Localization and Sub-Cellular Shuttling of HTLV-1 Tax with the miRNA Machinery

Abstract

The innate ability of the human cell to silence endogenous retroviruses through RNA sequences encoding microRNAs, suggests that the cellular RNAi machinery is a major means by which the host mounts a defense response against present day retroviruses. Indeed, cellular miRNAs target and hybridize to specific sequences of both HTLV-1 and HIV-1 viral transcripts. However, much like the variety of host immune responses to retroviral infection, the virus itself contains mechanisms that assist in the evasion of viral inhibition through control of the cellular RNAi pathway. Retroviruses can hijack both the enzymatic and catalytic components of the RNAi pathway, in some cases to produce novel viral miRNAs that can either assist in active viral infection or promote a latent state. Here, we show that HTLV-1 Tax contributes to the dysregulation of the RNAi pathway by altering the expression of key components of this pathway. A survey of uninfected and HTLV-1 infected cells revealed that Drosha protein is present at lower levels in all HTLV-1 infected cell lines and in infected primary cells, while other components such as DGCR8 were not dramatically altered. We show colocalization of Tax and Drosha in the nucleus in vitro as well as coimmunoprecipitation in the presence of proteasome inhibitors, indicating that Tax interacts with Drosha and may target it to specific areas of the cell, namely, the proteasome. In the presence of Tax we observed a prevention of primary miRNA cleavage by Drosha. Finally, the changes in cellular miRNA expression in HTLV-1 infected cells can be mimicked by the add back of Drosha or the addition of antagomiRs against the cellular miRNAs which are downregulated by the virus.

Ebola VP40 in Exosomes Can Cause Immune Cell Dysfunction

“…Given that we observed apoptosis in cells treated with purified VP40 protein and supernatants from cells transfected with VP40 DNA, we next asked whether Ebola VP40 was able to affect the cellular miRNA machinery of transfected cells, as well as cells receiving exosome-laden transfected cell supernatant treatments. It has been previously shown that some viruses interfere with host miRNA machinery as a means of evading host immune responses important for cell signaling and regulation of apoptosis (Coley et al, 2010; Van Duyne et al, 2012). Additionally, it has been observed that EBOV proteins VP35, VP30, and potentially VP40 are capable of deregulating the host RNAi pathway as suppressors of RNA silencing (Fabozzi et al, 2011).…”

“…Infection with flaviviruses, including Dengue virus and Kunjin virus, has been previously shown to result in the inhibition of RNAi machinery and a downregulation of Drosha and DGCR8 (Moon et al, 2015; Casseb et al, 2016). We also have previously shown that the HTLV-1 protein Tax can directly modulate the RNAi pathway through the shuttling of Drosha and Dicer to the proteasome for degradation with no induction of apoptosis (Van Duyne et al, 2012). Alternatively, the downregulation of various miRNA machinery components including Dicer, Drosha, Exportin 5 and Ago proteins have, in many cases, been linked to the induction of apoptosis (Su et al, 2009; Han et al, 2013; Bian et al, 2014; Lombard et al, 2015).…”

HTLV-1 bZIP Factor HBZ Promotes Cell Proliferation and Genetic Instability by Activating OncomiRs

“…For example, miR181 modulates the T-cell antigen receptor sensitivity (44); miR150 regulates the transcription factor c-Myb13 and miR155 affects the differentiation of CD4 þ T lymphocytes into T helper type I (Th1) cells, regulatory T-cell development (45,46), and the promotion of T-cell-dependent tissue inflammation (46). HTLV-1 was previously shown to modify the pattern of miRNA expression in cell lines and ATLL samples (30)(31)(32)(33)(47)(48)(49)(50). Our results here add to these findings, as the miRNA expression profile of nonimmortalized infected T cells derived from carriers without malignancies was significantly distinct from that of uninfected cells.…”

“…The pleiotropic effects of Tax were previously thought to govern HTLV-1-associated miRNA dysregulation. By targeting Drosha to the proteasome, Tax influences global miRNA biogenesis (49). Furthermore, Tax activates the transcription of miR130b (33) and miR146a (32), and downregulates miR149 and miR873, which both directly target the chromatin-remodeling factors p300 and p/CAF that are known to play a critical role in HTLV-1 pathogenesis (47).…”

RNA stability regulates human T cell leukemia virus type 1 gene expression in chronically-infected CD4 T cells

“…Previous reports have suggested that the HTLV-1 tax/rex mRNA maybe the target of cellular miRNAs (Hakim et al, 2008; Ruggero et al, 2010), which could lead to instability of this message. On the other hand, both Tax and Rex have been shown to interfere with the cellular microRNA machinery (Ruggero et al, 2010; Van Duyne et al, 2012), again suggesting potential positive feedback loops that would enhance tax/ rex mRNA stability.…”