Serum and extracellular calcium modulate induction of cytochrome P-450IA1 in human keratinocytes.

Culture conditions allowing for cytochrome P-450IA1 induction by 2,3,7,8-tetrachlorodibenzofuran (TCDF) in normal human keratinocytes (HK) were investigated. HK grown in serum-free low extracellular Ca2+ (0.1 mM) medium did not accumulate P-450IA1 mRNA in response to TCDF. If, however, the cultures were pretreated for more than 24 h with either serum or elevated extracellular Ca2+ (2.0 mM), induction of P-450IA1 was obtained by TCDF. Serum and elevated Ca2+ concentrations were found to be additive in this respect. When analyzing HK derived from five individuals, no apparent difference was found in the relative induction of P-450IA1 by increasing concentrations of TCDF, giving an EC50 of approximately 2 nM. The permissive effect of serum and elevated Ca2+ could be conferred to a reporter gene by the -1140 to +2435 part of the human CYPIA1 gene. Culture conditions allowing for P-450IA1 induction correlated with conditions that induced mRNA corresponding to the differentiation specific enzyme epidermal transglutaminase. This finding, together with the known differentiation promoting effects of serum and elevated Ca2+, suggest that terminal differentiation is necessary for P-450IA1 induction in HK by Ah receptor ligands.