Biological Activity Characterization of the Diagnostically Relevant Human Papillomavirus 16 E1C RNA

The spliced human papillomavirus 16 (HPV16) E1C RNA is associated with high-grade precursor lesions and cervical cancer. This qualifies E1C as a biomarker for high-grade lesions in HPV-based cervical cancer precursor screening. Here, we aimed to characterize the biological activity of HPV16 E1C RNA. In HEK-293T cells overexpressing HPV16 E1C RNA, we detected 9 kDa E1C protein in the cytoplasm using immunological assays with a newly generated E1C-specific monoclonal antibody or in mass spectrometry only after proteasome inhibition with MG132, indicating instability of the E1C protein. In HPV16-transformed cervical cancer cell lines in which the level of endogenous E1C RNA is much lower, E1C protein was not detected even after proteasome inhibition. Transient E1C overexpression in HEK-293T cells, co-transfected with a firefly luciferase reporter gene under the control of the HPV16 upstream regulatory region (URR), activated the HPV16 URR by 38%. This activation was also present when E1C translation was abolished by mutation. However, a construct expressing a random RNA sequence with similar GC content and 45% homology to the E1C RNA sequence also stimulated URR activity, indicating that special E1C RNA motifs might be responsible for the activation. In HPV16-transformed cell lines W12-episomal (W12-epi), W12-integrated HPV (W12-int), CaSki and SiHa stably overexpressing E1C RNA from lentiviral transduction, levels of endogenous HPV16 RNAs E6*I and E7 remained unchanged, while E1^E4 levels were significantly reduced by 20–30% in W12-epi, W12-int and CaSki cells. Overall, our study shows that E1C RNA is active and might contribute to transformation independent of the E6*I or E7 pathways. However, E1C overexpression resulted in only subtle changes in HPV16 RNA expression and very low copies of endogenous E1C RNA were detected in cervical cancer cell lines. This could weigh towards a less prominent role of E1C RNA in natural HPV transformation.