CKD-MBD- II Dr. Sandeep G Huilgol MBBS., DNB (Internal Medicine)., MMedSci(Nephrology)
FGF-23• Seven known subfamilies of human FGFs have been defined.• The FGF-19 subfamily is composed of three proteins—FGF- 19, FGF-21, and FGF-23.• FGF-23 is a central regulator of phosphate homeostasis and calcitriol blood levels;• FGF-19 inhibits the expression of enzyme cholesterol 7- ahydroxylase (CYP7A1), which is the first and rate-limiting• step in bile acid synthesis• FGF-21 stimulates insulin independent glucose uptake in adipocytes and lowers triglycerides.• FGF-19, FGF-21, and FGF-23 contain a disulfide bond that is absent inmost other subfamilies hence these are present in blood stream to mediate their functions.
• FGF-23 is a 251-amino acid protein (MW 26 kDa) synthesized and secreted by bone cells, mainly osteoblast.• Amino-terminal signal peptide (residues 1–24), “FGF- like sequence” (residues 25–180) and a carboxyl- terminal extended sequence (residues 181– 251) that is unique compared with other members of the FGF family .• The half-life of intact FGF-23 in the circulation of healthy individuals is 58min.• FGF-23 exerts its biological effects through activation of FGF receptors (FGF-Rs).• This activation is Klotho dependent as a Klotho/FGF-R complex binds to FGF-23 with higher affinity than does FGF-R or Klotho alone.
Physiological Functions of FGF-23• Renal phosphate excretion is physiologically regulated mainly by proximal tubular cells, which express both Na/Pi Type IIa and Na/Pi IIc cotransporters at their apical membrane• FGF-23 reduces the action of both cotransporters; in addition, it may inhibit gastrointestinal phosphate absorption by reducing intestinal Na/Pi IIb cotransporter activity in a vitamin D-dependent manner.• The principal physiological stimuli for increased FGF-23 expression both in vitro and in vivo are 1,25(OH)2D3 and high dietary phosphate intake
• Persistent hyperphosphataemia triggers FGF-23, while rapid changes in serum phosphate concentrations does not induce an acute increment in serum FGF-23 levels .• Therefore FGF-23 responds to the net phosphate balance rather than to the serum phosphate level.
FGF-23, PTH, and Calcitriol• FGF-23, PTH, and calcitriol influence each other in opposite manner.• FGF-R and Klotho are expressed in parathyroid glands.• FGF-23 might decrease PTH mRNA transcription• FGF-23 activity is not dependent on PTH as the phosphaturic effects of FGF-23 are maintained in animals after parathyroidectomy .• Conversely, PTH may stimulate FGF-23 secretion by osteoblast.• Experimental injection of recombinant FGF-23 reduces calcitriol levels within hours by decreasing renal expression of 1a-hydroxylase (CYP27B1) and increasing the expression of 24-hydroxylase (CYP24A1).• Conversely, calcitriol itself stimulates FGF-23 generation by binding to a vitamin D response region in the FGF-23 gene promoter.
FGF-23 in Subjects with Intact Renal Function• Physiological role of FGF-23 in healthy subjects is to regulate urinary phosphate excretion to maintain stable serum phosphate levels.• However, no correlation between FGF-23 and serum phosphate levels has been found in individuals without overt renal disease.
FGF-23 in CKD• In CKD, circulating FGF-23 levels gradually increase with declining renal function.• In end-stage renal disease, FGF23 levels can be up to 1000-fold above the normal range .• The increase in FGF-23 begins at a very early stage of CKD as a physiological compensation to stabilize serum phosphate levels as the number of intact nephrons declines.• In contrast, it was hypothesized that increased FGF- 23 levels in CKD result primarily from decreased renal clearance.
• However, there is no increase in the accumulation of degraded FGF-23 in advanced CKD.• FGF-23 levels also depend on an increased secretion due to an end-organ resistance to the phosphaturic stimulus of FGF-23 because of a deficiency of the necessary Klotho cofactor.• Release of unidentified FGF-23 stimulatory factors or loss of a negative feedback factor(s) that normally suppress FGF-23, by the failing kidney.
FGF-23,Mortality, and Cardiovascular End Points• In patients starting hemodialysis, higher FGF-23 levels were strongly associated with increased risk of 1-year mortality.• In early CKD, it is observed that higher FGF-23 is linked to several dynamic measurements of vascular dysfunction like arterial stiffness measured by pulse wave velocity and endothelial dysfunction.• FGF-23 levels can be assosiated with vascular dysfunction, atherosclerosis, and left ventricular in patients with a lower eGFR despite normal phosphate levels.
Klotho• Klotho is a gene that encodes a novel protein regulating multiple functions.• Discovered in 1997 by Kuroo and colleagues and named after the goddess who spins the thread of life in Greek mythology.• In mice, the deletion of Klotho gene causes a – phenotype of premature human aging – vascular calcification, – altered calcium/phosphate metabolism with hyperphosphataemia, – shortened lifespan.
• Klotho protein exists in two forms: – Type I transmembrane protein (1014 amino acids) with a large extracellular domain and a short intracellular portion (10 amino acids), predominantly expressed in the renal tubules. – A circulating soluble factor detectable in blood and in lesser extent in other biological fluids.
• Soluble Klotho is produced either by proteolytic cleavage of the extracellular domain of the transmembrane form (130 kDa isoform) operated by the membrane-anchored proteases ADAM10 and ADAM17 .• Or by alternative mRNA splicing (isoform 70 kDa).• The systemic effects of this protein appear to be predominantly due to the circulating form.• The transmembrane protein forms a complex with fibroblast growth factor (FGF) receptors and works as a coreceptor for FGF23 for phosphate excretion into urine.
• Hyperphosphataemia in Klotho and FGF23- mutant mice is due to hypervitaminosis D and increased expression/activity of renal sodium- dependent phosphate cotransporters.• Klotho-deficient mice display higher levels of FGF23, and a low-phosphate diet reduces the levels of FGF23 and results in a rescue of the features of premature aging.• This suggests that FGF23 per se cannot promote a phosphaturic effect in absence of Klotho.
• Klotho/FGF23 signalling induces phosphaturia by suppressing the sodium-dependent phosphate cotransporters type IIa (NPT2a) expressed on the brush border membrane of renal tubular cells.• Soluble Klotho has also been found to regulate directly the phosphate transport, in the proximal tubule of the kidney by deglycosylation of NaPi-2a cotransporters .• The resulting reduction in number and activity of NaPi-2a promotes phosphaturia independently of FGF-23.• Soluble Klotho also inhibits type III sodiumdependent phosphate cotransporters (Pit1 and Pit2) which are ubiquitously expressed and mediate phosphate uptake
• High FGF23 levels in patients with chronic kidney disease are due to – Declining renal clearance – Compensatory response to hyperphosphataemia.
Role of Klotho in Cardiovascular and Renal Disease• Reduction in renal, serum, and urine levels of Klotho has been observed with – Normal ageing. – Diabetes – Hypertension – Chronic kidney disease – Acute kidney injury – Kidney ischemia – Glomerulonephritis
• There is evidence in animals that the overexpression of soluble Klotho can – Reverse the ageing process – Provides cardiovascular-renal protection – Mechanism • inducing resistance to oxidative stress and protecting tissues from oxidative damage
Nephroprotective Effects of Klotho• Nephroprotective effects of this protein are mostly attributable to the antioxidant properties of its soluble form.• Klotho expression is reduced experimentally in renal distal tubules, urine, and blood of rats subjected to bilateral renal ischemia• The injection of an adenovirus harbouring the Klotho or the administration of recombinant soluble Klotho protein prior to the induction of the ischemic insult blunts the increase in creatinine and attenuates the tubulointerstitial damage.
• Klotho expression is also downregulated in an animal model of spontaneous hypertension.• The delivery of Klotho has been shown to prevent the progression of hypertension, renal damage, and the proteinuria.• Possible explanations for these observations are – Reduction of renal superoxide – Suppression of NADPH oxidase activity that is the main source of reactive oxygen species (ROS).
• Nephroprotective in animal model of glomerulonephritis.• In glomerulonephritis Klotho over expression results in – increased survival, – attenuated glomerular and tubulointerstitial changes, and – reduced proteinuria and blood urea nitrogen.
• It prevents the acute renal fibrosis through the inhibition of TGF-β1 signalling.• Klotho binds to the type II receptor (TGFβR2) suppressing the activation of the type I receptor (TGFβR1) that phosphorylates Smad2/3 proteins (transcription factors regulating the expression of TGFβ1 target genes
Vascular Protective Effects of Klotho• Soluble Klotho has an important role in maintaining endothelial wall homeostasis and promoting the health of the vasculature.• Endothelial dysfunction results from the imbalance between the release of vasodilator and vasoconstrictor factors.• Klotho gene increases endothelium dependent NO synthesis and prevents adverse vascular remodelling.
• Involved in the modulation of endothelial inflammation by suppressing the expression of adhesion molecules involved in the pathogenesis of vascular disease – Intracellular adhesion molecule-1 (ICAM-1) – vascular cell adhesion molecule-1 (VCAM-1)• Klotho is a direct inhibitor of vascular smooth muscle cell (VSMC) calcification.
References• Giuseppe Maltese and Janaka Karalliedde. The Putative Role of the Antiageing Protein Klotho in Cardiovascular and Renal Disease. International Journal of Hypertension. Volume 2012.• Domenico Russo and Yuri Battaglia. Clinical Significance of FGF-23 in Patients with CKD. International Journal of Nephrology. Volume 2011.