Pathophysiology of Renal Disease and Progression

Extracellular Signal–Regulated Kinase Inhibition Slows Disease Progression in Mice with Polycystic Kidney Disease

Sayu Omori, Mariko Hida, Hisayo Fujita, Hisahide Takahashi, Susumu Tanimura, Michiaki Kohno and Midori Awazu
JASN June 2006, 17 (6) 1604-1614; DOI: https://doi.org/10.1681/ASN.2004090800

Abstract

The expression of mitogen-activated protein kinases (MAPK) in DBA/2-pcy/pcy (pcy) mice, a murine model of polycystic kidney disease was investigated. Proliferating cell nuclear antigen–positive cells were recognized in cyst epithelium from embryonic day 14.5 to 25 wk of age. Extracellular signal–regulated kinase (ERK) was expressed in the renal tubules of control and pcy mice, but stronger immunostaining was observed in cyst epithelium. Phosphorylated ERK was detected only in pcy mice and was localized predominantly in the cysts. p38 MAPK (p38) was no longer expressed after birth in controls but was detected in the cyst epithelium and in occasional tubular cells of pcy mice at all stages examined. c-Jun N-terminal kinase (JNK) was expressed in all tubular segments of controls after neonatal day 7, whereas in pcy kidneys, tubules became positive for JNK after 8 wk, and the cysts expressed little JNK. Administration of an oral MAP/ERK kinase inhibitor, PD184352, 400 mg/kg per d, to 10-wk-old pcy mice daily for the first week and then every third day for 6 additional weeks significantly decreased BP, kidney weight, serum creatinine level, and water intake and significantly increased urine osmolality. The cystic index and expression of phosphorylated ERK and ERK were significantly lower in PD184352-treated pcy mice. These results demonstrate that the expression of MAPK is dysregulated in cyst epithelium and that inhibition of ERK slowed the progression of renal disease in pcy mice.

The mitogen-activated protein kinase (MAPK) family of serine/threonine kinases includes extracellular signal–regulated protein kinase (ERK), p38 MAPK (p38), and c-Jun N-terminal kinase (JNK) (1). ERK is activated by various growth factors and promotes cell proliferation and differentiation. p38 and JNK are activated by cytokines and cell stress and have been implicated in stress response, apoptosis, and hypertrophy (2,3). We previously reported that MAPK expression is developmentally regulated in rat kidney (4), with ERK and p38 being expressed predominantly in the embryonic kidney and JNK being expressed in the adult kidney. These findings pointed to an important role of ERK and p38 in renal development, and a subsequent organ culture study demonstrated that ERK inhibition attenuates nephron formation with a minimal effect on kidney size (5). p38 inhibition, however, suppresses both nephrogenesis and kidney growth.

Because each subgroup of MAPK seems to possess a specific role in renal development, it was suspected that dysregulation of MAPK might lead to kidney abnormality. We therefore examined the expression and localization of MAPK in human renal dysplasia, one of the most common kidney malformations. The results showed that p38 and phosphorylated p38 (P-p38) were strongly upregulated in dysplastic epithelium, whereas JNK and phosphorylated JNK (P-JNK) were barely detectable in dysplastic tubules and cysts (6). ERK was expressed in all tubular segments, and phosphorylated ERK (P-ERK) was detected in the distal tubules and collecting ducts of normal kidneys, but dysplastic kidney epithelium stained exclusively positive for ERK and P-ERK. The results suggested that dysregulated MAPK might mediate the hyperproliferation of dysplastic tubules that results in cyst formation.

Polycystic kidney disease (PKD) is characterized by the progressive expansion of multiple cystic lesions that leads to a decline in renal function. The epithelium of the cysts in PKD exhibits similar features to those of dysplastic epithelium: Increased proliferation; apoptosis; and expression of a transcription factor, PAX2 (7–9). In this study we investigated the expression of MAPK in DBA/2-pcy/pcy (pcy) mice, a murine model of childhood PKD, juvenile nephronophthisis. Pcy mice have a missense mutation in NPHP3, the causal gene for human adolescent-type nephronophthisis (10). At an early stage, pcy mice are characterized by cysts that are derived predominantly from collecting ducts and urine concentration defect (11,12). Later in the course, all nephron segments become cystically dilated. In this study, we found enhanced expression of ERK, P-ERK, and p38 as well as downregulation of JNK in the cyst epithelium of pcy mice, that was associated with increased proliferation. Administration of an inhibitor of MAP/ERK kinase (MEK), an upstream kinase of ERK, to pcy mice demonstrated a role of activated ERK in cyst formation, kidney enlargement, and loss of renal function.

Materials and Methods

Anti-ERK (erk1/2-CT, rabbit polyclonal IgG) was from Upstate Biotechnology (Lake Placid, NY). Anti-p38 (C-20) and anti-JNK (FL) were from Santa Cruz Biotechnology (Santa Cruz, CA). Monoclonal anti-MAPK, activated (diphosphorylated ERK-1 and -2) was purchased from Sigma (St. Louis, MO). Anti–phospho-p38 (P-p38) and anti–phospho-JNK (P-JNK) were from New England Biolab (Beverly, MA). A mAb specific for proliferating cell nuclear antigen (PCNA)/cyclin, peroxidase-conjugated rabbit anti-mouse Ig, peroxidase-conjugated swine anti-rabbit Ig, and the DAKO Protein K Enzyme Digestion were from DAKO A/S (Glostrup, Denmark). CD10 was from Novocastra Laboratories Ltd (Newcastle upon Tyne, UK). The ApopTag Peroxidase in situ apoptosis detection kit was from Intergen Company (Purchase, NY). PD184352 was synthesized by coupling 2-chloro-4-iodoaniline with 2,3,4-trifluorobenzoic acid after treatment with cyclopropylmethoxyamine in the presence of benzotriazol-1-yl-oxytripyrrolidinephosphonium hexafluorophosphate in 62% yield, and the compound obtained was fully characterized by 1H nuclear magnetic resonance and mass spectroscopy (13).

Experimental Animals and Sampling

The Keio University Ethics Committee for Animal Experiments approved all experiments in this study. Pregnancy was determined by the detection of a vaginal plug. Before removal of the embryos, pregnant mice were sedated with an intraperitoneal injection of sodium pentobarbital. Embryos were removed and decapitated on day 14.5 of gestation (E14.5) and then fixed with neutral-buffered formalin. Kidneys from postnatal days 1 and 7 (N1, N7) and 8- and 25-wk-old pcy mice and from age-matched controls were harvested and fixed with neutral-buffered formalin.

Immunohistochemistry

After fixation, kidneys were embedded in paraffin, and immunohistochemical staining was performed by the enzyme-labeled antibody method on serial sections that were 3 μm thick. Paraffin sections were deparaffinized and rehydrated, and endogenous peroxidase activity was quenched by incubating sections in 0.3% H2O2/methanol for 15 min. For unmasking antigens, slides were boiled at 100°C for 10 min in 10% citrate buffer (pH 6.0)/methanol. Sections were incubated with antibodies against PCNA (dilution 1:100), ERK (dilution 1:200), p38 (dilution 1:200), JNK (dilution 1:50), or P-ERK (dilution 1:100). The incubation time was 60 min at room temperature or overnight at 4°C. After incubation with secondary antibody at a concentration of 1:100, immunoreaction products were identified by reaction with 3,3′-diaminobenzidine as the chromogen. A Vectastain ABC kit (Vector Laboratories, Burlingame, CA) was used for CD10 staining. Sections were counterstained with hematoxylin acetate or methyl green. Positive controls (brain for ERK and JNK and bone marrow for p38) were run simultaneously. Negative controls included omitting the primary antibody or substituting rabbit serum for the primary antibody.

Detection of Apoptosis

Paraffin-embedded sections were deparaffinized, and terminal deoxynucleotide transferase-mediated nick-end labeling (TUNEL) staining was performed with the Apoptag kit. Sections were counterstained with methyl green. TdT was omitted from the staining procedure in negative controls.

Administration of PD184352

An oral MEK inhibitor, PD184352, 400 mg/kg per d, mixed in ground food, was administered to 10-wk-old male pcy mice daily for the first week and then every third day for 6 additional weeks (n = 7). To ensure complete consumption, the amount of food was adjusted according to the intake on the previous day. Eight age-matched male pcy mice served as controls. Before the mice were killed, they were placed in metabolic cages to measure daily water intake and urine osmolality. BP was measured by the tail-cuff method using BP-98A (Softron, Tokyo, Japan). Blood was obtained by decapitation for determination of serum creatinine. Kidneys were removed, weighed, and fixed with neutral-buffered formalin for histologic evaluation or snap-frozen for immunoblot analysis.

Morphometric Studies

Morphometric analyses were performed on three randomly selected sections that contained cortex, medulla, and papilla stained with hematoxylin and eosin. The cystic index, a measure of the fraction of the cross-sectional area that was cystic (0, no cyst; 1+, <25% cystic; 2+, 25 to 50% cystic; 3+, 50 to 75% cystic; 4+, >75% cystic), was determined by a standard point-counting method in at least five fields on each section (magnification, ×40) and averaged.

Immunoblot Analysis

Kidneys were thawed and lysed in solubilization buffer that contained 20 mM HEPES (pH 7.2), 1% Triton-100, 10% glycerol, 20 mM sodium fluoride, 1 mM sodium orthovanadate, 1 mM PMSF, 10 μg/ml aprotinin, and 10 μg/ml leupeptin. Insoluble material was removed by centrifugation (10,500 × g, 10 min). The protein content in kidney lysates was measured by a DC protein assay (Bio-Rad Laboratories, Tokyo, Japan). Lysates were resolved by SDS-PAGE and transferred to polyvinylidene difluoride membranes (Immobilon, Millipore Corp., Bedford, MA). Nonspecific binding sites were blocked in TBS buffer (10 mM Tris-Cl [pH 7.4] and 0.15 M NaCl) that contained 5% skim milk either overnight at 4°C or for 1 h at 25°C. Antibodies were added to TBS that contained 0.1% Tween 20 in saturating titers and incubated with mixing for 2 h at 25°C. Bound antibodies were detected by the ECL Western blotting system (Amersham, Arlington Heights, IL).

Statistical Analyses

The data are expressed as means ± SEM. Statistical analysis was performed by unpaired t test. P < 0.05 was considered significant.

Results

Immunohistochemistry

The results for the immunohistochemistry data are summarized in Table 1. Cysts had already formed in the kidneys of pcy mice at E14.5 (Figure 1A). The cysts were negative for the proximal tubule marker CD10. Sporadic enlargement of Bowman’s capsules also was observed. Both CD10-positive and CD10-negative cysts were observed in the neonatal kidney (Figure 1B). At 8 wk, dilated cysts were detected in the cortex and the medulla (Figures 1C and 2B). At 25 wk, numerous cysts dominated the kidney (data not shown).

Figure 1.
Figure 1.

Immunohistochemical staining for CD10 in a DBA/2-pcy/pcy (pcy) kidney. Pcy kidneys were stained for CD10, a marker of proximal tubules. Cysts that are negative for CD10 are seen already at embryonic day 14.6 (E14.5; (A; *). Enlargement of Bowman’s capsules also can be seen. At postnatal day 7 (N7) and 8 wk, both CD10-positive and CD10-negative cysts (*) are present (B and C; a cross-section of the cortex). CD10-positive cells (arrowheads) decrease in number as cyst size increases. Magnifications: ×200 in A and B; ×100 in C.

Figure 2.
Figure 2.

Distribution of proliferation and apoptosis in the kidney of 8-wk-old control and pcy mice. In the control kidney, proliferating cell nuclear antigen (PCNA)-positive (A) or terminal deoxynucleotide transferase-mediated nick-end labeling (TUNEL)-positive cells (C) are rarely detected. In the pcy kidney, PCNA staining is visible in the cyst epithelium (B). TUNEL-positive cells are numerous in the interstitium and also are found in cyst epithelium (D). Magnifications: ×100 in A and B; ×200 in C and D.

View this table:
Table 1.

Proliferation, apoptosis, and expression of MAPK in the kidneys of control and pcy micea

PCNA

In the control kidneys, PCNA-positive cells were found in the nephrogenic zone during the fetal and neonatal periods (data not shown) but were no longer detected at 8 wk (Figure 2A). In the pcy kidney, PCNA staining was noted in dilated glomerular parietal epithelial cells and cyst epithelium at all ages (Figure 2B).

TUNEL

In the normal kidneys, TUNEL-positive cells were observed only during development in the nephrogenic zone and ureteric buds, and they were rarely detected beyond the neonatal period (Figure 2C). In the pcy kidney, abundant apoptotic cells were observed in the interstitium and occasionally in the cyst epithelium (Figure 2D). These findings are consistent with the results of previous studies in other animal models of PKD as well as in human PKD (8,9).

ERK and P-ERK

ERK was expressed in uninduced and induced mesenchyme, ureteric buds, and immature tubules in the nephrogenic zone of both control and pcy kidneys at E14.5 and N1 (data not shown). After N7, ERK was detected in tubular segments distal to the proximal tubules of both control (Figure 3A) and pcy kidneys (Figure 3B). Cyst epithelium stained positive for ERK throughout all stages of development, and the staining became more intense at 8 wk (Figure 3B). P-ERK was not detected in the control kidneys at any age examined (Figure 3C). In pcy kidneys, cyst epithelium and occasional tubular cells stained positive for P-ERK (Figure 3D).

Figure 3.

Immunohistochemical localization of mitogen-activated protein kinases (MAPK) in the kidney of 8-wk-old control and pcy mice. Extracellular signal–regulated kinase (ERK) is expressed in tubular segments distal to the proximal tubules of both the control kidney (A) and the pcy kidney (B). Stronger immunostaining for ERK is observed in the cyst epithelium. Phosphorylated ERK (P-ERK) is not expressed in the control kidney (C) but is detected in the cyst epithelium and in occasional tubular cells of the pcy kidney (D). p38 is not expressed in the control kidney (E). In the pcy kidney, p38 is detected in the cyst epithelium and in occasional tubular cells (F). c-Jun N-terminal kinase (JNK) is abundantly expressed in all tubular segments of the control kidney (G). JNK is weakly expressed in the tubules of the pcy kidney and not expressed at all in the cyst epithelium (H). Magnification, ×200.

p38

In control kidneys, p38 was expressed only at E14.5 in the mesenchyme and ureteric buds and was not detected thereafter (Figure 3E). In the pcy kidney, persistent expression of p38 was observed after birth in cyst epithelium and occasional tubular cells (Figure 3F). Expression of p38 became stronger as the cysts increased in number and size.

JNK

In the control kidneys, weak JNK staining was observed at N7, but abundant expression was seen in all tubular segments at 8 wk (Figure 3G). In the pcy kidneys, the tubules became weakly positive for JNK at 8 wk (Figure 3H). At 25 wk, the extent of JNK expression in the tubules of the pcy mice was the same as in the controls (data not shown). Cyst epithelium, however, were entirely negative for JNK at all ages (Figure 3H).

Immunoblot Analysis

The altered expression of MAPK in pcy mice was confirmed by immunoblot analysis (Figure 4, F through J). P-ERK, ERK, and p38 were upregulated in the pcy kidney, and P-p38 also was increased in the pcy kidney compared with the control kidney. At 25 wk, JNK expression was similar in pcy and control mice, probably because JNK was abundantly expressed in tubules and the downregulation of JNK in cyst epithelial cells could not be detected by immunoblot analysis of whole-kidney lysate. P-JNK expression, however, was downregulated in the pcy kidney.

Figure 4.

Effect of PD184352 in pcy mice. Ten-week-old pcy mice were treated with PD184352 (PD) for 7 wk. Cyst formation was reduced in the PD-treated mice compared with the control pcy mice. (A) Representative cross-sections of kidneys from the control mice (CON), pcy mice (pcy), and PD-treated pcy mice (pcy+PD). Immunostaining of P-ERK and ERK was absent and reduced, respectively, in the kidneys of the PD-treated pcy mice (C and E, respectively) compared with immunostaining in the control pcy mice (B and D, respectively). (F) Immunoblot analysis of whole-kidney lysate confirmed the upregulation of P-ERK and ERK in the pcy mice compared with the control mice, as well as suppression of P-ERK and ERK upregulation in PD-treated pcy mice. (G) Phosphorylated p38 and p38 were upregulated and phosphorylated JNK was downregulated in the pcy mice, and PD had no effect on these alterations. There was no difference in JNK expression among kidneys of the CON, pcy mice, and PD-treated pcy mice. Means ± SEM are shown for a composite of three experiments (H through J). *P < 0.05 versus CON; **P < 0.05 versus pcy. Magnifications: ×100 in B and C; ×40 in D and E.

Role of Activated ERK in the pcy Kidney

To examine the functional role of activated ERK in the pcy kidney, we investigated the effect of PD184352, an oral inhibitor of MEK, on functional and morphologic abnormalities in the pcy kidney. Ten-week-old pcy mice received oral administration of PD184352 daily for the first week and then every third day for 6 additional weeks (n = 7). The body weight of the PD184352-treated pcy mice was slightly but significantly lower than that of the control pcy mice (n = 8; Table 2). The kidney weight, kidney weight to body weight ratio, BP, and serum creatinine levels of the PD184352-treated pcy mice also were significantly less at the end of treatment. Water intake of PD184352-treated pcy mice was reduced, and urine osmolality was increased (both P < 0.05). The cystic index also was significantly lower in the PD184352-treated pcy mice than in the control pcy mice. Representative photographs are shown in Figure 4A.

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Table 2.

Characteristic of control mice, pcy mice, and pcy mice treated with PD184352 at 17 wka

To confirm that the effect of PD184352 was mediated by inhibition of ERK, we examined the expression of P-ERK and ERK. As shown in Figure 4, B and C, P-ERK was expressed in the cysts of the control pcy kidneys but not detected in the kidneys of the PD184352-treated mice. The expression of ERK also was less in PD184352-treated mice (Figure 4, D and E). The reduced expression of P-ERK and ERK in the PD184352-treated mice may be due, at least in part, to the reduced cyst formation in the PD184352-treated mice. The reduction in P-ERK and ERK expression in the kidneys of the PD184352-treated mice was confirmed by immunoblot analysis (Figure 4, F and H). PD184352 had no effect on P-p38 and p38 upregulation or P-JNK downregulation (Figure 4, G, I, and J).

Discussion

The results of this study demonstrate dysregulated expression of MAPK in the cyst epithelium of pcy mice, a model of PKD. More specific, ERK, P-ERK, and p38 were found to be upregulated and JNK downregulated in cyst epithelium. The functional role of activated ERK was investigated by using an inhibitor of MEK, an upstream kinase of ERK, and the results showed that inhibition of ERK activity attenuated cyst formation, renal enlargement, and the loss of kidney function.

We previously reported excessive expression and activation of p38 and ERK in the cyst epithelium in human renal dysplasia, another disease that is characterized by multiple cysts (6). That JNK is not expressed in the normal immature kidney or in the epithelium of cysts in dysplastic kidneys suggests a role of JNK in differentiation at a later stage (4,6). Our study showed that the cyst epithelium of pcy mice has the same pattern of dysregulated MAPK expression as that of dysplasia and increased expression of PCNA and apoptosis in the cyst epithelium, a common feature of cystic diseases (7,8). Cyst epithelium in PKD is thought to have a persistent developmental phenotype as evidenced by altered expression of PAX2, vimentin, clusterin, Na+/K+-ATPase, and the EGF receptor (14). The dysregulated MAPK expression may be another characteristic that reflects the undifferentiated state of cyst epithelium.

Pax2 is a transcription factor that plays an important role during kidney development, and the expression and the function of p38 and ERK in dysplasia and PKD are reminiscent of those of Pax2. Pax2 is thought to be required for mesenchymal cell growth and prevention of apoptosis (15,16) and has been found to be overexpressed in cyst epithelium of both dysplasia and PKD (8). The reduced Pax2 gene dosage reduced cyst formation in cpk mice, a murine model of rapid-onset PKD (17). Pax2 knockout mice are characterized by hypoplasia and agenesis of the urogenital system (15). Woolf (18) hypothesized that excessive Pax2 results in cystic diseases and insufficient Pax2 leads to hypoplastic kidneys. In a previous study we showed that p38 and ERK are upregulated in cyst epithelium and that blockade of p38 and ERK inhibits metanephros growth and/or nephron formation (5). The components of the Pax2-mediated pathway are not well defined, but it is conceivable that p38 and ERK are involved in signaling events upstream or downstream of Pax2.

Nagao et al. (19) reported increased expression of P-ERK in the cyst epithelium of Han:SPRD rats, a model of autosomal dominant PKD (ADPKD). Increased ERK activity in cyst epithelium therefore may be a common feature of cystic diseases. Several explanations for the ERK activation in cyst epithelia in PKD are possible. First, it may be a reflection of the persistent fetal phenotype, as discussed above. Second, ERK may be activated through various signaling pathways that are known to be stimulated in PKD. EGF and the TGF-α/EGF receptor axis have been shown to be upregulated in the cyst epithelium in PKD (14). EGF activates ERK through the ras/Raf-1 pathway. Also, the intracellular cAMP level has been reported to be increased in various models of PKD (20). Although cAMP inhibits the Raf-1/ERK pathway in normal epithelial cells, Yamaguchi et al. (21,22) found that cAMP and adenylyl cyclase agonists stimulated the proliferation of cyst epithelial cells that were cultured from kidneys of patients with ADPKD by activating ERK via B-Raf. Furthermore, Chatterjee et al. (23) reported that a glycosphingolipid lactosylceramide is upregulated in proximal tubular cells from human PKD kidneys and may stimulate cell proliferation via ERK.

An alternative but not mutually exclusive explanation for the ERK activation in PKD is cell stretch secondary to transepithelial fluid secretion. Stretching of kidney epithelial cells has been reported to cause ERK activation as well as proliferation, a crucial factor in cyst development (24,25). Alexander et al. (25) recently demonstrated that cyclic stretch activates ERK in rabbit proximal tubule cells by releasing arachidonic acid through calcium channel activation followed by cPLA2 phosphorylation, and subsequent events involved activation of c-Src and phosphorylation of EGF receptor, leading to ERK activation. Stretch-induced activation of p38 also has been reported in a type II–like alveolar epithelial cell line (26). Therefore, the increased expression and/or activation of ERK and p38 that was observed in our study may be secondary to cell stretch. Because weak expression of p38 and P-ERK was found in the tubular cells of the pcy kidney, the preexisting fetal pattern of MAPK expression may be enhanced by cell stretch, resulting in cell proliferation and cyst enlargement.

Our studies have extended the earlier observations made by other investigators and demonstrated that ERK mediates progression of PKD. PD184352 is an orally active, highly specific, small-molecule inhibitor of MEK (27), and it mitigated cystogenesis, renal enlargement, and kidney dysfunction. There was no mortality associated with PD184352, but the body weight of the PD184352-treated mice was lower than that of the control pcy mice. The lower kidney weight in the PD184352-treated mice contributed to the difference, because there was no statistically significant difference between the PD184352-treated and untreated mice in body weight minus kidney weight (22.6 ± 0.5 versus 24.9 ± 0.8 g). Limiting food intake to ensure consumption of the full dose of PD184352 also may explain the lower weight of the PD184352-treated mice. BP was significantly lower in the PD184352-treated mice. The lower BP in the PD184352-treated mice at 17 wk probably was due to the retarded progression of the disease. An alternative but not mutually exclusive explanation is that PD184352 inhibited vascular contraction, because ERK-dependent signaling pathways have been reported to play a role in vascular hyperresponsiveness in hypertension (28).

We also observed a lower serum creatinine level in the PD184352-treated pcy mice. The cause of decrease in glomerular filtration in PKD was suggested to be disconnection of the glomeruli from the tubules by enlarging cysts (29). Therefore, the preservation of renal function by PD184352 may be attributable to the reduced cyst formation. In addition, high urine volume has been suggested as a risk factor for faster progression of PKD (30), and because PD184352 ameliorated the concentrating defect and reduced urine volume, it may have slowed the decrease in GFR via this mechanism. PD184352 has been evaluated already in clinical trials as a potential anticancer agent, and other more potent MEK inhibitors now are being tested (31). The concept that a polycystic kidney is a neoplasm in disguise is becoming increasingly accepted (32), and some cancer chemotherapeutic agents may be effective against PKD.

Although activation of ERK has been demonstrated previously by other investigators, this is the first study to report upregulation of p38 and downregulation of JNK. It would be interesting to determine whether blocking p38 would affect the course of PKD. The deficiency of JNK also may contribute to the increased proliferation of cyst epithelium. Although JNK originally was implicated in apoptosis, recent reports suggest that it may mediate survival signaling under specific circumstances (3). The role of JNK in renal tubular epithelial cells remains largely unknown, and the significance of the absence of this enzyme in the cyst epithelium in PKD needs to be clarified. It is interesting that a recent study by Le et al. (33) demonstrated increased activity of activator protein-1 (AP-1) transcription factor components in human and mouse PKD. AP-1 is a downstream target of MAPK, and PKD1 and PKD2, gene products of ADPKD, have been reported to modulate AP-1 activity through JNK and JNK/p38, respectively (34,35).

Gattone et al. (12) and Torres et al. (36) recently, showed that a vasopressin V2 receptor antagonist inhibited disease development and even caused regression in various animal models of PKD. Its mechanism of action is thought to be blockade of cAMP. ERK is the downstream target of cAMP in the cyst epithelium in PKD (20,21). Because targeting a downstream step generally is more efficient and more specific, inhibition of the ERK pathway may provide a promising approach to the treatment of PKD. Notably, lovastatin and probucol, which have been reported to slow the progression of cystic growth, are known to inhibit ERK (37,38). Future studies are warranted to clarify the role of ERK in other animal models of PKD as well as to develop efficient therapeutic modalities for inhibition of ERK in humans.

Acknowledgments

This study was supported by grants from Ministry of Education, Science, and Culture of Japan (12770401, 13770404, 14770576, 14570772, and 17591115) and from the Pharmacia-Upjohn Fund for Growth and Development Research.

Portions of this work were presented at the 2001, 2003, and 2005 National Meetings of the American Society of Nephrology.

Footnotes

  • Published online ahead of print. Publication date available at www.jasn.org.

    See related editorial, “MEK Inhibition Holds Promise for Polycystic Kidney Disease,” on pages 1498-1500.

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Extracellular Signal–Regulated Kinase Inhibition Slows Disease Progression in Mice with Polycystic Kidney Disease
Sayu Omori, Mariko Hida, Hisayo Fujita, Hisahide Takahashi, Susumu Tanimura, Michiaki Kohno, Midori Awazu
JASN Jun 2006, 17 (6) 1604-1614; DOI: 10.1681/ASN.2004090800
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