Different mechanisms of inhibition by alkyl-lysophospholipid and phorbol ester of granulocyte-macrophage colony-stimulating factor binding to human leukemic cell lines

Abstract

We investigated the effects of rac-1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine (ET-18-OCH3) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on granulocyte-macrophage colony-stimulating factor (GM-CSF) binding to human leukemic cell lines HL60, U937, KG-1, KG-1a, and K562. HL60, U937, and KG-1 exhibited the high-affinity receptors, but KG-1a and K562 revealed no demonstrable receptors. ET-18-OCH3 inhibited GM-CSF binding to HL60, U937, and KG-1 cells with a half-maximal inhibitory concentration of 16, 10, and 78 microM, respectively. ET-18-OCH3 at 10 microM reduced GM-CSF binding sites on HL60, U937, and KG-1, but had little effect on the dissociation constant (Kd). ET-18-OCH3 at 10 and 30 microM significantly (p < 0.01) decreased, in a dose-dependent manner, the total uptake, surface binding, and internalization of GM-CSF. The internalization of GM-CSF was more profoundly inhibited than its surface binding. TPA at 1 and 10 nM inhibited GM-CSF binding. Inhibition of GM-CSF binding by a combination of ET-18-OCH3 (10 microM) and TPA (1 or 10 nM) was less than additive, and ET-18-OCH3 partially inhibited TPA-induced protein kinase C (PKC) depletion in the cytosol and translocation to the particulate fractions. It is suggested that the inhibition of GM-CSF binding by ET-18-OCH3 is due in part to disruption of the plasma membrane and that the inhibition of GM-CSF binding by TPA is due to activation of PKC.