Growth of long bones in renal failure: roles of hyperparathyroidism, growth hormone and calcitriol

Recombinant growth hormone improves growth and adult height in patients with maternal inactivating GNAS mutations

BACKGROUND

Maternal inactivating GNAS mutations lead to pseudohypoparathyroidism 1A (PHP1A), newly classified as inactivating parathyroid hormone (PTH)/PTHrP-signaling disorder type 2 of maternal inheritance (iPPSD2). Patients present with resistance to PTH and other hormones, subcutaneous ossifications, brachydactyly, short stature, and early-onset obesity. They can be born small for gestational age (SGA) and may present with growth hormone (GH) deficiency. The use of recombinant human GH (rhGH) therapy has been sporadically reported, yet we lack data on the long-term efficacy and safety of rhGH, as well as on adult height.

OBJECTIVE

Our multicenter, retrospective, observational study describes growth in patients treated with rhGH in comparison with untreated iPPSD2/PHP1A controls.

METHODS

We included 190 patients, of whom 26 received rhGH. Height, weight, body mass index at various time points, and adult height were documented. We analyzed the effect of rhGH on adult height by using linear mixed models.

RESULTS

Adult height was available for 11/26 rhGH-treated individuals and for 69/164 controls. Patients treated with rhGH showed a gain in height of 0.7 standard deviation scores (SDS) after 1 year (CI +0.5 to +0.8, P < .001) and of 1.5 SDS after 3 years (CI +1.0 to +2.0, P < .001). Additionally, there was a clear beneficial impact of rhGH on adult height when compared with untreated controls, with a difference of 1.9 SDS (CI +1.1 to +2.7, P < .001). Body mass index SDS did not vary significantly upon rhGH therapy.

CONCLUSION

Recombinant human growth hormone treatment of iPPSD2/PHP1A patients with short stature improves growth and adult height. More studies are needed to confirm long-term efficacy and safety.

Growth patterns and outcomes of growth hormone therapy in patients with acrodysostosis

INTRODUCTION

Severe short stature is a feature of acrodysostosis, but data on growth are sparse. Treatment with recombinant human growth hormone (rhGH) is used in some centers to increase final height, but no studies have been published so far. Our objective was to conduct a multicenter, retrospective, cohort study to investigate growth in individuals with both types of acrodysostosis, treated with rhGH or not; we used the new nomenclature to describe acrodysostosis, as this disease belongs to the large group of inactivating PTH/PTHrP signaling disorders (iPPSD); acrodysostosis refers to iPPSD4 (acrodysostosis type 1 due to PRKAR1A mutations) and iPPSD5 (acrodysostosis type 2, due to PDE4D mutations).

METHODS

We present auxological data from individuals with genetically characterized iPPSD4, and participants with clinical features of iPPSD5.

RESULTS

We included 20 and 17 individuals with iPPSD4 and iPPSD5, respectively. The rhGH-treated iPPSD4 patients (n = 9) were smaller at birth than those who did not receive rhGH (median - 2.2 SDS vs. - 1.7 SDS); they showed a trend to catch-up growth during rhGH therapy (median 0.5 SDS in the first year). The rhGH-treated patients (n = 5) reached a better final height compared to those who did not receive rhGH (n = 4) (median - 2.8 SDS vs. - 3.9 SDS), suggesting that rhGH is efficient to increase height in those patients. The difference in target height to final height ranged between 1.6 and 3.0 SDS for iPPSD4 not treated with rhGH (n = 4), 2.1-2.8 SDS for rhGH-treated iPPSD4 (n = 5), 0.6-5.5 SDS for iPPSD5 not treated with rhGH (n = 5) and 2.5-3.1 for rhGH-treated iPPSD5 (n = 2).

CONCLUSION

Final height may be positively influenced by rhGH in patients with acrodysostosis/iPPSD. Our rhGH-treated cohort started therapy relatively late, which might explain, at least in part, the limited effect of rhGH on height.

Collagen X Biomarker (CXM), Linear Growth, and Bone Development in a Vitamin D Intervention Study in Infants

Collagen X biomarker (CXM) is suggested to be a biomarker of linear growth velocity. However, early childhood data are limited. This study examines the relationship of CXM to the linear growth rate and bone development, including the possible modifying effects of vitamin D supplementation. We analyzed a cohort of 276 term-born children participating in the Vitamin D Intervention in Infants (VIDI) study. Infants received 10 μg/d (group-10) or 30 μg/d (group-30) vitamin D₃ supplementation for the first 2 years of life. CXM and length were measured at 12 and 24 months of age. Tibial bone mineral content (BMC), volumetric bone mineral density (vBMD), cross-sectional area (CSA), polar moment of inertia (PMI), and periosteal circumference (PsC) were measured using peripheral quantitative computed tomography (pQCT) at 12 and 24 months. We calculated linear growth as length velocity (cm/year) and the growth rate in length (SD unit). The mean (SD) CXM values were 40.2 (17.4) ng/mL at 12 months and 38.1 (12.0) ng/mL at 24 months of age (p = 0.12). CXM associated with linear growth during the 2-year follow-up (p = 0.041) but not with bone (p = 0.53). Infants in group-30 in the highest tertile of CXM exhibited an accelerated mean growth rate in length compared with the intermediate tertile (mean difference [95% CI] -0.50 [-0.98, -0.01] SD unit, p = 0.044) but not in the group-10 (p = 0.062) at 12 months. Linear association of CXM and growth rate until 12 months was weak, but at 24 months CXM associated with both length velocity (B for 1 increment of √CXM [95% CI] 0.32 [0.12, 0.52] cm/yr, p = 0.002) and growth rate in length (0.20 [0.08, 0.32] SD unit, p = 0.002). To conclude, CXM may not reliably reflect linear growth from birth to 12 months of age, but its correlation with growth velocity improves during the second year of life. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Growth plate alterations in chronic kidney disease

Growth retardation is a major feature of chronic kidney disease (CKD) of onset in infants or children and is associated with increased morbidity and mortality. Several factors have been shown to play a causal role in the growth impairment of CKD. All these factors interfere with growth by disturbing the normal physiology of the growth plate of long bones. To facilitate the understanding of the pathogenesis of growth impairment in CKD, this review discusses cellular and molecular alterations of the growth plate during uremia, including structural and dynamic changes of chondrocytes, alterations in their process of maturation and hypertrophy, and disturbances in the growth hormone signaling pathway.

Cortical and trabecular bone are equally affected in rats with renal failure and secondary hyperparathyroidism

Background

Changes in mineral metabolism and bone structure develop early in the course of chronic kidney disease and at end-stage are associated with increased risk of fragility fractures. The disruption of phosphorus homeostasis leads to secondary hyperparathyroidism, a common complication of chronic kidney disease. However, the molecular pathways by which high phosphorus influences bone metabolism in the early stages of the disease are not completely understood. We investigated the effects of a high phosphorus diet on bone and mineral metabolism using a 5/6 nephrectomy model of chronic kidney disease.

Methods

Four-week old rats were randomly assigned into groups: 1) Control with standard diet, 2) Nephrectomy with standard rodent diet, and 3) Nephrectomy with high phosphorus diet. Rats underwent in vivo imaging at baseline, day 14, and day 28, followed by ex vivo imaging.

Results

Cortical bone density at the femoral mid-diaphysis was reduced in nephrectomy-control and nephrectomy-high phosphorus compared to control rats. In contrast, trabecular bone mass was reduced at both the lumbar vertebrae and the femoral secondary spongiosa in nephrectomy-high phosphorus but not in nephrectomy-control. Reduced trabecular bone volume adjusted for tissue volume was caused by changes in trabecular number and separation at day 35. Histomorphometry revealed increased bone resorption in tibial secondary spongiosa in nephrectomy-control. High phosphorus diet-induced changes in bone microstructure were accompanied by increased serum parathyroid hormone and fibroblast growth factor 23 levels.

Conclusion

Our study demonstrates that changes in mineral metabolism and hormonal dysfunction contribute to trabecular and cortical bone changes in this model of early chronic kidney disease.

Growth and Nutrition in Pediatric Chronic Kidney Disease

Children with chronic kidney disease (CKD) feature significant challenges to the maintenance of adequate nutrition and linear growth. Moreover, the impaired nutritional state contributes directly to poor growth. Therefore, it is necessary to consider nutritional status in the assessment of etiology and treatment of sub-optimal linear growth. The major causes of poor linear growth including dysregulation of the growth hormone/insulin-like growth factor-I (IGF-I) axis, nutritional deficiency, metabolic acidosis, anemia, renal osteodystrophy/bone mineral disease, and inflammation. This review summarizes the causes and assessment tools of growth and nutrition while providing a summary of state of the art therapies for these co-morbidities of pediatric CKD.

Chronic kidney disease in children

Chronic kidney disease (CKD) is a major health problem worldwide. Although relatively uncommon in children, it can be a devastating illness with many long-term consequences. CKD presents unique features in childhood and may be considered, at least in part, as a stand-alone nosologic entity. Moreover, some typical features of paediatric CKD, such as the disease aetiology or cardiovascular complications, will not only influence the child's health, but also have long-term impact on the life of the adult that they will become. In this review we will focus on the unique issues of paediatric CKD, in terms of aetiology, clinical features and treatment. In addition, we will discuss factors related to CKD that start during childhood and require appropriate treatments in order to optimize health outcomes and transition to nephrologist management in adult life.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

Endurance exercise and growth hormone improve bone formation in young and growth-retarded chronic kidney disease rats

BACKGROUND

Childhood chronic kidney disease (CKD) is associated with both short stature and abnormal bone mineralization. Normal longitudinal growth depends on proper maturation of epiphyseal growth plate (EGP) chondrocytes, leading to the formation of trabecular bone in the primary ossification centre. We have recently shown that linear growth impairment in CKD is associated with impaired EGP growth hormone (GH) receptor signalling and that exercise improved insulin-like growth factor I (IGF-I) signalling in CKD-related muscle atrophy.

METHODS

In this study, 20-day-old rats underwent 5/6 nephrectomy (CKD) or sham surgery (C) and were exercised with treadmill, with or without GH supplementation.

RESULTS

CKD-related growth retardation was associated with a widened EGP hypertrophic zone. This was not fully corrected by exercise (except for tibial length). Exercise in CKD improved the expression of EGP key factors of endochondral ossification such as IGF-I, vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and osteocalcin. Combining GH treatment with treadmill exercise for 2 weeks improved the decreased trabecular bone volume in CKD, as well as the expression of growth plate runt-related transcription factor 2, RANKL, metalloproteinase 13 and VEGF, while GH treatment alone could not do that.

CONCLUSIONS

Treadmill exercise improves tibial bone linear growth, as well as growth plate local IGF-I. When combined with GH treatment, running exercise shows beneficial effects on trabecular bone formation, suggesting the potential benefit of this combination for CKD-related short stature and bone disease.

Defective skeletal mineralization in pediatric CKD

Although traditional diagnosis and treatment of renal osteodystrophy focused on changes in bone turnover, current data demonstrate that abnormalities in skeletal mineralization are also prevalent in pediatric chronic kidney disease (CKD) and likely contribute to skeletal morbidities that continue to plague this population. It is now clear that alterations in osteocyte biology, manifested by changes in osteocytic protein expression, occur in early CKD before abnormalities in traditional measures of mineral metabolism are apparent and may contribute to defective skeletal mineralization. Current treatment paradigms advocate the use of 1,25(OH)2vitamin D for the control of secondary hyperparathyroidism; however, these agents fail to correct defective skeletal mineralization and may exacerbate already altered osteocyte biology. Further studies are critically needed to identify the initial trigger for abnormalities of skeletal mineralization as well as the potential effects that current therapeutic options may have on osteocyte biology and bone mineralization.

Immune, metabolic and epidemiological aspects of vitamin D in chronic kidney disease and transplant patients

Chronic kidney disease strongly impacts on mineral and bone metabolism. Despite numerous medications, the biological targets recommended by international guidelines are often unmet. Among the treatment armamentarium, native or nutritional vitamin D (25OHD3) has been rediscovered in the early 2000s, and its general and specific actions further studied. Effects on bone, immunity, infection prevention, muscle function and phosphocalcic metabolism have been reviewed. Assessment of nutritional vitamin D status showed very low serum 25OHD3 levels and increase in nutritional vitamin D prescription led to improvement in these levels. However, about 45% of adult CKD patients still have insufficient serum 25OHD3 levels. Epidemiological studies should be enforced to describe further the mineral and bone disease management in CKD.

The consequences of chronic kidney disease on bone metabolism and growth in children

Growth retardation, decreased final height and renal osteodystrophy (ROD) are common complications of childhood chronic kidney disease (CKD), resulting from a combination of abnormalities in the growth hormone (GH) axis, vitamin D deficiency, hyperparathyroidism, hypogonadism, inadequate nutrition, cachexia and drug toxicity. The impact of CKD-associated bone and mineral disorders (CKD-MBD) may be immediate (serum phosphate/calcium disequilibrium) or delayed (poor growth, ROD, fractures, vascular calcifications, increased morbidity and mortality). In 2012, the clinical management of CKD-MBD in children needs to focus on three main objectives: (i) to provide an optimal growth in order to maximize the final height with an early management with recombinant GH therapy when required, (ii) to equilibrate calcium/phosphate metabolism so as to obtain acceptable bone quality and cardiovascular status and (iii) to correct all metabolic and clinical abnormalities that can worsen bone disease, growth and cardiovascular disease, i.e. metabolic acidosis, anaemia, malnutrition and 25(OH)vitamin D deficiency. The aim of this review is to provide an overview of the mineral, bone and vascular abnormalities associated with CKD in children in terms of pathophysiology, diagnosis and clinical management.

Parathyroid hormone and growth in chronic kidney disease

Growth failure is common in children with chronic kidney disease, and successful treatment is a major challenge in the management of these children. The aetiology is multi-factorial with "chronic kidney disease-metabolic bone disorder" being a key component that is particularly difficult to manage. Parathyroid hormone is at the centre of this mineral imbalance, consequent skeletal disease and, ultimately, growth failure. When other aetiologies are treated, good growth can be achieved throughout the course of the disease when parathyroid hormone (PTH) levels are in the normal range or slightly elevated. A direct correlation between PTH levels and growth has not been convincingly established, and the direct effect of PTH on growth has not been adequately described; furthermore, direct actions of PTH on the growth plate are unproven. The effects of PTH on growth stem from the pivotal role that PTH plays in the development of renal osteodystrophy. In severe secondary hyperparathyroidism, the growth plate is altered and growth is affected. At the other end of the spectrum, with an over-suppressed parathyroid gland, the rate of bone turnover and remodelling is markedly diminished, and some data suggest this is associated with poor growth. Most of the data available suggests that avoiding the development of significant bone disease through the strict control of PTH levels permits good growth. Absolute optimal ranges for PTH that maximise growth or minimise growth failure are not yet established.

Trabecular bone volume and osteoprotegerin expression in uremic rats given high calcium

Calcium (Ca)-containing phosphate binders have been recommended for the treatment of hyperphosphatemia in children with chronic kidney disease. To study the effects of high Ca levels on trabecular bone volume (BV) and osteoprotegerin (OPG) expression in uremic young rats, a model of marked overcorrection of secondary hyperparathyroidism was created by providing a diet of high Ca to 5/6 nephrectomized young rats (Nx-Ca) for 4 weeks. The results of chondrocyte proliferation and apoptosis, osteoclastic activity, OPG expression and BV were compared among intact rats given the control diet, intact rats given a high Ca diet and 5/6 nephrectomized rats given the control diet (Nx-Control) and the high Ca diet (Nx-Ca). Ionized Ca levels were higher and parathyroid hormone levels were lower in Nx-Ca rats than in the other groups. Final weight, final length and final tibial length of Nx-Ca rats were significantly less than those of the other groups, although the length gain did not differ among the groups. The hypertrophic zone width was markedly enlarged in Nx-Ca rats. Chondrocyte proliferation rates did not differ among the groups, whereas osteoclastic activity was decreased in Nx-Ca rats compared with the Nx-Control animals. The OPG expression and BV were increased in Nx-Ca rats compared with the Nx-Control rats. Increased BV should improve bone strength, whereas disturbance of osteoclastogenesis interferes with bone remodeling. Bone quality has yet to be determined in high Ca-fed uremic young rats.

Bone growth during rapamycin therapy in young rats

BACKGROUND

Rapamycin is an effective immunosuppressant widely used to maintain the renal allograft in pediatric patients. Linear growth may be adversely affected in young children since rapamycin has potent anti-proliferative and anti-angiogenic properties.

METHODS

Weanling three week old rats were given rapamycin at 2.5 mg/kg daily by gavage for 2 or 4 weeks and compared to a Control group given equivalent amount of saline. Morphometric measurements and biochemical determinations for serum calcium, phosphate, iPTH, urea nitrogen, creatinine and insulin-growth factor I (IGF-I) were obtained. Histomorphometric analysis of the growth plate cartilage, in-situ hybridization experiments and immunohistochemical studies for various proteins were performed to evaluate for chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption.

RESULTS

At the end of the 2 weeks, body and tibia length measurements were shorter after rapamycin therapy associated with an enlargement of the hypertrophic zone in the growth plate cartilage. There was a decrease in chondrocyte proliferation assessed by histone-4 and mammalian target of rapamycin (mTOR) expression. A reduction in parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) and an increase in Indian hedgehog (Ihh) expression may explain in part, the increase number of hypertrophic chondrocytes. The number of TRAP positive multinucleated chondro/osteoclasts declined in the chondro-osseous junction with a decrease in the receptor activator of nuclear factor kappa beta ligand (RANKL) and vascular endothelial growth factor (VEGF) expression. Although body and tibial length remained short after 4 weeks of rapamycin, changes in the expression of chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption which were significant after 2 weeks of rapamycin improved at the end of 4 weeks.

CONCLUSION

When given to young rats, 2 weeks of rapamycin significantly decreased endochondral bone growth. No catch-up growth was demonstrated at the end of 4 weeks, although markers of chondrocyte proliferation and differentiation improved. Clinical studies need to be done to evaluate these changes in growing children.

Bone histomorphometry in children prior to commencing renal replacement therapy

Renal osteodystrophy (ROD) develops early in the course of chronic kidney disease (CKD). With improving patient survival it's importance and relevance has increased. The last published bone biopsy data in children prior to renal replacement therapy (RRT) was in 1982, which demonstrated abnormal histology in all patients with a glomerular filtration rate (GFR) <20 ml/min per 1.73 m(2). Studies investigating the relationship between bone histology and parathyroid hormone levels (PTH) and/or growth in children with CKD are few (seven). These were mostly undertaken in patients already initiated on RRT-dialysis. We investigated the presence of ROD in children at the commencement of RRT and to investigate any relationship between histology, growth and PTH levels. Following double tetracycline labelling, bone biopsies were taken from patients at the time of RRT surgery. Histological classification was based on the newly proposed turnover/mineralisation/volume (TMV) system. Eleven patients underwent bone biopsy. Patients were followed for an average of 1.1 years (0.5-1.8) prior to biopsy over an average of eight clinic visits (3-14), when routine biochemical data were collected. Time-integrated median calcium, phosphate and PTH levels were calculated. PTH levels were within the normal range in two patients with low turnover, 1.1-1.4 times the upper limit of normal (ULN) in three with mixed osteodystrophy and >2.9 times the ULN in four patients with high bone turnover. There was no relationship between bone turnover and growth. The presence of ROD was universal in these children with severe CKD. Low bone turnover was associated with normal-range mean PTH levels, and high bone turnover occurred at lower PTH levels than current guidelines would suggest.

Cinacalcet does not affect longitudinal growth but increases body weight gain in experimental uraemia

BACKGROUND

Cinacalcet (CIN) efficiently suppresses parathyroid hormone (PTH) secretion by the activation of the calcium-sensing receptor (CaR). Epiphyseal chondrocytes also express the CaR and its activation promotes cell proliferation and differentiation in vitro. Hence, the impact of CIN on the growth plate function requires assessment before routine administration in children.

METHODS

We treated subtotally nephrectomized (SNX) and sham-operated, ad lib and pair-fed Sprague-Dawley rats with CIN (15 mg/kg day) or solvent (S) for 14 days p.o. and assessed whole body and tibia length gain, growth plate morphology, osseous front advance (OFA) (calcein staining) and chondrocyte proliferation rate [5-bromo-2'-deoxyuridine (BrdU) staining].

RESULTS

Total body length gain did not differ after 7 and 14 days (SNX + CIN 2.9 +/- 0.6, SNX + S 3.0 +/- 0.7; sham + CIN 4.2 +/- 0.4, sham + S 4.5 +/- 0.4; sham pair-fed + CIN 3.3 +/- 0.5, sham pair-fed + S 3.5 +/- 0.6 cm/14 days; P = n.s.). Tibia length, the height of the total growth plate and the hypertrophic zone, OFA and chondrocyte proliferation rate were similar with CIN and S. Serum Ca(2+) declined with CIN treatment; PTH was 61% lower in CIN- compared to S-treated SNX (P < 0.05). Food intake was similar, whereas body weight gain (21.6 +/- 8.7 versus 12.7 +/- 11.2 g) and body weight gain per food intake (141 +/- 50 versus 77 +/- 70 g/kg) improved in CIN- versus S-treated SNX animals (P < 0.05).

CONCLUSION

CIN treatment does not impact on growth plate chondrocyte function in uraemic rats, but improves food efficiency and body weight gain.

Chronic kidney disease mineral and bone disorder in children

Childhood and adolescence are crucial times for the development of a healthy skeletal and cardiovascular system. Disordered mineral and bone metabolism accompany chronic kidney disease (CKD) and present significant obstacles to optimal bone strength, final adult height, and cardiovascular health. Decreased activity of renal 1 alpha hydroxylase results in decreased intestinal calcium absorption, increased serum parathyroid hormone levels, and high-turnover renal osteodystrophy, with subsequent growth failure. Simultaneously, phosphorus retention exacerbates secondary hyperparathyroidism, and elevated levels contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves growth and high-turnover bone disease. However, target ranges for serum calcium, phosphorus, and parathyroid hormone (PTH) levels vary according to stage of CKD. Since over-treatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy must be carefully adjusted to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents, including calcium-free phosphate binding agents and new vitamin D analogues, effectively suppress serum PTH levels while limiting intestinal calcium absorption and may provide future therapeutic alternatives for children with CKD.

Growth of prepubertal children on dialysis

Growth failure is a common and significant clinical problem for children on dialysis and often remains a major impediment to their rehabilitation. Early referral to a paediatric nephrology centre and appropriate management before the initiation of dialysis may significantly prevent growth deterioration. Growth in children on dialysis can be affected by nutritional, metabolic, and hormonal changes. Early diagnosis of malnutrition and aggressive management should be a priority. Gastrostomy feeding should be used when adequate oral intake to maintain normal height and weight velocity cannot be achieved. Active vitamin D metabolites should be used carefully, to prevent low-turnover bone disease. All children should have an adequate regimen of dialysis and an appropriate management of malnutrition, renal osteodystrophy, metabolic acidosis, salt wasting and anaemia, before recombinant human growth hormone (rhGH) administration is considered. The current challenge of reversing growth impairment in children on dialysis can only be achieved by optimization of their care.

Bone growth during daily or intermittent calcitriol treatment during renal failure with advanced secondary hyperparathyroidism

Calcitriol is a standard therapy for secondary hyperparathyroidism in chronic renal failure. We evaluated whether the effect of daily or intermittent calcitriol administration is more efficient in enhancing bone growth in renal failure with advanced secondary hyperparathyroidism in weanling 5/6 nephrectomized rats loaded with phosphorus to induce severe secondary hyperparathyroidism. The animals were treated daily or three times weekly with calcitriol for 4 weeks but the total weekly dose of calcitriol was the same. Although calcitriol increased the serum calcium, it did not lower parathyroid hormone (PTH) or improve tibia and body length. Animals with renal failure and advanced secondary hyperparathyroidism had decreased PTH/PTHrP, which was accompanied by an increase in the cyclin kinase inhibitor p57(Kip2). Calcitriol treatment upregulated the PTH/PTHrP receptor but also increased inhibitors of cell proliferation such as p21(Waf1/Cip1), IGFBP3, and FGFR3. Calcitriol also enhanced markers of chondrocyte differentiation, such as IGF1, Vitamin D receptor, FGF23, and bone morphogenetic protein-7. Receptor activator of nuclear factor-kappabeta ligand levels improved with calcitriol treatment but without changes in osteoprotegerin suggesting an enhancement of osteo/chondroclastogenesis and mineralization. Overall, both daily and intermittent calcitriol had similar effects on endochondral bone growth in phosphorus-loaded rats with renal failure.

Response of the growth plate of uremic rats to human growth hormone and corticosteroids

Children with chronic renal failure in general present growth retardation that is aggravated by corticosteroids. We describe here the effects of methylprednisolone (MP) and recombinant human growth hormone (rhGH) on the growth plate (GP) of uremic rats. Uremia was induced by subtotal nephrectomy in 30-day-old rats, followed by 20 IU kg-1 day-1 rhGH (N = 7) or 3 mg kg-1 day-1 MP (N = 7) or 20 IU kg-1 day-1 rhGH + 3 mg kg-1 day-1 MP (N = 7) treatment for 10 days. Control rats with intact renal function were sham-operated and treated with 3 mg kg-1 day-1 MP (N = 7) or vehicle (N = 7). Uremic rats (N = 7) were used as untreated control animals. Structural alterations in the GP and the expression of anti-proliferating cell nuclear antigen (PCNA) and anti-insulin-like growth factor I (IGF-I) by epiphyseal chondrocytes were evaluated. Uremic MP rats displayed a reduction in the proliferative zone height (59.08 +/- 4.54 vs 68.07 +/- 7.5 microm, P < 0.05) and modifications in the microarchitecture of the GP. MP and uremia had an additive inhibitory effect on the proliferative activity of GP chondrocytes, lowering the expression of PCNA (19.48 +/- 11.13 vs 68.64 +/- 7.9% in control, P < 0.0005) and IGF-I (58.53 +/- 0.96 vs 84.78 +/- 2.93% in control, P < 0.0001), that was counteracted by rhGH. These findings suggest that in uremic rats rhGH therapy improves longitudinal growth by increasing IGF-I synthesis in the GP and by stimulating chondrocyte proliferation.

Quantitative Microcomputed Tomography Assessment of Intratrabecular, Intertrabecular, and Cortical Bone Architecture in a Rat Model of Severe Renal Osteodystrophy

OBJECTIVE

To determine the effects of renal osteodystrophy (ROD) on bone microarchitecture in growing rats.

METHODS

A total of 24 rats underwent 5/6 nephrectomy (NX) and were fed a high-phosphorus diet to induce ROD; another 6 underwent sham NX. In vitro microcomputed tomography images (GEMS, London, Ontario, Canada) were obtained in the femoral metaphysis and midshaft.

RESULTS

Trabecular and cortical bone volume/total volume (BV/TV) were significantly lower in NX specimens because of pores within the trabeculae and along the endosteal surface. Topological analysis using component labeling in 3-dimensions verified that trabecular pores connected to the marrow space. After the trabecular pores were filled using a morphological filter, trabecular thickness was significantly increased in NX. In contrast, cortical thickness was significantly decreased in NX compared with controls; however, after filling the endocortical pores, thickness did not differ.

CONCLUSIONS

The ROD resulted in decreased cortical and trabecular BV/TV, increased porosity, and increased trabecular thickness. Advanced image processing algorithms demonstrated the effects of cortical and trabecular porosity on BV/TV and structure in ROD.

Catch-up growth follows an abnormal pattern in experimental renal insufficiency and growth hormone treatment normalizes it

The primary goal of this study was to determine if the ability to undergo catch-up growth following a transient injury is preserved in an experimental model of moderate chronic renal failure (CRF) and the effect of growth hormone (GH) administration on such phenomenon. Young rats were subtotally nephrectomized (days 0 and 4) (Nx). From days 11 to 13, food intake was restricted in subgroups of Nx and control (C) rats (NxR and CR). After refeeding, subgroups of NxR and CR rats received GH from days 14 to 20 (NxRGH and CRGH). Rats were killed on days 14 (C, CR, Nx, NxR), 17 and 21 (C, CR, CRGH, Nx, NxR, NxRGH), and 36 (C, CR, Nx, NxR). Longitudinal growth rate was measured by osseous front advance in the proximal tibiae. With refeeding, growth rate of CR, NxR, and NXrGH rats became significantly greater than that of C, indicating catch-up growth. This occurred later and with lower growth rate in NxR than in CR rats, whereas the characteristics of catch-up growth in CR and NxRGH animals were similar. Changes in growth rate were associated with modifications in the morphology and proliferative activity of growth cartilage. We conclude that catch-up growth occurs in renal insufficiency but follows a different pattern from that observed with normal renal function. GH treatment normalizes the pattern of catch-up growth in CRF. Changes in growth velocity are associated to modifications in the structure and dynamics of growth cartilage.

Impaired growth plate chondrogenesis in children with chronic illnesses

In mammals, statural growth is primarily accomplished by endochondral ossification, which takes place at the growth plate. Growth plate chondrocyte proliferation, hypertrophy/differentiation, apoptosis, and cartilage matrix synthesis all contribute to chondrogenesis or cartilage formation, a process tightly coupled to the simultaneous remodeling of the cartilage into bone at the metaphyseal border of the growth plate. Growth plate chondrogenesis is regulated by the complex interaction of molecular signals acting systemically as well locally within the growth plate. This network is often dysregulated during chronic illnesses, thus resulting in impaired growth plate chondrogenesis and, in turn, growth failure. The principal events responsible for altered growth plate chondrogenesis in chronic illness are inflammation, protein/calorie deprivation, uremia/metabolic acidosis, glucocorticoids, and impaired GH/IGF-I axis.

Chronic acidosis-induced growth retardation is mediated by proton-induced expression of Gs protein

The etiology of skeletal growth retardation accompanying metabolic acidosis is not clear. Using ex vivo models for endochondral ossification, we showed that the cAMP/PKA pathway, probably triggered by proton sensitive G-protein-coupled receptors, is responsible for impaired skeletal growth in acidosis.

INTRODUCTION

Chronic metabolic acidosis (CMA) is very often accompanied by skeletal growth retardation. We have previously shown in an ex vivo model of endochondral ossification that murine mandibular condyles subjected to acidic conditions exhibit growth retardation accompanied by a decline of insulin-like growth factor-I (IGF-I) and its receptors. PTH-induced ameliorative effects on the CMA-induced growth retardation of the mandibular condyle are partially mediated by protein kinase C (PKC). In this study we explored the mechanisms underlying the acidosis-induced growth retardation; in particular, the involvement of the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) cellular pathway in the process.

MATERIALS AND METHODS

Mandibular condyles from neonatal mice or mandibular condyle derived chondrocytes (MCDCs) were incubated for 3 days under either control or acidic conditions or in the presence of cAMP-regulating factors (cAMPrf) such as forskolin, iso-butyl methyl xanthine (IBMX), or 8-Br cAMP. The effects on proliferation and differentiation of the cultures as well as on phosphorylation of cAMP responsive element binding protein (CREB) and increased expression of the alpha subunit, Gs were determined. The intracellular pH was detected using the acridine orange assay.

RESULTS

Our results show that, under acidic conditions, PKA levels were increased. H89 abolished the adverse effects of acidosis on condylar development and restored IGF-I and IGF-I receptors (IGF-IR) levels. The inhibitory effects of acidosis on proliferation and differentiation of cartilaginous cells were mimicked by cAMPrf. We have also shown that acidosis stimulates activation of Gs trimeric protein and CREB phosphorylation. GDPbetaS--a Gs antagonist--abolished the acidosis-induced condylar growth arrest. Using an acridine orange assay, we showed that the intracellular environment is not acidified under acidic conditions.

CONCLUSIONS

Our results indicate that the adverse effects of acidosis on skeletal growth centers are mediated at least in part by the cAMP/PKA cellular pathway. We speculate that high proton concentrations exerted by acidosis conditions stimulate proton sensitive G-protein-coupled receptors, which are mediated by the cellular cAMP/PKA pathway and induce skeletal growth retardation.

Alterations of the growth plate in chronic renal failure

Chronic renal failure modifies the morphology and dynamics of the growth plate (GP) of long bones. In young uremic rats, the height of cartilage columns of GP may vary markedly. The reasons for this variation are unknown, although the severity and duration of renal failure and the type of renal osteodystrophy have been shown to influence the height of GP cartilage. Expansion of GP cartilage is associated with that of the hypertrophic stratum. The interference of uremia with the process of chondrocyte differentiation is suggested by some morphological features. However, analysis by immunohistochemistry and/or in situ hybridization of markers of chondrocyte maturation in the GP of uremic rats has yielded conflicting results. Thus, there have been reported normal and reduced mRNA levels for collagen X, parathyroid hormone/parathyroid hormone-related peptide receptor, and matrix metalloproteinase 9, as well as normal mRNA and protein expression for vascular endothelial growth factor and chondromodulin I, peptides related to the control of angiogenesis. In addition, a decreased immunohistochemical signal for growth hormone receptor and low insulin-like growth factor I mRNA in the proliferative zone of uremic GP are supportive of reduced chondrocyte proliferation. Growth hormone treatment improves chondrocyte maturation and activates bone metabolism in the primary spongiosa.

Daily or intermittent calcitriol administration during growth hormone therapy in rats with renal failure and advanced secondary hyperparathyroidism

Growth hormone (GH) improves growth in children with chronic renal failure. The response to GH may be affected by the degree of secondary hyperparathyroidism and concurrent treatment with vitamin D. Forty-six rats underwent 5/6 nephrectomy (Nx) and were given a high-phosphorus diet (Nx-Phos) to induce advanced secondary hyperparathyroidism and divided into the following groups: (1) Nx-Phos (n = 10) received saline, (2) GH at 10 IU/kg per d (Nx-Phos+GH; n = 9), (3) GH and daily calcitriol (D) at 50 ng/kg per d (Nx-Phos+GH+daily D; n = 8), (4) GH and intermittent D (three times weekly) at 350 ng/kg per wk (Nx-Phos+GH+int D; n = 9), and (5) intact-control (n = 10). Serum parathyroid hormone (PTH) levels were elevated in Nx-Phos, but IGF-I levels did not change with growth hormone. Body length, tibial length, and growth plate width did not increase with either GH or calcitriol. Proliferating cell nuclear antigen staining, PTH/PTHrP receptor, bone morphogenetic protein-7, and fibroblast growth factor receptor-3 expression increased with GH alone or with intermittent calcitriol but were slightly diminished during daily calcitriol administration. GH enhanced IGF-I, IGF binding receptor-3, and GH receptor but declined with daily and intermittent calcitriol. Overall, there was no improvement in body length, tibial length, and growth plate width at the end of GH therapy, but selected markers of chondrocyte proliferation and chondrocyte differentiation increased, although these changes were attenuated by calcitriol. The combination of GH and calcitriol that is frequently used in children with renal failure and secondary hyperparathyroidism require further studies to evaluate the optimal dose and frequency of administration to increase linear growth and prevent bone disease.

Bone elongation in rats with renal failure and mild or advanced secondary hyperparathyroidism

BACKGROUND

Impairment of growth in children with chronic renal failure may be due, in part to the insensitivity to the actions of growth hormone by insulin-like growth factor-I (IGF-I) because of accumulations of IGF binding proteins. There are a few studies describing the changes that occur in the growth plate in renal failure. None of these studies has simultaneously compared the modifications in the expression of selected markers of endochondral bone formation in renal failure with mild or advanced secondary hyperparathyroidism.

METHODS

Forty-six rats that underwent 5/6 nephrectomy (Nx) were fed either standard rodent diet (Nx-control) or high phosphorus diet to induce advanced secondary hyperparathyroidism (Nx-phosphorus) for 4 weeks. Sections of the tibia were obtained for growth plate histomorphometry, immunohistochemistry studies, and in situ hybridization experiments for selected markers of endochondral bone formation.

RESULTS

Weight gain, gain in length, and tibial length were less in Nx animals. Serum parathyroid hormone (PTH) and phosphorus levels were higher and serum calcium levels were lower in the Nx-phosphorus group. The width of the growth plate was much shorter in the Nx-phosphorus group due to a decrease in both proliferative and hypertrophic zones. IGF-I protein and IGF binding protein-3 staining were diminished in both Nx groups without changes in the IGF-I receptor expression; the decline in IGF-I protein expression was much lower in the Nx-phosphorus group. PTH/PTH receptor protein (PTHrP) receptor mRNA transcripts decline and tartrate-resistant acid phosphastase (TRAP) staining increased only in the Nx-phosphorus group.

CONCLUSION

The growth impairment in renal failure may be worsened by the severity of secondary hyperparathyroidism.

Chondromodulin-I expression in the growth plate of young uremic rats

BACKGROUND

Growth retardation of chronic renal failure is associated with alterations in the growth plate suggestive of a disturbed chondrocyte maturation process and abnormal vascular invasion at the chondro-osseous interphase. Chondromodulin I (ChM-I) is a potent cartilage-specific angiostatic factor. Its pattern of expression in the uremic rat growth plate is unknown. Persistence of ChM-I synthesis and/or imbalance between ChM-I and vascular endothelial growth factor (VEGF) expressions might play a role in the alterations of uremic growth plate.

METHODS

Growth cartilage ChM-I expression was investigated by immunohistochemistry, in situ hybridization, and reverse transcription-polymerase chain reaction (RT-PCR) in growth-retarded young uremic rats (UREM), control rats, fed ad libitum (SAL) or pair-fed with the UREM group (SPF), and uremic rats treated with growth hormone (UREM-GH). VEGF expression was analyzed by immunohistochemistry.

RESULTS

ChM-I and ChM-I mRNA were confined to the proliferative and early hypertrophic zones of growth cartilage. A similar number of chondrocytes per column was positive for ChM-I in the 4 groups. In accordance with the elongation of the hypertrophic stratum in uremia, the distance (X+/-SEM, microm) between the extracellular ChM-I signal and the metaphyseal end of growth cartilage was higher (P < 0.003) in UREM (236 +/- 40) and UREM-GH (297 +/- 17) than in SAL (92 +/- 7) and SPF (113 +/- 6). No differences in ChM-I expression were appreciated by RT-PCR. Similar VEGF positivity was observed in the hypertrophic chondrocytes of all groups.

CONCLUSION

In experimental uremia, expansion of growth cartilage does not result from increased or persistent expression of ChM-I or from reduced VEGF expression at the cartilage-metaphyseal bone interphase. GH treatment does not modify ChM-I and VEGF expressions.

Growth hormone therapy in calcium-loaded rats with renal failure

GH increases linear growth in children with chronic renal failure, but the response remains suboptimal in some patients. Some of the factors that may explain the poor response to GH include high doses of calcitriol and exogenous calcium loading to prevent hyperphosphatemia. High doses of exogenous calcium adversely affect chondrocyte proliferation and delay mineralization in the growth plate of rats with renal failure; bone histomorphometric changes in these animals are comparable to adynamic bone. To evaluate GH effects on adynamic bone in renal failure, 48 weanling rats underwent sham nephrectomy (Intact-Control) or 5/6 nephrectomy (Nx). Nx animals were fed a high-calcium diet (Nx-Ca(2+)) to induce adynamic bone. After 4 wk, the Nx-Ca(2+) animals were treated with GH (Nx-Ca(2+) + GH), calcitriol (Nx-Ca(2+) + D), or a combination of GH and calcitriol (Nx-Ca(2+)GH + D) for 2 wk. Serum intact PTH and IGF-I levels did not differ among all nephrectomized groups given high calcium. GH did not increase body length or tibial length at the end of study period. In the proximal tibia, the width of the growth plate and the growth plate architecture did not improve with GH. There was a decline in histone-4 expression, IGF-I protein, IGF binding protein-3, and bone morphogenetic protein-7 staining and a mild increase in IGF-I receptor, GH receptor, and gelatinase B expression in the Nx-Ca(2+) + GH group when compared with the Intact-Control group. Calcitriol blunted some of the mitogenic effects of GH in the growth plate. Thus, there was a poor response to GH therapy in calcium-loaded animals with renal failure.

Secondary hyperparathyroidism in children with chronic renal failure: pathogenesis and treatment

Despite advances in the management of patients with chronic renal failure, histologic features associated with secondary hyperparathyroidism remain the predominant skeletal findings; however, over the last decade the prevalence of adynamic bone has increased in both adult and pediatric patients with chronic renal failure. The management of children with secondary hyperparathyroidism and mild to moderate chronic renal failure should be started early, and should include correction of hypocalcemia and metabolic acidosis, maintenance of age-appropriate serum phosphorus levels, and institution of vitamin D therapy when serum intact parathyroid hormone (PTH) measurements are elevated to maintain the blood levels within normal limits; however, in children undergoing chronic dialysis therapy, the current recommendation is to maintain the serum intact PTH levels at least 2-4 times the upper limits of normal to prevent the development of low bone turnover disease. Serum calcium, phosphorus, alkaline phosphatase, and PTH levels should be monitored frequently, especially in infants and very young children. Discontinuation or reduction of vitamin D should be considered when there is a rapid decline in PTH levels, persistent elevation in serum calcium and serum phosphorus levels, and a significant diminution in alkaline phosphatase levels. In addition, a reduction in the calcium concentration of the dialysis fluid, and judicious use of calcium-containing salts as phosphate binding agents should also be performed in these patients. Although not yet extensively used in pediatric patients with secondary hyperparathyroidism, several therapeutic alternatives, such as the less calcemic vitamin D analogs, including paricalcitol [19-nor-1,25-(OH)(2)D(2)] and doxercalciferol [1-alpha-(OH)(2)D(2)], calcimimetics, and the availability of a calcium-free, aluminum-free phosphate binder such as sevelamer hydrochloride and lanthanum carbonate, may play significant roles in the future management of children with secondary hyperparathyroidism to promote linear growth, prevent parathyroid gland hyperplasia, avoid calciphylaxis and, in the long run, avert vascular calcifications.

Special aspects of renal osteodystrophy in children

Renal osteodystrophy represents a spectrum of skeletal lesions that range from high-turnover to low-turnover bone disease. Similar factors are involved in the pathogenesis of renal osteodystrophy in adult and pediatric patients with chronic kidney disease (CKD). However, growth retardation and the development of bone deformities are specific complications that occurred in pediatric patients with CKD. Metabolic acidosis, renal osteodystrophy, malnutrition, and disturbances in the insulin growth factor (IGF)/growth hormone (GH) are among the main factors involved and they are discussed briefly in this article. In addition to disturbances in bone remodeling, longitudinal bone growth occurs at the growth plate cartilage by endochondral ossification. Although young rats with experimental CKD have growth retardation, the characteristics of the growth plate are markedly different between animals with severe secondary hyperparathyroidism and those with calcium-induced adynamic osteodystrophy. These disturbances may suggest potential molecular mechanisms by which endochondral bone formation may be altered in renal failure, consequently leading to growth retardation.

Similar predictive value of bone turnover using first- and second-generation immunometric PTH assays in pediatric patients treated with peritoneal dialysis

BACKGROUND

Accurate measurements of the concentration of parathyroid hormone (PTH) in serum or plasma are essential for the proper assessment of renal osteodystrophy. The first-generation immunometric PTH assay (1st PTH-IMA) not only detects the intact hormone, but also additional PTH fragments truncated at the amino N-terminally truncated PTH-derived fragments [ntPTH(1-84)]. A second-generation immunometric PTH assay (2nd PTH-IMA) recognizes only PTH(1-84) and possibly PTH fragments that are truncated at the carboxyl-terminus but not PTH(7-84). Whether estimates of the ratio between PTH(1-84) and ntPTH(1-84) fragments are a better predictor of bone turnover remains controversial.

METHODS

Thirty-three patients aged 12.8 +/- 4.4 years treated with continuous cycling peritoneal dialysis (CCPD) for 13 +/- 9 months underwent iliac crest bone biopsy. PTH levels were measured by two newly developed first-generation and second-generation PTH-IMA. The ntPTH(1-84) fragments were calculated by subtracting PTH values determined using the 2nd PTH-IMA from values obtained using 1st PTH-IMA that detects both PTH(1-84) and relatively large ntPTH(1-84).

RESULTS

Determinations of PTH levels by both assays were highly correlated (r = 0.89, P < 0.001). The relationships between first-generation and second-generation PTH-IMA and bone formation were similar (r = 0.67, P < 0.0001 and r = 0.64, P < 0.0001, respectively). When patients were grouped according to the presence or absence of secondary hyperparathyroidism, the ratio PTH(1-84) to ntPTH(1-84) did not differ between groups.

CONCLUSION

PTH concentrations determined by either the first- or the second-generation PTH-IMA were found to be better predictors of bone formation than the PTH(1-84) to ntPTH(1-84) fragments ratio.

Effects of thyroparathyroidectomy, exogenous calcium, and short-term calcitriol therapy on the growth plate in renal failure

Several factors have been implicated in the development of adynamic bone, including the use of calcium-containing phosphate binding agents, aggressive calcitriol therapy, and parathyroidectomy. To evaluate the effects of these interventions on the growth plate, weanling rats underwent sham nephrectomy (Control, n = 10) and 5/6 nephrectomy (Nx). In the nephrectomized group, animals underwent (a) thyroparathyroidectomy (Nx-TPTX, n = 7), (b) received exogenous calcium (Nx-Calcium, n = 10), (c) received short-term calcitriol therapy (Nx-D, n = 10), or (d) nephrectomized control (Nx-Control, n = 10). Higher serum calcium and lower PTH levels were demonstrated in Nx-Calcium and Nx-D animals. A decline in growth was demonstrated in Nx-Calcium and Nx-TPTX accompanied by shorter tibial lengths. The width of the growth plate was wider in Nx-Calcium animals due to an increase in the width of the hypertrophic zone and a decrease in the proliferative zone; these changes were accompanied by an impairment of chondroclastic resorption, lower gelatinase B/MMP-9 activity, decline in insulin-like growth factor-I (IGF-I) receptor, and lower histone-4 mRNA expression. Such findings in the growth plate, may partially contribute to the diminution of growth in these animals. Although growth was impaired in the Nx-TPTX animals, there were no significant changes demonstrated in the growth plate cartilage. Histone-4 transcripts, IGF-I receptor expression, and histochemical staining for chondroclasts were decreased in Nx-D animals. Thus, treatments used in the management of secondary hyperparathyroidism in renal failure have diverse effects on the growth plate of the young skeleton, and concurrent use of these interventions needs further evaluation.

Alterations in the growth plate cartilage of rats with renal failure receiving corticosteroid therapy

Growth retardation is a complication often associated with corticosteroid therapy. Corticosteroids are frequently used in the treatment of children with chronic renal failure. To examine the effects of corticosteroids on the growth plate cartilage in renal failure, selected markers of chondrocyte function and phenotype were evaluated in the proximal tibia of subtotally nephrectomized rats treated with corticosteroid. Serum parathyroid hormone (PTH), urea nitrogen, and creatinine levels were higher in the nephrectomized animals. Weight gain was less in the corticosteroid-treated animals; however, linear growth and tibial length did not differ among the groups after 10 days of corticosteroid therapy. The total width of the growth plate and the width of the proliferative zone were much smaller in corticosteroid-treated nephrectomized (Nx-MP) animals. Type II collagen mRNA expression was lower in animals treated with corticosteroids, and proliferating-cell nuclear antigen staining, histone-4, and insulin-like growth factor-1 (IGF-1)-receptor mRNA expression were further decreased in the Nx-MP group. There was an increase in TUNEL-positive cells in the corticosteroid-treated rats with normal renal function (intact-MP), associated with an increase in Bax and a decrease in Bcl-2 protein expression. In the Nx-MP group, both Bax and Bcl-2 protein staining was much less frequent, and TUNEL-positive cells were lower in number compared with the intact-MP group. Vascular endothelial growth factor expression in the hypertrophic chondrocytes was lower in corticosteroid-treated animals. There was less gelatinase B/matrix metalloproteinase-9 expression in the Nx-MP group, which was not associated with a decrease in tartrate-resistant acid phosphatase (TRAP) staining in the chondro-osseous junction. Inhibition of chondrocyte proliferation, diminishing of apoptosis, and lower angiogenic activity may contribute to the alterations in growth plate architecture and the significant reduction in growth plate width in rats with renal failure receiving corticosteroid therapy.

Histologic and dynamic changes induced by chronic metabolic acidosis in the rat growth plate

To understand better the pathophysiology of growth impairment in persistent metabolic acidosis, the morphology and dynamics of the growth plate were studied in young rats grouped as follows: rats that were made acidotic by oral administration of ammonium chloride for 14 d (AC), nonacidotic rats that were fed ad libitum (control [C]), and nonacidotic rats that were pair-fed with the AC group (PF). AC rats became markedly acidotic and growth retarded. The volume of newly formed bone per day (mean +/- SEM) was significantly lowered (P < 0.05) in AC rats (AC, 3.4 +/- 0.4; C, 8.4 +/- 0.6; PF, 6.4 +/- 0.5 mm(3)/d). Growth plate height was lower in AC rats (303.8 +/- 12.7 microm) than in either C (478.0 +/- 16.0 microm) or PF rats (439.0 +/- 21.4 microm). The processes of chondrocyte proliferation (assessed by bromodeoxyuridine labeling) and maturation (assessed by stereologic estimators of size and shape of chondrocytes and the volume of matrix per cell) were not impaired by acidosis. By contrast, the dynamics of hypertrophic chondrocytes were altered significantly: both cell turnover per column per day (AC, 4.4 +/- 0.4; C, 8.0 +/- 0.8; PF, 6.2 +/- 0.6) and linear velocity of advance of chondrocytes (AC, 5.7 +/- 0.5; C, 11.2 +/- 0.9; PF, 9.4 +/- 0.8 microm/h) were lowered significantly. The study presented here shows the inhibitory effect of metabolic acidosis on cartilage cell progression and endochondral bone formation. Finally, the data show that metabolic acidosis caused a marked shortening of the growth plate because chondrocyte turnover was affected to a greater extent than bone tissue formation.

Preservation of bone mass in pediatric dialysis and transplant patients

Renal osteodystrophy continues to be a major challenge to the physician treating the child with end-stage renal disease (ESRD). The gold standard for the assessment of bone status is bone histomorphometry, which divides bone pathology into 3 main types; high-turnover, low-turnover, and mixed disease. The high-turnover disease, related to hyperparathyroidism, has been the one most extensively investigated; however, optimal therapy, especially in the growing child, is yet unclear. Overzealous treatment might result in adynamic bone disease (an extreme example of low-turnover disease), and further interference with statural growth. Pre-existent bone disease after kidney transplantation seems to worsen immediately, probably because of the high dose of corticosteroids used. In children who attain normal kidney function in the allograft, bone status seems to improve over time. Little is known about bone in transplanted patients with reduced glomerular filtration rate (GFR). The correlation between bone histology and its main surrogates, bone remodeling markers and bone mineral density, is yet unclear, but it might serve to follow the progress of an individual patient. New therapeutic modalities aimed at suppressing hyperparathyroidism, and consequently bone resorption, as well as agents directly attenuating bone resorption, should be further investigated for their effect on bone in patients with ESRD or after transplantation. Similarly, agents stimulating bone formation, particularly growth hormone, require further attention for their potential to improve bone status. Bone health and the child's somatic growth at ESRD or after kidney transplantation are closely related, and therapy should be aimed at achieving optimal results for both.

Is there an optimal parathyroid hormone level in end-stage renal failure: the lower the better?

Skeletal resistance to parathyroid hormone is well defined in patients with chronic renal failure. In recent years, with the increased frequency of development of adynamic bone disease, it has been recognized that secondary hyperparathyroidism must exist as a 'trade off' mechanism to maintain skeletal bone remodeling in this patient population. An optimal level of intact parathyroid hormone to maintain the normal skeletal bone turnover is believed to be between 2.0 and 2.5 times the upper limit of normal parathyroid hormone. It has very recently been argued that the optimal parathyroid hormone level for maintenance of skeletal bone remodeling may be insufficient to prevent the extraskeletal complications of coronary artery calcifications, calcific valvular heart disease, and cardiac death. To provide optimal health care for these patients several new treatments have been developed, including use of new vitamin D analogs, calcimimetic agents, and noncalcium-based phosphorus binders. It is anticipated that with lower suppression of parathyroid hormone by these vitamin D analogs, intermittent suppression of parathyroid hormone with calcimimetic agents, and the use of noncalcium phosphorus binders (Renageltrade) by regulating serum calcium, the resultant phosphorus concentrations will provide an optimal parathyroid hormone activity to maintain skeletal bone remodeling, while preventing extraskeletal complications.

Collagen metabolism is markedly altered in the hypertrophic cartilage of growth plates from rats with growth impairment secondary to chronic renal failure

Skeletal growth depends on growth plate cartilage activity, in which matrix synthesis by chondrocytes is one of the major processes contributing to the final length of a bone. On this basis, the present work was undertaken to ascertain if growth impairment secondary to chronic renal insufficiency is associated with disturbances of the extracellular matrix (ECM) of the growth plate. By combining stereological and in situ hybridization techniques, we examined the expression patterns of types II and X collagens and collagenase-3 in tibial growth plates of rats made uremic by subtotal nephrectomy (NX) in comparison with those of sham-operated rats fed ad libitum (SAL) and sham-operated rats pair-fed with NX (SPF). NX rats were severely uremic, as shown by markedly elevated serum concentrations of urea nitrogen, and growth retarded, as shown by significantly decreased longitudinal bone growth rates. NX rats showed disturbances in the normal pattern of chondrocyte differentiation and in the rates and degree of substitution of hypertrophic cartilage with bone, which resulted in accumulation of cartilage at the hypertrophic zone. These changes were associated with an overall decrease in the expression of types II and X collagens, which was especially marked in the abnormally extended zone of the hypertrophic cartilage. Unlike collagen, the expression of collagenase-3 was not disturbed severely. Electron microscopic analysis proved that changes in gene expression were coupled to alterations in the mineralization as well as in the collagen fibril architecture at the hypertrophic cartilage. Because the composition and structure of the ECM have a critical role in regulating the behavior of the growth plate chondrocytes, results obtained are consistent with the hypothesis that alteration of collagen metabolism in these cells could be a key process underlying growth retardation in uremia.

Regulation of bone remodeling: impact of novel therapies

The kidneys serve as both an endocrine organ and as a target of endocrine action, with the aim of controlling mineral and water balance. Hormones and other key metabolites regulate mineral homeostasis by altering gene function directly or by initiating a sequence of events, leading ultimately to a change in enzyme function. Two of these hormones, parathyroid hormone (PTH) and calcitriol (the active form of vitamin D), interact in multiple tissues in the body to regulate the flux of calcium and phosphorus between extra- and intracellular compartments. Changes in the concentration of PTH and vitamin D, or the interaction of these with other factors, lead to the aberrant regulation of calcium and phosphorus. Among other effects, this aberrant regulation leads to pathologic changes in bone metabolism. The pathology of renal bone disease varies along a spectrum from disorders of low turnover to those of high turnover. This spectrum reflects the results of therapeutic intervention, hormone balances, and other causes. Effective management of renal bone disease therefore requires thorough evaluation of relevant risk factors, measurement of biochemical markers of bone remodeling, and determination of the physical status of bone tissue either by bone mineral density or bone biopsy. Subsequent therapeutic intervention with newer vitamin D compounds, novel phosphate binders, calcimimetics, and the use of alternative dialysis modalities offer hope in normalizing bone remodeling and mineral balance. The human skeleton functions in two capacities: the storage of minerals and structural support of the body. It is the only tissue that behaves as both a major source and a sink of calcium (Ca) and phosphorus (P). Healthy bone is a composite of a collagenous matrix embedded with crystals of hydroxyapatite. On the surface of bone and within the calcified matrix are specialized cells that build and maintain the tissue, and facilitate the movement of Ca and P into and out of serum. Bone undergoes remodeling in response to either damage from mechanical strain or as part of the normal cycle of bone renewal. The process involves distinct steps of cellular activation, bone resorption, and subsequent bone formation. It is a relatively slow process that takes several months, and at any one time occurs at many different sites along the bone surface. Systemic factors, such as PTH and vitamin D, regulate the resorption and formation of bone, and thus the systemic movement of Ca and P. However, during conditions of stress and disease, other factors may also play a role. In patients with chronic renal disease, the balance of Ca and P is profoundly disturbed. This disruption and the compensatory changes that occur in response alter the normal processes of bone metabolism.

Insulin sensitivity is inversely correlated with plasma intact parathyroid hormone level

Abnormal glucose metabolism and a high prevalence of diabetes have been reported in patients with primary and secondary hyperparathyroidism. We hypothesize that plasma intact parathyroid hormone (iPTH) level is a determinant of either insulin sensitivity or beta-cell function. The study included 52 normotensive, healthy subjects with glucose tolerance. Insulin sensitivity and beta-cell function were assessed using a hyperglycemic clamp. Fasting plasma iPTH was determined. The relationships between its level and insulin sensitivity index and beta-cell function were examined. Insulin sensitivity index was inversely correlated with plasma iPTH level (r2 = .104, P = .020). The first phase insulin response was positively correlated with plasma iPTH level (r2 = .098, P = .023), but no correlation existed with the second phase insulin response. After adjusting for age, gender, ethnicity, and waist-to-hip ratio, plasma iPTH level was an independent determinant of insulin sensitivity index (P = .019). However, no independent relationship between plasma iPTH level and beta-cell function (the first phase and second phase insulin response) was found. In normotensive, glucose-tolerant, and healthy subjects, plasma iPTH level accounts for 10.4% of the variation in insulin sensitivity index. For each pg/mL increment in plasma iPTH level, there is a decrease of 0.247 micromol/L/m2/min/pmol/L in insulin sensitivity index. Although the molecular basis of this relationship is not clear, our results indicate that plasma iPTH level is inversely correlated with insulin sensitivity index.

Is aplastic osteodystrophy a disease of malnutrition?

Adynamic bone disease is emerging as a major type of renal osteodystrophy in chronic dialysis patients. Relative hypoparathyroidism is one of the important abnormalities underlying this disease. Recently, several reports have suggested that hypoparathyroidism reflects, at least in part, a state of malnutrition and contributes to the poor prognosis of patients on hemodialysis and chronic ambulatory peritoneal dialysis. Such a risk of survival may result not only from the malnutritional state, but also from unknown mechanisms resulting from parathyroid hormone (PTH) deficiency, or from other abnormalities that suppress PTH secretion. Another major abnormality underlying adynamic bone disease is the skeletal resistance to PTH in patients with uremia. Owing to the recent research on bone turnover at the molecular level, several new mechanisms for this abnormality have been elucidated. Correction of this 'skeletal resistance to PTH' will lead to the optimal management of parathyroid function and bone turnover in the future.

Effect of metabolic acidosis on the growth hormone/IGF-I endocrine axis in skeletal growth centers

BACKGROUND

Chronic metabolic acidosis (CMA) adversely affects bone metabolism and skeletal growth. Given the cardinal role played by the local growth hormone (GH)/insulin-like growth factor-I (IGF-I) in promoting cell proliferation and differentiation in growth plates, we tested the effect of CMA on the GH/IGF-I axis in a skeletal growth center.

METHODS

We employed an in vitro organ culture system using the murine mandibular condyle as a model for endochondral active growth center. Condyles from six-day-old ICR mice were cultured in BGJb medium of either neutral pH (pH approximately 7.4) or acidic pH (pH approximately 7.15). After 24, 48, 72, and 96 hours of culture, the condyles were washed, fixed in formaldehyde, and processed for paraffin embedding. We assessed histologic markers of the growth center. In addition, the protein level and mRNA expression for the different components of the GH/IGF-I axis were evaluated by immunohistochemistry and in situ hybridization, respectively. Finally, we evaluated the effect of acidosis on the biological functions mediated by GH and IGF-I (namely, proliferation and differentiation of cartilage cells in the active growth center).

RESULTS

Following three to four days in acidic conditions, there was a marked reduction in the size of young chondrocytic population, suggesting a defect in the process of endochondral differentiation. Immunohistochemistry and in situ hybridization analyses revealed a marked reduction in the expression of the IGF-I receptor, as well as in the GH receptor. These changes were already evident after 48 hours of incubation in acidic conditions. At 48 hours of acidosis, there was also a marked reduction in the expression of IGF-I both under basal conditions (nonstimulated) and following stimulation with GH. The expression of IGF binding protein 2 (IGFBP-2) and IGFBP-4, which serve as negative modulators of IGF-I, was enhanced in CMA. IGF-I markedly stimulated chondrocytic proliferation (assessed by BrdU incorporation into DNA) and differentiation (assessed as cartilage specific proteoglycan expression). These responses were markedly attenuated in acidic conditions.

CONCLUSION

CMA exerts an anti-anabolic effect in bone growth centers, which is partly related to a state of resistance to GH and IGF-I, created by CMA. This phenomenon may underlie the disturbance in longitudinal bone growth in CMA (that is, renal tubular acidosis) and may contribute to renal osteodystrophy in patients suffering from chronic renal failure.

Impaired growth, delayed ossification, and reduced osteoclastic activity in the growth plate of calcium-supplemented rats with renal failure

Linear growth is reduced in prepubertal children with adynamic renal osteodystrophy, suggesting that the proliferation and/or differentiation of epiphyseal growth plate chondrocytes is abnormal in this disorder. To examine this issue, in situ hybridization and histochemistry were used to measure selected markers of endochondral bone formation and bone resorption in the proximal tibia of subtotally nephrectomized rats fed a high calcium diet to induce biochemical changes consistent with adynamic osteodystrophy. Blood ionized calcium concentrations were higher and serum PTH levels were lower in nephrectomized, calcium-supplemented rats than in either intact or nephrectomized control animals. Linear growth and tibial length were reduced, but messenger RNA levels for type II collagen, type X collagen, and the PTH/PTHrP receptor did not differ from control values in nephrectomized rats given supplemental calcium. In contrast, both the width of epiphyseal cartilage and the height of the zone of hypertrophic chondrocytes were greater in calcium-supplemented nephrectomized rats. These morphological changes were associated with decreases in histochemical staining for tartrate-resistant acid phosphatase and lower levels of messenger RNA expression for the matrix metalloproteinase MMP-9/gelatinase B immediately adjacent to the epiphyseal growth plate. Diminished chondroclastic/osteoclastic activity alters growth plate morphology and adversely affects linear bone growth in calcium-supplemented, nephrectomized rats.

Cellular mechanisms of renal osteodystrophy

Renal osteodystrophy affects all patients with end-stage renal failure, resulting in significant skeletal and extra-skeletal morbidity. The patterns of disease seen in bone are the result of changes in calcium, phosphate, parathyroid hormone (PTH), and vitamin D metabolism, as well as the effects of uremia. Standard histological techniques, however, give little insight into the altered biological activity or mechanisms of disease at the cellular level. In order to examine the cellular abnormalities in renal bone disease we have performed a series of in situ hybridization studies to examine renal bone cell expression of genes for PTH receptor (PTHR1), transforming growth factor beta (TGF-beta) and insulin growth factor 1 (IGF-I). PTHR1 mRNA was expressed predominantly by osteoblasts, but also by resorbing osteoclasts, suggesting that these cells may be stimulated directly by PTH. Semi-quantitative analysis of gene expression showed down-regulation of PTHR1 mRNA by osteoblasts in renal bone compared with normal, fracture and Pagetic bone. This may be important in the pathogenesis of skeletal resistance seen in end-stage renal failure, altering the "threshold" at which PTH has its effects on bone cells. TGF-beta and IGF-I mRNA expression was also decreased, suggesting that synthesis of these factors, postulated to be mediators of PTH, is also downregulated.

Growth retardation in children with chronic renal failure

Growth retardation is a major obstacle to full rehabilitation of children with chronic renal failure (CRF). Several factors have been identified as contributors to impaired linear growth and they include protein and calorie malnutrition, metabolic acidosis, growth hormone resistance, anemia, and renal osteodystrophy. Although therapeutic interventions such as the use of recombinant human growth hormone, recombinant human erythropoietin, and calcitriol have made substantial contributions, the optimal therapeutic strategy remains to be defined. Indeed, growth failure persists in a substantial proportion of children with renal failure and those treated with maintenance dialysis. In addition, the increasing prevalence of adynamic lesions of renal osteodystrophy and its effect on growth have raised concern about the continued generalized use of calcitriol in children with CRF. Recent studies have shown the critical roles of parathyroid hormone-related protein (PTHrP) and the PTH/PTHrP receptor in the regulation of endochondral bone formation. The PTH/PTHrP receptor mRNA expression has been shown to be down-regulated in kidney and growth plate cartilage of animals with renal failure. Differences in the severity of secondary hyperparathyroidism influence not only growth plate morphology but also the expression of selected markers of chondrocyte proliferation and differentiation in these animals. Such findings suggest potential molecular mechanisms by which cartilage and bone development may be disrupted in children with CRF, thereby contributing to diminished linear growth.