PI3K a Selective Inhibitor Induces Growth Inhibition In Mantle Cell Lymphoma

Abstract 1812

Deregulation of the phosphatidylinositol 3-kinase (PI3K)-mediated signaling plays an important role in the development of cell proliferation of mantle cell lymphoma (MCL).

The PI3K pathway activation in MCL has been shown to result from constitutive B cell receptor (BCR) activation which is directly mediated by the Class IA PI3K p110 isoforms (a, β, and d). However, their relative contribution in MCL is not fully understood. In this study, the activity and molecular mechanisms of isoform-selective PI3K inhibitors which target different isoforms of the p110-kDa subunit has been investigated. We utilized the isoform-selective PI3K inhibitors; PI3-Ka inhibitor IV (p110a), TGX115 (p110b), IC87114 (p110d) and the non-specific PI3K inhibitor LY294002 (all inhibitors were purchased commercially).

The p110a and p110d but not p110b isoform protein expression was detected in all tested MCL cell lines (Granta 519, JVM-2, Z138, Jeko-1, MINO). PI3-Ka inhibitor IV as well as non-specific PI3K inhibitor LY294002 induced cell growth inhibition with dose-dependent manner (IC₅₀ at 48 hrs; PI3-Ka inhibitor IV: 17.5 μM for Granta 519, 14.3 μM for Jeko-1, 16.5 μM for Z138, LY294002: 14.8 μM for Granta 519, 19.4 μM for Jeko-1, 15.0 μM for Z138, MTT test). However, neither IC87114 nor TGX115 showed significant cell growth inhibition up to 40mM. Low dose of PI3-Ka inhibitor IV (5 μM) or LY294002 (5 μM) induced G₀/G₁ cell cycle arrest (increase of G₀/G₁ phase: PI3-Ka inhibitor IV 17.9 % for Granta 519, 28.2 % for Jeko-1, LY294002 19.3 % for Granta 519, 14.5 % for Jeko-1), and the higher dose (10 μM) increased apoptosis(specific apoptosis: PI3-Ka inhibitor IV 10.8 % for Granta 519, 15.3 % for Jeko-1, LY294002 13.6 % for Granta 519, 19.6 % for Jeko-1). No induction of cell cycle arrest/apoptosis by IC87114 or TGX115 treatment was observed. We then tried to assess the inhibition of PI3K/Akt signaling activation by p110a and p110d inhibitors. PI3-Ka inhibitor IV (10 μM) completely diminished phosphorylated (p-) Akt in all cell lines analyzed. Further investigation with 1–10 μM PI3-Ka inhibitor IV or IC87114 in Granta 519 and Jeko-1 cells declared that 1 μM PI3-Ka inhibitor IV almost diminished p-Akt and p-S6rp in both cells. The phosphorylation level of other PI3K/Akt signaling downstream substrates, GSK3-b and 4E-BP1, were down-regulated in dose dependent manner. Recently, GSK3-b kinase has been shown to negatively regulate cell cycle progression through Cyclin D1 repression in MCL. We observed that PI3-Ka inhibitor IV decreased Cyclin D1 expression and active pRb which are responsible for G₀/G₁ cell cycle arrest. The treatment with IC87114 (10 μM) performed moderate decrease of p-Akt, p-S6rp, and p-4E-BP, while no change in the levels of p-GSK3-b, Cyclin D1, or p-pRb was observed in both Granta 519 and Jeko-1 cells. We also tested whether the combination of PI3-Ka inhibitor IV or IC87114 with the proteasome inhibitor bortezomib induces synergistic cytotoxicity in MCL. No synergistic anti-proliferative effect was observed in any of the MCL cell lines analyzed. These findings demonstrate that p110a may be the responsible Class IA PI3K isoform for the development of MCL cell proliferation, and p110a isoform-selective PI3K inhibitor but not p110d or p110b inhibitors may provide a better therapeutic index relative to pan-PI3K inhibitors.