S100A6 Overexpression Is Associated with Poor Prognosis and Is Epigenetically Up-Regulated in Gastric Cancer

S100A6 has been implicated in a variety of biological functions as well as tumorigenesis. In this study, we investigated the expression status of S100A6 in relation to the clinicopathological features and prognosis of patients with gastric cancer and further explored a possible association of its expression with epigenetic regulation. S100A6 expression was remarkably increased in 67.5% of gastric cancer tissues as compared with matched noncancerous tissues. Statistical analysis demonstrated a clear correlation between high S100A6 expression and various clinicopathological features, such as depth of wall invasion, positive lymph node involvement, liver metastasis, vascular invasion, and tumor-node metastasis stage (P < 0.05 in all cases), as well as revealed that S100A6 is an independent prognostic predictor (P = 0.026) significantly related to poor prognosis (P = 0.0004). Further exploration found an inverse relationship between S100A6 expression and the methylation status of the seventh and eighth CpG sites in the promoter/first exon and the second to fifth sites in the second exon/second intron. In addition, the level of histone H3 acetylation was found to be significantly higher in S100A6-expressing cancer cells. After 5-azacytidine or trichostatin A treatment, S100A6 expression was clearly increased in S100A6 low-expressing cells. In conclusion, our results suggested that S100A6 plays an important role in the progression of gastric cancer, affecting patient prognosis, and is up-regulated by epigenetic regulation.S100A6, also known as calcyclin, is a low-molecular-weight acidic protein containing 2 EF-hand calcium-binding motifs.^1,2 It was discovered on the basis of its cell cycle-dependent expression and is preferentially expressed in G₁ phase of the cell cycle after mitogenic stimuli.^3,4 S100A6 is a member of the S100 family, which are found localized to the cytoplasm and/or nucleus in a wide range of cell types.⁵ S100A6 may interact with binding or target proteins, thereby regulating dynamics of cytoskeleton constituents, cell growth and differentiation, and calcium homeostasis.^5,6,7,8,9,10Subsequent studies showed that S100A6 may also be involved in the regulation of cancer progression.¹¹ The deregulation of S100A6 expression during malignant transformation has thus far been described in human pancreatic cancer, malignant thyroid neoplasms, malignant melanoma, breast cancer, hepatocellular carcinoma, lung cancer, prostate cancer, and colorectal carcinoma.^{12,13,14,15,16,17,18,19,20,21} S100A6 overexpression has been reported to link with metastasis in colon cancer^14,15 and is a well-established marker of melanoma in which its level correlates with tumor invasiveness and poor prognosis. Although the high expression of S100A6 was reported in gastric cancer, its correlation with patient prognosis and clinicopathological features has not been fully investigated.²² Like other S100 proteins, S100A6 may promote cancer progression through specific roles in cell survival and apoptotic pathways,⁶ however the exact mechanism is unclear.In this study, we performed a detailed analysis of S100A6 expression in primary gastric cancer, matched metastatic lymph nodes, and liver metastatic nodules. Then we analyzed the relationship between S100A6 overexpression and clinicopathological features and patients prognosis. And to gain insight into the mechanism of the regulation of S100A6 expression in gastric cancer, we examined DNA methylation and histone modifications along the S100A6 gene, which may affect S100A6 expression in cancer cell lines by previous reports.^23,24