An investigation into the functional roles of protein phosphatase 4 regulatory subunits PP4R3A and PP4R3B in leukemic T-cells

Abstract

The serine/threonine protein phosphatase 4 holoenzyme consists of PP4 catalytic subunit (PP4c), which interacts with four different regulatory subunits (PP4R1, PP4R2, PP4R3, PP4R4). Previous studies have shown that PP4c acts as a tumour suppressor and controls cell fate of both leukemic T-cells and untransformed human peripheral blood T-cells. Emerging evidence suggests that PP4 regulatory subunits might also regulate cell fate independently of PP4c. In line with this, this study investigates the role of PP4R3A (SMEK1) and PP4R3B (SMEK2) in leukaemia.

SMEK1 and SMEK2 overexpression decreased cell growth, increased spontaneous apoptosis rate, and reduced colony forming ability of leukemic cells, while siRNA-mediated silencing of SMEK1 and SMEK2 led to increased short and long-term survival in these cells. Phospho-protein arrays analyses revealed that increased expression of SMEK1 and SMEK2 affected the phosphorylation of key proteins involved in five core cancer signalling pathways: MAPK3, AKT, JAK/STAT, NF?B and TGFß. Most of the changes induced by SMEK1 and SMEK2 were mirrored in the cells overexpressing PP4c, apart from the effects on the TGFß signalling pathway, which were SMEK1/2 specific. Eight transcription factors were confirmed to show changes in their phosphorylation in cells overexpressing SMEK1/2, suggesting a role for SMEK1/2 in the regulation of gene expression. Indeed, RNA sequencing confirmed the role of SMEK1/2 in the regulation of gene expression at epigenetic, transcriptional, and posttranscriptional levels. RNA sequencing also confirmed the tumour suppressor role of SMEK1 and the dual function of SMEK2 as an oncogene and tumour suppressor.

Taken together, this study suggests that both SMEK1/2 exert a key role in regulating cell fate of leukemic T cells, indicating that SMEK1/2 dysfunction may be important in the development and progression of leukaemia. The study also confirmed that both proteins have some non-overlapping functions and are not functionally redundant.