Prostaglandin E and 12-O-Tetradecanoylphorbol-13-acetate Are Negative Modulators of Retinoic Acid Synthesis

Although retinoic acid is an important modulator of gene transcription that affects diverse processes during embryonic development and in the adult, no regulators of retinoic acid synthesis have been identified. This work will show that deletion of prostaglandin E, (PGEL) from the defined medium of confluent Madin-Darby canine kidney (MDCK) cells increased retinoic acid synthesis from retinol as much as twofold. Omitting any one of the other four growth factors (cortisol, insulin, transferrin, triiodothyronine) had no effect. Adding PGE1 to confluent cells maintained in its absence caused 71 ± 17% (mean ± SE, 10 experiments) inhibition of the conversion of retinol into retinoic acid. The ED50 of PGE1 was 70 nM and a maximum effect was observed by 1.5 h. 12-O-Tetradecanoylphorbol-13-acetate (TPA), an inducer of PGE synthesis in MDCK cells, decreased retinoic acid synthesis by 73 ± 14% (mean ± SE, 10 experiments). The ED50 of TPA was 5 nM and at least 4 h were required for a maximum effect. TPA inhibited the first and rate-limiting step, the conversion of retinol into retinal, and did not decrease either the conversion of retinal into retinoic acid or the elimination t1/2 of retinoic acid. PGE1 and TPA did not inhibit retinoic acid synthesis completely, nor were their actions additive or synergistic, consistent with more than one pathway of retinoic acid synthesis. Indomethacm did not prevent the TPA effect, suggesting that TPA acts independently of prostaglandin generation. MDCK cells expressed mRNA for retinoic acid receptor-α and retinoic acid receptor-β constitutively, and neither was induced by retinoic acid, consistent with an advanced state of differentiation. These data identify two antagonists of retinoid action, TPA and PGE, as modulators of retinoic acid synthesis in a retinoic acid-responsive cell. They also demonstrate the utility of MDCK cells for studying interactions among retinoic acid synthesis, the mechanism(s) of retinoic acid action, and the effects of TPA on retinoic acid synthesis and action