Serum amyloid A 1 (SAA1) polymorphisms are associated with variation in tumor suppressive activities in esophageal squamous cell carcinoma (ESCC)

Abstract

Esophageal cancer (EC), a malignancy originated from the epithelium of the esophagus, ranks eighth in incidence rate of cancer, sixth as the most common cause of death worldwide, and causes the eighth highest mortality rate in Hong Kong cancer patients. High prevalence areas, called “esophageal cancer belt”, are located along Northern Iran through the Central Asian Republics to North-Central China. Esophageal squamous cell carcinoma (ESCC) occurs as the dominant type of EC worldwide. Using the functional complementation approach, SAA1 was identified as one of the candidate tumor suppressive genes in ESCC. SAA1 is expressed as a secretary protein and is present in most histologically normal human epithelial tissues. The three SAA1 isoforms (SAA1.1, 1.3, 1.5) with two single nucleotide polymorphisms were observed in the ESCC patients and healthy individuals. The expression of SAA1 was frequently down-regulated in ESCC cell lines. Therefore, the functional roles of the three SAA1 isoforms in tumor suppression in ESCC were investigated. The restoration of SAA1.1 and 1.3 isoforms in the SAA1-null ESCC cell lines showed suppression of tumor growth and angiogenesis. The proteins of these two isoforms could inhibit tumor angiogenesis by their strong binding affinity to block the integrin αVβ3 and the downstream activation of FAK of the vascular endothelial cells. After that, these two SAA1 variant proteins abolished the assembly of stress fiber and focal adhesions, and led to the detachment of the vascular endothelial cells and eventually resulted in cell apoptosis. In contrast, SAA1.5 had weaker binding affinity to the integrin αVβ3 of the vascular endothelial cell surface and it could only delay the cytoskeleton arrangement and cell adhesion but could not induce cell death, demonstrating that SAA1.5 is a defective in the anti-angiogenic function.

The restoration of SAA1.1 and 1.3 gene expression in the ESCC cell lines could down-regulate the integrin αV (ITGAV) expression, and subsequently resisted to the epithelial-mesenchymal transition (EMT) induced by TGF-β. On the other hand, the SAA1.5 expression could not reverse this effect induced by TGF-β. In the reverse experiments, when SAA1 was depleted in the SAA1-positive cell lines, both the expression of ITGAV and N-cadherin (CDH2) were induced, showing that ITGAV might play a vital role in EMT in ESCC. Silencing of ITGAV gene expression resisted to the TGF-β-induced EMT morphology in ESCC. At the same time, the ITGAV could positively regulate the CDH2 expression in ESCC to control the cell migration. It is likely that SAA1.1 and 1.3 might suppress the ITGAV-mediated EMT by down-regulation of CDH2. In summary, this slightly change of amino acid of the current SAA1 polymorphisms can significantly affect their function in tumor suppression, tumor angiogenesis, and EMT in ESCC.