Chronic Uremia Attenuates Growth Hormone–Induced Signal Transduction in Skeletal Muscle
Di Fei Sun*,
Zhilan Zheng*,
Padmaja Tummala*,
Jun Oh,
Franz Schaefer and
Ralph Rabkin*
*Veterans Affairs Palo Alto Health Care System and Department of Medicine, Stanford University, Palo Alto, California; and Division of Pediatric Nephrology, University Children’s Hospital, Heidelberg, Germany
Correspondence to Dr. Ralph Rabkin, VAPAHCS (111R), 3801 Miranda Avenue, Palo Alto, CA 94304; Phone: 650-858-3985; Fax: 650-849-0213; E-mail: rabkin{at}stanford.edu
Malnutrition and muscle wasting are common in chronic renalfailure (CRF) and adversely affect morbidity and mortality.Contributing to the muscle wasting is resistance to growth hormone(GH). For testing whether impaired GH signaling is a cause ofthe skeletal muscle GH resistance and for elucidating its mechanisms,muscle GH signaling and action were studied in GH-deficientrats with surgically induced CRF and sham-operated pairfed controlrats. GH treatment increased gastrocnemius muscle IGF-1 mRNAlevels significantly in control but not in CRF rats. GH-activatedJanus-associated kinase 2 (JAK2)-signal transducers and activatorsof transcription 5 (STAT5) signaling was impaired in CRF rats,despite normal GH receptor (GHR), JAK2, and STAT5 protein levels.Phosphorylation of the GHR, JAK2, and STAT5 in response to GHwas depressed by nearly half in CRF (P < 0.05), and nuclearphospho-STAT5 levels were depressed by approximately one third(P < 0.01). GH-stimulated suppressors of cytokine signaling2 mRNA levels were significantly higher in CRF. This may berelated to inflammatory cytokine activity because C-reactiveprotein levels were elevated. Muscle protein-tyrosine phosphataseactivity was also increased significantly by twofold. In conclusion,rats with CRF acquire skeletal muscle resistance to GH thatis caused at least in part by impaired JAK2-GHR-STAT5 phosphorylationand nuclear STAT5 translocation. Furthermore, it seems thatthe attenuated JAK2-STAT5 phosphorylation may be caused by atleast two different processes. One involves depressed phosphorylationof the signaling proteins because of increased suppressors ofcytokine signaling 2 expression that may be linked to low-gradeinflammation. The other may involve increased signaling proteindephosphorylation because of heightened protein-tyrosine phosphataseactivity.
Advanced chronic renal failure (CRF) is often complicated bythe appearance of muscle wasting and malnutrition, conditionsthat have a distinct adverse effect on morbidity and mortality(1,2). Several factors contribute to the development and perpetuationof the wasted state, including anorexia with reduced calorieand protein intake; acidosis; inadequate dialysis with accumulationof toxic products; inflammatory cytokines; diabetes; cardiacfailure; loss of amino acids in the dialysate; and resistanceto anabolic hormones such as insulin, growth hormone (GH), andIGF-I which is worsened by malnutrition and inflammation (3–5).To some extent, insensitivity to GH can be overcome by pharmacologicdoses of recombinant GH, but this does increase the risk ofadverse events. Several small studies have demonstrated thatGH treatment of adult ESRD patients produces a salutary effecton urea kinetics, protein turnover, and lean body mass (6,7).In children with advanced CRF, GH resistance is a major causeof impaired body growth, and this can largely be corrected byGH treatment (8–10).
The mechanisms that account for the development of GH resistancein uremia are not well understood (10–12) and includeinsensitivity to IGF-1, the mediator of most of the actionsof GH, decreased IGF-I expression, possibly although controversialdepressed GH receptor (GHR) expression, and impaired GH signaltransduction through the Janus-associated kinase 2 (JAK2) andsignal transducers and activators of transcription (STAT) pathwaythat we recently uncovered in the liver of uremic rats (11).Whether a similar defect is present in skeletal muscle is thesubject of this study. When GH binds to its receptor, it causesreceptor dimerization followed by autophosphorylation of JAK2,a tyrosine kinase associated with the intracellular domain ofthe receptor (13). The activated JAK2 in turn phosphorylatesthe GHR, creating binding sites for selective members of theSTAT family of proteins, namely STAT1, STAT3, and STAT5a, andSTAT5b, which are then phosphorylated by JAK2 (14,15). The phosphorylatedSTAT form dimers that translocate into the nucleus, bind tospecific DNA sequences, and modulate gene transcription. Malemice with STAT5b deficiency and female mice with a combineddeletion of STAT5a and STAT5b, homologues with >90% sequencehomology, are severely growth retarded (16), whereas in humans,a STAT5b mutation causes growth retardation (17). It turns outthat STAT5b is important for mediating GH stimulation of IGF-1gene expression (18,19). Regulation of GH-mediated signal transductionis a complex process that involves several physiologic mechanisms,including protein-tyrosine phosphatases (PTPase) that dephosphorylatesignaling proteins and a family of suppressors of cytokine signaling(SOCS) that inhibit protein phosphorylation (20). In this study,we tested the postulate that in uremia, there is acquired resistanceto the induction of IGF-1 expression in skeletal muscle thatis caused by a defect in GH-stimulated JAK2-STAT5 signal transductionand then explored the pathomechanism of the defect. We foundthat GH-induced IGF-1 expression is indeed impaired in skeletalmuscle of uremic rats and that this seems to be caused, at leastin part, by upregulated SOCS expression and increased PTPaseactivity.
Experimental Animals and Protocols
GH-deficient dwarf (dw/dw) male rats were used as they are farmore sensitive to the action of exogenous GH than hormone-repleteanimals and avoid any confounding effects that may arise inpituitary-intact rats because of the frequent spontaneous GHsecretory activity. CRF was created by a two-step 5/6 nephrectomyprocedure with ketamine (80 mg/kg) and xylazine (10 mg/kg) anesthesiaas before (11). Sham operations were performed in control animalsthat were pairfed (PF) with the CRF rats.
GH-Stimulated Gene Expression
Thirteen days after the final surgery was completed, the CRFand control animals were each divided into two groups that weretreated for 8 d with either vehicle (V) or recombinant bovineGH (bGH; gift from Monsanto Corp., St. Louis, MO). GH was givenat a dose of 25 µg/100 g body wt in the morning and inthe evening for 4.5 d and then at 12.5 µg/100 g in themorning and in the evening for 3.5 d. On the last day, the ratsreceived two doses of GH, 12.5 µg/100 g each, or V 5 hapart and killed 1 h after the last injection to detect GH-stimulatedIGF-1 and SOCS expression, respectively. Gastrocnemius musclewas excised and stored at –80°C. Serum was collectedand frozen. A prolonged course of GH was given to ensure stimulationof IGF-1 expression. The doses used were based on pilot studiesof GH-induced IGF-1 expression.
GH-Activated JAK2/STAT5 Signal Transduction
After 16 d of CRF and after an overnight fast, the control andCRF rats were anesthetized and via a midline incision, a submaximaldose of GH (3 µg/100 g) or V was injected into the inferiorvena cava. Fifteen minutes thereafter, the gastrocnemius musclewas excised, frozen, and stored at –80°C. In anotherset of animals, to determine whether the resistant state couldbe overcome, a supramaximal dose of GH (25 µg/100 g) wasadministered and muscle was collected 10 min later. The GH dosesand times of killing were chosen on the basis of a pilot dose-responseexperiment in which maximal STAT5 phosphorylation occurred at12.5 µg GH/100 g body wt and plateaued between 10 and15 min after intravenous injections. Submaximal doses were chosen,as maximal or supramaximal doses may override resistance tosignal transduction.
Real-Time Quantitative Reverse Transcriptase–PCR Assay
Real-time quantitative reverse transcriptase–PCR withSYBR green dye as the detection agent was performed using theABI Prism 7900 Sequence Detection System (Applied Biosystems,Foster City, CA) using protocols established by the manufacturer(21). The primers (synthesized by Applied Biosystems) for thequantification of the IGF-1, SOCS1, SOCS2, SOCS3, and CIS andthe internal control genes glyceraldehyde-3-phosphate dehydrogenaseand ribosomal L7 were designed using the primer design softwarePrimer Express (Applied Biosystems) and from published sequencesand were synthesized by Qiagen Inc. (Alameda, CA; see Table 1).For IGF-1, detection primers and probes were designed toamplify and detect all splice variants of the IGF-1 mRNA. TotalRNA was extracted from gastrocnemius muscle and used for cDNAsynthesis by reverse transcription (Applied Biosystems); thecDNA samples were then subjected to PCR analysis. The resultswere quantified using the relative standard curve method asdescribed by the supplier. An internal control gene standardcurve was also generated, and the target gene was normalizedfor this endogenous control. Each sample was analyzed in triplicatein individual assays performed on two or more occasions.
Table 1. Primer sequences for quantitative real-time PCR analysisa
Western Immunoblotting and Immunoprecipitation
The STAT5 antibody that detects both STAT5a and STAT5b, theanti-JAK2 antibody, and protein A agarose were obtained fromSanta Cruz Biotechnology (Santa Cruz, CA). The antiphosphotyrosineantibody (clone 4G10) and the phospho-STAT5 antibody directedagainst the tyrosine-phosphorylated forms of both STAT5a andSTAT5b were from Upstate Biotechnology (Lake Placid, NY). TheGHR antibody, directed against the receptor extracellular domain,was a gift of W.R. Baumbach (22). Lysates and nuclear extractswere prepared from frozen muscle and used directly for Westernimmunoblot analysis or subjected to immunoprecipitation withantibodies against the GHR or JAK2 before analysis as describedpreviously (11). Immunoprecipitates or muscle lysates (for directassay of proteins) were heated in Laemmli buffer, separatedby electrophoresis on a 7.5% SDS polyacrylamide gel, and electroblottedonto nitrocellulose membranes and then immunodetected with appropriateantibodies and visualized by enhanced chemiluminescence as before(11). Protein expression was quantified with a Fluor-S digitalimage analyzer and Multianalyst software (Bio-Rad, Hercules,CA). Relative density units refer to mean pixel density withlocal background subtraction.
Biochemistry
Muscle tissue protein content by the Bradford method (ProteinAssay Kit; Bio-Rad) and serum creatinine and CO2 (mmol/L) weremeasured with a Beckman LX 20 Analyzer (Beckman Coulter, Inc.,Fullerton, CA). Serum c-reactive protein (CRP) levels were measuredwith a highly sensitive Rat CRP ELISA Kit (Alpha DiagnosticInternational, Inc., San Antonio, TX) according to the company’sinstruction.
Protein-Tyrosine Phosphatase Assay
This was performed with the Promega Tyrosine Phosphatase AssaySystem according to the manufacturer’s instructions (PromegaCorporation, Madison, WI). The assay system determines the amountof free phosphate generated from a phosphopeptide substratein a reaction by measuring the absorbance of a molybdate:malachitegreen:phosphate complex that is formed. Two synthesized phosphopeptides,END(pY)INASL and DADE(pY)LIPQQG, acted upon by a broad arrayof tyrosine phosphatases, served as substrates (23). Frozengastrocnemius muscle was homogenized in cold buffer A (1 mMDTT, 4 mM EDTA, 2.5 mM benzamidine, 1 µM leupeptin, 1µM pepstatin, 0.15 µM aprotinin, and 2 mM PMSF in25 mM HEPES [pH 7.4]) and centrifuged at 350,000 x g for 30min, and the supernatant was removed. The pellet was solubilizedby incubation in 250 µl of buffer B (Buffer A containing1% Triton X-100 and 0.6 M KCL) on ice for 30 min and recentrifugedat 150,000 x g for 1 h. The supernatant was collected as thesolubilized "particulate" fraction, which contains 90% of thePTPase activity in muscle (24), and this fraction was assayedand corrected for endogenous phosphate.
Statistical Analyses
Specific mRNA were normalized for the internal control geneand are expressed as transcript/housekeeping ratios. Tyrosine-phosphorylatedprotein levels were normalized for the respective protein levels.The control V-treated PF group mean was given a value of 100,and individual values are expressed relative to this value.Data are given as mean ± SEM. Two-tailed unpaired t testswere applied for comparison of two normally distributed groups;comparisons between more than two normally distributed groupswere made by one-way ANOVA followed by pairwise multiple comparisonwith the Holms t test (25). For more than two nonnormally distributedgroups, the Kruskal-Wallis statistic was applied followed bythe Student Newman Keuls test to distinguish between groups(25). P < 0.05 was considered statistically significant.
Serum Biochemistry
The serum creatinine and urea nitrogen levels were increasedsignificantly in the CRF rats (1.18 ± 0.2 and 71.7 ±15.5 mg/dl) compared with the PF controls (0.4 ± 0.02and 17.2 ± 0.6 mg/dl). However, serum total CO2 levelsin the CRF rats were similar to those in the controls (22.1± 0.7 versus 20.6 ± 0.5 mmol/L; P > 0.05).There was a significant (19%) increase in the serum CRP levelsin the CRF rats compared with the PF controls (204 ±7 versus 171 ± 5 pmol/L; P < 0.05).
IGF-1 and Suppressor of Cytokine Signaling mRNA Levels
The results of the real-time PCR assays for IGF-1 are shownin Figure 1. Basal IGF-1 mRNA levels measured in gastrocnemiusmuscle of the V-treated CRF and control rats were similar. However,whereas GH treatment induced a significant (42%) increase inIGF-1 expression in the controls, the response to GH was mutedand insignificant in the CRF group, reflecting resistance tothe hormone (Figure 1). SOCS1 and SOCS3 mRNA levels did notdiffer significantly between the CRF and control groups (Table 2).The basal expression of SOCS2 and CIS was increased by 55%in CRF muscle, but this did not reach statistical significance.GH treatment caused a significant two- to threefold increasein SOCS2, SOCS3, and CIS mRNA levels in both groups with a notablygreater increase in SOCS2 expression in CRF compared with thePF control group (465 ± 104 versus 266 ± 35 relativearbitrary units; P < 0.05).
Figure 1. Growth hormone (GH)-induced IGF-1 gene transcription is impaired in skeletal muscle of uremic rats. Rats with chronic renal failure (CRF) or pairfed controls were treated with bovine GH (bGH) or vehicle for 8 d. On the final day, GH or vehicle was administered 6 h and then again 1 h before the rats were killed. IGF-1 mRNA levels in gastrocnemius muscle were measured by quantitative real-time PCR and corrected for the internal housekeeping gene glyceraldehyde-3-phosphate dehydrogenase. The results, mean ± SEM of 6 to 8 rats/group, are expressed relative to the pairfed vehicle-treated control group, assigned a mean value of 100. *P < 0.05 versus all other groups.
Table 2. Effect of 8-day GH treatment and CRF on skeletal muscle CIS, SOCS-1, SOCS-2, and SOCS-3 mRNA levelsa
GH-Mediated JAK2/STAT5 Signal Transduction
The GHR, JAK2, and STAT5 protein levels were similar in thegastrocnemius muscle of CRF and PF control groups (Figure 2A).In the V-treated rats, tyrosine phosphorylation of these proteinswas negligible or absent, whereas 15 min after an intravenousbolus of bGH, 3 µg/100 g body wt, tyrosine phosphorylationof these proteins was marked (Figure 2A). Of note, however,in the CRF group, phosphorylation of the GHR, JAK2, and STAT5proteins was depressed significantly and to a similar extent(40 to 48%), compared with the control values (Figure 2, B and C).We then set out to determine whether the translocation ofphospho-STAT5 into the nucleus was also reduced. Phospho-STAT5levels were measured in nuclear extracts obtained 15 min afterGH administration. As shown in Figure 3, the nuclear phospho-STAT5levels were significantly lower in the CRF group (68 ±11 versus 100 ± 8 relative arbitrary units; P < 0.01).It is interesting that there was a significant (P < 0.05)linear relationship between the serum creatinine and the relativephospho-STAT5 levels in muscle and the relative phospho-JAK2and relative phospho-STAT5 and phospho-GHR levels (Figure 4).This illustrates the interrelationship between the differentsignaling molecules and suggests that the signaling defect arisesat the level of JAK2 phosphorylation and furthermore that itis influenced by the degree of renal failure. Finally, to determinewhether the resistance to GH can be overcome by a supramaximaldose of GH, we measured STAT5 phosphorylation in gastrocnemiusmuscle collected 10 min after an intravenous bolus of bGH 25µg/100 g. At this GH dose, relative phospho-STAT5 levelswere similar in the CRF and control groups (107 ± 4 versus100 ± 5 relative arbitrary units, respectively; Figure 5).
Figure 2. GH receptor (GHR), Janus-associated kinase 2 (JAK2), and signal transducers and activators of transcription 5 (STAT5) protein levels are unchanged in skeletal muscle of CRF rats, but GH-induced protein tyrosine phosphorylation of these proteins is attenuated. (A) Western immunoblots of GHR, JAK2, and STAT5 protein levels in lysates prepared from the gastrocnemius muscle of CRF and control pairfed (Con) rats obtained 15 min after an intravenous bolus of GH (3 µg/100 g) or vehicle. The samples from vehicle-treated animals were pooled for this immunoblot. (B) Tyrosine-phosphorylated GHR, JAK2, and STAT5 levels in the same samples as shown in A. The phospho-GHR and phospho-JAK2 were detected in immunoblots of GHR and JAK2 immunoprecipitates with antiphosphotyrosine antibody (clone 4G10). Phospho-STAT5 was detected without immunoprecipitation using a phospho-STAT5 specific antibody (n = 5 to 8 rats /group). (C) Relative phosphorylation of GHR, JAK2, and STAT5 proteins 15 min after bGH bolus in CRF and control animals. *P < 0.05. Phospho-protein signals were corrected for the specific protein levels, and the ratios were normalized to the pairfed control mean, which was assigned a value of 100. Bars indicate mean ± SEM.
Figure 3. Decreased nuclear accumulation of phosphorylated STAT5 in skeletal muscle of rats with CRF 15 min after GH administration. bGH (3 µg/100 g) was given intravenously as a bolus. Western immunoblots of nuclear protein extracts were performed with phosphotyrosine-specific antibodies against phospho-STAT5.
Figure 4. (A) Relationship between relative phospho-STAT5 and the serum creatinine level. (B) Relationship between relative phospho-STAT5 and relative phospho-JAK2 levels. (C) Relationship between relative phospho-GHR level and relative phospho-JAK2 levels.
Figure 5. STAT5 phosphorylation after high-dose GH treatment. CRF and pairfed control (Con) rats were given an intravenous bolus of GH (25 µg/100 g), and the gastrocnemius muscle was collected 10 min later. Bar graphs represent p-STAT5 levels corrected for STAT5 protein levels.
Protein-Tyrosine Phosphatase Activity
Tissue tyrosine phosphoprotein levels were determined by thebalance between the rate of phosphorylation and dephosphorylation.Accordingly, to determine whether an increase of PTPase activitymight be a cause of the reduced level of GH-induced phosphorylatedproteins, we measured PTPase activity in solubilized particulatefractions, where most of the cellular PTPase activity resides(24). A significant increase in the PTPase activity in uremicskeletal muscle was evident (Figure 6). The dephosphorylationof two synthesized phosphopeptide substrates, END(pY)INASL andDADE(pY)LIPQQG, acted upon by a broad array of tyrosine phosphatases,was increased by twofold (P < 0.05).
Figure 6. Tyrosine phosphatase activity is increased in skeletal muscle of CRF rats. As described in Materials and Methods, tyrosine phosphatase activity in the particulate fraction prepared from skeletal muscle was assayed with two synthesized phosphopeptides, END(pY)INASL (type I) and DADE(pY)LIPQQG (type II), as substrates. These are acted upon by a broad array of tyrosine phosphatases.
Skeletal muscle wasting and weakness are common in patientswith advanced renal failure, and as many of these patients areelderly and inactive, the changes induced by renal failure canbe especially disabling (26,27). Contributing to the musclewasting is resistance to the action of GH (28), and in thisstudy, we set out to define the mechanisms that account forthe development of GH resistance in renal failure. We confirmedthat in rats with surgically induced CRF, there is indeed skeletalmuscle resistance to the action of GH, for GH-induced IGF-1gene expression was depressed significantly. Because most ofthe actions of GH are mediated through IGF-1, a potently anabolichormone essential for maintaining normal muscle mass (29), attenuatedGH-induced skeletal muscle IGF-1 expression may be one way wherebyGH resistance contributes to uremic muscle wasting. We thenstudied the effect of uremia on GH-mediated JAK2-STAT5 signaltransduction as this pathway mediates IGF-1 transcription andbody growth (16–19). We found that despite normal GHR,JAK2, and STAT5 protein levels, activation of these proteins,as measured by the level of protein tyrosine phosphorylation,was impaired by nearly half. Nuclear translocation of the phosphorylatedSTAT5 was also depressed significantly, and as this would beexpected to reduce gene transcription, it provides an explanationfor the failure of GH to stimulate IGF-1 gene expression.
Two potential mechanisms that account for the attenuated GH-mediatedJAK2-STAT5 signal transduction were uncovered in this study.First, the basal mRNA level of the intracellular feedback regulatoryprotein SOCS2 and CIS were elevated by 55% in skeletal muscleof CRF animals, which did not reach statistical significance,whereas after treatment with GH, there was an exaggerated increasein SOCS2 mRNA level. SOCS proteins are a family of cytosolicproteins that are induced by cytokines and serve as negativefeedback regulators of the JAK-STAT pathway by binding to cytokinereceptor-JAK signaling complexes (20,30). Among the membersof the SOCS family of proteins, GH stimulates SOCS-1, -2, and-3 and CIS expression. These GH-inducible proteins partiallyor completely inhibit GH-dependent JAK2 activation. In SOCS2-deletedmice, GH-mediated STAT5 activation is enhanced and the animalsgrow larger than the wild-type mice (31). Proinflammatory cytokinesalso stimulate SOCS expression and in this way can inhibit GHaction (32,33). Because the serum CRP levels were elevated significantlyin the CRF animals, the increase in SOCS2 and CIS expressionmay have arisen in part because of heightened inflammatory cytokineactivity (20). This may be relevant to the patient with ESRD(34), for there is evidence that chronic inflammation is presentin 30 to 60% of all ESRD patients (5). However, it is unclearhow GH causes an exaggerated increase in SOCS2 expression inCRF when GH-activated JAK2-STAT5 signal transduction is impaired.One possibility is that this response is induced through a non-STAT–mediatedsignaling pathway.
The other key step in the regulation of JAK2/STAT5 that maybe altered in the skeletal muscle of uremic rats is dephosphorylationof the signaling molecules by protein tyrosine phosphatases;skeletal PTPase activity was increased significantly by twofoldin the uremic rats. If this general increase of PTPase activityincludes specific PTPases responsible for the dephosphorylationof proteins in the JAK2-STAT5 signaling pathway, then this couldpotentially lead to more rapid dephosphorylation and thus inactivationof the signaling proteins (20). Several PTPases, including SHP-1,SHP-2, and PTP1B, are involved in the regulation of GH-stimulatedJAK-STAT signal transduction, and we are currently attemptingto identify whether any of these PTPases could be a cause ofthe increased activity in uremic skeletal muscle. In this context,it is interesting to note that PTPase overactivity has beenidentified as a cause of insulin resistance in obesity and type2 diabetes (24). Other regulatory processes may also be alteredin CRF, for the regulation of GH-activated JAK2-STAT signaltransduction involves multiple biochemical events that are currentlynot well defined. These include ubiquitination of the dephosphorylatedGHR leading to receptor internalization and degradation, reducedaffinity of activated STATS for their DNA binding sequencesthrough the action of protein inhibitors of activated STATS(PIAS), and cross-talk between other cytokines and GH (20,35,36).It is of interest to note that when a supramaximal dose of GHwas administered to the uremic rats, the defect in signal transductionwas overcome and the phopho-STAT5 levels increased to a similarextent in CRF and control rats. This may explain the effectivenessof the clinical use of GH in the treatment of growth retardationin uremic children and muscle wasting in adults with ESRD (9,28).
The changes in the GH signal transduction that we observed inskeletal muscle are consistent with our previous descriptionof impaired GH-mediated JAK2-STAT signal transduction in theliver of the uremic rat (11). In that study, JAK2 and STAT5phosphorylation was impaired, and there was upregulation ofGH-stimulated SOCS2 expression, but unlike the current findingsin muscle, basal SOCS3 mRNA levels were elevated. GHR phosphorylationand hepatic PTPase activity were not examined. We have alsofound that GH-mediated JAK2-STAT5 signaling is impaired in theheart of uremic rats (37). In contrast to the changes observedin skeletal muscle and liver of uremic rats, a somewhat differentresponse occurs in sepsis, a condition that also causes GH resistance.In the septic rat, GH-mediated JAK2-STAT5 signaling is impairedin liver, whereas in skeletal muscle, the signaling is intact(38).
In conclusion, it seems that skeletal muscle resistance to GHarises in uremia because of impaired JAK2-STAT5 signal transduction.This may be caused by at least two exaggerated regulatory processes.One is an increase in protein tyrosine phosphatase activitythat could potentially increase the rate of dephosphorylationand thus inactivation of the signaling proteins. The other isoverexpression of the negative regulator of GH signaling, SOCS2,and this may in part reflect a response to chronic inflammation.In turn, the attenuated GH-activated JAK2-STAT5 signal transductionpathway leads to impaired GH-stimulated IGF-1 expression, whichlikely contributes to the muscle wasting of uremia.
Acknowledgments
This study was supported by a Merit Review Grant from the ResearchService of the Department of Veterans Affairs.
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Received for publication March 6, 2004.
Accepted for publication June 24, 2004.
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Abstract
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