title: 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-alpha and ERK1/2 phosphorylation) was activated by conditioned media from KLEC-producing 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.
authors.affiliation: Department of Cell Physiology and Metabolism, Faculty of Medicine, University of Geneva, Geneva, Switzerland.
authors.forename: Olivier
authors.initials: O
authors.lastname: Lorenzi
authors.forename: Christelle
authors.initials: C
authors.lastname: Veyrat-Durebex
authors.forename: Claes B
authors.initials: CB
authors.lastname: Wollheim
authors.forename: Pascal
authors.initials: P
authors.lastname: Villemin
authors.forename: Françoise
authors.initials: F
authors.lastname: Rohner-Jeanrenaud
authors.forename: Anne
authors.initials: A
authors.lastname: Zanchi
authors.forename: Ulrich M
authors.initials: UM
authors.lastname: Vischer
authors.forename: Ulrich
authors.initials: U
authors.lastname: Vischer
mesh_terms: D000818:Animals; D002460:Cell Line; D004041:Dietary Fats; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000378:metabolism; D005786:Gene Expression Regulation; D005966:Glucuronidase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007004:Hypoglycemic Agents / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D007333:Insulin Resistance / Q000502:physiology*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D000090265:Klotho Proteins; D008297:Male; D051379:Mice; D000077205:Pioglitazone; D051381:Rats; D017208:Rats, Wistar; D011924:Rats, Zucker; D015398:Signal Transduction / Q000502:physiology; D045162:Thiazolidinediones / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D004041:Dietary Fats; C000717427:FGF23 protein, human; C000717428:Fgf23 protein, mouse; C505605:Fgf23 protein, rat; D007004:Hypoglycemic Agents; D007328:Insulin; D045162:Thiazolidinediones; D005346:Fibroblast Growth Factors; D007334:Insulin-Like Growth Factor I; D000089703:Fibroblast Growth Factor-23; D005966:Glucuronidase; D000090265:Klotho Proteins; D000077205:Pioglitazone