During a century of vitamin A research in nutrition much detailed knowledge on the mechanism of action has accrued. Yet several aspects of nutritional vitamin A deficiency remain unexplained by the reigning paradigm of transcription control via retinoic acid. In particular, the regulation of cytoplasmic events by vitamin A and metabolites, postulated by many, has remained elusive. We have identified a family of likely receptors by showing that retinol and the metabolite, 14-hydroxy-retro-retinol (14HRR), bind the cysteine-rich, zinc-finger subdomains of the regulatory domains of several PKC isoforms and other serine/threonine kinases. Our hypothesis is that the retinoid/zinc-finger complex functions as reversible switch during redox activation of the kinase. The primary event is oxidation of selected cysteine residues tagged by a retinoid bound nearby. The retinoid acts as catalyst to facilitate oxidation. Release of zinc from cysteines and loss of coordination of an otherwise rigid structure leads to the changed conformational state in PKC that ushers in its activation. Thus, a zinc-coordinated structure with its bound retinoid could serve as a sensor and actuator, directly linked to the redox state of the cytoplasm, allowing cells to maintain a steady-state level of active PKC as well as to respond quickly to oxidative stress. It is proposed to determine the influence of retinol on the redox potential and the reversibility of redox activation of PKC alpha (Aim #1); to study the chemistry of the zinc finger with respect to redox changes that cause Zn2+ release/binding (Aim # 2); to investigate the fine-tuning of redox potential by different retinoids bound to the zinc-finger (Aim # 3); to verify by imaging techniques the binding of retinoids to PKC in vivo (Aim # 4). The study will move the field of vitamin A forward. Furthermore, a new paradigm would be created how redox regulation, an every-day requirement for cells, connects to the general signalling apparatus and on to the actuators of metabolism and transcription. These fundamental insights, will impact on inflammatory processes and contribute to understand how reactive oxygen promotes cancer progression.
