Study of TM4SF19 in GABP-dependent YAP transcriptional regulation under oxidative stress in head and neck cancer

Abstract

Head-and-neck squamous cell carcinoma (HNSCC), the sixth leading cancer worldwide, shows high levels of YAP expression compared to other human cancers. YAP is a transcriptional regulator that plays a critical role in cancer cell survival and proliferation. However, the molecular mechanisms explaining upregulation of YAP1 expression are poorly understood. Excessive consumption of tobacco and alcohol are key risk factors of human papillomavirus-negative (HPV$^-$) HNSCC. These substances generate metabolites that increase reactive oxygen species (ROS) to abnormally high levels. It is unclear whether YAP1 expression and function are linked to the oxidative stress response in HPV$^-$ HNSCC. Here, I aimed to elucidate the mechanism of YAP1 upregulation involved in progression of HPV$^-$ HNSCC under oxidative stress condition. I identified TM4SF19 as a candidate for mediating transcriptional regulation of YAP in HPV$^-$ HNSCC, based on the analysis of previous whole-genome siRNA library screening and TCGA genomics data. TM4SF19 is one of the transmembrane 4 L six family proteins that have topological similarities to tetraspanins. Little is known about its biological function. I found that TM4SF19 controls the expression of YAP1 gene by regulating GABPβ1, a subunit of the GABP transcription factor complex that binds and activates the YAP1 gene promoter. I observed that the depletion of TM4SF19 reduces GABPβ1, thereby decreasing the transcriptional activity of GABP complex. Importantly, TM4SF19, which is localized to the ER membrane, undergoes dimerization and inversion in response to oxidative stress and consequently mediates the upregulation of YAP1 transcription by ROS elevation. I further found that TM4SF19 knockdown reduces key characteristics of malignant cells, such as active proliferation and migration. These results suggest that the novel transcriptional regulator TM4SF19 is a promising therapeutic target to suppress the oncogenic activity of YAP and also provide a new clue to understand HPV$^-$ HNSCC tumorigenesis involving YAP and oxidative stress.