 or premutant PTHrPSM-/-), which allows temporally controlled, smooth muscle-targeted PTHrP knockdown in adult mice. Tamoxifen treatment induced efficient recombination of PTHrP-floxed alleles and decreased PTHrP expression in vascular and visceral smooth muscle cells of PTHrPSM-/- mice. Blood pressure remained unchanged in PTHrPSM-/- mice, but plasma renin concentration and creatinine clearance were reduced. Renal hemodynamics were further analyzed during clearance measurements in anesthetized mice. Conditional knockdown of PTHrP decreased renal plasma flow and glomerular filtration rate with concomitant reduction in filtration fraction. Similar measurements were repeated during acute saline volume expansion. Saline volume expansion induced a rise in renal plasma flow and reduced filtration fraction; both were blunted in PTHrPSM-/- mice leading to impaired diuresis. These findings show that endogenous vascular smooth muscle PTHrP controls renal hemodynamics under basal conditions, and it is an essential factor in renal vasodilation elicited by saline volume expansion.

Concurrent primary hyperparathyroidism and humoral hypercalcemia of malignancy in a patient with multiple endocrine neoplasia type 1

We report a patient with multiple endocrine neoplasia type 1 presenting with elevation of parathyroid hormone-related protein (PTHrP) from a metastatic pancreatic neuroendocrine tumor (PNET), and parathyroid hormone (PTH) from primary hyperparathyroidism, resulting in severe hypercalcemia. Parathyroid hormone-related protein production by the PNET was confirmed by immunohistochemical analysis. Hypercalcemia and elevated PTHrP improved markedly with hepatic artery chemoembolization of liver metastasis. Thus, in multiple endocrine neoplasia type 1, correct identification of the cause of hypercalcemia as PTHrP production from a PNET or PTH production from a parathyroid tumor has important therapeutic implications.

Hypercalcemia and local production of parathyroid hormone-related protein by a perisellar rhabdomyosarcoma after remote pituitary irradiation

OBJECTIVE

To describe a case of metastatic rhabdomyosarcoma originating from the sphenoid sinus in a patient previously treated with conventional irradiation for a prolactinoma, presenting as hypercalcemia in the setting of a normal level of serum parathyroid hormone-related protein (PTHrP).

METHODS

We report the case of a patient who underwent remote pituitary irradiation for a prolactinoma and then presented decades later with hypercalcemia of unknown cause. His clinical course, the initial biochemical and radiologic investigations, and the results of examination of pathology specimens are reviewed.

RESULTS

The patient was found to have a mass in the sphenoid sinus. The pathologic features were consistent with alveolar rhabdomyosarcoma. Although he had a normal serum PTHrP level, staining of his tumor with an antibody against PTHrP revealed local production of PTHrP at the tumor margins. His bone marrow biopsy specimen showed 100% involvement with rhabdomyosarcoma.

CONCLUSION

PTHrP staining of pathology specimens might explain hypercalcemia of undetermined cause in patients with a known malignant lesion, in whom elevated serum PTHrP levels cannot be demonstrated.

Parathyroid hormone-related protein production in the lamprey Geotria australis: developmental and evolutionary perspectives

This study explored the distribution of parathyroid hormone-related protein (PTHrP) and its mRNA in tissues of the lamprey Geotria australis, a representative of one of the two surviving groups of an early and jawless stage in vertebrate evolution. For this purpose, antibodies to N-terminal and mid-molecule human PTHrP were used to determine the locations of the antigen. Sites of mRNA production were demonstrated by in situ hybridisation with a digoxigenin-labelled riboprobe to exon VI of the human PTHrP gene. The results revealed that antigen and its mRNA were widely distributed among similar sites of tissue localisation to those described for mammalian and avian species. However, some novel sites of localisation, such as in the gill and notochord, were also found. Some differences in PTHrP localisation were noted among individuals at different intervals of the life cycle, indicating that the distributions of PTHrP, and possibly its roles, change with the stage of development in this species. The widespread tissue distribution in G. australis implies diverse physiological roles for this protein. The presence of PTHrP in the lamprey, a representative of a group of vertebrates, which apparently evolved over 540 million years ago, strongly suggests that it is a protein of ancient origin. In addition, the successful use of antibodies and probes based on the human sequence in the lamprey also provides evidence that the PTHrP molecule may have been conserved from lampreys through to humans.

Parathyroid hormone-related peptide improves contractile responsiveness of adult rat cardiomyocytes with depressed cell function irrespectively of oxidative inhibition

Parathyroid hormone-related peptide (PTHrP) was found to improve contractile function of stunned myocardium in pigs. The peptide is released from coronary endothelial cells during ischemia and significantly improves post-ischemic recovery. The present study was aimed to decide whether such an induction of contractile responsiveness of the heart requires co-activation of adjacent cells or is a genuine phenomenon of cardiomyocytes. A second aim of this study was to decide whether such an improvement is linked to depressed cell function in general or oxidative inhibition. Isolated adult ventricular cardiomyocytes from rats were constantly paced at 0.5 Hz for 10 min. Cells were exposed to a brief oxidative inhibition by addition of 0.5 mmol/l potassium cyanide (KCN) in the presence of glucose. Under these conditions, cells stopped beating after 280 s on average. 30 s before they stopped to beat, cells had already developed a reduction in cell shortening, maximal relaxation and contraction velocity. In the co-presence of PTHrP (1-34) (100 nmol/l) cells continued to beat regular and did not develop reduced cell shortening, irrespectively of oxidative inhibition. In a second attempt, cells were exposed to the NO donor SNAP (100 micromol/l) or 8-bromocGMP (1 mmol/l). As expected both agents reduced cell shortening significantly. This reduction in cell shortening was attenuated in co-presence of PTHrP, too. Finally, we investigated the effect of PTHrP on cell shortening at different extracellular concentrations of calcium. Although, PTHrP increased intracellular calcium at 2 and 5 mmol/l extracellular calcium, respectively, it improved cell shortening only at 5 mmol/l extracellular calcium. Thus, the beneficial effect of PTHrP on cell shortening was independent from intracellular calcium but dependent on the steepness of the calcium gradient between intra- and extracellular calcium. In conclusion, our study shows that PTHrP is able to improve cell shortening rapidly and directly irrespectively of the reason for the reduced cell function. Improved electromechanical coupling rather than intracellular calcium handling seems to be the most important mechanism.

Altered renal hemodynamics in mice overexpressing the parathyroid hormone (PTH)/PTH-related peptide type 1 receptor in smooth muscle

PTH-related protein (PTHrP) is an autocrine/paracrine peptide expressed in renal tubules and vasculature and may play an important role in regulating overall renal function. To evaluate the potential role of endogenous PTHrP in the control of renal hemodynamics, we performed clearance measurements in transgenic (TG) mice in which the SMP8 alpha-actin promoter was used to drive overexpression of the PTH/PTHrP type 1 receptor in smooth muscle. In protocol I, responses to acute saline volume expansion (SVE, 0.75 microl/min.g body weight) were measured in TG and nontransgenic (NTG) mice. Mean arterial pressure was significantly lower in TG mice throughout the experiment, and it decreased comparably in both groups in response to SVE. SVE significantly increased effective renal plasma flow in both groups of mice, but the increase was greater in TG than in NTG. Glomerular filtration rate decreased in response to SVE in NTG but did not change in TG animals. In protocol II, renal responses to angiotensin II (ANG II) infusion were determined (0.5 ng/min.g body weight). Baseline arterial pressure was again significantly lower in TG, compared with NTG mice, and TG mice had a blunted pressor response to ANG II. Also, ANG II decreased effective renal plasma flow and glomerular filtration rate in both groups of animals, but the reductions were less in TG than in NTG mice. Our findings indicate that smooth-muscle-specific overexpression of the PTH/PTHrP type 1 receptor resulted in augmentation of the vasodilatory response to SVE and attenuation of the vasoconstrictor response to ANG II. We conclude that endogenous PTHrP can act as an endogenous vasorelaxant factor to modulate renal responses to vasoactive stimuli.

Parathyroid hormone-related peptide improves contractile function of stunned myocardium in rats and pigs

The effect of synthetic parathyroid hormone (PTH)-related peptide [PTHrP(1-34)] on regional myocardial function was studied in 11 anesthetized pigs. Intracoronary infusion of PTHrP (cumulative dose: 14 +/- 1 microg) decreased coronary resistance to 33 +/- 2% of baseline (P < 0.05) and regional myocardial function to 90 +/- 3% of baseline (not significant). Ischemia-reperfusion alters the activity of several kinases and therefore possibly the myocardial effects of PTHrP. In stunned myocardium, induced by 20-min ischemia and 30-min reperfusion, the dose of PTHrP reducing coronary resistance to a minimum of 29 +/- 2% was decreased to 8 +/- 2 microg (P < 0.05). Regional myocardial function was no longer decreased but increased to 132 +/- 9% (P < 0.05). The increase in regional myocardial function during PTHrP was inversely related to baseline function at 30-min reperfusion in vivo (r = 0.9) as well as in myocytes isolated from stunned pig hearts (r = 0.7). In isolated rat hearts subjected to 30-min global ischemia followed by 30-min reperfusion, blockade of endogenous PTHrP by d-Trp(12)-Tyr(34)-PTH(7-34) attenuated the recovery of left ventricular developed pressure by 30 +/- 14% (P < 0.05). Thus endogenous and exogenous PTHrP impact on the function of stunned myocardium.

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.

Arterial calcification: a review of mechanisms, animal models, and the prospects for therapy

The causes of arterial calcification are beginning to be elucidated. Macrophages, mast cells, and smooth muscle cells are the primary cells implicated in this process. The roles of a variety of bone-related proteins including bone morphogenetic protein-2 (BMP-2), matrix Gla protein (MGP), osteoprotegerin (OPG), osteopontin, and osteonectin in regulating arterial calcification are reviewed. Animals lacking MGP, OPG, smad6, carbonic anhydrase isoenzyme II, fibrillin-1, and klotho gene product develop varying extents of arterial calcification. Hyperlipidemia, vitamin D, nicotine, and warfarin, alone or in various combinations, produce arterial calcification in animal models. MGP has recently been discovered to be an inhibitor of bone morphogenetic protein-2, the principal osteogenic growth factor. Many of the forces that induce arterial calcification may act by disrupting the essential post-translational modification of MGP, allowing BMP-2 to induce mineralization. MGP requires gamma-carboxylation before it is functional, and this process uses vitamin K as an essential cofactor. Vitamin K deficiency, drugs that act as vitamin K antagonists, and oxidant stress are forces that could prevent the formation of GLA residues on MGP. The potential role of arterial apoptosis in calcification is discussed. Potential therapeutic options to limit the rate of arterial calcification are summarized.

TGF-beta(1) downregulates PTHrP in coronary endothelial cells

Parathyroid hormone-related peptide (PTHrP) is expressed throughout the cardiovascular system including coronary endothelial cells. Factors involved in the regulation of cardiac PTHrP expression have not been examined before. This study investigates the influence of transforming growth factor (TGF)-beta(1)on ventricular PTHrP expression. Coronary endothelial cells were isolated from ventricles of adult rats and PTHrP protein expression in these cultures was analysed by immunoblotting. TGF-beta(1)caused a concentration-dependent reduction in PTHrP protein within 24 h. In transgenic mice over-expressing TGF-beta(1)ventricular PTHrP protein expression and release was reduced compared to non-transgenic littermates. Similar concerns hold for PTHrP mRNA content (RT-PCR). Since ventricular TGF-beta(1)expression increases under pathophysiological conditions like arterial hypertension, ventricular PTHrP expression was further determined in aging spontaneously hypertensive (SHR-SP) and normotensive rats. TGF- beta(1)expression was increased in SHR-SP and ventricular PTHrP mRNA expression was downregulated at the age of 10 months. PTHrP expression did not recover in elder SHR-SP in which TGF-beta(1)expression was normalized again. Finally, we investigated ventricular PTHrP expression in rats after banding of the ascending aorta which generates a pressure induced hypertrophy without an induction of TGF-beta(1)expression. In ventricles from these animals, PTHrP expression was transiently increased and normalized at day 3. In conclusion, PTHrP expression was reduced under all conditions in which coronary endothelial cells were exposed to TGF-beta(1). PTHrP expression does not correlate with cardiac hypertrophy. Since coronary endothelial cells represent the majority of PTHrP producing cells in the ventricle its downregulation by TGF- beta(1)seems to be relevant for the paracrine effects of PTHrP.

Expression, release, and biological activity of parathyroid hormone-related peptide from coronary endothelial cells

Ventricular cardiomyocytes have previously been identified as potential target cells for parathyroid hormone-related peptide (PTHrP). Synthetic PTHrP peptides exert a positive contractile effect. Because systemic PTHrP levels are normally negligible, this suggests that PTHrP is expressed in the ventricle and acts as a paracrine mediator. We investigated the ventricular expression of PTHrP and its expression in cultured cells isolated from the ventricle, studied the release of PTHrP from hearts and cultures, and investigated whether this authentic PTHrP mimics the biological effects previously described for synthetic PTHrP on ventricular cardiomyocytes. We found PTHrP expressed in ventricles of neonatal and adult rat hearts. In cells isolated from adult hearts, we found PTHrP expression exclusively in coronary endothelial cells but not in cardiomyocytes. The latter, however, are target cells for PTHrP. PTHrP was released from isolated perfused hearts during hypoxic perfusion and from cultured coronary endothelial cells under energy-depleting conditions. This PTHrP was biologically active; ie, it exerted a positive contractile and lusitropic effect on cardiomyocytes. Authentic PTHrP was glycosylated and showed a slightly higher potency than synthetic PTHrP. These results suggest that PTHrP is an endothelium-derived modulator of ventricular function.

Targeted overexpression of parathyroid hormone-related protein (PTHrP) to vascular smooth muscle in transgenic mice lowers blood pressure and alters vascular contractility

PTH-related protein (PTHrP) and its receptor are expressed in vascular smooth muscle cells and are believed to participate in the local regulation of vascular tone. To explore the function of locally produced PTHrP in vascular smooth muscle in vivo, we developed transgenic mice that overexpress PTHrP in smooth muscle using a smooth muscle alpha-actin promoter to direct expression of the transgene. In the PTHrP-overexpressing mice, messenger RNA expression was mainly restricted to smooth muscle-containing tissues. Several founders also expressed the transgene in bone and heart and exhibited striking abnormalities in the development of these tissues. In PTHrP-overexpressing mice, blood pressure was significantly lower than that in wild-type controls (121 +/- 3 vs. 135 +/- 2 mm Hg; P < 0.01). Moreover, the magnitude of the vasorelaxant response to iv infusions of PTHrP-(1-34)NH2 was significantly attenuated in the transgenic animals. A similar desensitization to PTHrP was observed in aortic ring and portal vein preparations. Surprisingly, PTHrP-overexpressing mice were also significantly less responsive to the hypotensive action of infused acetylcholine in vivo and to the relaxant actions of acetylcholine on aortic vessel preparations in vitro. In summary, we have successfully targeted overexpression of PTHrP to the smooth muscle of transgenic mice. When expressed in its normal autocrine/paracrine setting, PTHrP lowers systemic blood pressure and decreases vascular responsiveness to further relaxation by PTHrP and other endothelium-dependent vasorelaxants such as acetylcholine. We postulate that the heterologous desensitization to acetylcholine-induced relaxation in PTHrP-overexpressing blood vessels involves desensitization of second messenger/effector signaling pathways common to PTHrP and acetylcholine.

Reduced blood pressure and increased sensitivity of the vasculature to parathyroid hormone-related protein (PTHrP) in transgenic mice overexpressing the PTH/PTHrP receptor in vascular smooth muscle

PTH-related protein (PTHrP) is produced in vascular smooth muscle, where it is postulated to exert vasorelaxant properties by activation of the PTH/PTHrP type 1 receptor. As a model for studying the actions of locally produced PTHrP in vascular smooth muscle in vivo, we developed transgenic mice that overexpress the PTH/PTHrP receptor (PTHrP-R) in smooth muscle. Oocyte injection with a SMP8-PTHrP-R fusion construct yielded six founder mice. F1 offspring were viable and demonstrated selective overexpression of the SMP8-PTHP-R messenger RNA in smooth muscle-rich tissues. Baseline blood pressure measured in conscious mice by tail sphygmomanometry was significantly lower in the receptor-overexpressing mice than that in controls (117 +/- 4 vs. 133 +/- 3 mm Hg; P < 0.05). In anesthetized animals, iv infusion of PTHrP-(1-34)NH2 caused a significantly greater reduction in blood pressure and total peripheral resistance in transgenic mice than in control animals. Vascular contractility was studied in paired, isometrically mounted aortas from 9-week-old transgenic and wild-type mice. The force of contraction in response to phenlyephrine was not significantly different between transgenic and wild-type mice. However, PTHrP-(1-34) NH2 relaxed aortic vessel preparations from transgenic mice to a greater extent than in controls (77.1 +/- 3% vs. 38.4 +/- 4%; P < 0.001). To determine the impact of overexpression of PTH/PTHrP type 1 receptor and its ligand on the development of the cardiovascular system, double transgenic mice were created by crossing SMP8-PTHrP-R transgenic mice with mice overexpressing PTHrP (SMP8-PTHrP). Double transgenic mice died around day E9 with abnormalities in the developing heart. In conclusion, overexpression of PTH/PTHrP type 1 receptor in vascular smooth muscle of transgenic mice reduces blood pressure, probably through sustained activation of the receptor by endogenous ligand. The cardiovascular defects observed in mice overexpressing both PTHrP and its receptor suggest that PTHrP may play a role in the normal development of the cardiovascular system.

Parathyroid hormone-related proteins is abundant in osteoarthritic cartilage, and the parathyroid hormone-related protein 1-173 isoform is selectively induced by transforming growth factor beta in articular chondrocytes and suppresses generation of extracellular inorganic pyrophosphate

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a major, locally expressed regulator of growth cartilage chondrocyte proliferation, differentiation, synthetic function, and mineralization. Because mechanisms that limit cartilage chondrocytes from maturing and mineralizing are diminished in osteoarthritis (OA), we studied PTHrP expression by articular chondrocytes.

METHODS

PTHrP was studied in normal knee cartilage samples and cultured articular chondrocytes, and in cartilage specimens from knees with advanced OA, obtained at the time of joint replacement.

RESULTS

PTHrP was more abundant in OA than in normal human knee articular cartilage. Both demonstrated PTH/PTHrP receptor expression. PTHrP 1-173, one of three alternatively spliced PTHrP isoforms, was exclusively expressed and induced by transforming growth factor beta in cultured chondrocytes. Chondrocytes mainly used the GC-rich P2 alternative promoter to express PTHrP messenger RNA. Inhibition by PTHrP 1-173, but not by PTHrP 1-146 or PTHrP 1-87, of inorganic pyrophosphate (PPi) elaboration suggested selective functional properties of the 1-173 isoform. Exposure to a neutralizing antibody to PTHrP increased PPi elaboration by articular chondrocytes.

CONCLUSION

Increased expression of PTHrP, including the 1-173 isoform, has the potential to contribute to the pathologic differentiated functions of chondrocytes, including mineralization, in OA.

Centrally administered parathyroid hormone (PTH)-related protein(1-34) but not PTH(1-34) stimulates arginine-vasopressin secretion and its messenger ribonucleic acid expression in supraoptic nucleus of the conscious rats

It has been suggested that PTH-related protein (PTHrP) is an endogenous modulator of cardiovascular systems. We have reported that PTHrP(1-34), but not PTH(1-34), causes the release of arginine-vasopressin (AVP) from the supraoptic nucleus (SON) of the hypothalamus in vitro through a novel receptor distinct from the PTH/PTHrP receptors (type I or type II) described previously. In this study, we have investigated the in vivo effects of PTHrP(1-34) on AVP secretion and its, messenger RNA (mRNA) expression in the SON in conscious rats. Intracerebroventricular (i.c.v.) administration of PTHrP(1-34) resulted in an increase in plasma AVP concentration in a dose-dependent manner (0-400 pmol/rat). The maximal effect was obtained at 15 min after i.c.v. administration of PTHrP(1-34). Neither PTHrP(7-34) nor PTH(1-34) had any effect on plasma AVP levels. PTHrP(1-34)-induced AVP secretion was antagonized by pretreatment with PTHrP(7-34) but not by that with PTH(1-34). In addition, in situ hybridization study revealed that AVP mRNA expression in the SON and paraventricular nucleus was significantly increased 30 min after i.c.v. administration of PTHrP(1-34) and reached a maximum at 180 min. Furthermore, in Northern blot analyses, AVP mRNA expression in the SON was increased to approximately a 2-fold of basal level by PTHrP(1-34). On the other hand, neither PTHrP(7-34) or PTH(1-34) had any effect on the mRNA expression. The PTHrP(1-34)-stimulated AVP mRNA expression was eliminated by pretreatment with PTHrP(7-34) but not with PTH(1-34). These results suggest that, in the central nervous system, PTHrP(1-34) is involved in AVP secretion through a novel receptor distinct from the PTH/PTHrP receptors reported previously, playing a role in the body water and electrolyte homeostasis.

Cardiovascular actions of parathyroid hormone and parathyroid hormone-related peptide

Cardiovascular cells (cardiomyocytes and smooth muscle cells) are target cells for parathyroid hormone (PTH) and the structurally related peptide parathyroid hormone-related peptide (PTH-rP). PTH activates protein kinase C (PKC) of cardiomyocytes via a PKC activating domain previously identified on chondrocytes. Activation of PKC leads to hypertrophic growth and re-expression of fetal type proteins in cardiomyocytes. This hypertrophic effect of PTH might contribute to left ventricular hypertrophy in hemodialysis patients with secondary hyperparathyroidism. PTH-rP is expressed in cardiovascular cells (endothelial cells and smooth muscle cells). It does not mimic the above described actions of PTH but exerts effects of its own on cardiomyocytes. These effects involve activation of protein kinase A, via a N-terminal domain distinct from that identified on PTH, and activation of PKC, via a C-terminally located domain distinct from that found on PTH. On smooth muscle cells PTH and PTH-rP reduce the influence of extracellular calcium, through cAMP-dependent mechanisms. These inhibitory effects on voltage-dependent L-type calcium channels of smooth muscle cells cause vasorelaxation. Present studies concerning cardiovascular actions of either PTH and PTH-rP suggest that increased plasma levels of PTH and PTH-rP influence cardiomyocyte and smooth muscle cell physiology. It can be assumed that PTH-rP acts as a paracrine or autocrine modulator in heart and vessels.

Positive chronotropic actions of parathyroid hormone and parathyroid hormone-related peptide are associated with increases in the current, I(f), and the slope of the pacemaker potential

BACKGROUND

The classic calciotropic hormone parathyroid hormone (PTH) and its paracrine factor parathyroid hormone-related protein (PTHrP) both increase heart rate.

METHODS AND RESULTS

We used standard electrophysiological techniques to study the effects of PTH and PTHrP on isolated rabbit sinus node, isolated canine Purkinje fibers, and disaggregated rabbit sinus node myocytes. Sinus node maximum diastolic potential, activation voltage, and amplitude were unchanged by PTH or PTHrP (P>.05). However, the slope of phase 4 and the automatic rate were increased at PTH and PTHrP > or = 10 nmol/L (P<.05). Comparable results were seen in canine Purkinje fibers. We then used the perforated-patch technique to study the I(f) pacemaker current in sinus node. PTH 12.5 nmol/L and PTHrP 12.5 to 18 nmol/L increased I(f) at -65 mV by 68+/-41% (n=5) and 69+/-50% (n=5), respectively. Actions of both agents were reversible. The increase in I(f) appeared to result from a change in maximal conductance and not a shift in the voltage dependence of activation.

CONCLUSIONS

These observations provide, for the first time, direct electrophysiological support for the chronotropic actions of PTH and PTHrP. They suggest that classic hormones and paracrine factors can have multiple functions and that in the case of PTH and PTHrP, a newly recognized action is to alter automaticity directly.

Increased expression of parathyroid hormone-related peptide gene in blood vessels of spontaneously hypertensive rats

We have shown recently that mechanical stretch of cultured rat aortic smooth muscle cells induces a marked increase in gene expression of the vasorelaxant parathyroid hormone-related peptide. In the present study, we investigated whether mechanical force affected the in vivo parathyroid hormone-related peptide gene expression in blood vessels. Northern blot analysis revealed that stretch of isolated rat aortic strips increased the expression level of parathyroid hormone-related peptide mRNA. The parathyroid hormone-related peptide transcript level in aorta and mesenteric vessels from 18-week-old spontaneously hypertensive rats (SHR) was 2.5- and 2.2-fold higher, respectively, compared with age-matched Wistar-Kyoto (WKY) controls, whereas the parathyroid hormone-related peptide mRNA level in aorta from normotensive 4-week-old SHR was similar to that of age-matched WKY controls. The aortic parathyroid hormone-related peptide content was higher in 18-week-old SHR than in age-matched WKY controls. Moreover, treatment of mature SHR with an angiotensin II type 1 receptor antagonist or hydralazine caused a concomitant decrease in the parathyroid hormone-related peptide transcript level in aorta with lowering of blood pressure. These results suggest that the in vivo parathyroid hormone-related peptide gene expression in blood vessels is under the control of mechanical force, pointing to a role of parathyroid hormone-related peptide in the regulation of vascular tone.

Hemodynamic effects of parathyroid hormone-related peptide-(1-34) in humans

It has been suggested that PTH-related peptide-(1-34) (PTHrP) is a regulator or modulator of regional or systemic cardiovascular function with varying vasodilating actions in different species. We have studied the cardiovascular pharmacodynamic profile of PTHrP in healthy humans. In a double blind, placebo-controlled, cross-over study design, eight healthy subjects were assigned to stepwise increased i.v. doses of PTHrP. In addition, a dose-response curve to PTHrP was constructed in a dorsal hand vein in eight subjects. PTHrP dose-dependently increased pulse rate and renal plasma flow by more than 50% (P < 0.0001 for both parameters, by ANOVA), but only a small venodilating response was seen in hand vein experiments, and no effect was noted on mean arterial blood pressure or cardiac inotropic performance. Although it is unlikely that PTHrP regulates systemic hemodynamics, its chronotropic effect and its potent action on renal plasma flow may represent the primary cardiovascular physiological targets of action.

Effect of acute endurance and strength exercise on circulating calcium-regulating hormones and bone markers in young healthy males

Physical activity plays a role in the maintenance of the skeleton but the mechanical, metabolic and hormonal mechanisms involved are largely unknown. The influence of acute endurance and strength exercise on circulating levels of calcitonin, parathyroid hormone (PTH), PTH-related peptide (PTHrP), osteocalcin, carboxyterminal cross-linked telopeptide of type I collagen (ICTP) and ionized calcium (Ca2+) was therefore evaluated. Eight healthy young males performed three exercise bouts on separate occasions: endurance exercise, i.e. cycling on a cycle ergometer for 45 min at 55% of Vo2max (E55%) and 15 min at 85% of Vo2max (E85%) and strength exercise at 85% of three repetitions maximum using a leg-press device (STR). Control experiments included the same subjects with the same time schedule but without exercise. Blood samples were taken before, immediately after exercise and during the recovery period. Hormones and bone markers were measured by use of various immunoassays. There was no obvious influence on calcitonin and PTHrP levels, whereas PTH was increased after strength exercise. ICTP and osteocalcin levels correlated positively at all times and showed regular variations. In comparison with the controls, ICTP levels showed a more pronounced decrease following physical activity whereas osteocalcin followed the same pattern as the controls except for after prolonged endurance exercise when a decrease was abolished. In conclusion, an increase in PTH after strength exercise and a pronounced decrease in ICTP after all exercise together with a relative increase in osteocalcin after prolonged endurance exercise might reflect some mechanisms involved in the positive effect of physical activity on bone mass.

Parathyroid hormone-related peptide as a local regulator of vascular calcification. Its inhibitory action on in vitro calcification by bovine vascular smooth muscle cells

In the present study, we investigated the role of parathyroid hormone-related peptide (PTHrP) in vascular calcification by using an in vitro calcification model. We demonstrated that the expression of PTHrP decreased in the progression of bovine vascular smooth muscle cell (BVSMC) calcification and that inhibition of calcification by etidronate (EHDP) and levamisole restored PTHrP secretion, suggesting that the expression of PTHrP is associated with calcification. PTHrP (1-34) and PTH (1-34) dose-dependently inhibited BVSMC calcification. Protein kinase A (PKA) and protein kinase C (PKC) inhibitors completely blocked the inhibitory effect of PTHrP, suggesting that both PKA and PKC may be involved in its signaling pathway. Moreover, PTHrP inhibited alkaline phosphatase (ALP) activity, implying that the impact on ALP may contribute to its action on calcification. Furthermore, the PTHrP antagonist, PTHrP (7-34), dose-dependently increased calcium deposition by BVSMC. Interestingly, PTHrP production by BVSMC dramatically increased in the presence of EHDP, and PTHrP (7-34) partially antagonized the inhibitory effect of EHDP on BVSMC calcification. These results suggest that PTHrP may regulate vascular calcification as an autocrine/paracrine factor.

Parathyroid hormone-related peptide-(1-34) [PTHrP-(1-34)] induces vasopressin release from the rat supraoptic nucleus in vitro through a novel receptor distinct from a type I or type II PTH/PTHrP receptor

PTH and PTH-related peptide (PTHrP) bind to a type I PTH/PTHrP receptor expressed in bone and kidney or a type II receptor in nonclassical target tissue with equal affinity and similar bioactivities. PTHrP is abundant in the central nervous system, but its physiological role remains unknown. Herein, we examined the role of PTHrP-(1-34) on arginine vasopressin (AVP) release from the rat supraoptic nucleus (SON). Application of PTHrP-(1-34) to SON slices caused an increase in AVP release in a concentration-dependent manner. Neither PTHrP-(7-34) nor PTH-(1-34) had any effect on AVP release from the SON. PTHrP-(1-34)-induced AVP release was antagonized by a large excess of PTHrP-(7-34) and by H89, an inhibitor of cAMP-dependent protein kinase (A kinase), but not by PTH-(1-34) or PTH-(13-34). PTHrP-(1-34), but not PTH-(1-34), also dose-dependently increased the levels of cAMP in the SON. 125I-Labeled PTHrP-(1-34) bound specifically to crude membranes isolated from the SON. Scatchard analysis showed a single class of binding sites for PTHrP-(1-34) with a Kd of 36.4 nM and a maximum binding capacity of 3.94 pmol/mg protein. No specific binding for 125I-labeled PTH-(1-34) was noted. The binding of 125I-labeled PTHrP-(1-34) was displaced by unlabeled PTHrP-(1-34) and unlabeled PTHrP-(7-34), but not by unlabeled PTH-(1-34). These findings suggest that PTHrP-(1-34), but not PTH-(1-34), causes the release of AVP from the SON through a novel receptor distinct from type I or II PTH/PTHrP receptors.

Parathyroid hormone-related protein is not an autocrine growth factor for normal prostatic epithelial cells

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is the primary factor responsible for humoral hypercalcemia of malignancy. The hypercalcemic actions of PTHrP occur via stimulation of renal distal tubular calcium reabsorption and increased osteoclastic bone resorption. These effects of PTHrP are thought to be mediated through a common parathyroid hormone (PTH)/PTHrP receptor. In addition to the well-established role of PTHrP in bone remodeling, PTHrP is believed to be an important mediator of cellular growth and differentiation in a number of nonbony tissues. We recently demonstrated abundant expression of PTHrP in normal and malignant human prostatic tissues, and in cultured prostatic epithelial cells.

METHODS

In vitro assays were used to test growth-regulatory activity of synthetic and endogenous PTHrP peptides on normal prostatic epithelial cells.

RESULTS

No growth-regulatory activity could be demonstrated.

CONCLUSIONS

PTHrP is not an autocrine growth factor for normal prostatic epithelial cells.

Hypercalcemia and systemic lupus erythematosus

Hypercalcemia is commonly caused by the increased production of parathyroid hormone-related protein (PTHrP) by a malignancy. In fact, the demonstration of increased PTHrP production in a patient with hypercalcemia is virtually pathognomonic of malignancy. We studied a patient with systemic lupus erythematosus (SLE), generalized lymphadenopathy, and hypercalcemia. Immunohistology of 2 biopsied lymph nodes revealed the abundant expression of PTHrP and the absence of malignant transformation. Although apparently rare, PTHrP production by non-malignant lymphoid tissue may occur in SLE and should be considered in the differential diagnosis of hypercalcemia.

Effect of intrarenally infused parathyroid hormone-related protein on renal blood flow and glomerular filtration rate in the anaesthetized rat

1. Parathyroid hormone-related protein (PTHrP) is expressed in the kidney and acts on vascular PTH/ PTHrP receptors to vasodilate the isolated kidney and to stimulate renin release. However, effects of PTHrP on renal blood flow (RBF) and glomerular filtration rate (GFR) in vivo have not been assessed in the absence of its cardiac, peripheral and central effects. We investigated the renal effects of PTH and PTHrP infused into the left renal artery of anaesthetized rats. 2. Intrarenal infusions, adjusted to generate increasing concentrations of human PTHrP(1-34) and rat PTH(1-34) in renal plasma (2 x 10(-11) to 6 x 10(-9) M) produced a comparable dose-dependent increase in RBF. The rise was 4% at the lowest and 34% at the highest concentrations of peptides. Up to a concentration of 2 x 10(-9) M, mean arterial pressure (MAP) and heart rate were not affected, but at 6 x 10(-9) M, intrarenally infused peptides reached the peripheral circulation, and caused a fall in MAP within a few minutes. While MAP returned to basal value after the last peptide infusion, RBF remained more than 10% above control for at least 30 min. 3. Two competitive PTH/PTHrP receptor antagonists, [Nle8,18, Tyr34]-bPTH(3-34)amide and [Leu11, D-Trp12]-hPTHrP(7-34)amide (2 x 10(-8) M) were devoid of agonist activity, but markedly antagonized the dose-dependent increase in RBF elicited by PTHrP. 4. GFR and urine flow were measured in left PTHrP-infused experimental kidney and right control kidney. Renal PTHrP concentration of 10(-10) M elevated left RBF by 10%, and GFR by 20% without significantly increasing filtration fraction, and increased urine flow by 57%. In the right control kidney GFR and diuresis did not change. 5. The results indicate that PTHrP has similar renal haemodynamic effects as PTH and increases RBF, GFR and diuresis in anaesthetized rats.

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.

Immunohistochemical localization of parathyroid hormone-related protein in prostatic intraepithelial neoplasia

Parathyroid hormone-related protein (PTHrP) is a regulatory protein hormone that has been associated with normal fetal growth and differentiation as well as fetal calcium regulation. Parathyroid hormone-related protein has been implicated in a variety of carcinomas as a major factor in the development of humoral hypercalcemia of malignancy and may also play a role as an autocrine growth factor. In a previous immunohistochemical study we found that all prostatic adenocarcinomas (CAP) express PTHrP. In the current study, we evaluated PTHrP in prostate intraepithelial neoplasia (PIN) in radical prostatectomy specimens. A validated mouse monoclonal antibody, 9H7, raised against fragment 109-141 of the carboxy-terminus of PTHrP was used for immunostaining. The results generally showed negative to weak staining of normal and hyperplastic tissue and strong staining in PIN. The staining intensity was further evaluated by computer based image analysis. The relative optical density in PIN (9.24 +/- 9.05) was significantly (P = .008) higher than that in normal gland (.00 +/- 3.6). These findings suggest that PTHrP may be involved in the pathogenesis of prostatic dysplasia, and its immunohistochemical evaluation may have diagnostic use in the evaluation of PIN.