Bone cells, sclerostin, and FGF23: what’s bred in the bone will come out in the flesh

Bone cells, sclerostin, and FGF23: what’s bred in the bone will come out in the flesh

commentary cemic rat with insulin resistance.6 They show a similar profile of NBCe1 activation by insulin in kidneys from humans with insulin resista...

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cemic rat with insulin resistance.6 They show a similar profile of NBCe1 activation by insulin in kidneys from humans with insulin resistance. They conclude that the preserved stimulation of proximal tubule transport through the insulin/IRS2/PI3K pathway may play an important role in the pathogenesis of hypertension associated with metabolic syndrome.6 The preservation of insulin-mediated salt absorption in the kidney is suggestive of the presence of salt-sensitive hypertension in conditions associated with insulin resistance. If so, shall we recommend that individuals with metabolic syndrome, obesity, or type 2 diabetes mellitus limit their salt intake even before the onset of hypertension? Naturally, the best approach to control hypertension in states associated with insulin resistance is to reverse the cause of resistance, that is, through weight loss, the use of insulin-sensitizing agents, or an assuage of IRS dysregulation. Alternatively, the use of diuretics that specifically inhibit salt reabsorption in the proximal tubule may look appealing in individuals with hypertension due to insulin resistance. A classic example of the latter would be carbonic anhydrase inhibitors, such as acetazolamide. This option is specifically appealing if it is combined with thiazide derivatives, such as hydrochlorothiazide, which has been shown to prevent the generation of metabolic acidosis from acetazolamide monotherapy while enhancing its diuretic potency.9 The best approach to treating patients with insulin resistance and hypertension requires long-term studies aimed at addressing the risk factors (insulin resistance, hypertension, and so on) and optimizing clinical outcomes. DISCLOSURE

The author declared no competing interests. REFERENCES 1.

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Nakamura M, Yamazaki O, Shirai A et al. Preserved Na/HCO3 cotransporter sensitivity to insulin may promote hypertension in metabolic syndrome. Kidney Int 2015; 87: 535–542. Guo S. Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms. J Endocrinol 2014; 220: T1–T23.

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3.

4.

5.

6.

Boller S, Joblin BA, Xu L et al. From signal transduction to signal interpretation: an alternative model for the molecular function of insulin receptor substrates. Arch Physiol Biochem 2012; 118: 148–155. Landsberg L, Aronne LJ, Beilin LJ et al. Obesity-related hypertension: pathogenesis, cardiovascular risk, and treatment. A position paper of The Obesity Society and the American Society of Hypertension. J Clin Hypertens (Greenwich) 2013; 15: 14–33. Horita S, Seki G, Yamada H et al. Insulin resistance, obesity, hypertension, and renal sodium transport. Int J Hypertens [online] 2011; 2011: 391762. Ito O, Kondo Y, Oba M et al. Tyrosine kinase, phosphatidylinositol 3-kinase, and protein

7.

8.

9.

kinase C regulate insulin-stimulated NaCl absorption in the thick ascending limb. Kidney Int 1997; 51: 1037–1041. Zheng Y, Yamada H, Sakamoto K et al. Roles of insulin receptor substrates in insulin-induced stimulation of renal proximal bicarbonate absorption. J Am Soc Nephrol 2005; 16: 2288–2295. Rocchini AP, Katch V, Kveselis D et al. Insulin and renal sodium retention in obese adolescents. Hypertension 1989; 14: 367–374. Zahedi K, Barone S, Xu J et al. Potentiation of the effect of thiazide derivatives by carbonic anhydrase inhibitors: molecular mechanisms and potential clinical implications. PLoS One [online] 2013; 8: e79327.

see clinical investigation on page 593

Bone cells, sclerostin, and FGF23: what’s bred in the bone will come out in the flesh Susan M. Ott1 Bone metabolism is linked to systemic diseases, and new research shows that the bone cells have endocrine functions that affect multiple organs. They secrete sclerostin, FGF23, prostaglandins, and osteocalcin. Pereira et al. examined gene expression of cells grown from bone biopsies of adolescents with renal osteodystrophy, as a first step to understanding how the bone-cell abnormalities contribute to cardiovascular and metabolic problems in these patients. Kidney International (2015) 87, 499–501. doi:10.1038/ki.2014.360

Nephrologists have been aware that there are important connections between skeletal physiology, the cardiovascular system, and mineral metabolism since the coining of the phrase ‘chronic kidney disease–mineral and bone disorder’ (CKD-MBD) for the mineralizing and bone disorder seen in our patients. In recent years even more data have highlighted the important role played by bone cells, not only in cardiovascular calcifications, but also in the function of the heart and skeletal muscle, the parathyroid glands, phos1 Department of Medicine, University of Washington, Seattle, Washington, USA Correspondence: Susan M. Ott, Department of Medicine, University of Washington, Box 356426, Seattle, Washington 98195-6426, USA. E-mail: [email protected]

phate homeostasis, acid–base balance, and energy metabolism (see Figure 1). Some of this regulation is secondary to bone turnover itself, which is primarily directed by the osteocytes. These permanent residents of the bone are more active than previously recognized, and not only do they sense mechanical loading on the bone and secrete local factors that cause bone resorption and formation in just the right location, but the osteocytes also form an extensive network that can sense molecules in the circulation and secrete hormones that affect distant organs.1 One of the most important of these hormones is sclerostin, an inhibitor of the critical Wnt signaling pathway. Sclerostin is expressed almost 499

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Bone turnover Inhibits transformation of vascular smooth muscle cells Causes hypertrophy of myocytes

Impairs endothelial vasorelaxation

H+

Sclerostin

Buffers loads of calcium, phosphate, PO4 and acid, and stabilizes serum minerals Ca

Local factors FGF23

Inhibits secretion of PTH

Osteoclast

Osteocyte

Increases phosphate excretion by inhibition of sodium–phosphate transporters and inhibits synthesis of calcitriol Prostaglandin Increases muscle mass Increases insulin sensitivity

Osteoblasts

Osteocalcin

Reduces fat in liver

Increases insulin secretion from pancreas

Figure 1 | Hormones secreted by bone cells affect multiple organs. FGF23, fibroblast growth factor 23; PTH, parathyroid hormone.

exclusively in mature osteocytes buried in the bone mineralized matrix. When the osteocytes detect loading, they reduce the secretion of sclerostin, releasing inhibition on the precursors to osteoblasts, and thus will initiate new bone formation. Human inactivating mutations in the gene for sclerostin result in dense, strong bones, and antibodies to this protein are in phase 3 development for potential new therapies for osteoporosis. Some of the sclerostin is secreted into the general circulation, where it may affect the vascular smooth muscle cells. These cells have some similarities to osteoblasts, and sclerostin (and other inhibitors of Wnt signaling) can suppress the transformation of vascular smooth muscle cells into osteoblast-like cells. The role of this signaling in the development of cardiovascular calcification is an active area of current 500

research.2 In CKD patients serum sclerostin is elevated, and a recent study showed that it could be produced ectopically in calcified aortic valves.2 It is not clear whether this is a protective mechanism or whether the sclerostin is just a marker of the terminal transformation of the vascular cells. The osteocytes are also the major source of fibroblast growth factor 23 (FGF23). This hormone, important for mineralization and for phosphate homeostasis, has very complex regulation, including stimulation by parathyroid hormone and iron deficiency as well as infusions of iron, 1,25(OH)2 vitamin D, and leptin; inhibition by DMP1 and PHEX; and indirect inhibition by phosphate loading, sclerostin, and MEPE.1,3 The FGF23 also can be phosphorylated to make it resistant to cleavage, and this post-translational

step adds another level of complexity to the regulation. To understand FGF23 it is necessary to measure both intact hormone and C-terminal cleavage products. The C-terminal fragment can act as an antagonist at the level of the receptor. Thus it is interesting and important that most of the FGF23 in renal patients is in the intact form. The FGF23 secreted by the osteocytes will participate in several negative-feedback loops: it inhibits phosphate excretion in the proximal tubule (acting with the coreceptor Klotho in tubular epithelial cells), and it inhibits renal 25OH vitamin D 1a-hydroxylase expression. Normally FGF23 will inhibit the secretion of parathyroid hormone, but this process fails in patients with CKD because there is inadequate Klotho. FGF23 is associated with mortality and heart failure in patients with or without kidney disease. Because FGF23 is regulated by phosphate (albeit in an indirect manner) and phosphate is correlated with mortality and heart failure, it is possible that this association is not causative. In this regard, there was no direct association of serum FGF23 with arterial calcification in the Chronic Renal Insufficiency Cohort Study. Moreover, the authors of this study found no evidence for a direct pro-calcifying effect of FGF23 in vascular smooth muscle cells in vitro.4 However, other groups have found associations of serum FGF23 with vascular calcifications in patients with CKD, and FGF23 enhances phosphateinduced aortic calcifications in a rat model.5 Also, new studies suggest that there is actually a direct action of FGF23 on the myocardial cells, leading to hypertrophy of these cells.6 Other indirect effects of FGF23 are related to the decreased 1,25(OH)2 vitamin D. Bone-derived hormones thus impact cardiac muscle, but osteocytes also play a role in skeletal muscle. They secrete Wnt3a, an activator of the Wnt signaling pathway, and prostaglandin E2. The result is enhancement of skeletal muscle mass.1 The osteocytes can resorb and restore bone from the cannicular surface, which is a much greater mineral–fluid Kidney International (2015) 87

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interface than the traditional bone surface. Thus, these cells directly release calcium, phosphate, and bicarbonate into the circulation. The cannicular mineral flux, along with the traditional bone turnover, is important for buffering calcium and phosphate loads and maintaining steady serum mineral levels. Although these molecules are not exactly hormones, they certainly have systemic effects. The loss of this buffering contributes to the increased vascular calcification in kidney patients with low bone turnover, because dietary load can cause higher increases in the serum calcium and phosphate. The osteoblasts, precursors to the osteocytes, are known to lay down the mineral matrix, which is predominantly type I collagen. Another matrix protein is osteocalcin, which is modified on specific glutamic acid residues by carboxylation to form g-carboxyglutamic acid residues that bind calcium.7,8 This reaction requires vitamin K. Recent studies have discovered that osteocalcin, in the undercarboxylated form, is another systemic hormone secreted by the bone cells, and it regulates energy metabolism. This is heuristically appropriate since bone formation requires a large energy expenditure and if the body is not getting adequate nutrition then energy utilization should be shunted to organs that have less reserve. The osteoblast control of undercarboxylated osteocalcin is complex. The carboxylated form is secreted into the matrix, and expression is regulated by the insulin receptor.8 When exposed to insulin the osteoblast downregulates osteoprotegerin secretion, which will result in increased activity of the osteoclasts that secrete acid to resorb bone. This acid will decarboxylate the osteocalcin and release the hormonal form into the circulation. Undercarboxylated osteocalcin also will be seen in patients who have vitamin K deficiency. Undercarboxylated osteocalcin has several endocrine actions: in the pancreas it stimulates release of insulin, in the skeletal muscle it increases insulin sensitivity, and in the liver it reduces lipid accumulation.7 Kidney International (2015) 87

Disturbances in this highly regulated and networked system of mineral metabolism in patients with CKD contribute to skeletal fractures, heart failure, and muscle weakness. In order to further explore the abnormalities of the bone cells, Pereira and colleagues9 (this issue) cultured human bone cells from adolescents with CKD stage 5. Consistent with past studies of osteoblast cultures from dialysis patients, they found a faster proliferative rate than in cultures from individuals without CKD. The authors also examined the expression of several genes in core samples taken from the iliac crest, on which standard histomorphometric parameters were measured. They grew some of the cells to confluence and found significant correlations between some of the cell signaling genes in the core and in the cultured cells. These genes were mainly those known to be expressed by osteoblasts, and there was no increased expression of osteocyte genes. Osteocytes are very difficult to grow in ex vivo conditions, and in this study the cells had not differentiated yet into osteocytes. The authors note that their cultures were not from fibroblasts, but it is worth noting that the ‘fibrosis’ in renal hyperparathyroidism is caused not by fibroblasts but rather by inchoate osteoblasts that have not achieved full differentiation because of inadequate bone morphogenetic protein 7. Probably these cells were able to grow in the milieu of the culture media, contributing to the rapid proliferation. Although one would expect a difference in some of the gene expression between cells grown from patients with osteitis fibrosa and cells from patients with adynamic bone disease, this was not seen. There may be other genes that would differentiate between these physiological situations with polar differences. One limitation of the report by Pereira and colleagues9 is that there were only four controls. The controls did not have CKD, but they did have scoliosis, and recent reports have found that patients with adolescent scoliosis frequently have bone disease, low bone density, or genetic abnormalities.10 The

control samples were taken from the vertebra at the time of surgery, whereas the CKD subjects had iliac crest bone biopsies, and there may be differences in the rate of bone loss and in the genes expressed by bone cells in different skeletal sites. The range of gene expression from the controls was not reported, and no histomorphometric measurements were made in the controls. Therefore, the controls may not have been representative of healthy adolescents. Further studies of the abnormalities in bone cells from CKD patients should be done, and our hope is that better understanding will eventually lead to therapies that can improve the clinical outcomes of these patients.

DISCLOSURE

The author declared no competing interests.

REFERENCES 1.

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7.

8.

9.

10.

Dallas SL, Prideaux M, Bonewald LF. The osteocyte: an endocrine cell ... and more. Endocr Rev 2013; 34: 658–690. Brandenburg VM, Kramann R, Koos R et al. Relationship between sclerostin and cardiovascular calcification in hemodialysis patients: a cross-sectional study. BMC Nephrol 2013; 14: 219–229. Wolf M. Update on fibroblast growth factor 23 in chronic kidney disease. Kidney Int 2012; 82: 737–747. Scialla JJ, Lau WL, Reilly MP et al. Fibroblast growth factor 23 is not associated with and does not induce arterial calcification. Kidney Int 2013; 83: 1159–1168. Jimbo R, Kawakami-Mori F, Mu S et al. Fibroblast growth factor 23 accelerates phosphate-induced vascular calcification in the absence of Klotho deficiency. Kidney Int 2014; 85: 1103–1111. Faul C, Amaral AP, Oskouei B et al. FGF23 induces left ventricular hypertrophy. J Clin Invest 2011; 121: 4393–4408. Ferron M, Lacombe J. Regulation of energy metabolism by the skeleton: osteocalcin and beyond. Arch Biochem Biophys 2014; 561C: 137–146. Lombardi G, Perego S, Luzi L et al. A fourseason molecule: osteocalcin. Updates in its physiological roles. Endocrine; e-pub ahead of print 27 August 2014. Pereira RC, Delany AM, Khouzam NM et al. Primary osteoblast-like cells from patients with end-stage kidney disease reflect gene expression, proliferation, and mineralization characteristics ex vivo. Kidney Int 2015; 87: 593–601. Schlosser TP, van der Heijden GJ, Versteeg AL et al. How ‘idiopathic’ is adolescent idiopathic scoliosis? A systematic review on associated abnormalities. PLoS One [online] 2014; 9: e97461.

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