title: Glucocorticoid Suppresses Connexin 43 Expression by Inhibiting the Akt/mTOR Signaling Pathway in Osteoblasts.
abstract: The inhibition of proliferation or functional alteration of osteoblasts by glucocorticoids (GCs) has been recognized as an important etiology of GC-induced osteoporosis (GIO). Connexin 43 (Cx43) is the most abundant connexin isoform in bone cells and plays important roles in bone remodeling. Despite the important role of Cx43 in bone homeostasis and the prevalence of GIO, the direct action of GCs on Cx43 expression in osteoblasts has been poorly described. The aim of the present study was to evaluate how GCs affect Cx43 expression in osteoblasts. Dexamethasone (Dex) treatment decreased expression of Cx43 RNA and protein in MC3T3-E1 mouse osteoblastic cells. Reduction of Cx43 expression by Dex was dependent on the glucocorticoid receptor (GR), as it was abolished by pretreatment with a GR blocker. Treatment with PTH (1-34), a medication used for GIO management, counteracted the suppression of Cx43 by Dex. Akt or mTOR signaling modulators revealed the involvement of the Akt/mTOR signaling pathway in Dex-induced reduction of Cx43 expression. Moreover, overexpression of Cx43 significantly attenuated Dex-inhibited cell viability and proliferation, as evidenced by MTT and bromodeoxyuridine (BrdU) incorporation assay of MC3T3-E1 cells. To account for possible species or cell type differences, human primary osteoblasts were treated with Dex and similar downregulation of Cx43 by Dex was observed. In addition, immunofluorescent staining for Cx43 further demonstrated an apparent decrease in Dex-treated human osteoblasts, while analysis of lucifer yellow propagation revealed reduced gap junction intercellular communication by Dex. Collectively, these findings indicate that GCs suppress Cx43 expression in osteoblasts via GR and the Akt/mTOR signaling pathway and overexpression of Cx43 may, at least in part, rescue osteoblasts from GC-induced reductions in proliferation.
authors.affiliation: Division of Endocrinology and Metabolism, Department of Internal Medicine, Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Chonbuk National University Medical School, Geonji-Ro 20, Deokjin-Gu, Jeonju, 561-712, Republic of Korea.
authors.forename: Chen
authors.initials: C
authors.lastname: Shen
authors.affiliation: Division of Endocrinology and Metabolism, Department of Internal Medicine, Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Chonbuk National University Medical School, Geonji-Ro 20, Deokjin-Gu, Jeonju, 561-712, Republic of Korea.
authors.forename: Mi Ran
authors.initials: MR
authors.lastname: Kim
authors.affiliation: Division of Endocrinology and Metabolism, Department of Internal Medicine, Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Chonbuk National University Medical School, Geonji-Ro 20, Deokjin-Gu, Jeonju, 561-712, Republic of Korea.
authors.forename: Jeong Mi
authors.initials: JM
authors.lastname: Noh
authors.affiliation: Division of Endocrinology and Metabolism, Department of Internal Medicine, Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Chonbuk National University Medical School, Geonji-Ro 20, Deokjin-Gu, Jeonju, 561-712, Republic of Korea.
authors.forename: Su Jin
authors.initials: SJ
authors.lastname: Kim
authors.affiliation: Department of Internal Medicine, Presbyterian Medical Center, Jeonju, Republic of Korea.
authors.forename: Sun-O
authors.initials: SO
authors.lastname: Ka
authors.affiliation: Department of Biochemistry, Chonbuk National University Medical School, Jeonju, Republic of Korea.
authors.forename: Ji Hye
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of Internal Medicine, Presbyterian Medical Center, Jeonju, Republic of Korea.
authors.forename: Byung-Hyun
authors.initials: BH
authors.lastname: Park
authors.affiliation: Division of Endocrinology and Metabolism, Department of Internal Medicine, Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Chonbuk National University Medical School, Geonji-Ro 20, Deokjin-Gu, Jeonju, 561-712, Republic of Korea. parkjh@jbnu.ac.kr.
authors.forename: Ji Hyun
authors.initials: JH
authors.lastname: Park
mesh_terms: D000818:Animals; D001842:Bone and Bones / Q000378:metabolism; D002460:Cell Line; D018031:Connexin 43 / Q000187:drug effects / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D017629:Gap Junctions / Q000378:metabolism; D005938:Glucocorticoids / Q000378:metabolism / Q000494:pharmacology*; D051379:Mice; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism; D010024:Osteoporosis / Q000188:drug therapy / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*
chemical_list: D018031:Connexin 43; D005938:Glucocorticoids
keywords: Akt; Connexin 43; Glucocorticoid; Osteoblast; mTOR