There is emerging evidence for an endocrine function of bone that plays a key role in regulating phosphate homeostasis and vitamin D metabolism through the release of the hormone FGF23 by osteoblasts and osteocytes. Defining the molecular mechanisms controlling FGF23 production by bone is of critical importance in understanding the physiological and pathological role of this hormone. Studies elucidating the genetic basis for hereditary hypophosphatemic disorders and mutant mouse models have identified the endopeptidase Phex, the SIBLING protein Dmp1, and the endonuclease Enpp1, as local regulators of both the mineralization of extracellular matrix and Fgf23 production in bone. Activation of FGFR1/PI3K/Akt/-catenin and HMW-FGF2-dependent integrative fibroblastic growth factor receptor signaling (INFS) appear to couple bone mineralization with FGF23 release as well as integrate the effects of systemic factors, such as the PTH receptor/GNAS pathway, which has context-dependent effects on FGF23 gene transcription in osteoblasts /osteocytes. In addition, 1,25(OH)2D functions as a counter-regulatory hormone for FGF23, directly regulates FGF23 gene transcription through VDR-mediated mechanisms and is required for PTH regulation of FGF23 expression. This project proposes to understand how FGF23, vitamin D, PTH, Phex, Dmp1, and FGFR1 are integrated into a biological network that permits cross talk between systemic factors, the extracellular matrix mineralization process and osteoblast/osteocyte production of FGF23 to coordinate systemic phosphate and vitamin D homeostasis and bone formation/ mineralization. We propose studies that investigate the convergence of these local bone and systemic factors through canonical FGFR1, INFS, PTH and VDR-dependent signaling pathways to control basal and stimulated transcription of FGF23 in osteoblasts and osteocytes. We also propose to assess the relative roles of canonical FGFR1 and INFS in regulating FGF23 gene transcription both in vitro using osteoblasts derived from Phex-deficient Hyp and Dmp1-/- mice and in vivo by the conditional deletion of FGFR1 from osteoblasts and osteocytes of Hyp and Dmp1-/- mice. In addition, we will investigate cross-talk between 1,25(OH)2D, PTH and FGFR1 pathways in regulating FGF23 gene transcription in osteoblasts in vitro. These studies will lead to new knowledge of a complex systems biology that has evolved to permit cross-talk between bone and kidney to coordinate phosphate balance, vitamin D metabolism and bone mineralization. Such knowledge will help us understand and better manage patients with hyperphosphatemic and hypophosphatemic disorders.

Public Health Relevance

We have discovered that bone is an endocrine organ in which osteoblasts and osteocytes release FGF23, a phosphaturic hormone that participates in a bone-kidney axis to regulate phosphate, vitamin D and mineral homeostasis. We are pursuing studies to define the precise signaling pathways linking the mineralization of extracellular matrix with FGF23 production in bone, to establish that a physiological function of FGF23 is to coordinate bone mineralization and renal handling of phosphate, and to understand the complex interactions between PTH and 1,25(OH)2D and actions of local bone derived factors through FGFR1 to regulate FGF23 gene transcription. Overall our work is establishing a new conceptual framework whereby bone communicates with other organs to regulate phosphate and vitamin D metabolism in response to changing physiological requirements and is leading to a better understanding of the pathogenesis, diagnosis and treatment of hyperphosphatemic and hypophosphatemic disorders.

Agency
National Institute of Health (NIH)
Institute
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Type
Research Project (R01)
Project #
5R01AR045955-17
Application #
8926667
Study Section
Skeletal Biology Structure and Regeneration Study Section (SBSR)
Program Officer
Chen, Faye H
Project Start
1999-04-05
Project End
2016-08-31
Budget Start
2015-09-01
Budget End
2016-08-31
Support Year
17
Fiscal Year
2015
Total Cost
Indirect Cost
Name
University of Tennessee Health Science Center
Department
Internal Medicine/Medicine
Type
Schools of Medicine
DUNS #
941884009
City
Memphis
State
TN
Country
United States
Zip Code
38103
Fitzpatrick, Elizabeth A; Han, Xiaobin; Xiao, Zhousheng et al. (2018) Role of Fibroblast Growth Factor-23 in Innate Immune Responses. Front Endocrinol (Lausanne) 9:320
Kovesdy, Csaba P; Lu, Jun Ling; Wall, Barry M et al. (2018) Changes With Lanthanum Carbonate, Calcium Acetate, and Phosphorus Restriction in CKD: A Randomized Controlled Trial. Kidney Int Rep 3:897-904
Pi, Min; Ye, Ruisong; Han, Xiaobin et al. (2018) Cardiovascular Interactions between Fibroblast Growth Factor-23 and Angiotensin II. Sci Rep 8:12398
Han, Xiaobin; Quarles, L Darryl (2018) Letter to the Editor: ""Increased Circulating FGF23 Does Not Lead to Cardiac Hypertrophy in the Male Hyp Mouse Model of XLH"". Endocrinology 159:3655-3656
Han, Xiaobin; Ross, Jed; Kolumam, Ganesh et al. (2018) Cardiovascular Effects of Renal Distal Tubule Deletion of the FGF Receptor 1 Gene. J Am Soc Nephrol 29:69-80
Xiao, Zhousheng; Baudry, Jerome; Cao, Li et al. (2018) Polycystin-1 interacts with TAZ to stimulate osteoblastogenesis and inhibit adipogenesis. J Clin Invest 128:157-174
Han, Xiaobin; Li, Linqiang; Yang, Jiancheng et al. (2016) Counter-regulatory paracrine actions of FGF-23 and 1,25(OH)2 D in macrophages. FEBS Lett 590:53-67
Han, Xiaobin; Quarles, L Darryl (2016) Multiple faces of fibroblast growth factor-23. Curr Opin Nephrol Hypertens 25:333-42
Han, Xiaobin; Yang, Jiancheng; Li, Linqiang et al. (2016) Conditional Deletion of Fgfr1 in the Proximal and Distal Tubule Identifies Distinct Roles in Phosphate and Calcium Transport. PLoS One 11:e0147845
Zhang, Qian; Doucet, Michele; Tomlinson, Ryan E et al. (2016) The hypoxia-inducible factor-1? activates ectopic production of fibroblast growth factor 23 in tumor-induced osteomalacia. Bone Res 4:16011

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