A novel study of melatonin diffusion in a 3D cell culture model

Melatonin is now considered a major physiological regulator of many different functions including synchronization of circadian rhythms, antioxidant defense at different levels, immunomodulation, cell growth control, neuroprotector and anti-tumor agent. In addition to its membrane receptor-dependent actions, it has been classically assumed that its diffusion through lipid bilayers contribute to its intracellular actions, including direct and indirect free radical scavenging activities. While pineal gland is the major site of nocturnal production of the indolamine, skin is considered an important source of melatonin synthesis. Here, using a 3-D culture model of HaCaT cells in an artificial scaffold (epidermal equivalents), we have quantified diffusion of melatonin in these cells and compared it to 2-D or spheroid cultures. Diffusion in 3-D scaffold cultures was similar to that found in 2-D culture and proportion of intracellular melatonin was low. AFMK, a major oxidative metabolite of melatonin, was also found and quantified. Redox parameters including total ROS, superoxide or mitochondrial mass were also assayed. We also report the effect of melatonin on the cytoskeleton of normal human keratinocyte HaCaT cells. We propose HaCaT epidermal equivalents as an affordable, easy-to-use, 3-D cell culture tool to test diffusion rates of melatonin but also other similar small molecules. This 3-D models can also be studied at cellular and molecular level, including redox parameters, and can provide important information regarding molecules that can be topically added to skin. Similarly, mechanisms of transportation can also be approached with this methodology.