New insights into the functional role of protein phosphatase 4 regulatory subunit PP4R3A/SMEK1 in the regulation of leukemic cell fate.

The serine/threonine protein phosphatase 4 holoenzyme consists of a PP4 catalytic subunit (PP4c), which interacts with four different regulatory subunits. Previous studies have shown that PP4c acts as a tumour suppressor. Emerging evidence suggests that the protein phosphatase 4 regulatory subunits might regulate cell fate independently of PP4c. To this end, we investigated the role of PP4R3A (SMEK1) in Jurkat and CEM-C7 leukemic cell lines. SMEK1 overexpression decreased cell growth, increased spontaneous apoptosis, and reduced the colony forming ability of leukemic cells. Conversely, siRNA-mediated silencing of SMEK1 led to increased short and long-term survival in these cells. Phospho-protein arrays revealed that increased expression of SMEK1 affected the phosphorylation of key proteins involved in MAPK3, AKT, JAK/STAT, NFκB and TGFβ signalling pathways. These proteins include transcription factors such as NFκB, STAT3, c-JUN, SMAD1, and SMAD5, suggesting a role for SMEK1 in the regulation of gene expression. RNA sequencing confirmed the role of SMEK1 in the regulation of gene expression. RNA sequencing also confirmed the tumour suppressor role of SMEK1. Taken together, this study shows that SMEK1 regulates leukemic T cell survival, indicating that SMEK1 dysfunction may be important in the development and progression of leukemia.