1. ¹Turku Centre for Biotechnology, University of Turku, Åbo Akademi University, POB 123, Turku FIN-20521, Finland
2. ²Department of Biochemistry and Pharmacy, Åbo Akademi University, POB 66, Turku FIN-20521, Finland
3. ³Department of Biology, Åbo Akademi University, Turku FIN-20520, Finland
4. ⁴Department of Biology, Laboratory of Animal Physiology, University of Turku, Turku FIN-20014, Finland
5. ⁵Glogozymes, 6351 corte del Albeto Suite 101, Carlsbad, California 92009, USA

Correspondence: JE Eriksson, Turku Centre for Biotechnology, University of Turku, Åbo Akademi University, POB 123, Turku FIN-20521, Finland. E-mail: john.eriksson@utu.fl

⁶These two authors contributed equally to this work

Received 2 January 2003; Revised 25 July 2003; Accepted 5 August 2003.

Abstract

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-, 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.