Institute for Cardiovascular Studies, College of Pharmacy, University of Houston, Houston, Texas 77204-5515
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ABSTRACT |
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The present study investigates the
cellular mechanisms responsible for dopamine D2-like
receptor-mediated stimulation of Na+-K+-ATPase
in the proximal tubules of the kidney. Previously, we showed that
D2-like receptor-mediated increase in
Na+-K+-ATPase involves an increase in the
maximum rate of Na+-K+-ATPase activity
(Vmax). Therefore, we tested the hypothesis that D2-like receptor-mediated stimulation of
Na+-K+-ATPase requires phosphorylation and
recruitment of 1-subunits of the enzyme from cytosol to
the membrane. This hypothesis was tested by Western blotting for
Na+-K+-ATPase
1-subunits in
proximal tubular membrane. Treatment of the proximal tubules with
bromocriptine (D2-like receptor agonist) caused an increase
in Na+-K+-ATPase
1-subunit
abundance in the membrane preparations. This effect was blocked by
genistein (tyrosine kinase inhibitor), suggesting a role for tyrosine
phosphorylation. Moreover, bromocriptine caused an increase in tyrosine
phosphorylation of membrane-bound Na+-K+-ATPase
1-subunits. This effect was blocked by bafilomycin A1 (vesicular trafficking inhibitor), which suggested that this increase was due to the recruitment of tyrosine-phosphorylated
Na+-K+-ATPase
1-subunits. In
conclusion, we have demonstrated that activation of D2-like
receptors increases Na+-K+-ATPase activity
by recruitment of the tyrosine-phosphorylated
1-subunits
in the proximal tubules of the kidney.
dopamine; bromocriptine; bafilomycin A1; genistein; proximal tubules; sodium/potassium/adenosine 5'-triphosphatase
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INTRODUCTION |
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DOPAMINE PLAYS AN
IMPORTANT role in the regulation of
Na+-K+-ATPase (NKA) in the proximal tubules of
the kidney. Activation of D1-like receptors is associated
with the inhibition of NKA (12, 13). On the other hand,
activation of D2-like receptors causes stimulation of NKA
in the proximal tubules (11, 16). The inhibition or
stimulation of NKA depends on several factors, such as 1)
concentration of substrates (Na+, K+,
Mg+), 2) covalent modification of NKA subunits,
and 3) number of NKA subunits in the plasma membrane
(9, 10). It has been shown that activation of
D1-like receptors causes initial Ser18 phosphorylation of
the NKA 1-subunits and their subsequent endocytosis, which is responsible for the inhibition of NKA (3).
However, the mechanism by which D2-like receptors stimulate
NKA is not known. One of the mechanisms reported for activation of NKA
is the recruitment of NKA
1-subunits to the plasma
membrane (1, 5, 7, 14). In addition, phosphorylation of
Tyr10 residue in the NKA
1-subunit causes activation of
NKA (8).
We have previously reported that activation of D2-like
receptors increases the maximum rate of NKA catalytic activity
(Vmax) in the proximal tubules of the kidney
(11). This observation suggests that there might be an
increase in the number of NKA 1-subunits in the
membranes on activation of D2-like receptors. Furthermore,
this D2-like receptor-mediated increase in
Vmax was dependent on activation of a tyrosine
kinase-p44/42 MAPK pathway (15). As tyrosine kinases
increase tyrosine phosphorylation of substrate proteins, it is possible
that D2-like receptor agonists may also cause the tyrosine
phosphorylation of NKA
1-subunits. Thus there are two
potential mechanisms by which D2-like receptor activation
may increase NKA activity in the proximal tubules, namely, recruitment
or tyrosine phosphorylation of NKA
1-subunits. Moreover,
it is possible that tyrosine phosphorylation and recruitment may be
interrelated phenomena, which are required together for NKA
stimulation by D2-like receptor agonist.
In this study, we have tested the hypothesis that activation of
D2-like receptors causes recruitment of NKA
1-subunits to the membranes in the renal proximal
tubules. Furthermore, we have also investigated whether activation of
D2-like receptors causes tyrosine phosphorylation of NKA
1-subunits in proximal tubular cell membranes as well as
in the cytosol. In addition, we have asked whether tyrosine
phosphorylation of NKA
1-subunits plays a role in
recruitment of these subunits to the plasma membranes of proximal
tubular cells. All the studies were performed in freshly isolated
proximal tubules of kidneys obtained from Sprague-Dawley rats.
Recruitment was measured by Western blotting for NKA
1-subunits in proximal tubular membranes. For tyrosine
phosphorylation, NKA
1-subunits were first
immunoprecipitated from membranes or cytosol and then probed for
tyrosine phosphorylation.
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EXPERIMENTAL PROCEDURES |
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Animals. Male Sprague-Dawley rats between 12 and 15 wk old and weighing 200-250 g were used (Harlen Sprague Dawley, Indianapolis, IN). The animals were housed in plastic cages in an air-conditioned animal care facility and had access to standard rat chow (Purina Mills, St. Louis, MO) and tap water ad libitum.
Isolation of proximal tubules from the kidney. The rats were anesthetized with pentobarbital sodium (50 mg/kg ip). The proximal tubules were isolated from the kidneys of the rats and enriched by a previously described method (11). The enriched proximal tubules were resuspended in 5 ml of modified Krebs-Henseleit buffer [(in mM) 118 NaCl, 4 KCl, 1.25 CaCl2, 1.2 MgCl2, 27.2 NaHCO3, 1 KH2PO4, 5 glucose, and 10 HEPES, pH 7.4] and further used for drug treatment.
Drug treatment. All the drug treatments were done in 10 ml total volume of proximal tubular suspension (0.75 mg/ml). The proximal tubules were treated without (basal) or with 100 µl of bromocriptine to reach a final concentration of 0.1 µM at 37°C for 15 min. The reaction was terminated by rapid freezing and thawing in a dry ice-acetone mixture. The drug-treated cell lysates were then further used to prepare proximal tubular membranes. Just before freezing, 400 µl of protease inhibitor cocktail (25×; Boehringer Mannheim) (along with 4 mM sodium orthovanadate when measuring tyrosine phosphorylation) were added to each tube.
Inhibitor treatment. For inhibitor studies, the proximal tubules were incubated with PD-98059 (10 µM), genistein (0.001 µM), or bafilomycin A1 (0.02 µM), at 37°C for 10 min, before treatment with agonist.
Preparation of proximal tubular membranes. The freeze-thawed cell lysates were used for membrane preparation. The cell lysates were first centrifuged at 36,000 g at 4°C for 20 min to obtain pellets, and the supernatant was saved as cytosolic fraction. The pellets were resuspended in 5 ml of homogenization buffer (50 mM Tris · HCl and 1 mM MgCl2, pH 7.4) containing protease inhibitor cocktail and homogenized with a Wheaton homogenizer (20 strokes at setting 7). The homogenate was then centrifuged at 36,000 g at 4°C for 20 min to obtain membrane pellets. The pellet was resuspended in homogenization buffer (5 ml) and centrifuged at 36,000 g at 4°C for 10 min to obtain the membrane pellet. This washing step was repeated three times. Finally, the pellet was resuspended in buffer (250 µl) containing 50 mM Tris · HCl and 5 mM MgCl2 at pH 7.5.
Determination of NKA 1-subunits in membrane
preparation.
Loading samples were prepared for Western blotting, from membrane
samples, such that the protein concentration was 0.1 µg protein/10
µl. Samples of 10 µl/lane were loaded and separated by gel
electrophoresis and then transferred to polyvinylidene difluoride
(PVDF) membranes (Millipore, Bedford, MA). The membranes were probed
for NKA
1-subunits with mouse monoclonal NKA
1-subunit primary antibody (1:1,000 dilution; Research
Diagnostics, Flanders, NJ). Furthermore, the membranes were probed with
rabbit anti-mouse secondary antibody (1:10,000 dilution; Santa Cruz
Biotechnology, Santa Cruz, CA), and NKA
1-subunit bands
were detected with a chemiluminescence kit (Santa Cruz Biotechnology).
Densitometric analysis of the bands was performed and data were
represented in arbitrary units. In a few experiments, Commassie blue
staining of the PVDF membrane was performed to confirm the similar
amounts of proteins loaded.
Immunoprecipitation of NKA 1-subunit from membrane
and cytosolic fraction.
The membrane and cytosolic proteins were dissolved in
immunoprecipitation buffer [(in mM) 20 Tris · HCl, 150 NaCl,
10 NaF, 10 Na2P4O7, and 2 EDTA, as
well as 1% Triton X-100 and 0.1% SDS, pH 8] containing protease
inhibitor cocktail and 0.1 mM PMSF at a concentration of 250 µg
protein/ml. Next, the membrane proteins were cleared with protein
A/G-agarose (Santa Cruz Biotechnology). Furthermore, an aliquot (1 ml)
of precleared supernatant was incubated with mouse monoclonal antibody
against NKA
1-subunit. Antigen (NKA
1-subunit)-antibody complex was precipitated overnight
with protein A/G-agarose. After precipitation, the complex was washed once with immunoprecipitation buffer followed by one wash with wash
buffer [(in mM) 20 Tris · HCl, 150 NaCl, and 5 EDTA, as well as 0.1% Triton X-100 and 0.1% SDS, pH 8 at 25°C] and 50 mM
Tris · HCl (pH 8). Finally, the antigen-antibody complex was
dissociated with 50 µl of 2× Laemmli buffer at 37°C for 1 h.
The protein A/G-agarose beads were removed by centrifugation, and the
supernatant was used for Western blotting. All the immunoprecipitation
steps were carried out at 4°C.
Western blotting.
The immunoprecipitated samples were resolved by gel electrophoresis
using SDS-PAGE and transferred onto PVDF membranes. The membranes were
then probed with either mouse phosphotyrosine antibody (1:1,000
dilution; Santa Cruz Biotechnology) or mouse monoclonal NKA
1-subunit primary antibody (1:1,000 dilution). The
primary antibodies bound to the membranes were detected by using rabbit anti-mouse secondary antibody (1:5,000 dilution for phosphotyrosine and
1:10,000 dilution for NKA
1-subunit antibody). The
immunoreactivity was detected with a chemiluminescence kit.
Densitometric analysis was performed on the bands, and data were
represented as the ratio of phosphotyrosine to total NKA
1-subunits.
Statistical analysis. Data are represented as means ± SE of several experiments. Where applicable, the data were analyzed with unpaired Student's t-test or one-way analysis of variance along with a suitable post hoc test. The difference was considered significant if P < 0.05.
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RESULTS |
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Effect of bromocriptine on NKA 1-subunit abundance
in proximal tubular membrane.
Effect of bromocriptine on NKA
1-subunit (the catalytic
subunit) abundance in proximal tubular membranes was measured by Western blotting with mouse monoclonal antibody against the NKA
1-subunits. It was found that treatment of proximal
tubules with bromocriptine caused an increase in immunoreactivity for
NKA
1-subunit (~100 kDa) in the membranes, suggesting
an increase in NKA
1-subunit abundance (Fig.
1). This effect of bromocriptine
was blocked by PD-98059 [10 µM, MEK1/2 inhibitor (6)],
genistein [0.001 µM, tyrosine kinase inhibitor (4)],
and bafilomycin A1 [0.02 µM, vesicular trafficking inhibitor
(2)] (Fig. 1, A and B). The densitometric analysis of the blots showed an increase in NKA
1-subunit abundance in proximal tubular membranes (Fig.
1C).
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Effect of bromocriptine on tyrosine phosphorylation of NKA
1-subunits in proximal tubular membrane in absence and
presence of genistein.
Next, we performed experiments to determine whether D2-like
receptor activation causes tyrosine phosphorylation of NKA
1-subunits in proximal tubular membranes. This was done
by immunoprecipitating NKA
1-subunits from the proximal
tubular membranes after treating the proximal tubules with
bromocriptine (0.1 µM). Furthermore, the immunoprecipitated samples
were resolved by gel electrophoresis and probed with either
phosphotyrosine or NKA
1-subunit antibody. We found that
treatment of the proximal tubules with bromocriptine caused an increase
in the tyrosine phosphorylation of NKA
1-subunits. Furthermore, this effect was blocked by pretreatment with genistein (0.001 µM) (Fig. 2A). In
parallel experiments, it was confirmed that similar amounts of NKA
1-subunits were immunoprecipitated from the membranes by
checking the immunoreactivity to NKA
1-subunit antibody
(Fig. 2B). The densitometric ratio of phosphotyrosine to
total NKA
1-subunits was also increased in the
bromocriptine-treated group (Fig. 2C).
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Effect of bafilomycin A1 on D2-like receptor-mediated
increase in tyrosine phosphorylation of NKA 1-subunit in
proximal tubular cell membranes.
Because D2-like receptors caused an increase in tyrosine
phosphorylation of membrane NKA
1-subunits, it was
important to determine whether this increase was due to direct
phosphorylation of membrane NKA
1-subunits or
recruitment of ones phosphorylated in the cytosol. To determine this,
we studied the effect of bafilomycin A1 (which in our previous
experiments blocked D2-like receptor-mediated recruitment
of NKA
1-subunits) on D2-like
receptor-mediated tyrosine phosphorylation of NKA. It was found that
bafilomycin A1 blocked bromocriptine-mediated tyrosine phosphorylation
of NKA
1-subunits in the membranes (Fig.
3A). Furthermore, the NKA
1-subunits immunoprecipitated from each sample were
similar (Fig. 3B). In addition, there was an increase in the
ratio of phosphorylated tyrosine to total NKA
1-subunit
protein in the membrane (Fig. 3C). Thus the increase in
membrane tyrosine phosphorylation may be due to the recruitment of
tyrosine-phosphorylated
1-subunits to the proximal
tubular membranes.
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Effect of bafilomycin A1 on D2-like receptor-mediated
increase in tyrosine phosphorylation of NKA 1-subunit in
proximal tubular cell cytosol.
Furthermore, we wanted to see the effect of D2-like
receptor activation on tyrosine phosphorylation of NKA
1-subunits in the cytosol of the proximal tubular cells.
We found that 15-min treatment of proximal tubules with bromocriptine
did not have any effect on tyrosine phosphorylation of cytosolic NKA
1-subunits (Fig.
4A). Surprisingly, when the
proximal tubules were pretreated with bafilomycin A1 (0.02 µM),
bromocriptine was able to stimulate tyrosine phosphorylation of NKA
1-subunits in the cytosol (Fig. 4A), an
observation reciprocal to that in the membranes. Furthermore, in
bafilomycin A1-treated proximal tubules, there was an increase in the
ratio of phosphorylated tyrosine to total NKA
1-subunit protein in the cytosol (Fig. 4C). The immunoprecipitated NKA
1-subunits were similar in all treatment groups (Fig.
4B).
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DISCUSSION |
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In the present study, we have shown that dopamine
D2-like receptor activation causes recruitment of NKA
1-subunits in the proximal tubular cells. Furthermore,
activation of D2-like receptors also causes tyrosine
phosphorylation of NKA
1-subunits in the proximal
tubules. This effect is blocked by bafilomycin A1 (inhibitor of
vesicular trafficking), which suggests that the increase in the
abundance of tyrosine-phosphorylated membrane NKA
1-subunits may have been caused by D2-like
receptor-mediated recruitment of tyrosine-phosphorylated NKA
1-subunits from cytosol. This is supported by the
observation that D2-like receptor activation causes an
increase in tyrosine phosphorylation of cytosolic NKA
1-subunits only in the presence of bafilomycin A1 (which
prevents the recruitment of these tyrosine-phosphorylated NKA
1-subunits).
We had previously observed that bromocriptine (D2-like
receptor agonist) caused an increase in the rate of hydrolytic activity of NKA under saturating concentrations of sodium, potassium, magnesium, and ATP (11) in the proximal tubules of the kidney. In
other words, D2-like receptor activation caused an increase
in Vmax of NKA enzymatic activity. Because an
increase in Vmax is associated with the increase
in the amount of functional enzyme, we tested the hypothesis that
D2-like receptors might cause an increase in NKA abundance
in the proximal tubular membranes. We tested this hypothesis with
Western blotting to measure the changes in NKA
1-subunits in the proximal tubular membranes on
activation of D2-like receptors. We found that the NKA
1-subunit antibody immunoreacted with a 100-kDa
(approximate) protein. In these experiments, bromocriptine increased
NKA
1-subunit immunoreactivity in the proximal tubular
membranes, which was blocked by PD-98059 and genistein. Thus activation
of D2-like receptors causes recruitment of NKA
1-subunits in the proximal tubules of the kidney, which requires activation of p44/42 MAPK and a tyrosine kinase. This finding
is in concert with our previous observation that p44/42 MAPK and
tyrosine kinase are required for D2-like
receptor-mediated activation of NKA (15).
It has been reported that tyrosine phosphorylation of the NKA
1-subunits leads to an increase in NKA activity
(8, 9). Furthermore, our findings show that
D2-like receptor-mediated recruitment of NKA
1-subunits requires genistein-sensitive tyrosine kinase
activity. Therefore, we wanted to test the possibility that activation
of D2-like receptors causes tyrosine phosphorylation of NKA
1-subunits, which may trigger recruitment. We found that bromocriptine caused an increase in tyrosine phosphorylation per NKA
1-subunit in proximal tubular membrane. Furthermore,
bafilomycin A1 blocked D2-like receptor-mediated increase
in tyrosine phosphorylation per NKA
1-subunit in the
membrane. Because bafilomycin A1 blocked the recruitment of NKA
1-subunits in our experiments, it indicates that
activation of D2-like receptors might cause the recruitment of tyrosine-phosphorylated NKA
1-subunits in proximal tubules.
When we measured D2-like receptor-mediated tyrosine
phosphorylation of cytosolic NKA 1-subunits, we found
that bromocriptine alone did not increase the tyrosine phosphorylation.
On the other hand, pretreatment of the proximal tubules with
bafilomycin A1 resulted in a bromocriptine-mediated increase in
tyrosine phosphorylation per NKA
1-subunits in cytosolic
preparations. This observation can be explained if the time point of
measurement is considered. We measured the increase in recruitment and
tyrosine phosphorylation in proximal tubules treated with bromocriptine
for 15 min, when the NKA was found to be maximally stimulated
(11). Therefore, it is possible that at 15 min the NKA
1-subunits, which are tyrosine phosphorylated via
D2-like receptors, have already been recruited to the
membrane. Hence, we do not see an increase in cytosolic tyrosine
phosphorylation of NKA
1-subunits. However, once
recruitment is blocked by bafilomycin A1 pretreatment,
D2-like receptor-mediated increase in tyrosine
phosphorylation of the NKA
1-subunits retained in the
cytosol is measurable. In other words, activation of
D2-like receptors causes tyrosine phosphorylation and
subsequent recruitment of NKA
1-subunits in proximal
tubules of the kidney.
Recruitment of NKA 1-subunits seems to be a common
mechanism for the receptor-mediated activation of NKA in several cell types (1, 5, 7, 14). However, the covalent modification of
NKA
1-subunits that drives recruitment may be varied.
For example, Efendiev et al. (7) reported that
simultaneous phosphorylation of Ser11 and Ser18 of the NKA
1-subunits is responsible for the PKC-mediated
recruitment of the NKA
1-subunits to the plasma membrane. On the other hand, in alveolar epithelium, the
D1-like receptor-mediated recruitment of NKA
1-subunits is attributed to activation of a
serine/threonine protein phosphatase 2A (14). Along the
same lines, it was recently reported that aldosterone-mediated recruitment of NKA
1-subunits in cortical collecting
duct required a dephosphorylation of these subunits (5).
In this study, we have particularly examined the role of tyrosine
phosphorylation of NKA 1-subunits in its
D2-like receptor-mediated recruitment. Here, we have shown
that blocking tyrosine phosphorylation in the proximal tubules blocked
the D2-like receptor-mediated recruitment of NKA
1-subunits. This result supports our previous observation that genistein completely inhibited the stimulation of NKA
via D2-like receptors (15). Whether tyrosine
phosphorylation is the only requirement for recruitment of NKA
1-subunits by D2-like receptors is not
known. As mentioned above, serine/threonine dephosphorylation may be
another mechanism for recruitment. To our knowledge, such an effect of
D2-like receptors is not reported in the proximal tubules
of the kidney. Nevertheless, tyrosine phosphorylation is critical for
the bromocriptine-mediated activation of NKA.
In summary, we have shown that activation of the D2-like
receptor causes recruitment of NKA 1-subunits in a
p44/42 MAPK-tyrosine kinase-dependent manner in the proximal tubules of
rat kidney. Moreover, D2-like receptor-mediated recruitment
requires tyrosine phosphorylation of cytosolic NKA
1-subunits (Fig. 5).
Therefore, the present study provides the cellular mechanisms for
D2-like receptor-mediated stimulation of NKA activity in
renal proximal tubules.
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ACKNOWLEDGEMENTS |
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We thank Dr. M. Asghar for suggestions relating to the immunoprecipitation experiments.
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FOOTNOTES |
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This study was supported by National Institute on Aging Grant AG-15031.
Address for reprint requests and other correspondence: M. F. Lokhandwala, College of Pharmacy, Univ. of Houston, Houston, TX 77204-5511 (E-mail: MLokhandwala{at}uh.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
August 6, 2002;10.1152/ajprenal.00039.2002
Received 29 January 2002; accepted in final form 26 July 2002.
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