Investigation into the role of protein phosphatase 4 catalytic subunit on the survival of leukaemic cell lines

Abstract

Background: Serine/Threonine protein phosphatase 4 (PP4) plays a crucial role in cell signalling, proliferation and apoptosis, and its catalytic subunit has been reported as an important regulator in many types of cancer, including breast and lung cancers.

Aims: Evaluate the effects of PP4c endogenous modulation in leukaemic cell lines; K562 for chronic myeloid leukaemia and HL-60 for acute promyelocytic leukaemia, on cell viability, apoptosis, and proliferation.

Methods: K562 and HL-60 cells were transfected with pcDNA3.1-PP4c to over-express or with specific siRNAs to down-regulate PP4c. Following transfection, cell viability, total apoptosis percentage, proliferation rate and cell cycle were assessed. The response of these cells to four different therapeutic drugs (imatinib, cisplatin, doxorubicin and rapamycin) was assessed post PP4c silencing.

Results: This study demonstrated that PP4c over-expression in K562 cells significantly reduce cell viability and cell proliferation, accompanied by a significant increase in apoptosis at 24 and 48 hours post re-plating. It also altered the cell cycle profile by increasing the number of cells in G0/G1 phase and reducing the cell number in S phase. On the other hand, PP4c silencing significantly increased cell viability in K562 cells at 48 hours. PP4c down-regulation also enhanced and promoted the action of both imatinib and rapamycin by increasing apoptosis in K562 cells. PP4c over-expression in HL-60 decreased the viable cell number at 48 hours, while PP4c down-regulation increased the viable cell number at 72 hours and altered the cell cycle profile by reducing the cell number in S phase and increasing the cells number in G2/M phase. PP4c silencing in HL-60 cells had no effect on the action of the chemotherapeutic drugs.

Conclusion: The results suggested that PP4c regulates cell proliferation, cell death, and cell cycle in K562 cell line in addition to enhancing their response to imatinib and rapamycin. Such studies are important to identify molecular regulators of the pathways involved in the pathogenesis of leukaemia and will contribute to greater understanding of the mechanism(s) by which phosphatases regulate cell survival in cancer and may allow design of optimised targeted therapies in the future.