Inhibition of protein phosphatase PPM1D enhances retinoic acid-induced differentiation in human embryonic carcinoma cell line

Role of active site arginine residues in substrate recognition by PPM1A

Reversible protein phosphorylation is a key mechanism for regulating numerous cellular events. The metal-dependent protein phosphatases (PPM) are a family of Ser/Thr phosphatases, which uniquely recognize their substrate as a monomeric enzyme. In the case of PPM1A, it has the capacity to dephosphorylate a variety of substrates containing different sequences, but it is not yet fully understood how it recognizes its substrates. Here we analyzed the role of Arg33 and Arg186, two residues near the active site, on the dephosphorylation activity of PPM1A. The results showed that both Arg residues were critical for enzymatic activity and docking-model analysis revealed that Arg186 is positioned to interact with the substrate phosphate group. In addition, our results suggest that which Arg residue plays a more significant role in the catalysis depends directly on the substrate.

PPM1D Knockdown Suppresses Cell Proliferation, Promotes Cell Apoptosis, and Activates p38 MAPK/p53 Signaling Pathway in Acute Myeloid Leukemia

OBJECTIVES

This study was to explore the effect of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D knockdown on proliferation and apoptosis as well as p38 MAPK/p53 signaling pathway in acute myeloid leukemia.

METHODS

The expression of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D was detected in acute myeloid leukemia cell lines including SKM-1, KG-1, AML-193, and THP-1 cells, and normal bone marrow mononuclear cells isolated from healthy donors. The knockdown of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D was conducted by transfecting small interfering RNA into AML-193 cells and KG-1 cells.

RESULTS

The relative messenger RNA/protein expressions of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D were higher in SKM-1, KG-1, AML-193, and THP-1 cells compared with control cells (normal bone marrow mononuclear cells). After transfecting protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D small interfering RNA into AML-193 cells and KG-1 cells, both messenger RNA and protein expressions of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D were significantly reduced, indicating the successful transfection. Most importantly, knockdown of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D suppressed cell proliferation and promoted cell apoptosis in AML-193 cells and KG-1 cells. In addition, knockdown of protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D enhanced the expressions of p-p38 and p53 in AML-193 cells and KG-1 cells. The above observation suggested that protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D knockdown suppressed cell proliferation, promoted cell apoptosis, and activated p38 MAPK/p53 signaling pathway in acute myeloid leukemia cells.

CONCLUSION

Protein phosphatase, Mg²⁺/Mn²⁺ dependent 1D is implicated in acute myeloid leukemia carcinogenesis, which illuminates its potential role as a treatment target for acute myeloid leukemia.

Metal-dependent Ser/Thr protein phosphatase PPM family: Evolution, structures, diseases and inhibitors

Protein phosphatases and kinases control multiple cellular events including proliferation, differentiation, and stress responses through regulating reversible protein phosphorylation, the most important post-translational modification. Members of metal-dependent protein phosphatase (PPM) family, also known as PP2C phosphatases, are Ser/Thr phosphatases that bind manganese/magnesium ions (Mn²⁺/Mg²⁺) in their active center and function as single subunit enzymes. In mammals, there are 20 isoforms of PPM phosphatases: PPM1A, PPM1B, PPM1D, PPM1E, PPM1F, PPM1G, PPM1H, PPM1J, PPM1K, PPM1L, PPM1M, PPM1N, ILKAP, PDP1, PDP2, PHLPP1, PHLPP2, PP2D1, PPTC7, and TAB1, whereas there are only 8 in yeast. Phylogenetic analysis of the DNA sequences of vertebrate PPM isoforms revealed that they can be divided into 12 different classes: PPM1A/PPM1B/PPM1N, PPM1D, PPM1E/PPM1F, PPM1G, PPM1H/PPM1J/PPM1M, PPM1K, PPM1L, ILKAP, PDP1/PDP2, PP2D1/PHLPP1/PHLPP2, TAB1, and PPTC7. PPM-family members have a conserved catalytic core region, which contains the metal-chelating residues. The different isoforms also have isoform specific regions within their catalytic core domain and terminal domains, and these regions may be involved in substrate recognition and/or functional regulation of the phosphatases. The twenty mammalian PPM phosphatases are involved in regulating diverse cellular functions, such as cell cycle control, cell differentiation, immune responses, and cell metabolism. Mutation, overexpression, or deletion of the PPM phosphatase gene results in abnormal cellular responses, which lead to various human diseases. This review focuses on the structures and biological functions of the PPM-phosphatase family and their associated diseases. The development of specific inhibitors against the PPM phosphatase family as a therapeutic strategy will also be discussed.