Aim: Serum amyloid A (SAA) is an important mammalian acute reactant. Here, we aim to investigate the effect of SAA on apoptosis and its mechanism of action in human amniotic WISH cells.
Methods: The expression of formyl peptide receptor (FPRL1), which is reported as a SAA receptor, was tested using RT-PCR and ligand binding assay with radio-labeled FPRL1 ligand. The effect of SAA on proliferating cell population was evaluated by thymidine incorporation assay. The protein phosphorylation levels and caspase-3 activity were detected by Western blot assay.
Results: SAA inhibits thymidine incorporation in human amniotic WISH cells. A SAA-induced decrease of proliferating cell population was accompanied with nuclear condensation and caspase-3 activation in WISH cells, suggesting that SAA induces WISH cell apoptosis. Since FPRL1 has been reported as a SAA receptor, we investigated the effects of several FRPL1 agonists on a proliferating cell population in WISH cells. Among the tested FPRL1 agonists, only SAA induced a decrease of proliferating cell population in WISH cells. On the downstream signaling of SAA, we found that SAA stimulated extracellular signal-regulated kinase and p38 kinase, which were not inhibited by pertussis toxin (PTX), ruling out the role of PTX-sensitive G-proteins. Furthermore a SAAinduced decrease of proliferating cell population was not affected by PTX, suggesting that SAA inhibits WISH cell apoptosis in a PTX-sensitive G-proteinindependent manner. A SAA-induced decrease of a proliferating cell population was completely blocked by PD98059 and SB203580, suggesting that mitogenactivated protein kinase activities are essentially required for the process.
Conclusion: SAA is a novel inducer for WISH cell apoptosis, and the PTX-insensitive pathway is involved in the process.