14-3-3 protein sigma isoform, also known as stratifin or human mammary epithelial marker (HME) 1, has been most directly linked to tumor development. In humans, it is expressed by the SFN gene, strictly in stratified squamous epithelial cells in response to DNA damage where it is transcriptionally induced in a p53-dependent manner, subsequently causing cell-cycle arrest at the G2/M checkpoint. Up-regulation and down-regulation of 14-3-3 sigma expression have both been described in tumors. For example, in human breast cancer, 14-3-3 sigma is predominantly down-regulated by CpG methylation, acting as both a tumor suppressor and a prognostic indicator, while in human scirrhous-type gastric carcinoma (SGC), it is up-regulated and may play an important role in SGC carcinogenesis and progression. Loss of 14-3-3 sigma expression sensitizes tumor cells to treatment with conventional cytostatic drugs, making this protein an attractive therapeutic target. 14-3-3 domains are an essential part of 14-3-3 proteins, a ubiquitous class of regulatory, phosphoserine/threonine-binding proteins found in all eukaryotic cells, including yeast, protozoa and mammalian cells.
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