Glucose Uptake and Runx2 Synergize to Orchestrate Osteoblast Differentiation and Bone Formation

Abstract

Summary The synthesis of Type I collagen, the main component of the bone matrix, precedes the expression of Runx2, the earliest determinant of osteoblast differentiation. We hypothesized that the osteoblast's energetic needs might explain this apparent paradox. We show here that glucose, the main nutrient of osteoblasts, is transported in these cells through Glut1 whose expression precedes that of Runx2. Glucose uptake favors osteoblast differentiation by suppressing the AMPK-dependent proteasomal degradation of Runx2 and promotes bone formation by inhibiting another function of AMPK. While Runx2 cannot induce osteoblast differentiation when glucose uptake is compromised, raising blood glucose levels restores collagen synthesis in Runx2-null osteoblasts and initiates bone formation in Runx2-deficient embryos. Moreover, Runx2 favors Glut1 expression, and this feed-forward regulation between Runx2 and Glut1 determines the onset of osteoblast differentiation during development and the extent of bone formation throughout life. These results reveal an unexpected intricacy between bone and glucose metabolism.

Vhl deletion in osteoblasts boosts cellular glycolysis and improves global glucose metabolism

“…AdCre cells also displayed elevated expression of several GLUTs ( Figure 6D), particularly GLUT1, the dominant glucose transporter in osteoblasts (18). In line with these changes at the gene expression level, Vhl-deficient cells showed increased uptake of glucose and production of lactate, as documented by analysis of conditioned culture media ( Figure 6E) and quantification of cellular uptake of 2-NBDG, a fluorescently labeled glucose analog ( Figure 6F) that is not metabolized.…”

“…ing to the regions of intense population by glucose-avid osteoblasts (see Figure 7, C-E). Three-compartment model kinetic analyses further substantiated the increased 18 F-FDG uptake in the mutant bones (Supplemental Figure 13C). In contrast to the skeleton, which represents the targeted organ in this Osx-Cre:GFP-driven genetic strategy, the liver showed no differences in 18 F-FDG uptake between the genotypes (Figure 8, B and F).…”

“…Three-compartment model kinetic analyses further substantiated the increased 18 F-FDG uptake in the mutant bones (Supplemental Figure 13C). In contrast to the skeleton, which represents the targeted organ in this Osx-Cre:GFP-driven genetic strategy, the liver showed no differences in 18 F-FDG uptake between the genotypes (Figure 8, B and F). Most interestingly, we found that the 18 F-FDG uptake values in the bones of individual mice negatively correlated with the animal's glycemic levels ( Figure 8G), whereas such correlation was not seen in the liver ( Figure 8H).…”

“…In vitro and in vivo data indicated that Vhl-deficient osteoblasts displayed massively increased glucose uptake and glycolysis associated with upregulated HIF-target gene expression, resembling the Warburg effect that typifies cancer cells. Overall, the glucose consumption by the skeleton was increased in the mutant mice, as revealed by 18 F-FDG radioactive tracer experiments. Moreover, the glycemia levels correlated inversely with the level of skeletal glucose uptake, and pharmacological treatment with the glycolysis inhibitor dichloroacetate (DCA), which restored glucose metabolism in Vhl-deficient osteogenic cells in vitro, prevented the development of the systemic metabolic phenotype in the mutant mice.…”

“…Moreover, these may change throughout their lifespan and differentiation progress, and are likely context dependent (16,17). Recent in vivo work using genetically modified mouse models has been actively investigating this topic, and revealed a prominent role for glucose metabolism during osteoblast differentiation (18,19). Moreover, aerobic glycolysis has been implicated as an important driver of bone formation and shown to be regulated by osteoanabolic pathways involving Wnt-LRP5 signaling, parathyroid hormone, and HIF (20)(21)(22)(23).…”