Frequent Elevation of Akt Kinase Phosphorylation in Mononuclear Cells from High-Risk Myelodysplastic Syndrome Patients.

Abstract

Myelodysplastic syndromes (MDS) comprise a heterogeneous group of clonal stem cell disorders characterized by ineffective and inadequate hematopoesis. Early stages of MDS display an increased apoptosis, while later stages of MDS present an excessive survival and decreased apoptosis of progressing abnormal clonal cells. The identification of the signaling pathways responsible for increased survival of MDS cells is of high importance as they might represent promising targets for novel forms of therapy aimed at preventing MDS evolution in to acute myeloid leukaemia (AML). However, to date, very limited information is available on survival pathways activated in MDS patients. The phosphatidylinositol 3-kinase (PI3K)/Akt signalling pathway has been implicated in the aggressiveness of a number of different human cancers, and constitutive activation of the PI3K/Akt pathway has been shown to be necessary for the survival of AML cells. In this study, we have performed an immunocytochemical and flow cytometric staining analysis, using an antibody to Ser473 phosphorylated-At (p-Akt) recognizing the activated form of the Akt kinase, to evaluate the levels of p-Akt in MDS mononuclear cells from either bone marrow or peripheral bllod. We observed a high Ser473 p-Akt staining in 85 % of the patients (n=22) diagnosed as high risk MDS, whereas mononuclear cells from healthy donors or low risk MDS patients showed very low or absent Ser473 p-Akt staining. Furthermore, high levels of the class I PI3K p110d isoform was consistently observed in MDS samples also demonstrating high levels of p-Akt. PI3K p110d was recently shown to have a higher expression in contrast to other class I PI3K isoforms in AML blasts. Finally, the expression of PTEN was correlated to the elevated levels of Ser473 p-AKT and PI3K p110d detected in myeloid cells. PTEN is a lipid phosphatase having the PI3K downstream product phosphatidylinositol 3,4,5 triphosphate (PIP3) as one of its primary targets, thus negatively regulating Akt activity. We have found that PTEN is expressed in mononuclear cells from high risk MDS patients, albeit with a lower staining intensity when compared with peripheral blood mononuclear cells from healthy donors. These findings identify activated Akt and PI3K p110d as potential survival factors in high risk MDS and could be important as therapeutic targets for innovative strategies to treat MDS patients.