Abstract LB111: Epigenetic mechanism underlying pathogenesis of skin photo-damage and actinic keratoses
Authors: Kashyap Mahendra P. , Sinha Rajesh , Shafi Rubina , Atigadda Venkatram R. , Elmets Craig A. , Athar Mohammad 
Published: April 2023

Abstract
Chronic skin exposure to solar ultraviolet B (UVB) radiation causes cutaneous inflammation, sun-burns and pre-malignant lesions known as Actinic Keratosis (AK). About 20% of AKs may progress to highly invasive malignant lesions known as Squamous Cell Carcinoma (SCC). These lesions carry multiple mutations in oncogenes and/or tumor suppressor genes. However, the molecular mechanism underlying AK is not incompletely defined. Here, we demonstrate that cutaneous UVB exposure leads to activation of bromodomain-4 (BRD4), an epigenetic modifier. We observed enhanced expression of BRD4 and its hyper-acetylation marks, H3K18ac and H3K27ac in AK, accompanied by elevated transcription of BRD4-target genes: MMP13, CXCL9, IL33, Apol19b, IL19 and Oasl2. Using confocal microscopy, we observed enhanced nuclear accumulation of BRD4 and H3K27ac in the sun-exposed lesional skin, AK and in SCC tissues from patients. Ptch+/−/SKH1 mice irradiated with to chronic UVB manifest similar molecular alterations in highly photo-damaged skin. By manipulating BRD4 expression in human skin SCC A431 cells, we identified two important BRD4-regulated signaling pathways, which may be involved in the molecular pathogenesis of AKs and SCCs. BRD4-mediated inflammatory cytokine signaling is regulated by EZH2-SOCS3-NFkB/STAT3 axis while Myc-Survivin-CDK4-E2F1 axis drives tumor cell hyper-proliferation. Co-localization of BRD4/H3K27ac and Survivin-E2F1 was contiguous from sun-burn skin to AK and SCC. We observed enhanced copy number of Myc in human AK lesional skin and associated SCCs. We also show that Myc-Survivin-CDK4-E2F1 axis is critical in driving the progression of tumor keratinocytes to highly invasive and fast growing SCCs. This signaling acts by enhancing the expression epithelial-mesenchymal transition transcription factors and other proteins involved in UVB-induced SCC development. 9cUAB30, a RXR agonist developed by us blocked BRD4 activation and its downstream signaling in A431 and SCC13 cells. Oral administration of 9cUAB30 abrogates UVB-induced inflammation and keratinocyte hyper-proliferation in murine skin in BRD4 dependent manner. 9cUAB30 treatment attenuates growth of human SCC xenograft tumors in highly immune-suppressed mice, a response which was significantly more profound when the drug was administered with a survivin inhibitor, LQZ-7I. These translational studies demonstrate that UVB-induced cutaneous BRD4 drives AK pathogenesis and their progression to invasive SCCs.
Citation Format: Mahendra P. Kashyap, Rajesh Sinha, Rubina Shafi, Venkatram R. Atigadda, Craig A. Elmets, Mohammad Athar. Epigenetic mechanism underlying pathogenesis of skin photo-damage and actinic keratoses [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB111.