FGF 23 and KLOTHO

FGF 23• Seven known subfamilies of human FGFs have been defined• FGF-23 is a 251-amino acid protein (MW 26 kDa)

synthesized and secreted by bone cells, mainly osteoblast

• It is composed of an a) amino-terminal signal peptide (residues 1–24), b) followed by an “FGF-like sequence” (residues 25–180) and c) a carboxyl-terminal extended sequence (residues 181–251) that is unique compared with other members of the FGF family

• T ½- 58 min

• 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

FGF-23- PTH- VitD3 axes

Active form of vitamin D (1,25-dihydroxyvitamin D3) binds to vitamin D receptor (VDR) in the bone (osteocytes).

The ligand-bound VDR forms a heterodimer with anuclear receptor retinoid X receptor (RXR) and transactivates expression of the FGF23 gene.

FGF23 secreted from bone acts on the Klotho–FGF receptor (FGFR) complex expressed in the

a) kidney (the bone−kidney axis) and

b) parathyroid gland (the bone−parathyroid axis)

In the kidney• Downregulates luminal sodium/phosphate

cotransporters in the proximal tubule NPt IIa, decreasing phosphorus reabsorption and therefore increasing its excretion• FGF23 suppresses synthesis of active vitamin D

by downregulating expression of the Cyp27b1 gene and

• Promotes its inactivation by upregulating expression of the Cyp24 a1 (24-hydroxylase)

In the parathyroid gland, • FGF23 suppresses production and• Secretion of parathyroid hormone (PTH).

PTH binds to the PTH receptor (PTHR) expressed on renal tubular cells, leading to upregulation of the Cyp27b1 gene expression.

Thus, suppression of PTH by FGF23 reduces expression of the Cyp27b1 gene (1alfa hydroxylase) and serum levels of 1,25-dihydroxyvitamin D3.

The principal physiological stimuli for increased FGF-23 expression both in vitro and in vivo area)1,25(OH)2D3 andb)high dietary phosphate intake

Persistent hyperphosphataemia is an effective trigger for FGF-23, while rapid changes in serum phosphate concentrations may not induce an acute increment in serum FGF-23 levels

It is therefore possible that FGF-23 responds to the netphosphate balance rather than to the serum phosphatelevel, but experimental data supporting this hypothesis arescarce.

FGF-23 CKD• In CKD, circulating FGF-23 levels gradually increase with

declining renal function 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 trading off with hypovitaminosis D and secondary HPT

• It is also likely that FGF-23 levels 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 .

FGF-23,Mortality,and Cardiovascular End Points

• Indeed, in patients starting hemodialysis, higher FGF-23 levels were strongly associated with increased risk of 1-year mortality, reaching a nearly 6-fold higher risk.

• FGF-23 was stronger predictor of mortality than the serum phosphate level.

• FGF23 did not associate to mortality in patients within the highest quartile of serum phosphate suggesting that the prognostic value of FGF23 is blunted in the presence of severe hyperphosphatemia N Engl J Med 2008;359:584-92.

• Higher FGF-23 was linked to several dynamic measurements of vascular function, includingarterial stiffness and endothelial dysfunction.• Higher FGF-23 level was associated with higher

atherosclerosis score and coronary and peripheral vasculature calcification score

• Consequent LVH noted

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 aphenotype of • premature human aging • including vascular calcification,• altered calcium/phosphate metabolism with

hyperphosphataemia,• and shortened lifespan.

Klotho protein exists in two forms: Type I transmembrane protein (1014 AA) • with a large extracellular domain and a short intracellular portion• predominantly expressed in the renal tubules,

Circulating soluble factor • detectable in blood and in lesser extent in other

biological fluids

• The systemic effects of this protein appear to be predominantly due to the circulating form.

• Klotho is predominantly expressed in the renal distal tubularcells

• The transmembrane protein forms a complex with fibroblast growth factor (FGF) receptors and works as an obligate coreceptor for FGF23

• FGF23 per se cannot promote a phosphaturic effect in absence of Klotho

• Klotho/FGF23 signalling induces phosphaturia by suppressing (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 sodium dependentphosphate cotransporters (Pit1 and Pit2) which are ubiquitously expressed and mediate phosphate uptake

• The reduction in Klotho expression observed in Chronic kidney disease may be an important event contributing to accumulation of FGF23 being a Compensatory mechanism to the increase of phosphate levels driven by the primary reduction in Klotho.

• Changes in Klotho levels appear to precede changes in phosphate levels, the key driver of FGF23 balance in renal disease

Nephroprotective Effects of Klotho

• Nephroprotective effects of this protein are mostly attributable to the antioxidant properties of its soluble form

• Causes reduction of renal superoxide and suppression of NADPH oxidase activity that is the main source of reactive oxygen species (ROS) which are all involved in the pathogenesis of renal disease.

• Decresed levels seen in ischemia,hypertension

• Delivery to distal tubule by adenovirus with klotho gene causes prevention of the progression of hypertension, renal damage, and the proteinuria

• Klotho may be an early clinical biomarker of acute and chronic renal injury CKD as its diminution precedes changes of other well-establishedmarkers/factors involved in the progression of renal failure.

• Urinary Klotho levels have been tested for the same.

Vascular Protective Effects of Klotho

In experimental models, the absence of Klotho gene is associated with

• endothelial dysfunction and • diffuse vascular calcification