Cooperative Inhibition of Wnt/β‐Catenin Signaling by Klotho and Vitamin D: Implications for Chemoprevention

Abstract

The Klotho gene has been closely associated with delayed onset of aging. Inhibition of the klotho protein promotes aging‐like phenotypes, while its overexpression has been found to extend lifespan in mice. The single‐pass transmembrane isoform of klotho (m‐klotho) forms a receptor complex with the fibroblast growth factor (FGF) receptor to create a high‐affinity binding site for FGF23, a hormone involved in phosphate homeostasis and suppressing vitamin D activation. In contrast, secreted klotho (s‐klotho) forms via ectodomain shedding of the membrane‐bound isoform or through alternative splicing of the Klotho gene. It exhibits pleiotropic functions presumably via its weak glycosidase activity, including the inhibition of the insulin/insulin‐like growth factor 1. The proposed anti‐aging properties of s‐klotho have already been demonstrated by its endocrine role in reducing oxidative stress and promoting cardiovascular protection. Due to its potential role as a tumor‐suppressor, we have focused on how s‐klotho interacts with a known chemopreventative agent and steroid hormone, 1,25‐dihydroxyvitamin D3 (1,25 D), and its nuclear receptor (VDR) to inhibit the activity of target proto‐oncogenes. One such target for 1,25D/klotho interaction is β‐catenin. Mutations in the Wnt/β‐catenin signaling pathway have been implicated in numerous cancers, particularly colorectal neoplasia. In this study, the activity of β‐catenin was analyzed in vitro using transcriptional luciferase assays and western blotting as a response to modulated levels of exogenous klotho, 1,25D, and VDR. In addition, quantitative polymerase chain reaction (qPCR) was utilized to evaluate the ability of s‐klotho and 1,25D to inhibit the expression of known β‐catenin target genes including cell cycle regulators such as cyclin D1 and c‐myc, as well as the apoptosis inhibitor survivin. Our results revealed that s‐klotho stimulated VDR transactivation while inhibiting β‐catenin activity independently as well as synergistically with VDR and 1,25D. These findings were further supported by the 1,25D/klotho‐mediated down‐regulation of cyclin D1, c‐myc, and survivin expression. The dual isoforms of klotho exhibited functional differences, as lower concentrations of m‐klotho were found to boost β‐catenin activity, implicating the secreted isoform of klotho in directing its hypothesized anti‐aging properties. Our findings indicate that the crosstalk between vitamin D and klotho may promote healthful aging in part due to their cooperative tumor‐suppressive role and inhibition of factors implicated in oncogenic signaling.Support or Funding InformationThis project was funded by the New College of Interdisciplinary Arts and Sciences at Arizona State University and the National Institute of Health.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.