Endocrine Abstracts

Folate and cobalamin are methyl donors (MD) needed for synthesis of methionine, precursor of S-adenosylmethionine (SAM), the substrate of methylation in epigenetic, and epigenomic pathways. The MD deficiency (MDD) leads to a decrease in SAM/SAH ratio and hyperhomocysteinemia, which has been related to osteoporosis in humans and disruption of development of epiphyseal cartilage in rat. However, the underlying molecular mechanisms remain elusive.

Method: We studied the consequences of MDD on proliferation and differentiation of MG-63 pre-osteoblasts.

Results: 1,25(OH)₂D₃ and vehicle treated MDD cells displayed a significantly decreased nuclear expression of PGC-1α/VDR complex comparing to controls. The impaired activation of PGC-1α in deficient cells resulted from its decreased methylation related to the blunted methyl-transferase activity, and increased cellular concentration of SAH, a potent inhibitor of PRMT1. Hypomethylation of PGC-1α dramatically affected expression of differentiation markers in deficient cells with a strong increase in PPARγ and its targets (adiponectin and ERRα), and a significantly decreased expression and activity of bALP. The MDD induced a marked increase in expression of the HSP90. Although primarily cytoplasmic, HSP90 has been shown to translocate to nucleus and play a role in regulation of NR activity. In this study, strong HSP90-VDR interactions in the MDD cells make VDR less likely to bind to the PGC-1α. We suppose that the HSP90-VDR impaired formation of the VDR/NR heterodimer, with an increased expression of PPARγ targets (adiponectin and ERRα) leading to an adipogenic phenotype. Treatment with SAM rescued effects of 1,25(OH)₂D₃ on VDR/NR normalizing expression of PPARλ, ERRα, adiponectin. Addition of AdoX, an inhibitor of methyltransferase activity, produced similar changes as MDD.

In conclusion, this is the first evidence demonstrating existence of functional interactions between PGC-1α and VDR, its key role for normal functioning of the VDR-NR transcriptional co-regulation during osteoblast differentiation, and a dramatic effect of MDD on this co-regulatory network.