Evidence against a direct role of klotho in insulin resistance

Abstract

The klotho gene may be involved in the aging process. Klotho is a coactivator of FGF23, a regulator of phosphate and vitamin D metabolism. It has also been reported to be downregulated in insulin resistance syndromes and paradoxically to directly inhibit IGF-1 and insulin signaling. Our aim was to study klotho's regulation and effects on insulin and IGF-1 signaling to unravel this paradox. We studied klotho tissue distribution and expression by quantitative real-time polymerase chain reaction and Western blotting in obese Zucker rats and high-fat fed Wistar rats, two models of insulin resistance. Klotho was expressed in kidneys but at much lower levels (<1.5%) in liver, muscle, brain, and adipose tissue. There were no significant differences between insulin resistant and control animals. We next produced human recombinant soluble klotho protein (KLEC) and studied its effects on insulin and IGF-1 signaling in cultured cells. In HEK293 cells, FGF23 signaling (judged by FRS2-α and ERK1/2 phosphorylation) was activated by conditioned media from KLECproducing cells (CM-KLEC); however, IGF-1 signaling was unaffected. CM-KLEC did not inhibit IGF-1 and insulin signaling in L6 and Hep G2 cells, as judged by Akt and ERK1/2 phosphorylation. We conclude that decreased klotho expression is not a general feature of rodent models of insulin resistance. Further, the soluble klotho protein does not inhibit IGF-1 and/or insulin signaling in HEK293, L6, and HepG2 cells, arguing against a direct role of klotho in insulin signaling. However, the hypothesis that klotho indirectly regulates insulin sensitivity via FGF23 activation remains to be investigated.

Klotho Lacks a Vitamin D Independent Physiological Role in Glucose Homeostasis, Bone Turnover, and Steady-State PTH Secretion In Vivo

“…In agreement with our study, two rat models of insulin resistance failed to detect a regulation of Klotho, and administration of insulin sensitizers to rats did not increase renal Klotho expression [26]. However, several open questions remain.…”

Growth hormone and Klotho

“…sKlotho could also act as an enzyme (b-glucuronidase-like) to induce phosphaturia in an FGF23-independent manner; however, the direct enzyme target (potentially NaPi2a and NaPi2c) has been less well defined than in the case of TRPV5 (Hu et al 2010). Disruption of the Klotho gene not only causes growth arrest, hyperphosphataemia and high calcitriol but also causes low glucose and insulin levels; however, whether loss of Klotho directly improves insulin sensitivity is still unclear (Utsugi et al 2000, Hesse et al 2007, Lorenzi et al 2010. The mice are lean and have A B Figure 2 Effect of GH-secreting pituitary adenomas on circulating levels of IGF1 and sKlotho.…”

Klotho locus, metabolic traits, and serum hemoglobin in hospitalized older patients: a genetic association analysis

“…Notwithstanding the present promising findings, between the two actions of KL, e.g., modulation of the insulin/IGF signaling pathway or regulation of calcium and phosphate homeostasis, the latter pathway appears to be more important in regulating aging. In fact, normalization of vitamin D activity in mice lacking (Lorenzi et al 2010). However, the hypothesis that KL indirectly regulates insulin sensitivity via FGF23 activation remains to be investigated.…”

“…Therefore, these proteins may be candidates as regulatory factors in various KL/ HDL pathways (Walter 2009). WBC white blood cells, ESR erythrocyte sedimentation rate, CRP C-reactive protein, TG tryglicerides, TC total cholesterol, HDL-C high-density lipoprotein cholesterol, LDL-C low-density lipoprotein cholesterol, FT3 free triiodothyronine, IGF-1 insulin-like growth factor 1 a The analysis was adjusted for age and gender and the presence of diabetes mellitus, neurodegenerative diseases, cerebrovascular disease, and cardiovascular diseases, as well as for the use of statins, thyroid hormone, anti-inflammatory, and hypoglycemic drugs b The analyses were adjusted for age and the presence of diabetes mellitus, neurodegenerative diseases, cerebrovascular disease, and cardiovascular diseases, as well as for the use of statins, thyroid hormone, anti-inflammatory, and hypoglycemic drugs Table 7 Association of Klotho genotypes with some laboratory parameters according to different Genetic Risk Scores in all hospitalized older patients and gender-segregated analyses Also, glucose metabolism has been associated with genetic KL variants (Rhee et al 2006a;Shimoyama et al 2009a;Lorenzi et al 2010). Although KL is a type 2 diabetes mellitus candidate gene due to its importance in insulin signaling, as demonstrated by murine models, and its potential role in longevity (Kuro-o et al 1997;Utsugi et al 2000;Kurosu et al 2005;Arking et al 2002Arking et al , 2005, results from several studies on the KL-VS variant gave no support for this (Freathy et al 2006;Arking et al 2002Arking et al , 2003.…”

“…In humans, KL polymorphisms have been extensively associated with glucose metabolism (Rhee et al 2006a;Shimoyama et al 2009a;Lorenzi et al 2010), lipid metabolism, serum levels of high-density lipoprotein cholesterol (HDL-C) (Arking et al 2005;Walter 2009;Shimoyama et al 2009a, b) and uric acid (Shimoyama et al 2009b). KL polymorphisms have been also variously associated with some features of the aging process such as osteopenia/osteoporosis (Kawano et al 2002;Shimoyama et al 2009a;Mullin et al 2005;Yamada et al 2005), atherosclerosis , coronary artery disease (Arking et al 2003;Low et al 2005;Imamura et al 2006;Rhee et al 2006a, b;Walter 2009), stroke (Arking et al 2005;Kim et al 2006), blood pressure (Arking et al 2005;Rhee et al 2006a;Shimoyama et al 2009a), type 2 diabetes mellitus (Freathy et al 2006) and more generally with human lifespan, although with contrasting results (Utsugi et al 2000;Arking et al 2002Arking et al , 2005Novelli et al 2008;Invidia et al 2010).…”