Phosphatidylinositol-3-phosphate kinase pathway activation protects leukemic large granular lymphocytes from undergoing homeostatic apoptosis

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Abstract
T-cell large granular lymphocytic leukemia (T-LGL) is characterized by chronic clonal lymphoproliferation of cytotoxic T lymphocytes (CTLs). Despite exhibiting phenotypic properties of antigen-activated cells, including expression of Fas and FasL, T-LGL cells accumulate and demonstrate resistance to apoptosis. We propose that increased activity of a prosurvival signaling pathway in T-LGL is responsible for attenuated apoptosis in T-LGL. Given the importance of the phosphatidylinositol-3 kinase (PI3K)–AKT pathway in regulating the balance between survival and apoptosis, we analyzed AKT activity in T-LGL cells. Compared with resting CTLs from healthy donors, patients' T-LGL cells showed higher levels of phosphorylated AKT. We demonstrate that phospho-AKT induction is dependent on the upstream activity of a Src family kinase. Since the PI3K-AKT pathway can antagonize the ability of Fas to initiate apoptosis, we hypothesized that inhibition of PI3K would lead to reacquisition of Fas sensitivity in T-LGL. Inhibition of the PI3K-AKT pathway alone led to brisk spontaneous apoptosis of T-LGL. These results suggest that T-LGL pathogenesis is dependent on activity of the PI3K-AKT pathway, without which the leukemic cells will begin to undergo spontaneous apoptosis. We propose that novel therapeutics inhibiting the PI3K-AKT axis may provide effective treatment for T-LGL.