Bovine aorta smooth muscle cells (SMC) incubated with a tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), released increased levels of prostaglandin I2 [measured as its stable hydrolytic product, 6-keto-prostaglandin F1α (6-keto-PGF1α)], and this response was inhibited by all-trans-retinoic acid (RA) at concentrations as low as 17 nm. Retinol and retinyl acetate, at concentrations as high as 1.7 and 1.5 µm, respectively, did not inhibit the TPA-stimulated 6-keto-PGF1α production. RA was not cytotoxic at 1.7 µm, as assayed by exclusion of trypan blue dye. Inhibition by RA was increased after preincubation of the SMC with RA prior to TPA stimulation. The inhibition of arachidonic acid (AA) metabolism by RA was not specific for TPA stimulation; RA inhibited prostaglandin production after SMCs were stimulated by serotonin; melittin; the Ca2+ ionophore, A23187; and fetal calf serum. RA had no effect on phorbol ester binding to SMC, nor did it inhibit increased 6-keto-PGF1α production in SMC treated with exogenous AA. While RA inhibited TPA-stimulated production of 14C-labeled 6-keto-PGF1α from [14C]AA-labeled cells, it did not inhibit the accumulation of [14C]AA in the culture medium. The data suggest that RA inhibits stimulated, rather than basal, levels of prostaglandin production. RA does not seem to act by inhibiting the deacylation of AA from cellular phospholipid pools, insofar as this is reflected in the accumulation of AA in the media, but may inhibit reactions at, or after, the generation of endoperoxides by cyclooxygenase.
This work was supported by Grants GM 27256, CA 17309, and NS 19038-01 from the NIH. Publication No. 1461 of the Department of Biochemistry, Brandeis University, Waltham, Mass. 02254.