We recently identified polynucleotide phosphorylase (PNPase) as a potential binding partner for the TCL1 oncoprotein. Mammalian PNPase exhibits exoribonuclease and poly(A) polymerase activities, and PNPase overexpression inhibits cell growth, induces apoptosis, and stimulates proinflammatory cytokine production. A physiologic connection for these anticancer effects and overexpression is difficult to reconcile with the presumed mitochondrial matrix localization for endogenous PNPase, prompting this study. Here we show that basal and interferon-␤-induced PNPase was efficiently imported into energized mitochondria with coupled processing of the N-terminal targeting sequence. Once imported, PNPase localized to the intermembrane space (IMS) as a peripheral membrane protein in a multimeric complex. Apoptotic stimuli caused PNPase mobilization following cytochrome c release, which supported an IMS localization and provided a potential route for interactions with cytosolic TCL1. Consistent with its IMS localization, PNPase knockdown with RNA interference did not affect mitochondrial RNA levels. However, PNPase reduction impaired mitochondrial electrochemical membrane potential, decreased respiratory chain activity, and was correlated with altered mitochondrial morphology. This resulted in F o F 1 -ATP synthase instability, impaired ATP generation, lactate accumulation, and AMP kinase phosphorylation with reduced cell proliferation. Combined, the data demonstrate an unexpected IMS localization and a key role for PNPase in maintaining mitochondrial homeostasis.

B-cell lymphoma is the most common immune system malignancy. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B-and T-cell leukemia and lymphoma subtypes, supporting an oncogenic role for TCL1 that probably involves AKT and MAPK-ERK signaling pathway augmentation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg Bcell tumors to discover tumor-associated IntroductionHyperactivation of the RAS-RAF-MEK-ERK signaling pathway (MAPK-ERK pathway) promotes a variety of cancers, including hematologic malignancies, by enhancing cell proliferation and cell survival and by affecting differentiation. 1,2 The MAPK-ERK pathway can be constitutively activated by tumor type-specific gain-of-function mutations in (or overexpression of) upstream pathway members, such as growth factor receptors, RAS, and RAF, and by loss-of-function mutations in (or repression of) negative pathway regulators, such as NF1, SPROUTY, or SPRED proteins. 3 Ectopic expression of T-cell leukemia 1 (TCL1), a gene that encodes a 14-kDa protein that augments AKT pathway signaling in lymphocytes, is common in mature B-cell leukemias and lymphomas. [4][5][6] A causative role for ectopic TCL1 expression in lymphocyte transformation is supported by the observation that a spectrum of Tand B-cell tumors form in TCL1 transgenic (TCL1-tg) mice, in which the human TCL1 gene is ectopically expressed under the control of an E-B29 promoter in mature lymphocytes. 7 The mechanism(s) by which ectopic TCL1 expression promotes T and B lymphocyte transformation is not fully understood. In T cells, TCL1 augments AKT and MAPK-ERK signaling with each pathway independently contributing to increase T-cell proliferation in TCL1-tg mice. 8 There is also evidence that the tumorigenic activity of TCL1 involves more than augmentation of AKT signaling because it has been shown that increasing AKT activity in B cells to levels higher than are detected in TCL1-tg B cells by deleting Pten, a negative regulator of AKT, fails to cause lymphocyte transformation. 9 Moreover, it is not known what effect TCL1 has on MAPK-ERK signaling in B cells. The observation that in humans and TCL1-tg mice there is a long latency before B-and T-cell tumors form after ectopic expression of TCL1 is consistent with the conclusion that additional changes beyond TCL1-mediated augmentation of AKT signaling are needed to transform lymphocytes.Because TCL1 expression is more frequently dysregulated in B-cell compared with T-cell tumors, we have focused on its role in B-cell malignancies. B-cell lymphomas from TCL1-tg mice display aneuploidy and chromosomal translocations. In many of these lymphomas, trisomy 15 and its associated c-MYC overexpression results in a lesion that histologically and molecularly resembles human Burkitt lymphoma (BL). 10,11 In addition to genetic alterations, TCL1-tg B-cell tumors also display reproducible genomewide p...

The results presented here elucidate key targets of RAPA-induced cell cycle arrest, provide insight into the growth pathways of EBV+ B-cell lymphomas, and demonstrate the potential for RAPA as a therapeutic option in the treatment of PTLD and other EBV+ lymphomas.

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Leukemias and lymphomas arise by genetic and epigenetic alterations of previously healthy lymphocytes. B-cell lymphoma is the most frequent lymphocyte malignancy, with certain tumor subtypes characterized by recurring genetic alterations that include reciprocal chromosome translocations between IG loci and CCND1, BCL2, MYC, and BCL6.1 These rearrangements place tumor-promoting genes under the control of IG rather than endogenous regulatory elements, leading to their dysregulated expression. Epigenetic changes, including DNA hypermethylation, aberrant histone modifications, and altered microRNA expression are also linked to B-cell transformation.2-4 In particular, DNA hypermethylation promotes chromatin compaction and gene silencing, with consistent repression of specific tumor suppressor genes associated with multiple types of cancer, including subtypes of B-cell leukemia and lymphoma. 5Aberrant expression of the TCL1 oncogene, first identified from rearrangements with TCR loci in T-cell prolymphocytic leukemia, also occurs frequently in mature B-cell leukemias and lymphomas, although not by gene rearrangement [reviewed in 6 ]. A causative role for ectopic TCL1 expression in lymphocyte transformation is supported by three different TCL1 transgenic (TCL1-tg) mouse models that develop mature B-and T-cell malignancies.6,7 TCL1-tg mice and patients with dysregulated TCL1 expression exhibit polyclonal lymphocyte hyperplasia and a long delay to tumor formation,

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