title: Okadaic acid suppresses TPA-induced differentiation by stimulating G1/S transition in human myeloblastic leukaemia ML-1 cells.
abstract: The association between the phosphorylation status of the retinoblastoma protein, pRb and changes in cell cycle control caused by either protein kinase C (PKC) or protein kinase A (PKA) stimulation was evaluated in human myeloblastic leukaemia ML-1 cells. TPA-induced PKC activation resulted in dephosphorylation of pRb and subsequently induced ML-1 differentiation based on morphological changes and CD14 expression. In the present study, we showed that inhibition of protein phosphatases (PP-1 and PP-2a) prevented the TPA-induced differentiation in ML-1 cells. Preinhibition of PP-1 and PP-2a activities with 1-100 nM okadaic acid dose-dependently blunted the decrease in the phosphorylation status of pRb obtained with TPA and overrode cell cycle arrest. PKA stimulation with 8-chlorophenylthio-cAMP (100 microM) decreased cell proliferation by 65% and the distribution of cells in the G1 phase significantly increased from 38% to 83% concomitant with a 34% decline in the number of cells present in the S phase. In addition, PKA stimulation significantly decreased the pRb phosphorylation status but did not elicit CD14 expression, indicating that cAMP-induced dephosphorylation of pRb cannot by itself trigger differentiation in ML-1 cells.
authors.affiliation: Department of Physiology and Biophysics, Wright State University School of Medicine, Dayton, OH, USA.
authors.forename: P S
authors.initials: PS
authors.lastname: Reinach
authors.forename: T
authors.initials: T
authors.lastname: Li
authors.forename: L
authors.initials: L
authors.lastname: Lu
mesh_terms: D002454:Cell Differentiation / Q000187:drug effects*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism* / Q000494:pharmacology; D016193:G1 Phase; D006801:Humans; D015470:Leukemia, Myeloid, Acute; D018950:Lipopolysaccharide Receptors / Q000096:biosynthesis; D019319:Okadaic Acid / Q000378:metabolism* / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010766:Phosphorylation; D016160:Retinoblastoma Protein / Q000378:metabolism*; D016196:S Phase; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D004791:Enzyme Inhibitors; D018950:Lipopolysaccharide Receptors; D016160:Retinoblastoma Protein; D019319:Okadaic Acid; D017868:Cyclic AMP-Dependent Protein Kinases; D010749:Phosphoprotein Phosphatases; D013755:Tetradecanoylphorbol Acetate