c-FLIP mediates resistance of Hodgkin/Reed-Sternberg cells to death receptor-induced apoptosis

Activation of mammalian target of rapamycin signaling promotes cell cycle progression and protects cells from apoptosis in mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterized by the t(11;14) and cyclin D1 overexpression. However, additional molecular events are most likely required for oncogenesis, possibly through cell cycle and apoptosis deregulation. We hypothesized that mammalian target of rapamycin (mTOR) is activated in MCL and contributes to tumor proliferation and survival. In MCL cell lines, pharmacological inhibition of the phosphoinositide 3-kinase/AKT pathway was associated with decreased phosphorylation (activation) of mTOR and its downstream targets phosphorylated (p)-4E-BP1, p-p70S6 kinase, and p-ribosomal protein S6, resulting in apoptosis and cell cycle arrest. These changes were associated with down-regulation of cyclin D1 and the anti-apoptotic proteins cFLIP, BCL-XL, and MCL-1. Furthermore, silencing of mTOR expression using mTOR-specific short interfering RNA decreased phosphorylation of mTOR signaling proteins and induced cell cycle arrest and apoptosis. Silencing of eukaryotic initiation factor (eIF4E), a downstream effector of mTOR, recapitulated these results. We also assessed mTOR signaling in MCL tumors using immunohistochemical methods and a tissue microarray: 10 of 30 (33%) expressed Ser473p-AKT, 13 of 21 (62%) Ser2448p-mTOR, 22 of 22 (100%) p-p70S6K, and 5 of 20 (25%) p-ribosomal protein S6. Total eIF4E binding protein 1 and eukaryotic initiation factor 4E were expressed in 13 of 14 (93%) and 16 of 29 (55%) MCL tumors, respectively. These findings suggest that the mTOR signaling pathway is activated and may contribute to cell cycle progression and tumor cell survival in MCL.

Expression of bcl2 family proteins and active caspase 3 in classical Hodgkin's lymphomas

The expression of various bcl2 family proteins has been reported in Hodgkin and Reed-Sternberg cells, but the proteins bad, bid, and bim have not been analyzed in classical Hodgkin's lymphomas (HLs). This study aimed to investigate the expression of the proteins bcl2, bcl-xl, mcl1, bax, bak, bad, bid, bim, and active caspase 3, and the TUNEL (terminal deoxynucleotidyl transferase-mediated in situ labeling) index to gain further insight into the apoptosis profile of classical HLs. A high expression of the proteins bcl2, bcl-xl, mcl1, bax, bak, bad, bid, and bim in HRS cells was found in 27 of 101 (27%), 95 of 101 (94%), 27 of 97 (29%), 73 of 95 (77%), 37 of 102 (36%), 85 of 94 (90%), 19 of 109 (17%), and 43 of 91 (47%) cases, respectively. The high expression of bcl-xl, bax, and bad in HRS cells in most classical HLs indicates that these proteins may play predominant roles in the regulation of apoptosis in classical HLs. Active caspase 3-positive and TUNEL-positive Reed-Sternberg cells were detected in 47 of 70 (67%; range, 0%-12%) and 60 of 71 (85%; range, 0%-19%) cases, respectively. Significant positive correlations were found between bax/bcl2 (P = .002), bad/bcl2 (P = .020), bad/bcl-xl (P = .003), and bim/mcl1 (P = .036). Based on these findings, it could be hypothesized that the antiapoptotic proteins bcl2, bcl-xl, and mcl1 may counteract the expression of the proapoptotic proteins bax, bad, and bim, thereby contributing to the survival of Reed-Sternberg cells.

Down-regulation of c-FLIP increases reactive oxygen species, induces phosphorylation of serine/threonine kinase Akt, and impairs motility of cancer cells

The role of c-FLIP in cell motility was investigated using RNA interference. Down-regulation of c-FLIP increased amounts of reactive oxygen species (ROS), while over-expression of c-FLIP produced opposite effect. ROS induced phosphorylation of Akt and impaired cell motility.

Pathobiology of Classical Hodgkin Lymphoma

The World Health Organization has acknowledged the malignant nature of classical Hodgkin lymphoma (cHL), which encompasses four histological subtypes. The diagnosis of cHL is based on the detection of malignant Hodgkin and Reed-Sternberg cells (HRSC) confirmed by immunophenotyping and the detection of growth patterns specific to each histological subtype. The pathologic HRSC arise from germinal center or immediate postgerminal cells that lack detectable immunoglobulin/B-cell antigen receptor expression, with a consequent loss of B-cell identity; very few cHL cases are of T-cell origin. To escape apoptosis, which normally occurs in B cells with nonfunctioning antigen receptor machinery, HRSC develop concurrent antiapoptotic mechanisms by activation of nuclear factor-kappaB or are rescued by Epstein-Barr virus infection. HRSC are characterized by a variable and inconstant immunophenotype, with a remarkable loss of lineage-specific cell antigens and expression of antigens of other cell lineages. The master plan of B-cell identity in HRSC is disturbed not only at the immunoglobulin expression level, but also at the transcriptional factor level. HRSC are further characterized by profound cell cycle deregulation with futile replication, multinucleation and poly- and aneuploidy. Here, we review pathobiological aspects of cHL with respect to lymphomagenesis and routine diagnostics.

EBV can protect latently infected B cell lymphomas from death receptor-induced apoptosis

The relationship between EBV infection and sensitivity to death receptor (DR)-induced apoptosis is poorly understood. Using EBV- and EBV+ BJAB cells, we provide the first evidence that EBV can protect latently infected B cell lymphomas from apoptosis triggered through Fas or TRAIL receptors. Caspase 8 activation was impaired and cellular FLIP recruitment was enriched in death-inducing signaling complexes formed in EBV-infected BJAB cells relative to parent BJAB cells. Furthermore, latent membrane protein 1 expression alone could reduce caspase activation and confer partial resistance to DR apoptosis in BJAB cells. This protective effect was dependent on C-terminal activating region 2-driven NF-kappaB activation, which in turn up-regulated cellular FLIP expression in latent membrane protein 1+ BJAB cells. Thus, the ability of latent EBV to block DR apoptosis may help to ensure the survival of host cells during B cell differentiation, and contribute to the development of B cell lymphomas, especially in immunocompromised individuals.

Mechanism of ASC-mediated apoptosis: bid-dependent apoptosis in type II cells

Apoptosis-associated speck-like protein containing a CARD (ASC) is an adaptor molecule that mediates apoptotic and inflammatory signals, and implicated in tumor suppression. However, the mechanism of ASC-mediated apoptosis has not been well elucidated. Here, we investigated the molecular mechanisms of ASC-mediated apoptosis in several cell lines using a caspase recruitment domain 12-Nod2 chimeric protein that transduces the signal from muramyl dipeptide into ASC-mediated apoptosis. Experiments using dominant-negative mutants, small-interfering RNAs and peptide inhibitors for caspases indicated that caspase-8 was generally required for ASC-mediated apoptosis, whereas a requirement for caspase-9 depended on the cell type. In addition, caspase-like apoptosis-regulatory protein (CLARP)/Fas-like inhibitor protein, a natural caspase-8 inhibitor, suppressed ASC-mediated apoptosis, and Clarp-/- mouse embryonic fibroblasts were highly sensitive to ASC-mediated apoptosis. Bax-deficient HCT116 cells were resistant to ASC-mediated apoptosis as reported previously, although we failed to observe colocalization of ASC and Bax in cells. Like Fas-ligand-induced apoptosis, the ASC-mediated apoptosis was inhibited by Bcl-2 and/or Bcl-XL in type-II but not type-I cell lines. Bid was cleaved upon ASC activation, and suppression of endogenous Bid expression using small-interfering RNAs in type-II cells reduced the ASC-mediated apoptosis. These results indicate that ASC, like death receptors, mediates two types of apoptosis depending on the cell type, in a manner involving caspase-8.

Inhibition of the phosphatidylinositol 3'-kinase pathway promotes autocrine Fas-induced death of phosphatase and tensin homologue-deficient prostate cancer cells

Rationally designed therapeutics that target the phosphatidylinositol 3'-kinase (PI3K) cell survival pathway are currently in preclinical and clinical development for cancer therapy. Drugs targeting the PI3K pathway aim to inhibit proliferation, promote apoptosis, and enhance chemosensitivity and radiosensitivity of cancer cells. The phosphatase and tensin homologue (PTEN) phosphatidylinositol 3'-phosphatase is a key negative regulator of the PI3K pathway. Inactivation of the PTEN tumor suppressor results in constitutive activation of the PI3K pathway and is found in approximately 50% of advanced prostate cancers, which correlates with a high Gleason score and poor prognosis. Inhibition of the PI3K pathway leads to apoptosis of prostate cancer cells; however, the precise mechanism by which this occurs is unknown. Here we report that apoptotic cell death of PTEN-deficient LNCaP and PC3 prostate cancer cells induced by the PI3K inhibitor LY294002 can be abrogated by disrupting Fas/Fas ligand (FasL) interactions with recombinant Fas:Fc fusion protein or FasL neutralizing antibody (Nok-1), or by expressing dominant-negative Fas-associated death domain. Furthermore, we find that apoptosis induced by expression of wild-type PTEN, driven by a tetracycline-inducible expression system in LNCaP cells, can be inhibited by blocking Fas/FasL interaction using Fas:Fc or Nok-1. These data show that apoptosis induced by blockade of the PI3K pathway in prostate tumor cells is mediated by an autocrine Fas/FasL apoptotic mechanism and the Fas apoptotic pathway is both necessary and sufficient to mediate apoptosis by PI3K inhibition.

Targeting cellular FLICE-like inhibitory protein as a novel approach to the treatment of Hodgkin's lymphoma

Hodgkin's lymphoma is one of the most common lymphoid cancers, particularly among young adults. Although there have been dramatic improvements in the treatment of Hodgkin's lymphoma, leading to high cure rates in some groups, current combination chemotherapy regimes are associated with significant secondary complications in long-term survivors. Furthermore, although a proportion of patients with Hodgkin's lymphoma will be cured, there still remains a significant rate of relapse and also a smaller proportion of poor responders who will go on to die of their disease. Therefore, developments in the treatment of Hodgkin's lymphoma must be directed at improving cure rates and reducing the burden of secondary complications. In recent years, the underlying pathogenesis of Hodgkin's lymphoma has become better understood. In particular, it is emerging that a key pathogenic event in Hodgkin's lymphoma is protection from Fas-induced cell death. Recent studies by the authors' group, and others, have demonstrated that this is, in part, due to the expression by Hodgkin/Reed-Sternberg cells of the cellular Fas-associated death domain-like IL-1 converting enzyme (FLICE)-like inhibitory protein molecule, a potent inhibitor of Fas-induced death. In this review, the role of cellular FLICE-like inhibitory protein in the pathogenesis of Hodgkin's lymphoma will be explored and also the possibility of targeting this molecule in order to provide an alternative and potentially safe approach to the treatment of Hodgkin's lymphoma will be investigated.

Epstein-Barr virus, cytokines, and inflammation: a cocktail for the pathogenesis of Hodgkin's lymphoma?

The association between chronic inflammation and cancer has been known for well over a century. However, direct evidence detailing the role of inflammation in carcinogenesis has been slow in forthcoming. A number of recent studies suggest that the gaps in our understanding of the molecular pathways bridging the link between inflammation and cancer are slowly beginning to close and that this relationship is more deep-rooted than had been previously believed. This review addresses the link between inflammation and Hodgkin's lymphoma (HL), a malignancy which has many features reminiscent of chronic inflammation. The role of Epstein-Barr virus (EBV) in the pathogenesis of HL is discussed, along with an outline of our current understanding of the cellular nature and development of Reed-Sternberg cells, the malignant cells of HL. The involvement of cytokines and chemokines as orchestrators of inflammation and vehicles for chemical cross-talk between the malignant cells and the reactive inflammatory infiltrate forms a major part of the review. It is suggested that chronic inflammation, triggered by factors such as EBV, is likely to contribute to tumor cell proliferation, progression, and inhibition of apoptosis. Furthermore, it is proposed that the pro-inflammatory transcription factor NF-kappaB plays a central role in many of these processes.

Molecular biology of Hodgkin's and Reed/Sternberg cells in Hodgkin's lymphoma

Hodgkin's and Reed/Sternberg (HRS) cells, the tumour cells in classical Hodgkin's lymphoma (HL), represent transformed B cells in nearly all cases. The detection of destructive somatic mutations in the rearranged immunoglobulin (Ig) genes of HRS cells in classical HL indicated that they originate from preapoptotic germinal centre (GC) B cells that lost the capacity to express a high-affinity B-cell receptor (BCR). Several aberrantly activated signalling pathways and transcription factors have been identified that contribute to the rescue of HRS cells from apoptosis. Among the deregulated signalling pathways, activation of multiple receptor tyrosine kinases in HRS cells appears to be a specific feature of HL. In about 40% of cases of classical HL the HRS cells are infected by Epstein-Barr virus (EBV), indicating an important role of EBV in HL pathogenesis. Interestingly, nearly all cases of HL with destructive Ig gene mutations eliminating BCR expression (e.g. nonsense mutations) are EBV-positive, suggesting that EBV-encoded genes have a particular function to prevent apoptosis of HRS-cell precursors that acquired such crippling mutations. This idea is further supported by the recent demonstration that isolated human GC B cells harbouring crippled Ig genes can be rescued by EBV from cell death, giving rise to lymphoblastoid cell lines. The molecular analysis of composite Hodgkin's and non-Hodgkin's lymphomas indicated that many cases develop from a common GC B-cell precursor in a multistep transformation process with both shared and distinct oncogenic events.

The proteasome is required for rapid initiation of death receptor-induced apoptosis

Due to their tremendous apoptosis-inducing potential, proteasomal inhibitors (PIs) have recently entered clinical trials. Here we show, however, that various PIs rescued proliferating tumor cells from death receptor-induced apoptosis. This protection correlated with the stabilization of X-linked IAP (XIAP) and c-FLIP and the inhibition of caspase activation. Together with the observation that PIs could not protect cells expressing XIAP or c-FLIP short interfering RNAs (siRNAs) from death receptor-induced apoptosis, our results demonstrate that PIs mediate their protective effect via the stabilization of these antiapoptotic proteins. Furthermore, we show that once these proteins were eliminated, either by long-term treatment with death receptor ligands or by siRNA-mediated suppression, active caspases accumulated to an even larger extent in the presence of PIs. Together, our data support a biphasic role for the proteasome in apoptosis, as they show that its constitutive activity is crucial for the rapid initiation of the death program by eliminating antiapoptotic proteins, whereas at later stages, the proteasome acts in an antiapoptotic manner due to the proteolysis of caspases. Thus, for a successful PI-based tumor therapy, it is crucial to carefully evaluate basal proteasomal activity and the status of antiapoptotic proteins, as their PI-mediated prolonged stability might even cause adverse effects, leading to the survival of a tumor.

E2F1 induces apoptosis and sensitizes human lung adenocarcinoma cells to death-receptor-mediated apoptosis through specific downregulation of c-FLIP(short)

E2F1 is a transcription factor that plays a well-documented role during S phase progression and apoptosis. We had previously postulated that the low level of E2F1 in primary lung adenocarcinoma contributes to their carcinogenesis. Here, we show that E2F1 triggers apoptosis in various lung adenocarcinoma cell lines by a mechanism involving the specific downregulation of the cellular FLICE-inhibitory protein short, leading to caspase-8 activation at the death-inducing signaling complex. Importantly, we also provide evidence that E2F1 sensitizes tumor as well as primary cells to apoptosis mediated by FAS ligand or tumor necrosis factor-related apoptosis-inducing ligand, and enhances the cytotoxic effect of T lymphocytes against tumor cells. Finally, we describe the specific overexpression of c-FLIP(S) in human lung adenocarcinomas with low level of E2F1. Overall, our data identify E2F1 as a critical determinant of the cellular response to death-receptor-mediated apoptosis, and suggest that its downregulation contributes to the immune escape of lung adenocarcinoma tumor cells.

Autocrine production of interleukin-4 and interleukin-10 is required for survival and growth of thyroid cancer cells

Although CD95 and its ligand are expressed in thyroid cancer, the tumor cell mass does not seem to be affected by such expression. We have recently shown that thyroid carcinomas produce interleukin (IL)-4 and IL-10, which promote resistance to chemotherapy through the up-regulation of Bcl-xL. Here, we show that freshly purified thyroid cancer cells were completely refractory to CD95-induced apoptosis despite the consistent expression of Fas-associated death domain and caspase-8. The analysis of potential molecules able to prevent caspase-8 activation in thyroid cancer cells revealed a remarkable up-regulation of cellular FLIP(L) (cFLIP(L)) and PED/PEA-15, two antiapoptotic proteins whose exogenous expression in normal thyrocytes inhibited the death-inducing signaling complex of CD95. Additionally, small interfering RNA FLIP and PED antisense sensitized thyroid cancer cells to CD95-mediated apoptosis. Exposure of normal thyrocytes to IL-4 and IL-10 potently up-regulated cFLIP and PED/PEA-15, suggesting that these cytokines are responsible for thyroid cancer cell resistance to CD95 stimulation. Moreover, treatment with neutralizing antibodies against IL-4 and IL-10 or exogenous expression of suppressor of cytokine signaling-1 of thyroid cancer cells resulted in cFLIP and PED/PEA-15 down-regulation and CD95 sensitization. More importantly, prolonged IL-4 and IL-10 neutralization induced cancer cell growth inhibition and apoptosis, which were prevented by blocking antibodies against CD95 ligand. Altogether, autocrine production of IL-4 and IL-10 neutralizes CD95-generated signals and allows survival and growth of thyroid cancer cells. Thus, IL-4 and IL-10 may represent key targets for the treatment of thyroid cancer.

Specific positive and negative effects of FLIP on cell survival in human prostate cancer

We demonstrate here for the first time novel positive and negative effects of the FLICE-like inhibitory protein (FLIP) on human prostate cancer cell survival. A proteaosome inhibitor, MG132, mediated cell cycle arrest at G2/M and apoptosis through p38 activation. Interestingly, FLIP was stabilized by MG132 and interacted with Raf-1, resulting in enhancement of p38 signals and cytotoxicity. In contrast, overexpression of FLIP inhibited ubiquitylation and proteasomal degradation of beta-catenin, resulting in increase of the target gene cyclin D1, colony formation and invasive activity. Immunohistochemical analysis and in vitro experiments in primary culture showed FLIP to be overexpressed, statistically associated with expression of beta-catenin/cyclin D1 in metastatic cells, the FLIP/beta-catenin/cyclin D1 signals contributing to colony formation and invasion, which were canceled by FLIP knock down. In contrast, MG132-induced cytotoxicity including apoptosis was strongly inhibited by reduction of FLIP. Taken together, the results indicate that FLIP plays an important role in development of metastatic prostate cancer by inhibiting proteasomal degradation of beta-catenin, whereas it is mainly involved in proteasome inhibitior-mediated cell cycle arrest and apoptosis through activating the Raf-1/p38 pathway. Furthermore, proteasome inhibitors may be effective drugs for advanced prostate cancers overexpressing FLIP.

cFLIP expression correlates with tumour progression and patient outcome in non-Hodgkin lymphomas of low grade of malignancy

The present study investigated whether the expression of cellular Fas-associated death domain-like interleukin-1beta-converting enzyme (FLICE) inhibitory protein (cFLIP) conveys prognostic information in non-Hodgkin lymphomas (NHLs). cFLIP expression was quantified by immunohistochemistry and immunofluorescence in biopsy specimens from 86 NHL patients for whom clinical information was available. NHL malignancy was graded as high/intermediate or low according to the World Health Organization Classification of Lymphoid Neoplasms. cFLIP was positive in 23 of 45 high-/intermediate-grade NHLs and in 25 of 41 low-grade NHLs. Negative expression of cFLIP was associated with the presence of apoptotic cells in the tumour mass, regardless of the histotype and of the malignancy grade. In NHLs positive for cFLIP, 11 of 23 (48%) high-/intermediate-grade cases and 18 of 25 (72%) low-grade cases showed a bad outcome. In NHLs negative for cFLIP, only four of 22 (18%) high-/intermediate-grade patients and 12 of 16 (75%) low-grade patients achieved complete remission. All these correlations were statistically significant. The correlation of cFLIP expression with clinical outcome was independent of therapy, whether or not it included anti-CD20 antibody (Rituximab). The present findings strongly indicate that cFLIP is a reliable predictor of tumour progression and clinical prognosis in NHLs of low grade of malignancy.

Targeting NF-κB in hematologic malignancies

The transcription factor nuclear factor kappa B (NF-kappaB) can intervene in oncogenesis by virtue of its capacity to regulate the expression of a plethora of genes that modulate apoptosis, and cell survival as well as proliferation, inflammation, tumor metastasis and angiogenesis. Different reports demonstrate the intrinsic activation of NF-kappaB in lymphoid and myeloid malignancies, including preneoplastic conditions such as myelodysplastic syndromes, underscoring its implication in malignant transformation. Targeting intrinsic NF-kappaB activation, as well as its upstream and downstream regulators, may hence constitute an additional approach to the oncologist's armamentarium. Several small inhibitors of the NF-kappaB-activatory kinase IkappaB kinase, of the proteasome, or of the DNA binding of NF-kappaB subunits are under intensive investigation. Currently used cytotoxic agents can induce NF-kappaB activation as an unwarranted side effect, which confers apoptosis suppression and hence resistance to these drugs. Thus, NF-kappaB inhibitory molecules may be clinically useful, either as single therapeutic agents or in combination with classical chemotherapeutic agents, for the treatment of hematological malignancies.

c-FLIP inhibits chemotherapy-induced colorectal cancer cell death

c-FLIP inhibits caspase 8 activation and apoptosis mediated by death receptors such as Fas and DR5. We studied the effect of c-FLIP on the apoptotic response to chemotherapies used in colorectal cancer (CRC) (5-fluorouracil, oxaliplatin and irinotecan). Simultaneous downregulation of both c-FLIP splice forms c-FLIP(L) and c-FLIP(S) with siRNA synergistically enhanced chemotherapy-induced apoptosis in p53 wild-type (HCT116p53(+/+), RKO), null (HCT116p53(-/-)) and mutant (H630) CRC cell lines. Furthermore, overexpression of c-FLIP(L), but not c-FLIP(S), potently inhibited apoptosis induced by chemotherapy in HCT116p53(+/+) cells, suggesting that c-FLIP(L) was the more important splice form in mediating chemoresistance. In support of this, siRNA specifically targeted against c-FLIP(L) synergistically enhanced chemotherapy-induced apoptosis in a manner similar to the siRNA targeted against both splice forms. Inhibition of caspase 8 blocked the enhanced apoptosis induced by c-FLIP-targeted (FT) siRNA and chemotherapy. Furthermore, we found that downregulating cell surface DR5, but not Fas, also inhibited apoptosis induced by FT siRNA and chemotherapy. Interestingly, these effects were not dependent on activation of DR5 by its ligand TRAIL. These results indicate that c-FLIP inhibits TRAIL-independent, DR5- and caspase 8-dependent apoptosis in response to chemotherapy in CRC cells. Moreover, targeting c-FLIP in combination with existing chemotherapies may have therapeutic potential for the treatment of CRC.

The role of cellular flice inhibitory protein (c-FLIP) in the pathogenesis and treatment of cancer

Protection from death receptor (DR)-mediated apoptosis has been proposed as an important step in the development of malignancy, enabling tumour cells not only to survive and escape antitumour immune responses, but also to develop resistance to chemotherapy or other cancer treatments. An important regulator of DR-induced death is the cellular FADD-like IL-1beta-converting enzyme inhibitory protein (c-FLIP) which, when overexpressed, can protect tumour cells from apoptosis. This review focuses on the role of c-FLIP as a tumour progression factor, with particular emphasis on recent work from the authors' laboratory concerning the contribution of c-FLIP to the pathogenesis of Hodgkin's lymphoma. The possibility of targeting c-FLIP as an approach to the treatment of cancer and, in particular, Hodgkin's lymphoma is discussed.

Inhibition of heat shock protein 90 function by 17-allylamino-17-demethoxy-geldanamycin in Hodgkin's lymphoma cells down-regulates Akt kinase, dephosphorylates extracellular signal-regulated kinase, and induces cell cycle arrest and cell death

PURPOSE

Heat shock protein 90 (HSP90) is a chaperone for several client proteins involved in transcriptional regulation, signal transduction, and cell cycle control. HSP90 is abundantly expressed by a variety of tumor types and has been recently targeted for cancer therapy. The objective of this study was to determine the role of HSP90 in promoting growth and survival of Hodgkin's lymphoma and to determine the molecular consequences of inhibiting HSP90 function by the small-molecule 17-allylamino-17-demethoxy-geldanamycin (17-AAG) in Hodgkin's lymphoma.

EXPERIMENTAL DESIGN

HSP90 expression in Hodgkin's lymphoma cell lines was determined by Western blot and in primary lymph node sections from patients with Hodgkin's lymphoma by immunohistochemistry. Cell viability was determined by the 3-(4,5-dimethyl-thiazol-2yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. Apoptosis and cell cycle fractions were determined by flow cytometry. Expression of intracellular proteins was determined by Western blot.

RESULTS

HSP90 is overexpressed in primary and cultured Hodgkin's lymphoma cells. Inhibition of HSP90 function by 17-AAG showed a time- and dose-dependent growth inhibition of Hodgkin's lymphoma cell lines. 17-AAG induced cell cycle arrest and apoptosis, which were associated with a decrease in cyclin-dependent kinase (CDK) 4, CDK 6, and polo-like kinase 1 (PLK1), and induced apoptosis by caspase-dependent and caspase-independent mechanisms. Furthermore, 17-AAG depleted cellular contents of Akt, decreased extracellular signal-regulated kinase (ERK) phosphorylation, and reduced cellular FLICE-like inhibitory protein levels (FLIP), and thus enhanced the cytotoxic effect of doxorubicin and agonistic anti-tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptor antibodies.

CONCLUSION

Inhibition of HSP90 function induces cell death and enhances the activity of chemotherapy and anti-tumor necrosis factor-related apoptosis-inducing ligand death receptor antibodies, suggesting that targeting HSP90 function might be of therapeutic value in Hodgkin's lymphoma.

HTLV-1 Tax protects against CD95-mediated apoptosis by induction of the cellular FLICE-inhibitory protein (c-FLIP)

The HTLV-1 transactivator protein Tax is essential for malignant transformation of CD4 T cells, ultimately leading to adult T-cell leukemia/lymphoma (ATL). Malignant transformation may involve development of apoptosis resistance. In this study we investigated the molecular mechanisms by which HTLV-1 Tax confers resistance toward CD95-mediated apoptosis. We show that Tax-expressing T-cell lines derived from HTLV-1-infected patients express elevated levels of c-FLIP(L) and c-FLIP(S). The levels of c-FLIP correlated with resistance toward CD95-mediated apoptosis. Using an inducible system we demonstrated that both resistance toward CD95-mediated apoptosis and induction of c-FLIP are dependent on Tax. In addition, analysis of early cleavage of the BH3-only Bcl-2 family member Bid, a direct caspase-8 substrate, revealed that apoptosis is inhibited at a CD95 death receptor proximal level in Tax-expressing cells. Finally, using siRNA we directly showed that c-FLIP confers Tax-mediated resistance toward CD95-mediated apoptosis. In conclusion, our data suggest an important mechanism by which expression of HTLV-1 Tax may lead to immune escape of infected T cells and, thus, to persistent infection and transformation.

Fas activation of a proinflammatory program in rheumatoid synoviocytes and its regulation by FLIP and caspase 8 signaling

OBJECTIVE

The expansion of an aggressive population of fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA) synovium occurs despite their expression of functional death receptors and exposure to death receptor ligands. FLS can survive Fas challenge because of the constitutive expression of FLIP apoptosis inhibitor. We investigated whether Fas signaling plays a pathogenetic role by activating a nonapoptotic proinflammatory program in RA FLS.

METHODS

Cultured RA FLS were stimulated with an agonistic anti-Fas antibody in the presence or absence of the caspase inhibitor Z-VAD-FMK or after RNA interference with a short hairpin RNA expression plasmid directed against FLIP. NF-kappaB and activator protein 1 (AP-1) activation was studied by electrophoretic mobility shift assays and p65 immunofluorescence analysis, and expression of messenger RNA (mRNA) for monocyte chemoattractant protein 1, interleukin-8, IkappaB alpha, and matrix metalloproteinases (MMPs) 1, 9, and 13 was examined by reverse transcription-polymerase chain reaction. Chemotactic activity of Fas-activated FLS-conditioned media was studied in Transwell migration assays.

RESULTS

Fas stimulation activated NF-kappaB and AP-1, and this response required caspase activity, since Z-VAD-FMK inhibitor precluded it. FLIP was processed to p43 protein after Fas stimulation in a caspase-dependent manner, and inhibition of FLIP expression resulted in reduced Fas-triggered NF-kappaB activation. Fas stimulation increased expression of mRNA for IkappaB alpha, MMPs, and chemokines, and Fas-activated RA FLS displayed increased chemotactic activity for monocytic cells.

CONCLUSION

Fas triggering may contribute to the proinflammatory features of RA FLS by activating NF-kappaB and AP-1 and by expression of relevant target genes, such as MMPs and chemokines. Fas proinflammatory signaling is dependent upon caspase activity and FLIP expression. These data implicate FLIP as a potentially important molecular switch that turns the Fas signaling away from apoptosis and toward induction of a proinflammatory phenotype in RA FLS.

Intrinsic inhibition of transcription factor E2A by HLH proteins ABF-1 and Id2 mediates reprogramming of neoplastic B cells in Hodgkin lymphoma

B cell differentiation is controlled by a complex network of lineage-restricted transcription factors. How perturbations to this network alter B cell fate remains poorly understood. Here we show that classical Hodgkin lymphoma tumor cells, which originate from mature B cells, have lost the B cell phenotype as a result of aberrant expression of transcriptional regulators. The B cell-specific transcription factor program was disrupted by overexpression of the helix-loop-helix proteins ABF-1 and Id2. Both factors antagonized the function of the B cell-determining transcription factor E2A. As a result, expression of genes specific to B cells was lost and expression of genes not normally associated with the B lineage was upregulated. These data demonstrate the plasticity of mature human lymphoid cells and offer an explanation for the unique classical Hodgkin lymphoma phenotype.

cIAP2 is highly expressed in Hodgkin–Reed–Sternberg cells and inhibits apoptosis by interfering with constitutively active caspase-3

In this study, the expression of activated caspase-3 by the tumor cells of classical Hodgkin lymphoma (cHL), the Hodgkin-Reed-Sternberg (HRS) cells, is confirmed. This raises the question why caspase-3 does not kill HRS cells. There are only a few molecules, which are able to directly inhibit caspase-3. One of them is cIAP2. We show that cIAP2 is expressed in the HRS cells in 20 of 23 cHL cases by in situ hybridization. Suppression experiments with cIAP2 antisense RNA show that down-regulation of cIAP2 significantly reduces apoptosis resistance in cHL cell lines. cIAP2 overexpression appears to be unique for HRS cells since the tumor cells of non-Hodgkin lymphomas are nearly cIAP2-negative. We demonstrate that cIAP2 is inducible by CD30 stimulation in cHL cell lines of T-cell origin and anaplastic large cell lymphoma cell lines, whereas cHL cell lines of B-cell origin constitutively express cIAP2. Inhibition of cIAP2 expression by cIAP2 antisense RNA decreases resistance to apoptosis. The results indicate that cIAP2 contributes to the apoptosis resistance of HRS cells, mainly by inhibiting effector caspases. According to these findings, a therapeutical application of inhibitors of apoptosis proteins antagonists in cHL appears promising.

c-FLIP confers resistance to FAS-mediated apoptosis in anaplastic large-cell lymphoma

We hypothesized that inhibition of the FAS-mediated apoptosis pathway by FLICE-like inhibitory protein (c-FLIP) may contribute to oncogenesis in ALK+ anaplastic large-cell lymphoma (ALCL). Treatment with increasing concentrations of CH-11 (CD95/FAS agonistic antibody) had no effect on cell viability of 2 ALK+ ALCL cell lines, Karpas 299 and SU-DHL1, each expressing high levels of c-FLIP. However, inhibition of endogenous c-FLIP expression by specific c-FLIP siRNA in Karpas 299 and SU-DHL1 cells treated with CH-11 resulted in FAS-mediated cell death associated with increased annexin V binding, apoptotic morphology, and cleavage of caspase-8. In 26 ALK+ ALCL tumors, assessed for expression of DISC-associated proteins, CD95/FAS and c-FLIP were commonly expressed, in 23 (92%) of 25 and 21 (91%) of 23 tumors, respectively. By contrast, CD95L/FASL was expressed in only 3 (12%) of 26 ALCL tumors, although it was strongly expressed by surrounding small reactive lymphocytes. Our findings suggest that overexpression of c-FLIP protects ALK+ ALCL cells from death-receptor-induced apoptosis and may contribute to ALCL pathogenesis.

Inhibition of Nuclear Translocation of Nuclear Factor-κB Despite Lack of Functional IκBα Protein Overcomes Multiple Defects in Apoptosis Signaling in Human B-Cell Malignancies

PURPOSE

Defective apoptosis signaling is a typical feature of classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. In these malignancies, the transcription factor nuclear factor-kappaB (NF-kappaB) is a critical mediator of apoptosis resistance and oncogenic growth, making it an attractive therapeutic target. Here, we sought to determine how to overcome apoptosis resistance experimentally in these malignancies by targeting NF-kappaB.

EXPERIMENTAL DESIGN

We investigated the effect of different inhibitors of NF-kappaB on classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma cell lines harboring different molecular defects in apoptosis signaling both quantitatively and qualitatively.

RESULTS

The cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J(2), a known inhibitor of NF-kappaB, induced caspase-dependent apoptosis; it restored mitochondrial apoptotic signaling by down-regulation of X-linked inhibitor of apoptosis protein and heat shock protein 27 and led to breakdown of the mitochondrial membrane potential and, finally, cleavage of caspase-3 irrespective of IkappaBalpha mutational status. Surprisingly, 15-deoxy-Delta12,14-prostaglandin J(2) and the IkappaB kinase inhibitor curcumin both reduced nuclear levels of p65 in cell lines lacking IkappaBalpha, suggesting that inhibition of nuclear translocation of NF-kappaB can occur in the absence of IkappaBalpha. Finally, a synthetic peptide that specifically abrogates the assembly of the IkappaB kinase complex killed IkappaBalpha-defective cells by induction of apoptosis, paralleled by reduction of nuclear NF-kappaB.

CONCLUSIONS

These results show that molecular defects in apoptotic signaling, such as IkappaBalpha mutations, can be circumvented by targeting NF-kappaB through inhibition of the IkappaB kinase complex followed by induction of apoptosis in classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. Thus, targeting IkappaB kinases may represent an attractive therapeutic approach against these malignancies regardless of the mutational status of IkappaBalpha.

Molecular pathogenesis of Hodgkin's lymphoma

According to the WHO classification, Hodgkin's lymphoma (HL) is subdivided into a classical variant and a nodular lymphocyte predominant variant which are characterized by the presence of Hodgkin's and Reed-Sternberg (H-RS) cells or lymphocytic and histiocytic (L&H) cells, respectively. This article reviews genetic characteristics and transcriptional changes of H-RS and L&H cells, including recent knowledge about transforming mechanisms and signaling pathways that contribute to the antiapoptotic phenotype displayed by H-RS and L&H cells. We also discuss major cellular and molecular mediators contributing to the establishment and maintenance of a reactive background in HL-affected tissues. We believe that an in-depth understanding of the pathogenesis of HL will eventually lead to the development of novel biologically based therapeutic strategies in the near future.

Methylseleninic acid sensitizes prostate cancer cells to TRAIL-mediated apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytotoxic agent that preferentially induces apoptosis in a variety of human cancer cells. Unfortunately, some tumor cells remain resistant to TRAIL. Therefore, agents that sensitize malignant cells to TRAIL-mediated cell death might be of particular importance for the development of novel antitumor therapeutic regimens. Recent studies establish a critical role of selenium in prostate cancer prevention in vitro and in vivo. Here, we demonstrate that concomitant administration of TRAIL and methylseleninic acid (MSA) produces synergistic effects on the induction of apoptosis in androgen-dependent LNCaP and androgen-independent DU-145 prostate cancer cells. MSA rapidly and specifically downregulates expression of the cellular FLICE inhibitory protein, a negative regulator of death receptor signaling. In addition, we demonstrate that the synergistic effects of MSA and TRAIL result from the activation of the mitochondrial pathway-mediated amplification loop. Addition of MSA effectively blocked TRAIL-mediated BAD phosphorylation at Ser112 and Ser136 in DU-145 cells and was accompanied by induction of the mitochondrial permeability transition and release of apoptogenic cytochrome c and Smac/DIABLO proteins from the mitochondria and into the cytosol. These results suggest that selenium-based dietary compounds may help to overcome resistance to TRAIL-mediated apoptosis in prostate cancer cells.

The fully human anti-CD30 antibody 5F11 activates NF-κB and sensitizes lymphoma cells to bortezomib-induced apoptosis

5F11, a fully human monoclonal antibody directed against CD30, effectively induces killing of CD30-expressing lymphoma cell lines in vitro and in animal models. A recently conducted phase 1/2 study shows that 5F11 is well tolerated in heavily pretreated patients with relapsed and refractory CD30+ lymphoma and has some clinical activity. In the present study, we demonstrate that 5F11 activates nuclear factor κB (NF-κB) and the anti-apoptotic protein cellular FLICE (Fas-associating protein with death domain-like interleukin-1β-converting enzyme) inhibitory protein (c-flip) in Hodgkin lymphoma (HD)-derived cell lines, which might cause apoptosis resistance, thus limiting the clinical use of 5F11. To overcome this resistance, we combined 5F11 with the proteasome inhibitor bortezomib, which has been shown to suppress NF-κB activity. This combination revealed a synergistic cytotoxic effect in vitro and in a human HD xenograft model provided that 5F11 precedes bortezomib treatment. We conclude that initial 5F11-mediated NF-κB signaling sensitizes the tumor cells to bortezomib-induced cell death. These data suggest a therapeutic value of this combination for HD patients. (Blood. 2005;106:1839-1842)

Elevated NF-kappaB p50 complex formation and Bcl-3 expression in classical Hodgkin, anaplastic large-cell, and other peripheral T-cell lymphomas

Transcription factor nuclear factor kappa B (NF-kappaB) plays a central role in the pathogenesis of classical Hodgkin lymphoma (cHL). In anaplastic large-cell lymphomas (ALCLs), which share molecular lesions with cHL, the NF-kappaB system has not been equivalently investigated. Here we describe constitutive NF-kappaB p50 homodimer [(p50)2] activity in ALCL cells in the absence of constitutive activation of the IkappaB kinase (IKK) complex. Furthermore, (p50)2 contributes to the NF-kappaB activity in Hodgkin/Reed-Sternberg (HRS) cells. Bcl-3, which is an inducer of nuclear (p50)2 and is associated with (p50)2 in ALCL and HRS cell lines, is abundantly expressed in ALCL and HRS cells. Notably, a selective overexpression of Bcl-3 target genes is found in ALCL cells. By immunohistochemical screening of 288 lymphoma cases, a strong Bcl-3 expression in cHL and in peripheral T-cell non-Hodgkin lymphoma (T-NHL) including ALCL was found. In 3 of 6 HRS cell lines and 25% of primary ALCL, a copy number increase of the BCL3 gene locus was identified. Together, these data suggest that elevated Bcl-3 expression has an important function in cHL and peripheral T-NHL, in particular ALCL.

Resistance to Fas-mediated apoptosis is restored by cycloheximide through the downregulation of cellular FLIPL in NK/T-cell lymphoma

Extranodal NK/T-cell lymphoma (NKTL), nasal type, is a highly aggressive neoplasm and is strongly associated with Epstein-Barr virus (EBV). In this study, we demonstrate that EBV-positive NKTL cell lines, namely, Hank-1, NK-YS, and NK-L, are resistant to Fas-mediated apoptosis induced by anti-Fas antibodies despite high levels of Fas surface expression and no mutation in the Fas gene. Fas stimulation of Hank-1 and NK-YS cells showed little processing of caspase 8, caspase 3, or bid, although the proximal signaling molecules of the death-inducing signaling complex, namely, Fas, Fas-associated protein with a death domain, caspase 8, and bid were present in these cells. Consistent with previous reports on the hypermethylation of death associated protein (DAP) kinase in NKTLs, the promoter of DAP kinase was methylated and its mRNA not detected in Hank-1 cells. However, the restoration of DAP kinase expression by 5-aza-2'-deoxycytidine did not sensitize Hank-1 to Fas-mediated apoptosis, indicating that DAP kinase deficiency does not contribute to resistance to Fas-mediated apoptosis. Since etoposide-induced apoptosis involved caspase 3 activation in Hank-1 and NK-YS cells, the caspase 3-dependent apoptotic machinery appears to be intact. Interestingly, cotreatment of Hank-1 with cycloheximide, a protein synthesis inhibitor, markedly sensitized cells to Fas-mediated apoptosis along with caspase 8 activation and c-FLIP(L) (cellular FLICE inhibitory protein long form) downregulation. Moreover, immunohistochemistry on paraffin-embedded tissue revealed c-FLIP expression in 39% (14 of 36) of NKTL patients. Taken together, these findings indicate that c-FLIP(L)-mediated resistance to Fas contributes to the development and progression of NKTLs. This study also suggests that agents capable of downregulating c-FLIP(L) could be used to treat NKTL.

From Hodgkin disease to Hodgkin lymphoma: biologic insights and therapeutic potential

Reclassification of Hodgkin disease as Hodgkin lymphoma (HL) represents a milestone in the lymphoma field, awarding recent insights in the molecular biology of Hodgkin and Reed-Sternberg (H-RS) cells and their environment. This review summarizes antiapoptotic and proproliferative pathways involved in the pathogenesis of this disease with the ultimate goal of translating laboratory knowledge into clinical decision making. The focus is on potential targets and novel drugs, which are discussed in the context of the complex biology of HL. Considering that HL patients are more likely to die from acute and late treatment-related toxicities than from HL itself, the introduction of targeted, biologically based therapies for HL patients with palliative and eventually curative intention might be justified. (Blood. 2005;105:4553-4560)

FLIP protects against hypoxia/reoxygenation-induced endothelial cell apoptosis by inhibiting Bax activation

Hypoxia/reoxygenation causes cell death, yet the underlying regulatory mechanisms remain partially understood. Recent studies demonstrate that hypoxia/reoxygenation can activate death receptor and mitochondria-dependent apoptotic pathways, involving Bid and Bax mitochondrial translocation and cytochrome c release. Using mouse lung endothelial cells (MLEC), we examined the role of FLIP, an inhibitor of caspase 8, in hypoxia/reoxygenation-induced cell death. FLIP protected MLEC against hypoxia/reoxygenation by blocking both caspase 8/Bid and Bax/mitochondrial apoptotic pathways. FLIP inhibited Bax activation in wild-type and Bid(-/-) MLEC, indicating independence from the caspase 8/Bid pathway. FLIP also inhibited the expression and activation of protein kinase C (PKC) (alpha, zeta) during hypoxia/reoxygenation and promoted an association of inactive forms of PKC with Bax. Surprisingly, FLIP expression also inhibited death-inducing signal complex (DISC) formation in the plasma membrane and promoted the accumulation of the DISC in the Golgi apparatus. FLIP expression also upregulated Bcl-X(L), an antiapoptotic protein. In conclusion, FLIP decreased DISC formation in the plasma membrane by blocking its translocation from the Golgi apparatus and inhibited Bax activation through a novel PKC-dependent mechanism. The inhibitory effects of FLIP on Bax activation and plasma membrane DISC formation may play significant roles in protecting endothelial cells from the lethal effects of hypoxia/reoxygenation.

The Hodgkin and Reed/Sternberg cell

Hodgkin and Reed/Sternberg (HRS) cells are the hallmark cells of Hodgkin's lymphoma (HL). They are large, often multinucleated with a peculiar morphology and an unusual immunophenotype, that does not resemble any normal cell in the body. Despite their rarity in HL tissues, HRS cells are the clonal tumour cells of HL. HRS cells in nearly all cases of HL derive from B cells, and only rarely from T cells. Notably, the pattern of somatic mutations in their rearranged immunoglobulin V genes suggests that they are derived from pre-apoptotic germinal center B cells. The pathogenesis of HL is still largely unresolved, but it is now clear that aberrant activation of several signalling pathways (such as the NFkappaB pathway) is of key importance for HRS cell survival. HRS or HRS-like cells are also found in several other diseases, e.g. as rare intermingled cells in some non-Hodgkin lymphomas and in infectious mononucleosis.

Trypanosoma cruzi posttranscriptionally up-regulates and exploits cellular FLIP for inhibition of death-inducing signal

Intracellular persistence of the protozoan parasite, Trypanosoma cruzi, is an aggravating cause of Chagas' disease, involving that the protozoan infection specifically inhibits death receptor-mediated apoptosis of host cells. Here we demonstrate that the parasite dramatically up-regulates cellular FLICE inhibitory protein (c-FLIP), the only known mammalian inhibitor specific for death receptor signaling, in infected cells by an unusual, posttranscriptional stabilization of the short-lived protein. We also show that c-FLIP is accumulated in T. cruzi-infected mouse heart muscle cells in vivo. Stimulation of death receptor Fas in infected cells induces recruitment of c-FLIP to block the procaspase-8 activation at the most upstream caspase cascade. c-FLIP knock-down with a small interfering RNA significantly restores Fas-mediated apoptosis in infected cells. Taken together, our findings indicate that T. cruzi posttranscriptionally up-regulates and exploits host c-FLIP for the inhibition of death-inducing signal, a mechanism that may allow parasites to persist in host cells.

Immunohistochemical profiling of caspase signaling pathways predicts clinical response to chemotherapy in primary nodal diffuse large B-cell lymphomas

We used biopsy specimens of primary nodal diffuse large B-cell lymphoma (DLBCL) to investigate whether the inhibition of caspase 8 and/or 9 apoptosis signaling pathways predicts clinical outcome. Expression levels of cellular FLICE inhibitory protein (c-Flip) and numbers of active caspase 3-positive lymphoma cells were used to determine the status of the caspase 8-mediated pathway. Expression levels of Bcl-2 and X-linked inhibitor of apoptosis (XIAP) were used to determine the status of the caspase 9-mediated pathway. Expression of c-Flip, XIAP, Bcl-2, and caspase 3 activity all provided prognostic information. According to these immunohistochemical parameters, inhibition of either or both caspase signaling pathways was detected in all patients. Three groups of patients were identified, one with a caspase 8 inhibition profile, one with caspase 8 and 9 inhibition profiles, and one with a caspase 9 inhibition profile. Caspase 9 inhibition was strongly associated with poor response to chemotherapy and usually with fatal outcome, whereas caspase 8 inhibition was associated with excellent clinical outcome. Thus, our data strongly suggest that inhibition of the caspase 9-mediated pathway, but not the caspase 8-mediated pathway, is a major cause for therapy resistance in patients with nodal DLBCL.

Selective knockdown of the long variant of cellular FLICE inhibitory protein augments death receptor-mediated caspase-8 activation and apoptosis

Death receptors trigger apoptosis by activating the apical cysteine proteases caspase-8 and -10 within a death-inducing signaling complex (DISC). c-FLIP (cellular FLICE inhibitory protein) is an enzymatically inactive relative of caspase-8 and -10 that binds to the DISC. Two major c-FLIP variants result from alternative mRNA splicing: a short, 26-kDa protein (c-FLIP(S)) and a long, 55-kDa form (c-FLIP(L)). The role of c-FLIP(S) as an inhibitor of death receptor-mediated apoptosis is well established; however, the function of c-FLIP(L) remains controversial. Although overexpression of transfected c-FLIP(L) inhibits apoptosis, ectopic expression at lower levels supports caspase-8 activation and cell death. Simultaneous ablation of both c-FLIP variants augments death receptor-mediated apoptosis, but the impact of selective depletion of c-FLIP(L) on caspase-8 activation and subsequent apoptosis is not well defined. To investigate this, we developed small interfering RNAs that specifically knock down expression of c-FLIP(L) in several cancer cell lines and studied their effect on apoptosis initiation by Apo2L/TRAIL (Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand). Knockdown of c-FLIP(L) augmented DISC recruitment, activation, processing, and release of caspase-8, thereby enhancing effector-caspase stimulation and apoptosis. Thus, endogenous c-FLIP(L) functions primarily as an inhibitor of death receptor-mediated apoptosis.

Exploiting death receptor signaling pathways for tumor therapy

Apoptosis or programmed cell death is a key regulator of physiological growth control and regulation of tissue homeostasis. Tipping the balance between cell death and proliferation in favor of cell survival may result in tumor formation. Moreover, current cancer therapies, e.g. chemotherapy, gamma-irradiation, immunotherapy or suicide gene therapy, primarily exert their antitumor effect by triggering an evolutionary conserved apoptosis program in cancer cells. For example, death receptor signaling has been implied to contribute to the efficacy of cancer therapy. Thus, failure to undergo apoptosis in response to anticancer therapy because of defects in death receptor pathways may result in resistance. Further insights into the mechanisms regulating apoptosis in response to anticancer therapy and how cancer cells evade cell death may provide novel opportunities for targeted therapeutics. Thus, agents designed to selectively activate death receptor pathways may enhance the efficacy of conventional therapies and may even overcome some forms of cancer resistance.

Immunobiology and pathophysiology of Hodgkin lymphomas

Classical Hodgkin lymphoma (HL) is characterized by the presence of Reed-Sternberg (RS) cells, which are transformed post-germinal center B cells destined for apoptosis since they have not undergone successful immunoglobulin gene rearrangement. Several mechanisms, including latent infection by Epstein-Barr virus (EBV), allow these cells to survive. It is remarkable that many of the signaling pathways that promote survival are shared between the EBV-induced proteins, such as EBNA1, LMP1, and LMP2, and other molecules that are upregulated in RS cells. A key role is played by the presence of constitutive nuclear factor (NF)-kappaB, which is induced by LMP1, as well as by CD30, CD40, tumor necrosis factor (TNF)-alpha, and Notch1 interactions, and results in the upregulation of at least 45 genes including chemokines, cytokines, receptors, apoptotic regulators, intracellular signaling molecules, and transcription factors. The other characteristic of classical HL is the presence of an extensive inflammatory infiltrate. Key features of this infiltrate are that it comprises Th2 and T regulatory cells and generally lacks Th1 cells, CD8 cytotoxic T cells, and natural killer (NK) cells. The RS cells appear to induce this infiltrate by the secretion of Th2 type chemokines such as TARC and MDC. The RS cells also produce cytokines that inhibit Th1 responses, as interleukin (IL)-10 and transforming growth factor (TGF)-beta express CD95 ligand, which induces apoptosis of activated Th1 and CD8 T cells. Other important mechanisms that allow the RS cells to escape an effective anti-EBV immune response include the downregulation of HLA class I in EBV-negative cases or the presence of a polymorphism in HLA class I in EBV-positive cases that allow escape from CD8-mediated cytotoxicity. On the other hand, expression of HLA-G allows the escape from NK cells that would normally recognize the HLA class I-negative RS cells. Overall, the cellular infiltrate in HL appears to play a decisive role in allowing the RS cells to survive by providing an environment that suppresses cytotoxic immune responses and providing cellular interactions and cytokines that support the growth and survival of RS cells. Future therapeutic strategies could focus directly on the NF-kappaB activation, on various receptors to ligand interactions, on the chemokine and cytokine network, or on the induction of effective anti-EBV latent protein immune responses.