*5.2. Klotho Pathway*

Kidney-derived klotho is of particular interest as a potential treatment for inflammation and premature ageing in CKD, because klotho-deficient mice and patients with CKD have similar phenotypes, such as EVA and a pro-inflammatory status, and klotho is related to ageing in both humans and mice [70,72]. Similar to the NRF2 system, klotho is repressed in CKD, and stimulation of klotho can ameliorate oxidative stress and mitochondrial function [108], renal fibrosis [152], inflammation [72], as well as premature ageing [108], including EVA [55,81–83,111–113].

Importantly, and possibly in contrast to the NRF2–KEAP1 signaling pathway, klotho is down-regulated in several cancers and is also recognized as a potential anti-tumor therapy [153].

Therapeutic approaches to stimulate klotho expression can aim for the reactivation of endogenous klotho or the administration of exogenous klotho [72]. Several drugs increase endogenous klotho [72]. For instance, the thiazolidinedione pioglitazone protects against renal injury in ageing by an increased expression of klotho [72]. Furthermore, the vitamin D analog paricalcitol induces the tissue-dependent expression of klotho in the kidneys and increases serum and urinary klotho levels in rodent CKD models [72]. In vitro data using murine internal medulla collecting duct epithelial cells further sugges<sup>t</sup> that statins induce *klotho* mRNA expression [72]. Moreover, because angiotensin II and aldosterone decrease *klotho* mRNA expression in vitro and in vivo [70], RAAS blockers potentially reverse the decrease in *klotho* expression in rodents [70]. Recently, testosterone was positively correlated with circulating klotho levels in both men and women, and the association sustained significance after adjustment for cortisol and markers of renal function but not chronological age [154]. However, and in contrast to this study, a randomized controlled trial of transdermal testosterone does not find changes in soluble klotho levels between the verum and placebo group [155]. Finally, direct administration of exogenous, soluble klotho has also been proven e ffective for increasing circulating klotho levels, as well as protecting against acute kidney injury and CKD [72,156]. It has to be pointed out that, except for direct klotho administration, each of the mentioned pharmacological compounds is already approved for the use in CKD, and for some but not all of these, positive renal outcome data in humans with CKD are available [157]. However, the relative contribution of increased klotho on the outcome of these human studies has not been analyzed thus far. Hence, the direct e ffect of klotho on uremic inflammation and premature ageing needs to be addressed. However, preliminary safety data appear to be more attractive compared to targeting the NRF2–KEAP1 signaling pathway.
