INHIBITION OF TUMOR PROMOTER-INDUCED TRANSFORMATION BY RETINOIDS THATTRANSREPRESS AP-1 WITHOUT TRANSACTIVATING RETINOIC ACID RESPONSE ELEMENT

Both retinoic acid (RA) treatment and dominant-negative c-Jun mutant e xpression effectively inhibit phorbol ester-induced AP-1 activity and induced neoplastic transformation in mouse epidermal JB6 cells, Howeve r, both reagents also target non-AP-1 molecules in addition, Because l iganded retinoic acid receptors interact with and transactivate RA res ponse elements (RAREs) on DNA, as well as interact with Jun protein to block AP-1 activity, the question arises as to which of these two act ivities of retinoids is responsible for antitumor-promoting activity, To address this question we generated JB6 promotion-sensitive (P+) cel l lines that are stably transfected with a construct containing the co llagenase promoter bearing one AP-1-binding site that drives a lucifer ase reporter gene. The stable collagenase-luciferase-transfected cell lines showed 1.5-3.5-fold enhanced AP-1 activity when treated with 12- O-tetradecanoyl-phorbol-13-acetate (TPA). Up to 90% of TPA-induced AP- 1 activity was blocked by retinoids SR11238, SR11302, or trans-RA, but not by retinoid SR11235, Of these retinoids, only RA and SR11235 were able to transactivate RARE-dependent gene expression. Transrepression of TPA-induced AP-1 and transactivation of RARE by RA, SR11238, and S R11302 were concentration dependent at 10(-10) to 10(-6) M retinoid, W hen tested for activity in inhibiting tumor promoter-induced transform ation in JB6 P+ cells, the retinoids specific for AP-1 transrepression were inhibitory, whereas SR11235, which only activated RARE, showed l ittle effect, We thus conclude that the AP-1-blocking activity of reti noids is likely to be responsible for the antitumor-promoting activity , This result, together with the observation that dominant-negative Ju n blocks transformation, argues for a requirement of induced AP-1 in t he tumor promoter-induced transformation process.