Regulative Loop between β‐Catenin and Protein Tyrosine Phosphatase Receptor Type γ (PTPRG) in Chronic Myeloid Leukemia
Authors: Tomasello Luisa , Vezzalini Marzia , Fiorini Zeno , Al‐Dewik Nader , Yassin Mohamed , Sorio Claudio 
Published: November 2023

Chronic Myeloid Leukemia (CML) is a myeloproliferative disease characterized by the presence of BCR/ABL1 tyrosine kinase. PTPRG (Protein Tyrosine Phoshatase Receptor type γ) is a tumor suppressor gene down‐regulated by hypermethylation of its promoter region in CML and other cancers. Previous studies demonstrated that a re‐expression of PTPRG protein is correlated with a decreased colony formation and clonogenic capability of CML cells. In addition, its restored expression was observed in patients after TKI therapy. In order to understand the mutual regulation between this phosphatase and BCR/ABL1, we searched for PTPRG putative interactors among proteins that are downstream BCR/ABL1 driven pathways and we focused on β‐Catenin, a key transcriptional regulator of cell cycle genes regulated by tyrosine phosphorylation.

Materials and Methods

Cells: PTPRG negative cell line K562, with a stable transfection of exogenous PTPRG, and PTPRG positive cell line LAMA‐84, treated with a specific siRNA and with a new PTPRG small drug inhibitor. Pull‐down assay with purified, recombinant intracellular domain of PTPRG demonstrated a direct interaction between PTPRG and β‐Catenin, while Western Blotting or Immunofluorescence were applied to detect a specific dephosphorylation pattern in presence of PTPRG. Chromatin Immunoprecipitation showed us the binding between DNMT1(β‐Catenin transcriptional target) and PTPRG promoter region.

Results

A colony assay performed using LAMA‐84 cells treated with different concentration of PTPRG specific inhibitor confirmed an inverse correlation between PTPRG activity and CML cells proliferative ability. PTPRG is able to bind and dephosphorylate the onco‐protein β‐Catenin causing its cytoplasmic destabilization and the consequent degradation in CML cell lines with an exogenous or endogenous expression of PTPRG (K562 and LAMA‐84 cell lines). Furthermore, we demonstrated that an increased expression of β‐Catenin in PTPRG negative CML cell lines is correlated with an over‐expression of the DNA (cytosine‐5)‐methyltransferase 1 (DNMT1) that is responsible of PTPRG promoter hypermethylation and that an inhibition after a treatment with 5‐Azacydine or a down‐regulation of this protein is correlated with a PTPRG re‐expression both at mRNA and protein levels.

Discussion

PTPRG is a tumor suppressor gene in Chronic Myeloid Leukemia and its presence is correlated with a decreased proliferative capability of these cells. Here we show for the first time a mechanism that involves β‐Catenin degradation control and the consequent down‐regulation of genes regulated by the TCF/β‐Catenin transcription complex. In return, β‐Catenin up‐regulation is correlated with an over‐expression of DNMT1 that contributes to an hypermethylation of PTPRG promoter region. We hypothesized that there is a regulative loop between PTPRG and β‐Catenin and that an imbalance of the system in favor of one or the other could determine a different proliferation fate of CML cells and their clinical aggressiveness.

Support or Funding Information

Qatar National Research Fund: NPRP 4‐157‐3‐052