Microarray profiling of diaphyseal bone of rats suffering from hypervitaminosis A

Vitamin A is the only known compound that produces spontaneous fractures in rats. In an effort to resolve the molecular mechanism behind this effect, we fed young male rats high doses of vitamin A and performed microarray analysis of diaphyseal bone with and without marrow after 1 week, i.e., just before the first fractures appeared. Of the differentially expressed genes in cortical bone, including marrow, 98% were upregulated. In contrast, hypervitaminotic cortical bone without marrow showed reduced expression of 37% of differentially expressed genes. Gene ontology (GO) analysis revealed that only samples containing bone marrow were associated with a GO term, which principally represented extracellular matrix. This is consistent with the histological findings of increased endosteal/marrow osteoblast number. Fourteen genes, including Cyp26b1, which is known to be upregulated by vitamin A, were selected and verified by real-time PCR. In addition, immunohistochemical staining of bone sections confirmed that the bone-specific molecule osteoadherin was upregulated. Further analysis of the major gene-expression changes revealed apparent augmented Wnt signaling in the sample containing bone marrow but reduced Wnt signaling in cortical bone. Moreover, induced expression of hypoxia-associated genes was found only in samples containing bone marrow. Together, these results highlight the importance of compartment-specific analysis of bone and corroborate previous observations of compartment-specific effects of vitamin A, with reduced activity in cortical bone but increased activity in the endosteal/marrow compartment. We specifically identify potential key osteoblast-, Wnt signaling-, and hypoxia-associated genes in the processes leading to spontaneous fractures.