RESUMO
Chronic Kidney Disease (CKD)-Mineral and Bone Disorder (CKD-MBD) is a complex disease that is not completely understood. However, some factors secreted by the osteocytes might play an important role in its pathophysiology. Therefore, we evaluated the bone expression of proteins in a group of patients with CKD 2-3, CKD 4, and CKD 5 on dialysis and healthy individuals. We also tested several bone remodeling markers, and correlated these levels with bone biopsy findings. As expected, as serum calcium decreased, serum phosphate, alkaline phosphatase, fibroblast growth factor-23 (FGF-23), parathyroid hormone, and osteoprotegerin increased, as CKD progressed. Additionally, there was a gradual increase in bone resorption associated with a decrease in bone formation and impairment in bone mineralization. Bone expression of sclerostin and parathyroid hormone receptor-1 seemed to be increased in earlier stages of CKD, whereas FGF-23 and phosphorylated ß-catenin had increased expression in the late stages of CKD, although all these proteins were elevated relative to healthy individuals. Immunohistochemical studies showed that FGF-23 and sclerostin did not co-localize, suggesting that distinct osteocytes produce these proteins. Moreover, there was a good correlation between serum levels and bone expression of FGF-23. Thus, our studies help define the complex mechanism of bone and mineral metabolism in patients with CKD. Linkage of serum markers to bone expression of specific proteins may facilitate our understanding and management of this disease.
Assuntos
Remodelação Óssea , Osso e Ossos/metabolismo , Distúrbio Mineral e Ósseo na Doença Renal Crônica/sangue , Osteócitos/metabolismo , Insuficiência Renal Crônica/sangue , Proteínas Adaptadoras de Transdução de Sinal , Adulto , Idoso , Biomarcadores/sangue , Biópsia , Proteínas Morfogenéticas Ósseas/metabolismo , Osso e Ossos/patologia , Cálcio/sangue , Estudos de Casos e Controles , Distúrbio Mineral e Ósseo na Doença Renal Crônica/diagnóstico , Distúrbio Mineral e Ósseo na Doença Renal Crônica/terapia , Feminino , Fator de Crescimento de Fibroblastos 23 , Fatores de Crescimento de Fibroblastos/sangue , Marcadores Genéticos , Humanos , Masculino , Pessoa de Meia-Idade , Osteócitos/patologia , Osteoprotegerina/sangue , Hormônio Paratireóideo/sangue , Fosforilação , Receptor Tipo 1 de Hormônio Paratireóideo/metabolismo , Diálise Renal , Insuficiência Renal Crônica/diagnóstico , Insuficiência Renal Crônica/terapia , Índice de Gravidade de Doença , beta Catenina/metabolismoRESUMO
Osteocytes respond to kidney damage by increasing production of secreted factors important to bone and mineral metabolism. These circulating proteins include the antianabolic factor, sclerostin, and the phosphaturic hormone, fibroblast growth factor 23 (FGF23). Elevated sclerostin levels correlate with increased FGF23, localized reduction in Wnt/ß-catenin signaling in the skeleton and reduced osteoblast differentiation/activity. Decreased Wnt/ß-catenin signaling occurs regardless of the overall changes in bone formation rates, suggesting that a reduction in the anabolic response may be a common feature of renal bone disorders but additional mechanisms may contribute to the diversity of osteodystrophy phenotypes. Recent preclinical studies support this hypothesis, as treatment with antisclerostin antibodies improved bone quality in the context of low but not high turnover renal osteodystrophy. Sclerostin also appears in the circulation suggesting additional roles outside the skeleton in normal and disease states. In patients with chronic kidney disease (CKD), serum levels are elevated several fold relative to healthy individuals. Emerging data suggest that these changes are associated with increased fracture rates but the relationship between sclerostin and cardiovascular disease is unclear. Additional epidemiologic studies that examine stage specific and patient sub-populations are needed to assess whether sclerostin elevations influence comorbidities associated with CKD.
Assuntos
Proteínas Morfogenéticas Ósseas/metabolismo , Distúrbio Mineral e Ósseo na Doença Renal Crônica , Osteócitos/metabolismo , Insuficiência Renal Crônica , Proteínas Adaptadoras de Transdução de Sinal , Densidade Óssea , Distúrbio Mineral e Ósseo na Doença Renal Crônica/etiologia , Distúrbio Mineral e Ósseo na Doença Renal Crônica/metabolismo , Distúrbio Mineral e Ósseo na Doença Renal Crônica/patologia , Progressão da Doença , Fator de Crescimento de Fibroblastos 23 , Marcadores Genéticos , Humanos , Osteócitos/patologia , Insuficiência Renal Crônica/complicações , Insuficiência Renal Crônica/metabolismo , Insuficiência Renal Crônica/patologiaRESUMO
High phosphate intake is known to aggravate renal osteodystrophy along various pathogenetic pathways. Recent studies have raised the possibility that dysregulation of the osteocyte Wnt/ß-catenin signaling pathway is also involved in chronic kidney disease (CKD)-related bone disease. We investigated the role of dietary phosphate and its possible interaction with this pathway in an experimental model of adynamic bone disease (ABD) in association with CKD and hypoparathyroidism. Partial nephrectomy (Nx) and total parathyroidectomy (PTx) were performed in male Wistar rats. Control rats with normal kidney and parathyroid function underwent sham operations. Rats were divided into three groups and underwent pair-feeding for 8 weeks with diets containing either 0.6% or 1.2% phosphate: sham 0.6%, Nx+PTx 0.6%, and Nx+PTx 1.2%. In the two Nx+PTx groups, serum creatinine increased and blood ionized calcium decreased compared with sham control group. They also presented hyperphosphatemia and reduced serum parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) levels. Fractional urinary excretion of phosphate increased in Nx+PTx 1.2% rats despite lower PTH and FGF23 levels than in sham group. These biochemical changes were accompanied by a decrease in bone formation rates. The Nx+PTx 1.2% group had lower bone volume (BV/TV), higher osteoblast and osteocyte apoptosis, and higher SOST and Dickkopf-1 gene expression than the Nx+PTx 0.6% group. Nx+PTx 0.6% rat had very low serum sclerostin levels, and Nx+PTx 1.2% had intermediate sclerostin levels compared with sham group. Finally, there was a negative correlation between BV/TV and serum sclerostin. These results suggest that high dietary phosphate intake decreases bone volume in an experimental model of CKD-ABD, possibly via changes in SOST expression through a PTH-independent mechanism. These findings could have relevance for the clinical setting of CKD-ABD in patients who low turnover bone disease might be attenuated by optimal control of phosphate intake and/or absorption.