Tumor suppressor Interferon Regulatory Factor 1 selectively blocks expression of endogenous retrovirus

Abstract

Endogenous retroviruses (ERVs) comprise 10% of the genome, with many of these transcriptionally silenced post early embryogenesis. Several stimuli, including exogenous virus infection and cellular transformation can reactivate ERV expression via a poorly understood mechanism. We identified Interferon Regulatory Factor 1 (IRF-1), a tumor suppressor and an antiviral host factor, as a suppressor of ERV expression. IRF-1 decreased expression of a specific mouse ERV in vitro and in vivo. IRF-3, but not IRF-7, also decreased expression of distinct ERV families, suggesting that suppression of ERVs is a relevant biological function of the IRF family. Given the emerging appreciation of the physiological relevance of ERV expression in cancer, IRF-1-mediated suppression of specific ERVs may contribute to the overall tumor suppressor activity of this host factor.

Keywords: Cancer; Endogenous retroviruses, Interferon Regulatory Factor.

Figure 1.. IRF-1 suppresses endogenous reverse transcriptase (RT) activity.

(A, B) Primary macrophages were derived from bone marrow of mice of indicated genotypes. A. Macrophages were mock-treated or infected with MHV68. Reverse transcriptase activity was measured in cell lysates isolated at indicated time post infection. B. Macrophages were mock treated or incubated in the presence of recombinant IFNβ for 8 hours and reverse transcriptase activity measured in cell lysates. C. Mouse embryonic fibroblasts of indicated genotypes were mock-treated or treated with IFNβ as in B. Reverse transcriptase activity was measured in cell lysates collected at 8 hours post treatment. Data were pooled from 2-4 independent experiments. In this and other figures the mean is shown with error bars representing standard error of mean (SEM). *p<0.05; **p<0.01, ****p<0.0001. Only those differences that have reached statistical significance are defined as such in all figures.

Figure 2.. IRF-1 selectively suppresses Emv2 expression.

(A-C, F, G). Primary macrophages of indicated genotypes were mock-treated or treated with IFNβ for 8 hours. Levels of indicated transcripts were measured by RT-qPCR. Data were pooled from at least two independent experiments. Dotted line in F and G indicates level of detection (no RT controls). (D, E). RNA was isolated from lungs and spleens harvested from naïve animals of indicated genotypes. Levels of indicated transcripts were measured by RT-qPCR. Each symbol represents an individual animal.

Figure 3.. IRF-3 suppresses expression of several ERV families.

A. Endogenous RT activity was measured in cell lysates harvested from bone marrow derived macrophages of indicated genotypes. (B-D). Macrophages of indicated genotypes were treated as in Fig. 2. Levels of indicated transcripts were measured by RT-qPCR. Data were pooled from at least two independent experiments. (E, F). RNA was isolated from lungs and spleens harvested from naïve animals of indicated genotypes. Levels of indicated transcripts were measured by RT-qPCR. Each symbol represents an individual animal.