Abstract 4647: Keratinocyte specific deletion of TWIST1 promotes differentiation and inhibits UVB-induced hyperproliferation
Authors: Eguiarte-Solomon Fernando , Rho Okkyung , DiGiovanni John 
Published: August 2019

Abstract
Non-melanoma skin cancers (NMSC) have one of the highest incidences in the United States with over 5 million diagnoses every year. The major risk factor for the development of NMSC is solar ultraviolet radiation consisting of both UVA and UVB. In epidermal keratinocytes, UV radiation results in DNA damage as well as deregulation of both proliferative and survival signaling pathways that contribute to tumorigenesis. The transcription factor TWIST1 has been reported to be essential for the formation and invasiveness of chemically-induced tumors in skin. TWIST1 deletion in mouse epidermis results in arrested cell cycle progression in response to treatment with the tumor promoter, TPA, which in turn prevents skin tumor growth in two-stage skin carcinogenesis assays. However, the impact of the keratinocyte specific TWIST1 deletion on skin carcinogenesis caused by UV radiation has not been clarified. In preliminary experiments, we found that primary mouse keratinocytes with TWIST1 knockout (KO) displayed an increase in the G2/M phase population after a single exposure to 20mJ/cm2 of UVB radiation. Accordingly, protein levels of cell cycle regulator p21 increased while those of pMDM2 decreased after UVB exposure. Moreover, we found that primary mouse keratinocytes with TWIST1 KO displayed an increase in apoptosis as shown by augmented Annexin V staining that corresponded with the reduction in levels of anti-apoptotic regulators Bcl-2 and Bcl-XL and concomitant upregulation of cleaved PARP and cleaved caspase-7 after UVB exposure. Furthermore, deletion of TWIST1 in skin keratinocytes in vivo (using K5-Cre x TWIST1flox/flox mice) led to a significant reduction in UVB-induced epidermal hyperproliferation compared to wild-type mice assessed by both Ki67 immunofluorescence staining as well as BrdU incorporation. Additionally, the protein level for p21 was increased while the levels of c-myc, and Cyclin B1 were reduced in the TWIST1 deficient keratinocytes in vivo. Interestingly, expression of differentiation markers such as Loricrin, Keratin-10, Sca-1, and transglutaminase-1 were also increased in the epidermis of TWIST1 deficient mice. These results from the in vivo model show that TWIST1 deletion results in an interruption of UVB-induced hyperproliferation via cell cycle arrest and induction of keratinocyte differentiation. Collectively, our findings indicate that the deficiency of TWIST1 has significant effects on the proliferation, survival and differentiation of keratinocytes; processes important for UVB skin tumor development. Future experiments will continue to assess the changes in oncogenic signaling in epidermal keratinocytes in response to UVB exposure and further investigate the impact of the deletion of TWIST1 on UV-induced skin carcinogenesis. Research supported by CPRIT 2018 Grant RP150247 and by CONACYT Fellowship 671661.
Citation Format: Fernando Eguiarte-Solomon, Okkyung Rho, John DiGiovanni. Keratinocyte specific deletion of TWIST1 promotes differentiation and inhibits UVB-induced hyperproliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4647.