Vinh Nguyen

Introduction: Keratinocyte carcinoma, comprised of cutaneous squamous cell carcinoma (cSCC) and basal cell carcinoma (BCC), are the most frequently diagnosed cancers in the Western world.¹ Ultraviolet (UV) radiation is the overwhelming causative environmental carcinogen in keratinocyte carcinoma.¹ Histopathology is the gold standard for the diagnosis of cSCC, and surgery remains the mainstay of treatment for invasive keratinocyte carcinoma.² Beta human papillomavirus (βHPV) DNA may play a role at the beginning of the tumorigenic process, facilitating the accumulation of UV-induced DNA mutations.³ The early region of the genome contains viral oncogenes E6 and E7.⁴ These oncoproteins target tumor-suppressor proteins for proteasomal degradation, which leads to suppression of apoptosis and promotion of cellular proliferation.⁴ This gives HPV its carcinogenic potential.⁴ Once UV-induced DNA damage becomes irreversible, the infected cell can progress towards transformation, and the expression of E6 and E7 is dispensable for the maintenance of the malignant phenotype.⁵ This behavior has been described as “hit-and-run.”^3,5 This paper aims to explore the mechanisms behind this behavior and potential therapeutic targets. Methods: PHKs expressing human telomerase reverse transcriptase gene (hTERT) were transduced with a recombinant pLXSN retroviral vector encoding HPV38 E6 and E7.⁶ These hTERT PHKs were then UVB irradiated and compared with hTERT PHKs transduced with an empty control vector that was also UVB irradiated.⁶ Total RNA was extracted, and TLR9 and viral mRNA levels were analyzed by RT-qPCR.⁶ In a second experiment, in order to examine ITGA1, PHKs were transduced with pLXSN HPV38 oncoprotein E6 and E7 or the empty retroviral vector pLXSN.⁷ mRNA levels were measured by RT-qPCR.⁷ In a third experiment, K14-HPV38 E6/E7^Tg mice were crossed with Pycard^KO mice (Pycard encodes ASC).⁸ Littermates not crossed with Pycard^KO were used as controls.⁸  Results: TLR9 expression levels and TLR9 protein levels in HPV38 E6 and E7 hTERT PHKs were lower compared to the control cells.⁶ ChIP experiments showed that E6 and E7 significantly affected p53 recruitment to the TLR9 promoter.⁶ This indicates that E6 and E7 can prevent UV-mediated activation of TLR9 expression.⁶ In the second experiment, the expression levels of ITGA1, a repressor of epidermal growth factor receptor signaling, were significantly downregulated by the oncoproteins.⁷ In the third experiment, the K14-HPV38 E6/E7^Tg mice crossed with Pycard^KO mice developed skin tumors significantly quicker than the littermate controls, which provides evidence that ASC provides a protective advantage against UV-induced skin carcinogenesis.⁸ Conclusion:  Together, these studies provide evidence that TLR9, ITGA1, and ASC can be potential therapeutic targets in preventing UV-induced skin carcinogenesis associated with HPV38 infection.

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