Control of Mitochondrial Electron Transport Chain Flux and Apoptosis by Retinoic Acid: Raman Imaging In Vitro Human Bronchial and Lung Cancerous Cells

SIMPLE SUMMARY: We have shown that a confocal Raman imaging provides an excellent tool to extend our knowledge on the redox status of iron ion of heme proteins inside specific organelles of bronchial normal epithelial lung (BEpC) and lung cancer (A549) cells. The role of retinoic acid in regulating respiration, apoptosis processes, and reactive oxygen species production has been discussed. We showed that retinoic acid plays a key role in the oscillations between reduced (Fe(2+)) and oxidized (Fe(3+)) form of cytochrome c in the electron transport chain that can activate or inhibit these physiological functions. The redox status of cytochrome c regulated by retinoic acid becomes a target for further development of robust therapeutic approaches. ABSTRACT: The multiple functions of cytochrome c (cyt c) and their regulation in life and death decisions of the mammalian cell go beyond respiration, apoptosis, ROS scavenging, and oxidation of cardiolipine. It has become increasingly evident that cyt c is involved in the propagation of mitogenic signals. It has been proposed that the mitogenic signals occur via the PKCδ-retinoic acid signal complex comprising the protein kinase Cδ, the adapter protein Src homologous collagen homolog (p66Shc), and cyt c. We showed the importance of retinoic acid in regulating cellular processes monitored by the Raman bands of cyt c. To understand the role of retinoids in regulating redox status of cyt c, we recorded the Raman spectra and images of cells receiving redox stimuli by retinoic acid at in vitro cell cultures. For these purposes, we incubated bronchial normal epithelial lung (BEpC) and lung cancer cells (A549) with retinoic acid at concentrations of 1, 10, and 50 µM for 24 and 48 h of incubations. The new role of retinoic acid in a change of the redox status of iron ion in the heme group of cyt c from oxidized Fe(3+) to reduced Fe(2+) form may have serious consequences on ATPase effectiveness and aborting the activation of the conventional mitochondrial signaling protein-dependent pathways, lack of triggering programmed cell death through apoptosis, and lack of cytokine induction. To explain the effect of retinoids on the redox status of cyt c in the electron transfer chain, we used the quantum chemistry models of retinoid biology. It has been proposed that retinol catalyzes resonance energy transfer (RET) reactions in cyt c. The paper suggests that RET is pivotally important for mitochondrial energy homeostasis by controlling oxidative phosphorylation by switching between activation and inactivation of glycolysis and regulation of electron flux in the electron transport chain. The key role in this process is played by protein kinase C δ (PKCδ), which triggers a signal to the pyruvate dehydrogenase complex. The PKCδ-retinoic acid complex reversibly (at normal physiological conditions) or irreversibly (cancer) responds to the redox potential of cyt c that changes with the electron transfer chain flux.