Interactions between vitamins A and D

[Amazoniac]

To start a discussion on these two toxins..

Vitamin D Interferes with Transactivation of the Growth Hormone Gene by Thyroid Hormone and Retinoic Acid

"It has been demonstrated that TR and RAR interact in solution not only with RXR but also with VDR (35–37). Therefore, it was possible that formation of inactive TR-VDR and RAR-VDR heterodimers which either do not bind or bind in a transcriptionally inactive form to the T3 and RA response elements could contribute to the transcriptional repression observed in pituitary cells. We were not able to find TR-VDR or RAR-VDR binding to the palindromic response element, showing a low affinity, if any, of these heterodimers for the element. However, VDR very effectively inhibited binding of TR-RXR and RAR-RXR by formation of VDR-RXR heterodimers which bind with a much lower affinity than does either TR-RXR or RAR-RXR to the TRE(PAL). More interestingly, VDR also competed TR monomeric or homodimeric binding to this element, demonstrating that VDR sequesters TR (and most likely also RAR) and prevents binding to the element and transcriptional activation."

"Schräder et al. (36) have reported that TR-VDR heterodimers bind almost as strongly as VDR-RXR heterodimers to the osteopontin VDRE. In contrast, we did not detect significant TR-VDR or RAR-VDR binding to this element. Also, with this VDRE we found that TR and RAR displaced not only VDR-RXR binding but also VDR homodimeric binding. Taken together, our data suggest the formation of TR-VDR and RAR-VDR heterodimers which have low affinities by either the TRE or the VDRE and can inhibit binding of more active heterodimers with RXR by competition."