Expressions of four major protein Ser/Thr phosphatases in human primary leukemic cells.

Activity and expression of four major protein serine/threonine (Ser/Thr) phosphatases, protein phosphatase type 1 (PP1), protein phosphatase type 2A (PP2A), protein phosphatase type 2B (PP2B) and protein phosphatase type 2C (PP2C) were evaluated in normal peripheral leukocytes, and in various leukemic cells from patients with acute myelogenous leukemia (AML), common acute lymphocytic leukemia (cALL), or chronic lymphocytic leukemia (CLL). PP1 was the most abundant phosphatase in blood cells, and relative abundance of each phosphatase was: PP1 > PP2A > PP2B approximately = PP2C. PP1 activity and its expressions were higher in blasts of AML-M4 and -M5 than in cells of AML-M1, cALL and CLL. PP2A activity and its expression were higher in blasts of AML-M3, -M4 and -M5 than in cells of AML-M1, cALL and CLL. Activity and expression of both PP1 and PP2A in normal monocytes were highest, and PP2A activity in normal neutrophils was lowest among normal leukocytes. PP2B activity and its expression were higher in blasts of AML-M2, -M3 and normal lymphocytes. PP2C activity and its expression were relatively constant in various leukemic cell types. Activities of PP1 and PP2A of AML blasts correlated positively with the expression of CD11b, whereas activities of PP1 and PP2B correlated negatively with the expression of CD7. Thus, each phosphatase was ubiquitously but differently expressed in various leukemic cell types and in normal leukocytes. These data also suggest that expressions of PP1, PP2A and PP2B are relatively low in leukemic blasts arresting at the stage of early pluripotent stem cells, and are differently modulated during the course of myelomonocytic commitment and maturation.