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Background: Class I phosphatidylinositol 3-kinases (PI3Ks) are heterodimeric lipid kinases consisting of a regulatory subunit and one of four catalytic subunits (p110α, p110β, p110γ or p110δ). p110γ/p110δ PI3Ks are highly enriched in leukocytes. In general, PI3Ks regulate a variety of cellular processes including cell proliferation, survival and metabolism, by generating the second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP3). Their activity is tightly regulated by the phosphatase and tensin homolog (PTEN) lipid phosphatase. PI3Ks are widely implicated in human cancers, and in particular are upregulated in T-cell acute lymphoblastic leukemia (T-ALL), mainly due to loss of PTEN function. At present different compounds which target single or multiple PI3K isoforms have entered clinical trials. Aims: To explore the role of the different PI3K isoforms in T-ALL, in order to identify the most effective targeted therapy strategy. Methods: Both PTEN wt (ALL-SIL and DND-41) and PTEN deleted (Jurkat and Loucy) T-ALL cell lines were treated with class I pan-PI3K inhibitors (BKM120 and ZSTK454) or p110α (A-66), p110β (TGX-221), p110γ (AS-605240), p110δ (CAL-101) and p110γ/δ (IPI-145) selective inhibitors (all purchased from Selleck Chemicals, Houston TX, USA), and their effects were evaluated. Results: In the tested cell lines, flow cytometry analysis evidenced a slight reduction in PIP3 levels after treatment with all the single-isoform inhibitors, whereas pan-PI3K and p110γ/δ inhibition significantly decreased PIP3 level. However, only pan-PI3K inhibition significantly affected cell viability in a dose and time-dependent manner after 48h treatment, with IC50 values ranging between 1 and 4 μM, for both BKM120 and ZSTK454, whereas for all the other drugs IC50 values were not attained. This finding was confirmed by a time-course analysis of viable cells, which revealed a rapid and drastic antiproliferative effect of the pan-PI3K inhibitors. Only after prolonged drug-exposure (64h) the combined p110γ/δ inhibition exerted a significant antiproliferative effect, in agreement with PIP3 decrease. Consistent with these results, Annexin V/PI staining analysis showed a significant increase of apoptosis after 48h treatment in both PTEN wt and deleted cell lines only after treatment with pan-PI3K inhibitors, whereas cell cycle was not affected either revealed the inactivation of the main downstream targets already after 1h treatment with pan-PI3K inhibitors, as evidenced by decrease of p-Akt, both at Thr308 and Ser473, p-P70S6K and p-S6RP. Of note, p110γ and p110δ inhibition slightly affected these PI3K targets, but only in some cell lines. Overall, no difference emerged in relationship with PTEN status. Summary and Conclusion: Due to the roles played by PI3Ks in cancer, many efforts have tried to address the identification of compounds which specifically target PI3K isoforms for a better effectiveness and a lower toxicity. Here we demonstrated that, irrespective of PTEN status, only pan-class I PI3K inhibition is cytotoxic in T-ALL cells, implying that any isoform could sustain leukemic cell proliferation, and suggesting a redundant role played by each isoform. Therefore, our findings strongly support clinical application of pan-class I PI3K rather than single-isoform inhibitors in T-ALL treatment.

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