Combined Inhibition of CDK and PI3K/Akt Pathways Induces Apoptosis of Mantle Cell Lymphoma.

Mantle cell lymphoma (MCL) arises from the neoplastic transformation of naïve B cells in the mantle zone of the B cell follicle. It is an aggressive and incurable B cell neoplasm. It accounts for 5% to 8% of non-Hodgkin’s lymphomas. Patients with MCL respond initially to chemotherapy but ultimately relapse. Mean survival time is only three to four years. Thus, the need for new treatments that effectively combat MCL is obvious and imperative. Defects in cell cycle regulation and apoptosis are primary events in MCL. It is characterized by the presence of a chromosomal translocation t (11:14)(q13:q32), which results in deregulated cyclin D1 expression. Cyclin D1 overexpression in MCL is thought to play a major role in lymphomagenesis, although the precise mechanisms by which tumor formation and progression occur are not fully understood. Constitutive activation of the PI3K/Akt pathway contributes to the pathogenesis and survival of MCL. Activated AKT was found in cultured MCLs as well as in 100% of aggressive-blastoid variants of MCL tumors, compared to 30% of typical MCLs. Towards the search for novel therapies for MCLs, we examined the potential of roscovitine (rosc) or flavopiridol (flav), inhibitors of cyclin dependent kinase (CDK), as a single agent or in combination with LY294002 (LY), a PI3K/Akt inhibitor, in inducing apoptosis of various MCL lines as well as in MCL patient samples. CDK inhibitors modestly increased the percentage of apoptotic cells (∼30%), whereas LY had no effect. However, when added in combination, rosc/LY or flav/LY induced apoptosis in more than 70% of cells. In an effort to understand the mechanism of apoptosis, we identified three targets: cyclin D1, the anti-apoptotic proteins myeloid cell leukemia-1 (Mcl-1) and X-linked Inhibitor of Apoptosis (XIAP). CDK inhibitors eliminated Mcl-1 expression, slightly reduced XIAP abundance and had very little effect on abundance of cyclin D1. Conversely, LY reduced cyclin D1 expression and slightly reduced the abundance of Mcl-1 and XIAP. A larger decrease in XIAP abundance is seen in cultures treated with a combination of rosc/LY or flav/LY. On the basis of these findings, we suggest that agents that target Mcl-1, XIAP and cyclin D1 will be most effective in inducing apoptosis of human MCLs.