UV Radiation Activates Toll-Like Receptor 9 Expression in Primary Human Keratinocytes, an Event Inhibited by Human Papillomavirus 38 E6 and E7 Oncoproteins

Abstract

Several lines of evidence indicate that cutaneous human papillomavirus (HPV) types belonging to the beta genus of the HPV phylogenetic tree synergize with UV radiation in the development of skin cancer. Accordingly, the E6 and E7 oncoproteins from some beta HPV types are able to deregulate pathways related to immune response and cellular transformation. Toll-like receptor 9 (TLR9), in addition to playing a role in innate immunity, has been shown to be involved in the cellular stress response. Using primary human keratinocytes as experimental models, we have shown that UV irradiation (and other cellular stresses) activates TLR9 expression. This event is closely linked to p53 activation. Silencing the expression of p53 or deleting its encoding gene affected the activation of TLR9 expression after UV irradiation. Using various strategies, we have also shown that the transcription factors p53 and c-Jun are recruited onto a specific region of the TLR9 promoter after UV irradiation. Importantly, the E6 and E7 oncoproteins from beta HPV38, by inducing the accumulation of the p53 antagonist ΔNp73␣, prevent the UV-mediated recruitment of these transcription factors onto the TLR9 promoter, with subsequent impairment of TLR9 gene expression. This study provides new insight into the mechanism that mediates TLR9 upregulation in response to cellular stresses. In addition, we show that HPV38 E6 and E7 are able to interfere with this mechanism, providing another explanation for the possible cooperation of beta HPV types with UV radiation in skin carcinogenesis.IMPORTANCE Beta HPV types have been suggested to act as cofactors in UVinduced skin carcinogenesis by altering several cellular mechanisms activated by UV radiation. We show that the expression of TLR9, a sensor of damage-associated molecular patterns produced during cellular stress, is activated by UV radiation in primary human keratinocytes (PHKs). Two transcription factors known to be activated by UV radiation, p53 and c-Jun, play key roles in UV-activated TLR9 expression. The E6 and E7 oncoproteins from beta HPV38 strongly inhibit UV-activated TLR9 expression by preventing the recruitment of p53 and c-Jun to the TLR9 promoter. Our findings provide additional support for the role that beta HPV types play in skin carcinogenesis by preventing activation of specific pathways upon exposure of PHKs to UV radiation.KEYWORDS UV radiation, Toll-like receptor 9, HPV38, primary keratinocytes, p53

Cross-talk of cutaneous beta human papillomaviruses and the immune system: determinants of disease penetrance

“…Beta-HPV38 E6 and E7 oncoproteins are able to inhibit the expression of TLR9, and they seem to share this function with mucosal HR-HPV [70,71]. In addition, HPV38 E6 and E7 oncoproteins are able to block the UV-mediated activation of TLR9 by preventing the recruitment of p53 and c-Jun to the TLR9 promoter [72].…”

Immunopathogenesis of HPV-Associated Cancers and Prospects for Immunotherapy

“…Although HPVs encode only a limited number of regulatory genes, they engage various active strategies to counteract immune recognition and cell-autonomous immune responses at different levels [ 54 ]. Mucosal as well as cutaneous HPV were shown to suppress recognition by the pattern recognition receptor toll-like receptor 9 [ 55 , 56 ]. Mucosal HPV also specifically inhibits interferon (IFN) expression, IFN signaling and downstream responses [ 57 , 58 , 59 , 60 ].…”

Significance and Role of Pattern Recognition Receptors in Malignancy

“…For example, Dewan et al (2012) applied topical TLR7 agonist in concern with low-dose cyclophosphamide and irradiation in mouse cutaneous breast cancer with apparently promising results. UV irradiation was shown to activate TLR9 in human keratinocytes (Pacini et al 2017). In mouse models activation by unmetylated CPG ODN synergized with a range of chemotherapy regimens including topotecan, 5-fluorouracil, gemcitabine and others in various mouse tumor models.…”