In vitro models for the assessment of skin xenobiotic metabolism
Skin subcellular fractions, keratinocytes, living skin equivalents (LSE) and percutaneous absorption using diffusion cells were examined as in vitro models for assessment of skin xenobiotic metabolism. Cytochrome P450 monooxygenase, esterase and glutathione-S-transferase (GST) activities were investigated in rodent, pig and human skin. CYP1A1 activity was detected in rat and pig skin microsomes by ethoxyresorufin-Odeethylation. CYP2B activity was detected in rat skin microsomes by pentoxyresorufin-Odepentylation. Neither were detected in human skin microsomes. Rat keratinocytes lost cytochrome P450 activity within several hours of being isolated from skin, and were not reliably induced following exposure to B-naphthoflavone. Cytochrome P450 activities were detected in the LSE after induction by 3-methylcholanthrene. Carboxylesterase activity was detected in skin, keratinocytes, and the LSE using 4- methylumbelliferyl heptanoate as the substrate. Induction of caboxylesterase activity by 3- methylcholanthrene was shown in the LSE but not keratinocytes. GST activity was shown in skin and keratinocytes, but induction was only shown in the LSE. The ability to induce xenobiotic metabolising activity suggests that enzyme induction may be linked to cell differentiation. GST's were localised at the basal layer of the epidermis. During percutaneous absorption, DNCB was metabolised to the glutathione (GSH) conjugate, limited by the GSH available in the skin. GSH conjugation of DNCB is thought to be a detoxification pathway preventing the immune response illicited by DNCB. Studies investigating the effect of age of rat on dermal xenobiotic metabolism revealed no differences between the neonate and mature rat with respect to cytochrome P450 monooxygenase activity or esterase activity. However, neonatal rat skin showed five fold lower GST activity and three fold higher reduced GSH levels. Pig skin showed similar levels of xenobiotic metabolising activity to human skin and showed a similar metabolic profile for DNCB during percutaneous absorption, supporting its use as a better model for human than rodent skin. The LSE was a good model for studies of human dermal xenobiotic metabolism particularly with the influence of inducing agents.