Heavy water labeling of keratin as a non-invasive biomarker of skin turnover in vivo in rodents and humans.

Measurement of skin turnover has been problematic in humans. Heavy water (2H2O) labeling has recently been developed as a safe, simple method to study in vivo kinetics of many biosynthetic processes, including DNA and protein synthesis. Here, we apply this approach to the measurement of 2H incorporation into skin keratin and show close agreement between keratin and keratinocyte turnover data in the epidermis of rodents. Elevated turnover rates of both keratin and keratinocytes were observed in the epidermis of the flaky skin mouse, although topical treatments effective in human psoriasis had no effect on either turnover rate in these mice. In humans, keratin turnover was monitored non-invasively by serial tape stripping during and after 2H2O labeling. Kinetic data were consistent with previous estimates of epidermal turnover, with a lag time of 18 days before label appeared at the skin surface and a transit time of 4-5 weeks. Variability in skin keratin turnover rates was present among healthy individuals. In summary, 2H2O labeling of skin keratin represents a non-invasive approach for assessing skin turnover dynamics in pre-clinical models and in human subjects.