All-trans-Retinoic acid (RA) is a potent inducer in vitro of the differentiation of the human acute myeloid leukemia cell line HL60. A mechanism for RA-induced differentiation of HL60 cells may involve retinoylation (RA acylation) which is a posttranslational modification of proteins occurring in many eukaryotic cell lines. Here, we found that differentiation by the synthetic retinoid (E)4-[3-(3,5-di-tert- butylphenyl)3-oxo-1-propenyl]benzoic acid (Ch55) was dose-dependent in se- rum-free medium. The synthetic retinoid 4(5,6,7,8-tetrahydro-5,5,8,8- tetramethyl-2-naphthalenylcarbamoyl) benzoic acid (Am80) did not induce differentiation. Ch55 bound covalently to proteins of HL60 cells. In contrast, covalent binding of Am80 to HL60 proteins was much lower. Two- dimensional gel electrophoresis patterns of proteins labeled covalently by RA and Ch55 were different with few proteins labeled by both retinoids. The level of retinoylation was increased by Am80 and combinations of RA with either Ch55 or Am80 synergistically induced differentiation of HL60 cells. These results suggest that covalent modification of proteins by a retinoid may play a role in inducing differentiation of HL60 cells. In addition, the synergy seen with combinations of RA and either Ch55 or Am80 suggests that some synthetic retinoids may be active because they displace RA from intracellular sites or because they inhibit RA catabolism.
