Departments of Medicine and of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555
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ABSTRACT |
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Growth factors
stimulate
Na+/H+
exchange activity in many cell types but their effects on acid
secretion via this mechanism in renal tubules are poorly understood. We
examined the regulation of HCO3
absorption by nerve growth factor (NGF) in the rat medullary thick
ascending limb (MTAL), which absorbs HCO
3
via apical membrane
Na+/H+
exchange. MTAL were perfused in vitro with 25 mM
HCO
3 solutions (pH 7.4; 290 mosmol/kgH2O). Addition of 0.7 nM
NGF to the bath decreased HCO
3
absorption from 13.1 ± 1.1 to 9.6 ± 0.8 pmol · min
1 · mm
1
(P < 0.001). In contrast, with
10
10 M arginine vasopressin
(AVP) in the bath, addition of NGF to the bath increased
HCO
3 absorption from 8.0 ± 1.6 to
12.5 ± 1.3 pmol · min
1 · mm
1
(P < 0.01). Both effects of NGF were
blocked by genistein, consistent with the involvement of tyrosine
kinase pathways. However, the AVP-dependent stimulation required
activation of protein kinase C (PKC), whereas the inhibition was PKC
independent, indicating that the NGF-induced signaling pathways leading
to inhibition and stimulation of HCO
3
absorption are distinct. Hypertonicity blocked the inhibition but not
the AVP-dependent stimulation, suggesting that hypertonicity and NGF
may inhibit HCO
3 absorption via a
common mechanism. These data demonstrate that NGF inhibits
HCO
3 absorption in the MTAL under
basal conditions but stimulates HCO
3 absorption in the presence of AVP, effects that are mediated through distinct signal transduction pathways. They also show that AVP is a
critical determinant of the response of the MTAL to growth factor
stimulation and suggest that NGF can either inhibit or stimulate
apical Na+/H+ exchange activity
depending on its interactions with other regulatory factors. Locally
produced growth factors such as NGF may play a role in regulating renal
tubule HCO
3 absorption.
sodium/hydrogen exchange; protein kinase C; signal transduction; hypertonicity; tyrosine kinases
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INTRODUCTION |
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GROWTH FACTORS INFLUENCE A variety of renal
processes, including cellular proliferation, development and
differentiation, hypertrophy, matrix production, and repair following
injury (29, 30, 44, 50). Growth factors that are produced by the kidney include insulin-like growth factor, platelet-derived growth factor, nerve growth factor (NGF), hepatocyte growth factor, transforming growth factor-, and epidermal growth factor (30). Much recent work
has focused on the involvement of these factors in the
pathophysiological control of glomerular function (1, 30, 50) and in
recovery from acute renal failure (8, 29, 30, 36). In contrast, with
the exception of epidermal growth factor (11, 28, 48), little is known
about the effects of locally produced growth factors on the function of
renal tubules, particularly with respect to the regulation of ion
transport and signal transduction pathways.
One of the major cellular actions of growth factors is stimulation of
Na+/H+
exchange activity. This stimulation is rapid, is observed in many
different cell types, and occurs with virtually all mitogens (27, 54).
Furthermore, growth factors activate the NHE1, NHE2, and NHE3
Na+/H+
exchanger isoforms (35, 54). NHE3 is the predominant exchanger isoform
in the apical membrane of the renal proximal tubule and thick ascending
limb, where it mediates the H+
secretion necessary for transepithelial
HCO3 absorption (2, 4, 7, 26). NHE1,
the ubiquitously expressed exchanger isoform (54), is localized in the
basolateral membrane of many nephron segments and also can influence
transepithelial HCO
3 absorption (25).
Based on these findings and on the abundance of growth factors produced
by the kidney, it might be expected that growth factors would play a
role in the regulation of renal tubule
Na+/H+
exchange activity and luminal proton secretion. Although a few studies
have examined the effects of growth factors on
HCO
3 absorption by segments of the
proximal tubule and collecting duct, the results of these studies
largely have been negative (28, 40, 46, 47). Furthermore, the
possibility that locally produced growth factors may interact with
other regulatory factors to control Na+/H+
exchange activity and HCO
3 transport
in renal tubules has not been explored.
The medullary thick ascending limb (MTAL) of the rat participates in
the renal regulation of acid-base balance by reabsorbing a sizable
fraction of the HCO3 filtered at the glomerulus (21). The H+ secretion
required for this HCO
3 reabsorption is
mediated virtually completely by apical membrane
Na+/H+
exchange (26). Furthermore, the regulation of
HCO
3 absorption is mediated through
regulation of this apical exchanger (19, 21, 26, 52, 55). Because of
their potent stimulation of
Na+/H+
exchange activity in other systems, we investigated whether growth factors might stimulate HCO
3
absorption in the MTAL. Our results demonstrate that NGF regulates
HCO
3 absorption in the MTAL in a
complex manner: it inhibits HCO
3 absorption under basal conditions but stimulates
HCO
3 absorption in the presence of
arginine vasopressin (AVP). In addition, we show that these regulatory
effects are mediated via distinct signal transduction pathways. These
findings indicate that AVP is a critical determinant of the response of
MTAL cells to NGF stimulation and suggest that locally produced growth
factors such as NGF may play a role in the regulation of renal tubule
acid excretion.
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METHODS |
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The methods for in vitro microperfusion of MTAL from Sprague-Dawley
rats (50-100 g body wt; Taconic, Germantown, NY) have been
described previously (19, 20). In brief, MTAL were dissected at
10°C from the inner stripe of the outer medulla in control bath
solution (see below). The tubules were then transferred to a bath
chamber on the stage of an inverted microscope, mounted on concentric
glass micropipettes, and perfused at 37°C. The length of the
perfused tubule segments ranged from 0.51 to 0.70 mm. In all
experiments, the luminal perfusion solution contained (in mM) 146 Na+, 4 K+, 122 Cl, 25 HCO
3, 2.0 Ca2+, 1.5 Mg2+, 2.0 phosphate, 1.2 SO2
4, 1.0 citrate, 2.0 lactate, and
5.5 glucose (osmolality = 290 mosmol/kgH2O). The bath solution
was identical except for addition of 0.2% fatty acid-free bovine
albumin (Sigma). In some experiments, hypertonic solutions were
prepared by adding 75 mM NaCl to the above control solution (final
osmolality = 425 mosmol/kgH2O).
All solutions were equilibrated with 95%
O2-5%
CO2 and were pH 7.45-7.47 at 37°C. Experimental agents were added to the bath solution as
described in RESULTS. Solutions
containing AVP, forskolin, 8-bromo-cAMP (8-BrcAMP), staurosporine,
chelerythrine chloride, and genistein were prepared as previously
described (19, 22, 23). NGF (7S NGF or NGF-
, Sigma) was prepared as
a 7 × 10
7 M stock solution in
140 mM NaCl plus 0.1% albumin and added to bath solution to achieve a
final concentration of 0.7 nM. Unless stated otherwise, experiments
were performed using the 7S NGF complex (53).
The protocol for study of transepithelial
HCO3 absorption was as described (19,
20, 22). The tubules were equilibrated for 20-30 min at 37°C
in the initial perfusion and bath solutions, and the luminal flow rate
(normalized to unit tubule length) was adjusted to 1.3-1.9
nl · min
1 · mm
1.
Two to four 10-min tubule fluid samples were then collected for each
period (initial, experimental, and recovery). The tubules were allowed
to reequilibrate for 5-15 min after an experimental agent was
added to or removed from the bath solution. The absolute rate of
HCO
3 absorption
(JHCO
3; pmol · min
1 · mm
1) was calculated from
the luminal flow rate and the difference between total
CO2 concentrations in perfused and
collected fluids (19, 20). An average
HCO
3 absorption rate was calculated
for each period studied in a given tubule. When repeat measurements
were made at the beginning and end of an experiment (initial and
recovery periods), the values were averaged. Single tubule values are
presented in Figs. 1-7. Mean values ± SE
(n = number of tubules) are presented
in the text. Differences between means were evaluated using the
Student's t-test for paired data, with P < 0.05 considered
statistically significant.
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RESULTS |
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Effects of NGF on HCO3
Absorption
NGF inhibits HCO3 absorption.
Based on the virtually universal finding that growth factors stimulate
Na+/H+
exchange (27, 54), it was anticipated that NGF would stimulate HCO
3 absorption. Instead, addition of
0.7 nM NGF to the bath decreased HCO
3
absorption from 13.1 ± 1.1 to 9.6 ± 0.8 pmol · min
1 · mm
1
(n = 6;
P < 0.001; Fig.
1). The inhibition by NGF was reversible, was observed within 15 min after addition of NGF to the bath, and was
stable for up to 2 h.
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Hypertonicity prevents inhibition by NGF.
In other systems, stimulation of
Na+/H+
exchange by hypertonicity precludes subsequent stimulation by growth
factors (16). In the MTAL, hypertonicity inhibits
HCO3 absorption (20, 22). Therefore,
we tested for possible interactions between hypertonicity and NGF.
Hypertonicity was produced by adding 75 mM NaCl to the lumen and bath
solutions. As shown in Fig. 2, addition of
0.7 nM NGF to the bath had no effect on
HCO
3 absorption in tubules studied in
hypertonic solutions [5.3 ± 0.1 pmol · min
1 · mm
1
for hypertonic vs. 5.2 ± 0.1 pmol · min
1 · mm
1
for hypertonic + NGF; n = 3; not
significant (NS)]. Thus inhibition of
HCO
3 absorption by hypertonicity
prevents subsequent inhibition by NGF.
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NGF stimulates HCO3
absorption in the presence of AVP.
AVP inhibits HCO
3 absorption in the
MTAL, and the inhibitory effects of AVP and hypertonicity are additive (19, 20). We therefore tested whether inhibition by NGF also is
additive to inhibition by AVP. Surprisingly, in MTAL bathed with
10
10 M AVP, addition of 0.7 nM NGF to the bath increased HCO
3 absorption from 8.0 ± 1.6 to 12.5 ± 1.3 pmol · min
1 · mm
1
(n = 4;
P < 0.01; Fig.
3A). NGF
also increased HCO
3 absorption in the
presence of AVP in hypertonic solutions (2.9 ± 0.2 pmol · min
1 · mm
1
for AVP + hypertonic vs. 5.7 ± 0.5 pmol · min
1 · mm
1
for AVP + hypertonic + NGF; n = 3;
P < 0.05; Fig.
3B). Thus NGF inhibits
HCO
3 absorption under basal conditions but stimulates HCO
3 absorption in the
presence of AVP. These results show that AVP is a critical determinant of the response of MTAL cells to NGF stimulation and suggest that NGF
may regulate HCO
3 absorption through
at least two distinct signal transduction pathways.
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Effects of NGF-.
To define further the regulatory action of NGF, we examined the effects
of NGF-
(or 2.5S NGF), the biologically active subunit of the 7S NGF
complex (53). In MTAL perfused and bathed in control solution, addition
of 0.7 nM NGF-
to the bath decreased
HCO
3 absorption from 11.4 ± 0.6 to
7.2 ± 0.8 pmol · min
1 · mm
1
(n = 3;
P < 0.05). In tubules bathed with
10
10 M AVP, addition of 0.7 nM NGF-
to the bath increased HCO
3 absorption from 6.5 ± 0.7 to 8.3 ± 0.7 pmol · min
1 · mm
1
(n = 3;
P < 0.001). Thus results with
NGF-
were similar to those obtained with 7S
NGF.1
Signaling Pathways Involved in Regulation by NGF
Previously, we demonstrated that cAMP, protein kinase C (PKC), and protein-tyrosine kinase pathways play key roles in the regulation of MTAL HCOInhibition by NGF does not involve cAMP or PKC.
cAMP inhibits HCO3 absorption in the
MTAL (19). We therefore tested whether this pathway is involved in inhibition by NGF. MTAL were bathed with forskolin or 8-BrcAMP, agents
that induce maximal cAMP-dependent inhibition of
HCO
3 absorption (19, 24). The results
in Fig. 4 show that, in the presence of
10
6 M forskolin or
10
4 M 8-BrcAMP, addition of
0.7 nM NGF to the bath decreased HCO
3 absorption from 7.2 ± 1.1 to 3.8 ± 1.0 pmol · min
1 · mm
1
(n = 4;
P < 0.001). Thus the inhibition by
NGF is not mediated by an increase in cell cAMP.
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Stimulation by NGF is mediated by PKC.
In the MTAL, prostaglandin E2
(PGE2) stimulates
HCO3 absorption in the presence of AVP
through activation of PKC (23). We therefore tested the hypothesis that
PKC mediates the AVP-dependent stimulation by NGF. In MTAL bathed with
AVP plus 10
7 M
staurosporine or 10
8 M
chelerythrine, addition of 0.7 nM NGF to the bath had no effect on
HCO
3 absorption (5.7 ± 0.5 pmol · min
1 · mm
1
for AVP + inhibitor vs. 5.8 ± 0.6 pmol · min
1 · mm
1
for AVP + inhibitor + NGF; n = 4; NS;
Fig.
6A).
Thus pretreatment with inhibitors of PKC abolished the stimulation of
HCO
3 absorption by NGF. In tubules
studied with AVP and NGF in the bath solution, addition of
staurosporine or chelerythrine to the bath decreased
HCO
3 absorption from 7.7 ± 0.5 to
4.4 ± 0.4 pmol · min
1 · mm
1
(n = 4;
P < 0.01; Fig.
6B). In contrast, addition of these
agents had no effect on HCO
3
absorption in the presence of NGF (Fig.
5B) or AVP (23) alone. Thus the PKC
inhibitors targeted specifically the interaction between NGF and AVP,
reversing the effect of NGF to stimulate
HCO
3 absorption when AVP is present.
Together, these results establish that PKC mediates the AVP-dependent
stimulation of HCO
3 absorption by NGF.
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Inhibition and stimulation by NGF are blocked by genistein.
Tyrosine kinases are important intermediates in the signal transduction
pathways activated by growth factors (17, 33) and play a key role in
mediating inhibition of HCO3 absorption by hypertonicity in the MTAL (22). We therefore examined whether tyrosine kinase pathways are involved in the regulation of
HCO
3 absorption by NGF. MTAL were
bathed with genistein, which selectively blocks tyrosine
kinase-dependent regulation of HCO
3
absorption in the MTAL (22). Under basal conditions, the inhibition of
HCO
3 absorption by NGF was eliminated
nearly completely by 7 µM genistein (15.9 ± 0.4 pmol · min
1 · mm
1
for genistein vs. 14.6 ± 0.6 pmol · min
1 · mm
1
for genistein + NGF; n = 4;
P < 0.05) and was abolished by 70 µM genistein (11.6 ± 0.8 pmol · min
1 · mm
1
for genistein vs. 11.5 ± 0.9 pmol · min
1 · mm
1
for genistein + NGF; n = 4; NS; Fig.
7A). The
stimulation of HCO
3 absorption by NGF
in the presence of AVP was abolished by either 7 or 70 µM genistein
(6.5 ± 1.4 pmol · min
1 · mm
1
for AVP + genistein vs. 6.4 ± 1.3 pmol · min
1 · mm
1
for AVP + genistein + NGF; n = 4; NS;
Fig. 7B). Thus tyrosine kinase
pathways appear to be involved in both the inhibition and the
AVP-dependent stimulation of HCO
3
absorption by NGF.
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DISCUSSION |
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Regulation of HCO3 absorption in the
MTAL by a variety of physiological factors, including vasopressin, changes in osmolality, and chronic metabolic acidosis, is achieved through regulation of apical membrane
Na+/H+
exchange activity (19, 26, 52, 55). Because growth factors are potent
activators of
Na+/H+
exchange in many other systems, we tested the hypothesis that they
would stimulate HCO
3 absorption in the isolated, perfused MTAL. Our results show that NGF regulates
HCO
3 absorption in the MTAL but that
its effects are complex: NGF inhibits HCO
3 absorption under basal conditions
but stimulates HCO
3 absorption in the
presence of AVP. Furthermore, hypertonicity blocks the inhibition by
NGF but has no effect on the AVP-dependent stimulation. We also found
that activation of PKC plays a key role in the AVP-dependent
stimulation of HCO
3 absorption but is
not involved in HCO
3 transport inhibition, indicating that the NGF-induced signaling pathways leading
to inhibition and stimulation of HCO
3 absorption are distinct. Together, these results identify a pattern of
growth factor regulation of acid-base transport unlike that described
in other cell types and establish an important role for AVP in
determining the response of MTAL cells to growth factor stimulation. As
discussed below, our findings suggest that NGF can either inhibit or
stimulate apical membrane
Na+/H+
exchange activity depending on its interactions with other regulatory factors.
Effects of NGF on HCO3
Absorption
Inhibition of basal HCO3
absorption.
Under a wide variety of experimental conditions, apical membrane
Na+/H+
exchange mediates virtually all of the
H+ secretion necessary for
HCO
3 absorption in the MTAL (26).
Hence, the rate of HCO
3 absorption serves as a measure of apical
Na+/H+
exchange activity under steady-state transporting conditions. In the
present study, we found that NGF inhibits
HCO
3 absorption, which suggests
strongly that NGF inhibits apical membrane Na+/H+
exchange. Mitogens have been shown to stimulate
Na+/H+
exchange activity in many systems; however, to our knowledge, inhibition of
Na+/H+
exchange by growth factors has not been reported. The predominant Na+/H+
exchanger isoform in the apical membrane of the MTAL is NHE3, which is
stimulated by growth factors when expressed in exchanger-deficient cell
lines (35). Thus inhibition by growth factors may be a novel functional
property of the apical
Na+/H+
exchanger in the MTAL. At this point, we do not know whether the NGF
pathway is coupled directly to inhibition of the apical exchanger or
whether NGF may act indirectly to reduce the driving force for the
exchanger through effects on other transporters such as the
Na+-K+-ATPase.
Relevant to this question, however, we found that the effects of NGF
and hypertonicity to inhibit HCO
3 absorption are not additive, suggesting that these factors may act via
a common mechanism. Hypertonicity inhibits the apical exchanger (NHE3)
directly by decreasing its sensitivity to intracellular H+ (55). Together, these data
suggest that NGF may activate signals that couple directly to
inhibition of the apical membrane
Na+/H+
exchanger. Study of the effects of NGF on the transport properties of
this exchanger, independent of effects on other transporters, will be
required to test this hypothesis.
Stimulation of HCO3
absorption in the presence of AVP.
In contrast to its inhibitory effect under basal conditions, NGF
stimulates HCO
3 absorption in MTAL
exposed to AVP. As discussed below, this stimulation likely occurs as the result of NGF inhibiting AVP-stimulated cAMP production, which results in an increase in apical membrane
Na+/H+
exchange activity (19, 23). Thus NGF apparently is capable of inducing
the classical growth factor stimulation of
Na+/H+
exchange activity in the MTAL; however, this stimulation requires the
interaction of NGF with AVP. These studies identify a potentially important physiological function of AVP in the kidney, namely, to
modify the response of tubule epithelial cells to growth factor stimulation. Our data establish that stimulation of luminal
acidification by NGF in the MTAL is dependent on AVP. In many systems,
stimulation of
Na+/H+
exchange activity by growth factors is an early event that may be
permissive for cell proliferation (18, 27, 34, 43). Based on our
results suggesting that NGF stimulates apical
Na+/H+
exchange only when AVP is present, we speculate that AVP could be a
key cofactor in determining the actions of growth factors to regulate
other cellular processes in the MTAL such as survival, growth, and
repair.
Signal Transduction by NGF
NGF activates at least two distinct signaling pathways in the MTAL: one that is PKC independent and leads to inhibition of HCONGF receptors.
NGF and other neurotrophins induce intracellular signals through two
classes of cell-surface receptors: the Trk family of receptor tyrosine
kinases (6, 33) and p75, a member of the tumor necrosis factor receptor
superfamily (6, 10). TrkA is a high-affinity NGF receptor
[dissociation constant
(Kd) of 1011 to
10
10 M] that is
tyrosine phosphorylated on NGF exposure and is coupled to the
downstream activation of multiple signaling proteins, including phosphatidylinositol 3-kinase, mitogen-activated protein kinases, and
phospholipase C-
(pathways that may lead to activation of PKC) (6,
33). In contrast, the p75 receptor binds NGF with low affinity
(Kd of
10
9 to
10
8 M), lacks intrinsic
tyrosine kinase activity, and couples to the activation of the
transcription factor NF-
B, ceramide production, and c-Jun
NH2-terminal kinase (JNK), signals thought to
participate in neuronal cell death (10, 12, 13). The receptor(s) that mediates NGF action in the MTAL is unknown; however, both the p75 and
TrkA receptors are expressed in the kidney (3, 38, 49). We found that
NGF altered HCO
3 absorption in the
MTAL at 7 × 10
10 M
(Figs. 1-5) but had no effect at 7 × 10
11 M (data not shown),
suggesting that the responses to NGF may be mediated via the
low-affinity receptor. On the other hand, our results show that NGF
signaling involves both tyrosine kinases and PKC, pathways more
consistent with signaling via high-affinity TrkA receptors. Evidence in
other systems indicates that the p75 and TrkA receptors may coexist and
interact functionally, with activation of one receptor altering the
affinity or signaling efficiency of the other receptor (6, 10).
Assessment of whether this type of interaction may explain our
observations will require the molecular identification of the
receptor(s) that mediates NGF signal transduction in the MTAL.
Role of PKC in regulation by NGF.
NGF stimulates HCO3 absorption in the
presence of AVP through activation of PKC. This conclusion is based on
the observations that pretreatment with inhibitors of PKC prevented the
stimulation of HCO
3 absorption and
that the stimulation was reversed when the inhibitors were added in the
presence of NGF (Fig. 6). Virtually identical results were obtained
with staurosporine and chelerythrine chloride, two chemically different
PKC inhibitors with different kinetic properties (23). Furthermore,
these inhibitors have no effect on
HCO
3 absorption under basal conditions
or in the presence of AVP alone (23) and do not prevent inhibition of
HCO
3 absorption by NGF alone (Fig.
5A). Thus the PKC inhibitors blocked specifically the signaling pathway through which NGF interacts with
AVP. Together, these data establish that the stimulation of
HCO
3 absorption occurs through a
PKC-dependent mechanism, whereas the inhibition of
HCO
3 absorption occurs independently
of PKC. This difference indicates that the NGF-induced signaling
pathways leading to stimulation and inhibition of
HCO
3 absorption are distinct.
Role of tyrosine kinase pathways in regulation by NGF.
Binding of NGF to its high-affinity receptor stimulates
receptor-tyrosine kinase activity and induces the tyrosine
phosphorylation and activation of a number of intracellular target
molecules, including phospholipase C- (33). We found that both the
inhibition and the stimulation of HCO
3
absorption by NGF were blocked by genistein, which selectively inhibits
tyrosine kinase-dependent regulation of
HCO
3 absorption in the MTAL (22) and
prevents NGF-stimulated tyrosine phosphorylation in PC-12 cells (45).
Thus tyrosine kinases appear to be components of the NGF signal
transduction pathways that lead to inhibition and PKC-dependent
stimulation of HCO
3 absorption. Of
interest, genistein did not block the PKC-dependent stimulation of
HCO
3 absorption by
PGE2 (22). This suggests that the
genistein-sensitive step in the NGF stimulatory pathway is upstream of
PKC activation. At this point, we do not know whether NGF activates two
separate genistein-sensitive pathways or activates a common pathway
that diverges to regulate distinct pathways coupled to
HCO
3 transport inhibition and
stimulation. A simple explanation for our results is that genistein
inhibits the tyrosine kinase activity of the NGF receptor, thereby
blocking both downstream signaling pathways. However, studies in PC-12 cells show that genistein concentrations in excess of 100 µM are needed to inhibit NGF-stimulated tyrosine phosphorylation via the
high-affinity (TrkA) receptor (45). In our study, complete or nearly
complete inhibition of NGF action was observed using only 7 µM
genistein (Fig. 7). Thus the effects of genistein in the MTAL may be
mediated through inhibition of NGF-stimulated tyrosine kinase(s)
distinct from the NGF receptor. Alternatively, it is possible that the
NGF receptor in the MTAL is much more sensitive to genistein than the
receptor in PC-12 cells. Resolution of these issues will require
molecular identification of the signaling proteins that undergo
tyrosine phosphorylation in response to NGF stimulation.
Physiological Implications
The kidney produces a number of growth factors that are known to influence the activities of acid-base transporters and intracellular pH in many cell types (18, 27, 32, 35, 42, 54). Despite this, growth factors generally have not been considered to play a role in the regulation of renal tubule acid excretion. Recent studies indicate, however, that levels of growth factors within the kidney are altered in a variety of pathophysiological conditions that are associated with changes in renal tubule acid-base transport, including reduction of renal mass, ureteral obstruction, K+ depletion, and diabetes (14, 28, 30, 31, 37, 44, 51). The results of the present study demonstrating regulation of HCONGF and its receptors are expressed in the kidney, but their functions
are unknown. Our results establish directly that NGF can influence the
function of renal tubules and identify a possible role for NGF in the
control of urinary acidification. In view of the close association
between effects of mitogens on acid-base transporters and cell
proliferation (18, 27, 34, 43) and the important role of NGF in
determining the survival and differentiation of nerve cells (33, 53),
it is also possible that the signaling pathways and acid-base transport
effects we identified may be indicative of a role for NGF in
controlling the growth and survival of MTAL cells. At present,
information is lacking on many critical issues, including sources of
NGF in the renal medulla, factors that regulate NGF levels in the
kidney, and effects of NGF on renal function. Further work on these and
other basic issues is needed before the physiological relevance of the
effects of NGF on signal transduction and
HCO3 transport in the MTAL can be
defined. An additional goal will be to examine the effects of other
growth factors in order to determine whether the unusual pattern of
HCO
3 transport regulation that we
observed is specific for NGF or represents a general response of MTAL
cells to growth factor stimulation.
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ACKNOWLEDGEMENTS |
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This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-38217.
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FOOTNOTES |
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1
The effect of NGF- to inhibit
HCO
3 absorption was not fully
reversible. The explanation for this difference from 7S NGF was not
investigated but may relate to differences in the extent of proteolysis
of the
-subunit when it is isolated from the 7S complex (53).
Address for reprint requests: D. W. Good, Division of Nephrology, 4.200 John Sealy Hospital 0562, University of Texas Medical Branch, Galveston, TX 77555.
Received 30 September 1997; accepted in final form 5 January 1998.
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