The functional role(s) of SAA1 polymorphisms in integrin-focal adhesion kinase-mediated cell adhesion and tumor metastasis

Our previous functional complementation approach and the subsequent cDNA microarray analysis have identified that Serum Amyloid A 1 (SAA1) is a nasopharyngeal carcinoma (NPC) candidate tumor suppressor gene. Although SAA is well-known as an acute phase protein, which is produced in the liver in response to a number of stimuli, our previous study has discovered its constitutive expression in the nasopharyngeal epithelium which contributes to novel tumor suppressive and anti-angiogenic functions. We clearly showed that the SAA1 proteins can directly function as endogenous angiogenesis inhibitors with adhesion-blocking activities. SAA1 has five polymorphic coding alleles and their gene products differ by a few amino acids. Only three SAA1 isoforms (SAA1.1, 1.3, and 1.5) were observed in both Hong Kong NPC patients and healthy individuals. Interestingly, the variant SAA1.5 has lost this novel anti- angiogenic function. The disproportionate frequency of the homozygous SAA1.5/1.5 genotype in the NPC patients indicates that this SAA1 genotype is likely a recessive susceptibility gene, which contributes to an increased NPC risk. Our preliminary results showed that the loss of SAA1 staining in the metastatic NPC tissues is associated with tumor progression. However, the functional roles of the three SAA1 variants in NPC metastasis are not yet studied. We first want to examine whether the reconstitution of the three SAA1 isoforms in the SAA1-null cell lines will affect the metastatic potential of these cell lines. Their metastatic potential will be examined by an in vivo lung metastatic model. In vascular endothelial cells, we have demonstrated that the two functional SAA1.1 and 1.3 isoforms could inhibit the assembly of focal adhesions by directly blocking the integrin V3-focal adhesion kinase (FAK) signaling pathway. Cell-matrix adhesion is one of the critical steps for the disseminated tumor cells to enter the lung in the in vivo metastasis model. We want to investigate whether the SAA1.1 and 1.3 isoforms can still suppress the NPC cell adhesion by antagonizing the integrin-FAK signaling pathway. We also plan to use an experimental animal model to assay the in vivo extravasation to compare NPC cell lines expressing the three SAA1 isoforms. As an alternative strategy, we also plan to use mass spectrometry to identify other SAA1-binding partner(s) which is/are responsible for the SAA1 protein(s) to antagonize the NPC cell adhesion and metastasis. Finally, we will summarize whether the in vivo extravasation assay results are in concordance with the in vitro cell adhesion and the in vivo metastasis assays and to determine whether these phenotypes are dependent or independent events and whether the integrin-FAK signaling axis plays a central role in these related events. In addition, the importance of the integrin-FAK signaling pathway in NPC progression will be implicated by this proposed study. Although integrins are well-known cancer-related proteins, their contributions to NPC progression are not yet tested. The results of integrin-FAK would shed some light on the therapeutic use of the integrin antagonists and FAK inhibitors in treating NPC metastasis. The SAA protein is also expressed in the normal epithelia of various tissues, so SAA1 and its genotypes may be involved in tumor development/progression of other cancer types of epithelial origin. Taken together, this proposed study of SAA1 polymorphisms in NPC progression will open up different research areas in the near future