PREFERENTIAL BINDING OF GROWTH INHIBITORY PROSTAGLANDINS BY THE TARGET PROTEIN OF A CARCINOGEN

Liver fatty acid binding protein (L-FABP) is the principal target protein of the hepatic carcinogen N-(2-fluorenyl)acetamide (2-acetylaminofluorene) in rat liver. In addition, the cyclopentenone prostaglandins (PG), PGA, PGJ2, and Δ12-PGJ2, inhibit the growth of many cell types in vitro. This report describes the preferential binding of the growth inhibitory prostaglandins by L-FABP and the reversible inhibition of thymidine incorporation into DNA by PGA2 and Δ12-PGJ2 in primary cultures of purified rat hepatocytes. As a model ligand, [3H]PGA1 bound to L-FABP specifically, reversibly, rapidly, and with high affinity. Its dissociation constants were 134 nM (high affinity) and 3.6 μM (low affinity). The high-affinity binding of [3H]PGA1 was 9- and ≃13-fold more avid than the binding of the conventional fatty acid ligands, oleic acid and arachidonic acid, respectively. The abilities of different prostaglandins to compete with the high-affinity binding of [3H]PGA1 correlated with their growth inhibitory activities reported previously and here. The growth inhibitory cyclopentenone prostaglandins (PGA1, PGA2, Δ12-PGJ2, and PGJ2) were the best competitive ligands, intermediate competitors were the weak growth inhibitors PGE1 and PGD2, and the poorest competitors were PGE2 and PGF(2α), which stimulate rather than inhibit DNA synthesis in rat hepatocytes in primary culture. The in vitro actions of L-FABP are compatible with those of a specific and dissociable carrier of growth inhibitory prostaglandins in rat hepatocytes and suggest that the carcinogen may usurp the cellular machinery of the growth inhibitory prostaglandins.