FORSKOLIN STIMULATES DETOXIFICATION OF BREFELDIN-A

Forskolin has been shown to prevent the effects brefeldin A (BFA) exer ts on many mammalian cells with respect to the disassembly of the Golg i apparatus as well as an increase of sphingomyelin synthesis (Lippinc ott, S, J,, Glickman, J,, Donaldson, J, G,, Robbins, J., Kreis, T, E., Seamen, K. B,, Sheetz, M, P,, and Klausner, R, D, (1991) J, Cell Biol , 112, 567-577), It has been speculated that forskolin interferes with the action of BFA by competition for the binding of BFA to its target protein, which is most likely the Golgi-localized nucleotide exchange factor specific for ADP-ribosylation factor 1, Here we show that in v itro forskolin does not prevent inhibition of Golgi-catalyzed nucleoti de exchange by BFA. Therefore it appears unlikely that forskolin and B FA bind to the same target protein, Using [H-3]BFA we have measured de toxification of BFA by Chinese hamster ovary (CHO) cells, BFA is secre ted from CHO cells as cysteine and glutathione conjugates (Bruning, A, Ishikawa, T., Kneusel, R, FF., Matern, U,, Lottspeich, F,, and Wielan d, F, T, (1992) J, Biol. Chem. 267, 7726-7732), We present evidence th at forskolin treatment of CHO cells results in increased levels of Cys -EFA, the major FFA conjugate secreted by CHO cells, in the medium, El evated levels of Cys-BFA are also found intracellularly, The effect of forskolin is shown to be independent of its ability to raise the intr acellular concentration of cyclic AMP, Therefore, we suggest that the effect of forskolin on BFA-induced disassembly of the Golgi apparatus might be due to an enhanced detoxification of the drug.