Introduction; interferons’ connection to cancer

Introduction; interferons’ connection to cancer

seminars in CANCER BIOLOGY, Vol. 10, 2000: pp. 69–72 doi: 10.1006/scbi.2000.0309, available online at http://www.idealibrary.com on Introduction; int...

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seminars in CANCER BIOLOGY, Vol. 10, 2000: pp. 69–72 doi: 10.1006/scbi.2000.0309, available online at http://www.idealibrary.com on

Introduction; interferons’ connection to cancer Paula M. Pitha

INTERFERONS (IFN) are a group of cellular proteins

assumed that IFN genes themselves may function as tumor supressor genes.5 The rearrangement of IFN genes including the translocation of (9;11) (p22; q23) was found in about 10% of the patients with acute myeoblastic leukemia.6 Samples of primary leukemia from ALL patients show high frequency of either homozygous or hemizygous deletion of IFN genes Deletion of IFN genes has been also noted in non-lymphoid neoplasia such as glioblastoma and melanoma.7 The direct function of IFN genes, as tumor suppressor genes was later questioned by finding that this region also contained other tumor suppressor genes. However, while the IFN genes were not yet shown to be the primary tumor suppressor genes the importance of IFNs and IFN induced proteins in tumor suppression has clearly emerged. Thus, a number of IFN induced proteins which are contributing to the antiviral effect of interferon, have tumor suppressor activity when over expressed in uninfected cells. These include, but are not limited to, double stranded RNA activated protein kinase (PKR), activated RNAse L, the proteins of the 200 gene family and some of the transcription factors of IRF family.8 The role of these proteins in tumorogenicity has been further supported by the observation that some of the tumorogenic DNA viruses target the function of the interferon stimulated genes. This viral mimicry involves several mechanisms. Human papiloma virus encoded E6 oncogene binds the cellular transcription factor IRF-3 that plays a critical role in the induction of type I IFN genes. This affects formation of the transcription complex-enhancoseome and consequently, induction of IFN B genes.9 The Adenovirus uses a similar mechanism where the E1A oncogene impairs IRF-3 and IRF-7 mediated activation of type I IFN genes10, 11 as well as formation of the transcription complex ISGF3 and the activation of IFN induced genes (ISG).12 The Epstein-Barr virus encoded EBNA2 protein, that is required for immortalization of B cells,inhibits the anti growth effect of IFN.13 The inhibition of the IFN effect seems to be a result of the EBNA2 association with the transcription

originally discovered by their ability to suppress viral infection in a cell type specific, but virus non-specific manner. These proteins are expressed in infected cells as an early response to viral infection and have been shown to play a major role in innate immunity. However it soon became clear that in addition to their antiviral activity, IFNs can also have a profound effect on cell growth.1 Both of these effects of IFNs are induced upon binding of IFN to the cell type specific receptors and by consequent activation of cellular signaling pathways. This results in the transcriptional activation of a large number of cellular genes.2 While some of these genes encode proteins which modulate viral replication, the effects of others extend beyond the infected cells. The antitumor effects of Type I IFNs (IFNsA and IFNB) are mediated both by its antiviral effect as well as by its ability to modulate expression of growthregulating/tumors suppressing genes. The ability of interferon to suppress viral infection was shown to be associated with its ability to limit virally induced malignancies. This includes hepatitis virus induced liver carcinomas, human papilomavirus induced juvenile laryngeal papillomatosis and condyloma acuminate. Treatment with IFN was also beneficial to HIV-1 and KS HSV associated Kaposi sarcoma. IFN can however also inhibit malignancies that are not virally induced and where the uncontrolled proliferation and cell growth are results of genetic alterations and consequent malfunction of growth regulating-tumor suppressor genes.3 Chromosomal abnormalities in lymphoid malignancies have been well documented. These include high incidence of chromosomal breakage on chromosome 9p22.4 Since the type I IFN locus is also localized in this region of Chromosome 9 it was

From the Oncology Center and Department of Molecular Biology and Genetics, Johns Hopkins University, Baltimore MD 21231, U.S.A.. c 2000 Academic Press 1044–579X / 00 / 020069+ 04 / $35.00 / 0

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coactivator CBP/p300. Finaly the KS HSV encodes analogues of cellular transcription factors of IRF family (vIRF1 and 2) which function as dominant negative mutants of the cellular IRFs factors and inhibit their activity.14–16 Thus the genes activated by IFN seem to play a direct role in the regulation of cell growth and as such are targeted by the oncogenic DNA viruses. The present collection of reviews focuses on various components of the interferon system that may relate to its anti-tumor activities. It clearly demonstrates that the antitumor effect of IFNs is complex and that many factors originally discovered as a part of the antiviral IFN system have a definite role outside viral infection. The transcription factors of the IRF family are presently composed of nine distinct, but related genes. IRF-1 and IRF-2 were originally assumed to have an essential role in the induction of IFN genes. However, the generation of mice with null deletion of these genes clearly indicated their role not only in innate immunity, but also in differentiation of immune cells and apoptosis. IRF-1 gene is localized on human chromosome 5 q31,3. Deletion of this region can be often found in patients with acute myelogenous leukemia or myelodisplasia.15 Mouse embryonic fibroblasts lacking the IRF-1 gene can not undergo UV induced cell arrest and are resistant to apoptotic death. When IRF-2 is overexpressed in mouse fibroblasts, these cells can grow as tumors in nude mice. IRF-3 that is constitutively expressed in all tissues and cell lines,17 together with IRF-7, plays a major role in the induction of type I IFN genes.10, 11, 18 IRF-3 gene was mapped to human chromosome 19q13.3.19 Deletions spanning this region have been associated with the development of gliomas.20 Chromosomal translocation t(6; 14) × (p25; q32) by which the immunoglobin heavy chain is juxtaposed next to multiple myeoloma oncogene 1/IRF-4 gene occurs with high incidence in multiple myeloma.21 Over-expression of IRF-8 inhibits Bcr-Abl induced CML like disease in mice.22 These results altogether indicate that several IRF genes modulate activity of genes involved in the control of cell growth and proliferation. Interestingly KS-HSV encoded v IRF-1 when overexpressed in mouse fibroblast cells confers tumor formation after transplantation to nude mice. This effect has been associated both with enhancement of c-myc expression in these cells as well as with their resistance to apoptosis16, 23 . Drs Tanaka and Tanaguchi summarized in their chapter our present knowledge

of the IRF factors with the focus on IRF-1 and IRF-2 and their role in regulation of cell proliferation and oncogenicity. They point out to the remarkable functional diversity and complexity of these factors. The interaction of IFN with cellular receptors generates multiple cellular signaling pathways. The antiviral effects of IFN are mediated generally by the JAK-STAT signaling pathway. This pathway, which has been studied in great detail, is also used by several other cytokines and growth factors. However, relatively very little is known about the molecular mechanisms that generate the anti-proliferative effect of IFNs. The null mutation in the alpha subunit of type I IFN receptor results in the diminished antiproliferative response to IFNs while the overexpression of this subunit has a major effect on cell proliferation even in the absence of IFN binding. Type I IFN receptors (binding IFNs alpha and IFN beta) and their structure-function properties are described by Drs. Prejean and Colaminici. These authors indicate that in addition to the well described JAK-STAT pathway, several other signaling pathways are induced upon binding of IFNs to their receptors. However, the role of these additional pathways in the anti-tumor effect of IFN remains to be clarified. IFNs generate their antiviral and antigrowth effects by induction of cellular genes that can modulate viral replication, activate the immune recognition of the infected cells, and control cell growth or cell death. It was recently shown, by using microarray analyses, that IFN treatment results in a major activation of cellular genes expression. However, the functions of the majority of these genes and their role in the antigrowth effects are presently unknown. While the anti-tumor effect of interferon can be clearly demonstrated, the resistance to IFN treatment can develop with a long-term IFN therapy. The proteins mediating the anti-tumor effect of IFN therefore need to be fully identified and the mechanism of development of the IFN resistance clarified. In his chapter, Dr. Sen describes four novel proteins encoded by IFN induced genes and the mechanism of their action. He points out that proteins encoded by the family of 200 genes have a major impact on the IFN mediated regulation of cell growth. One of the most interesting features of the IFN system and the individual components of this system, is their relation to programmed cell death-apoptosis. Many of the IFN induced proteins were shown to have a critical role in apoptosis. Overexpression of some of 70

Introduction; interferons’ connection to cancer

these proteins has proaptotic effects,while homozygous deletion confers resistance to apoptosis.Thus IFN activated signal transducer and activator-1 (STAT1) was shown to be required for the expression of the proapototic gene ICE (caspase1). The critical role of two interferon-induced genes Rnase L and PKR, as well as IRF-1 in apoptosis has been well established. These findings may have implications for cancer therapy, since they suggest that the combination of chemotherapy with IFN treatment may increase the effectiveness of the proapototic drugs in cancer patients. Dr. Barber describes the current knowledge of the relationship between cell death and interferon system. He however points out to the complexity of the IFN regulation and function. He suggests that IFN has a bifunctional role and that, in addition to its proapoptotic effect, IFNs can have also antiapoptotic effects. As soon as IFNs were discovered,their clinical potential has been realized at first as a general antiviral drug and later as an anticancer drug. Recombinant DNA technology generated sufficient amounts of interferon protein for clinical use and IFNs were the first recombinant cytokines used therapeutically in cancer treatment. Initially however the therapeutic expectations were not realized and it has been only recently when we have learned more how the IFNs work in vitro and that IFNs have been used more successfully therapeutically. Currently there are several approved therapeutic IFN therapies such as use of IFN beta for multiple sclerosis, IFNA for the chronic hepatitis B and hepatitis C infections. Clinical effectiveness of different IFN subtypes in treatment of various forms of cancers are summarized by Dr. Borden et al in their review. However as these authors point out a number of questions still remain open. Are the currently used recombinant IFN subtypes more effective then the other IFN subtypes? Should a distinct subtype be used for the treatment of viral infections and cancer? Are the individual subtypes more effective then their mixture? As pointed out by Dr. Borden et al some of these questions will now be answered by testing a second generation of therapeutic IFNs. The clinical testing of this second generation of IFNs will hopefully result in more effective IFN therapy and its expanded use in treatment of malignancies. The IFNs are presently the most widely clinically used cytokines. However it is not clear whether the method of therapy currently used is the most effective strategy for achieving of the optimal responses.Long term treatment, such as required for treatment of can-

cers, is often accompanied by toxicity and severe discomfort which both objectively and subjectively overshadow its benefits. The new approach to IFN therapy, would be to use the principle of gene therapy, and transfer the IFN genes directly into tumor cells. This would offer many advantages to the currently used methods. It would allow the targeting of high levels of interferon synthesis to tumor cells and eliminate exposure of other host cells to IFN. It is expected that this approach may eliminate the toxicity seen with the currently used IFN therapy. Furthermore, it was shown that even very low levels of autocrine IFN production can confer resistance to viral replication.23 Also recent studies with cells producing IFN constitutively revealed a novel effects of IFN on dentritic cells and T cells which may contribute to its antitumor effects. Drs. Ferrantini and Belardelli summarized in their comprehensive review various studies with interferon that are based on the gene transfer approach. They describe two major strategies 1. Use of tumor cells expressing IFNs, in the vaccine-like approach and 2. Use of IFN gene transfer in the gene therapy strategies. Should these be feasible in clinical setting, they would provide a novel, more effective IFN therapy. Since its discovery, it has been clear that IFNγ plays a unique role among the IFN family since in additional to its antiviral activity it has also an important proinflammatory function and is a very effective immune modulator. The IFNG null mice have been shown not only to be very sensitive to various types of infection but also defective in immune mediated tumor surveillance and destruction. Drs. Tanenabaum and Hamilton described in their comprehensive review the multiple mechanisms that contribute to antitumor effect of IFNγ . These authors point out that although the IFNγ plays a critical role in the immune destruction there are three requirements that needed to be fulfilled should IFNγ be effectively used in clinics. One is the necessity to produce high levels of IFNγ in the tumors itself, the second is that the tumor itself should be sensitive to IFNγ . And the third that immunosuppressive activity of tumor needs to be overcome. Thus this issue of Seminars in Cancer biology describes both the known principles and mechanisms of the antitumor effects of IFNs as well as the current use of IFNs in anticancer therapy and its possible future development. The author regrets, that because of space limitation many valuable references that are listed elsewhere in this issue could not be included.

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