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

The Molecular Mechanisms Underlying the Systemic Effects Mediated by Parathormone in the Context of Chronic Kidney Disease

Chronic kidney disease (CKD) stands as a prominent non-communicable ailment, significantly impacting life expectancy. Physiopathology stands mainly upon the triangle represented by parathormone-Vitamin D-Fibroblast Growth Factor-23. Parathormone (PTH), the key hormone in mineral homeostasis, is one of the less easily modifiable parameters in CKD; however, it stands as a significant marker for assessing the risk of complications. The updated "trade-off hypothesis" reveals that levels of PTH spike out of the normal range as early as stage G2 CKD, advancing it as a possible determinant of systemic damage. The present review aims to review the effects exhibited by PTH on several organs while linking the molecular mechanisms to the observed actions in the context of CKD. From a diagnostic perspective, PTH is the most reliable and accessible biochemical marker in CKD, but its trend bears a higher significance on a patient's prognosis rather than the absolute value. Classically, PTH acts in a dichotomous manner on bone tissue, maintaining a balance between formation and resorption. Under the uremic conditions of advanced CKD, the altered intestinal microbiota majorly tips the balance towards bone lysis. Probiotic treatment has proven reliable in animal models, but in humans, data are limited. Regarding bone status, persistently high levels of PTH determine a reduction in mineral density and a concurrent increase in fracture risk. Pharmacological manipulation of serum PTH requires appropriate patient selection and monitoring since dangerously low levels of PTH may completely inhibit bone turnover. Moreover, the altered mineral balance extends to the cardiovascular system, promoting vascular calcifications. Lastly, the involvement of PTH in the Renin-Angiotensin-Aldosterone axis highlights the importance of opting for the appropriate pharmacological agent should hypertension develop.

Uric Acid Deteriorates Load-Free Cell Shortening of Cultured Adult Rat Ventricular Cardiomyocytes via Stimulation of Arginine Turnover

Hyperuricemia is a risk factor for heart disease. Cardiomyocytes produce uric acid via xanthine oxidase. The enzymatic reaction leads to oxidative stress in uric-acid-producing cells. However, extracellular uric acid is the largest scavenger of reactive oxygen species, specifically to nitrosative stress, which can directly affect cells. Here, the effect of plasma-relevant concentrations of uric acid on adult rat ventricular cardiomyocytes is analyzed. A concentration- and time-dependent reduction of load-free cell shortening is found. This is accompanied by an increased protein expression of ornithine decarboxylase, the rate-limiting enzyme of the polyamine metabolism, suggesting a higher arginine turnover. Subsequently, the effect of uric acid was attenuated if other arginine consumers, such as nitric oxide synthase, are blocked or arginine is added. In the presence of uric acid, calcium transients are increased in cardiomyocytes irrespective of the reduced cell shortening, indicating calcium desensitization. Supplementation of extracellular calcium or stimulation of intracellular calcium release by β-adrenergic receptor stimulation attenuates the uric-acid-dependent effect. The effects of uric acid are attenuated in the presence of a protein kinase C inhibitor, suggesting that the PKC-dependent phosphorylation of troponin triggers the desensitizing effect. In conclusion, high levels of uric acid stress cardiomyocytes by accelerating the arginine metabolism via the upregulation of ornithine decarboxylase.

Parathyroid Hormone-Related Peptide: A Novel Endocrine Cardioprotective "Conditioning Mimetic"

An as-yet limited body of evidence suggests that calcium-regulating endocrine hormones-in particular, parathyroid hormone-related peptide (PTHrP)-may have unappreciated cardioprotective effects. The current review focuses on the concept that PTHrP may, via modulation of classic cardioprotective signaling pathways, provide a novel strategy to attenuate myocardial ischemia-reperfusion injury.

The Effects of Swiprosin-1 on the Formation of Pseudopodia-Like Structures and β-Adrenoceptor Coupling in Cultured Adult Rat Ventricular Cardiomyocytes

BACKGROUND

Recent findings suggest that adult terminally differentiated cardiomyocytes adapt to stress by cellular de- and redifferentiation. In the present study we tested the hypothesis that swiprosin-1 is a key player in this process. Furthermore, the relationship between swiprosin-1 and β-adrenoceptor coupling was analyzed.

METHODS

In order to study the function of swiprosin-1 in adult rat ventricular cardiomyocytes (ARVC) they were isolated and cultured in a medium containing 20% fetal calf serum (FCS). Changes in cell morphology of ARVC during cultivation were quantified by light and confocal laser scan microscopy. Small interfering RNA (siRNA) was used to reduce the expression of swiprosin-1. The impact of calcium on swiprosin-1 dependent processes was investigated with Bapta-AM. Immunoblot techniques and qRT-PCR were performed to measure mRNA and protein expression.

RESULTS

In culture, ARVC first lost their contractile elements, which was followed by a formation of pseudopodia-like structures (spreading). Swiprosin-1 was detected in ARVC at all time points. However, swiprosin-1 expression was increased when ARVC started to spread. Reduction of swiprosin-1 expression with siRNA delayed ARVC spreading. Similarly, Bapta-AM attenuated swiprosin-1 expression and spreading of ARVC. Furthermore, swiprosin-1 expression correlated with the expression of G protein-coupled receptor kinase 2 (GRK2). Moreover, silencing of swiprosin-1 was associated with a down regulation of GRK2 and caused a sensitization of β-adrenergic receptors.

CONCLUSION

Swiprosin-1 is required for ARVC to adapt to culture conditions. Additionally, it seems to be involved in the desensitization of β-adrenergic receptors. Assuming that ARVC adapt to cardiac stress in a similar way, swiprosin-1 may play a key role in cardiac remodeling.

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

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

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.

Mechanisms by which calcium receptor stimulation modifies electromechanical coupling in isolated ventricular cardiomyocytes

The calcium-sensing receptor (CaR) is widely expressed throughout the entire cardiovascular system and is capable of activating signaling pathways in different cells. Alongside calcium, the CaR also responds to physiological polycations such as putrescine underlining a participation in physiological and pathophysiological processes. Here, we aimed to determine mechanisms as to how CaR activation affects the contractile responsiveness of ventricular cardiomyocytes under basal and stimulated conditions. For that purpose, cardiac myocytes from 3-month-old male Wistar rats were isolated, and the acute effects of an antagonist (NPS2390), agonists (putrescine and gadolinium), or of downregulation of the CaR by siRNA on cell shortening were recorded in a cell-edge-detection system. In addition, experiments were performed on muscle stripes and Langendorff preparations. Mechanistic insights were taken from calcium transients of beating fura-2 AM-loaded cardiomyocytes and western blots. Isolated ventricular cardiomyocytes constitutively express CaR. The expression in the atria is less pronounced. Acute inhibition of CaR reduced basal cell shortening of ventricular myocytes at nearly physiological levels of extracellular calcium. Inhibition of CaR strongly reduced contractility of ventricular muscle stripes but not of atria. Activation of CaR by putrescine and gadolinium influences the contractile responsiveness of isolated cardiomyocytes. Increased calcium mobilization from the sarcoplasmic reticulum via an IP3-dependent mechanism was responsible for amplified systolic calcium transients and a subsequent improvement in cell shortening. Alongside with these effects, activation of CaR increased relaxation velocity of the cells. In conclusion, ventricular CaR expression affects contractile parameters of ventricular heart muscle cells and modifies electromechanical coupling of cardiomyocytes.

Recent insights in the paracrine modulation of cardiomyocyte contractility by cardiac endothelial cells

The cardiac endothelium is formed by a continuous monolayer of cells that line the cavity of the heart (endocardial endothelial cells (EECs)) and the luminal surface of the myocardial blood vessels (intramyocardial capillary endothelial cells (IMCEs)). EECs and IMCEs can exercise substantial control over the contractility of cardiomyocytes by releasing various factors such as nitric oxide (NO) via a constitutive endothelial NO-synthase (eNOS), endothelin-1, prostaglandins, angiotensin II, peptide growth factors, and neuregulin-1. The purpose of the present paper is actually to shortly review recent new information concerning cardiomyocytes as effectors of endothelium paracrine signaling, focusing particularly on contractile function. The modes of action and the regulatory paracrine role of the main mediators delivered by cardiac endothelial cells upon cardiac contractility identified in cardiomyocytes are complex and not fully described. Thus, careful evaluation of new therapeutic approaches is required targeting important physiological signaling pathways, some of which have been until recently considered as deleterious, like reactive oxygen species. Future works in the field of cardiac endothelial cells and cardiac function will help to better understand the implication of these mediators in cardiac physiopathology.

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.

Parathyroid hormone stimulates juxtaglomerular cell cAMP accumulation without stimulating renin release

Parathyroid hormone (PTH) is positively coupled to the generation of cAMP via its actions on the PTH1R and PTH2R receptors. Renin secretion from juxtaglomerular (JG) cells is stimulated by elevated intracellular cAMP, and every stimulus that increases renin secretion is thought to do so via increasing cAMP. Thus we hypothesized that PTH increases renin release from primary cultures of mouse JG cells by elevating intracellular cAMP via the PTH1R receptor. We found PTH1R, but not PTH2R, mRNA expressed in JG cells. While PTH increased JG cell cAMP content from (log(10) means ± SE) 3.27 ± 0.06 to 3.92 ± 0.12 fmol/mg protein (P < 0.001), it did not affect renin release. The PTH1R-specific agonist, parathyroid hormone-related protein (PTHrP), also increased JG cell cAMP from 3.13 ± 0.09 to 3.93 ± 0.09 fmol/mg protein (P < 0.001), again without effect on renin release. PTH2R receptor agonists had no effect on cAMP or renin release. PTHrP increased cAMP in the presence of both low and high extracellular calcium from 3.31 ± 0.17 to 3.83 ± 0.20 fmol/mg protein (P < 0.01) and from 3.29 ± 0.18 to 3.63 ± 0.22 fmol/mg protein (P < 0.05), respectively, with no effect on renin release. PTHrP increased JG cell cAMP in the presence of adenylyl cyclase-V inhibition from 2.85 ± 0.17 to 3.44 ± 0.14 fmol/mg protein (P < 0.001) without affecting renin release. As a positive control, forskolin increased JG cell cAMP from 3.39 ± 0.13 to 4.48 ± 0.07 fmol/mg protein (P < 0.01) and renin release from 2.96 ± 0.10 to 3.29 ± 0.08 ng ANG I·mg prot(-1)·h(-1) (P < 0.01). Thus PTH increases JG cell cAMP via non-calcium-sensitive adenylate cyclases without affecting renin release. These data suggest compartmentalization of cAMP signaling in JG cells.

Cell-specific effects of nitric oxide deficiency on parathyroid hormone-related peptide (PTHrP) responsiveness and PTH1 receptor expression in cardiovascular cells

The missing influence of estrogen on endothelial nitric oxide (NO) synthase often forms the basis for a worsening of the cardiac risk profile for women in postmenopause. Various studies have shown that decreasing estrogen levels also directly effect the expression of PTHrP and TGFbeta1. PTHrP is involved in the endothelium-dependent regulation of coronary resistance and cardiac function. The current study investigates to what extent chronic NO deficit affects the cardiac effects of PTHrP. NO deficit was achieved in female adult rats by feeding them the NO synthase inhibitor N-omega-nitro-L-arginine methyl ester over a period of 4 wk. Isolated hearts of the conditioned animals were investigated in Langendorff technique and perfused for 3 min with 100 nM PTHrP. The contraction behavior of isolated cardiomyocytes was registered in a cell-edge detection system. Hearts from untreated animals displayed a significant drop in left ventricular developed pressure and a pronounced increase in heart rate in consequence of short term PTHrP stimulation. In hearts from NO-deficient rats PTHrP no longer affected the inotropy and chronotropy. The vasodilating effect of PTHrP on coronary vessels was, however, independent of the NO level. These changes were accompanied by a differing expression of the PTH1 receptor. TGFbeta1 was identified as an important mediator for the regulation of the PTH1 receptor in myocytic but not endothelial cells. These results indicate that chronic NO deficit down-regulates the PTH1 receptor in a TGFbeta1-dependent way. These findings are important with respect to the relatively new therapy of postmenopausal osteoporosis with PTHrP analogs.

Parathyroid hormone improves contractile performance of adult rat ventricular cardiomyocytes at low concentrations in a non-acute way

AIMS

In patients with congestive heart failure, plasma parathyroid hormone (PTH) levels are positively associated with cardiac function. PTH, used to mobilize stem cells from the bone marrow after myocardial infarction, causes an increased left ventricular ejection fraction. The aim of this study was to investigate whether low but plasma-relevant concentrations of PTH directly influence the contractile properties of cardiomyocytes.

METHODS AND RESULTS

Isolated adult rat ventricular cardiomyocytes were exposed to PTH(1-34) or full-length PTH at picomolar concentrations for 24 h. Cell shortening was measured at 2 Hz as a cellular correlate of inotropic responsiveness. Intracellular calcium was measured in Fura-AM-loaded cells. PTH(1-3) (20-200 pM) and full-length PTH (200 pM) increased cell shortening within 24 h. PTH had no effect on cell size, but resting and peak systolic calcium concentrations were elevated. The beneficial effect of PTH was mediated via its cAMP/protein kinase A-activating domain and attenuated by addition of a protein kinase A inhibitor. In contrast, PTH peptides representing a protein kinase C-activating domain but not a cAMP/protein kinase A-activating domain or peptides that represent none of these domains had no effect on cell shortening. The effect of PTH on cell shortening was strong at low concentrations of extracellular calcium but declined at higher calcium concentrations. PTH downregulated the expression of the calcium sensing receptor, a receptor known to antagonize the action of PTH on calcium transport. Furthermore, PTH antagonized the angiotensin II-induced loss of cell function.

CONCLUSION

Low concentrations of PTH improve cell shortening by increasing calcium load at rest. By this mechanism cardiomyocytes compensate reduced extracellular calcium levels as they occur in patients with heart failure.

Experimental hyperthyroidism increases expression of parathyroid hormone-related peptide and type-1 parathyroid hormone receptor in rat ventricular myocardium of the Langendorff ischaemia-reperfusion model

Parathyroid hormone-related peptide (PTHrP) is released under ischaemic conditions and it improves contractile function of stunned myocardium. The actions of PTHrP are mediated primarily by the type 1 parathyroid hormone receptor (PTH.1R), while PTHrP and PTH.1R expression levels are increased in ventricular hypertrophy associated with experimental hyperthyroidism. Since chronic administration of thyroxine (T4) improves postischaemic recovery in isolated heart models subjected to ischaemia-reperfusion stress, we tested the hypothesis that experimentally induced hyperthyroidism is associated with elevated expression of PTHrP and PTH.1R in rat myocardium. Hyperthyroid and control male Wistar rats were subjected to ischaemia-reperfusion stress using the Langendorff technique, and the PTHrP and PTH.1R expression was assessed by relative quantitative reverse transcriptase-polymerase chain reaction, Western blot analysis and immunohistochemistry. In the Langendorff model, the recovery of left ventricular developed pressure at the end of the stablization period and 45 min into the reperfusion period was used to assess the cardioprotective actions of T4 administration. Our data show that hyperthyroid animals had increased tolerance to the ischaemia-reperfusion stress and that this was associated with an increase of PTHrP and PTH.1R expression levels compared with those of control animals. In the control animals, the expression of PTHrP was increased 45 min into the reperfusion phase, while the PTH.1R expression pattern was significantly and gradually decreased throughout the ischaemia and reperfusion phases. In the hyperthyroid animals, the PTHrP and PTH.1R expression pattern was significantly higher throughout the ischaemia and reperfusion phases compared with that of control hearts. Our data suggest that increasing levels of PTHrP and PTH.1R expression can mediate, at least in part, the T4 administration-induced cardioprotection in rat ventricular myocardium.

Calcium-regulating peptide hormones and blood electrolytic balance in chronic heart failure

Calcium-regulating system is important for the functional activity of myocardium. However, little is known about the role of this system in the pathogenesis of cardiovascular diseases. Blood samples from the patients with chronic heart failure (CHF) caused by ischaemic disease (coronary artery disease) (NYHA class I-IV) were used to analyze the levels of calcium, inorganic phosphate, sodium, potassium, parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP). The heart beat rate and arterial blood pressure were chosen as additional tests for the functional status of cardiovascular system. The alteration of electrolytes homeostasis was found dependent on the severity of the pathology being maximally expressed in the NYHA class IV patients. Similar tendency was demonstrated for circulating PTH and PTHrP with the highest blood concentrations observed in patients of the NYHA class III and IV. The extent of these changes was found more pronounced in the female patients. It is suggested that the calcium-regulating hormonal system is involved in the pathogenesis of the ischaemic heart disease; however the sharp increase of PTH and PTHrP at the severe stages of pathology may play a compensatory role in maintaining the heart function.

Vasodilatory effect of tuberoinfundibular peptide (TIP39): requirement of receptor desensitization and its beneficial effect in the post-ischemic heart

Tuberoinfundibular peptide of 39 residues (TIP39) is a member of the parathyroid hormone (PTH) family and a highly specific ligand of the PTH-receptor type 2 (PTH-2r). Recent studies have shown vasoactive properties of TIP39 in the kidney. This effect was stronger after desensitization of the parathyroid hormone-receptor type 1 (PTH-1r). The aims of our study were three-fold: (1) to investigate the influence of TIP39 on coronary resistance (CR), (2) to investigate a possible cross-talk between vascular PTH-receptors in the cardiovascular system, and (3) to investigate whether the endogenously released PTHrP during ischemia induces such a desensitizing effect. Experiments were performed on isolated rat hearts that were perfused with a constant pressure (Langendorff mode) and the coronary flow was determined. Under basal conditions, TIP39 showed no influences on CR. However, TIP39 reduced the CR by approximately 22% after pre-treatment of the hearts with a PTH-1r agonist. This TIP39 effect was abolished either by co-administration of a PTH-2r antagonist or by inhibition of nitric oxide (NO) formation. In an ischemia-reperfusion model endogenously released PTHrP desensitized the PTH-1r and pre-ischemic addition of TIP39 reduced post-ischemic CR by about 28%. Again, this effect was completely abolished in the presence of the PTH-2r antagonist or the PTH-1r-antagonist or by inhibition of NO formation. However, no effect was observed when TIP39 was washed-out prior to ischemia or if the treatment with TIP39 was restricted to the reperfusion. Furthermore, a pre-ischemic application of the NO-dependent vasorelaxant bradykinin provoked a similar effect on the post-ischemic CR than TIP39. In conclusion, a NO-dependent vasodilatory effect of TIP39 was demonstrated if the PTH-1r is desensitized by either exogenously applicated PTHrP peptides or endogenously released PTHrP.

Impaired release of bioactive parathyroid hormone-related peptide in patients with pulmonary hypertension and endothelial dysfunction

BACKGROUND

Parathyroid hormone-related protein (PTHrP) is an endothelial-derived vasoactive peptide. This study investigated whether bioactive PTHrP is locally released in a pressure-dependent way.

METHODS

A PTHrP antibody directed against the midregional part of PTHrP was used to analyze PTHrP in plasma samples. The biological activity of this PTHrP-like peptide was investigated in vitro. Plasma values were determined in samples from the left pulmonary artery and the arteria femoralis, taken under basal conditions and after the application of oxygen or iloprost to lower the pulmonary pressure. Twenty young patients (mean age 6.5 years), who were catheterized for an analysis of the reactivity of the pulmonary bed, were investigated. Endothelial function was investigated by acetylcholine responsiveness.

RESULTS

The antibody recognized a 30-kDa protein with in vitro PTHrP-like activity. In 11 patients (responders) with intact endothelial function, the PTHrP values determined in the left pulmonary artery were higher than those in the arteria femoralis. The local increase in the PTHrP concentration was reduced when either oxygen or iloprost lowered the pressure. Nine patients with endothelial dysfunction did not show any concentration gradients at any time (nonresponders).

CONCLUSIONS

The local concentration of bioactive PTHrP is increased in patients with pulmonary hypertension and normal endothelial function.

N-terminal parathyroid hormone-related peptide hyperpolarizes endothelial cells and causes a reduction of the coronary resistance of the rat heart via endothelial hyperpolarization

Parathyroid hormone-related peptide (PTHrP) is known to be a strong vasorelaxant peptide. The mechanisms by which PTHrP reduces the coronary resistance of the rat heart have not been worked out but seem to be independent of the classical PTH/PTHrP receptor-mediated, cAMP-dependent effect. In this study we hypothesized that PTHrP reduces the coronary resistance of the rat heart via endothelial cell hyperpolarization. Isolated microvascular endothelial cells from rat heart were incubated with PTHrP(1-36), and changes in the membrane potential were recorded via DiBAC fluorescence. Cells exposed to PTHrP showed a hyperpolarization of approximately 7mV. In the isolated Langendorff preparation, PTHrP-dependent vasodilatation of l-nitro-arginine-exposed hearts was abolished under depolarizing conditions (high potassium). Denudation of the endothelial cell layer significantly impaired the vasodilatory effect of PTHrP. In the presence of H89 (a cAMP/protein kinase A pathway antagonist) and indomethacin (a cyclooxygenase inhibitor), PTHrP dilated the vessels. In conclusion, PTHrP exerted a nitric oxide-independent vasodilatory effect that depends on endothelial cell hyperpolarization.

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.

VEGF165 transfection decreases postischemic NF-kappa B-dependent myocardial reperfusion injury in vivo: role of eNOS phosphorylation

Endothelial nitric oxide synthase (eNOS) phosphorylation increases nitric oxide formation, for example, after VEGF stimulation. We investigated whether nitric oxide formed after overexpression of VEGF or of phosphomimetic eNOS (S1177D) affects PMN-induced myocardial detriment after ischemia and reperfusion. Pigs (n=8 per group) were subjected to percutaneous liposome-based gene transfer by retroinfusion of the anterior interventricular vein 48 h before LAD occlusion (60 min) and reperfusion (24 h). Thereafter, regional myocardial function was assessed as subendocardial segment shortening (SES), and infarct size was determined. Tissue from the infarct region, the noninfarcted area at risk, and a control region was analyzed for NF-kappaB activation (EMSA), tumor necrosis factor (TNF)-alpha, and E-selectin mRNA and infiltration of polymorphonuclear neutrophils (PMN). L-NAME was applied in one group of VEGF-transfected animals. NF-kappaB activition, PMN infiltration in the infarct region, and AAR were reduced after transfection of VEGF or eNOS S1177D, but not after VEGF+L-NAME coapplication. Infarct size decreased, whereas SES improved after either VEGF or eNOS S1177D transfection, an effect inhibited by L-NAME coapplication. Retroinfusion of liposomal VEGF cDNA reduces NF-kappaB-dependent postischemic inflammation and subsequent myocardial reperfusion injury, an effect mediated at least in part by enhanced eNOS phosphorylation.

Parathyroid hormone-related protein induction in focal stroke: a neuroprotective vascular peptide

Parathyroid hormone-related protein (PTHrP) is a multifunctional peptide that enhances blood flow in non-central nervous system (CNS) vascular beds by causing vasodilation. PTHrP expression is induced in non-CNS organs in response to ischemia. Experiments were therefore undertaken to determine whether PTHrP can be induced in brain in response to ischemic injury and whether PTHrP can act locally as a vasodilator in the cerebral vasculature, an effect that could be neuroprotective in the setting of stroke. PTHrP expression was examined by Northern analysis and immunohistochemical staining in male Sprague-Dawley rats subjected to permanent middle cerebral artery occlusion (MCAO). Vasodilatory effects of superfused PTHrP(1-34) on pial arterioles were determined by intravital fluorescence microscopy. Effects of PTHrP(1-34) peptide administration on MCAO infarction size reduction were assessed. PTHrP expression was induced in the ischemic hemisphere as early as 4 h after MCAO and remained elevated for up to 24 h. Increased immunoreactive PTHrP at sites of ischemic tissue injury was located in the cerebral microvessels. Superfusion with PTHrP(1-34) peptide for up to 25 min increased pial arteriolar diameter by 30% in normal animals. In animals with permanent MCAO, PTHrP(1-34) peptide treatment significantly decreased cortical infarct size (-47%). In summary, PTHrP expression increases at sites of ischemic brain injury in the cerebrovasculature. This local increase in PTHrP could be an adaptive response that enhances blood flow to the ischemic brain, thus limiting cell injury.

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.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Parathyroid hormone-related peptide is induced by stimulation of alpha 1A-adrenoceptors and improves resistance against apoptosis in coronary endothelial cells

Parathyroid hormone-related peptide (PTHrP) is expressed throughout the vascular system, including coronary endothelial cells. The regulation of endothelial PTHrP expression and the role of PTHrP expression in endothelial cells is not clear. This study investigates the question of whether the stimulation of alpha-adrenergic or angiotensin II receptors increases endothelial expression of PTHrP and whether endogenously expressed PTHrP exerts intracrine effects in coronary endothelial cells. We found that the stimulation of alpha 1A-adrenoceptors, but not that of angiotensin II, increases cellular expression of PTHrP in growing, but not in growth-arrested, coronary endothelial cells. Angiotensin II increases the expression of PTHrP in smooth muscle cells but not in endothelial cells. PTHrP enters the nucleus of endothelial cells at the stadium of confluence, which suggests an intracrine effect of PTHrP. It was further investigated whether the down-regulation of endogenous PTHrP expression by transfection with antisense oligonucleotides alters cell proliferation or apoptosis resistance in growing or nongrowing endothelial cells. Down-regulation of PTHrP did not modify cell proliferation, but it increased the amount of UV-induced apoptosis. An increased expression of PTHrP in cells pretreated with an alpha-adrenoceptor agonist reduced the basal rate of apoptosis and improved resistance against UV-induced apoptosis. These results indicate a novel intracrine effect of PTHrP in coronary endothelial cells that improves cell survival. In endothelial cells, its expression is regulated by alpha-adrenoceptor stimulation in a cell-cycle-dependent and cell-type-specific manner.

Expression of PTHrP and its cognate receptor in the rheumatoid synovial microcirculation

Parathyroid hormone-related protein (PTHrP), a multifunctional peptide that acts as a vasodilator as well as possible regulator of vascular development, is produced in increased amounts in the rheumatoid synovium. To understand whether PTHrP can contribute to the development and function of the rheumatoid microcirculation, studies were undertaken to identify and compare vascular sites of expression of PTHrP and its cognate receptor in the rheumatoid synovium and/or in cultured rheumatoid synovial endothelial cells. Endothelial cells, including apoptotic cells, as determined by TUNEL staining, were the primary site of vascular PTHrP expression in the rheumatoid synovium, a result confirmed in vitro in rheumatoid synovial microvascular endothelial cells. In contrast, the PTH/PTHrP receptor was primarily located in pericytes and smooth muscle cells within the vasculature. These results are consistent with a possible paracrine pathway for PTHrP action in the synovial microcirculation, wherein PTHrP peptides secreted by the synovial endothelium could act on surrounding PTH1R-positive pericytes and smooth muscle cells.

Mechanosensitive release of parathyroid hormone-related peptide from coronary endothelial cells

10.1152/ajpheart.00925. 2001.-Parathyroid hormone-related peptide (PTHrP) is expressed throughout the cardiovascular system and is able to dilate vessels. This study investigated whether mechanical forces generated by changes in regional perfusion influence PTHrP release from the coronary vascular bed. Experiments were performed in vitro on saline-perfused rat hearts or isolated coronary endothelial cells exposed to cyclic strain and in vivo in anesthetized pigs. In vitro, PTHrP release from saline-perfused rat hearts was strongly correlated with coronary flow (r = 0.84). Increasing coronary flow from 5 to 10 ml/min increased PTHrP release from 442 +/- 42 to 1,563 +/- 167 pg/min. Increasing the viscosity of the perfusate did not change basal PTHrP release. Increasing flow without a concomitant increase in pressure did not lead to an increase in release rate, but reduction in pressure under flow-constant conditions reduced PTHrP release rate. Cyclic strain induced a strain-dependent release of PTHrP from endothelial cells that was inhibited by the addition of a calcium-chelating agent. In vivo, there was a net release of PTHrP in the coronary circulation and decreases in coronary flow and pressure decreased the PTHrP release rate. Bradykinin in the presence of constant pressure increased PTHrP release, probably by increasing the intracellular calcium concentration in coronary endothelial cells. In summary, mechanical forces evoked by blood flow can trigger a constant PTHrP release.

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.

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.

A N-terminal PTHrP peptide fragment void of a PTH/PTHrP-receptor binding domain activates cardiac ET(A) receptors

1. Adult ventricular cardiomyocytes show an unusual structure-function relationship for cyclic AMP-dependent effects of PTHrP. We investigated whether PTHrP(1 - 16), void of biological activity on classical PTHrP target cells, is able to mimic the positive contractile effect of PTHrP(1 - 34), a fully biological agonist on cardiomyocytes. 2. Adult ventricular cardiomyocytes were paced at a constant frequency of 0.5 Hz and cell contraction was monitored using a cell-edge-detection system. Twitch amplitudes, expressed as per cent cell shortening of the diastolic cell length, and rate constants for maximal contraction and relaxation velocity were analysed. 3. PTHrP(1 - 16) (1 micromol l(-1)) mimicked the contractile effects of PTHrP(1 - 34) (1 micromol l(-1)). It increased the twitch amplitude from 5.33+/-0.72 to 8.95+/-1.10 (% dl l(-1)) without changing the kinetic of contraction. 4. PTH(1 - 34) (10 micromol l(-1)) affected the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 5. RpcAMPS (10 micromol l(-1)) inhibited the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 6. The positive contractile effect of PTHrP(1 - 16) was antagonized by the ET(A) receptor antagonist BQ123. 7. Sarafotoxin 6b and PTHrP(1 - 16), but not PTHrP(1 - 34), replaced (3)H-BQ123 from cardiac binding sites. 8. We conclude that N-terminal PTHrP peptides void of a PTH/PTHrP-receptor binding domain are able to bind to, and activate cardiac ET(A) receptors.