UP-REGULATION OF PROTEIN-SERINE THREONINE PHOSPHATASE TYPE 2C DURING 1-ALPHA,25-DIHYDROXYVITAMIN D-3-INDUCED MONOCYTIC DIFFERENTIATION OF LEUKEMIC HL-60 CELLS

Treatment with 20 nM 1 alpha,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) caused a progressive increase in the activity of Mg2+-dependent protein serine/threonine phosphatase type 2C (PP2C) in subcellular fractions of HL-60 cells, whereas PP2C activity was relatively constant throughout all-trans retinoic acid-induced (1 mu M) granulocytic differentiation. The increase in PP2C activity appeared to parallel the 1,25(OH)(2)D-3-induced phenotypic and functional changes in HL-60 cells, Immunoblot and Northern blot analysis indicated that the increase in PP2C activity corresponded to the increased expression of PP2C protein, which was preceded by an increase in the level of mRNA for PP2C beta, No mRNA for PP2C alpha was detected in resting or 1,25(OH)(2)D-3-stimulated HL-60 cells. These results suggest that the increased expression of PP2C is related with the 1,25(OH)(2)D-3-induced monocytic differentiation of HL-60 cells.