Citrullination is a post-translational modification catalyzed by peptidyl-arginine deiminases (PADs), which convert peptidyl-arginine into peptidyl-citrulline. The PAD family in humans comprises five isozymes (PADs 1-4 and 6), which are implicated in various diseases, including cancer. Our previous findings revealed a novel mechanism by which high-risk human papillomaviruses (HPVs) manipulate host regulatory pathways involved in the cell cycle and survival to enhance viral fitness, implying that PADs may be promising targets for developing new host-targeting antivirals to prevent cervical cancer progression. In this study, we further explore the impact of PAD-mediated protein citrullination on HPV infection, in the context of the head and neck squamous cell carcinoma (HNSCC).
Here we demonstrate using RT-qPCR that PAD1, PAD2, and PAD4 are expressed and modulated in HNSCC biopsies compared to normal mucosa, though their regulation is not influenced by HPV status as tested by nested-PCR. Additionally, we conducted an immunohistochemical (IHC) analysis focusing on PAD1 and PAD4 in a cohort of formalin-fixed paraffin-embedded HNSCC samples.
According to preliminary results, PAD1 expression appears to be associated with tumor differentiation; specifically, we observed intense IHC positivity in poor differentiated HNSCC. Additional investigation is necessary for PAD4; however, its expression seems to overlap with PAD1 tumor expression. To further corroborate these data, we took advantage of an in vitro cellular model of immortalized keratinocytes (NOKs) expressing the major HPV oncoproteins E6 and E7. Preliminary results revealed that both PAD expression and overall citrullination are not significantly modulated by HPV oncoproteins.
In conclusion, in the context of HNSCC, the expression of PAD1 and PAD4 is modulated according to tumor differentiation, although HPV infection does not appear to significantly influence this regulation.