Bone and Mineral Metabolism Laboratory and Renal Research Laboratory, Fundación Jiménez Díaz and Universidad Autónoma, Madrid, Spain
Introduction
Parathyroid hormone (PTH)-related protein (PTHrP) was initially isolated from tumours associated with humoural hypercalcaemia of malignancy [1]. Both PTH and PTHrP share homology in their N-terminal region, and bind to the type 1 PTH/PTHrP receptor (PTHR), first cloned in osteoblasts and renal tubular cells, activating both adenylate cyclase and phospholipase C/protein kinase C [2]. However, while PTH is a regulator of mineral homeostasis, acting mainly on bone and kidney, PTHrP is now known to be widely expressed in non-malignant foetal and adult tissues [3]. Furthermore, PTHrP post-translational processing generates various fragments, including the PTH-like region and other mid- and C-terminal domains lacking PTH homology [4,5]. These fragments, interacting with specific receptors, appear to act as autocrine/paracrine regulators of cell growth and/or differentiation, at least in some tissues [1,3]. Although knowledge of the physiological role of PTHrP and its fragments in the kidney is in its infancy, the present review focuses on recent hypotheses regarding the putative actions of this protein as a complex renal cytokine.
PTHrP and its receptors in renal tissue
Targeted disruption of the PTHrP gene in mice has not been shown to be associated with abnormalities in kidney development [6]. A more recent study has examined the localization of PTHrP and PTHR mRNA in the developing mouse kidney [7]. High PTHrP mRNA levels were found in the collecting duct, urothelium of the pelvis, and immature elements of the glomeruli in this model. In contrast, PTHR mRNA increased in association 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 recent 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 [8,9]. Using these techniques as well as reverse transcription followed by polymerase chain reaction of microdissected nephron segments, the PTHR was detected in convoluted and straight proximal tubules, cortical straight ascending limbs, and distal convoluted tubules, in both rats and humans [912]. This localization is consistent with known sites of PTH action, such as 25-hydroxycholecalciferol 1-hydroxylase activation, and tubular calcium and phosphate reabsorption [13]. In addition, a PTHrP receptor whose transcript has at least partial sequence homology with the PTHR was found to be abundant in human glomerular podocytes [10]. It is presently unknown whether this receptor results from the PTHR by alternative splicing, as splice variants of this receptor mRNA have been detected in the human kidney cortex [14]. Furthermore, the putative presence of PTHrP receptors different from the PTHR, which appear to occur in other cells, is uncertain for the glomerulus or other parts of the nephron [3,15].
PTHrP and renal regeneration after acute renal failure
PTHrP is mitogenic for various renal cell types in culture, including renal carcinoma cells, mesangial cells, distal tubule-like cells MDCK, and subconfluent proximal tubule cells [1519]. This effect is similar to that induced by PTH in these cells, and, at least in the latter cells, appears to involve both adenylate cyclase and protein kinase C activation [20]. Furthermore, PTHrP mRNA increases, together with a decreased PTHR gene expression, in the renal cortex during the recovery phase after renal ischaemic injury [18]. 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 [21]. Moreover, both PTHrP mRNA and secreted protein increase in proximal tubule-like cells HK-2 shortly following severe ATP depletion [22]. These findings suggest that PTHrP is an autocrine factor that might participate in the renal regenerative process after acute injury. Our preliminary findings after acute administration of PTHrP to folic acid-treated rats support this hypothesis [21]. Therefore, this autocrine effect of PTHrP is different from that of other well-characterized renal tubular mitogens, such as epidermal growth factor and hepatocyte growth factor, which act in a paracrine manner after acute renal failure [23]. Moreover, considering the rapid and transient response of the PTHrP gene in this situation, it is unlikely that these factors are involved in PTHrP overexpression. Additional studies are required to assess the interaction between the mitogenic effect of any of the aforementioned growth factors and PTHrP in renal cells, and its possible role in the process of renal regeneration after acute renal failure.
PTHrP and renal disease progression
Other recent data suggest a role of PTHrP in the mechanisms associated with progression of renal damage. Thus, chronic cyclosporin administration rats, which induces interstitial fibrosis and tubular atrophy [24], upregulates renal PTHrP mRNA and leads to a dramatic increase of PTHrP immunostaining in the renal cortex [25]. Furthermore, since proteinuria correlates with the progression to end-stage renal disease [26], the renal expression of PTHrP and PTHR was recently evaluated in a rat model of intense proteinuria and tubulointerstitial nephropathy after protein overload [12]. In this animal model, PTHrP mRNA was found to increase sequentially in the renal cortex during the development of proteinuria. In contrast, PTHR mRNA decreased in these animals, possibly as a consequence of the increase in intracellular calcium concentration observed in chronic renal failure [12,27]. After protein overload PTHrP immunostaining also increased in both proximal and distal tubules and in the glomerulus, where PTHrP positivity was found to reappear in mesangial and endothelial cells [12]. The significance of the latter finding is unknown, but might be related to the increased mesangial growth and/or matrix expansion observed in this animal model [12].
The mechanisms responsible for the observed PTHrP upregulation in renal tissue during chronic renal injury are ill-defined at present. The renin-angiotensin system is known to play an important pathogenic role in the development of kidney damage [28]. In fact, angiotensin II (Ang II) induces either proliferation or hypertrophy in mesangial and tubular epithelial cells and in interstitial fibroblasts, in part through TGF-ß synthesis [2931]. Interestingly, an increase in angiotensin-converting enzyme and in pre-proendothelin-1 gene expression was observed in the renal cortex of protein-overloaded rats [12]. Both Ang II and endothelin-1, as well as TGF-ß, rapidly induce PTHrP gene expression in various cells, including vascular smooth-muscle cells [3,32,33]. The possibility that any of these factors might be responsible for the increased PTHrP in the chronically damaged kidney has not yet been tested. Our own as yet unpublished observations, using Ang II infusions into rats, suggest that at least for this factor this is likely to be the case.
PTHrP, acting through PTHR, is a potent renal vasodilator, and has been shown to inhibit Ang II-induced DNA synthesis in smooth-muscle cells from rat aorta [1,3,8,32]. In addition, recent studies have demonstrated that PTHrP antagonizes the effects of platelet-activating factor on mesangial cell contraction [15]. Thus, PTHrP overexpression in the renal cortex during chronic renal damage could be part of a feed-back mechanism limiting the renal effects of Ang II and maybe other vasoactive factors. However, other PTHrP effects have opposite consequences on glomerular haemodynamics, such as those on renin production and mesangial cell proliferation [15,18,34]. Therefore, current data point to PTHrP as a factor involved in the complex mechanisms associated with progression of renal disease.
Future perspectives
Much work has still to be done to fully understand the role of PTHrP as a renal regulatory cytokine. Specifically, the following issues should be addressed in future studies: (i) identification and localization of possible PTHrP receptors different from the PTHR in the kidney; (ii) assessment of possible effects of other PTHrP fragments, distinct from the PTH-like fragment, on growth and differentiation of the different renal cell types; (iii) analysis of the relationship between PTHrP and other characterized renal growth factors in the regulation of renal growth and function; (iv) evaluation of the relative significance of the haemodynamic effects of PTHrP vs its proliferative effects on different cell types in the injured kidney; (v) clarification of the putative role of PTHrP as a mediator for the action of Ang II and perhaps other vasoactive factors in the renal tissue; and (vi) assessment of a possible therapeutic role for PTHrP as a renal mitogen in renal failure. The development of transgenic mice overexpressing PTHrP in different nephron segments would be a valuable tool to elucidate at least some of these issues.
Notes
Correspondence and offprint requests to: Dr J. Egido, Renal Research Laboratory, Fundación Jiménez Díaz, Avda Reyes Católicos 2, E-28040 Madrid, Spain.
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