TI - Phosphatase inhibitors differentially affect ERK activation in MCF-7 cells enriched and depleted for membrane ER-alpha . AB - We next asked whether the observed decrease in phosphorylaTED ERK1/2 after 20 min in both subpopulations of cells , and the continued low PHOSphorylation levels after 60 min in mERlow cells , could successfully be abrogated with specific phosphatase inhibitors ( Fig 10 ) . We tested inhibitors of protein phosphatase ( PP ) 1 , PP1A and PP2B . These phosphatases can be considered principal enzymes of this class , based on their general abundance and broad specificity [30] . Okadaic acid ( OA ) , an inhibitor of PP1 and PP1A , and cyclosporin A , an inhibitor of PP2B , were both able to reverse the ERK inactivation in mERhigh cells ( Fig 10a ) . In mERlow cells , both the 20 min and continued 60 min dePHOSphorylation were abrogated only with the PP2B inhibitor ( Fig 10b and 10c ) . Because of the known apoptotic effect of OA at some concentrations [31] , it is important to stress that we applied it at a low concentration ( 50 nmol/l ) . In addition , OA does not have toxic effects in short-term incubations [32] . These results suggest that dephosphorylaTION of ERKs is an important component of their process of action and that coordinated phosphorylaTION/dePHOSphorylation is required for strong signaling through this pathway .