1Nephrological Center, Villingen-Schwenningen and 2Department of Internal Medicine, University of Heidelberg, Germany
Correspondence and offprint requests to: Dr Helmut Reichel, Nephrological Center, Schramberger Strasse 28, D-78054 Schwenningen, Germany. Email: helmut.reichel{at}dialyse-schwenningen.de
Keywords: chronic allograft rejection; hypertension; proteinuria; renal transplantation
Proteinuria: marker or culprit?
The amount of protein excreted in the urine was previously shown to be a powerful predictor of renal outcome [1]. More recently it has been proposed that proteinuria per se promotes progression of renal disease by conferring to proximal tubular epithelial cells, after uptake of proteins by endocytosis, an inflammatory phenotype with increased production of ANG II, endothelin 1, cytokines and chemokines. All these agonists, preferentially secreted in an upluminal direction [2] stimulate interstitial fibroblasts, and potentially cause trans-differentiation of epithelial cells into fibroblasts, thus promoting interstitial fibrosis. Direct evidence that proteinuria leads to interstitial fibrosis of the kidney has recently been provided by studies on the amphibian kidneys of axolotl [3]. These kidneys have two types of nephrons: closed nephrons like the mammalian kidney, and open nephrons that communicate with the peritoneal cavity via an orifice. Injection of protein into the peritoneal cavity causes fibrosis selectively around the open, but not the closed, nephrons.
It has recently been argued [4] that the allograft is particularly vulnerable to the adverse effect of proteinuria for several reasons. Protein-loaded epithelial cells express more MHC-2 antigens, rendering them potentially susceptible to immune reactions. It has also been shown that proximal tubular cells from patients with cyclosporine nephrotoxicity express more endothelin I, a potent agonist in the generation of renal fibrosis [5].
Against this background, it appears reasonable to discuss the role of proteinuria in the pathogenesis and management of chronic allograft nephropathy. It is of particular interest to review the evidence concerning whether proteinuria is only a marker of allograft dysfunction, or whether it should be a target for therapeutic intervention. Proteinuria as a consequence of recurrent or de novo glomerulonephritis will not be discussed in detail.
How frequent is microalbuminuria/proteinuria after renal transplantation?
Immediately after transplantation, transient high molecular weight proteinuria and more persistent but ultimately evanescent molecular weight proteinuria are noted [6]. This presumably reflects perioperative ischaemic damage to the graft.
In the long run, however, microalbuminuria or proteinuria are more frequent in allograft recipients that in the normal population. Halimi et al. [7] examined cross-sectionally 75 hypertensive non-proteinuric first graft recipients who had not received antihypertensive medication for 1 month. Of the 75 hypertensive graft recipients, 46 had microalbuminuria. Microalbuminuric patients were characterized by higher systolic blood pressure and a higher frequency of acute rejection. Acute rejection was found in 45.7% of the microalbuminuric patients as compared to 17.2% of normoalbuminuric individuals.
Does proteinuria predict graft loss in patient survival?
How frequent is proteinuria after renal transplantation? In a retrospective analysis, Hohage et al. [8] examined 327 patients transplanted between 1980 and 1990. Proteinuria (0.251.0 g/day) at 6 months after transplantation was noted in 25.5% of the patients. Non-proteinuric patients had a 5 year graft survival of 85.6% as compared to a much lower graft survival in proteinuric patients (58.9%). Interestingly, no correlation was found with regard to gender or age of recipient, to duration of haemodialysis, to age of the donor, cold ischaemia time or mismatches. It appears that mild proteinuria at 6 months after transplantation is a predictor of decreased long-term graft function. McLaren et al. [9] retrospectively examined 862 primary renal allograft recipients over a 10 year period. In this multivariate logistic regression analysis, technique graft failure was predicted by a history of rejection (with acute rejections occurring after 3 months being more predictive than early acute rejections), by proteinuria at 1 year after transplantation and by triglyceride concentration. Taken together, there is no doubt that proteinuria is a marker for graft survival.
Moreover, proteinuria is also indicative of patient survival. Roodnat et al. [10] assessed graft function in 722 graft recipients 1 year or longer after transplantation. Proteinuria was assessed by taking proteinuria as a continuous variable as well as applying categorical analysis. The relative risk (RR) of graft failure was elevated in proteinuric patients (RR 2.03) as compared to non-proteinuric patients. A correlation was also found with the type of underlying disease (higher risk of graft failure in patients with glomerulonephritis), history of hypertension and presence of systemic disease. Interestingly, patient death was also higher in graft recipients with proteinuria (RR 1.98) as compared to non-proteinuric patients. This finding was confirmed by the analysis of Fernandez-Fresnedo et al. [11]. In a cohort of graft recipients, persistent proteinuria was associated with graft loss (RR 4.18), patient death (RR 1.92), and cardiovascular events (RR 2.45).
What are the pathomechanisms underlying post-transplant proteinuria?
There is no doubt that in the absence of recurrent or de novo glomerulonephritis allograft nephropathy is the ultimate reason for proteinuria. Nevertheless, in recent years the intermediate steps culminating in proteinuria of graft recipients have been more clearly analysed. One of the most important causes of proteinuria is hypertension, but nephron underdosing (or more specifically kidney/recipient size mismatch), quality of the graft and activation of the intrarenal renin angiotensin system have also been identified and assessed with regard to influence on proteinuria.
In most studies [12,13] hypertension emerged as a powerful predictor of proteinuria. This is of course reminiscent of the role of blood pressure in determining proteinuria in primary chronic renal disease, particularly glomerulonephritis [14]. This finding has been reproduced in experimental studies where superimposition of DOCA-salt hypertension in allograft recipients, using the Fisher to Lewis renal allograft model, was shown to be associated with more severe proteinuria and presumably more severe glomerular hypertension [15].
Brenner et al. [16] have proposed the hypothesis that nephron underdosing, i.e. a deficit in glomerular number, predisposes the recipient to proteinuria, hypertension and progressive renal disease. In this context, it is of interest to assess urinary albumin excretion in living donors in whom the nephron number is reduced by one-half. Saran et al. [17] followed 75 living kidney donors 1231 years after nephrectomy. Urinary albumin excretion >20 µg/min was found in 34% of the donors, most of whom were hypertensive.
A long-term assessment of living kidney donors was also carried out by Eberhard et al. [18]. Twenty-nine donors who had been nephrectomized between 1973 and 1990 (median age 54 years) were assessed 11 years after nephrectomy. Only 10% had elevated serum creatinine, i.e. >1.3 mg%, but 24% exhibited microalbuminuria. This percentage was higher than that observed in the age-matched background population [19]. An interesting comparison of urinary protein excretion between living kidney donors and graft recipients was made by Borchhardt et al. [20] who measured glomerular filtration rate, renal plasma flow and fractional dextran clearance. As anticipated, graft recipients had higher proteinuria (0.39 vs 0.07 g/day) and albuminuria (137 vs 26 mg/day). Of the 22 donors, 5 (22.7%) had microalbuminuria. There was also no change in fractional dextrane clearance, i.e. no change in glomerular permselectivity.
The above data are in contrast to the study of Bertolatus et al. [21] who assessed kidney donors up to 3 years after transplantation. Estimated single nephron filtration rate was increased by 33% as compared to normals. There were transient episodes of increased albumin and protein excretion after transplantation, which did not persist after 3 years.
The role of diminished nephron supply as a determinant of proteinuria and long-term graft outcome is exemplified by the experimental findings of the Boston group [22,23] in the Fisher to Lewis model. Uninephrectomized rats received orthotopic renal allotransplants. The remaining normal kidney was either removed or preserved. If the remaining kidney had been removed, proteinuria was significantly increased (35 ± 2 vs 7 ± 1 mg/day) and the glomerulosclerosis index was significantly higher (24 ± 8 vs 4 ± 1%). There is also evidence that poor graft quality, e.g. elderly donors or donors with a history of cardiovascular disease, predispose recipients to post-transplantation proteinuria [24], although this has not been consistently found in all studies [25]. Under experimental conditions, prolonged cold ischaemia time resulted in increased post-transplantation proteinuria [26]. Against the background of the nephron underdosing hypothesis, an effort has been made to transplant two kidneys when they were of poor qualitya procedure that yielded surprisingly good results [27].
Which interventions affect proteinuria after transplantation?
Of particular interest for the selection of appropriate interventions is the role of angiotensin II, possibly also of aldosterone, and endothelin-1 in the genesis of albuminuria/proteinuria after renal allograft transplantation.
A theoretical basis for the use of ACE inhibitors after transplantation was provided, amongst others, by the study of Richter et al. [28]. In the Fisher to Lewis model, heterotopic heart transplant animals received Candesartan (20 mg/kg/day) or Enalapril (40 mg/kg/day). This caused significant reduction of neointimal proliferation in small arteries in the myocardium, but not in the epicardial conduit arteries. Since the small arteries are a target of allograft nephropathy, this finding was of interest with respect to renal allografts as well. The study of Smit-van Oosten et al. [29] injected a note of caution. In the renal allograft Fisher to Lewis model, Lisinopril prevented glomerular damage and had some, but considerably less, effect on interstitial fibrosis. Unexpectedly, however, Lisinopril caused dramatic intimal hyperplasia of the renal arteries. It was also seen in control animals receiving syngeneic grafts, but was particularly severe in recipients of allografts. This finding is reminiscent of vascular lesions in the kidney in the ACE knock-out mouse [30].
What is the clinical experience with ACE inhibitors? Borchhardt et al. [31] examined 25 stable renal graft recipients with proteinuria. Thirteen patients received low dose Lisinopril, while 12 patients did not receive an ACE inhibitor. At a stable elevated mean arterial pressure (112 mmHg), Lisinopril decreased the filtration fraction as expected (from 22 to 19%). The fractional protein excretion remained stable in patients on Lisinopril, but it increased in the control patients. The size selectivity was decreased in control patients, but remained stable in Lisinopril-treated patients. In patients with advanced allograft nephropathy, the findings were somewhat disappointing [32]. Fosinopril (1015 mg/day) was administered to 28 allograft recipients with persisting proteinuria (>1.25 g/day). The mean value of protein excretion was 2.94 g/day at baseline, decreased to 1.82 g/day at 3 months, but increased again to 2.48 g/day at 8 months. An inverse correlation between the antiproteinuric effect of ACE inhibition and histological lesions was noted. The observation presumably indicates that the effect of ACE inhibition on proteinuria is no longer impressive when renal lesions are advanced.
In a retrospective analysis of 63 patients with biopsy-proven chronic allograft nephropathy [33] it has been claimed that ACE inhibitors lower renal insufficiency and improve survival in graft recipients. In this study, 32 of the 63 patients had received ACE inhibitors. There was significantly less doubling of serum creatinine (19.0 vs 39.0%) in ACE inhibitor recipients. The combined endpoint of allograft failure or death had occurred in only 9.4% of patients who had received ACE inhibitors compared to 36% of patients without ACE inhibitors. Retrospective studies, however, are potentially subject to many confounders. In a double-blind randomized prospective study, Hausberg et al. [34] assessed hypertensive renal allograft recipients. Patients were randomized to Quinalapril or Atenolol 612 weeks after transplantation. With a similar reduction in diastolic blood pressure, mean albumin excretion decreased by 10 ± 15 mg/day with Quinalapril and increased by 52 ± 32 mg/day with Atenolol. There was no effect on long-term graft outcome.
When one evaluates the impact of ACE inhibitors, one has to take into account that the effects of ACE inhibitor treatment after renal transplantation extend beyond renal effects. Hernandez et al. [35] studied 52 stable non-diabetic renal graft recipients with hypertension and left ventricular hypertrophy by echocardiography. Lisinopril (10 mg/day), as compared to a placebo, caused a significant decrease in left ventricular mass index. After adjustment for confounders, a decrease of 9.5 ± 3.5% was noted compared to an increase of 3.0 ± 3.2% in patients receiving a placebo. Whether this will ultimately translate into lower patient mortality is currently unknown.
The experience with angiotensin receptor blockers is incomplete, and only small series have been reported. The results show that angiotensin receptor blockers, as one would anticipate, reduce microalbuminuria [36] or proteinuria [13].
In two recent reviews [37,38] the authors drew the cautious and sensible conclusion that ACE inhibitors after renal transplantation are safe and effective. There is, however, no evidence from controlled studies for the superiority of ACE inhibitors over other antihypertensive agents with respect to graft and patient survival. Nevertheless, it is clear that ACE inhibitors improve cardiovascular risk indicators, but whether this will ultimately translate into fewer cardiovascular events is uncertain. The authors also acknowledged that controversies persist with respect to the indication of ACE inhibitor use.
These controversies are based on the old observations of Curtis et al. [39] that in the renal allograft recipient hypertension is characterized by sodium retention and vasoconstriction, the latter induced by cyclosporine A. It was also shown that diuretics and calcium antagonists were highly effective, as one would anticipate from the pathophysiology involved. The authors suggested that these agents are the drugs of the first choice for post-transplant hypertension [40].
Van der Schaaf et al. [41] assessed the acute effects of calcium channel blockers vs ACE inhibitors in 20 hypertensive stable graft recipients receiving cyclosporine A in a double-blind, cross-over trial comparing Amlodipine 10 mg/day with Lisinopril 10 mg/day. Lisinopril did not change GFR, renal plasma flow and renal vascular resistance. In contrast, Amlodipine increased GFR by 10%, renal plasma flow by 27% and decreased renal vascular resistance by 23%. The authors concluded that the renin angiotensin system does not play a major role in cyclosporine A-associated changes of renal haemodynamics and that the antihypertensive potency of ACE inhibitors was inferior to that of Amlodipine, at least in this single dose comparison.
In a prospectively carried out trial Midtvedt et al. [42] compared the calcium channel blocker Nifedipine (30 mg/day) with Lisinopril (10 mg/day) in 154 graft recipients on cyclosporine A. Mean GFR at baseline was similar in both groups (46 vs 43 ml/min). Nevertheless, GFR after 1 year was higher in graft recipients treated with Nifedipine as compared to Lisinopril (56 vs 44 ml/min). The authors concluded that both calcium channel blockers and ACE inhibitors are safe and effective, but they underlined the better renal haemodynamic effects of calcium channel blockers. Whether the presumed renal vasodilatation which carries the potential risk of glomerular hypertension will turn out to be beneficial in the long run is undecided. Based on observations in patients with primary glomerular disease it will presumably be essential for the long-term effects of calcium channel blockers disease that blood pressure is sufficiently lowered [43,44]. It also appears that co-administration of ACE inhibitors with calcium channel blockers will provide additional safety [43]. At any rate, retrospective observations in patients with primary chronic renal disease suggest that one should lower blood pressure to values as low as 120/70 mmHg [45,46].
Conclusions
The presently available evidence on proteinuria after renal transplantation shows that microalbuminuria and more significantly proteinuria after transplantation are markers of graft dysfunction, correlated both to graft failure and to cardiovascular risk indicators (and possibly the risk of cardiovascular death). In patients with proteinuria one has to exclude recurrent glomerulonephritis (which is surprisingly frequent [47]) in order to avoid unnecessary immunosuppression. Whilst there is no doubt that proteinuria is a sensitive marker of allograft dysfunction, possibly inferior only to protocolled biopsies, there is currently insufficient evidence to assume that it plays a major causal role in graft loss.
Conflict of interest statement. None declared.
References