ESPE Abstracts

Background: Folate and cobalamin are needed for synthesis of methionine, a substrate of methylation in epigenetic, and epigenomic pathways. Methyl donor deficiency (MDD) leads to hyperhomocysteinemia, which has been related to osteoporosis in humans and disruption of epiphyseal cartilage and bone development in rodents. Recent studies have revealed evidence for association between 25(OH)D₃ and homocysteine levels, however, underlying mechanisms remain elusive.

Objective: We aimed to elucidate molecular mechanisms linking MDD and VDR signalling, and the role of MDD in VDR-mediated transcription during osteoblast differentiation.

Method: Proliferation and differentiation of MG-63 pre-osteoblasts were studied under normal conditions and under experimental conditions of MDD.

Results: The 1,25(OH)2D₃ and vehicle treated MDD cells displayed significantly decreased expression of VDR comparing to controls and impaired nuclear activation of the transcriptional co-activator PGC-1α, through its decreased methylation related to blunted expression of PRMT1 and ADMA, decreased S-adenosylmethionine:S-adenosylhomocysteine ratio, and increased cellular concentration of S-adenosylmethionine, a potent inhibitor of PRMT1. The mechanistic studies further revealed a markedly decreased interaction between VDR and PGC-1α and a strong interaction between protein chaperon HSP90 and VDR in MDD cells. We speculate that HSP90 sequestrates VDR and prevents the hormone binding to PGC-1α leading to altered expression of downstream genes, in particular increased expression of pro-adipogenic PPARγ, adiponectin and ERRα, and significantly decreased expression and activity of bone ALP.

Conclusion: The data presented here provide new mechanistic insights into regulation of human osteoblast differentiation demonstrating for the first time a crucial role of PGC-1α-VDR functional interaction, dramatic effect of MDD on the nuclear action of 1,25(OH)2D₃ through hypomethylation of PGC-1α and sequestration of VDR in a form of the HSP90-VDR complex. This prevents the hormone binding to PGC-1α and leads to impaired expression of target genes ultimately directing cell differentiation toward a more adipogenic phenotype.