FoxO1 protein cooperates with ATF4 protein in osteoblasts to control glucose homeostasis

The Forkhead transcription factor FoxO1 inhibits through its expression in osteoblasts β-cell proliferation, insulin secretion, and sensitivity. At least part of the FoxO1 metabolic functions result from its ability to suppress the activity of osteocalcin, an osteoblast-derived hormone favoring glucose metabolism and energy expenditure. In searching for mechanisms mediating the metabolic actions of FoxO1, we focused on ATF4, because this transcription factor also affects glucose metabolism through its expression in osteoblasts. We show here that FoxO1 co-localizes with ATF4 in the osteoblast nucleus, and physically interacts with and promotes the transcriptional activity of ATF4. Genetic experiments demonstrate that FoxO1 and ATF4 cooperate to increase glucose levels and decrease glucose tolerance. These effects result from a synergistic effect of the two transcription factors to suppress the activity of osteocalcin through up-regulating expression of the phosphatase catalyzing osteocalcin inactivation. As a result, insulin production by β-cells and insulin signaling in the muscle, liver and white adipose tissue are compromised and fat weight increases by the FoxO1/ATF4 interaction. Taken together these observations demonstrate that FoxO1 and ATF4 cooperate in osteoblasts to regulate glucose homeostasis.

Differentiation and proliferation of periosteal osteoblast progenitors are differentially regulated by estrogens and intermittent parathyroid hormone administration

The periosteum is now widely recognized as a homeostatic and therapeutic target for actions of sex steroids and intermittent PTH administration. The mechanisms by which estrogens suppress but PTH promotes periosteal expansion are not known. In this report, we show that intermittent PTH(1-34) promotes differentiation of periosteal osteoblast precursors as evidenced by the stimulation of the expression or activity of alkaline phosphatase as well as of targets of the bone morphogenetic protein 2 (BMP-2) and Wnt pathways. In contrast, 17beta-estradiol (E2) had no effect by itself. However, it attenuated PTH- or BMP-2-induced differentiation of primary periosteal osteoblast progenitors. Administration of intermittent PTH to ovariectomized mice induced rapid phosphorylation of the BMP-2 target Smad1/5/8 in the periosteum. A replacement dose of E2 had no effect by itself but suppressed PTH-induced phosphorylation of Smad1/5/8. In contrast to its effects to stimulate periosteal osteoblast differentiation, PTH promoted and subsequently suppressed proliferation of periosteal osteoblast progenitors in vitro and in vivo. E2 promoted proliferation and attenuated the antiproliferative effect of PTH. Both hormones protected periosteal osteoblasts from apoptosis induced by various proapoptotic agents. These observations suggest that the different effects of PTH and estrogens on the periosteum result from opposing actions on the recruitment of early periosteal osteoblast progenitors. Intermittent PTH promotes osteoblast differentiation from periosteum-derived mesenchymal progenitors through ERK-, BMP-, and Wnt-dependent signaling pathways. Estrogens promote proliferation of early osteoblast progenitors but inhibit their differentiation by osteogenic agents such as PTH or BMP-2.