Secretory Phospholipase A2, Group IIA Is a Novel Serum Amyloid A Target Gene ACTIVATION OF SMOOTH MUSCLE CELL EXPRESSION BY AN INTERLEUKIN-1 RECEPTOR-INDEPENDENT MECHANISM

Atherosclerosis is a multifactorial vascular disease characterized by formation of inflammatory lesions. Elevated circulating acute phase proteins indicate disease risk. Serum amyloid A (SAA) is one such marker but its function remains unclear. To determine the role of SAA on aortic smooth muscle cell gene expression, a preliminary screen of a number of genes was performed and a strong up-regulation of expression of secretory phospholipase A(2), group IIA (sPLA(2)) was identified. The SAA-induced increase in sPLA(2) was validated by real time PCR, Western blot analysis, and enzyme activity assays. Demonstrating that SAA increased expression of sPLA(2) heteronuclear RNA and that inhibiting transcription eliminated the effect of SAA on sPLA(2) mRNA suggested that the increase was transcriptional. Transient transfections and electrophoretic mobility shift assays identified CAAT enhancer-binding protein (C/EBP) and nuclear factor kappa B (NF kappa B) as key regulatory sites mediating the induction of sPLA2. Moreover, SAA activated the inhibitor of NF-kappa B kinase (IKK) in cultured smooth muscle cells. Previous reports showed that interleukin (IL)-1 beta up-regulates Pla2g2a gene transcription via C/EBP beta and NF kappa B. Interestingly, SAA activated smooth muscle cell IL-1 beta mRNA expression, however, blocking IL-1 receptors had no effect on SAA-mediated activation of sPLA(2) expression. Thus, the observed changes in sPLA(2) expression were not secondary to SAA-induced IL-1 receptor activation. The association of SAA with high density lipoprotein abrogated the SAA-induced increase in sPLA(2) expression. These data suggest that during atherogenesis, SAA can amplify the involvement of smooth muscle cells in vascular inflammation and that this can lead to deposition of sPLA(2) and subsequent local changes in lipid homeostasis.