Expression Analysis and Prognostic Significance of the PHLPP Gene in Acute Leukemias and Myelodysplastic Syndromes.

[Background] The proto-oncogene Akt/PKB is activated in many human cancers including leukemias, and regulates the cell survival and apoptosis. Akt/PKB is activated by growth factors or the phosphatidylinositol-3,4,5-triphosphate via PI3K, phosphorylates many substrates for cell survival or proliferation. The tumor suppressor PTEN prevents Akt/PKB phosphorylation and activation. On the other hand, the PH domain leucine-rich repeat protein phosphatase (PHLPP) dephosphorylates Akt/PKB, induces apoptosis, and suppresses tumor growth. It has been reported that the expression levels of PHLPP are significantly reduced in several colon cancer and glioblastoma cell lines, and Akt/PKB phosphorylation is elevated. However, we found that PHLPP gene was overexpressed in MNCs derived from patients with acute leukemias and MDS. In this study, we have investigated the expression analysis and prognostic significance of the PHLPP gene in patients with acute leukemias and MDS.

[Methods] The cells used in this study were human acute leukemia cell lines, HL60, U937, YRK2, CEM, and MOLT4 cells. Primary acute lymphoblastic leukemia (8 ALL (8 L2)), myelodysplastic syndromes (32 MDS (18 RAEB-I, 14 REAB-II)), and acute myeloblastic (25 AML (4 M1, 11 M2, 6 M4, 4 M5)) cells were obtained from the peripheral blood. Human normal mononuclear cells (MNCs) were isolated from peripheral blood (PB) of healthy volunteers after obtaining informed consents. For analysis of PHLPP mRNA, quantitative RT-PCR was performed in all cell lines and clinical samples.

[Results] In all cell lines, the expressions of PHLPP mRNA were significantly increased more than normal MNCs. In HL60, U937, YRK2, CEM and MOLT4 cells transfected with the siRNA PHLPP, it is shown that the phosphorylation state of Akt was markedly increased compared to the untransfected cells. In the clinical samples obtained from patients with acute leukemias and myelodisplastic syndromes, the PHLPP gene was overexpressed in 7/8 (87.5%) of ALL, 14/25 (56%) of AML, and 26/32 (81.3%) of MDS. This higher expression was not associated to the counts of blasts of PB (P < 0.01). The relative folds of PHLPP gene expression were for AML: 3.02, MDS: 4.16, and ALL 3.62 compared to normal MNCs. Kaplan-Meier survival analysis revealed that lower PHLPP expression increases the probability of the longer overall and the progression free survival of the patients. Moreover, when CR status after chemotherapy, mean PHLPP expression levels were significantly reduced compared to unCR status after chemotherapy in MNCs derived from PB of patients with acute leukemias and myelodysplastic syndromes.

[Conclusions] Our results suggest that the PHLPP gene was overexpressed in MNCs derived from the patients with acute leukemias and myelodysplastic syndromes and may possibly be characterized as a new marker of unfavorable prognosis for ALL, AML, and MDS.