hrHPV E5 oncoprotein: immune evasion and related immunotherapies

Abstract

The immune response is a key factor in the fight against HPV infection and related cancers, and thus, HPV is able to promote immune evasion through the expression of oncogenes. In particular, the E5 oncogene is responsible for modulation of several immune mechanisms, including antigen presentation and inflammatory pathways. Moreover, E5 was suggested as a promising therapeutic target, since there is still no effective medical therapy for the treatment of HPV-related pre-neoplasia and cancer. Indeed, several studies have shown good prospective for E5 immunotherapy, suggesting that it could be applied for the treatment of pre-cancerous lesions. Thus, insofar as the majority of cervical, oropharyngeal and anal cancers are caused by high-risk HPV (hrHPV), mainly by HPV16, the aim of this review is to discuss the immune pathways interfered by E5 oncoprotein of hrHPV highlighting the various aspects of the potential immunotherapeutic approaches.

Keywords: E5 oncoprotein; HPV immune evasion; HPV-related cancer; Human Papillomavirus; Immune response modulation; Immunotherapy.

Fig. 1

EGF-R and TGF-β signaling pathways interfered by E5. EGF-R and TGF-β share several signal transduction pathways, so that the inteference of hrHPV E5 on any of these receptors have effects on each other. TGF-β is an important immunosuppressive cytokine, which activates different transduction pathways interposed by E5 activity. In a natural condition (without any infection), TGF-β prioritizes SMAD pathway, which results in expression of several tumor suppressive proteins. In the HPV infection, SMAD pathway is hampered by HPV oncoproteins, whereas other alternative pathways are stimulated. (1) E5 is capable of increasing the EGF-R levels (by preventing the activities of c-Cbl and V-ATPase) and activating MAPK-ERK, NF-κB and PI3K pathways. (2 and 3) These pathways interact each other in an intricate regulation way. Ras protein stabilizes TGIF [75], a co-inhibitor of SMAD pathway. Moreover, ERK activation inhibits the SMAD activity through phosphorylation [77] and activates PI3K and NF-κB pathways [78]. In turn, activation of NF-κB and PI3K pathways by EGF-R cause a negative feedback on SMAD pathway. (4) E5 also inhibits TGF-βRII expression, (5) SMAD2 phosphorylation and (6) SMAD2-SMAD4 complex translocation to the nucleus [68]. (7) The activation of non-SMAD pathways leads to cell proliferation and disruption of cytokines synthesis, which stimulate tumor progression. (8) E5 can also can stimulates VEGF through EGF-R-PI3K-Akt signaling inducing angiogenesis [144]

Fig. 2

E5 immune evasion mechanisms. E5 induces COX-2 expression through: i) the activation of the epidermal growth factor receptor (EGF-R) signalling pathway and the interaction with the nuclear factor 1 (NF-1), which results in the induction of ii) AP-1 and iii) NF-κB transcription, which has the strongest effect on COX-2 transcription. E5 upregulation of EGF-R expression also leads to an increase of the vascular endothelial growth factor (VEGF), through COX-2 [48]. Following this, COX-2 and EP4 stimulate PGE2 signalling pathway through a feedback mechanism involving increased levels of cAMP, PKA and CREB (cyclic adenosine monophosphate response element binding protein), binding to the EP4 promoter [53]. This leads to a rise in the expression of this receptor [87, 88], which is associated with breast and colon carcinogenesis [145, 146]. Moreover, E5 impairs MHC I [36] and II [44] surface cell expression

Fig. 3

Interferon synthesis and signaling pathways. Interferons are crucial molecules for creating antiviral status. E5 stimulates IFN synthesis through activation of NF-κB signalling pathway and IRF-1 protein. (1) PRR/MAVS (Mitochondrial antiviral signaling) activates IKK that liberates NF-κB. (2) NF-κB, IRF-3 and ATF-2/c-Jun form a transcriptional complex that recruits the (3) CPB/P300 enhancer to IFN-β promoter. (4) This complex (along with IRF-1) binds to (5) particular DNA regions (PRDI, PRDII and PRDIV) which results in (6) IFN-β gene transcription. In IFN-independent way, (7) viral dsRNA induces (8) PKR-IRF3 signalling. The produced IFNs exert their activities in keratinocytes by (9) interacting with specific IFN-type receptor and (10) triggering JAK/STAT signalling pathways. IFN-α, IFN-β and IFN-λ interact with receptors associated with JAK1 and Tyk2 tyrosine kinases which induce the activation and dimer formation of the transcriptional factor STAT1/STAT2 by phosphorylation. This dimer forms a complex with IFN-stimulatory gene factor- 3γ (ISGF-3γ), also called IFN regulatory factor-9 (IRF-9) or P48, which bind to the ISRE sequence of DNA. In turn, IFN-γ binds to receptors associated with Jak1 and Jak2 tyrosine kinases that induce the formation of the Stat1/Stat1 homodimer. Finally, (11) the binding of transcription factors to specific responsive elements (i.e. ISRE and GAS) of DNA leads to (12) ISGs expression [99]. This IFN-induced activities create an antiviral state which leads to the destruction of infected cells with episomal HPV [90], whereas cells with integrated viral DNA can survive and transcription of E6-E7 oncogenes are no longer regulated by E2 leading to persistent infection and cancer formation [90]