J. Biol. Chem., Vol. 268, Issue 5, 3245-3250, Feb, 1993
Okadaic acid induces both augmentation and inhibition of beta 2- adrenergic stimulation of cAMP accumulation in S49 lymphoma cells
RB Clark, J Friedman, MW Kunkel, BG January and S Shenolikar
Graduate School of Biomedical Sciences, University of Texas Health Science Center, Houston 77225-0334.
To address the role of protein phosphatases in regulating hormonal
responses in mammalian cells, we investigated the effects of okadaic acid,
a potent inhibitor of protein phosphatases 1 and 2A, on epinephrine and
prostaglandin E1 stimulation of cAMP accumulation and adenylyl cyclase in
S49 WT and kin- lymphoma cells. Depending on the dose and time of okadaic
acid pretreatment of both cell lines, there were two distinguishable
effects on cAMP accumulation, an augmentation and an inhibition. The
augmentation occurred rapidly (t1/2 < 1 min), was maximal with 3 microM
okadaic acid, and was observed with concentrations of okadaic acid as low
as 0.3 microM. Prolonged (t1/2 of 5-15 min) pretreatment of cells with
okadaic acid caused an inhibition of epinephrine-stimulated cAMP
accumulation, which was characterized by a 2-3-fold increase in the EC50
for the response to epinephrine. The EC50 for the okadaic acid-mediated
inhibition was similar to that for the augmentation. In assays of adenylyl
cyclase in membrane fractions prepared from okadaic acid-pretreated cells
the inhibitory, but not the stimulatory, effects of okadaic acid
pretreatment were observed. The data demonstrate that protein phosphatases
play an important role in regulating adenylyl cyclase and suggest that
cAMP-dependent protein kinase is not involved in either of its actions.
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