Serum Amyloid A Stimulates PKR Expression and HMGB1 Release Possibly through TLR4/RAGE Receptors

High-Density Lipoprotein (HDL) Counter-Regulates Serum Amyloid A (SAA)-Induced sPLA2-IIE and sPLA2-V Expression in Macrophages

“…To test this possibility, we stimulated murine macrophages with SAA in the absence or presence of HDL, and measured the levels of HMGB1 release by Western blotting analysis. Consistent with our recent report [29], SAA effectively induced HMGB1 release in murine macrophage cultures (Fig 4). As predicted, HDL effectively prevented SAA-induced HMGB1 release in a dose-dependent fashion (Fig 4), supporting a previous notion that active HMGB1 release might be dependent on the prerequisite sPLA 2 secretion in innate immune cells.…”

“…Despite the high homology between these SAA isomers, their capacities in inducing late proinflammatory mediators (e.g., HMGB1) are dramatically different. We recently discovered that human SAA isomer might be specifically expressed in a subset of septic patients [29], but was capable of inducing HMGB1 release in macrophage and monocyte cultures [29]. In the present study, we provided the first evidence that human SAA also upregulated the expression and secretion of sPLA 2 -IIE and sPLA 2 -V in macrophage cultures.…”

“…These findings further support the possibility that sPLA 2 may potentiate the release of other late mediators including HMGB1 [7] and NO [6]. Notably, the SAA-induced NO production was entirely dependent on functional TLR4 signaling [15,29], because it was almost completely abolished in TLR4-deficient macrophages. In agreement with the notion that NO serves as a signaling molecule for autophagy induction [38], we found for the first time, that SAA effectively induced LC3-II production and aggregation—two markers of autophagy.…”

“…Interestingly, these HDL-inhibitable cytokines/chemokines may be similarly dependent on functional TLR4 signaling, since TLR4 disruption resulted in a marked reduction of SAA-induced secretion of IL-6, MCP-1 and RANTES [29]. In a sharp contrast, HDL did not markedly inhibit SAA-induced secretion of LIX, MIP-2 or G-CSF, which might depend on other receptor-dependent signaling pathways.…”

“…SC-16755) was obtained from Santa Cruz Biotechnology. TLR2, TLR4 and RAGE KO, and TLR2/RAGE and TLR4/RAGE-double KO mice on a C57BL/6 genetic background were maintained at The Feinstein Institute for Medical Research as previously described [29]. Because the KO mice were derived from C57BL/6 mice, small colonies of wild-type C57BL/6 (Jackson Laboratory) were maintained under the same conditions.…”