Assessment of tumor-associated trypsin O

O RIGINAL INVESTIGATION
J NEPHROL 2003; 16: 663-672
Assessment of tumor-associated trypsin
inhibitor (TATI) as a marker of renal function
Gianfranco Tramonti 1, Marco Ferdeghini 2, Carmela Annichiarico 2, Carlo Donadio 1, Maria Norpoth 1,
Emanuela Mantuano1, Claudio Bianchi1
Department of Internal Medicine, Nephrology Unit
Department of Oncology, Nuclear Medicine, University of Pisa, Pisa - Italy
1
2
ABSTRACT: Background: Low molecular weight (LMW) proteins have been proposed for renal function assessment. This
study aimed to ascertain the usefulness of tumor-associated trypsin inhibitor (TATI), a LMW protein (6.200 d), as a
glomerular filtration rate (GFR) marker. The results were compared with those of β2-microglobulin and of creatinine
(Cr).
Methods: Renal handling of TATI labelled with 125I was first studied in rats. Then, in 198 patients, serum TATI levels and
GFR (99mTc-DTPA clearance, bladder cumulative method) were determined. To evaluate urine excretion, the fractional
TATI clearance was determined in 63 patients.
Results: In rats, total body scan showed a large amount of radioactivity in the kidneys, but not in other organs. The duration of radioactivity demonstrated a peak-time of 11 min. In human beings, the relationship between TATI and GFR was
similar to that of β2-microglobulin and Cr. The increase in TATI with declining renal function was statistically significant,
vs. patients with GFR >100 mL/min, already in the group with GFR 80-100 mL/min (p<0.05, Bonferroni-Dunn test). The
β2-microglobulin increase was significant in the group with GFR 60-80 mL/min and of Cr in the group with GFR 40-60
mL/min. In patients with renal failure (GFR <20 mL/min) TATI increased, vs. patients with GFR >100 mL/min, 13x,
β2-microglobulin 8x and Cr 5x. Urinary excretion of TATI, expressed as fractional clearance, was very low increasing
when GFR fell <40 mL/min.
Conclusions: The kidney plays an important role in the handling of TATI. When GFR fell, the increase in blood levels of
TATI was sooner and higher than that of β2-microglobulin and CR. Consequently, TATI can be added to the group of
renal function markers.
Key words: Tumor-associated trypsin inhibitor, β2-microglobulin, Creatinine, Glomerular filtration rate, Low molecular weight proteins
INTRODUCTION
Renal function assessment is very important in clinical
practice, since it allows nephrologists to evaluate renal
damage at the moment of diagnosis or the effect of
treatment on renal disease progression. Glomerular
filtration rate (GFR) is usually estimated by measuring
the blood level of substances eliminated by the kidneys. The most common are blood urea and creatinine (Cr). The usefulness of these parameters to detect renal function is under debate. Both urea and Cr
increase when renal function is reduced and therefore, they are not useful in detecting variations occurring in the range from normal GFR to 40 mL/min.
Furthermore, many factors influence their blood levels. The clearance measure of these substances supplies a renal function estimate, but it is cumbersome
and correct urine collection is often difficult. Methods
that are more accurate imply the continuous infusion
of exogenous tracers for prolonged time to collect
urine correctly. Consequently, their use is restricted.
In order to improve and simplify renal function
assessment, many substances have been investigated.
Renal function markers recently proposed belong to
the family of low molecular weight (LMW) proteins.
They are proteins of MW lower than that of albumin
(69.000 D). Due to their low weight, these proteins are
freely filtered by the glomeruli and subsequently reabsorbed by proximal tubular cells where they undergo
metabolic degradation (1, 2). Lysozyme, α2-microglobulin and ß2-microglobulin were first proposed as renal
function markers (3-11). Recently, other small proteins
entered this group, the latter being cystatin C (12).
Urinary excretion of these proteins is very low, but in
renal failure or in tubular diseases it increases. Therefore, LMW proteins are also used to detect tubular
damage (LMW proteinuria).
Recently, in the urine of women affected by ovarian
www.sin-italia.org/jnonline/vol16n5/
663
TATI as a marker of renal function
cancer, another LMW protein was discovered (13, 14).
This protein was called tumor-associated trypsin inhibitor (TATI), and proposed as a tumor marker (1521). TATI has a ver y low MW, 6.200 D, and its
aminoacid sequence is the same as pancreatic secretory
trypsin inhibitor (PSTI), formerly known as Kazal inhibitor. The name TATI is misleading since this small
protein is not produced solely by the tumor, but circulates in the blood of healthy people being mainly produced by the pancreas (22). The aminoacid sequence
of TATI/PSTI presents an homology of 48% to the
EGF. TATI exhibits some growth promoting properties in cell cultures (23-24). Previous studies demonstrated an increase in serum TATI levels in renal failure patients or those undergoing hemodialysis (HD)
(15, 25-26). We recently reported the relationship between serum TATI levels and renal function (27-29).
The results of the above studies suggest that TATI can
be employed as a renal function marker. Nevertheless,
the studies were not exhaustive.
This study aimed to ascertain the usefulness of TATI as
a renal function marker. For this purpose, we studied
in the rat the renal handling of TATI labelled with 125I
and in a large number of renal patients the relationship with GFR of both serum levels and urinary excretion of TATI. Blood levels of TATI were compared
with those of Cr and ß2-microglobulin (MW 11,800
D), the LMW protein most extensively employed for
the assessment of renal function.
SUBJECTS AND METHODS
Studies in rat
We used four male Sprague-Dawley adult rats (Morini,
S. Polo d’Enza, Italy) with a body weight of 325-335 g.
TATI was labeled with 125I by the Chloramine T method
(30). Anesthesia was induced by i.p. injecting penthotal
sodium. The administered dose was 5 mg/100 g body
weight. 125I-TATI was injected as a bolus in a vein of the
tail at a dose of 80 µCi. In one rat, a total body scan was
performed using a rectilinear scanner (Italelettronica,
Roma, Italy). The scan was recorded from 3-26 min after the tracer injection. The scanner ran at a speed of
100 cm/min using a focusing collimator. In three other
rats, the curve of renal radioactivity was recorded. After
the labeled TATI injection, the collimator was focused
over the left kidney and the radioactivity was recorded
each minute for 30 min.
Studies in humans
The study consisted of 198 patients, 90 males and 108
females. Their ages ranged from 14-81 yrs, mean age
53 yrs. In all cases, informed consent was obtained. Renal function ranged from normal values to advanced
renal failure (plasma Cr from 0.8-8.1 mg/dL). All patients underwent renal function measurements because of kidney disease, and there were no signs or
symptoms of neoplastic disease present at the time of
the study. To exclude ovarian cancer, female patients
underwent ultrasound or gynecological examinations.
Table I reports the diagnoses of the patients.
GFR was determined in the morning by the bladder
cumulative method, using 99mTc-DTPA as a glomerular
tracer. This method is non-invasive and provides the
direct measure of renal clearance. Briefly, the tracer was
injected i.v. as a bolus. Ninety minutes after the injection, radioactivity was recorded over the bladder for approximately 30 min. At the middle point, a blood sample was taken and finally, urine was collected by spontaneous voiding. Previous studies using 131I-diatrizoate
showed that this method provides results in agreement
with those obtained using inulin (31, 32). The reliability
of 99mTc-DTPA as a glomerular tracer for this method was
subsequently demonstrated (33).
TATI, ß2-microglobulin and Cr were determined in
TABLE I - DIAGNOSES OF PATIENTS STUDIED
Diagnosis
Glomerulonephritis
Essential hypertension
Diabetic nephropathy
Chronic pyelonephritis
Vascular nephropathy
Nephrolithiasis
Congenital abnormalities
IgA nephropathy
Urinary tract infection
Undetermined
664
Number
53
38
22
12
11
10
10
8
7
11
Diagnosis
Cystic disease
Reflux nephropathy
Tubular-interstitial nephritis
Angiomyolipoma
Prostatitis
Genito-urinary tuberculosis
Sarcoidosis
Amyloidosis
SLE
Number
4
3
2
2
1
1
1
1
1
Percent uptake
Tramonti et al
Time (min)
Fig. 2 - Time course of renal radioactivity recorded in a rat after the 125I-TATI injection (80 µCi i.v.). The radioactivity is expressed as a percentage of the maximum value (peak-time).
RESULTS
Studies in rat
Fig. 1 - Total body scan of a rat after the 125I-TATI injection (80
µCi i.v.). The scan was recorded starting 3 min after the tracer injection.
the same blood sample taken during GFR measurement. TATI was measured by RIA (Spectria TATI Update, Orion Diagnostica, Oulunsalo, SF, reference
range =5-15 mg/L). ß2-microglobulin was measured
by RIA (ß2-Microglobuline, Immunotech, Marseilles,
France, reference range =1-2.4 mg/L). Plasma Cr was
determined by an autoanalyzer (Hitachi 717, 911 N;
Tokyo, Japan).
In 63 patients, urinary excretion of TATI was also
determined. The urine was from the same sample
collected for GFR measurement. We used these
urine samples to avoid mistakes, either due to the
wrong urine collection or to the permanence of
TATI for a long period in the urine. The results were
expressed as fractional TATI clearance (U TATI/P
TATI x P 99mTc-DTPA/U 99mTc-DTPA x 100). In this
way, the volume of urine was not necessary to calculate urinary excretion of TATI.
Statistical analysis
The Bonferroni-Dunn test was used. Receiver operating characteristic (ROC) curves for GFR values <70
and 80 mL/min were also evaluated. A value p<0.05
was considered statistically significant.
Figure 1 shows a total body scan, recorded after the
125
I-TATI injection. In both kidneys, a large amount of
radioactivity is visible. Conversely, no significant
radioactivity was present in organs other than the
kidneys. Therefore, labeled TATI was accumulated by
the kidneys. No other organs seemed to be responsible for the clearance of this small protein.
Figure 2 shows a time-course curve of radioactivity
recorded over the left kidney. The values are expressed as the percentage of the maximum value
(peak-time). After the 125I-TATI injection, renal
radioactivity rapidly increased and reached the peaktime at 11 min. The following decrease was almost
rapid and within 30 min the radioactivity was <50% of
the maximum value.
Studies in humans
Figure 3 shows the relationship between serum TATI
levels and GFR, including all 198 patients. No significant
variations of TATI were observed until GFR was >40
mL/min. When GFR fell <40 mL/min, serum TATI levels progressively increased. The shape of this curve was
the same as that of other substances eliminated by the
kidneys.
Figure 4 shows the relationship between serum ß2-microglobulin and GFR. Also ß2-microglobulin increased
when GFR fell <40 mL/min. The curve was similar to
that of TATI.
Figure 5 shows the relationship between plasma Cr
and GFR in the same patients. Our results confirmed that the curve was the well-known hyperbole
equilateral.
665
Tati (µg/L)
β2 microglobulin (mg/L)
TATI as a marker of renal function
GFR (mL/min)
GFR (mL/min)
Based on the above results we concluded that the
behavior of the three studied substances was similar.
However, some differences were found with further
data analysis.
Figure 6 reports the mean values of serum levels of
TATI, β2-microglobulin and Cr clustered based on
GFR. Each value represents the mean of the group
(GFR <20 mL/min, n=33; 20-40 mL/min, n=33; 40-60
mL/min, n=23; 60-80 mL/min, n=48; 80-100
mL/min, n=33; GFR >100 mL/min, n=28). Each
group was tested vs. the group with normal renal function, i.e. GFR >100 mL/min. Interestingly, serum
TATI levels showed a statistically significant difference
vs. the group with normal renal function, already in
the group with GFR 80-100 mL/min. The ß2-microglobulin increase was statistically significant starting
from the group with GFR 60-80 mL/min. The Cr results were statistically significant starting solely from
the group with GFR 40-60 mL/min. The mean values
of TATI, ß2-microglobulin and Cr in groups with different GFR were normalized considering 1, the value
in the group with normal renal function. The other
points indicate the ratio with the mean value of normal renal function. Therefore, we compared the
amount of increase in all three parameters. The decrease in GFR was accompanied by an increase in
Fig. 4 - Relationship between serum β2-microglobulin levels
and GFR.
Creatinine (mg/dL)
Fig. 3 - Relationship between serum TATI level and GFR.
GFR (mL/min)
Fig. 5 - Relationship between plasma Cr levels and GFR.
blood level of TATI, ß2-microglobulin and Cr. However, the amount of increase was different. In fact, in
patients with advanced renal failure, i.e. with GFR <20
mL/min, plasma Cr increased approximately 5 times
compared to the group with normal renal function,
and ß2-microglobulin approximately 8 times. The
TATI increase in the same group was definitely higher
than that of Cr and ß2-microglobulin. The increase
was approximately 13 times (Tab. II).
TABLE II - MEAN VALUES (±SD) PLASMA CONCENTRATIONS OF CREATININE, TATI AND β2-MICROGLOBULIN DIVIDED
IN GROUPS ACCORDING TO GFR
GFR (mL/min)
<20
TATI (µg/L)
102.31 ± 51.69*
Creatinine (mg/dL)
5.13 ± 1.73*
β2-microglobulin (mg/L) 12.21 ± 5.44*
20-40
40-60
60-80
80-100
>100
30.90 ± 12.95*
2.04 ± 0.71*
3.93 ± 1.89*
15.00 ± 7.70*
1.29 ± 0.40*
2.37 ± 1.00*
10.44 ± 3.23*
1.07 ± 0.26
2.07 ± 0.79*
10.33 ± 3.77*
1.03 ± 0.13
1.73 ± 0.41
7.81 ± 3.58
0.99 ± 0.18
1.61 ± 0.32
∗ indicates the significance of the difference vs. group of patients with GFR >100 mL/min (Bonferroni-Dunn test, p<0.05).
666
β2 microglobulin (mg/L)
GFR (mL/min)
1/Tati
β2 microglobulin
1/β
GFR (mL/min)
GFR (mL/min)
Normalized values
GFR (mL/min)
1/Creatinine
Fig. 7 - Relationship between reciprocal of
serum concentrations
of TATI (y=0.001x +
0.006; r=0.76), β 2-microglobulin (y=0.005x +
0.148; r=0.70), and Cr
(y=0.008x + 0.316;
r=0.78) and GFR. Reciprocal of TATI, β 2-microglobulin and Cr vs.
GFR are also reported
(last panel) together
normalized considering
1 the value of the group
with GFR >100 mL/min.
▲ TATI (1, y=0.009x +
0.01), ■ β2-microglobulin (2, y=0.008x + 0.25),
•Cr (3, y=0.007x + 0.27).
GFR (mL/min)
Normalized values
GFR (mL/min)
Creatinine (mg/L)
Fig. 6 - Relationship between serum levels of
TATI, β2-microglobulin,
and Cr and GFR. The
experimental points
represent the mean ±
SD of patients clustered in groups according to their GFR. * =p<
0.05. (Bonferroni-Dunn
test).
In the last panel the relationships between
serum TATI, serum β2microglobulin, plasma
Cr and GFR are shown
together normalized
considering 1 the value
of the group with GFR
>100 mL/min. ▲ TATI, ■
β2-microglobulin, •Cr
Tati (µg/L)
Tramonti et al
GFR (mL/min)
Figure 7 shows the relationship between the reciprocal of serum levels of TATI, β2-microglobulin, Cr and
of GFR. The intercept of the regression line of 1/TATI
was close to zero (0.006) and the correlation coefficient was 0.76. The intercept of 1/β2-microglobulin
was 0.148 and the correlation coefficient was 0.70,
while those of 1/Cr were 0.316 and 0.78. In order to
compare the slopes of the reciprocals, the results have
been normalized considering 1, the mean value of the
patients with GFR >100 mL/min. The other groups
clustered according to their GFR and represented the
ratio with normal values. The parameters of these re-
GFR (mL/min)
gression lines were y=0.009x + 0.01 for TATI, y=0.008x
+ 0.25 for β2-microglobulin and y=0.007x + 0.27 for Cr.
Figure 8 represents the relationship between urinary
excretion of TATI and GFR. The results are expressed
as a fractional clearance, i.e. the ratio between urinary
clearance and GFR. The fractional clearance expressing the urinary excretion of TATI was very low, but
when GFR fell <40 mL/min it rapidly increased.
Figure 9 reports the results as mean values based on
different GFR. The increase in fractional TATI clearance was statistically significant, with respect to the
group with GFR >100 mL/min, starting from the
667
Tati fractional CI x 100
Tati fractional CI x 100
TATI as a marker of renal function
GFR (mL/min)
GFR (mL/min)
group with GFR 40-20 mL/min. In the group with advanced renal failure, i.e. with GFR <20 mL/min, the
mean value was approximately 45%. Therefore, in
these patients the average tubular reabsorption was
approximately 55% of the filtered load.
Figure 10 reports both serum TATI values and fractional clearance, again in groups with different GFR.
Both the parameters were normalized considering 1,
the value in patients with normal renal function, i.e.
with GFR >100 mL/min. The other experimental
points represent the ratio between the mean value of
each group and those of patients with normal renal
function. When GFR fell, serum TATI values increased, while the fractional clearance remained
almost stable, at least until GFR was >40 mL/min. In
the group with GFR 40-60 mL/min, the value (normalized) of serum TATI levels was 2.3, while the fractional clearance was still 1.
The ROC curves for GFR <70 - 80 mL/min were also evaluated. Table III summarizes the results. The
area under the ROC curve of TATI was similar to
that of both Cr and β2-microglobulin. Only the difference between TATI and β2-microglobulin for
GFR <70 mL/min was statistically significant
(p=0.012).
Fig. 9 - Relationship between fractional TATI clearance and
GFR. The experimental points represent the mean ± SD of patients clustered in groups according to their GFR. *=p<0.05
(Bonferroni-Dunn test).
Normalized values
Fig. 8 - Relationship between fractional TATI clearance and
GFR. Fractional clearance expresses urinary excretion of
TATI.
GFR (mL/min)
Fig. 10 - Relationship of serum level and fractional clearance
of TATI vs. GFR. The values are normalized considering 1
those of the group with GFR >100 mL/min. ▲ serum values, •
fractional clearance.
DISCUSSION
The results of this study indicate that the kidney plays
an important role in TATI metabolism. The rat studies
showed that TATI was actively taken up by the kidneys,
while no significant amount of labeled TATI was ob-
TABLE III - ROC CURVES OF CREATININE, TATI AND β2-MICROGLOBULIN. AREA UNDER THE CURVE FOR GFR<70 AND
<80 mL/MIN IS REPORTED
GFR <70 mL/min
GFR <80 mL/min
GFR <70 mL/min TATI vs. β2-microglobulin p=0.012.
668
TATI
CREATININE
β2-MICROGLOBULIN
0.876
0.825
0.857
0.786
0.811
0.799
Tramonti et al
served in other organs. The renal radioactivity curve
recorded over a rat kidney represents the renal
kinetics of this small protein. The comparison with
other low MW proteins studied with the same
method (2) suggests that renal TATI metabolism is
rather fast. In fact, maximum activity peaked at 11
min after the labeled TATI injection and the following decline was rapid. Taken together, the results in
the rat indicated that TATI was taken up by the kidneys where it under went metabolic degradation.
This behavior is similar to that of other LMW proteins. Organs other than the kidney do not seem to
play a role in the metabolism of this small protein.
The results in humans show that the relationship
between serum TATI levels and GFR were similar to
that of other substances, such as ß2-microglobulin
and Cr, acknowledged to be mainly cleared by
glomerular filtration. To detect renal function we
used the bladder cumulative method. This method
provides the actual renal clearance and therefore, it
is preferred to methods based on the plasma disappearance curve of a tracer. Previously, this method
was compared with inulin clearance performed by
vesical catheterization and there was good agreement (31, 32). It is thought that vesical catheterization for diagnostic purposes is, at present, not feasible and we believe that the bladder cumulative
method represents the best available method for research purposes in a large number of patients.
Urinary excretion of TATI behaves like that of the
LMW proteins. In fact, it is very low when renal function is normal or slightly reduced while in renal failure it progressively increases. In renal failure, the
number of nephrons decreases and the filtered load
for the remaining nephrons increases up to overcome the reabsorption capacity. This explains why in
renal failure the urinary excretion of TATI increases
and indicates that, like other low MW proteins, TATI
is filtered by the glomeruli and then reabsorbed by
proximal tubular cells.
Taken together, the rat and human studies suggest
that TATI is handled by the kidneys using the classical pathway of glomerular filtration and tubular
reabsorption. It should be noted that urine loss,
whatever the amount of protein excreted in the
urine, does not influence the serum concentration
of LMW proteins, which depends only on glomerular
filtration.
It is well known that many renal diseases relentlessly
progress towards renal failure even when the initial
cause has disappeared. Great efforts are being made
to search for new methods which allow nephrologists
to evaluate any variation of renal function in the simplest and most accurate way. It is preferable to estimate renal function by tests based on a simple blood
test, so avoiding urine collection and/or exogenous
substance injections. Our results demonstrate that
blood levels of TATI increased sooner and to a higher extent than those of ß2-microglobulin and Cr, the
parameters most frequently used for this purpose.
The increase in blood levels of TATI was already statistically significant in the group of patients with GFR
80-100 mL/min. Furthermore, when renal function
fell <60 mL/min, blood levels of TATI greatly increased. In fact, in patients with advanced renal failure TATI levels increased, compared to those with
GFR >100 mL/min, approximately 13 times, while
ß2-microglobulin increased in the same patients 8
times and Cr 5 times.
The reasons why TATI increases at a different rate
with respect to ß2-microglobulin and Cr deserve clarification. As far as Cr was concerned, its smaller increase can be explained by the presence of tubular
secretion as well as of glomerular filtration. In renal
failure, tubular secretion of Cr, which is usually
approximately 10% of the total excretion, can increase to overcome the filtered amount. Furthermore, in advanced renal failure, Cr clears via the intestine where it is metabolized by bacteria (34). Due
to both these mechanisms, the Cr increase in renal
failure underestimates the reduction in renal function. More difficult is understanding the difference
between TATI and ß2-microglobulin. In fact, both
are proteins with a very LMW and in renal failure
they must increase to the same extent. ß2-microglobulin elimination is acknowledged to be solely by
glomerular filtration and the same should be the fate
of TATI. To explain the difference between the two
proteins we can hypothesize that in renal failure the
filterability of ß2-microglobulin is retained while that
of TATI is impaired. This hypothesis disagrees with
the fact that TATI has a lower MW (6,200 D) than
that of ß2-microglobulin (11,800 D) and it is likely
that it continues to be more freely filtered than ß2microglobulin. Another possibility is that in renal
failure some organs other than the kidney are involved in ß2-microglobulin clearance. Finally, TATI
production can increase since it is considered a protein of reactive phase and production could consequently rise due to the uremic environment (15).
More conclusions can be drawn by studying the figures where the relationships between the reciprocals
of the studied parameters and GFR are reported.
The reciprocal of the plasma level of a substance
behaves like its clearance. The results of the normalized values demonstrate that the intercept value of
the reciprocal of Cr was 0.27. This suggests that when
GFR was zero CR elimination was by another pathway, otherwise the intercept must be close to zero.
Concerning ß2-microglobulin, our results show that
the intercept of its normalized reciprocal was 0.25.
This indicates that in renal failure another elimina669
TATI as a marker of renal function
tion route is present and, since the value of the intercept of ß2-microglobulin was lower than that of
Cr, explains why it increases more than Cr. TATI exhibited an intercept of its normalized reciprocal that
was very close to zero (0.01). This suggests that TATI
is eliminated mainly (or exclusively) via glomerular
filtration and for this reason in advanced renal failure it increases more than ß2-microglobulin and Cr.
Furthermore, these results allowed us to exclude the
hypothesis of an increase in TATI production occurring in renal failure. In fact, if this was the case, the
intercept should be lower than zero. Therefore,
TATI elimination seems to be solely by glomerular
filtration with no other existing pathways. It should
be noted that TATI has the lowest MW (6,200 D) and
therefore, a reduction in its filterability is not likely
to occur. The slope of the line of TATI was 0.009 times,
while those of β2-microglobulin and Cr were 0.008
and 0.007, respectively. This confirmed that TATI increased more than the other studied parameters.
Other studies confirm our results. Donadio (35) reported an intercept of 0.429 for the reciprocal of Cr.
Another low MW protein recently proposed as a
renal function marker is cystatin C (MW 13,000 D)
(12, 36). The results reported indicated that the reciprocal of cystatin C was far from zero (Donadio 0.425
and Plebani 0.487). In Risch et al (37) this value was
lower (0.19) than those of the above studies, but they
used 51Cr-EDTA plasma clearance to detect GFR. It is
well known that in renal failure plasma clearance
overestimates GFR. In the same study, the intercept of
the reciprocal of Cr was close to zero (0.006) suggesting that, like Cr, another elimination route of the
glomerular tracer exists. In addition, in renal failure
cystatin C increases less than plasma Cr (35). Therefore, cystatin C does not seem to be a better GFR
marker than Cr or β2-microglobulin. Therefore, we
preferred to compare the results of TATI with those of
β2-microglobulin, instead of cystatin C, although the
latter has been claimed as the best GFR marker.
We evaluated the behavior in two different groups,
diabetic nephropathy and chronic interstitial
nephropathy and there were no differences between
these two groups (data not shown). In addition,
there was no difference between males and females.
Concerning the possibility that other diseases influence the blood level of TATI, it should be noted that
TATI increase can solely reflect trypsin expression
and occurs in the advanced disease (39). Furthermore, the diseases in addition to renal failure in
which TATI increases, such as acute pancreatitis or
ovarian cancer are very severe and easily recognized
(16, 38). Regarding inflammatory diseases, it should
be noted that only a strong acute-phase reaction appears to trigger TATI expression (39).
The increase in blood levels of TATI due to renal insuf670
ficiency induced an increase in the filtered load for the
remaining nephrons. Consequently, the content of this
protein in the tubular cells increased. We can ascertain
this information by studying the figure showing where
blood levels of TATI and its fractional clearance are together normalized. In the group with GFR 40-60
mL/min, the fractional TATI clearance was still 1, while
its serum concentration was increased up to 2.3 times.
This suggests that the whole burden of filtered TATI
was reabsorbed totally by tubular cells where it accumulated. Further increases in blood levels observed in the
patients with more advanced renal failure were followed by an increase in urine excretion. However, the
tubular content was still higher than in the group with
normal renal function in which urine excretion was
close to zero. We can attempt to quantify the amount of
tubular content of TATI in renal failure. In patients
with GFR <20 mL/min fractional TATI clearance,
shown in Figure 9, was approximately 45%. Therefore,
55% of the filtered load was reabsorbed. In the same
group the blood level of TATI, and consequently the filtered load, increased approximately 13 times. In combining these two results we concluded that in the single
nephron the tubular content was approximately 7 times
more than in normal conditions (0.55 x 13 = 7.15). It
has been reported that the renal content of other low
MW proteins, such as α1-microglobulin, lysozyme and
α2-microglobulin (40-42), increases in the remaining
nephrons in the setting of a reduction in renal mass.
Our results suggest that in renal failure an increase in
tubular cell content of TATI does exist and that such an
increase is very high. It is acknowledged that the increase in protein content in tubular cells represents
one of the factors involved in renal damage progression
(43). Therefore, this small protein, the plasma level of
which sharply increases in renal insufficiency, might
play a role in renal damage progression.
In conclusion, the results of this study demonstrate
that TATI can be considered another renal function
marker, like plasma ß2-microglobulin, cystatin C and
Cr. Further larger studies are advisable to establish its
accuracy in clinical settings and to ascertain the role
(favorable or not) played by TATI in the uremic syndrome or in renal disease progression.
Preliminary results of this study were presented at the Eighth International Symposium of Nephrology at Montecatini, Montecatini Terme,
Italy, 1996 and previously published in part in the proceedings (Ren
Fail 1998; 20: 295-302.
ACKNOWLEDGEMENTS
We are indebted to Cristina Consani, Nicola D’Onza and Giulietta Sbragia for their valuable technical assistance.
This work was supported in part by a fund from MURST.
Tramonti et al
Address for correspondence:
Gianfranco Tramonti, M.D.
Dipartimento di Medicina Interna
Unità di Nefrologia
Azienda Ospedaliera Pisana
Via Roma, 67
56126 Pisa, Italy
g.tramonti@med.unipi.it
g.tramonti@ao-pisa.toscana.it
16.
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Received: December 13, 2002
Revised: June 24, 2003
Accepted: July 03, 2003
© Società Italiana di Nefrologia