Trimethylation of histone H3 lysine 4 by Set1 in the lytic infection of human herpes simplex virus 1

Abstract

Human herpes simplex virus 1 (HSV-1) is a double-stranded DNA virus that causes facial, ocular, and encephalitic disease in humans. Previous work showed that the genome of HSV-1 is associated with acetylated and methylated histones during lytic infection. However, the physiological role of histone modifications in lytic infection of HSV-1 is unclear. We examined the role of protein methylation in lytic infection of HSV-1 using a protein methylation inhibitor, 5'-deoxy-5'-methylthioadenosine (MTA). We found that MTA strongly reduces the transcription and replication of HSV-1. Moreover, MTA treatment decreases the level of trimethylation of lysine 4 in histone H3 (H3K4me3) on the HSV-1 genome. These results suggest that protein methylation, and in particular, histone methylation, is involved in the lytic infection of HSV-1. To delineate the underlying mechanism, we investigated the role of two H3K4 methyltransferases, Set1 and Set7/9, in the lytic infection of HSV-1. Using small interference RNA, we found that the reduction of Set1, but not Set7/9, reduces the transcription and replication of HSV-1 and specifically decreases H3K4me3 on the virus genome. These results indicate that H3K4me3 mediated by Set1 is required for optimal gene expression and replication of HSV-1 during lytic infection and suggest that this pathway could be a potential point of pharmacological intervention during HSV-1 infection.

FIG. 1.

MTA treatment reduces histone modifications in HeLa cells. (A) Cells were treated with 1 mM MTA for 24 h and then infected with HSV-1 for 3 h. Histones were extracted from cells, and Western blot analysis was performed with H3K4me3 and H3K9me2 antibodies. (B) After treatment with 1 mM MTA for 24 h, lactate dehydrogenase release was measured as the index for cytotoxicity. DMSO, dimethyl sulfoxide. (C) Cells were infected with HSV-1 for 1, 3, 6, and 10 h. Equal amounts of mRNA from each sample were reverse transcribed, and the level of 28S RNA was analyzed by quantitative real-time PCR.

FIG. 2.

MTA treatment decreases the recruitment of H3K4me3 on the genome of HSV-1. A ChIP assay was performed with cells treated with MTA (+MTA) (black bar) or without MTA (−MTA) (white bar) for 24 h, followed by HSV-1 infection for 1 h. The recruitment of H3K4me3 and H3K4me2 at the ICP0, TK, and VP16 promoters was measured by real-time PCR.

FIG. 3.

MTA inhibits gene expression and replication of HSV-1. Cells were treated with MTA (+MTA) (black bar) or without MTA (−MTA) (white bar), followed by HSV-1 infection for 1, 3, 6, and 10 h. (A) mRNA levels for ICP0, TK, and VP16 were measured by real-time PCR and normalized to that of 28S RNA. (B) The genome copy number of HSV-1 was quantified by real-time PCR and normalized to cell numbers.

FIG. 4.

siRNA-mediated knockdown of Set1 and Set7/9. Cells were transfected with two rounds of siRNA against luciferase (control) (white bar), Set1 (shaded bar), or Set7/9 (black bar) for 48 h. (A) mRNA levels for Set1 and Set7/9 were measured and normalized to glyceraldehyde-3-phosphate dehydrogenase. (B) Western blot analysis was performed with Set1 or Set7/9 antibody.

FIG. 5.

Role of Set1 and Set7/9 in gene expression and replication of HSV-1. Cells were treated with siRNA against the control, Set1, or Set7/9 for 2 days, followed by HSV-1 infection for 3, 6, and 10 h. (A) mRNA levels of ICP0, TK, and VP16 were measured as described in the legend of Fig. 3. (B) The genome copy number of HSV-1 was measured as described in the legend of Fig. 3.

FIG. 6.

siRNA of Set1 reduces the recruitment of H3K4me3 on the genome of HSV-1. Cells were transfected with siRNA of the control, Set1, or Set7/9 for 2 days. A ChIP assay was performed to examine the recruitment of H3K4me3 or H3 on the genome of HSV-1 as described in the legend of Fig. 2. IgG, immunoglobulin G.

FIG. 7.

Model for the role of Set1 in the lytic infection by HSV-1. See Discussion for a description.