Differential Inhibition and Posttranslational Modification of Protein Phosphatase 1 and 2A in MCF7 Cells Treated with Calyculin-A, Okadaic Acid, and Tautomycin*

Calyculin-A (CA), okadaic acid (OA), and tautomycin (TAU) are potent inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) and are widely used on cells in culture. Despite their well characterized selectivity in vitro, their exact intracellular effects on PP1 and PP2A cannot be directly deduced from their extracellular concentration because their cell permeation properties are not known. Here we demonstrate that, due to the tight binding of the inhibitors to PP1 and/or PP2A, their cell penetration could be monitored by measuring PP1 and PP2A activities in cell-free extracts. Treatment of MCF7 cells with 10 nm CA for 2 h simultaneously inhibited PP1 and PP2A activities by more than 50%. A concentration of 1 μm OA was required to obtain a similar time course of PP2A inhibition in MCF7 cells to that observed with 10 nm CA, whereas PP1 activity was unaffected. PP1 was predominantly inhibited in MCF7 cells treated with TAU but even at 10 μm TAU PP1 inhibition was much slower than that observed with 10 nm CA. Furthermore, binding of inhibitors to PP2Ac and/or PP1c in MCF7 cells led to differential posttranslational modifications of the carboxyl termini of the proteins as demonstrated by Western blotting. OA and CA, in contrast to TAU, induced demethylation of the carboxyl-terminal Leu309 residue of PP2Ac. On the other hand, CA and TAU, in contrast to OA, elicited a marked decrease in immunoreactivity of the carboxyl terminus of the α-isoform of PP1c, probably reflecting proteolysis of the protein. These results suggest that in MCF7 cells OA selectively inhibits PP2A and TAU predominantly affects PP1, a conclusion supported by their differential effects on cytokeratins in this cell line.