Expression of parathyroid hormone-related protein in the rat glomerulus and tubule during recovery from renal ischemia

Urinary excretion of parathyroid hormone-related protein correlates with renal function in control rats and rats with cisplatin nephrotoxicity

Parathyroid hormone-related protein (PTHrP) and its receptor are abundantly expressed throughout the renal parenchyma, where PTHrP exerts a modulatory action on renal function. PTHrP upregulation is a common event associated with the mechanism of renal injury and repair. However, no study has yet explored the putative excretion of PTHrP in urine, including its potential relationship with renal function. In the present study, we tested this hypothesis by studying the well-known rat model of acute renal injury induced by the chemotherapeutic agent cisplatin. Using Western blot analysis, we could detect a single protein band, corresponding to intact PTHrP, in the urine of both control and cisplatin-injected rats, whose levels were significantly higher in the latter group. PTHrP was detected in rat urine by dot blot, and its quantification with two specific ELISA kits showed that, compared with control rats, those treated with cisplatin displayed a significant increase in urinary PTHrP (expressed as the PTHrP-to-creatinine ratio or 24-h excretion). In addition, a positive correlation between urinary PTHrP excretion and serum creatinine was found in these animals. In conclusion, our data demonstrate that PTHrP is excreted in rat urine and that this excretion is higher with the decrease of renal function. This suggests that urinary PTHrP levels might be a renal function marker.

Parathyroid hormone-related protein is a mitogenic and a survival factor of mesangial cells from male mice: role of intracrine and paracrine pathways

Glomerulonephritis is characterized by the proliferation and apoptosis of mesangial cells (MC). The parathyroid-hormone related protein (PTHrP) is a locally active cytokine that affects these phenomena in many cell types, through either paracrine or intracrine pathways. The aim of this study was to evaluate the effect of both PTHrP pathways on MC proliferation and apoptosis. In vitro studies were based on MC from male transgenic mice allowing PTHrP-gene excision by a CreLoxP system. MC were also transfected with different PTHrP constructs: wild type PTHrP, PTHrP devoid of its signal peptide, or of its nuclear localization sequence. The results showed that PTHrP deletion in MC reduced their proliferation even in the presence of serum and increased their apoptosis when serum-deprived. PTH1R activation by PTHrP(1-36) or PTH(1-34) had no effect on proliferation but improved MC survival. Transfection of MC with PTHrP devoid of its signal peptide significantly increased their proliferation and minimally reduced their apoptosis. Overexpression of PTHrP devoid of its nuclear localization sequence protected cells from apoptosis without changing their proliferation. Wild type PTHrP transfection conferred both mitogenic and survival effects, which seem independent of midregion and C-terminal PTHrP fragments. PTHrP-induced MC proliferation was associated with p27(Kip1) down-regulation and c-Myc/E2F1 up-regulation. PTHrP increased MC survival through the activation of cAMP/protein kinase A and PI3-K/Akt pathways. These results reveal that PTHrP is a cytokine of multiple roles in MC, acting as a mitogenic factor only through an intracrine pathway, and reducing apoptosis mainly through the paracrine pathway. Thus, PTHrP appears as a probable actor in MC injuries.

Novel role of parathyroid hormone-related protein in the pathophysiology of the diabetic kidney: evidence from experimental and human diabetic nephropathy

Parathyroid hormone-related protein (PTHrP) and its receptor type 1 (PTH1R) are extensively expressed in the kidney, where they are able to modulate renal function. Renal PTHrP is known to be overexpressed in acute renal injury. Recently, we hypothesized that PTHrP involvement in the mechanisms of renal injury might not be limited to conditions with predominant damage of the renal tubulointerstitium and might be extended to glomerular diseases, such as diabetic nephropathy (DN). In experimental DN, the overexpression of both PTHrP and the PTH1R contributes to the development of renal hypertrophy as well as proteinuria. More recent data have shown, for the first time, that PTHrP is upregulated in the kidney from patients with DN. Collectively, animal and human studies have shown that PTHrP acts as an important mediator of diabetic renal cell hypertrophy by a mechanism which involves the modulation of cell cycle regulatory proteins and TGF- β 1. Furthermore, angiotensin II (Ang II), a critical factor in the progression of renal injury, appears to be responsible for PTHrP upregulation in these conditions. These findings provide novel insights into the well-known protective effects of Ang II antagonists in renal diseases, paving the way for new therapeutic approaches.

Receptor activator of NF-kappaB and podocytes: towards a function of a novel receptor-ligand pair in the survival response of podocyte injury

BACKGROUND

Glomerulosclerosis correlates with reduction in podocyte number that occurs through mechanisms which include apoptosis. Podocyte injury or podocyte loss in the renal glomerulus has been proposed as the crucial mechanism in the development of glomerulosclerosis. However, the mechanism by which podocytes respond to injury is poorly understood. TNF and TNF receptor superfamilies are important in the pathogenesis of podocyte injury and apoptosis. The ligand of receptor activator of NF-kappaB (RANKL) and receptor activator of NF-kappaB (RANK) are members of the TNF and receptor superfamilies. We investigated whether RANK-RANKL is a receptor-ligand complex for podocytes responding to injury.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, RANKL and RANK were examined in human podocyte diseases and a rat model of puromycin aminonucleoside nephrosis (PAN). Compared with controls, RANK and RANKL were increased in both human podocyte diseases and the rat PAN model; double immunofluorescence staining revealed that RANK protein expression was mainly attributed to podocytes. Immunoelectron microscopy showed that RANK was localized predominantly at the top of the foot process membrane and the cytoplasm of rat podocyte. In addition, RANK was upregulated in mouse podocytes in vitro after injury induced by puromycin aminonucleoside (PA). Knockdown of RANK expression by small interference RNA (siRNA) exacerbated podocyte apoptosis induced by PA. However, RANKL inhibited significantly the apoptosis of podocytes induced by PA.

CONCLUSIONS/SIGNIFICANCE

These findings suggest the increase in RANK-RANKL expression is a response to podocyte injury, and RANK-RANKL may be a novel receptor-ligand complex for the survival response during podocyte injury.

A transgenic mouse model for studying the role of the parathyroid hormone-related protein system in renal injury

Parathyroid hormone- (PTH-) related protein (PTHrP) and its receptor, the PTH1 receptor (PTH1R), are widely expressed in the kidney, where PTHrP exerts a modulatory action on renal function. PTHrP is known to be upregulated in several experimental nephropathies such as acute renal failure (ARF), obstructive nephropathy (ON) as well as diabetic nephropathy (DN). In this paper, we will discuss the functional consequences of chronic PTHrP overexpression in the damaged kidney using a transgenic mouse strain overexpressing PTHrP in the renal proximal tubule. In both ARF and ON, PTHrP displays proinflammatory and profibrogenic actions including the induction of epithelia to mesenquima transition. Moreover, PTHrP participates in the mechanisms of renal hypertrophy as well as proteinuria in experimental DN. Angiotensin II (Ang II), a critical factor in the progression of renal injury, appears to be, at least in part, responsible for endogenous PTHrP upregulation in these pathophysiological settings. These findings provide novel insights into the well-known protective effects of Ang II antagonists in renal diseases, paving the way for new therapeutic approaches.

Parathyroid hormone-related protein induces hypertrophy in podocytes via TGF-beta(1) and p27(Kip1): implications for diabetic nephropathy

BACKGROUND

Hypertrophy of podocytes is characteristic in diabetic nephropathy (DN). Previously, we observed the upregulation of parathyroid hormone-related protein (PTHrP) and its receptor PTH1R, in experimental DN, associated with renal hypertrophy. Herein, we test the hypothesis that PTHrP participates in the mechanism of high glucose (HG)-induced podocyte hypertrophy.

METHODS

On mouse podocytes, hypertrophy was assessed by protein content/cell and [H(3)]leucine incorporation. Podocytes were stimulated with HG (25 mM), PTHrP(1-36) (100 nM), angiotensin II (AngII) (100 nM) or TGF-beta(1) (5 ng/mL) in the presence or absence of PTHrP-neutralizing antibodies (alpha-PTHrP), the PTH1R antagonist JB4250 (10 microM), PTHrP silencer RNA (siRNA) or TGF-beta(1) siRNA. Protein expression was analysed by western blot and immunohistochemistry.

RESULTS

HG-induced hypertrophy was abolished in the presence of either alpha-PTHrP or PTHrP siRNA. This effect was associated with an inhibition of the upregulation of TGF-beta(1) and p27(Kip1). JB4250 also inhibited HG-induced p27(Kip1) upregulation. Interestingly, whilst HG and AngII were unable to stimulate the expression of p27(Kip1) on PTHrP siRNA-transfected podocytes, TGF-beta(1) was still able to upregulate p27(Kip1) in these cells. Moreover, HG and PTHrP-induced hypertrophy as well as p27(Kip1) upregulation were abolished on TGF-beta(1) siRNA-transfected podocytes. Furthermore, the glomeruli of transgenic PTHrP-overexpressing mice showed a constitutive overexpression of TGF-beta(1) and p27(Kip1) to a degree similar to that of diabetic animals.

CONCLUSIONS

PTHrP seems to participate in the hypertrophic signalling triggered by HG. In this condition, AngII induces the upregulation of PTHrP, which might induce the expression of TGF-beta(1) and p27(Kip1). These findings provide new insights into the protective effects of AngII antagonists in DN, opening new paths for intervention.

The parathyroid hormone 1 receptor directly binds to the FERM domain of ezrin, an interaction that supports apical receptor localization and signaling in LLC-PK1 cells

PTH 1 receptor (PTH1R) regulates mineral ion homeostasis. Both apical and basolateral PTH1R subpopulations exist within the renal proximal tubule. The purpose of this research was to examine determinants within the PTH1R that direct apical localization. When expressed in LLC-PK1 cells, a proximal tubule cell model, the PTH1R localizes to both apical and basolateral membranes. The C terminus of the PTH1R contains a psd-95, discs large, ZO-1 domain interaction motif that binds the sodium-hydrogen exchanger regulatory factor 1 (NHERF-1), a renal tubule scaffold protein. Receptors lacking the psd-95, discs large, ZO-1 domain interaction motif (PTH1R-CDelta4) partly localize to apical membranes, suggesting that additional factors may be involved. Ezrin, a membrane-cytoskeleton linking protein, directly binds NHERF-1 and thus links assembled complexes to actin. In vitro, subdomain C of the ezrin band 4.1, ezrin, radixin, domain interacts with the C-terminal tail of the PTH1R on a site that is mutually exclusive from the NHERF-1 interaction domain, suggesting the presence of a ternary complex. Mutating the lysine-arginine-lysine motif within the juxtamembrane region of the PTH1R C-terminal tail to alanines markedly disrupts interactions with the band 4.1, ezrin, radixin, domain of ezrin both in vitro and within cells. Inclusion of these mutations in the context of the full-length PTH1R disrupts apical localization with no effect on basolateral expression. Expression of a dominant-negative ezrin selectively disrupts apical expression and signaling of the PTH1R. However, dominant-negative ezrin does not affect expression or signaling of the basolateral PTH1R subpopulation. These findings reveal that direct ezrin interactions promote PTH1R apical localization and signaling in LLC-PK1 cells.

Cellular mechanism through which parathyroid hormone-related protein induces proliferation in arterial smooth muscle cells: definition of an arterial smooth muscle PTHrP/p27kip1 pathway

Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27(kip1). Adenoviral restoration of p27(kip1) fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27(kip1). In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27(kip1) expression and accelerates cell cycle progression. p27(kip1) is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27(kip1) pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury.

Abnormal renovascular parathyroid hormone-1 receptor in hypertension: Primary defect or secondary to angiotensin ii type 1 receptor activation?

We previously reported that PTHrP-induced renal vasodilation is impaired in mature spontaneously hypertensive rats (SHR) through down-regulation of the type 1 PTH/PTHrP receptor (PTH1R), a feature that contributes to the high renal vascular resistance in SHR. Here we asked whether this defect represents a prime determinant in genetic hypertension or whether it is secondary to angiotensin II (Ang II) and/or the mechanical forces exerted on the vascular wall. We found that the treatment of SHR with established hypertension by the Ang II type 1 receptor antagonist, losartan, reversed the down-regulation of PTH1R expression in intrarenal small arteries and restored PTHrP-induced vasodilation in ex vivo perfused kidneys. In contrast, the PTH1R deregulation was not found in intrarenal arteries isolated from prehypertensive SHR. Moreover, this defect, which is not seen in extrarenal vessels (aorta, mesenteric arteries) from mature SHR appeared kidney specific in accordance with the acknowledged enrichment of interstitial Ang II in this organ and its enhancement in SHR. In deoxycorticosterone-acetate-salt rats, an Ang II-independent model of hypertension, renovascular PTH1R expression and related vasodilation were not altered. In SHR-derived renovascular smooth muscle cells (RvSMCs), the PTH1R was spontaneously down-regulated and its transcript destabilized, compared with Wistar RvSMCs, both effects being antagonized by losartan. Exogenous Ang II elicited down-regulation of PTH1R mRNA in RvSMCs from Wistar rats. Together, these data demonstrate that Ang II acts via the Ang II type 1 receptor to destabilize PTH1R mRNA in the renal vessel in the SHR model of genetic hypertension. This process is kidney specific and independent from blood pressure increase.

The parathyroid hormone-related protein system and diabetic nephropathy outcome in streptozotocin-induced diabetes

The pathophysiology of the diabetic kidney (e.g., hypertrophy, increase urinary albumin excretion (UAE) is still ill-defined. Parathyroid hormone-related protein (PTHrP) is overexpressed in several nephropathies, but its role remains unclear. We evaluated the effect of high glucose on PTHrP and the PTH1 receptor (PTH1R) protein (by Western blot and immunohistochemistry) in the kidney of mice ith streptozotocin-induced diabetes, and in several mouse renal cells in vitro. Diabetic mice showed a significantly increased renal expression of PTHrP and PTH1R proteins with 2-8 weeks from the onset of diabetes. These animals exhibited an intense immunostaining for both proteins in the renal tubules and glomeruli. Using transgenic mice overexpressing PTHrP targeted to the renal proximal tubule, we found a significant increase in the renal hypertrophy index and in UAE in these diabetic mice relative to their control littermates. Moreover, logistic regression analysis showed a significant association between both PTHrP and PTH1R protein levels and UAE in all diabetic mice throughout the study. High-glucose (25 mm) medium was found to increase PTHrP and PTH1R in tubuloepithelial cells, mesangial cells and podocytes in vitro. Moreover, this increase in PTHrP (but not that of PTH1R) was inhibited by the AT1 receptor antagonist losartan. Collectively, these results indicate that the renal PTHrP/PTH1R system is upregulated in streptozotozin-induced diabetes in mice, and appears to adversely affect the outcome of diabetic renal disease. Our findings also suggest that angiotensin II might have a role in the PTHrP upregulation in this condition.

Role of parathyroid hormone-related protein in tubulointerstitial apoptosis and fibrosis after folic acid-induced nephrotoxicity

Parathyroid hormone-related protein (PTHrP) is shortly upregulated in acute renal injury, but its pathophysiologic role is unclear. Investigated was whether PTHrP might act as a profibrogenic factor in mice that do or do not overexpress PTHrP in the proximal tubule after folic acid (FA) nephrotoxicity, a model of acute renal damage followed by partial regeneration and patchy tubulointerstitial fibrosis. It was found that constitutive PTHrP overexpression in these animals conveyed a significant increase in tubulointerstitial fibrosis, associated with both fibroblast activation (as alpha-smooth muscle actin staining) and macrophage influx, compared with control littermates at 2 to 3 wk after FA damage. Cell proliferation and survival was higher (P<0.01) in the renal interstitium of PTHrP-overexpressing mice than in control littermates within this period after injury. Moreover, the former mice had a constitutive Bcl-XL protein overexpression. In vitro studies in renal tubulointerstitial and fibroblastic cells strongly suggest that PTHrP (1-36) (100 nM) reduced FA-induced apoptosis through a dual mechanism involving Bcl-XL upregulation and Akt and Bad phosphorylation. PTHrP (1-36) also stimulated monocyte chemoattractant protein-1 expression in tubuloepithelial cells, as well as type-1 procollagen gene expression and fibronectin (mRNA levels and protein secretion) in these cells and renal fibroblastic cells. Our findings indicate that this peptide, by interaction with the PTH1 receptor, can increase tubulointerstitial cell survival and seems to act as a proinflammatory and profibrogenic factor in the FA-damaged kidney.

Antitumor effect of parathyroid hormone-related protein neutralizing antibody in human renal cell carcinoma in vitro and in vivo

Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene occurs in 40-80% of human conventional renal cell carcinomas (RCCs). We showed recently that VHL-deficient RCCs expressed large amounts of parathyroid hormone-related protein (PTHrP), and that PTHrP, acting through the PTH1 receptor (PTH1R), plays an essential role in tumor growth. We also showed that PTHrP expression is negatively regulated by the VHL gene products (pVHL). Our goal was to determine whether blocking the PTHrP/PTH1R system might be of therapeutic value against RCC, independent of VHL status and PTHrP expression levels. The antitumor activity of PTHrP neutralizing antibody and of PTH1R antagonist were evaluated in vitro and in vivo in a panel of human RCC lines expressing or not pVHL. PTHrP is upregulated compared with normal tubular cells. In vitro, tumor cell growth and viability was decreased by up to 80% by the antibody in all cell lines. These effects resulted from apoptosis. Exogenously added PTHrP had no effect on cell growth and viability, but reversed the inhibitory effects of the antibody. The growth inhibition was reproduced by a specific PTH1R antagonist in all cell lines. In vivo, the treatment of nude mice bearing the Caki-1 RCC tumor with the PTHrP antibody inhibited tumor growth by 80%, by inducing apoptosis. Proliferation and neovascularization were not affected by the antiserum. Anti-PTHrP treatment induced no side effects as assessed by animal weight and blood chemistries. Current therapeutic strategies are only marginally effective against metastatic RCC, and adverse effects are common. This study provides a rationale for evaluating the blockade of PTHrP signaling as therapy for human RCC in a clinical setting.

Role of the Renin-Angiotensin System on the Parathyroid Hormone–Related Protein Overexpression Induced by Nephrotoxic Acute Renal Failure in the Rat

Parathyroid hormone-related protein (PTHrP), a mitogenic factor for renal cells, is overexpressed in acute renal failure (ARF). Recent data support an association between PTHrP and the renin-angiotensin system in the damaged kidney. The effects of angiotensin II (Ang II) inhibitors (quinapril, enalapril, and/or losartan) on PTHrP and the PTH1 receptor (PTH1R) expression in rats with either folic acid (FA)- or gentamicin-induced ARF were analyzed. The decreased renal function and the PTHrP upregulation and PTH1R downregulation induced by the nephrotoxins were inhibited by the Ang II blockers. In tubuloepithelial cells NRK-52E, the rapid (10 min) increase in PTHrP mRNA by FA, associated with a perinuclear relocalization of Ang II/AT1 receptor, was inhibited by losartan but not candesartan, which traps Ang II receptors at the cell surface. Maximal PTHrP protein overexpression by FA (at 24 to 72 h)-or by exogenous Ang II-was abolished by both Ang II antagonists. PTHrP upregulation by FA was preceded by increased extracellular signal-regulated kinase (ERK) phosphorylation and inhibited by the ERK inhibitor PD098059. FA also activated cAMP response element-binding (CREB) protein, and this was prevented by losartan in these cells. Moreover, PTHrP mRNA overexpression by either FA or Ang II occurred in NRK 52E that were transfected with a CREB construct but not the dominant-negative CREB133 construct. These findings demonstrate that the decreased renal function and PTHrP overexpression in nephrotoxin-damaged kidney depends on renin-angiotensin system. In this setting, intracellular Ang II/AT1 receptor recycling seems to be related to PTHrP induction through ERK and CREB activation in tubuloepithelial cells.

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 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.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

The parathyroid hormone-related protein system: more data but more unsolved questions

PURPOSE OF REVIEW

The present review focuses on recent studies that might be considered as the most relevant advances in the parathyroid hormone-related protein field, with special emphasis on proven functions in renovascular and cardiovascular systems, in physiological as well as pathological conditions. Thus, the questions as to whether and how parathyroid hormone-related protein intervenes in vascular development and homeostasis and in vascular diseases such as hypertension, atherosclerosis, restenosis and heart failure have begun to be unraveled.

RECENT FINDINGS

Since its discovery from hypercalcemia-associated tumors in 1987, it has become clear that parathyroid hormone-related protein is a ubiquitously expressed poly-hormone and plays crucial roles in normal life. The early lethality to parathyroid hormone-related protein knockout mice emphasizes the crucial roles of the protein in development but has limited the use of these models. However, data accumulated from transgenic animals overexpressing the protein in particular cells have provided considerable support to its physiological and pathological relevance. The recent demonstration that nascent parathyroid hormone-related protein not only follows the secretory pathways, but also directly translocates to the nucleus, is beginning to uncover new actions for the protein in a number of physiological systems such as bone, mammary gland and vascular smooth muscle, as well as in pathological situations, such as cancer, osteoporosis, sepsis, atherosclerosis and hypertension.

SUMMARY

The development of mice with conditionally deleted parathyroid hormone-related protein or parathyroid hormone-1 receptor alleles will allow the creation of cell- or tissue-specific parathyroid hormone-related protein knockout mice which will greatly facilitate the determination of the biological relevance of this protein in a specific cell or tissue type, particularly in the cardiovascular system.

Parathyroid hormone-related protein is a gravisensor in lung and bone cell biology

Parathyroid Hormone-related Protein (PTHrP) has been shown to be essential for the development and homeostatic regulation of lung and bone. Since both lung and bone structure and function are affected by microgravity, we hypothesized that 0 x g down-regulates PTHrP signaling. To test this hypothesis, we suspended lung and bone cells in the simulated microgravity environment of a Rotating Wall Vessel Bioreactor, which simulates microgravity, for up to 72 hours. During the first 8 hours of exposure to simulated 0 x g, PTHrP expression fell precipitously, decreasing by 80-90%; during the subsequent 64 hours, PTHrP expression remained at this newly established level of expression. PTHrP production decreased from 12 pg/ml/hour to 1 pg/ml/hour in culture medium from microgravity-exposed cells. The cells were then recultured at unit gravity for 24 hours, and PTHrP expression and production returned to normal levels. Based on these findings, we have obtained bones from rats flown in space for 2 weeks (Mission STS-58, SL-2). Analysis of PTHrP expression by femurs and tibias from these animals (n=5) revealed that PTHrP expression was 60% lower than in bones from control ground-based rats. Interestingly, there were no differences in PTHrP expression by parietal bone from space-exposed versus ground-based animals, indicating that the effect of weightlessness on PTHrP expression is due to the unweighting of weight-bearing bones. This finding is consistent with other studies of microgravity-induced osteoporosis. The loss of the PTHrP signaling mechanism may be corrected using chemical agents that up-regulate this pathway. In conclusion, PTHrP represents a stretch-sensitive paracrine signaling mechanism that may sense gravity.

Biological action of parathyroid hormone (PTH)-related peptide (PTHrP) mediated either by the PTH/PTHrP receptor or the nucleolar translocation in chondrocytes

Parathyroid hormone (PTH)-related peptide (PTHrP) has been believed to act by binding the common receptor to PTH (PTH/PTHrP receptor). However, PTHrP is localized not only in the secretory pathway, but also in nucleoli by virtue of its nucleolar targeting signal (NTS). This review demonstrates the bipartite action of PTHrP on chondrocytes, the receptor-mediated and -independent signaling pathway. Mice with deletion of the PTHrP gene were characterized by a chondrodysplasia due to markedly reduced proliferation of epiphyseal chondrocytes. The PTH/PTHrP receptor was localized mainly in proliferative chondrocytes in the epiphyseal cartilage, indicating that PTHrP modulates normal proliferation via the receptor. In contrast to the receptor-mediated action, the mid-region of the amino acid sequence of PTHrP contains an NTS. The PTHrP-translation was found to initiate from both methionine-coding AUG and downstream leucine-coding CUGs in its signal sequence. When translated from CUGs, PTHrP accumulated in the nucleoli, and the translation from AUG localized PTHrP in both the Golgi apparatus and nucleoli. Therefore, nucleolar PTHrP appears to be derived from the translation initiating from both AUG and CUGs. A chondrocytic cell line expressing a full-length PTHrP, but not PTHrP lacking NTS, were resistant to apoptosis caused by serum depletion, suggesting that the nucleolar PTHrP in chondrocytes serves as a survival factor against apoptosis. Thus, PTHrP regulates chondrocyte proliferation, differentiation and apoptosis by mediating its receptor or acting directly on the nucleolus.

Angiotensin II increases parathyroid hormone-related protein (PTHrP) and the type 1 PTH/PTHrP receptor in the kidney

Angiotensin II (AngII) participates in the pathogenesis of kidney damage. Parathyroid hormone (PTH)-related protein (PTHrP), a vasodilator and mitogenic agent, is upregulated during renal injury. The aim of this study was to investigate the potential relation between AngII and PTHrP system in the kidney. Different methods were used to find that both rat mesangial and mouse tubuloepithelial cells express PTHrP and the type 1 PTH/PTHrP receptor (PTH1R). In these cells, AngII increased PTHrP mRNA and protein production. In contrast, PTH1R mRNA was increased in mesangial cells and downregulated in tubular cells, but its protein levels were unmodified in both cells. AT(1) antagonist, but not AT(2), abolished AngII effects on PTHrP/PTH1R. The in vivo effect of AngII was further investigated by systemic infusion (a low dose of 50 ng/kg per min) into normal rats. In controls, PTHrP immunostaining was mainly detected in renal tubules. In AngII-infused rats, PTHrP staining increased in renal tubules and appeared in the glomerulus and the renal vessels. After AngII infusion, PTHR1 staining was markedly increased in all these renal structures at day 3 but remained elevated only in tubules at day 7. The AT(1) antagonist, but not the AT(2), significantly diminished AngII-induced PTHrP and PTHR1 overexpression in the renal tissue, associated with a decrease in tubular damage and fibrosis. The results indicate that AngII regulates renal PTHrP/PTH1R system via AT(1) receptors. These findings demonstrate that PTHrP upregulation occurs in association with the mechanisms of AngII-induced kidney injury.

cAMP pathway in podocytes

Podocytes possess receptors for a variety of hormones. The following receptors whose stimulation results in increased cAMP levels have been detected in podocytes: adrenergic beta(2) receptor, dopamine D(1) receptor, prostaglandin IP and EP(4) receptors, and parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor. Besides activating protein kinase A, increased levels of cAMP depolarize podocytes via opening of chloride channels. Relatively little is known about the impact of the cAMP pathway on podocyte function. Results obtained in a limited number of studies indicate that cAMP in podocytes may regulate cell morphology, actin assembly, and matrix production. In addition, cAMP seems to attenuate the action of hormones, which activate the Ca(2+)/protein kinase C pathway. Effects of the cAMP pathway on further aspects of podocyte biology, such as contractility, phosphorylation state of slit membrane-associated proteins, glomerular permeability, cell cycle control, and synthesis of reactive oxygen species can be anticipated from studies on other cell types and from studies on isolated glomeruli. In summary, the data available indicate that the cAMP pathway affects several aspects of podocyte biology in an overall glomerulo-protective manner.

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.

Up-regulation of parathyroid hormone-related protein in folic acid-induced acute renal failure

BACKGROUND

Parathyroid hormone (PTH)-related protein (PTHrP) is present in many normal tissues, including the kidney. Current evidence supports that PTHrP is involved in renal pathophysiology, although its role on the mechanisms of renal damage and/or repair is unclear. Our present study examined the changes in PTHrP and the PTH/PTHrP receptor (type 1) in folic acid-induced acute renal failure in rats. The possible role of PTHrP on the process of renal regeneration following folic acid administration, and potential interaction between angiotensin II (Ang II) and endothelin-1, and PTHrP, were examined in this animal model.

METHODS

PTHrP, PTH/PTHrP receptor, ACE, and preproendothelin-1 (preproET-1) mRNA levels in the rat kidney were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and/or RNase protection assay. Immunohistochemistry also was performed for PTHrP, the PTH/PTHrP receptor, and Ang II in the renal tissue of folic acid-injected rats. The role of PTHrP on tubular cell proliferation following folic acid injury was investigated in vitro in rat renal epithelial cells (NRK 52E). PTHrP secretion in the medium conditioned by these cells was measured by an immunoradiometric assay specific for the 1-36 sequence.

RESULTS

Using RT-PCR, PTHrP mRNA was rapidly (1 hour) and maximally increased (3-fold) in the rat kidney after folic acid, decreasing after six hours. At 72 hours, renal function was maximally decreased in these rats, associated with an increased PTHrP immunostaining in both renal tubules and glomeruli. In contrast, the PTH/PTHrP receptor mRNA (RNase protection assay) decreased shortly after folic acid administration. Moreover, PTH/PTHrP receptor immunostaining dramatically decreased in renal tubular cell membranes after folic acid. A single subcutaneous administration of PTHrP (1-36), 3 or 50 microg/kg body weight, shortly after folic acid injection increased the number of tubular cells staining for proliferating cell nuclear antigen by 30% (P < 0.05) or 50% (P < 0.01), respectively, in these rats at 24 hours, without significant changes in either renal function or calcemia. On the other hand, this peptide failed to modify the increase (2-fold over control) in ACE mRNA, associated with a prominent Ang II staining into tubular cell nuclei, in the kidney of folic acid-treated rats at this time period. The addition of 10 mmol/L folic acid to NRK 52E cells caused a twofold increase in PTHrP mRNA at six hours, without significant changes in the PTH/PTHrP receptor mRNA. The presence of two anti-PTHrP antibodies, with or without folic acid, in the cell-conditioned medium decreased (40%, P < 0.01) cell growth.

CONCLUSIONS

Renal PTHrP was rapidly and transiently increased in rats with folic acid-induced acute renal failure, featuring as an early response gene. In addition, changes in ACE and Ang II expression were also found in these animals. PTHrP induces a mitogenic response in folic acid-damaged renal tubular cells both in vivo and in vitro. Our results support the notion that PTHrP up-regulation participates in the regenerative process in this model of acute renal failure and is a common event associated with the mechanisms of renal injury and repair.

Induction of osteoblast differentiation indexes by PTHrP in MG-63 cells involves multiple signaling pathways

Parathyroid hormone (PTH)-related peptide (PTHrP) can modulate the proliferation and differentiation of a number of cell types including osteoblasts. PTHrP can activate a G protein-coupled PTH/PTHrP receptor, which can interface with several second-messenger systems. In the current study, we have examined the signaling pathways involved in stimulated type I collagen and alkaline phosphatase expression in the human osteoblast-derived osteosarcoma cells, MG-63. By use of Northern blotting and histochemical analysis, maximum induction of these two markers of osteoblast differentiation occurred after 8 h of treatment with 100 nM PTHrP-(1-34). Chemical inhibitors of adenylate cyclase (H-89) or of protein kinase C (chelerythrine chloride) each diminished PTHrP-mediated type I collagen and alkaline phosphatase stimulation in a dose-dependent manner. These effects of PTHrP could also be blocked by inhibiting the Ras-mitogen-activated protein kinase (MAPK) pathway with a Ras farnesylation inhibitor, B1086, or with a MAPK inhibitor, PD-98059. Transient transfection of MG-63 cells with a mutant form of Galpha, which can sequester betagamma-subunits, showed significant downregulation of PTHrP-stimulated type I collagen expression, as did inhibition of phosphatidylinositol 3-kinase (PI 3-kinase) by wortmannin. Consequently, the betagamma-PI 3-kinase pathway may be involved in PTHrP stimulation of Ras. Collectively, these results demonstrate that, acting via its G protein-coupled receptor, PTHrP can induce indexes of osteoblast differentiation by utilizing multiple, perhaps parallel, signaling pathways.

Developmental aspects of parathyroid hormone-related protein biology

Parathyroid hormone-related protein (PTHrP) has been discovered as a parathyroid hormone (PTH)-like factor responsible for the humoral hypercalcaemia of malignancies. Further studies revealed that PTHrP is ubiquitously expressed, in mature as well as in developing normal tissues from various species. Although not completely understood, the biological roles of PTHrP concern a variety of domains, including calcium phosphorus metabolism and bone mineralization, smooth muscle relaxation, cell growth and differentiation, and embryonic development. As a poly-hormone, PTHrP is now acknowledged to act via the paracrine, autocrine, and even the intracrine pathways. This review focuses on the main developmental features of the biology of PTHrP. During embryonic development, PTHrP is considered to be involved as a growth factor that promotes cell proliferation and delays cell terminal maturation. PTHrP has been shown to intervene in the development of various tissues and organs such as the skeleton, skin, hair follicles, tooth, pancreas, and the kidney. In addition, through its midregion sequence, which is able to promote an active transplacental calcium transport, PTHrP may intervene indirectly in the mineralization of the foetal skeleton. PTHrP has also been shown to be necessary for the normal development of the mammary gland, while huge amounts of PTHrP are found in the human milk. Finally, observations of physiologic, vasodilating effects of PTHrP in the kidney suggest its involvment in the control of renal hemodynamics, especially in the perinatal period.

Cell biology and molecular mechanisms of injury in ischemic acute renal failure

The pathogenesis of acute renal failure has been attributed to persistent vasoconstriction and leukocyte-endothelial interactions, resulting in inflammation and compromise of local blood flow to the outer medulla, the loss of tubular epithelial cell polarity with multiple functional sequelae, necrosis or apoptosis of epithelial cells, and the de-differentiation, migration and proliferation of surviving cells. In this paper, the authors present their views of pathophysiology of ischemic acute renal failure.

Acute renal failure. III. The role of growth factors in the process of renal regeneration and repair

This review, which is the final installment in a series devoted to controversial issues in acute renal failure (ARF) (3, 47), will examine available information regarding the role of growth factors in ARF. In general, studies in this area have fallen into two broad categories: 1) those that have examined the renal expression of genes encoding growth factors or transcriptional factors associated with the growth response that is induced after ARF, and 2) those that have examined the efficacy of exogenously administered growth factors in accelerating recovery of renal function in experimental models of ARF. Despite the vast amount of information that has accumulated in these two areas of investigation, our understanding of the mechanisms involved in the process of regeneration and repair after ARF, and the role of growth factors in this response, remains rudimentary. This overview, contributed to by a number of experts in the field, is designed to summarize present knowledge and to highlight potentially fertile areas for future research in this area.

Hepatocyte growth factor overexpression in the islet of transgenic mice increases beta cell proliferation, enhances islet mass, and induces mild hypoglycemia

Hepatocyte growth factor (HGF) is produced in pancreatic mesenchyme-derived cells and in islet cells. In vitro, HGF increases the insulin content and proliferation of islets. To study the role of HGF in the islet in vivo, we have developed three lines of transgenic mice overexpressing mHGF using the rat insulin II promoter (RIP). Each RIP-HGF transgenic line displays clear expression of HGF mRNA and protein in the islet. RIP-mHGF mice are relatively hypoglycemic in post-prandial and fasting states compared with their normal littermates. They display inappropriate insulin production, striking overexpression of insulin mRNA in the islet, and a 2-fold increase in the insulin content in islet extracts. Importantly, beta cell replication rates in vivo are two to three times higher in RIP-HGF mice. This increase in proliferation results in a 2-3-fold increase in islet mass. Moreover, the islet number per pancreatic area was also increased by approximately 50%. Finally, RIP-mHGF mice show a dramatically attenuated response to the diabetogenic effects of streptozotocin. We conclude that the overexpression of HGF in the islet increases beta cell proliferation, islet number, beta cell mass, and total insulin production in vivo. These combined effects result in mild hypoglycemia and resistance to the diabetogenic effects of streptozotocin.

Effects of parathyroid hormone-related protein on human mesangial cells in culture

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) produce similar biological effects through the PTH/PTHrP receptor. Because PTHrP exhibits vasodilatory properties, we evaluated the hypothesis that this hormone interacts with human mesangial cells (HMC). The PTHrP prevented both the expected reduction in the planar cell surface area and the increase in myosin light-chain phosphorylation induced by platelet-activating factor (PAF) on HMC, in a dose-dependent manner. This effect was completely blocked by pertussis toxin and dideoxyadenosine, suggesting that a G protein-coupled receptor and cAMP are important in the PTHrP transduction mechanism. Moreover, PTHrP increased cAMP synthesis and thymidine incorporation in HMC. However, whereas RT-PCR and Southern and Northern blot analyses demonstrated the expression of human PTH/PTHrP receptor in human kidney cortex, no expression could be demonstrated in HMC. These results show that PTH and PTHrP directly interact with mesangial cells. These effects might be mediated by a receptor different from the PTH/PTHrP receptor.

Expression of parathyroid hormone-related peptide messenger ribonucleic acid in developing kidney

BACKGROUND

Parathyroid hormone (PTH)-related peptide (PTHrP), originally identified as a causative agent of hypercalcemia of malignancy, has been implicated in the regulation of growth and differentiation of endochondral bone, hair follicle, and breast as an autocrine/paracrine factor. Although some experiments indicate that PTHrP works as a growth factor for primary renal cells in vitro, the role of PTHrP in the kidney in vivo is not yet known.

METHODS

We examined the amounts of PTHrP and PTH/ PTHrP receptor (PTHR) mRNA in the mouse kidney developmental process by reverse transcription-polymerase chain reaction, and investigated which cells produce PTHrP and PTHR in vivo by in situ hybridization.

RESULTS

We observed high levels of PTHrP mRNA during mouse kidney maturation. PTHrP mRNA was expressed in the collecting duct, urothelium of the pelvis, and immature elements in the cortex of the developing kidney, including the S-shaped body, ureteric bud, and glomerulus. However, the expression of PTHR mRNA was lower during maturation than after the completion of the maturation process, and it was not detected in the collecting duct, urothelium of the pelvis, or nephrogenic zone in embryonic day 16 or 0-day-old mouse kidneys.

CONCLUSION

These findings suggest that PTHrP has a role in mouse kidney maturation or glomerular development.

Expression and role of parathyroid hormone-related protein in human renal proximal tubule cells during recovery from ATP depletion

Parathyroid hormone (PTH)-related protein (PTHrP) is widely expressed in normal fetal and adult tissues and regulates growth and differentiation in a number of organ systems. Although various renal cell types produce PTHrP, and PTHrP expression in rat proximal renal tubules is upregulated in response to ischemic injury in vivo, the role of PTHrP in the kidney is unknown. To study the effects of injury on PTHrP expression and its consequences in more detail, the immortalized human proximal tubule cell line HK-2 was used in an in vitro model of ATP depletion to mimic in vivo renal ischemic injury. These cells secrete PTHrP into conditioned medium and express the type I PTH/PTHrP receptor. Treatment of confluent HK-2 cells for 2 h with substrate-free, glucose-free medium containing the mitochondrial inhibitor antimycin A (1 microM) resulted in 75% depletion of cellular ATP. After an additional 2 h in glucose-containing medium, cellular ATP levels recovered to approximately 75% of baseline levels. PTHrP mRNA levels, as measured in RNase protection assays, peaked at 2 h into the recovery period (at four times baseline expression). The increase in PTHrP mRNA expression was correlated with an increase in PTHrP protein content in HK-2 cells at 2 to 6 h into the recovery period. Heat shock protein-70 mRNA expression was not detectable under baseline conditions but likewise peaked at 2 h into the recovery period. Treatment of HK-2 cells during the recovery period after injury with an anti-PTHrP(1-36) antibody (at a dilution of 1:250) resulted in significant reductions in cell number and uptake of [3H]thymidine, compared with nonimmune serum at the same titer. Similar results were observed in uninjured HK-2 cells. It is concluded that this in vitro model of ATP depletion in a human proximal tubule cell line reproduces the pattern of gene expression previously observed in vivo in rat kidney after ischemic injury and that PTHrP plays a mitogenic role in the proliferative response after energy depletion.

Renal expression of parathyroid hormone-related protein (PTHrP) and PTH/PTHrP receptor in a rat model of tubulointerstitial damage

BACKGROUND

PTHrP, which appears to act as a growth/differentiation factor in a variety of tissues, is present in the kidney; however, its role is unclear.

METHODS

The expression of PTHrP and the PTH/PTHrP receptor were investigated by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry in the remnant kidney of uninephrectomized (UNX) rats after protein overloading [1 g/day of bovine serum albumin (BSA)].

RESULTS

Compared with UNX-control rats, proteinuria in BSA-overloaded animals was detected within the first 24 hours and increased during the entire study period (28 days). Kidney examination by light microscopy showed no significant renal lesions at day 1 of BSA treatment, whereas at days 8 and 28, tubular lesions, infiltration of mononuclear cells, and mesangial expansion were observed. PTHrP mRNA expression in the renal cortex was already increased at day 1 (fourfold) and plateaued between days 8 and 28 (12- and 15-fold, respectively) in BSA-overloaded animals compared with UNX-control rats. At day 8, immunohistochemical analysis with two different anti-PTHrP antibodies showed a dramatic increase of PTHrP staining in the damaged proximal and distal tubules from BSA-overloaded rats with respect to UNX-control rats. Moreover, intense PTHrP immunostaining was also observed in glomerular mesangial and endothelial cells in BSA-overloaded rats, but not in the UNX-control rats. A reciprocal decrease of PTH/PTHrP receptor mRNA and immunostaining, without significant changes in the cellular localization (proximal and distal tubule, and glomerular mesangial and epithelial cells) of the PTH/PTHrP receptor positivity was found to occur in the renal cortex of BSA-overloaded rats. At day 8, coinciding with the up-regulation of PTHrP, an increase in the angiotensin converting enzyme and preproendothelin-1 gene expression was observed in the renal cortex of BSA-overloaded rats compared with UNX-control rats.

CONCLUSIONS

These results indicate that PTHrP can be added to the group of genes that are up-regulated in proximal tubular cells in response to intense proteinuria. Our results, together with previous findings, suggest that the vasoactive hormones angiotensin II and endothelin-1 could participate in the PTHrP production in the renal cortex of BSA-overloaded rats. Further experiments are required to clarify the mechanisms of PTHrP up-regulation and its possible role in the response to renal damage in this animal model.

Parathyroid hormone-related protein increases DNA synthesis in proximal tubule cells by cyclic AMP- and protein kinase C-dependent pathways

The present study was performed to characterize the possible involvement of cAMP synthesis and protein kinase C (PKC) activation in the DNA synthesis-stimulating effect of parathyroid hormone-related protein (PTHrP) in proximal tubule cells. We found that DNA synthesis was stimulated by 10 microM 8BrcAMP, and 1 microM Sp-cDBIMPS, two cAMP analogs, and also by 1 microM phorbol 12-myristate 13-acetate (PMA) and 100 microM 1,2-dioctanoyl-sn-glycerol, two PKC activators, and 10 nM [Cys23] human (h)PTHrP (24-35) amide in rabbit proximal tubule cells (PTC). Both Sp-cDBIMPS and PMA, at 1 microM, also increased DNA synthesis in SV40-immortalized mouse proximal tubule cells MCT. Human PTHrP (7-34) amide [PTHrP (7-34)] dose dependently stimulated DNA synthesis in a similar manner as [34Tyr]PTHrP (1-34) amide [PTHrP (1-34)], in PTC. PMA pre-treatment for 20 h, which downregulates PKC, completely blocked the effect induced by PTHrP (7-34), but not that of PTHrP (1-34), in the latter cells. In contrast, the same PMA pre-treatment abolished the DNA synthesis stimulation by PTHrP (1-34) and PTHrP (7-34) in MCT cells, which appear to have PTH receptors mainly coupled to phospholipase C and not adenylate cyclase. Our results indicate that the stimulatory effect of PTHrP on DNA synthesis in proximal tubule cells is mediated by a cAMP- and PKC-dependent mechanism.

Cyclosporine increases renal parathyroid hormone-related protein expression in vivo in the rat

BACKGROUND

Clinical use of cyclosporine (CsA) is limited by its known nephrotoxicity. Parathyroid hormone (PTH)-related protein (PTHrP) increases after acute renal ischemia and stimulates proliferation of renal cells in culture. Herein, we have examined whether the renal expression of PTHrP and its PTH/PTHrP receptor is affected by chronic CsA nephrotoxicity.

METHODS

Rats were randomly assigned to receive daily intramuscular injections of either CsA (25 mg/kg) or the same volume of the vehicle olive oil (control) for 3 weeks. At this time interval, under ether anesthesia, rat blood and kidneys were obtained for analytical determinations, and total RNA isolation or immunohistochemistry, respectively.

RESULTS

Serum urea was 11+/-2 and 6+/-1 mmol/L (P < 0.01) in CsA-treated and control rats, respectively. We found that PTH/PTHrP receptor mRNA was unchanged, but PTHrP mRNA, and also transforming growth factor-beta1 mRNA expression as positive control, was about twofold increased in the kidney of CsA-treated rats. This was accompanied by increased PTHrP immunostaining in renal cortical tubules, associated with tubule vacuolation.

CONCLUSION

This study demonstrates an up-regulation of PTHrP, associated with chronic CsA-induced nephrotoxicity. Our findings support a role for PTHrP in the CsA-injured kidney.

Parathyroid hormone-related peptide--a smooth muscle tone and proliferation regulatory protein

Parathyroid hormone-related protein (PTHrP) appears to play crucial roles in the cardiovascular system. Over the past few years it has become apparent that there is more than one receptor recognizing parathyroid hormone or PTHrP, or both, and that PTHrP is not only a potent vasodilator of vascular smooth muscle cell tone, but is also a regulator of vascular smooth muscle cell proliferation and a secretagogue of renin and vasopressin. Investigators in several laboratories have started to query whether PTHrP intervenes in vascular diseases such as hypertension, (re)stenosis-atherosclerosis and endotoxaemia.

Renal regulation of phosphate excretion in endotoxaemic rats

1. Maintenance of phosphate homeostasis is essential for energy producing and oxygen delivery systems, particularly, when the energy requirements are increased in certain conditions, such as septicaemia. We investigated the phosphaturic response to parathyroid hormone (PTH) in endotoxin (ETx)-treated rats in order to clarify the renal regulation of phosphate excretion during endotoxaemia. 2. Wistar rats that had undergone thyroparathyroidectomy were challenged with either Escherichia coli ETx (n = 8) or saline vehicle (n = 9). Thirty-minute renal clearance tests were done before and after PTH infusion. Rats infused with saline instead of PTH served as time controls for the ETx- (n = 7) and saline-treated (n = 8) rats. 3. In time control rats, ETx administration enhanced phosphate excretion progressively and this was associated with an obvious increase in the level of kidney adenosine 3', 5'-cyclic mono-phosphate (P < 0.005) compared with levels following saline vehicle administration. However, this phosphaturia in late-phase endotoxaemia was not observed in rats infused with PTH; ETx, but not saline vehicle, blunted the PTH-mediated increase in phosphate excretion (P < 0.005). Increased urinary noradrenaline and constant dopamine excretion were observed in endotoxaemic rats. Endotoxin administration produced marked metabolic acidosis and hypocapnia in comparison with the administration of the saline vehicle. 4. To test whether renal tubular sensitivity to parathyroid hormone related-protein (PTHrP) was enhanced during endotoxaemia, phosphaturic response to PTHrP in ETx- (n = 7) and saline-treated rats (n = 7) was examined. Parathyroid hormone related-protein infusion produced phosphaturia in both groups. However, the severity of the phosphaturia after PTHrP infusion was less in ETx-than in saline-treated rats. 5. In summary, although ETx administration causes a progressive increase in phosphate excretion in the absence of PTH, this is overcome by the antiphosphaturic effect of ETx, attenuating PTH-mediated phosphaturia after PTH infusion.

Parathyroid hormone-related protein detection and interaction with NO and cyclic AMP in the renovascular system

The presence of parathyroid hormone-related protein (PTHrP) in human kidney vasculature and the signal transduction pathways stimulated during PTHrP-induced vasodilation of the rabbit kidney were investigated. Immunostaining of human kidney revealed the abundant presence of PTHrP in media and intima of all microvessels as well as in macula densa. In isolated perfused rabbit kidney preconstricted with noradrenaline, 10(-5) M Rp-cAMPS, a direct inhibitor of protein kinase A, produced comparable inhibition of 2.5 x 10(-7) M forskolin- and 10(-7) M PTHrP-induced vasorelaxations. Renal vasorelaxation and renal microvessel adenylyl cyclase stimulation underwent comparable desensitization following exposure to PTHrP. Nitric oxide (NO)-synthase inhibition by L-NAME (10(-4) M), NO scavenging by an imidazolineoxyl N-oxide (10(-4) M) and guanylyl cyclase inhibition by methylene blue (10(-4) M) decreased PTHrP-induced vasorelaxation by 27 to 53%, abolished bradykinin-induced vasorelaxation and did not affect forskolin-induced vasorelaxation. The effects of Rp-cAMPS and L-NAME were not additive on PTHrP-induced vasorelaxation. Damaging endothelium by treating the kidney with either anti-factor VIII-related antibody and complement, gossypol or detergent, did not affect PTHrP- or forskolin-induced vasorelaxations but reduced bradykinin-induced vasorelaxation by 53 to 92%. Conversely, endothelial damage did not alter the inhibitory action of L-NAME on PTHrP-induced vasorelaxation. In conclusion, PTHrP is present throughout the human renovascular tree and juxtaglomerular apparatus. Activation of both adenylyl cyclase/protein kinase A and NO-synthase/guanylyl cyclase pathways are directly linked to the renodilatory action of PTHrP in a way that does not require an intact endothelium in the isolated rabbit kidney.

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.

Overexpression of parathyroid hormone-related protein in the pancreatic islets of transgenic mice causes islet hyperplasia, hyperinsulinemia, and hypoglycemia

Parathyroid hormone-related protein (PTHrP) is produced by the pancreatic islet. It also has receptors on islet cells, suggesting that it may serve a paracrine or autocrine role within the islet. We have developed transgenic mice, which overexpress PTHrP in the islet through the use of the rat insulin II promoter (RIP). Glucose homeostasis in these mice is markedly abnormal; RIP-PTHrP mice are hypoglycemic in the postprandial and fasting states and display inappropriate hyperinsulinemia. At the end of a 24-hour fast, blood glucose values are 49 mg/dl in RIP-PTHrP mice, as compared to 77 mg/dl in normal littermates; insulin concentrations at this time are 6.3 and 3.9 ng/ml, respectively. Islet perifusion studies failed to demonstrate abnormalities in insulin secretion. In contrast, quantitative islet histomorphometry demonstrates that the total islet number and total islet mass are 2-fold higher in RIP-PTHrP mice than in their normal littermates. PTHrP very likely plays a normal physiologic role within the pancreatic islet. This role is most likely paracrine or autocrine. PTHrP appears to regulate insulin secretion either directly or indirectly, through developmental or growth effects on islet mass. PTHrP may have a role as an agent that enhances islet mass and/or enhances insulin secretion.

Parathyroid hormone-related protein is an autocrine modulator of rabbit proximal tubule cell growth

Parathyroid hormone-related protein (PTHrP), a likely mediator for humoral hypercalcemia of malignancy, is also synthesized in various normal tissues. In the kidney, PTHrP, mainly detected in proximal and distal tubules, has been shown to stimulate proliferation of rat mesangial cells in culture. Experiments were carried out to investigate the possible mitogenic effect of PTHrP in cultures of rabbit proximal tubule cells (PTC). Immunocytochemical analysis, using antihuman (h)PTHrP antibodies to (38-64) and (107-111) epitopes in the PTHrP molecule, showed strong cytoplasmic staining in PTC and proximal tubule-like LLC-PK1 cells. PTC secreted immunoreactive PTHrP (54.8 +/- 7.0 fmol/10(6) cells) into the culture medium. Human PTHrP(1-141) stimulated proliferation in subconfluent cultures of these cells dose-dependently. This effect was similar to that induced by [Tyr34]hPTHrP(1-34) amide (hPTHrP[1-34]), hPTHrP(1-86), and bovine (b)PTH(1-34), while hPTHrP(38-64) amide, hPTHrP9107-111) amide, and hPTHrP(107-139) amide were ineffective. Addition of anti-hPTHrP neutralizing antibodies to (1-34), (38-64), and (107-111) epitopes of PTHrP decreased PTC growth. The mitogenic effect of these agonists was abolished in confluent PTC. In contrast, [Nle8,18, Tyr34]bPTH(3-34)amide (bPTH[3-34]) increased DNA synthesis in either subconfluent or confluent PTC. In LLC-PK1 cells, which also secreted PTHrP and are devoid of PTH receptors, none of these peptides affected proliferation. Forskolin (10 microM) or H-8 (2 microM), a protein kinase A inhibitor, did not affect basal or hPTHrP(1-34)-stimulated DNA synthesis, respectively, in subconfluent PTC. On the other hand, 10 nM staurosporine and 100 nM calphostin C, protein kinase C (PKC) inhibitors, blunted the effects of hPTHrP(1-34) or bPTH(3-34) on DNA synthesis in these cells. These studies suggest that PTHrP may function as an autocrine factor in the regulation of proximal tubule cell growth by a PKC-mediated mechanism.

PTH/PTHrP receptor expression on osteoblasts and osteocytes but not resorbing bone surfaces in growing rats

Using in situ hybridization, we correlated the expression of mRNA for the parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) receptor with bone formation and resorption in undecalcified serial sections of bones from growing rats. In addition we investigated the presence of biologically active receptors in the same locations using an in vivo autoradiographic technique. In the ulnae of growing rats, there are well defined zones of cortical bone formation and resorption. These contribute to the modeling drifts by which the bone achieves its adult shape. Forming surfaces incorporate fluorochrome labels, are lined with osteoid, and have a layer of cuboidal osteoblasts that have a high alkaline phosphatase activity. Resorbing surfaces have no fluorochrome incorporation, no osteoid, and are lined with resorbing cells with high tartrate-resistant acid phosphatase (TRAP) activity. PTH/PTHrP receptor mRNA was expressed predominantly on forming but not on resorbing bone surfaces and colocalized with sites of binding of radiolabeled PTH after intravenous injection. PTH/PTHrP mRNA expression on osteocytes was inconclusive but radiolabeled PTH bound to a proportion of osteocytes in all regions of the cortex although binding was not specifically related to areas of bone formation or resorption. These results suggest that in growing animals the actions of PTH or PTHrP are connected more with bone formation than resorption. Such a role may be linked to the ability of PTH to induce bone formation in adults but does not explain the actions of the hormone in regulating resorption. Binding of PTH to osteocytes increases the evidence for a physiological role for these cells.

Parathyroidectomy does not prevent the renal PTH/PTHrP receptor down-regulation in uremic rats

In a recent study we demonstrated that the PTH/PTHrP receptor (PTH-R) mRNA was markedly down-regulated in the remnant kidney of uremic rats with severe secondary hyperparathyroidism. Among the factors potentially implicated in this down-regulation, to date only PTH has been demonstrated to modulate PTH-R expression. Here, we examined the effect of thyroparathyroidectomy (TPTX) on the renal expression of PTH-R in rats with normal renal function or with chronic renal failure (CRF) induced by 5/6 nephrectomy. Four groups of rats were studied: control, TPTX, CRF, and CRF + TPTX. Moderate-degree renal failure was documented by mean (+/- SD) creatinine clearances (microliter/min/100 g body wt) of 259 +/- 40 and 212 +/- 45 in CRF and CRF + TPTX rats, compared with 646 +/- 123 and 511 +/- 156 in control and TPTX rats, respectively. Plasma phosphorus, calcitriol, and ionized calcium were significantly lower in CRF and CRF + TPTX than in control animals. Plasma ionized calcium and calcitriol were also lower in TPTX than in control rats. Plasma PTH levels (pg/ml) were increased in CRF rats (41.8 +/- 29.4), and markedly decreased in TPTX (10.1 +/- 7.8) and CRF + TPTX (8.0 +/- 3.8) rats compared with control rats (21.7 +/- 7.5). Northern blot analysis showed that the level of the steady-state PTH-R mRNA in the kidney of CRF and CRF + TPTX rats was markedly decreased compared with that of control rats, the ratios of PTH-R mRNA/beta-actin mRNA being 0.28 +/- 0.04 and 0.27 +/- 0.03 versus 0.54 +/- 0.05, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)