Intriguingly, we also found that the E7-induced transcriptional activation of the rDNA promoter in MCF7 and H358 cells was moderately but significantly increased upon p14ARF co-expression. We cannot exclude a slight difference in E7 expression at the protein level. However, the decrease in rDNA promoter activity upon silencing of p14ARF in CaSki cells provides further evidence of this finding. This effect was not clearly observed with the E7C24G mutant, PF-5274857 suggesting an involvement of pRb binding and degradation. We found no significant difference in levels of phosphorylated UBF1 in cells expressing E7 or E7C24G, compared to cells expressing E7 and p14ARF or cells expressing E7C24G and p14ARF. Thus, a difference of phosphorylated UBF1 levels does not account for the increased activity of the rDNA promoter observed upon E7 and p14ARF co-expression. Pan et al showed that the introduction of the murine protein p19ARF in ARF-deficient U2OS cells induced PF-3758309 nucleolar localization of E7. Moreover, immunofluorescence and electron microscopy with immunogold staining revealed the presence of E7 within the nucleolus of the HPV16-positive cervical carcinoma cell line CaSki, that expresses endogenous p14ARF. Accordingly, we show that E7 accumulates in the nucleolar compartment of U2OS and MCF7 cells upon p14ARF overexpression. The E7C24G mutant exhibited a similar nucleolar accumulation upon p14ARF expression. Together, these results suggest that the nucleolar localization of E7 facilitates its enhancement of rDNA transcription by a mechanism involving pRb degradation. In the nucleolus, Rb directly represses transcription of the rRNA genes by binding to UBF1 and inhibiting its DNA binding activity. We provided evidence that E7 interacts with UBF1 and with p14ARF. These interactions are physiologically relevant, as they were also observed in immunoprecipitation experiments in CaSki cells. However, we detected no direct interaction between E7 and p14ARF, suggesting that the interaction occurs through a protein complex, probably involving UBF1. Thus, a possible explanation of the increased E7-activation of rDNA transcription is that the nucleolar accumulation of E7 could allow E7 to inhibit the binding of pRb to UBF1 and facilitate the degradation of nucleolar pRb.