POST-TRANSLATIONAL MODIFICATIONS OF PROTEIN PHOSPHATASE 2A (PP2A) IN MIXED LINEAGE LEUKEMIA (MLL)
Author(s): ,
Yoana Arroyo-Berdugo
Affiliations:
Life Sciences,University of Roehampton,London,United Kingdom
,
Antonella Di Mambro
Affiliations:
Life Sciences,University of Roehampton,London,United Kingdom
,
Yolanda Calle-Patino
Affiliations:
Life Sciences,University of Roehampton,London,United Kingdom
,
Bela Wrench
Affiliations:
Hemato-Oncology,Barts Cancer Institute,London,United Kingdom
,
John Gribben
Affiliations:
Hemato-Oncology,Barts Cancer Institute,London,United Kingdom
Maria Teresa Esposito
Affiliations:
Life Sciences,University of Roehampton,London,United Kingdom
EHA Library. ARROYO Y. Jun 15, 2019; 266617; PS1000
Dr. Yoana ARROYO
Dr. Yoana ARROYO
Contributions
Abstract

Abstract: PS1000

Type: Poster Presentation

Presentation during EHA24: On Saturday, June 15, 2019 from 17:30 - 19:00

Location: Poster area

Background
Mixed lineage leukemia (MLL) is the most common leukemia in infant patients. This leukemia is due to chromosomal translocations at locus 11q23 and is associated with poor clinical outcome.  Novel, targeted therapeutic approaches are needed. Protein phosphatase 2A (PP2A) is a serine threonine phosphatase which regulates the phosphorylation of several kinases, including Erk, Akt, GSK3 which are fundamental for MLL cells' survival. PP2A is a trimeric protein complex in which a core dimer formed between the scaffold subunit and the catalytic subunit is associated with one of the many regulatory subunits that facilitate and direct the interaction of the trimer with substrate proteins. PP2A activity is regulated by interaction with PP2A inhibitor proteins (PIPs), as well as by post-translational modifications of PP2A complex components.

Aims
In this context, our aim is to understand how PP2A post-translational modifications affect the activity of PP2A on its downstream targets and to investigate whether PP2A re-activation, by preventing phosphorylation and activation of collateral pathways, might represent a valid therapeutic strategy for MLL.

Methods
12 different leukemic cell lines and 8 primary samples from MLL patients were included in this study. We measured the PP2A phosphatase activity by a colorimetric assay using a synthetic phospho-peptide and malachite green reagent. The protein levels of PP2A, phospho-PP2A, demethyl-PP2A, LCMT-1, PME-1, Akt, phospho-Akt, Erk, and phospho-Erk were determined by western blot using specific antibodies.

Results
Compared to healthy bone marrow, we found an increase in the phosphorylation at Tyrosine 307 (Tyr307) and a decrease in the methylation at Leucine 309 (Leu309) of the catalytic subunit of PP2A in MLL samples. These changes in the post-translational modifications of the catalytic subunit correlated with a decrease in the phosphatase activity of PP2A. Methylation at Leu309 has been described as an absolute requirement for the binding of the regulatory subunit B55α to PP2A core. B55α was found up-regulated in MLL cell lines and primary samples compared to healthy bone marrow.The methylation of the catalytic subunit of PP2A is catalyzed by leucine carboxyl methyltransferase (LCMT-1), which we found down-regulated in MLL samples, whereas demethylation is catalyzed by the phosphatase methylesterase (PME-1) that was found up-regulated in MLL. In line with the inactivation of PP2A in the MLL samples, PP2A downstream targets such as Akt and Erk were found hyper-phosphorylated in  MLL samples. 

Conclusion
Our results suggest that PP2A phosphatase activity inhibition might sustain in MLL samples a persistent serine/threonine phosphorylation of PP2A substrates, such as Akt and Erk that mediate pro-survival and anti-apoptotic signals. This inhibition might be explained, at least in part, by phosphorylation and demethylation of PP2A catalytic subunit. A better understanding of the mechanisms that regulate PP2A activity could provide new strategies to rescue PP2A phosphatase activity and target fundamental mechanisms of leukemic survival and chemotherapy resistance.

Session topic: 3. Acute myeloid leukemia - Biology & Translational Research

Keyword(s): Leukemia, MLL, Phosphorylation

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