A potential biomarker for CKD
CKD is not easy to detected at early stage of CKD and thus it is very difficult to make an early and accurate diagnosis. And there are no biomarkers which are able to be measured easily, sensitively, reliably, and specially, in correlation with presence, development, and complications of CKD [
135]. As described previously, renal klotho deficiency is highly associated with ion disorders, VC, inflammation, renal fibrosis, and mineral bone disorder, which are all characteristics of CKD. It has been shown that soluble klotho in the circulation starts to decline early in stage 2 CKD and urinary klotho possibly declines even earlier 1 [
14]. In addition, data show that klotho deficiency in CKD can enhance the renal tubular and vascular cell senescence induced by oxidative stress [
136,
137] and can result in defective endothelial function and impaired vasculogenesis [
138]. Together, these findings indicate that klotho deficiency is closely correlated with the development and progression of CKD and extrarenal complications. Thus, soluble klotho deficiency seems to have diagnostic potential, serving as an early and sensitive biomarker of CKD.
Many researchers have investigated the possibility of using klotho as a biomarker for CKD. CKD-MBD is one of the striking features associated with the high morbidity and mortality of cardiovascular events in CKD and ESRD [
51,
139]. Abnormal mineral metabolism includes high serum phosphate, FGF23, and PTH levels, which are closely associated with or even induced by klotho deficiency [
14,
140‐
142]. Clinical studies in patients with CKD have shown that soluble klotho is lower than normal (519 ± 183 versus 845 ± 330 pg/mL,
P < .0001) in renal patients, and soluble klotho is positively correlated with serum calcium and negatively correlated with serum phosphate, PTH, and FGF23, suggesting that soluble klotho might reflect the ensuing tubular resistance to FGF23, which could be an early marker of CKD-MBD [
143,
144]. Recently, another clinical study suggested that soluble klotho is significantly associated with phosphate reabsorption independently of FGF-23, which may be a marker of phosphate reabsorption [
145]. Therefore, soluble klotho seems to be a marker for disorders of phosphate and bone metabolism in CKD.
GFR, the gold standard for assessing kidney function, is significantly decreased in CKD [
112]. Clinical and experimental studies have shown that this significant decrease in klotho in the kidneys is positively associated with estimated GFR (eGFR) in CKD samples [
144‐
147]. Several other studies have confirmed the positive correlation between klotho levels (in serum and urine) and eGFR in adult patients with CKD [
7,
33]. Moreover, both serum and urine klotho levels are independently associated with eGFR in patients with CKD [
33,
148]. Another study showed that serum klotho levels are progressively lower with advancing CKD stage, with an adjusted mean decrease of 3.2 pg/mL for each 1 mL/min/1.73 m
2 eGFR decrease [
149]. Consistently, a similar positive correlation between plasma klotho levels and eGFR was shown in children with CKD [
150]. These results suggest that the decrease in soluble klotho may mirror an eGFR decrease in patients with CKD.
However, some researchers obtained adverse results. Sarah Seiler et al. analysed a large cohort of 312 patients with stage 2-4 CKD and found that plasma klotho levels were not significantly associated with eGFR or other calcium-phosphate metabolism parameters in these patients [
151]. Similarly, in a prospective observational study among 444 patients with CKD stages 2-4, klotho levels were not significantly related to cardiovascular outcomes [
152]. These results indicate that plasma klotho levels are not related to kidney function and do not predict adverse outcome in patients with CKD. There may be two reasons for this contradictory data. One is age. Yamazaki et al. suggested that soluble klotho levels are correlated with age, finding that klotho levels are higher in children (mean age 7.1±4.8 years) than in adults [
153]. Shimamura et al. also reported significantly lower klotho levels in CKD stage 2-5 patients than in CKD stage 1 patients. Moreover, this finding was largely based on data from four young individuals with normal eGFR and extremely high klotho levels, whereas klotho levels in the remaining participants did not predict adverse outcome of CKD [
143,
151]. Furthermore, a recent clinical study found that an allele of the G-395A klotho gene polymorphism has a significantly higher frequency among children with CKD, suggesting that this mutant allele of klotho can be used as a risk marker for the development of ESRD and as a predictor of CVD in children [
154]. Another reason may be the differences in sample size. The results obtained from some studies with small cohorts of CKD patients [
155‐
157] were different from those obtained with a large cohort [
151]. The idea of a decline in klotho levels with impaired kidney function has been further disputed by smaller studies [
151,
155,
158].
Although the results of relations between circulating klotho levels and outcomes of CKD are contradictory, three commonly used commercial immunoassay products for measuring soluble klotho-- are available from IBL (IBL International GmbH, Hamburg, Germany), Cusabio (Cusabio Biotech, Wuhan, China), and USCN (USCN Life Science Inc., Wuhan, China) [
159]. Only the IBL kit provides information on epitope specificity [
159]. However, researchers have found that these assays exhibited poor performance, including a lack of unit standardization in readouts, and the assays have to be improved to produce accurate results before they can provide reliable conclusions [
160].
As a potential treatment strategy for CKD
Although the causes of CKD are multifactorial, klotho deficiency is significantly associated with the development and progression of CKD and extrarenal complications. Many clinical and animal studies have suggested that when the klotho-deficient state in CKD is rescued, the renal function, morphologic lesion, and complications of CKD are obviously improved [
4,
14,
16,
135,
148,
161]. For example, the administration of soluble klotho protein significantly attenuated UUO-induced renal fibrosis and suppressed the expression of fibrosis markers and TGF-β1 target genes, such as
Snail and
Twist [
125]. Furthermore, klotho connected intermedin 1-53 to the suppression of VC in CKD rats [
162], and klotho supplementation suppressed the renin-angiotensin system to ameliorate Adriamycin nephropathy. In addition, klotho protein appeared to suppress the epithelial-mesenchymal transition by inhibiting TGF-β and Wnt signalling [
163]. Therefore, klotho deficiency may not only be a pathogenic intermediate in the acceleration of CKD progression but may also be a major contributor to chronic complications, such as CKD-MBD and cardiovascular diseases in CKD. Conceivably, any therapy that restores the klotho level by supplementation with exogenous klotho and/or the up-regulation of endogenous klotho production might be a novel treatment strategy for CKD [
14].
Several methods are dependent on various mechanisms to increase klotho expression (Table
1) [
14]; these includethe following: (1) Demethylation. Methylation of the klotho gene promoter reduces its activity by 30% to 40%, whereas DNA demethylation increases klotho expression 1.5-fold to threefold [
164]. (2) Deacetylation. Data show that the TNF and TWEAK-induced down-regulation of klotho expression in the kidney and kidney cell lines can be blunted by the inhibition of histone deacetylase [
74]. (3) Drugs. Several drugs on the market have been shown to up-regulate klotho expression in vivo and/or in vitro, including PPAR-γ agonists [
165], angiotensin II-type I receptor antagonists [
166], vitamin D active derivatives [
167,
168], and intermedin [
98]. (4) Klotho gene delivery. Klotho gene delivery through a viral carrier has been shown to effectively improve multiple pathophysiological phenotypes in klotho-deficient mice [
169], thereby preventing the progression of kidney damage in rat models [
170] and improving VC and endothelial function in CKD [
80]. (5) Administration of soluble klotho protein. Increasing circulating klotho levels through the administration of soluble klotho protein, which is the cleaved, full-length extracellular domain of membrane klotho, is more direct, safer, and an easier modality to restore endocrine klotho deficiency [
14,
72]. Animal studies have shown that the bolus administration of soluble klotho protein is a safe and effective means for protecting against kidney injury and preserving renal function [
14,
72].
Table 1
Potential treatment strategies for CKD via the up-regulation of klotho
DNA demethylation | Methylation of the klotho gene promoter reduces its activity by 30-40% |
Histone deacetylation | Hyperacetylation of histone in the klotho promoter down-regulates klotho expression |
Drugs: PPAR-γ agonists, angiotensin II-type I receptor antagonists, statin, vitamin D active derivatives, intermedin | These drugs can up-regulate klotho expression in vitro and in vivo |
Delivery of klotho cDNA | The klotho gene is transfected by viral carrier into target cells or animal models |
Soluble klotho protein administration | Recombinant klotho protein, which is the cleaved, full-length extracellular domain of membrane klotho, can be injected |