fThe Wnt coreceptors Lrp5 and Lrp6 are essential for normal postnatal bone accrual and osteoblast function. In this study, we identify a previously unrecognized skeletal function unique to Lrp5 that enables osteoblasts to oxidize fatty acids. Mice lacking the Lrp5 coreceptor specifically in osteoblasts and osteocytes exhibit the expected reductions in postnatal bone mass but also exhibit an increase in body fat with corresponding reductions in energy expenditure. Conversely, mice expressing a high bone mass mutant Lrp5 allele are leaner with reduced plasma triglyceride and free fatty acid levels. In this context, Wnt-initiated signals downstream of Lrp5, but not the closely related Lrp6 coreceptor, regulate the activation of -catenin and thereby induce the expression of key enzymes required for fatty acid -oxidation. These results suggest that Wnt-Lrp5 signaling regulates basic cellular activities beyond those associated with fate specification and differentiation in bone and that the skeleton influences global energy homeostasis via mechanisms independent of osteocalcin and glucose metabolism.
Wnt signaling regulates nearly all aspects of osteoblast function, from initial fate specification (1) to the control of osteoclast differentiation (2). In this pathway, low-density-lipoprotein (LDL)-related receptor 5 (Lrp5) and the closely related Lrp6 participate in the stabilization and activation of the transcription factor -catenin by facilitating the interaction of Wnt ligands with frizzled receptors (3, 4). Osteoblasts express all the components of the Wnt/-catenin pathway, and most have now been linked with bone development and maintenance in humans and mouse models (5-7). Mutations in the LRP5 gene, in particular, can result in premature and generalized osteoporosis as in the rare condition osteoporosis pseudoglioma (8) or a high-bone-mass phenotype (9, 10) likely due to an increase in the number of mineralizing osteoblasts (11).Like other metabolically active cells, osteoblasts require a supply of energy-rich molecules to fuel the synthesis, deposition, and mineralization of bone matrix (12). When energy input fails to meet demand, normal bone accrual ceases, a phenomenon that is evident clinically by the arrest of longitudinal bone growth and osteopenia observed in undernourished children and adults (13, 14). Therefore, osteoblasts must possess mechanisms to acquire and regulate the utilization of fuel macromolecules, as well as the ability to communicate energy needs with other tissues.Recent studies have delineated a role for the osteoblast in a bone-pancreas endocrine loop that contributes to the regulation of glucose metabolism, as well as bone acquisition. Insulin receptor signaling in the osteoblast regulates the activity of the osteogenic transcription factor Runx2 and is required for the attainment of a mature phenotype as well as normal postnatal bone acquisition (15). In addition, insulin actions regulate the production and bioavailability of osteocalcin (15, 16), a bone-derived hormone that in ...