PARATHYROID HORMONE-RELATED PROTEIN AS A RENAL REGULATORY FACTOR: FROM VESSELS TO GLOMERULI AND TUBULAR EPITHELIUM

Pedro Esbrit*, Soledad Santos*, Arantxa Ortega*, Teresa Fernández-Agulló, Begonia Gea Contreras, M. Antonia Gutiérrez-Tárres, Jordi Bover**, and Ricardo J. Bosch.

Bone and Mineral Metabolism Laboratory, Research Unit, Fundación Jiménez Díaz*, Madrid; Department of Nephrology, Prínceps d´Espanya Hospital, L´Hospitalet de Llobregat**, Barcelona; and Department of Physiology, Alcalá School of Medicine, Alcalá de Henares, Spain.

Introduction Parathyroid hormone (PTH)-related protein (PTHrP) was initially isolated from tumors associated with humoral hypercalcemia of malignancy [1]. Both PTH and PTHrP share homology in their N terminal region, and bind to the type 1 PTH/PTHrP receptor (PTHR), cloned in osteoblasts and renal tubular cells, leading to activation of both adenylate cyclase and phospholipase C/protein quinase C [2]. However, while PTH is a regulator of mineral homeostasis, acting mainly on bone and kidney, PTHrP is found in many nonmalignant fetal and adult tissues [3]. Furthermore, PTHrP post-translational processing generates various fragments lacking PTH homology, whose biological activity remains to be clarified [3,4].

Despite the widespread production of PTHrP in healthy individuals, its circulating concentration is below the detectable limit of the majority of current assays [5]. Thus, in contrast to the situation of humoral hypercalcemia of malignancy in which PTHrP plays the role of a classical "endocrine" hormone, under normal circumstances, PTHrP seems to play a paracrine and/or autocrine role. These physiological functions appear to include: 1) regulation of smooth muscle (vascular, intestinal, uterine, bladder) tone; 2) modulation of transepithelial (renal, placental, oviduct, mammary gland) calcium transport; and 3) regulation of tissue and organ development, differentiation, and proliferation [1,3]. The latter lends support to the concept considering PTHrP as a developmental, and/or growth-regulating factor, much like other well-known cytokines and growth factors than PTH.

Current knowledge of the physiological role of PTHrP in the kidney is limited. The present review focuses on recent research supporting the role of this protein as a renal regulating factor, from the renal vessels to the glomeruli and the tubular epithelium, with impact in the physiology and pathophysiology of the kidney.

PTHrP as a vasoactive hormone

PTH has long been known to have acute hypotensive vasodilatory effects [6]. However, the putative physiological role of PTH as a systemic vasoregulatory hormone has been difficult to understand in homeostatic terms, since PTH synthesis is confined to the parathyroid gland. On the other hand, it is now clear that PTHrP is produced throughout the cardiovascular system, and binds to vascular smooth muscle cells through the PTHR, acting in a paracrine/autocrine, and perhaps even intracrine manner [1,3,6]. PTHrP has been shown to be a potent smooth muscle relaxant in every tissue examined, including vascular smooth muscle cells. Interestingly, in these tissues, both PTHrP mRNA and protein are dramatically upregulated in response to mechanical stretch [1]. Pirola et al [7] demonstrated that vasoconstrictors such as angiotensin II, serotonin, and bradykinin markedly induce PTHrP gene expression in the vascular tree, whereas other vasoactive substances such as atrial natriuretic peptide, neurokinin, and substance P are ineffective. Moreover, angiotensin II evokes a rapid and transient expression of PTHrP in a fashion reminiscent of that of immediate response genes in smooth muscle cells [7]. In addition, PTHrP inhibits angiotensin II-induced smooth muscle cell growth, suggesting that local production of PTHrP may serve as a counterbalancing modulator of the contractile and/or growth-promoting effects of angiotensin II, and possibly of other vasoconstrictors. Thus, PTHrP might serve as a local peptide, limiting or antagonizing the biological activity of at least some contractile stimuli in the arterial wall.

PTHrP and its receptors in renal tissue

Targeted disruption of the PTHrP gene in mice is not shown to be associated with abnormalities in kidney development [8]. Recently, the localization of PTHrP and PTHR mRNA in the developing mouse kidney has been examined [9]. High PTHrP mRNA levels were found in the collecting duct, urothelium of the pelvis, and immature elements of the glomeruli in this model. PTHR mRNA increased, associated with the maturation process, in the developing tubules and glomeruli, but was not found in urothelium of the pelvis or the collecting duct [7]. These findings suggest a role for PTHrP in renal development. In the adult kidney, PTHrP has been identified by immunohistochemistry or in situ hybridization in the glomerular podocytes, and proximal, distal, and collecting tubules, as well as in the intrarenal arterial tree, including afferent and efferent arterioles, and in renal macula densa [10,11].

Using immunohistochemistry or various mRNA detection methods, the PTHR was detected in convoluted and straight proximal tubules, cortical straight ascending limbs, and distal convoluted tubules, consistent with known sites of PTH action [10-14]. In addition, a PTHrP receptor whose transcript has at least partial sequence homology with the PTHR was found to be highly present in human glomerular podocytes [12]. Whether this receptor results from alternative splicing of the PTHR mRNA, as shown to occur in the human kidney cortex [15], is unknown. Recently, we have found that both PTH and PTHrP counteract the contracting effects of platelet activating factor on human mesangial cells in vitro [16]. However, while we could identify the PTHR mRNA in the human kidney cortex by Northern blot analisys and RT-PCR, no PTHR transcript was found in mesangial cells [16]. Our results strongly suggest that the PTHR is not responsible for the effects of PTH and PTHrP in the human mesangium.

Glomerular actions of PTHrP

Several lines of evidence support a direct action of PTH and PTHrP on the glomerulus. Micropuncture studies in rats have shown that both PTH and cAMP cause a reduction in the glomerular ultrafiltration coefficient (Kf). In addition, parathyroid glands removal increases the Kf, suggesting that PTH, at physiological concentrations, might play a regulatory role on glomerular ultrafiltration [17]. It is unclear whether the effects of PTH on Kf would affect the glomerular filtration rate (GFR) and the single nephron GFR, since this is determined by various factors in addition to Kf. PTH may stimulate, via cAMP, the glomerular production of angiotensin II, thereby causing mesangial cell contraction, and then a reduction of the available filtration surface and a fall in Kf [17]. In a more recent study, using a hydronephrotic rat kidney model, local administration of PTH and PTHrP induced a similar pattern of vasodilatation of all preglomerular vascular segments, including the afferent arteriole, and an increase in renal blood flow [18]. Furthermore, Massfelder et al [19] studied the renal effects of PTHrP infused directly into the left renal artery of anaesthetized rats. PTHrP, at 100 pM, increased renal blood flow by 10%, and GFR by 20%, without significantly increasing the filtration fraction, and it increased urine flow by 57% in the left kidney. Meanwhile, in the right control kidney, GFR and diuresis did not change. These findings support the renal vasodilatory effect of PTHrP.

More recently, we found a direct relaxant effect of PTHrP on mesangial cells in vitro, which seems to involve cAMP and G-proteins [16]. Our findings suggest a modulatory effect of PTHrP on glomerular function by counteracting the effects of vasoconstrictor agents on mesangial cells. Moreover, these in vitro data support the notion that PTHrP (and PTH as well) has a direct relaxant effect on the mesangium, which would tend to increase both Kf and GFR [20]. Thus, taken together, the results from these studies suggest that the local action of PTHrP would predominantly induce an increase of both Kf and GFR.

Physiology and pathophysiology of PTHrP in the tubular epithelium

PTHrP is mitogenic for various renal cells, including renal carcinoma cells, mesangial cells, distal tubule-like cells MDCK, and subconfluent proximal tubule cells [21-25]. Furthermore, PTHrP mRNA increases, associated with a decreased PTHR gene expression, in renal tubular cells during the recovery phase after ischemic injury [23,24]. We recently found a similar response pattern for the renal expression of PTHrP and the PTHR in folic acid-injected rats, another model of acute renal failure, which is associated with mild kidney damage but dramatic tubular hyperplasia [26]. These findings suggest that PTHrP is an autocrine factor that might participate in the renal regenerative process after acute injury. This autocrine effect of PTHrP is different from that of other well characterized renal tubular mitogens, which act in a paracrine manner after acute renal failure [27]. Additional studies should assess the putative interaction between the mitogenic effect of these known growth factors and PTHrP in the process of renal regeneration after acute renal failure.

PTHrP and renal disease progression

Recent data suggest a role of PTHrP in the mechanisms associated to progression of renal damage. In this regard, PTHrP mRNA was found to increase sequentially in the renal cortex during the development of proteinuria in a rat model of tubulointerstitial nephropathy after protein overload [13]. PTHrP immunostaining also increased in both proximal and distal tubules, and in the glomerulus, where PTHrP positivity was found in both mesangial and endothelial cells [13]. The significance of the latter finding is intriguing, and might be related to the increased mesangial growth and/or matrix expansion observed in protein-overloaded animals [13].

The mechanisms responsible for the observed PTHrP upregulation in the renal tissue during progression of renal injury are yet unknown. Interestingly, an increase in angiotensin converting enzyme, an important factor in the mechanisms associated with the development of renal damage [28], and in preproendothelin-1 mRNA occurred in the renal cortex of protein-overloaded rats [13]. The possibilty that angiotensin II or endothelin-1, which rapidly induce PTHrP mRNA in vascular smooth muscle cells [7], would be responsible for the increased PTHrP in the chronically damaged kidney has not yet been tested. Renal PTHrP overexpression during chronic renal damage could be part of a feedback mechanism to counteract the effects of other vasoactive factors such as angiotensin II, considering those of PTHrP on vascular tone and mesangial contraction mentioned above. However, other PTHrP effects have opposite consequences on glomerular hemodynamics, such as those on renin production and mesangial cell proliferation [16,23,28]. Thus, PTHrP seems to be a factor with complex and partly defined roles in the mechanisms associated with renal disease progression.

Summary

Current data support the notion that PTHrP can be considered as a renal regulatory factor that may limit or antagonize the biological activity of contractile stimuli in the renal arterial wall. Therefore, locally produced PTHrP could participate in the regulation of renal blood flow and glomerular filtration rate. Moreover, PTHrP, besides mimicing PTH actions, has important effects on the growth of both glomerular and tubular cells. Finally, although much more work needs to be done to further characterize the emerging role of PTHrP as a renal regulating factor, studies of the renal effects of PTHrP may provide new insights for a better understanding of the normal and the injured kidney.

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