Innate Immune Signaling Induces High Levels of TC-specific Deaminase Activity in Primary Monocyte-derived Cells through Expression of APOBEC3A Isoforms

Abstract

In HIV-1-infected individuals, G-to-A hypermutation is found in HIV-1 DNA isolated from peripheral blood mononuclear cells (PBMCs). These mutations are thought to result from editing by one or more host enzymes in the APOBEC3 (A3) family of cytidine deaminases, which act on CC (APOBEC3G) and TC (other A3 proteins) dinucleotide motifs in DNA (edited cytidine underlined). Although many A3 proteins display high levels of deaminase activity in model systems, only low levels of A3 deaminase activity have been found in primary cells examined to date. In contrast, here we report high levels of deaminase activity at TC motifs when whole PBMCs or isolated primary monocyte-derived cells were treated with interferon-␣ (IFN␣) or IFN␣-inducing toll-like receptor ligands. Induction of TC-specific deaminase activity required new transcription and translation and correlated with the appearance of two APOBEC3A (A3A) isoforms. Knockdown of A3A in monocytes with siRNA abolished TC-specific deaminase activity, confirming that A3A isoforms are responsible for all TC-specific deaminase activity observed. Both A3A isoforms appear to be enzymatically active; moreover, our mutational studies raise the possibility that the smaller isoform results from internal translational initiation. In contrast to the high levels of TC-specific activity observed in IFN␣-treated monocytes, CC-specific activity remained low in PBMCs, suggesting that A3G deaminase activity is relatively inhibited, unlike that of A3A. Together, these findings suggest that deaminase activity of A3A isoforms in monocytes and macrophages may play an important role in host defense against viruses. A33 proteins are cytidine deaminases that are capable of inhibiting replication of retroviruses, DNA viruses, and retroelements (reviewed in Refs. 1-3). The seven A3 proteins in the human genome share a strong preference for deaminating cytidines that are in a CC or TC context in single-stranded DNA (edited cytidine underlined). Although APOBEC3G (A3G), the best understood A3 protein, acts preferentially on CC motifs, the six other human A3 proteins display varying degrees of preference for TC motifs (4 -13). Because other cytidine deaminases, such as activation-induced deaminase and APOBEC1, either display a strong preference for editing deoxycytidines in other dinucleotide contexts or act on TC motifs in RNA (4), mutations that arise in a CC or TC context in DNA constitute signatures of editing by A3 proteins.Much headway has been made in understanding how A3 proteins act on HIV-1 DNA, in part through transfection of A3 plasmids into human epithelium-derived cell lines, such as 293T or HeLa cells, which lack significant endogenous expression of most A3 proteins. From such studies, A3G and APOBEC3F (A3F) are known to be packaged into virus particles when the HIV-1 Vif protein is not expressed and to cause a substantial reduction in virus infectivity (5, 12, 14 -16). Reduced virus infectivity results in part from non-enzymatic inhibition of reverse transcription by A3G and A3F (17-2...

TRIM22 Inhibits HIV-1 Transcription Independently of Its E3 Ubiquitin Ligase Activity, Tat, and NF-κB-Responsive Long Terminal Repeat Elements

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A Biochemical Analysis Linking APOBEC3A to Disparate HIV-1 Restriction and Skin Cancer

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