Kaposi's Sarcoma-Associated Herpesvirus Open Reading Frame 57 Encodes a Posttranscriptional Regulator with Multiple Distinct Activities

Abstract

Open reading frame (ORF) 57 of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a homolog of known posttranscriptional regulators that are essential for replication in other herpesviruses. Here, we examined the expression of this gene and the function(s) of its product. KSHV ORF 57 is expressed very early in infection from a 1.6-kb spliced RNA bearing several in-frame initiation codons. Its product is a nuclear protein that, in transient assays, has little effect on the expression of luciferase reporter genes driven by a variety of KSHV and heterologous promoters. However, ORF 57 protein enhances the accumulation of several viral transcripts, in a manner suggesting posttranscriptional regulation. These transcripts include not only known cytoplasmic mRNAs (e.g., ORF 59) but also a nuclear RNA (nut-1) that lacks coding potential. Finally, ORF 57 protein can also modulate the effects of the ORF 50 gene product, a classical transactivator known to be required for lytic induction. The expression from some (e.g., nut-1) but not all (e.g., tk) ORF 50-responsive promoters can be synergistically enhanced by coexpression of ORF 50 and ORF 57. This effect is not due to upregulation of ORF 50 expression but rather to a posttranslational enhancement of the transcriptional activity of ORF 50. These data indicate that ORF 57 is a powerful pleiotropic effector that can act on several posttranscriptional levels to modulate the expression of viral genes in infected cells.Kaposi's sarcoma (KS), an endothelial tumor with neoangiogenic and inflammatory components, is a common neoplasm of AIDS patients. Recent evidence strongly implicates a novel lymphotropic herpesvirus, KS-associated herpesvirus (KSHV; also called human herpesvirus 8), in the pathogenesis of KS (5, 12; for reviews, see references 14 and 41). KSHV DNA is found in virtually all of the spindle (endothelial) cells of clinically apparent KS lesions (8, 46), as well as in tumorinfiltrating monocytes (6) and circulating B cells (1,27,50). Consistent with its classification as a lymphotropic gammaherpesvirus, KSHV is also tightly linked to certain B-cell lymphomas, termed primary effusion lymphomas (9, 45). Although KSHV infection of KS spindle cells is predominantly latent, lytic replication is also evident in the tumor (28,35,36,46), and growing evidence suggests that the lytic cycle contributes importantly to tumorigenesis. For example, the incidence of KS is greatly decreased when AIDS patients at risk for KS are treated with ganciclovir, a drug that specifically blocks lytic viral replication (25). Moreover, several lytic-cycle gene products can stimulate inflammatory and angiogenic responses in surrounding cells and tissues (2, 3, 7). Lytic replication has also been posited to be required for KSHV spread from its presumed lymphoreticular reservoir to its endothelial targets (24). Thus, the study of the KSHV lytic cycle (and the switch from latency to lytic growth) is important not only to fully characterize the molecular basis of viral replication but also to f...

Rhesus Monkey Rhadinovirus ORF57 Induces gH and gL Glycoprotein Expression through Posttranscriptional Accumulation of Target mRNAs

“…However, it has been difficult to demonstrate experimentally that this enhancement of mRNA accumulation is independent of nuclear export. Still, the fact that nut-1/PAN RNA, which always localizes to the nucleus and is not exported to the cytoplasm (9,39), is enhanced by ORF57 (15,20,25) supports the idea that enhanced RNA accumulation in this case is independent of nuclear export (35). Furthermore, the nuclear and cytoplasmic fractionation of KSHV ORF59 RNA showed no sign of enhancement of nuclear RNA export (32).…”

“…RRV ORF57 did not activate the transcription of the renilla luciferase gene driven from the same CMV promoter (data not shown). Furthermore, it has been shown in previous reports that KSHV ORF57 did not activate transcription from a CMV promoter (14,20). We therefore hypothesized that gH and gL mRNAs are naturally unstable and that ORF57 allows gH, gL, c.o.…”

“…The timing and intensity of viral gene expression is orchestrated by comprehensive, interrelated regulatory mechanisms. ORF57 is expressed in the immediate-early phase of lytic replication (20,24,28) and is a key orchestrator of the viral lytic cycle by inducing delayed early and late gene expression (15,20,25,26,32). The extremely low levels of expression of envelope glycoproteins until the final stage of lytic replication may serve in part to limit any cytotoxicity of virus-encoded glycoproteins and may also aid virus replication by limiting the exposure of these antigens to the host immune response.…”

“…KSHV ORF57 has been shown to enhance, by as-yet unknown mechanisms, mRNA levels of ORF59 and nut-1/polyadenylated nuclear (PAN) RNA (20,26). However, it has been difficult to demonstrate experimentally that this enhancement of mRNA accumulation is independent of nuclear export.…”

“…The transcriptional activation of these promoters could be accomplished by ORF57 protein alone, or it could be further enhanced by the cooperation with the ORF50 gene product (also known as RTA) (20,28,33). However, as the name MTA (mRNA transcript accumulation) in KSHV implies, a posttranscriptional mechanism of activation by ORF57 orthologues has been studied and documented most extensively (4,14,21,29).…”