Abstract
In order to develop cancer treatments, it is essential to study the molecular regulation of cancer cells. One possible gene of recent interest that could be used as a cancer drug target is Testis Expressed 19 (TEX19), which is a cancer-testis antigen (CTA) gene found to be expressed in healthy individuals in human germline cells and stem cells. CTA genes are known to be activated in cancer cells, which allows them to be used as biomarkers for cancer and possible therapeutic targets. Some CTA genes have also been found to be associated with stemness, although it is so far unclear what role they play in stem cell biology.
In this study, we have verified that TEX19 is expressed in various cancer cells and normal testes tissues, which matches with its identification as a CTA gene, and we also found TEX19 is expressed in human embryonic cells (hESCs). We also found that TEX19 is localised to both the nucleus and the cytoplasm in a number of cancer cell lines, but is mostly nuclear in hESCs and the cancer stem-like cell line NTERA2, suggesting a specific nuclear function in stemness control.
Murine Tex19.1 is known to modulate both meiotic recombination and inhibition of the mobility of transposable elements, including LINE-1 (L1) in the germline and placenta, where it regulates transposition via interaction with the E3 ubiquitin ligase Ubr2, and it is postulated that Tex19.1 interacts with both Ubr2 and L1-Orf1 protein to stimulate the polyubiquitination of L1-Orf1 driving proteasome-mediated degradation and thus inhibiting L1 transposition, for which L1-Orf1 protein is required. Here we employ siRNAs to deplete TEX19/UBR2 levels in a range of cancer cell lines and stem cells to assess functional roles of human TEX19. We use various protein detection and interaction protocols to explore the relationship between TEX19, histones, L1-ORF1 and UBR2. Depletion of TEX19 in cancer cells in vitro results in: reduced cell proliferation, reduction of acetylated histone 3 lysine 9, reduction of LINE1-ORF1 protein and in addition to this, TEX19 and UBR2 interact both physically and mechanistically.
Collectively, the data presented lead to the postulate that TEX19 is essential for cancer cell proliferation, so can serve as an intervention point, TEX19/UBR2 exhibit a stem/germline-like interaction in cancer cells and stem cell, TEX19/UBR2 is required for the stability of LINE1-ORF1 protein in cancer cells and TEX19/UBR2 have a uniform role in controlling histone H3K9 acetylation, but possess plasticity of other functions.