Treatment of SV40-transformed keratinocytes (Z114) with epidermal growth factor (EGF) resulted in an increase in ornithine decarboxylase (ODC) activity and a dose-dependent increase in ODC mRNA levels. Pretreatment of keratinocytes with all-trans-retinoic retinoic acid inhibited the EGF induction of ODC activity. In both quiescent and EGF-stimulated cells, all-trans-retinoic acid inhibited ODC gene transcription and lowered ODC mRNA levels, whereas glyceraldehyde phosphate dehydrogenase expression remained unaffected. Treatment with all-trans-retinoic acid for 24 h resulted in a dose- and time-dependent decrease of up to 52% in EGF binding to EGF receptors and a 30-75% decrease in EGF-receptor quantity. In addition, when cells were treated with both UV radiation and all-trans-retinoic acid, their effects were additive in causing a decrease in EGF binding. Blocking of EGF receptors with a neutralizing antibody for EGF receptors inhibited the induction of ODC activity by EGF. The effects of several other retinoids, including Ro15-0778, etretinate, Ro13-7410, etarotene, Ro40-8757, 13-cis-retinoic acid and acitretin, were also studied to determine their effects on EGF binding and ODC activity. Two of these other retinoids, 13-cis-retinoic acid and Ro13-7410, inhibited EGF binding the most (35-46%, P < 0.001); several others (etarotene, Ro40-8757 and etretinate) were less effective (7-16%), but significantly decreased EGF binding (P < 0.05), and two retinoids (Ro15-0778 and acitretin) showed no significant effect on EGF binding. In contrast, all of the retinoids tested inhibited the induction of ODC activity by EGF, although etretinate and Ro15-0778 were less effective. EGF signal transduction is important in ODC gene regulation, and retinoids are significant modulators of this pathway.