Increased expression of cellular RNA‐binding proteins in HPV‐induced neoplasia and cervical cancer

Self Cite

Our results presented here demonstrate that the most abundant human papillomavirus type 16 (HPV-16) mRNAs expressing the viral oncogenes E6 and E7 are regulated by cellular ASF/SF2, itself defined as a proto-oncogene and overexpressed in cervical cancer cells. We show that the most frequently used 3-splice site on the HPV-16 genome, site SA3358, which is used to produce primarily E4, E6, and E7 mRNAs, is regulated by ASF/SF2. Splice site SA3358 is immediately followed by 15 potential binding sites for the splicing factor ASF/SF2. Recombinant ASF/SF2 binds to the cluster of ASF/SF2 sites. Mutational inactivation of all 15 sites abolished splicing to SA3358 and redirected splicing to the downstream-located, late 3-splice site SA5639. Overexpression of a mutant ASF/SF2 protein that lacks the RS domain, also totally inhibited the usage of SA3358 and redirected splicing to the late 3-splice site SA5639. The 15 ASF/SF2 binding sites could be replaced by an ASF/SF2-dependent, HIV-1-derived splicing enhancer named GAR. This enhancer was also inhibited by the mutant ASF/SF2 protein that lacks the RS domain. Finally, silencer RNA (siRNA)-mediated knockdown of ASF/SF2 caused a reduction in spliced HPV-16 mRNA levels. Taken together, our results demonstrate that the major HPV-16 3-splice site SA3358 is dependent on ASF/SF2. SA3358 is used by the most abundantly expressed HPV-16 mRNAs, including those encoding E6 and E7. High levels of ASF/SF2 may therefore be a requirement for progression to cervical cancer. This is supported by our earlier findings that ASF/SF2 is overexpressed in high-grade cervical lesions and cervical cancer.Human papillomavirus type 16 (HPV-16) is the foremost cause of cervical cancer, which is one of the most common cancers in women globally (10, 37). Persistence of high-risk HPV types, such as HPV-16, is the highest risk factor for the development of cervical cancer. The majority of all DNA viruses that establish persistence have evolved a highly organized gene expression program, often divided into clear early and late phases. The HPV-16 genome contains an early promoter that could potentially express mRNAs encoding all viral gene products, and a late differentiation-dependent promoter that specifically excludes expression of E6 and E7 (21). The switch from early to late gene expression includes a promoter switch as well as derepression and activation of the late poly(A) signal and late splice sites (16). To activate late splice sites and the late poly(A) signal, many early splice sites and the early poly(A) signal must be downregulated to allow for competition from mutually exclusive late splice sites and poly(A) signal (8,26,36). Other HPV-16 splice sites are used by both early and late mRNAs and should function well in both mitotic cells and terminally differentiated cells. One of the major splice sites used by both early and late mRNAs is SA3358 (Fig. 1A). This splice site is outstanding in that it is used to produce the majority of all HPV-16 mRNAs, including the mRNAs of the oncogenes E6 and E...

Section: Discussionsupporting

confidence: 92%

Section: Discussionmentioning

confidence: 99%

Multiple ASF/SF2 Sites in the Human Papillomavirus Type 16 (HPV-16) E4-Coding Region Promote Splicing to the Most Commonly Used 3′-Splice Site on the HPV-16 Genome

Somberg

Schwartz

2010

Self Cite

“…Using cell line models of epithelial differentiation and HPV infection and backed up by data from virus-infected patient tissue, we and others demonstrated that SRSF1 (SF2/ASF), SRSF2 (SC35), and SRSF3 (SRp20) are specifically upregulated in HPVinfected, differentiated epithelial cells (24,25,31,32). For SRSF1, we showed previously that HPV16 E2 controlled the promoter of the SRSF1 gene by interacting within a region from nt Ϫ565 to nt Ϫ363 with respect to the transcription initiation site (this was originally cited as nt Ϫ689 to nt Ϫ482 in our previous paper [25] according to the numbering of a previously sequenced version of the SRSF1 promoter).…”

Section: Resultsmentioning

confidence: 98%

Human Papillomavirus E2 Regulates SRSF3 (SRp20) To Promote Capsid Protein Expression in Infected Differentiated Keratinocytes

Klymenko

Hernandez-Lopez

MacDonald

et al. 2016

The human papillomavirus (HPV) life cycle is tightly linked to differentiation of the infected epithelial cell, suggesting a sophisticated interplay between host cell metabolism and virus replication. Previously, we demonstrated in differentiated keratinocytes in vitro and in vivo that HPV type 16 (HPV16) infection caused increased levels of the cellular SR splicing factors (SRSFs) SRSF1 (ASF/SF2), SRSF2 (SC35), and SRSF3 (SRp20). Moreover, the viral E2 transcription and replication factor that is expressed at high levels in differentiating keratinocytes could bind and control activity of the SRSF1 gene promoter. Here, we show that the E2 proteins of HPV16 and HPV31 control the expression of SRSFs 1, 2, and 3 in a differentiation-dependent manner. E2 has the greatest transactivation effect on expression of SRSF3. Small interfering RNA depletion experiments in two different models of the HPV16 life cycle (W12E and NIKS16) and one model of the HPV31 life cycle (CIN612-9E) revealed that only SRSF3 contributed significantly to regulation of late events in the virus life cycle. Increased levels of SRSF3 are required for L1 mRNA and capsid protein expression. Capsid protein expression was regulated specifically by SRSF3 and appeared independent of other SRSFs. Taken together, these data suggest a significant role of the HPV E2 protein in regulating late events in the HPV life cycle through transcriptional regulation of SRSF3 expression. IMPORTANCEHuman papillomavirus replication is accomplished in concert with differentiation of the infected epithelium. Virus capsid protein expression is confined to the upper epithelial layers so as to avoid immune detection. In this study, we demonstrate that the viral E2 transcription factor activates the promoter of the cellular SRSF3 RNA processing factor. SRSF3 is required for expression of the E4^L1 mRNA and so controls expression of the HPV L1 capsid protein. Thus, we reveal a new dimension of virus-host interaction crucial for production of infectious virus. SRSF proteins are known drug targets. Therefore, this study provides an excellent basis for developing strategies to regulate capsid protein production in the infected epithelium and the production of new virions.

“…Apart from RNA processing-related functions, SR proteins have also been shown to be involved in chromatin remodelling, transcriptional regulation, genome stability maintenance, nucleolar stress, cell cycle progression, apoptosis control, and protein sumoylation (15)(16)(17)(18)(19)(20). Unsurprisingly, due to their diverse functions, many SR proteins are overexpressed in a range of tumors (21)(22)(23)(24)(25). Importantly, SRSF1 (ASF/SF2), SRSF3 (SRp20), and SRSF9 (SRp30c) have been shown to possess oncogenic properties (22)(23)(24)(25)(26)(27)(28)(29)(30)(31).…”

mentioning

confidence: 99%

“…Unsurprisingly, due to their diverse functions, many SR proteins are overexpressed in a range of tumors (21)(22)(23)(24)(25). Importantly, SRSF1 (ASF/SF2), SRSF3 (SRp20), and SRSF9 (SRp30c) have been shown to possess oncogenic properties (22)(23)(24)(25)(26)(27)(28)(29)(30)(31). Increased SRSF levels can result in the production of alternatively spliced RNA isoforms encoding key antiapoptotic, cell proliferation, and epithelial-mesenchymal transition (EMT)-inducing proteins (18).…”

mentioning

confidence: 99%

Human Papillomavirus 16 Oncoprotein Expression Is Controlled by the Cellular Splicing Factor SRSF2 (SC35)

McFarlane

MacDonald

Stevenson

et al. 2015

High-risk human papillomaviruses (HR-HPV) cause anogenital cancers, including cervical cancer, and head and neck cancers. Human papillomavirus 16 (HPV16) is the most prevalent HR-HPV. HPV oncogenesis is driven by two viral oncoproteins, E6 and E7, which are expressed through alternative splicing of a polycistronic RNA to yield four major splice isoforms (E6 full length, E6*I, E6*II, E6*X). The production of multiple mRNA isoforms from a single gene is controlled by serine/arginine-rich splicing factors (SRSFs), and HPV16 infection induces overexpression of a subset of these, SRSFs 1, 2, and 3. In this study, we examined whether these proteins could control HPV16 oncoprotein expression. Small interfering RNA (siRNA) depletion experiments revealed that SRSF1 did not affect oncoprotein RNA levels. While SRSF3 knockdown caused some reduction in E6E7 expression, depletion of SRSF2 resulted in a significant loss of E6E7 RNAs, resulting in reduced levels of the E6-regulated p53 proteins and E7 oncoprotein itself. SRSF2 contributed to the tumor phenotype of HPV16-positive cervical cancer cells, as its depletion resulted in decreased cell proliferation, reduced colony formation, and increased apoptosis. SRSF2 did not affect transcription from the P 97 promoter that controls viral oncoprotein expression. Rather, RNA decay experiments showed that SRSF2 is required to maintain stability of E6E7 mRNAs. These data show that SRSF2 is a key regulator of HPV16 oncoprotein expression and cervical tumor maintenance. H uman papillomaviruses (HPV) infect mucosal and cutaneous epithelia. At least 13 so-called "high-risk" HPV (HR-HPV) infect the anogenital epithelium and can cause persistent lesions that may progress to cancer (1). For example, around 500,000 women worldwide experience anogenital HPV infection, and nearly 300,000 die per annum from cervical cancer. Increasingly, HPV infection is also being linked to oropharyngeal cancer, whereby incidence of this disease is increasing rapidly (2). HPV16 is the most prevalent HR-HPV. HPV-associated tumorigenesis is driven by increased expression of the HPV E6 and E7 oncoproteins (3). E6 promotes ubiquitin-mediated degradation of p53 to inhibit apoptosis, modulates transcription of cell cycle-related genes, induces telomerase activity, controls cell shape and polarity, and activates cap-dependent translation (4). E7 binds and degrades Rb to promote S phase entry and cell division, controls transcription of cell cycle-related genes, and acts as a mitotic mutator (4). IMPORTANCE Expression of the HPV16 oncoproteins E7 and E6 drives HPV-associated tumor formation. Although increased transcription may yield increased levels of E6E7 mRNAs, it is known that theHPV E6 and E7 oncoproteins are expressed from a polycistronic transcript that for HPV16 can potentially produce four different alternatively spliced mRNAs (E6 full length [E6fl], E6*I, E6*II, and E6*X [also called E6*III]) (5, 6). The putative E6* proteins all share the first 44 amino acids of full-length E6 with C-terminal trun...