Based on these observations, we concluded that the VDR inhibited the Dinaciclib mitochondrial membrane potential and likely restrained ROS production, EX 527 protecting the cell from additional oxidative stress. On the contrary, VDR loss increased the respiratory potential, but rendered cells more prone to an oxidant-driven potential collapse. This possibility was supported by the significantly lower glutathione consumption in wild type cells, as revealed by the higher levels of the antioxidant molecule that were measured in wild type cells compared to silenced cells. Mitochondrial potential is sustained by the proton gradient that is created by respiratory chain activity; therefore, we decided to examine the expression of two subunits of complex IV: Cytochrome c oxidase subunits II and IV, whose transcripts are of mitochondrial and nuclear origin. Both nuclear- and mitochondrially encoded proteins are required for the formation of active respiratory complexes. Mitochondrial RNAs are transcribed as long, polycistronic precursor transcripts that are later processed to release individual rRNAs and mRNAs. Therefore, we considered COX II to be a marker of mitochondrial transcription activity and COX IV to be a marker of the nuclear contribution to respiratory chain modulation. Increased expression of both subunits in silenced cells compared to control cells was observed using real-time PCR. In order to confirm that the VDR negatively affected COX transcription, we treated wild type HaCaT cells with vitamin D and observed that the levels of all of the transcripts were decreased. Given the fact that the transcription of subunit IV is nuclear, whereas that of subunit II is encoded by mitochondrial DNA, we concluded that vitamin D transcriptional control is exerted at both levels, which is not surprising given the fact that nuclear and mitochondrial transcription of respiratory chain proteins is finely tuned. Because the modulation of mitochondrial transcription by the VDR has not been previously described, we considered the possibility of direct binding of the receptor to mtDNA. In silico analysis was conducted with the aim of screening mtDNA to identify vitamin D responsive element sites. We used a VDRE sequence represented by a collection of positional weight matrices to compute the affinity of the VDR for the mtDNA sequence. Only two VDRE sites were found to have high affinity cutoffs and both were located in the displacement loop, a non-coding and regulatory region. We also identified a total of 40 VDRE sites with low affinity scores clustered in a few regions.