FoxO1: a molecule for all seasons.

The FoxO family of forkhead transcription factors is at the crossroads of many signal transduction pathways that are evolutionarily conserved. Such pathways have been co-opted in differentiated tissues for a variety of vital and specialized functions, such as differentiation, proliferation, and survival in cells as diverse as adipocytes, hepatocytes, β-cells, myoblasts, thymocytes, and cancer cells. FoxO metabolic functions are relevant to glucose metabolism, tumor suppression, hematopoiesis, angiogenesis, and antioxidant defense. Among the FoxO isoforms, FoxO1 is a main target of insulin signaling and regulates metabolic homeostasis and organismal survival at many different levels. FoxO1 entered into the field of skeletal biology by a property that is unique among its functions in other organs. With the osteoblast as its target cell, FoxO1 not only acts on it to regulate bone homeostasis but also through it as a transcriptional modulator of the endocrine function of the skeleton in regulating glucose metabolism. Through its direct skeletal actions, FoxO1 promotes osteoblast proliferation by maintaining protein synthesis and redox balance. Through its endocrine actions on target tissues of insulin, FoxO1 acts by way of osteocalcin to suppress glucose production by pancreatic beta cells and hepatocytes and to decrease insulin production and sensitivity. These two parallel but opposing actions, one in favor of the skeleton and the other in disadvantage of glucose-regulating tissues, may signify an adaptive mechanism that integrates responses between different organs and is beneficial for whole-body physiology during stress and aging.