Collectively, the data indicate that FoxO1, through its expression in osteoblasts, inhibits insulin secretion. == Increased insulin sensitivity in Foxo1ob/mice. results indicate that FoxO1 expression in osteoblasts contributes to FoxO1 control of glucose homeostasis and identify FoxO1 as a key modulator of the ability of the skeleton to function as an endocrine organ regulating glucose metabolism. == Introduction == Insulin, a hormone whose main function is to regulate blood glucose level, is a major endocrine regulator of energy metabolism. Insulin resistance plays a fundamental role in the pathogenesis of a host of metabolic diseases, ranging from type 2 diabetes mellitus to hypertension, lipid disorders, atherosclerosis, and reproductive dysfunction (1). Given the paramount importance of insulin, its signaling pathway has been studied intensively since its receptor was identified and molecularly cloned (2). This body of work has highlighted the biological importance of FoxO1, as the major transcriptional mediator of insulin signaling in many cells, including the cell, the adipocyte, the hepatocyte, and the myoblast (38). FoxO1, which belongs to the Forkhead family of transcription factors, is a negative regulator of insulin sensitivity in cells, hepatocytes, and adipocytes. Loss- or gain-of-function animal models of FoxO1 have clearly demonstrated that FoxO1 suppresses cell proliferation and function (4,9). For instance, FoxO1 haploinsufficiency partially reinstates the decreased cell proliferation observed in insulin receptor substrate-2knockout (Irs2-knockout) mice (10), as well as in mice with cell specific deletion ofPdk1, a PI3K-dependent protein kinase that is important in maintenance of cell mass (11). In the hepatocyte, FoxO1 promotes gluconeogenesis by acting in concert with the PPAR coactivator PGC1 to stimulate the expression of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate kinase 1 (Pck1) (6). Thus, FoxO1 controls at least 2 important processes in glucose metabolism: cell proliferation and hepatic glucose production. However, FoxO1 is expressed in many other cell types, where its functions are not known. Recently, it was shown that, in addition to the cell and the hepatocyte, the skeleton may be a regulator of glucose metabolism (12). Osteoblasts, the bone-forming cells, secrete a protein, osteocalcin, that when uncarboxylated acts as a hormone to favor cell proliferation, insulin secretion, insulin sensitivity, and energy expenditure. The function of this bone-derived hormone is negatively regulated by another gene expressed in osteoblasts,Esp. Protein tyrosine phosphatase (OST-PTP), the product ofEsp, decreases through an as-yet-unknown mechanism the bioactivity of the osteocalcin protein by favoring its carboxylation. Because FoxO1 is the main target of insulin action in other tissues, we examined whether it fulfills Levocetirizine Dihydrochloride its metabolic function, in part, through its osteoblastic expression. In this report, we show that FoxO1 helps to control glucose metabolism through 2 regulatory effects in the osteoblast: stimulation of osteocalcin and inhibition ofEspexpression. As a result of these complementary functions, FoxO1 through its osteoblastic expression is a negative regulator of energy metabolism. == Results == == Inactivation of Foxo1 in osteoblasts leads to perinatal lethality. == To determine which member of the FoxO family was most highly expressed in osteoblasts, we used mouse primary osteoblastic cells.Foxo1was the most abundant member among the 3Foxoisoforms of this family of proteins in osteoblasts (Figure1A). Because of these observations and evidence from several reports identifying the pivotal role of FoxO1 in insulin action and development of type 2 diabetes, angiogenesis, organismal growth, cell differentiation, and tumorigenesis (3), we focused our subsequent studies on FoxO1. == Levocetirizine Dihydrochloride Figure 1. Perinatal lethality inFoxo1ob/mice. == (A) Real-time PCR analysis of the expression of the 3Foxoisoforms in primary osteoblasts;n= Levocetirizine Dihydrochloride 3. (B) Real-time PCR analysis ofFoxo1expression in bone and other tissues of WT andFoxo1ob/mice;n= 3 mice/group. Data are presented as mean SEM; **P< 0.01 by Studentsttest. (C) Western blot analysis of FoxO1 protein levels in osteoblasts. (D) Real-time PCR analysis Rabbit polyclonal to USP22 of the expression ofFoxo3andFoxo4in the femur of WT andFoxo1ob/mice;n= 3/group. InBandD, mice were 2 months of age. To conditionally inactivateFoxo1in osteoblasts (Foxo1ob/), we bred floxedFoxo1(Foxo1fl/fl) mice (13) with transgenic mice expressingCreunder the control of the osteoblast-specific.