ABSTRACT
Bookmarking factors are transcriptional regulators involved in the mitotic transmission of epigenetic information via their ability to remain associated with mitotic chromatin. The mechanisms through which bookmarking factors bind to mitotic chromatin remain poorly understood. HNF1ß is a bookmarking transcription factor that is frequently mutated in patients suffering from renal multicystic dysplasia and diabetes. Here, we show that HNF1ß bookmarking activity is impaired by naturally occurring mutations found in patients. Interestingly, this defect in HNF1ß mitotic chromatin association is rescued by an abrupt decrease in temperature. The rapid relocalization to mitotic chromatin is reversible and driven by a specific switch in DNA-binding ability of HNF1ß mutants. Furthermore, we demonstrate that importin-ß is involved in the maintenance of the mitotic retention of HNF1ß, suggesting a functional link between the nuclear import system and the mitotic localization/translocation of bookmarking factors. Altogether, our studies have disclosed novel aspects on the mechanisms and the genetic programs that account for the mitotic association of HNF1ß, a bookmarking factor that plays crucial roles in the epigenetic transmission of information through the cell cycle.
Subject(s)
Epigenesis, Genetic , Hepatocyte Nuclear Factor 1-beta/genetics , Mutation/genetics , Animals , Cells, Cultured , Chromatin/metabolism , DNA/metabolism , Diabetes Mellitus, Type 2/genetics , Dogs , Epigenesis, Genetic/drug effects , Epithelial Cells/drug effects , Epithelial Cells/metabolism , Gene Deletion , Green Fluorescent Proteins/metabolism , Hepatocyte Nuclear Factor 1-beta/chemistry , Heterozygote , Humans , Kidney/cytology , Madin Darby Canine Kidney Cells , Mitosis/genetics , Models, Biological , Protein Binding/drug effects , Protein Domains , Quinazolines/pharmacology , Recombinant Fusion Proteins/metabolism , TemperatureABSTRACT
The synthesis of type I collagen, the main component of bone matrix, precedes the expression of Runx2, the earliest determinant of osteoblast differentiation. We hypothesized that the energetic needs of osteoblasts 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 feedforward 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.
Subject(s)
Cell Differentiation , Core Binding Factor Alpha 1 Subunit/metabolism , Glucose/metabolism , Osteoblasts/metabolism , Osteogenesis , AMP-Activated Protein Kinases/antagonists & inhibitors , AMP-Activated Protein Kinases/genetics , Amino Acid Sequence , Animals , Embryo, Mammalian/cytology , Embryo, Mammalian/metabolism , Glucose Transporter Type 1/metabolism , Homeostasis , Mice , Osteoblasts/cytology , Sequence Alignment , Skull/cytologyABSTRACT
Early B-cell factor 1 (Ebf1) is a transcription factor whose inactivation in all cells results in high bone mass because of an increase in bone formation. This observation suggests Ebf1 may be an inhibitor of osteoblast differentiation. To test this contention, we analyzed Ebf1 pattern of expression and function in osteoblasts ex vivo and in vivo through osteoblast-specific inactivation in the mouse. We show here that in vivo deletion of Ebf1 in osteoblast progenitors does not affect osteoblast differentiation or bone formation accrual post-natally. These observations indicate that the phenotype described in Ebf1(-/)(-) mice is not osteoblast-autonomous.
