Type-2A protein phosphatase activity is required to maintain death receptor responsiveness.
Type-2A protein phosphatase (PP2A) is a key regulator in many different cell signaling pathways and an important determinant in tumorigenesis. One of the signaling targets of PP2A is the mitogen-activated protein kinase (MAPK/ERK) cascade. In this study, we wanted to determine whether PP2A could be involved in regulation of death receptor activity through its capacity to regulate MAPK/ERK. To this end, we studied the effects of two different routes of protein phosphatase inhibition on death receptor-mediated apoptosis. We demonstrated that the apoptosis mediated by Fas, TNF-alpha, and TRAIL in U937 cells is suppressed by calyculin A, an inhibitor of type-1 and type-2A protein phosphatases. The inhibition of the protein phosphatase activity was shown to subsequently increase the MAPK activity in these cells, and the level of activation corresponded to the degree of suppression of cytokine-mediated apoptosis. A more physiological inhibitor, the intracellular PP2A inhibitor protein I2(PP2A), protected transfected HeLa cells in a similar way from Fas-mediated apoptosis and induced activation of MAPK in I2(PP2A) transfected cells. A corresponding inhibition could also be obtained by stable transfection with a constitutively active form of the MAPK kinase, MKK1 (also referred to as MEK1). The inhibitor-mediated protection was highly efficient in preventing early stages of apoptosis, as no caspase-8 cleavage occurred in these cells. The observed apoptosis suppression is likely to facilitate the tumor-promoting effect of a range of different type-2A protein phosphatase inhibitors, and could explain the reported tumor association of I2(PP2A).
Härmälä-Braskén
Ann-Sofi
AS
Turku Centre for Biotechnology, University of Turku, Abo Akademi University, POB 123, Turku FIN-20521, Finland.
Mikhailov
Andrey
A
Söderström
Thomas S
TS
Meinander
Annika
A
Holmström
Tim H
TH
Damuni
Zahi
Z
Eriksson
John E
JE
D016475:3T3 Cells; D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D051017:Apoptosis Regulatory Proteins; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D048369:MAP Kinase Kinase 1; D020935:MAP Kinase Signaling System / Q000187:drug effects; D008387:Marine Toxins; D008562:Membrane Glycoproteins / Q000494:pharmacology; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D010080:Oxazoles / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D053221:TNF-Related Apoptosis-Inducing Ligand; D014162:Transfection; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology; D020298:U937 Cells; D019014:fas Receptor / Q000378:metabolism
D051017:Apoptosis Regulatory Proteins; D004791:Enzyme Inhibitors; D008387:Marine Toxins; D008562:Membrane Glycoproteins; D010080:Oxazoles; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; C505654:Tnfsf10 protein, mouse; D014409:Tumor Necrosis Factor-alpha; D019014:fas Receptor; C059041:calyculin A; D017346:Protein Serine-Threonine Kinases; D048369:MAP Kinase Kinase 1; C482125:MAP2K1 protein, human; C482127:Map2k1 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases; D010749:Phosphoprotein Phosphatases