Abstract B12: Expression of Integrated Stress Response proteins during the progression of UVB-induced squamous cell carcinoma

Abstract

Abstract The major environmental risk factor for developing non-melanoma skin cancers (NMSCs) is exposure to the ultraviolet-B (UVB) wavelengths found in sunlight. Unlike other types of cancer where the occurrence each year is precisely known, so many NMSCs occur each year in the United States that the incidence of NMSC is not accurately reported. Treatment of NMSC costs the U.S. healthcare system over 1 billion dollars annually. Therefore while NMSC is rarely lethal, the expense of NMSC treatment is the fifth highest among all cancer treatments. In response to a myriad of environmental stresses, including UVB irradiation, eukaryotic cells rapidly modulate protein synthesis. An important mechanism for translation regulation involves phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α), which results in a prompt decrease in global protein synthesis, concurrent with preferential translation of cytoprotective gene transcripts. Because a range of different stresses can activate one of four eIF2 kinases, this pathway is referred to as the Integrated Stress Response (ISR). Although the ISR pathway has been implicated in carcinogenesis in a variety of tissues, little is known about whether the ISR can modulate the development of NMSC in human skin. We previously defined the cytoprotective role of the ISR in response to UVB-irradiation of human keratinocytes in vitro. Following UVB exposure, keratinocytes lacking an ISR response have a shortened G1 arrest, a decreased ability to repair UVB-damaged DNA, a diminished senescence response, and an increased susceptibility to apoptosis. Collectively, these functions attributed to the ISR response in keratinocytes following UVB-irradiation are tumor suppressive. To address how the ISR is involved during photocarcinogenesis, we analyzed the expression of ISR proteins (eIF2α-phosphorylation, ATF4, CHOP, GADD34) in the progression of a subtype of NMSC, squamous cell carcinoma (SCC). The progression ranges from normal skin, to a precursor lesion of SCC (actinic keratosis, AK), and finally to SCC tumors. ISR marker proteins were detected using immunofluorescence on formalin-fixed, paraffin-embedded tissue from at least four distinct specimens in each category. In normal skin, expression of ISR proteins were found in differentiating layers of the epidermis. eIF2α phosphorylation was predominantly observed in both the nucleus and cytoplasm of keratinocytes in the squamous and granular layers of the skin. Similarly, GADD34, ATF4, and CHOP were expressed in normal differentiating keratinocytes. In contrast, using a human skin model of AK, ATF4 expression was greatly increased in all but the basal cell layer of the epidermis. CHOP and GADD34 expression, as well as eIF2α phosphorylation, in human AKs followed a similar pattern. Progression to SCC resulted in a striking decrease in ISR protein expression. ATF4 and CHOP expression were absent, no eIF2α phosphorylation was detected, and GADD34 expression was greatly diminished. Therefore, the standard ISR response in differentiating keratinocytes is enhanced in AK lesions but silenced as the tumor progresses to SCC. These data suggest that sustaining the ISR response in UVB-damaged epidermis could be a valuable therapeutic target for the prevention of SCC development. Citation Format: Dan F. Spandau, Ann Collier, Ron Wek. Expression of Integrated Stress Response proteins during the progression of UVB-induced squamous cell carcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B12.