Cardiac resistance to growth hormone in uremia

The Role of Growth Hormone in Chronic Kidney Disease

Growth hormone (GH) has become a critical therapy for treating growth delay and failure in pediatric chronic kidney disease. Recombinant human GH treatment is safe and significantly improves height and height velocity in these growing patients and improved growth outcomes are associated with decreased morbidity and mortality as well as improved quality of life. However, the utility of recombinant human GH in adults with chronic kidney disease and end-stage renal disease for optimization of body habitus and reducing frailty remains uncertain. Semin Nephrol 41:x-xx © 2021 Elsevier Inc. All rights reserved.

Impaired renal growth hormone JAK/STAT5 signaling in chronic kidney disease

BACKGROUND

Treatment with recombinant human growth hormone (GH) is the standard therapy for short stature in children with chronic kidney disease (CKD). However, concerns have been raised on the potential renal fibrogenic effects of GH. There is no information regarding the renal GH receptor (GHR)-JAK-STAT signaling pathway in CKD.

METHODS

Subtotal nephrectomized (CKD) and pair-fed sham-operated control (C) juvenile rats were treated with subcutaneous GH or saline for 2 weeks. A single intravenous GH bolus or vehicle was provided prior to euthanasia.

RESULTS

Reduced body weight in CKD was improved with GH therapy. The remnant kidney showed glomerular hypertrophy and early interstitial fibrosis without inflammatory infiltration. Treatment of CKD rats with GH did not worsen renal function or fibrosis. Kidney GHR mRNA and protein levels were reduced and basal phosphorylation of JAK2 and STAT5 was significantly impaired. However, intravenous GH administration prior to sacrifice normalized STAT5 phosphorylation. Basal renal IL6 mRNA and phosphorylation of its downstream signaling molecule STAT3 were increased as was the product of its action, the suppressor of cytokine signaling 3 (SOCS3) mRNA.

CONCLUSIONS

Despite known unaltered circulating GH levels, remnant kidneys of uremic growth retarded juvenile rats show impaired basal signaling along the GH-activated JAK2/STAT5 signaling pathway. This may well be a consequence of the reduced GHR level and the inhibitory effect of the increase in IL-6-mediated SOCS3 expression. This renal GH insensitivity, if present in humans, may protect against the potential adverse renal effects of GH administration in CKD patients.

OPPORTUNITY™: a large-scale randomized clinical trial of growth hormone in hemodialysis patients

BACKGROUND

Adult maintenance hemodialysis (MHD) patients experience high mortality and morbidity and poor quality of life (QoL). Markers of protein-energy wasting are associated with these poor outcomes. The OPPORTUNITY™ Trial examined whether recombinant human growth hormone (hGH) reduces mortality in hypoalbuminemic MHD patients. Secondary end points were effects on number of hospitalizations, cardiovascular events, lean body mass (LBM), serum proteins, exercise capacity, QoL and adverse events.

METHODS

We performed a randomized, double-blind, placebo-controlled, multicenter multinational trial stratified for diabetic status. Clinically, stable adult MHD patients with serum albumin <4.0 g/dL were randomized to subcutaneous injections of hGH, 20 μg/kg/day, or placebo. Planned treatment duration was 24 months for 2500 patients. The trial was terminated early due to slow recruitment.

RESULTS

Seven hundred and twelve patients were randomized until trial termination; 695 patients received at least one dose of trial medication. Mean treatment duration was 20 weeks (no completers). There were no differences between groups in all-cause mortality, cardiovascular morbidity or mortality, serum albumin, LBM, physical exercise capacity or QoL. The hGH group, compared to placebo, displayed a reduction in body weight, total body fat, serum high-sensitivity C-reactive protein and possibly homocysteine and an increase in serum high-density lipoprotein-cholesterol and transferrin levels.

CONCLUSIONS

Although the OPPORTUNITY™ Trial was terminated early, treatment with hGH, compared to placebo, improved certain cardiovascular risk factors but did not reduce mortality, cardiovascular events or improve nutritional factors or QoL. The power for showing differences was substantially reduced due to the marked decrease in treatment duration and sample size.

Uremia attenuates growth hormone-stimulated insulin-like growth factor-1 expression, a process worsened by inflammation

Growth hormone (GH) resistance is common in uremia and together with resistance to insulin-like growth factor-1 (IGF-1) contributes to uremic growth retardation and muscle wasting. Previously, we found decreased GH-stimulated janus-kinase 2-signal transducers and activators of transcription 5 (STAT5) phosphorylation and nuclear translocation in uremia; however, it is unclear whether there are more distal defects. Therefore, we tested whether the binding of phosphorylated STAT5b to DNA is intact in uremia. Using uremic rats we found that in addition to impaired hepatic STAT5b phosphorylation, the binding of available phospho-STAT5b to DNA is decreased thus contributing to impaired IGF-1 gene expression. As sepsis-induced inflammation causes a loss of body protein and as Gram-negative infections are relatively common in uremia, we also characterized mechanisms in which acute inflammation might contribute to GH resistance in uremia. Endotoxin-induced inflammation markedly increased the resistance to GH-mediated STAT5b signaling, and further decreased STAT5b binding to DNA and IGF-1 gene expression. These perturbations appear to be related to increased cytokine expression. Thus, our findings indicate that hepatic resistance to GH-induced IGF-1 expression in uremia arises due to defects in STAT5b phosphorylation and its impaired binding to DNA, processes further aggravated by inflammation.

Low-dose growth hormone is cardioprotective in uremia

Growth hormone (GH) is required to maintain normal cardiac structure and function and has a positive effect on cardiac remodeling in experimental and possibly human disease. Cardiac resistance to GH develops in the uremic state, perhaps predisposing to the characteristic cardiomyopathy associated with uremia. It was hypothesized that administration of low-dosage GH may have a salutary effect on the cardiac remodeling process in uremia, but because high levels of GH have adverse cardiac effects, administration of high-dosage GH may worsen uremic cardiomyopathy. In rats with chronic renal failure, quantitative cardiac morphology revealed a decrease in total capillary length and capillary length density and an increase in mean intercapillary distance and fibroblast volume density evident. Low-dosage GH prevented these changes. Collagen and TGF-beta immunostaining, increased in chronic renal failure, were also reduced by GH, suggesting a mechanism for its salutary action. Low-dosage GH also prevented thickening of the carotid artery but did not affect aortic pathology. In contrast, high-dosage GH worsened several of these variables. These results suggest that low-dosage GH may benefit the heart and possibly the carotid arteries in chronic renal failure.

Work-induced changes in skeletal muscle IGF-1 and myostatin gene expression in uremia

Resistance to growth hormone (GH)-induced insulin-like growth factor-1 (IGF-1) gene expression contributes to uremic muscle wasting. Since exercise stimulates muscle IGF-1 expression independent of GH, we tested whether work overload (WO) could increase skeletal muscle IGF-1 expression in uremia and thus bypass the defective GH action. Furthermore, to provide insight into the mechanism of uremic wasting and the response to exercise we examined myostatin expression. Unilateral plantaris muscle WO was initiated in uremic and pairfed (PF) normal rats by ablation of a gastrocnemius tendon and adjoining part of this muscle with the contralateral plantaris as a control. Some rats were GH treated for 7 days. WO led to similar gains in plantaris weight in both groups and corrected the uremic muscle atrophy. GH increased plantaris IGF-1 mRNA >twofold in PF rats but the response in uremia was severely attenuated. WO increased the IGF-1 mRNA levels significantly in both uremic and PF groups, albeit less brisk in uremia; however, after 7 days IGF-1 mRNA levels were elevated similarly, >2-fold, in both groups. In the atrophied uremic plantaris muscle basal myostatin mRNA levels were increased significantly and normalized after an increase in WO suggesting a myostatin role in the wasting process. In the hypertrophied uremic left ventricle the basal myostatin mRNA levels were reduced and likely favor the cardiac hypertrophy. Together the findings provide insight into the mechanisms of skeletal muscle wasting in uremia and the hypertrophic response to exercise, and suggest that alterations in the balance between IGF-1 and myostatin play an important role in these processes.

Circulating growth hormone binding protein levels and mononuclear cell growth hormone receptor expression in uremia

BACKGROUND

Resistance to growth hormone (GH) in end-stage renal disease (ESRD) causes growth retardation and muscle wasting. In humans, circulating GH binding protein (GHBP), the extracellular domain of the GH receptor that is shed into the circulation and is believed to reflect tissue GH receptor levels, is reduced in uremia and suggests that cellular GH receptor levels are correspondingly reduced. If true, this could be a cause of GH resistance. We set out to establish whether serum GHBP levels reflect cellular GH receptor levels and whether changes in serum GHBP levels are related to nutritional or inflammatory status.

METHODS

GH receptor protein expression in peripheral blood mononuclear cells (PBMC) from 21 ESRD and 14 normal subjects were analyzed by fluorochrome flow cytometry.

RESULTS

The GH receptor density and percent total PBMCs expressing the GH receptor were similar in the 2 groups, and there was no difference in percent GH receptor positive T or B cells or monocytes. In contrast, serum GHBP levels were 80% lower in ESRD. GHBP levels did not correlate with serum albumin, body mass index, or muscle mass but seemed to be partly related to the log serum C-reactive protein levels.

CONCLUSIONS

Serum GHBP levels are markedly reduced in ESRD; this seems to occur independent of nutritional status and may in part be caused by inflammation. Because GH receptor expression on PBMC of ESRD and control subjects was similar, our findings argue against a reduction in GH receptor as a cause of GH resistance and the use of serum GHBP levels as a reliable marker of specific tissue GH receptor levels.

Suppressors of cytokine signaling in health and disease

Cytokines consist of a large family of secreted proteins, including pro-inflammatory agents, growth hormone and erythropoietin, that utilize the Janus kinase (JAK) signal transducer and activator of transcription (STAT) signal transduction pathway to mediate many of their key physiologic and pathologic actions. These actions include cytokine-mediated inflammation, immunoregulation, hematopoiesis and growth. The JAK-STAT pathway is regulated by several processes, among which negative feedback regulation by the suppressors of cytokine signaling (SOCS), members of a family of eight proteins, is particularly important. Each cytokine induces one or more specific SOCS proteins that in turn down-regulate the signal initiated by the cytokine. Through their impact on the cytokine-activated JAK-STAT pathway, the SOCS proteins are involved in many diseases that come to the attention of the pediatric nephrologist. For example, an increase in the expression of SOCS-2 and -3 may be a cause of growth hormone resistance and thus may contribute to the growth retardation that affects children with chronic renal failure. Because of their obvious biologic importance, the SOCS proteins have been the subject of intense research that includes the development of strategies to utilize these proteins to control cytokine-induced JAK/STAT signal transduction for therapeutic purposes.

Growth hormone resistance in uremia, a role for impaired JAK/STAT signaling

Resistance to growth hormone (GH) is a significant complication of advanced chronic renal failure. Thus while the circulating GH levels are normal or even elevated in uremia, resistance to the hormone leads to stunting of body growth in children and contributes to muscle wasting in adults. Insensitivity to GH is the consequence of multiple defects in the GH/insulin-like growth factor-1 (IGF-1) system. Expression of the GH receptor may be reduced, although this is not a consistent finding, GH activation of the Janus kinase 2-signal transducer (JAK2) and activator of transcription (STAT) signal transduction pathway is depressed and this leads to reduced IGF-1 expression, and finally there is resistance to IGF-1, a major mediator of GH action. We review these various defects with an emphasis on the GH-activated JAK2-STAT5 pathway, since this pathway is essential for normal body growth and there has been recent progress in our understanding of the perturbations that occur in uremia.