Modulation of the rhythmic patterns of expression of phosphoprotein phosphatases in human leukaemia cells

Temporal variations in the expression of phosphoprotein phosphatase 1 (PP1), phosphoprotein phosphatase 2A (PP2A) and protein tyrosine phosphatase 1B (PTP1B) were monitored in the human acute, promyelocytic leukaemia cell line, HL60. Granulocytic differentiation was induced using all-trans retinoic acid (ATRA) and monocytic differentiation by phorbol-12-myristate-13-acetate (PMA). Expression of the enzyme proteins in cell extracts was determined by SDS-PAGE and Western immunoblotting using specific antibodies. For PP1, a single immunospecific band of molecular mass 38 kDa was detected corresponding to the catalytic subunit; induction of differentiation with either ATRA or PMA showed differences in the patterns of expression and, in the case of the latter, the mean value. Two immunospecific bands, of mass 34 and 37 kDa, possibly corresponding to dephosphorylated and phosphorylated forms, respectively, were detected for PP2A, as well as a minor band of mass 46 kDa; dynamic variations in the expression of all 3 forms were observed and there were differences between the control and treated cells. The catalytic domain of PTP1B was detected as a 46 kDa band. A 42 kDa form of the protein was also seen, which may represent a change in phosphorylation state, or be the result of proteolytic cleavage; usually the 46 kDa band was the major form, but on occasion there was a change to predominance of the 42 kDa band.