Towards a genetic definition of cancer-associated inflammation: role of the IDO pathway

Chronic inflammation drives the development of many cancers, but a genetic definition of what constitutes 'cancer-associated' inflammation has not been determined. Recently, a mouse genetic study revealed a critical role for the immune escape mediator indoleamine 2,3-dioxygenase (IDO) in supporting inflammatory skin carcinogenesis. IDO is generally regarded as being immunosuppressive; however, there was no discernable difference in generalized inflammatory processes in IDO-null mice under conditions where tumor development was significantly suppressed, implicating IDO as key to establishing the pathogenic state of 'cancer-associated' inflammation. Here we review recent findings and their potential implications to understanding the relationship between immune escape and inflammation in cancer. Briefly, we propose that genetic pathways of immune escape in cancer are synonymous with pathways that define 'cancer-associated' inflammation and that these processes may be identical rather than distinct, as generally presumed, in terms of their genetic definition.

Induction of IDO-1 by immunostimulatory DNA limits severity of experimental colitis

The chronic inflammatory bowel diseases are characterized by aberrant innate and adaptive immune responses to commensal luminal bacteria. In both human inflammatory bowel disease and in experimental models of colitis, there is an increased expression of the enzyme IDO. IDO expression has the capacity to exert antimicrobial effects and dampen adaptive immune responses. In the murine trinitrobenzene sulfonic acid model of colitis, inhibition of this enzyme leads to worsened disease severity, suggesting that IDO acts as a natural break in limiting colitis. In this investigation, we show that induction of IDO-1 by a TLR-9 agonist, immunostimulatory (ISS) DNA, critically contributes to its colitis limiting capacities. ISS DNA induces intestinal expression of IDO-1 but not the recently described paralog enzyme IDO-2. This induction occurred in both epithelial cells and in subsets of CD11c(+) and CD11b(+) cells of the lamina propria, which also increase after ISS-oligodeoxynucleotide. Signaling required for intestinal IDO-1 induction involves IFN-dependent pathways, as IDO-1 was not induced in STAT-1 knockout mice. Using both the trinitrobenzene sulfonic acid and dextran sodium sulfate models of colitis, we show the importance of IDO-1s induction in limiting colitis severity. The clinical parameters and histological correlates of colitis in these models were improved by administration of the TLR-9 agonist; however, when the function of IDO is inhibited, the colitis limiting effects of ISS-oligodeoxynucleotide were abrogated. These findings support the possibility that targeted induction of IDO-1 is an approach deserving further investigation as a therapeutic strategy for diseases of intestinal inflammation.

Immunotherapeutic suppression of indoleamine 2,3-dioxygenase and tumor growth with ethyl pyruvate

Efforts to improve cancer care in the developing world will benefit from the identification of simple, inexpensive, and broadly applicable medical modalities based on emergent innovations in treatment, such as targeting mechanisms of tumoral immune tolerance. In this report, we offer preclinical evidence that the low-cost, anti-inflammatory agent ethyl pyruvate elicits a potent immune-based antitumor response through inhibition of indoleamine 2,3-dioxygenase (IDO), a key tolerogenic enzyme for many human tumors. Consistent with its reported ability to interfere with NF-kappaB function, ethyl pyruvate blocks IDO induction both in vitro and in vivo. Antitumor activity was achieved in mice with a noncytotoxic dosing regimen of ethyl pyruvate shown previously to protect against lethality from sepsis. Similar outcomes were obtained with the functional ethyl pyruvate analogue 2-acetamidoacrylate. Ethyl pyruvate was ineffective at suppressing tumor outgrowth in both athymic and Ido1-deficient mice, providing in vivo corroboration of the importance of T-cell-dependent immunity and IDO targeting for ethyl pyruvate to achieve antitumor efficacy. Although ethyl pyruvate has undergone early-phase clinical testing, this was done without consideration of its possible applicability to cancer. Our findings that IDO is effectively blocked by ethyl pyruvate treatment deepen emerging links between IDO and inflammatory processes. Further, these findings rationalize oncologic applications for this agent by providing a compelling basis to reposition ethyl pyruvate as a low-cost immunochemotherapy for clinical evaluation in cancer patients.

Loss of RhoB expression enhances the myelodysplastic phenotype of mammalian diaphanous-related Formin mDia1 knockout mice

Myelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis and hyperplastic bone marrow. Complete loss or interstitial deletions of the long arm of chromosome 5 occur frequently in MDS. One candidate tumor suppressor on 5q is the mammalian Diaphanous (mDia)-related formin mDia1, encoded by DIAPH1 (5q31.3). mDia-family formins act as effectors for Rho-family small GTP-binding proteins including RhoB, which has also been shown to possess tumor suppressor activity. Mice lacking the Drf1 gene that encodes mDia1 develop age-dependent myelodysplastic features. We crossed mDia1 and RhoB knockout mice to test whether the additional loss of RhoB expression would compound the myelodysplastic phenotype. Drf1^−/−RhoB^−/− mice are fertile and develop normally. Relative to age-matched Drf1^−/−RhoB^+/− mice, the age of myelodysplasia onset was earlier in Drf1^−/−RhoB^−/− animals—including abnormally shaped erythrocytes, splenomegaly, and extramedullary hematopoiesis. In addition, we observed a statistically significant increase in the number of activated monocytes/macrophages in both the spleen and bone marrow of Drf1^−/−RhoB^−/− mice relative to Drf1^−/−RhoB^+/− mice. These data suggest a role for RhoB-regulated mDia1 in the regulation of hematopoietic progenitor cells.

Tissue inhibitor of metalloproteinase-4 is elevated in early-stage breast cancers with accelerated progression and poor clinical course

An increasing number of breast cancer patients are diagnosed with small, localized, early-stage tumors. These patients are typically thought to have a good prognosis for long-term disease-free survival, but epidemiological studies indicate that up to 30% may have a recurrence within 3 to 5 years of diagnosis. Identifying patients with a high risk of recurrence and/or progression is important because they could be more aggressively treated at diagnosis to improve their chances for disease-free survival. Recent evidence suggests that elevated levels of the matrix metalloproteinase inhibitor, tissue inhibitor of metalloproteinase (TIMP)-4, are associated with malignant progression of ductal carcinoma in situ, a precancerous lesion. To examine the association of TIMP-4 with survival outcomes, we conducted a retrospective immunohistochemical analysis of 314 cases from patients with early-stage disease, defined as tumors smaller than 2 cm and no spread to lymph nodes (tumor-node-metastasis staging: T1N0MX). We found that tumors with elevated levels of TIMP-4 were correlated with a reduced probability of long-term disease-free survival, especially in patients with estrogen receptor-negative tumors. Our findings prompt further evaluation of TIMP-4 as a simple prognostic marker that may help identify patients with early-stage breast cancer who could benefit from more aggressive treatment at diagnosis.

The immunoregulatory enzyme IDO paradoxically drives B cell-mediated autoimmunity

Rheumatoid arthritis (RA) is a chronic and debilitating inflammatory autoimmune disease of unknown etiology. As with a variety of autoimmune disorders, evidence of elevated tryptophan catabolism has been detected in RA patients indicative of activation of the immunomodulatory enzyme IDO. However, the role that IDO plays in the disease process is not well understood. The conceptualization that IDO acts solely to suppress effector T cell activation has led to the general assumption that inhibition of IDO activity should exacerbate autoimmune disorders. Recent results in cancer models, however, suggest a more complex role for IDO as an integral component of the inflammatory microenvironment necessary for supporting tumor outgrowth. This has led us to investigate the involvement of IDO in the pathological inflammation associated with RA. Using the K/BxN murine RA model and IDO inhibitor 1-methyl-tryptophan, we found that inhibiting IDO activity had the unexpected consequence of ameliorating, rather than exacerbating arthritis symptoms. 1-Methyl tryptophan treatment led to decreased autoantibody titers, reduced levels of inflammatory cytokines, and an attenuated disease course. This alleviation of arthritis was not due to an altered T cell response, but rather resulted from a diminished autoreactive B cell response, thus demonstrating a previously unappreciated role for IDO in stimulating B cell responses. Our findings raise the question of how an immunosuppressive enzyme can paradoxically drive autoimmunity. We suggest that IDO is not simply immunosuppressive, but rather plays a more complex role in modulating inflammatory responses, in particular those that are driven by autoreactive B cells.

Genotyping and expression analysis of IDO2 in human pancreatic cancer: a novel, active target

BACKGROUND

The recently discovered indoleamine 2,3-dioxygenase-2 (IDO2) gene has 2 functional polymorphisms that abolish its enzymatic activity. We hypothesize that expression of the IDO2 enzyme in primary pancreatic ductal adenocarcinomas (PDA) can help cancer cells evade immune detection.

STUDY DESIGN

Because the IDO2 enzyme might be the preferential target of d-1-methyl-tryptophan, a clinical lead inhibitor of IDO currently being evaluated in phase I trials, we sequenced IDO2 in 36 pancreatic specimens and evaluated its expression.

RESULTS

We found that 58% (21 of 36) of cases were heterozygous for the R248W polymorphism; 28% (10 of 36) were homozygous wild-type; and only 14% (5 of 36) were homozygous for the functionally inactive polymorphism. As for the Y359STOP polymorphism, we found that 27% (10 of 36) of cases were heterozygous, 62% (22 of 36) were homozygous wild-type, and only 11% (4 of 36) were homozygous for this functionally inactive allele. Ruling out the possibility of compound polymorphic variants, we estimated 75% of our resected patient cohort had an active IDO2 enzyme, with a conservative estimate that 58% of the patients had at least 1 functional allele. IDO2 was expressed in PDA tissue from each genetically polymorphic subgroup. We also detected IDO2 protein expression in the genetically distinct pancreatic cancer cell lines after exposure with interferon-gamma.

CONCLUSIONS

This is the first study to report IDO2 expression in PDA and related cancers indicating that IDO2 genetic polymorphisms do not negate interferon-gamma-inducible protein expression. Taken together, our data strongly suggest that the clinical lead compound d-1-methyl-tryptophan might be useful in treatment of PDA.

Indoleamine 2,3-dioxygenase in T-cell tolerance and tumoral immune escape

Indoleamine 2, 3-dioxygenase (IDO) degrades the essential amino acid tryptophan in mammals, catalyzing the initial and rate-limiting step in the de novo biosynthesis nicotinamide adenine dinucleotide (NAD). Broad evidence implicates IDO and the tryptophan catabolic pathway in generation of immune tolerance to foreign antigens in tissue microenvironments. In particular, recent findings have established that IDO is overexpressed in both tumor cells and antigen-presenting cells in tumor-draining lymph nodes, where it promotes the establishment of peripheral immune tolerance to tumor antigens. In the normal physiologic state, IDO is important in creating an environment that limits damage to tissues due to an overactive immune system. However, by fostering immune suppression, IDO can facilitate the survival and growth of tumor cells expressing unique antigens that would be recognized normally as foreign. In preclinical studies, small-molecule inhibitors of IDO can reverse this mechanism of immunosuppression, complementing classical cytotoxic cancer chemotherapeutic agents' ability to trigger regression of treatment-resistant tumors. These results have encouraged the clinical translation of IDO inhibitors, the first of which entered phase I clinical trials in the fall of 2007. In this article, we survey the work defining IDO as an important mediator of peripheral tolerance, review evidence of IDO dysregulation in cancer cells, and provide an overview of the development of IDO inhibitors as a new immunoregulatory treatment modality for clinical trials.

A role for candidate tumor-suppressor gene TCEAL7 in the regulation of c-Myc activity, cyclin D1 levels and cellular transformation

The pathophysiological mechanisms that drive the development and progression of epithelial ovarian cancer remain obscure. Recently, we identified TCEAL7 as a transcriptional regulatory protein often downregulated in epithelial ovarian cancer. However, the biological significance of such downregulation in cancer is not currently known. Here, we show that TCEAL7 is downregulated frequently in many human cancers and that in immortalized human ovarian epithelial cells this event promotes anchorage-independent cell growth. Mechanistic investigations revealed that TCEAL7 associates with cyclin D1 promoter containing Myc E-box sequence and transcriptionally represses cyclin D1 expression. Moreover, downregulation of TCEAL7 promotes DNA-binding activity of Myc-Max, and upregulates the promoter activity of c-Myc-target gene, ornithine decarboxylase (ODC), whereas enhanced expression of TCEAL7 inhibits Myc-induced promoter activity of ODC. Our findings suggest that TCEAL7 may restrict ovarian epithelial cell transformation by limiting Myc activity. These results also suggest a potential, alternative mechanism by which c-Myc activity may be deregulated in cancer by the downregulation of TCEAL7.

BAR the door: cancer suppression by amphiphysin-like genes

The evolutionarily conserved amphiphysin-like genes Bin1 and Bin3 function in membrane and actin dynamics, cell polarity, and stress signaling. Recent genetic studies in mice discriminate non-essential roles in endocytic processes commonly ascribed to amphiphysins from essential roles in cancer suppression. Bin1 acts in default pathways of apoptosis and senescence that are triggered by the Myc and Raf oncogenes in primary cells, and Bin1 gene products display a 'moonlighting function' in the nucleus where a variety of other 'endocytic' proteins are also found. Together, genetic investigations in yeast, flies, and mice suggest that amphiphysin-like adapter proteins may suppress cancer by helping integrate cell polarity signals generated by actin and vesicle dynamics with central regulators of cell cycle arrest, apoptosis, and immune surveillance.

Structure based development of phenylimidazole-derived inhibitors of indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase (IDO) is emerging as an important new therapeutic target for the treatment of cancer, chronic viral infections, and other diseases characterized by pathological immune suppression. With the goal of developing more potent IDO inhibitors, a systematic study of 4-phenylimidazole (4-PI) derivatives was undertaken. Computational docking experiments guided design and synthesis efforts with analogues of 4-PI. In particular, three interactions of 4-PI analogues with IDO were studied: the active site entrance, the interior of the active site, and the heme iron binding. The three most potent inhibitors (1, 17, and 18) appear to exploit interactions with C129 and S167 in the interior of the active site. All three inhibitors are approximately 10-fold more potent than 4-PI. The study represents the first example of enzyme inhibitor development with the recently reported crystal structure of IDO and offers important lessons in the search for more potent inhibitors.

Indoleamine 2,3-dioxygenase is the anticancer target for a novel series of potent naphthoquinone-based inhibitors

Indoleamine 2,3-dioxygenase (IDO) is emerging as an important new therapeutic target for the treatment of cancer, chronic viral infections, and other diseases characterized by pathological immune suppression. While small molecule inhibitors of IDO exist, there remains a dearth of high-potency compounds offering in vivo efficacy and clinical translational potential. In this study, we address this gap by defining a new class of naphthoquinone-based IDO inhibitors exemplified by the natural product menadione, which is shown in mouse tumor models to have similar antitumor activity to previously characterized IDO inhibitors. Genetic validation that IDO is the critical in vivo target is demonstrated using IDO-null mice. Elaboration of menadione to a pyranonaphthoquinone has yielded low nanomolar potency inhibitors, including new compounds which are the most potent reported to date (K(i) = 61-70 nM). Synthetic accessibility of this class will facilitate preclinical chemical-genetic studies as well as further optimization of pharmacological parameters for clinical translation.

Immune escape as a fundamental trait of cancer: focus on IDO

Immune escape is a critical gateway to malignancy. The emergence of this fundamental trait of cancer represents the defeat of immune surveillance, a potent, multi-armed and essential mode of cancer suppression that may influence the ultimate clinical impact of an early stage tumor. Indeed, immune escape may be a central modifier of clinical outcomes, by affecting tumor dormancy versus progression, licensing invasion and metastasis and impacting therapeutic response. Although relatively little studied until recently, immune suppression and escape in tumors are now hot areas with clinical translation of several new therapeutic agents already under way. The interconnections between signaling pathways that control immune escape and those that control proliferation, senescence, apoptosis, metabolic alterations, angiogenesis, invasion and metastasis remain virtually unexplored, offering rich new areas for investigation. Here, an overview of this area is provided with a focus on the tryptophan catabolic enzyme indoleamine 2,3-dioxygenase (IDO) and its recently discovered relative IDO2 that are implicated in suppressing T-cell immunity in normal and pathological settings including cancer. Emerging evidence suggests that during cancer progression activation of the IDO pathway might act as a preferred nodal modifier pathway for immune escape, for example analogous to the PI3K pathway for survival or the VEGF pathway for angiogenesis. Small molecule inhibitors of IDO and IDO2 heighten chemotherapeutic efficacy in mouse models of cancer in a nontoxic fashion and an initial lead compound entered phase I clinical trials in late 2007. New modalities in this area offer promising ways to broaden the combinatorial attack on advanced cancers, where immune escape mechanisms likely provide pivotal support.

Alternate cyclin D1 mRNA splicing modulates p27KIP1 binding and cell migration

Cyclin D1 is an important cell cycle regulator, but in cancer its overexpression also increases cellular migration mediated by p27 KIP1 stabilization and RhoA inhibition. Recently, a common polymorphism at the exon 4-intron 4 boundary of the human cyclin D1 gene within a splice donor region was associated with an altered risk of developing cancer. Altered RNA splicing caused by this polymorphism gives rise to a variant cyclin D1 isoform termed cyclin D1b, which has the same N terminus as the canonical cyclin D1a isoform but a distinct C terminus. In this study we show that these different isoforms have unique properties with regard to the cellular migration function of cyclin D1. Although they displayed little difference in transcriptional co-repression assays on idealized reporter genes, microarray cDNA expression analysis revealed differential regulation of genes, including those that influence cellular migration. Additionally, whereas cyclin D1a stabilized p27 KIP1 and inhibited RhoA-induced ROCK kinase activity, promoting cellular migration, cyclin D1b failed to stabilize p27 KIP1 or inhibit ROCK kinase activity and had no effect on migration. Our findings argue that alternate splicing is an important determinant of the function of cyclin D1 in cellular migration.

A key in vivo antitumor mechanism of action of natural product-based brassinins is inhibition of indoleamine 2,3-dioxygenase

Agents that interfere with tumoral immune tolerance may be useful to prevent or treat cancer. Brassinin is a phytoalexin, a class of natural products derived from plants that includes the widely known compound resveratrol. Brassinin has been demonstrated to have chemopreventive activity in preclinical models but the mechanisms underlying its anticancer properties are unknown. Here, we show that brassinin and a synthetic derivative 5-bromo-brassinin (5-Br-brassinin) are bioavailable inhibitors of indoleamine 2,3-dioxygenase (IDO), a pro-toleragenic enzyme that drives immune escape in cancer. Like other known IDO inhibitors, both of these compounds combined with chemotherapy to elicit regression of autochthonous mammary gland tumors in MMTV-Neu mice. Furthermore, growth of highly aggressive melanoma isograft tumors was suppressed by single agent treatment with 5-Br-brassinin. This response to treatment was lost in athymic mice, indicating a requirement for active host T-cell immunity, and in IDO-null knockout mice, providing direct genetic evidence that IDO inhibition is essential to the antitumor mechanism of action of 5-Br-brassinin. The natural product brassinin thus provides the structural basis for a new class of compounds with in vivo anticancer activity that is mediated through the inhibition of IDO.

RhoB regulates PDGFR-beta trafficking and signaling in vascular smooth muscle cells

OBJECTIVE

RhoB is a small GTPase localized at the plasma membrane and endosomes that participates in the regulation of endocytic trafficking of the epidermal growth factor (EGF) receptor and the nonreceptor kinases Src and Akt. This study was performed to determine whether RhoB plays a critical role in trafficking and signaling by the platelet-derived growth factor receptor-beta (PDGFR-beta) in vascular smooth muscle cells.

METHODS AND RESULTS

Cells derived from RhoB knockout mice failed to proliferate in response to PDGF, and downstream signaling was compromised as reflected by reduced phosphorylation of the effector kinases Akt and ERK1/2. In normal cells, PDGF stimulated trafficking of PDGFR-beta into a perinuclear late endosomal compartment and triggered entry of Src, Akt, extracellular signal-regulated kinase (ERK) into the cell nucleus. In contrast, PDGF treatment of RhoB null cells resulted in neither PDGFR-beta trafficking to late endosomes nor nuclear localization of Src, Akt, or ERK. In support of an essential function in these processes, restoring expression of RhoB in null cells rescued these defects and restored cell proliferation in response to PDGF.

CONCLUSIONS

Our findings establish RhoB as a critical regulator of PDGFR-beta trafficking and signaling in vascular smooth muscle cells.

Bin1 ablation increases susceptibility to cancer during aging, particularly lung cancer

Age is the major risk factor for cancer, but few genetic pathways that modify cancer incidence during aging have been described. Bin1 is a prototypic member of the BAR adapter gene family that functions in vesicle dynamics and nuclear processes. Bin1 limits oncogenesis and is often attenuated in human cancers, but its role in cancer suppression has yet to be evaluated fully in vivo. In the mouse, homozygous deletion of Bin1 causes developmental lethality, so to assess this role, we examined cancer incidence in mosaic null mice generated by a modified Cre-lox technology. During study of these animals, one notable phenotype was an extended period of female fecundity during aging, with mosaic null animals retaining reproductive capability until the age of 17.3 +/- 1.1 months. Through 1 year of age, cancer incidence was unaffected by Bin1 ablation; however, by 18 to 20 months of age, approximately 50% of mosaic mice presented with lung adenocarcinoma and approximately 10% with hepatocarcinoma. Aging mosaic mice also displayed a higher incidence of inflammation and/or premalignant lesions, especially in the heart and prostate. In mice where colon tumors were initiated by a ras-activating carcinogen, Bin1 ablation facilitated progression to more aggressive invasive status. In cases of human lung and colon cancers, immunohistochemical analyses evidenced frequent attenuation of Bin1 expression, paralleling observations in other solid tumors. Taken together, our findings highlight an important role for Bin1 as a negative modifier of inflammation and cancer susceptibility during aging.

Novel tryptophan catabolic enzyme IDO2 is the preferred biochemical target of the antitumor indoleamine 2,3-dioxygenase inhibitory compound D-1-methyl-tryptophan

Small-molecule inhibitors of indoleamine 2,3-dioxygenase (IDO) are currently being translated to clinic for evaluation as cancer therapeutics. One issue related to trials of the clinical lead inhibitor, D-1-methyl-tryptophan (D-1MT), concerns the extent of its biochemical specificity for IDO. Here, we report the discovery of a novel IDO-related tryptophan catabolic enzyme termed IDO2 that is preferentially inhibited by D-1MT. IDO2 is not as widely expressed as IDO but like its relative is also expressed in antigen-presenting dendritic cells where tryptophan catabolism drives immune tolerance. We identified two common genetic polymorphisms in the human gene encoding IDO2 that ablate its enzymatic activity. Like IDO, IDO2 catabolizes tryptophan, triggers phosphorylation of the translation initiation factor eIF2alpha, and (reported here for the first time) mobilizes translation of LIP, an inhibitory isoform of the immune regulatory transcription factor NF-IL6. Tryptophan restoration switches off this signaling pathway when activated by IDO, but not IDO2, arguing that IDO2 has a distinct signaling role. Our findings have implications for understanding the evolution of tumoral immune tolerance and for interpreting preclinical and clinical responses to D-1MT or other IDO inhibitors being developed to treat cancer, chronic infection, and other diseases.

mDia function is critical for the cell suicide program triggered by farnesyl transferase inhibition

Farnesyl transferase inhibitors (FTIs) exhibit limited cytotoxic effects against human cancer cells, perhaps explaining the limited efficacy of FTIs in clinical trials. Learning how these well-tolerated drugs trigger p53-independent apoptosis in mouse models of cancer might therefore benefit efforts to leverage their utility in clinic. Recent clinical findings indicate that the oncogenic Rho guanine nucleotide exchange factor AKAP13/Lbc is associated with clinical responsiveness, in support of an earlier genetic proof in mice that gain of the geranylgeranylated isoform of RhoB which blocks oncogenic Rho signaling is essential for FTI-induced apoptosis. Here we offer evidence that the RhoB effector mDia is a critical downstream player in this death program. Dominant inhibition of mDia ablated FTI-induced apoptosis but not actin reorganization or growth inhibition, the latter of which has been linked previously to interactions with a RhoB effector kinase pathway that downregulates c-Myc. In nude mice, dominant inhibition of mDia promoted tumor formation and ablated FTI antitumor efficacy. Our findings suggest that the RhoB-mDia pathway is critical for the cell death mechanism engaged by FTI. Further, they suggest that mDia may be important for Rho-dependent survival of oncogenically transformed cells, perhaps driven by AKAP13/Lbc.

Cancer Immunologists and Cancer Biologists: Why We Didn't Talk Then but Need to Now

What is cancer? Cancer is a disease initiated by a series of cumulative genetic and epigenetic changes that occur in a normal cell. However, in addition to the malignant cell itself, cancer is a disease of microenvironment and immunity. Although genetic and epigenetic alterations drive cellular transformation, genomic plasticity, and evolution, it has become increasingly apparent that multiple signals delivered within the tumor microenvironment by modifier genes, stromal and endothelial cells, and immune cells are critical factors in determining the progression versus dormancy or destruction of an initiated lesion and also whether metastasis may occur. With regard to the important roles of immune cells in cancer, a chasm exists between immunologists and biologists: although sharing a common disease interest, there is little history for workers to draw on based on shared perspectives or understanding. How did this disconnect arise? Here, we look at how these workers became separated in the past and address why it has now become critical to spur greater cross-fertilization. In particular, we highlight three ideas that we believe are important for discussion and debate. The first idea is that therapeutic strategies that fail to harness the immune system will always be defeated by tumor resistance, due to the large "genomic space" that genetically plastic tumor cells can readily access to evolve resistance mechanisms. Because all therapies drive tumor progression by imposing a selection for resistant cells, harnessing the adaptivity of the immune system will be indispensable to ultimately stanching the deadly adaptability of the tumor cell. The second idea is that using molecular targeted agents to reverse tumoral immune suppression may offer a powerful method to leverage the efficacy of most if not all therapeutic agents. We suggest that the mechanisms that support evolution of a "smoldering" inflammatory environment in cancer overlap with those that support evolution of tumoral immune escape. If true, relieving immune suppression will switch the inflammatory state from supportive to destructive for the tumor. The third idea is that by ablating immunosuppression mechanisms, cytotoxic chemotherapy might synergize with, rather than antagonize, active immunotherapy. Provocative preclinical studies in this area prompt clinical attention. We believe that increased efforts to intermingle the perspectives and work of cancer immunologists with cancer biologists and pharmacologists will be needed to realize the National Cancer Institute's goal of managing cancer in the clinic by 2015.

Indoleamine 2,3-dioxygenase in immune suppression and cancer

The extrahepatic enzyme indoleamine 2,3-dioxygenase (IDO) catalyzes tryptophan degradation in the first and rate-limiting step towards biosynthesis of the central metabolic co-factor nicotinamide adenine dinucleotide (NAD). While this pathway has been known for decades, the actual physiological role for IDO in mammals remained obscure, because (i.) most cell types do not express the downstream enzymes in the NAD biosynthesis pathway and (ii.) mammals salvage rather than synthesize NAD to meet their metabolic needs. An immunological role for IDO was hinted at with the observation that IDO expression is stimulated by interferon-gamma and subsequently confirmed by the discovery of its physiological importance in protecting the fetus from maternal immunity. Similarly, elevations in tryptophan catabolism in cancer patients were known since the 1950s, but the basis and meaning of this phenomenon were uncertain until it was shown that IDO, which is commonly elevated in tumors and draining lymph nodes, suppresses T cell immunity in the tumor microenvironment. Indeed, by creating peripheral tolerance to tumor antigens, IDO can undermine immune responses that thwart tumor cell survival in the context of an underlying inflammatory environment that facilitates tumor outgrowth. In preclinical studies, small molecule inhibitors of IDO compromise this mechanism of immunosuppression and strongly leverage the efficacy of a variety of classical chemotherapeutic agents, supporting the clinical development of IDO inhibitors as a therapeutic goal. This essay summarizes key findings that implicate IDO as an important mediator of peripheral tolerance and discusses the development of anti-cancer modalities that incorporate the use of IDO inhibitors.

Inhibition of indoleamine 2,3-dioxygenase in dendritic cells by stereoisomers of 1-methyl-tryptophan correlates with antitumor responses

Indoleamine 2,3-dioxygenase (IDO) is an immunosuppressive enzyme that contributes to tolerance in a number of biological settings. In cancer, IDO activity may help promote acquired tolerance to tumor antigens. The IDO inhibitor 1-methyl-tryptophan is being developed for clinical trials. However, 1-methyl-tryptophan exists in two stereoisomers with potentially different biological properties, and it has been unclear which isomer might be preferable for initial development. In this study, we provide evidence that the D and L stereoisomers exhibit important cell type-specific variations in activity. The L isomer was the more potent inhibitor of IDO activity using the purified enzyme and in HeLa cell-based assays. However, the D isomer was significantly more effective in reversing the suppression of T cells created by IDO-expressing dendritic cells, using both human monocyte-derived dendritic cells and murine dendritic cells isolated directly from tumor-draining lymph nodes. In vivo, the d isomer was more efficacious as an anticancer agent in chemo-immunotherapy regimens using cyclophosphamide, paclitaxel, or gemcitabine, when tested in mouse models of transplantable melanoma and transplantable and autochthonous breast cancer. The D isomer of 1-methyl-tryptophan specifically targeted the IDO gene because the antitumor effect of D-1-methyl-tryptophan was completely lost in mice with a disruption of the IDO gene (IDO-knockout mice). Taken together, our findings support the suitability of D-1-methyl-tryptophan for human trials aiming to assess the utility of IDO inhibition to block host-mediated immunosuppression and enhance antitumor immunity in the setting of combined chemo-immunotherapy regimens.

Bin1 attenuation in breast cancer is correlated to nodal metastasis and reduced survival

Clinical outcomes in breast cancer are likely influenced by modifier genes that affect tumor dormancy versus progression. The Bin1 gene encodes a nucleocytosolic adapter protein that suppresses neoplastic cell transformation and that is often attenuated in human breast carcinoma. Recent mouse genetic studies indicate that Bin1 loss cooperates with ras activation to drive progression of mammary carcinoma, establishing Bin1 as a negative modifier of tumor progression in breast cancer. In this study, we investigated whether immunohistochemical losses of nuclear Bin1 proteins in cases of human breast cancer were correlated to progression status. In American and Japanese groups of low or middle grade breast cancers, losses were associated with reduced survival and increased nodal metastasis, respectively. Taken together with recent findings from mouse genetic studies, these findings encourage further evaluation of the potential utility of Bin1 as a clinical prognostic marker in breast cancer.

Bin1 ablation in mammary gland delays tissue remodeling and drives cancer progression

Genes that modify oncogenesis may influence dormancy versus progression in cancer, thereby affecting clinical outcomes. The Bin1 gene encodes a nucleocytosolic adapter protein that interacts with and suppresses the cell transforming activity of Myc. Bin1 is often attenuated in breast cancer but its ability to negatively modify oncogenesis or progression in this context has not been gauged directly. In this study, we investigated the effects of mammary gland-specific deletion of Bin1 on initiation and progression of breast cancer in mice. Bin1 loss delayed the outgrowth and involution of the glandular ductal network during pregnancy but had no effect on tumor susceptibility. In contrast, in mice where tumors were initiated by the ras-activating carcinogen 7,12-dimethylbenz(a)anthracene, Bin1 loss strongly accentuated the formation of poorly differentiated tumors characterized by increased proliferation, survival, and motility. This effect was specific as Bin1 loss did not accentuate progression of tumors initiated by an overexpressed mouse mammary tumor virus-c-myc transgene, which on its own produced poorly differentiated and aggressive tumors. These findings suggest that Bin1 loss cooperates with ras activation to drive progression, establishing a role for Bin1 as a negative modifier of oncogenicity and progression in breast cancer.

The crystal structure of the BAR domain from human Bin1/amphiphysin II and its implications for molecular recognition

BAR domains are found in proteins that bind and remodel membranes and participate in cytoskeletal and nuclear processes. Here, we report the crystal structure of the BAR domain from the human Bin1 protein at 2.0 A resolution. Both the quaternary and tertiary architectures of the homodimeric Bin1BAR domain are built upon "knobs-into-holes" packing of side chains, like those found in conventional left-handed coiled-coils, and this packing governs the curvature of a putative membrane-engaging concave face. Our calculations indicate that the Bin1BAR domain contains two potential sites for protein-protein interactions on the convex face of the dimer. Comparative analysis of structural features reveals that at least three architectural subtypes of the BAR domain are encoded in the human genome, represented by the Arfaptin, Bin1/Amphiphysin, and IRSp53 BAR domains. We discuss how these principal groups may differ in their potential to form regulatory heterotypic interactions.

Cyclin D1 Functions in Cell Migration

Cell migration is essential for developmental morphogenesis, tissue repair, and tumor metastasis. A recent study reveals that cyclin D1 acts to promote cell migration by inhibiting Rho/ROCK signaling and expression of thrombospondin-1 (TSP-1), an extracellular matrix protein that regulates cell migration in many settings including cancer. Given the frequent overexpression of cyclin D1 in cancer cells, due to its upregulation by Ras, Rho, Src, and other genes that drive malignant development, the new findings suggest that cyclin D1 may have a central role in mediating invasion and metastasis of cancer cells by controlling Rho/ROCK signaling and matrix deposition of TSP-1.

Structure-activity study of brassinin derivatives as indoleamine 2,3-dioxygenase inhibitors

A screen of indole-based structures revealed the natural product brassinin to be a moderate inhibitor of indoleamine 2,3-dioxygenase (IDO), a new cancer immunosuppression target. A structure-activity study was undertaken to determine which elements of the brassinin structure could be modified to enhance potency. Three important discoveries have been made, which will impact future IDO inhibitor development: (i) The dithiocarbamate portion of the brassinin lead is a crucial moiety, which may be binding to the heme iron of IDO; (ii) an indole ring is not necessary for IDO inhibition; and (iii) substitution of the S-methyl group of brassinin with large aromatic groups provides inhibitors that are three times more potent in vitro than the most commonly used IDO inhibitor, 1-methyl-tryptophan.

RhoB in cancer suppression

RhoB is a mainly endosomal small GTPase that regulates actin organization and vesicle trafficking. Expression of RhoB is elevated rapidly by many stimuli, including growth factors, cytokines, and genotoxic stress. In cancer, RhoB can limit cell proliferation, survival, invasion, and metastasis, and during malignant progression its levels are attenuated commonly. In support of its role as a negative modifier of cancer progression, targeted deletion of RhoB in mice can increase tumor formation initiated by Ras mutation. How RhoB acts to suppress different aspects of cancer pathophysiology has emerged as a question of significant interest.

Reduction of hepatitis C virus NS5A phosphorylation through its interaction with amphiphysin II

Hepatitis C virus non-structural protein 5A (NS5A) is a pleiotropic protein with key roles in viral RNA replication, modulation of cellular-signaling pathways and interferon (IFN) responses. To search for possible host factors involved in mediating these functions of NS5A, we adopted an affinity purification approach coupled with mass spectrometry to examine protein-protein interactions, and found that human amphiphysin II (also referred to as Bin1) specifically interacts with NS5A in mammalian cells. Pull-down assays showed that the Src homology 3 (SH3) domain of amphiphysin II is required for NS5A interaction and that c-Src also interacts with NS5A in cells. IFN-alpha treatment reduced the interaction of NS5A with c-Src, but not amphiphysin II, suggesting that the latter is independent of the IFN-signaling pathway. NS5A is a phosphoprotein and its phosphorylation status is considered to have an effect on viral RNA replication. In vitro kinase assays demonstrated that its interaction with amphiphysin II inhibits phosphorylation of NS5A. These results suggest that amphiphysin II participates in the HCV life cycle by modulating the phosphorylation of NS5A.

RhoB facilitates c-Myc turnover by supporting efficient nuclear accumulation of GSK-3

The small GTPase RhoB suppresses cancer in part by limiting cell proliferation. However, the mechanisms it uses to achieve this are poorly understood. Recent studies link RhoB to trafficking of Akt, which through its regulation of glycogen synthase kinase-3 (GSK-3) has an important role in controlling the stability of the c-Myc oncoprotein. c-Myc stabilization may be a root feature of human tumorigenesis as it phenocopies an essential contribution of SV40 small T antigen in human cell transformation. In this study we show that RhoB directs efficient turnover of c-Myc in established or transformed mouse fibroblasts and that the attenuation of RhoB which occurs commonly in human cancer is a sufficient cause to elevate c-Myc levels. Increased levels of c-Myc elicited by RhoB deletion increased the proliferation of nullizygous cells, whereas restoring RhoB in null cells decreased the stability of c-Myc and restrained cell proliferation. Mechanistic analyses indicated that RhoB facilitated nuclear accumulation of GSK-3 and GSK-3-mediated phosphorylation of c-Myc T58, the critical site for ubiquitination and degradation of c-Myc. RhoB deletion restricted nuclear localization of GSK-3, reduced T58 phosphorylation, and stabilized c-Myc. These effects were not associated with changes in phosphorylation or localization of Akt, however, differences were observed in phosphorylation and localization of the GSK-3 regulatory Akt-related kinase, serum- and glucocorticoid-inducible protein kinase (SGK). The ability of RhoB to support GSK-3-dependent turnover of c-Myc offers a mechanism by which RhoB acts to limit the proliferation of neoplastically transformed cells.

Marrying Immunotherapy with Chemotherapy: Why Say IDO?: Figure 1

Activation of the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) in cancer cells facilitates immune escape. A recent study now shows how small-molecule inhibitors of IDO can be used to leverage the efficacy of traditional chemotherapeutic drugs that are used to treat cancer in the clinic. By promoting antitumor immune responses in combination with cytotoxic chemotherapy, IDO inhibitors may offer a drug-based strategy to more effectively attack systemic cancer.

Indoleamine 2,3-dioxygenase in cancer: targeting pathological immune tolerance with small-molecule inhibitors

Indoleamine 2,3-dioxygenase (IDO) is an interferon (IFN)-gamma-inducible, extrahepatic enzyme that catalyses the initial and rate-limiting step in the degradation of the essential amino acid tryptophan. Elevated tryptophan catabolism mediated by IDO is associated with a wide variety of human cancers and has historically been thought to be a tumoricidal consequence of IFN-gamma exposure. Evidence of a physiological requirement for IDO activity in protecting the allogeneic fetus from rejection by the maternal immune system has stimulated a radical shift in thinking about the role of IDO in cancer. Evidence now suggests that tumours can exploit IDO-mediated peripheral tolerance to promote immune escape. This review summarises key studies that implicate IDO as an important mediator of peripheral immune tolerance as well as the development of a promising new anticancer modality that incorporates the use of IDO inhibitors. The second part focuses on the current state of development of IDO inhibitory compounds as potential pharmaceutical agents.

Farnesyltransferase inhibitors: potential therapeutic for inflammatory breast cancer?

Inflammatory breast cancer (IBC) is an aggressive and poorly managed disease that accounts for up to 6% of new breast cancer cases in the United States annually. Recent preclinical findings suggest that IBC might be treated with farnesyltransferase inhibitors (FTIs), a novel class of experimental therapeutics that are currently in clinical trials. FTIs exhibit considerable selectivity and efficacy against tumors in preclinical models, and they have been well-tolerated in Phase I human trials. How FTIs target cancer cells has emerged as an important question, with the somewhat disappointing results from initial Phase II efficacy trials. FTI development was predicated on Ras inhibition but there is considerable evidence that other mechanisms are important for mediating the antineoplastic activities of these drugs. One important mediator that has emerged is RhoB, a small GTPase that is recruited by FTIs to trigger growth inhibition and apoptosis. Recently, van Golen and Merajver and their colleagues have demonstrated that FTIs will recruit RhoB to suppress IBC cell phenotypes or neoplastic transformation of human mammary epithelial cells by RhoC, a key oncogenic driver in IBC. A mechanistic implication of this study is that RhoB can interfere with RhoC-dependent signals required for neoplastic pathophysiology. Further preclinical and clinical investigations to test the potential therapeutic utility of FTIs for treating RhoC-driven breast cancers such as IBC would seem warranted.

Cyclin B1 is a critical target of RhoB in the cell suicide program triggered by farnesyl transferase inhibition

Farnesyl transferase inhibitors (FTIs) have displayed limited efficacy in clinical trials, possibly because of their relatively limited cytotoxic effects against most human cancer cells. Therefore, efforts to leverage the clinical utility of FTIs may benefit from learning how these agents elicit p53-independent apoptosis in mouse models of cancer. Knockout mouse studies have established that gain of the geranylgeranylated isoform of the small GTPase RhoB is essential for FTI to trigger apoptosis. Here we demonstrate that Cyclin B1 is a crucial target for suppression by RhoB in this death program. Steady-state levels of Cyclin B1 and its associated kinase Cdk1 were suppressed in a RhoB-dependent manner in cells fated to undergo FTI-induced apoptosis. These events were not derivative of cell cycle arrest, because they did not occur in cells fated to undergo FTI-induced growth inhibition. Mechanistic investigations indicated that RhoB mediated transcriptional suppression but also accumulation of Cyclin B1 in the cytosol at early times after FTI treatment, at a time before the subsequent reduction in steady-state protein levels. Enforcing Cyclin B1 expression attenuated apoptosis but not growth inhibition triggered by FTI. Moreover, enforcing Cyclin B1 abolished FTI antitumor activity in graft assays. These findings suggest that Cyclin B1 suppression is a critical step in the mechanism by which FTI triggers apoptosis and robust antitumor efficacy. Our findings suggest that Cyclin B1 suppression may predict favorable clinical responses to FTI, based on cytotoxic susceptibility, and they suggest a rational strategy to address FTI nonresponders by coinhibition of Cdk1 activity.

Inhibition of indoleamine 2,3-dioxygenase, an immunoregulatory target of the cancer suppression gene Bin1, potentiates cancer chemotherapy

Immune escape is a crucial feature of cancer progression about which little is known. Elevation of the immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) in tumor cells can facilitate immune escape. Not known is how IDO becomes elevated or whether IDO inhibitors will be useful for cancer treatment. Here we show that IDO is under genetic control of Bin1, which is attenuated in many human malignancies. Mouse knockout studies indicate that Bin1 loss elevates the STAT1- and NF-kappaB-dependent expression of IDO, driving escape of oncogenically transformed cells from T cell-dependent antitumor immunity. In MMTV-Neu mice, an established breast cancer model, we show that small-molecule inhibitors of IDO cooperate with cytotoxic agents to elicit regression of established tumors refractory to single-agent therapy. Our findings suggest that Bin1 loss promotes immune escape in cancer by deregulating IDO and that IDO inhibitors may improve responses to cancer chemotherapy.

RhoB and actin polymerization coordinate Src activation with endosome-mediated delivery to the membrane

We have used a c-Src-GFP fusion protein to address the spatial control of Src activation and the nature of Src-associated intracellular structures during stimulus-induced transit to the membrane. Src is activated during transit, particularly in RhoB-containing cytoplasmic endosomes associated with the perinuclear recycling compartment. Knocking out RhoB or expressing a dominant-interfering Rab11 mutant suppresses both catalytic activation of Src and translocation of active kinase to peripheral membrane structures. In addition, the Src- and RhoB-containing endosomes harbor proteins involved in actin polymerization and filament assembly, for example Scar1, and newly polymerized actin can associate with these endosomes in a Src-dependent manner. This implies that Src may regulate an endosome-associated actin nucleation activity. In keeping with this, Src controls the actin dependence of RhoB endosome movement toward the plasma membrane. This work identifies RhoB as a component of "outside-in" signaling pathways that coordinate Src activation with translocation to transmembrane receptors.

Targeted deletion of the suppressor gene bin1/amphiphysin2 accentuates the neoplastic character of transformed mouse fibroblasts

The Bin1/Amphiphysin2 gene encodes several alternately spliced BAR adapter proteins that have been implicated in membrane-associated and nuclear processes. Bin1 expression is often attenuated during tumor progression and Bin1 splice isoforms that localize to the nucleus display tumor suppressor properties. While these properties may reflect the ability of these isoforms to interact with and suppress the cell transforming activity of c-Myc, the effects of Bin1 deletion on the oncogenicity of c-myc or other transforming genes has not been gauged directly. Here we report that targeted deletion of Bin1 enhances the neoplastic character of primary murine embryo fibroblasts (MEFs) cotransformed by c-myc and mutant grasg. Specifically, Bin1 loss accentuated the spindle morphology of transformed cells, increased anchorage-independent proliferation, and promoted tumor formation in syngeneic hosts. These effects were specific as they were not recapitulated in cells transformed by viral oncoproteins and mutant ras. Although some Bin1 splice isoforms associate with endocytotic complexes the effects of Bin1 loss were not correlated with a generalized defect in receptor-mediated endocytosis. However, Bin1 loss increased sensitivity to paclitaxel, a drug that can affect endocytotic trafficking by disrupting microtubule dynamics. In E1A?transformed MEFs, Bin1 loss reduced the susceptibility to apoptosis triggered by tumor necrosis factor-alpha, an effect that was associated with precocious nuclear trafficking of NF-kappaB. These findings offer a novel line of support for the hypothesized role of Bin1 in limiting malignant growth, possibly as a negative modifier or anti-progression gene.

RhoB controls Akt trafficking and stage-specific survival of endothelial cells during vascular development

Blood vessel formation is a complex morphological process that is only beginning to be understood at the molecular level. In this study, we demonstrate a novel and critical role for the small GTPase, RhoB, in vascular development. RhoB null mice have retarded vascular development in the retina characterized by altered sprout morphology. Moreover, pharmaceutical means to deplete RhoB in neonatal rats is associated with apoptosis in the sprouting endothelial cells of newly forming vessels. Similarly, acute depletion of RhoB by antisense or dominant-negative strategies in primary endothelial cell culture models led to apoptosis and failures in tube formation. We identified a novel link between RhoB and the Akt survival signaling pathway to explain these changes. Confocal microscopy revealed that RhoB is highly localized to the nuclear margin with a small percentage found inside the nucleus. Similarly, total Akt is throughout the cell but has increased accumulation at the nuclear margin, and active phosphorylated Akt is found primarily inside the nucleoplasm, where it partially colocalizes with the RhoB therein. We show that this colocalization is functionally relevant, because when RhoB was depleted, Akt was excluded from the nucleus and total cellular Akt protein was decreased in a proteosome-dependent manner. Because the function of RhoB in vivo appears to only be rate limiting for endothelial cell sprouting, we propose that RhoB has a novel stage-specific function to regulate endothelial cell survival during vascular development. RhoB may offer a therapeutic target in diseases such as cancer, diabetic retinopathy, and macular degeneration, where the disruption of sprouting angiogenesis would be desirable.

Signal transduction: putting translation before transcription

A recent microarray-based study shows that Ras/Akt signaling rapidly alters the pattern of existing mRNAs that are recruited to polysomes. This response precedes the effects of transcription on total cellular RNA, suggesting that the primary effect of Ras/Akt signaling on gene expression may occur mainly at the level of translation.

Expression of a MYCN-interacting isoform of the tumor suppressor BIN1 is reduced in neuroblastomas with unfavorable biological features

PURPOSE

Amplification of the MYCN proto-oncogene is strongly correlated with poor outcome in neuroblastoma (NB), although deregulated MYCN is a potent inducer of apoptosis. BIN1 (2q14) encodes multiple isoforms of a Myc-interacting adaptor protein that has features of a tumor suppressor, including the ability to inhibit Myc-mediated cell transformation and to promote apoptosis. We hypothesized that BIN1 may function as a suppressor gene in NB, because Bin1 is highly expressed in neural tissues and binds the Myc Box motifs that are conserved in MycN. EXPERMENTAL DESIGN: Expression of MYCN, total BIN1, and BIN1 isoforms were determined in 56 primary NBs using the real-time PCR. Expression was correlated with biological and genetic features. To determine the functional significance of BIN1 expression we ectopically expressed BIN1 isoforms in NB cell lines with and without MYCN amplification, and assessed clonogenic growth.

RESULTS

Four predominant BIN1 isoforms resulting from alternative splicing of exon 12A (a neural tissue-specific exon) and exon 13 (a Myc-binding domain encoding exon) were variably expressed in the 56 primary NBs. Expression of BIN1 was lower in: NBs with MYCN amplification (n = 10) compared with those without, P < 0.03; in International Neuroblastoma Risk Group high-risk NB (n = 19) compared with low- or intermediate-risk NB, P < 0.01; and in metastatic NB (n = 21) compared with localized NB, P < 0.06. BIN1 inactivation by deletion or genomic rearrangement was identified infrequently. Forced expression of BIN1 isoforms containing the Myc-binding domain (with or without exon 12A) inhibited colony formation in NB cell lines with MYCN amplification (P < 0.01) but not in those without. Forced expression of BIN1 isoforms with a MBD deletion did not inhibit colony formation in any cell line assessed.

CONCLUSIONS

These data support that reduced BIN1 expression contributes to the malignant phenotype of childhood NB. As we reported previously, BIN1 may function to circumvent MycN-mediated apoptosis in NBs with MYCN amplification.