Membrane-associated serine/threonine protein phosphatase in endometrial cancer

Objective Reversible serine/threonine protein phosphorylation catalyzed by kinases and phosphatases plays a crucial role in cellular growth and differentiation. We attempted to determine the subcellular location of serine/threonine phosphatase (protein phosphatase type 2A [PP2A]) in endometrial cancer. Study design Endometrial cancers surgically removed were examined. PP2A activity was assessed by measuring the dephosphorylation of phosphopeptide highly selective for the PP2A in cytosol and membranes fractionated on a continuous sucrose density gradient. Its protein level was detected by immunoblotting with a specific antibody. Results There were three peaks of PP2A enzyme activity and immunoreactivity corresponding by marker enzyme analysis to the cytoplasm, plasma membrane, and endoplasmic reticulum fractions. An enzyme kinetic analysis showed the different activity in cytosol and plasma membrane; Km values of 98±12 μmol/L for cytosol and 32±6.2 μmol/L for plasma membrane (P<.01), respectively. The membrane phosphatase was sensitive to inhibition by okadaic acid and sodium fluoride, characteristics suggestive of PP2A activity. Conclusion PP2A activity in the plasma membrane of endometrial cancers might be distinct from that present in the cytosol. The plasma membrane PP2A may be responsible for a rapid and initial decrease in intracellular level of phosphoserine and phosphothreonine, interfering with serine/threonine protein phosphorylation–mediated growth of endometrial cancer.