TLR2 stimulates ABCA1 expression via PKC-η and PLD2 pathway.

ATP-binding cassette transporter A1 (ABCA1) is a membrane-bound protein that regulates cardiovascular disease including atherosclerosis by the efflux of excess cholesterol from cells and by suppression of inflammation. Using a mouse macrophage cell line Raw264.7, we studied the importance of toll-like receptor 2 (TLR2) on ABCA1 expression and the signaling pathway responsible for TLR2-mediated ABCA1 expression. Interestingly, our data demonstrated that treatment of macrophages with TLR2 agonist Pam(3)CSK(4) significantly increased ABCA1 mRNA and protein levels. We found that ABCA1 induction is myeloid differentiation primary response gene 88 (MyD88)-dependent as well as TLR2-dependent. ABCA1 induction upon Pam(3)CSK(4) is controlled by protein kinase C-η (PKC-η) and phospholipase D2 (PLD2). Furthermore, direct treatment of dioctanoyl phosphatidic acid (diC(8)PA) into cells also induced ABCA1 mRNA and protein indicating that PLD2-mediated PA involve in the TLR2-stimulated ABCA1 expression. Cumulatively, these results demonstrate for the first time that activation of PKC-η and PLD2 signaling pathway is an important mechanism for regulation of TLR2-induced ABCA1 expression.

Protein kinase C-eta and phospholipase D2 pathway regulates foam cell formation via regulator of G protein signaling 2.

Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for Galpha(q), which is involved in regulating various vascular functions. To understand how RGS2 regulates foam cell formation, the present study identified signaling pathways controlled by lipopolysaccharide (LPS) and discovered new mechanisms whereby protein kinase C (PKC)-eta and phospholipase D (PLD) 2 regulate RGS2 expression. The toll-like receptor (TLR) 4 agonist LPS caused foam cell formation of Raw264.7 macrophages and dramatically decreased RGS2 mRNA expression. RGS2 down-regulation by LPS was partially recovered by TLR4 small interfering RNA (siRNA). Peritoneal macrophages were separated from wild-type and TLR4 mutant mice, and treatment with LPS showed RGS2 expression decrease in wild-type macrophages but no change in TLR4 mutant macrophages. RGS2 overexpression was suppressed, whereas RGS2 down-regulation accelerated foam cell formation by LPS. Treatment of PKC-eta pseudosubstrate weakened foam cell formation and recovered RGS2 down-regulation by LPS. In addition, LPS or phorbol 12-myristate 13-acetate stimulated PLD activity, and the pretreatment of PLD inhibitor weakened foam cell formation and recovered RGS2 down-regulation. Inhibition of PLD2 expression by siRNA also weakened foam cell formation and partially recovered LPS-mediated RGS2 down-regulation. On the other hand, PLD2 overexpression intensified RGS2 down-regulation and foam cell formation by LPS. These results suggest that LPS causes foam cell formation by increasing PKC-eta and PLD2 activity by down-regulating RGS2 expression via TLR4 dependently.

Early growth response-1 is involved in foam cell formation and is upregulated by the TLR9-MyD88-ERK1/2 pathway.

Early growth response-1 (Egr-1), a zinc finger transcription factor, plays a key regulatory role in pathological cardiovascular processes including atherosclerosis. Here, we investigate whether Egr-1 expression and foam cell formation require toll-like receptor 9 (TLR9) and myeloid differentiation factor 88 (MyD88). CpG DNA and its related synthetic CpG oligodeoxynucleotides (CpG ODN) play an important role in immunity and inflammation. CpG ODN increased expression of Egr-1 and formation of foam cells in Raw264.7 cells or bone marrow-derived macrophages. Egr-1 siRNA decreased foam cell formation by CpG ODN compared to that of control siRNA. In addition, when TLR9 or MyD88 was knocked down, CpG ODN-induced Egr-1 expression was also attenuated. CpG ODN increased ERK1/2 phosphorylation. U0126, a MEK pathway inhibitor, suppressed activation of Egr-1 expression by CpG ODN. CpG ODN-induced expression of tissue factor (TF) and NGFI-A binding protein 2 (Nab2), and the expression of both genes is blocked by knockdown of TLR9 or MyD88 siRNA or MEK inhibition. These results suggest that CpG ODN activates the TLR9-MyD88-ERK1/2 pathway causing expression of Egr-1 and its target genes such as TF and Nab2, thus inducing foam cell formation.

The JAK2-Akt-glycogen synthase kinase-3ß signaling pathway is involved in toll-like receptor 2-induced monocyte chemoattractant protein-1 regulation.

Monocyte chemoattractant protein-1 (MCP-1) is an essential cytokine for the migration of monocytes into vessels, and is also involved in the pathogenesis of atherosclerosis. In this study, we investigated the importance of janus kinase 2 (JAK2) and the function of the Akt and glycogen synthase kinase-3ß (GSK3ß) pathway in toll-like receptor (TLR2)-mediated MCP-1 expression. The TLR2 agonist, Pam3CSK4, induced MCP-1 expression in the Raw264.7 cell line. The induction of MCP-1 was seen in the bone marrow-derived macrophages of wild-type mice but not in TLR2 knockout mice. The TLR2-mediated MCP-1 induction was myeloid differentiation primary response gene 88 (MyD88)-independent. By contrast, the inactivation of JAK2 attenuated TLR2-mediated MCP-1 expression. The JAK inhibitor suppressed the phosphorylation of GSK3ß as well as Akt by Pam3CSK4 stimulation. While the inactivation of Akt by LY294002 suppressed TLR2-mediated MCP-1 induction, the inactivation of GSK3ß by LiCl potentiated TLR2-mediated MCP-1 induction. Furthermore, Akt inhibitor suppressed TLR2-mediated phosphorylation of GSK3ß. Taken together, these results suggest that a MyD88-independent pathway exists in TLR2 signaling; the JAK2-Akt-GSK3ß pathway is a novel MyD88-independent pathway for MCP-1 induction.

RGS2 is a negative regulator of STAT3-mediated Nox1 expression.

NADPH oxidase 1 (Nox1) is essential for reactive oxygen species production in the innate immune responses mediated by toll-like receptor (TLR), but the mechanism regulating its expression remains uncertain. Here, we find that Nox1 induction is TLR2-dependent, but independent of myeloid differentiation primary response gene 88 (MyD88). We demonstrate the capacity of signal transducer and activator of transcription 3 (STAT3) to activate Nox1's transcription, as well as cooperative regulation by janus kinase 1 and 3 (JAK1 and JAK3). We find that regulator of G-protein signaling 2 (RGS2) inhibits STAT3-mediated Nox1 transcription, and can itself be repressed by TLR2; Nox1 induction upon RGS2 down-regulation is controlled by protein kinase C-η (PKC-η) and phospholipase D2 (PLD2). A GFP-tagged version of RGS2 concentrates in the nucleus; RGS2 additionally directly binds STAT3 to regulate its transcriptional activity through TLR2 stimulation. Cumulatively, these results suggest that TLR2 signaling enhances Nox1 expression through the JAK1/3-STAT3 pathway, and that RGS2, through its regulation by the PKC-η/PLD2 pathway, represses STAT3's transcriptional activation of Nox1.