High-Risk Mucosal Human Papillomavirus 16 (HPV16) E6 Protein and Cutaneous HPV5 and HPV8 E6 Proteins Employ Distinct Strategies To Interfere with Interferon Regulatory Factor 3-Mediated Beta Interferon Expression.

Persistent infection with some mucosal α-genus human papillomaviruses (HPVs; the most prevalent one being HPV16) can induce cervical carcinoma, anogenital cancers, and a subset of head and neck squamous cell carcinoma (HNSCC). Cutaneous ß-genus HPVs (such as HPV5 and HPV8) associate with skin lesions that can progress into squamous cell carcinoma with sun exposure in Epidermodysplasia verruciformis patients and immunosuppressed patients. Here, we analyzed mechanisms used by E6 proteins from the α- and ß-genus to inhibit the interferon-ß (IFNB1) response. HPV16 E6 mediates this effect by a strong direct interaction with interferon regulatory factor 3 (IRF3). The binding site of E6 was localized within a flexible linker between the DNA-binding domain and the IRF-activation domain of IRF3 containing an LxxLL motif. The crystallographic structure of the complex between HPV16 E6 and the LxxLL motif of IRF3 was solved and compared with the structure of HPV16 E6 interacting with the LxxLL motif of the ubiquitin ligase E6AP. In contrast, cutaneous HPV5 and HPV8 E6 proteins bind to the IRF3-binding domain (IBiD) of the CREB-binding protein (CBP), a key transcriptional coactivator in IRF3-mediated IFN-ß expression. IMPORTANCE Persistent HPV infections can be associated with the development of several cancers. The ability to persist depends on the ability of the virus to escape the host immune system. The type I interferon (IFN) system is the first-line antiviral defense strategy. HPVs carry early proteins that can block the activation of IFN-I. Among mucosal α-genus HPV types, the HPV16 E6 protein has a remarkable property to strongly interact with the transcription factor IRF3. Instead, cutaneous HPV5 and HPV8 E6 proteins bind to the IRF3 cofactor CBP. These results highlight the versatility of E6 proteins to interact with different cellular targets. The interaction between the HPV16 E6 protein and IRF3 might contribute to the higher prevalence of HPV16 than that of other high-risk mucosal HPV types in HPV-associated cancers.

The unfolded protein response-glutathione metabolism axis: A novel target of a cycloruthenated complexes bypassing tumor resistance mechanisms.

Platinum-based drugs remain the reference treatment for gastric cancer (GC). However, the frequency of resistance, due to mutations in TP53 or alterations in the energy and redox metabolisms, impairs the efficacy of current treatments, highlighting the need for alternative therapeutic options. Here, we show that a cycloruthenated compound targeting the redox metabolism, RDC11, induces higher cytotoxicity than oxaliplatin in GC cells and is more potent in reducing tumor growth in vivo. Detailed investigations into the mode of action of RDC11 indicated that it targets the glutathione (GSH) metabolism, which is an important drug resistance mechanism. We demonstrate that cycloruthenated complexes regulate the expression of enzymes of the transsulfuration pathway via the Unfolded Protein Response (UPR) and its effector ATF4. Furthermore, RDC11 induces the expression of SLC7A11 encoding for the cystine/glutamate antiporter xCT. These effects lead to a lower cellular GSH content and elevated oxygen reactive species production, causing the activation of a caspase-independent apoptosis. Altogether, this study provides the first evidence that cycloruthenated complexes target the GSH metabolism, neutralizing thereby a major resistance mechanism towards platinum-based chemotherapies and anticancer immune response.