A keratinocytes-melanocytes coculture system for the evaluation of active ingredients' effects on UV-induced melanogenesis.

A new experimental design, more reliable for in vitro testing of active ingredients' effect on ultraviolet (UV)-induced melanogenesis has been carried out. It uses a bicompartmental coculture system where cell communication between keratinocytes and melanocytes can take place. Thus, this experimental situation enables to monitor the effect of biological agents released by both cell types on melanogenesis and the interference of tested compounds with this 'paracrine linkage'. Experiments with UVB-irradiated cocultures show the importance of cell communication in the melanogenic response. In this model, the endogenous mediator, nitric oxide (NO), increased melanin production. Different compounds were tested in the coculture system, and comparison with data obtained from irradiated monocultures of melanocytes enables to distinguish a specific effect on cell communication. In addition, this more close-to-reality experimental model proved to provide a valuable first approach for the assessment of the 'bioavailability' of the tested substances. Finally, the effect of an innovative photoprotective agent capable of 'boosting' UV-induced melanogenic cell communication is presented.

Oxidative stress and cutaneous ageing: the 'toxic second messengers' concept and an interesting family of products, 'pseudodipeptides'.

There are a number of factors responsible for cutaneous ageing. Apart from genetically programmed cell ageing, different external aggressors, such as solar radiation, pollution and stress, can accelerate this phenomenon. It is now accepted that deleterious oxidations of live mediums are of major importance. This so-called 'oxidative stress' is a very complex situation that covers a large number of chemical reactions, and involves very diverse oxidizing species. Being aware of this complexity, and aiming to obtain a 'universal protection' against oxidative stress, we have tailored specific protective agents. At the same time, it was necessary to design new experimental models for the evaluation of antioxidants' efficiency. Our studies have revealed that cutaneous tissue also needs protection against non-radicalar, toxic-oxidizing molecules, which we called 'toxic second messengers' of oxidative stress [1]. These molecules, against which live mediums have very poor self-defence mechanisms, are key intermediates in the oxidative reactions. They are responsible for irreversible damage, such as the deactivation of enzymatic complexes and loss of the skin's elasticity, or even DNA damage. This better understanding of oxidative stress mechanisms of action allowed us to design original protective agents against toxic second messengers, called 'pseudodipeptides'. In order to obtain a topically active compound, we have synthesized several 'biomimetic and biocompatible molecules' with improved activities and a longer life on the skin. It was also necessary to design specific tests in order to prove the unique properties of pseudodipeptides. Several experiments have been carried out, with the aim being to be as close as possible to conditions in vivo and more particularly to demonstrate their 'lipid peroxidase-like' activity or their efficiency against 'toxic aldehydes': HNE, from the oxidation of phospholipids [1], and glucosone, from that of sugars. More recent experimental designs have revealed the involvement of transition metals in oxidative stress and demonstrated that pseudodipeptides can 'detoxify' transition metals without interfering with useful biological mechanisms that also use transition metals. Finally, the anti-aging efficiency of pseudodipeptides was proven ex vivo and allowed us to present them as biocompatible and biomimetic compounds with a real polyvalent protective and repairing anti-oxidative activity.