In the last few years, our studies of mantle cell lymphoma (MCL) have been focused on developing a cellular signaling map of this lymphoma to better understand the factors driving cell growth, and to identify potential therapeutic targets and prognostic markers. As a first approach, we initiated studies investigating the potential significance of important cellular kinases known to be involved in proliferation and survival pathways in cell line models as well as in primary samples. We recently published our first study showing that aggressive (blastoid) MCL differs from typical MCL as a result of PI3K/AKT pathway activation (Blood, 2006). Cell lines that simulate blastoid MCL and primary blastoid MCL cases show phosphorylation of multiple downstream AKT targets, while typical MCL do not. In trying to understand the proximal cause of AKT activation in this subset of MCL cases, we have found that activity of the BCR-associated SYK kinase is necessary for activation of AKT to occur. However, SYK activity alone is insufficient and in addition to SYK signaling, inactivation of PTEN, whether through loss of PTEN or phosphorylation at Ser 380, is required. These data suggest a model in which PTEN plays a gatekeeper role in the activation of AKT by SYK, by controlling the absolute levels of the phospholipid activator phosphatidylinositol 3,4,5-trisphosphate (PIP3). In related studies, we demonstrate that combinations of drugs that target both the PI3K/AKT pathway and the B-cell receptor (BCR) signaling pathway have synergistic activity against MCL, suggesting a therapeutic approach. We have recently submitted one manuscript detailing these investigations, and have a second one in preparation. In a related study focused on MCL therapy (Clinical Cancer Research), we have shown that Nutlin-3, a small molecular inhibitor of the ubiquitin-ligase MDM2, is highly effective in inducing cell death in MCL with wild-type TP53 and also has activity in MCL with mutant TP53. We also provide data concerning the mechanism of activity of Nutlin-3 in both wild-type TP53 and mutant TP-53 bearing MCL cells. Furthermore, we show that the combination of Nutlin-3 with the proteasome inhibitor Bortezomib results in synergistic cytotoxicity in MCL cells harboring either wild-type or mutant TP53. These results provide a basis for the rational use of Nutlin-3 in MCL either alone or in combination with other compounds known to have activity in MCL. We continue to work with Dr. Leticia Quintanilla-Martinez (Tuebingin, Germany), a former fellow in my laboratory, on the role of Cyclin D1 in mantle cell lymphomagenesis. In these studies we have shown that Cyclin D1 is not necessary for mantle cell lymphoma survival (Leukemia 2008). Furthermore, through the use of si-RNA targeting Cyclin D1, we demonstrate the existence of a compensatory feedback loop with Cyclin D2 that allows cells to continue growing following Cyclin D1 knockdown. These studies, initiated in our laboratory several years ago, are now primarily being performed in Dr. Quintanilla-Martinez's laboratory. In anaplastic large cell lymphoma (ALCL), we have focused on uncovering the signaling links between the abnormally activated ALK kinase and the development of the lymphoma through a series of targeted inhibition experiments, combined with expression analysis. We recently showed that the abnormally expressed ALK kinase is linked to the activation of the C/EBP beta transcription factor, not previously known to be expressed in ALCL (Blood. 2006). We also found that the LIP isoform was preferentially expressed in ALCL, which also occurs in other aggressive cancers. Our current studies indicate that C/EBP beta is critical for ALCL cell growth, and that its activation occurs through two independent convergent pathways activated by the ALK kinase (manuscript under review). Other investigators have reported that ALK activates STAT3, and that STAT3 is necessary for ALCL survival. Our study indicates that STAT3 is required for C/EBP beta gene expression, suggesting that at least one of the critical STAT3 targets is C/EBP beta. Biological activity of C/EBP beta also requires critical phosphoylation events, and our studies indicate that MAPK activation by ALK is also necessary for C/EBP beta activity. This is an unusual example of a proximal oncogenic event activating a downstream target through two convergent pathways, one acting at the transcriptional level (STAT3) and the second, at the post-transcriptional level (MAPK). Studies are ongoing in collaboration with Dr. Leticia Quintanilla-Martinez to identify genes controlled by C/EBP beta using targeted knockdown experiments, combined with expression analysis.
