Sirtuin/FOXO Signaling in the Regulation of Bone Mass

Columbia University Health Sciences, New York NY

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Abstract

DESCRIPTION (provided by applicant): Component of the application in the parent application we have described the FoxO family of ubiquitous transcription factors which regulate development and metabolic homeostasis. Therein we proposed to study the role of FoxO1, one of three FoxO proteins, and its interaction with Sirtuin or 2-catenin in the control of bone homeostasis. However FoxO1 is known to regulate glucose homeostasis and insulin resistance through its expression in the pancreas and liver. Because of these observations and given the recent realization that osteoblasts are endocrine cells favoring glucose homeostasis, in studies outside the specific aims of the parent application, we examined potential metabolic functions that FoxO1 may exert through its osteoblastic expression. We have found that deletion of FoxO1 specifically from osteoblasts (FoxO1ob-/- mice) decreases osteoblast numbers, bone formation rate and bone volume due to decreased osteoblast proliferation. Osteoclast numbers were not affected. Remarkably, many FoxO1ob-/- mice died before weaning. We found that perinatal lethality could be explained by the fact that Foxo1ob-/- mice were hypoglycemic and displayed improved glucose tolerance. These effects appeared to stem from an increase in 2-cell proliferation, insulin secretion and insulin sensitivity. Additional molecular as well as metabolic studies show that FoxO1, through its expression in osteoblasts, inhibits 2-cell proliferation, insulin secretion and insulin sensitivity. Based on these and additional findings we hypothesize that FoxO1, through its expression in osteoblasts, regulates glucose homeostasis by controlling the expression or activity of two osteoblast-produced proteins Esp and Osteocalcin. To test this hypothesis we will determine through genetic means whether FoxO1 in osteoblasts controls energy metabolism by acting through Esp. We will also examine whether FoxO1 in osteoblasts suppresses insulin secretion and sensitivity by limiting the bioactivity of osteocalcin. These studies will provide novel insights into the mechanisms by which the skeleton controls signals that affect glucose homeostasis. PUBLIC HEALTH RELEVANCE: Component of the application As described in the parent application we deleted Foxo1 specifically from osteoblasts (FoxO1ob-/- mice) and found decreased osteoblast numbers, bone formation rate and bone volume. In addition, Foxo1ob-/- mice were hypoglycemic and displayed improved glucose tolerance. These effects stem from an increase in 2-cell proliferation, insulin secretion and insulin sensitivity. Based on these and additional findings we hypothesize that FoxO1, through its expression in osteoblasts, regulates glucose homeostasis by controlling the expression or activity of two osteoblast-produced proteins Esp and Osteocalcin. These studies will provide novel insights into the mechanisms by which the skeleton controls signals that affect glucose homeostasis under the control of FoxO1.