Combined Effect of Mtor Inhibitors and FTI for the Growth Inhibition of T Cell Lymphoma - Involvement of Different Molecular Mechanisms According to the Lymphoma Subtype

Abstract 4870

Since the succesful treatment of T cell lymphoma remains to be problematic, identification of new pharmacological targets in this malignancies are desperately needed. The AMPK-Rheb-mTOR signaling pathway plays an important role in regulating processes such as proliferation and proteinsynthesis according to energy and nutrient levels in normal and malignant T cells. Inhibitors of mTOR have shown promising results in clinical trials in several lymphoma types. Similarly, recent data could prove inhibitors of farnesyltransferase (FTI) to be effective as a single agent in certain subtypes of T cell lymphoma. Despite divergent data regarding the molecular target of FTI action, recently published work suggest inhibition of prenylation of the GTPase Rheb as putative mechanism for the antineoplastic effects of FTI (Basso et al., J Biol Chem, 2005). Therefore, combining inhibition of mTOR and Rheb might result in increased inhibition of T cell lymphoma proliferation. To investigate this hypothesis, human T cell lymphoma cell lines DERL-2 (originated from hepatosplenic gamma-delta T cell lymphoma), Karpas-299 (originated from anaplastic large cell T cell lymphoma) and normal human CD4+ T cells were incubated with a combination of everolimus as mTOR inhibitor and FTI (lonafarnib, SCH-66336) or the single agents. While both substances showed an additive combined anti-proliferative effect in DERL-2 cells, proliferation of Karpas cells were more susceptible to inhibition by FTI. On a molecular level, despite substantial growth inhibition in both cell lines by everolimus alone, phosphorylation of 4EBP1 and p70S6K remained unaffected, while FTI mediated reduction of Karpas cell proliferation was associated with a substantial decrease in AMPK phosphorylation together with an overexpression of p27kip, which could not be observed in DERL-2 cells. In contrast, incubation of stimulated human CD4+ T cells with the drugs alone or in combination did not result in changes in the phosporylation status of AMPK. Nevertheless, in contrast to everolimus, FTI induced a reduction of total protein expression of AMPK and other proteins, e.g. AKT. In addition, contrary to the observations in the malignant T cells, FTI treatment of unstimulated human CD4+ T cells resulted even in an increase of AMPK-phosphorylation. A hint for the explanation of these conflicting data came from analyses of Rheb expression in the examined cell types. While Rheb was easily detectable in the malignant T cell lines and the stimulated CD4+ T cells, it was almost absent in unstimulated CD4+ T cells. A model derived from this findings is that FTI effects depend on different targets available for inhibition of prenylation according to the activation or differentiation status of the T cells. While Rheb might be the target in malignant or activated T cells, another target, e.g. phosphatases, might be responsible for the FTI effect in resting T cells where Rheb is not available. In Karpas cells a particular connection between Rheb and AMPK might exist, as described for other cell lines (Lacher et al., Oncogene, 2010). Inhibition of this Rheb-AMPK axis might explain the particular gowth inhibiting effect of FTI in this model of anaplastic large T cell lymphoma. Nevertherless, the presented data show a combined effect of mTOR inhibitors and FTI for the potent treatment of T cell lymphoma involving different molecular mechanisms according to the lymphoma subtype.