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Tumor cells display a different profile of gene expression than their normal counterparts. Perturbations in the levels of cellular splicing factors can alter gene expression, potentially leading to tumorigenesis. We found that splicing factor SRp20 (SFRS3) is highly expressed in cancers. SRp20 regulated the expression of Forkhead box transcription factor M1 (FoxM1) and two of its transcriptional targets, PLK1 and Cdc25B, and controlled cell cycle progression and proliferation. Cancer cells with RNAi-mediated reduction of SRp20 expression exhibited G2/M arrest, growth retardation, and apoptosis. Increased SRp20 expression in rodent fibroblasts promoted immortal cell growth and transformation. More importantly, we found that SRp20 promoted tumor induction and the maintenance of tumor growth in nude mice and rendered immortal rodent fibroblasts tumorigenic. Collectively, these results suggest that increased SRp20 expression in tumor cells is a critical step for tumor initiation, progression, and maintenance.

High-risk human papillomaviruses (HPVs) encode two viral oncoproteins, E6 and E7, from a single bicistronic pre-mRNA containing three exons and two introns. Retention of intron 1 in the E6 coding region is essential for production of the full-length E6 oncoprotein. However, splicing of intron 1 is extremely efficient in cervical cancer cells, leading to the production of a spliced transcript, E6*I, of E6. Here, we investigated whether this splicing of intron 1 might benefit E7 production. Using RNA interference as a tool, we targeted the intron 1 region using small interfering RNAs (siRNAs) in HPV-positive cell lines. At an effective low dose, the siRNAs specifically suppressed E6 expression but not E7 expression, as demonstrated by the stabilization of p53. However, at high doses the HPV18 intron 1-specific siRNA substantially and specifically reduced the level of the 18E6*I mRNA lacking the intron region in HeLa cells, implying its nuclear silencing on the pre-mRNA before RNA splicing. Two other siRNAs targeting the exon 2 regions of HPV16 and -18, which encode the E7 oncoprotein, reduced the E6*I mRNAs to a remarkable extent and preferentially suppressed expression of E7, leading to accumulation of hypophosphorylated p105Rb and cell cycle arrest, indicating that the majority of E7 proteins are the translational products of E6*I mRNAs. This was confirmed by transient transfection in 293 cells: E7 could be translated only from the E7 open reading frame (ORF) on E6*I mRNA in a distance-dependent matter of upstream E6*I ORF by translation reinitiation. The data thus provide direct evidence that the E6*I mRNAs of high-risk HPVs are responsible for E7 production.Human papillomavirus (HPV) infection of epidermal or mucosal epithelial cells causes benign and sometimes malignant neoplasms. Certain HPVs, such as HPV16, -18, -31, and -45, are detected frequently in anogenital cancers, particularly cancer of the cervix and anus, and are thus considered to be high risk or oncogenic (22,42). Among the high-risk HPVs, HPV16 and -18 cause Ͼ90% of cervical cancers (22) and Ͼ20% of oral cancers (35). High-risk HPVs encode two potent viral oncoproteins, E6 and E7, that mediate, respectively, degradation of the cellular proteins p53 and retinoblastoma protein (pRb), two tumor suppressor proteins that are essential for cell cycle control (21).In both HPV16 and HPV18, E6 and E7 are transcribed as a single bicistronic E6E7 transcript using a common promoter and a common early polyadenylation site (1). Promoter p97, upstream of the HPV16 E6 (16E6) open reading frame (ORF), and promoter p105, upstream of the HPV18 E6 (18E6) ORF, are responsible for the initiation of transcription in each virus genome. The bicistronic E6E7 pre-mRNAs of HPV16 and HPV18 contain three exons and two introns. One of the two introns, intron 1 (the cap-proximal intron), is positioned in the E6 ORF in both the HPV16 and HPV18 E6E7 pre-mRNAs ( Fig. 1A and B). Conceivably, intron 1 removal by RNA splicing would disrupt the E6 ORF and prevent full-length E6 from...