Interferon: no magic bullet against cancer

ATXN3 Positively Regulates Type I IFN Antiviral Response by Deubiquitinating and Stabilizing HDAC3

Ataxin-3 (ATXN3) belongs to the Josephin family of deubiquitinases. So far, ATXN3 is majorly linked to the neurodegenerative disease, Machado-Joseph disease. The role of ATXN3 in the antiviral function has not been explored, and the in vivo deubiquitinating activity of ATXN3 remains largely unknown. In this study, we report that ATXN3 is an important positive regulator of type I IFN (IFN-I)-mediated antiviral activity in murine primary lung cells and human epithelial and fibroblast cell lines. We clarify that ATXN3 does not promote IFN-I production, but enhances the IFN-I-mediated signaling pathway. Furthermore, ATXN3 physically interacts with histone deacetylase 3 (HDAC3) and upregulates the level of HDAC3 protein. Moreover, ATXN3 deubiquitinates HDAC3, thereby enhancing HDAC3 protein stability. Interestingly, the interaction between ATXN3 and HDAC3 increases during viral infection, which promotes IFN-I-induced signaling in murine primary lung cells. Finally, we reveal the ATXN3/HDAC3 axis-mediated regulation of IFN-I antiviral response. These findings reveal a novel biological function of ATXN3 and an important antiviral mechanism by which the deubiquitinase ATXN3 positively regulates IFN-I antiviral response, and they may provide a novel strategy for enhancing IFN-based antiviral therapy.

Requirement of Histone Deacetylase Activity for Signaling by STAT1

STAT1 is a transcription factor that plays a crucial role in signaling by interferons (IFNs). In this study we demonstrated that inhibitors of histone deacetylase (HDAC) activity, butyrate, trichostatin A, and suberoylanilide hydroxamic acid, prevented IFNgamma-induced JAK1 activation, STAT1 phosphorylation, its nuclear translocation, and STAT1-dependent gene activation. Furthermore, we showed that silencing of HDAC1, HDAC2, and HDAC3 through RNA interference markedly decreased IFNgamma-driven gene activation and that overexpression of HDAC1, HDAC2, and HDAC3 enhanced STAT1-dependent transcriptional activity. Our data therefore established the essential role of deacetylase activity in STAT1 signaling. Induction of IRF-1 by IFNgamma requires functional STAT1 signaling and was abrogated by butyrate, trichostatin A, suberoylanilide hydroxamic acid, and STAT1 small interfering RNA. In contrast, silencing of STAT1 did not interfere with IFNgamma-induced expression of STAT2 and caspase-7, and HDAC inhibitors did not preclude IFNgamma-induced expression of STAT1, STAT2, and caspase-7, suggesting that HDAC inhibitors impede the expression of IFNgamma target genes whose expression depends on STAT1 but do not interfere with STAT1-independent signaling by IFNgamma. Finally, we showed that inhibitors of deacetylase activity sensitized colon cancer cells to IFNgamma-induced apoptosis through cooperative negative regulation of Bcl-x expression, demonstrating that interruption of the balance between STAT1-dependent and STAT1-independent signaling significantly alters the biological activity of IFNgamma.

Oncogenic Ki-ras inhibits the expression of interferon-responsive genes through inhibition of STAT1 and STAT2 expression

Endogenous interferon gamma (IFNgamma) promotes the host response to primary tumors, and IFNgamma-insensitive tumors display increased tumorigenicity and can evade tumor surveillance mechanisms. Here we demonstrate that activating mutations of Ki-ras are sufficient to inhibit the expression of STAT1 and STAT2, transcription factors required for signaling by IFNs, providing a potential mechanism for the insensitivity of tumors to IFNs. We demonstrated that colon cancer cell lines with Ki-ras mutations display reduced expression of IFN-responsive genes compared with the cell lines that have retained wild type Ras and that inactivation of the mutant Ki-ras allele in the HCT116 colon cancer cell line is sufficient to restore the expression of STAT1, STAT2, and IRF-9. Accordingly, the expression of 27 interferon-inducible genes was reduced in HCT116 cells compared with the isogenic clones with targeted deletion of the mutant Ki-ras allele, Hkh2 and Hke-3. The expression of IFNgamma receptors did not differ among the isogenic cell lines. IFNgamma stimulated transcription of a STAT1-dependent reporter gene was impaired by RasV12, demonstrating a transmodulation of IFN/STAT signaling by activated Ras. Finally, we demonstrated that the expression of RasV12 in 293T cells is sufficient to inhibit the endogenous expression of STAT1 and STAT2, confirming the negative regulation of IFN signaling by oncogenic Ras. Our data demonstrate that the signaling initiated by activated Ki-ras interferes with the IFN/STAT signaling pathway and modulates the responsiveness of cancer cells to interferons. Furthermore, the data suggest that tumors harboring activating Ki-ras mutations may escape tumor surveillance mechanisms due to reduced responsiveness to IFNgamma.

Interferon and cis-DDP: combination chemotherapy for P388 leukemia in CDF1 mice

CDF1 mice were implanted with 10(6) P388 leukemia cells. One day postimplantation, when interferon (IFN) therapy was begun, the tumor burden was ca. 4.8 X 10(6) cells. Therapy consisted of nine consecutive daily intraperitoneal (i.p.) injections of 10(6) units of Ehrlich ascites tumor cell IFN (specific activity 5 X 10(7) IU/mg). The median life span for control animals was 10.0 days versus 18.5 days for the treated animals. It was estimated that about 2 X 10(6) viable P388 cells were present on Day 9 when therapy was discontinued. There was no evidence of toxicity in animals receiving 10(6) units of IFN/day for nine consecutive days. In a separate experiment, control animals challenged as above had a median life span of 9.5 days of versus 15 days for animals receiving 10(6) units of IFN qd on Days 2-10. A third group of animals receiving a single injection of cis-DDP on Day 1 had a median survival of 18.5 days, while a fourth group receiving both cis-DDP on Day 1 and subsequent treatment with IFN survived 26 days. The results are discussed in terms of impact on tumor cell population.

Progress towards a unified theory of the mechanisms of carcinogenesis, III: Circumvention of proliferation controls

Circumvention of part of the normal controls of proliferation permits the relatively autonomous proliferation of cells which is characteristic of tumors. Several different mechanisms exist which permit circumvention of proliferation controls. RNA tumor viruses probably induce carcinogenesis by producing a viral kinase which is similar to a cAMP independent tyrosine kinase that is activated by epidermal growth factor. Circumvention of proliferation controls is permitted by modification of gene expression induced by genetic and/or epigenetic mechanisms. There does not appear to be one single mechanism for circumventing proliferation controls present in all tumors. Circumvention of proliferation controls is the basic cause of carcinogenesis.