NIH Research Festival
The molecular mechanisms that control transformation and tumorigenicity of human papillomavirus-associated (HPV+) head and neck squamous cell carcinomas (HNSCCs) are currently being investigated. The number of HPV+ HNSCC cases has increased sharply in recent years, specifically in tonsil, oropharyngeal, and oral cavity cancers. These HPV+ HNSCCs often respond differently to treatments. Recently, The Cancer Genome Atlas (TCGA, 2015) identified novel loss-of-function genomic alterations of TNF receptor-associated factor 3 (TRAF3) in HPV+ HNSCCs. TRAF3 is a ring-finger E3 Ubiquitin Ligase which inhibits downstream alternative NF-¿B signaling and promotes anti-viral immunity by promoting degradation of ubiquitinated proteins. To assess TRAF3’s role in HPV+ HNSCCs, we identified cell lines with lower level of TRAF3 protein, consistent with deficient TRAF3 expression identified in TCGA data. Functional studies showed that TRAF3 expression led to decreased steady-state protein levels of the alternative NF-¿B pathway components Rel B and p52, as assessed by Western blot, reporter assays and immunofluorescence. Additionally, TRAF3 expression led to decreased cell proliferation, tumorigenic activity and migration, and increased sensitivity to chemotherapy agent cisplatin. Conversely, decreased TRAF3 expression by RNAi led to the opposite effect. Interestingly, TRAF3 increases the steady state protein level of the classical tumor suppressors RB and p53 in HPV+ HNSCC cell lines as assessed by Western blot. Preliminary data suggests that this novel result is through degradation of the viral oncogenes E6/E7 encoded by the HPV genome. Further in vitro characterization of TRAF3’s function in HPV+ HNSCCs was assessed using clinically identified TRAF3 loss-of-function mutagenic isoforms (mimicking both TCGA data and HNSCC cell line data for TRAF3 loss-of-function) in vitro. These mutant forms of TRAF3 suppress wild-type TRAF3 phenotypes, including p53 induction and decreased Rel B and p52 protein levels. Further experiments will be performed to assess mutant TRAF3’s ability to reverse TRAF3’s tumor suppression phenotypes in vitro and in vivo. An inducible HPV+ HNSCC line expressing TRAF3 using the TET-ON system was generated for long-term studies of TRAF3 gain-of-function in vitro and in vivo in HNSCCs. In addition, TRAF3 Knockout HPV+ HNSCC line as well as a TRAF3 knockout Human Oral Keratinocyte (HOK) line using CRISPR-Cas9 genomic editing tools was generated for long-term studies of TRAF3 loss-of-function in vitro and in vivo. These data taken together with TRAF3’s role in inhibiting the pro-survival alternative NF-¿B signaling pathway strongly suggest that TRAF3 acts as a tumor suppressor in HPV+ HNSCC. In conclusion, the ring-finger E3 ubiquitin ligase TRAF3 serves as a critical tumor suppressor in HPV-associated HNSCCs and can serve as a potential therapeutic target for treatment of these cancers.
Scientific Focus Area: Cell Biology
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