Apolipoprotein E interrupts interleukin-1beta signaling in vascular smooth muscle cells

Area of Science:

• Cardiovascular Biology
• Immunology
• Molecular Signaling

Background:

• Apolipoprotein E (apoE) is known for its antiatherogenic properties, but its precise molecular mechanisms, particularly in regulating inflammatory signaling, are not fully understood.
• Interleukin-1beta (IL-1beta), a key proinflammatory cytokine, is implicated in the pathogenesis of atherosclerotic lesions.
• Understanding how apoE interacts with IL-1beta signaling pathways is crucial for developing novel therapeutic strategies against atherosclerosis.

Purpose of the Study:

• To investigate the effect of apolipoprotein E (apoE) on intracellular signaling pathways activated by interleukin-1beta (IL-1beta) in vascular smooth muscle cells (VSMCs).
• To elucidate the molecular mechanisms by which apoE modulates IL-1beta-induced inflammatory responses.
• To determine if apoE's anti-inflammatory effects are linked to specific signaling intermediates and receptors.

Main Methods:

• Vascular smooth muscle cells (VSMCs) were treated with IL-1beta, and the effects of apoE on the expression and activation of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were assessed.
• NF-kappaB and activating protein-1 (AP-1) transactivation assays were performed to evaluate the impact of apoE on transcription factor activity.
• Studies involved VSMCs overexpressing IL-1beta signaling intermediates (MyD88, IRAK1, TRAF6) and siRNA-mediated silencing of LDL receptor-related protein-1 (LRP1).
• Adenylyl cyclase and protein kinase A (PKA) activity were measured, and the effects of their modulators on IL-1beta signaling in the presence of apoE were examined.

Main Results:

• ApoE inhibited IL-1beta-induced expression and activation of iNOS and COX-2 in VSMCs.
• These inhibitory effects were associated with the suppression of NF-kappaB and AP-1 transactivation, indicating apoE acts upstream of these transcription factors.
• ApoE prevented IRAK1 phosphorylation and IRAK1-TRAF6 complex formation, but not MyD88-IRAK1 complex formation, suggesting a specific targeting of IRAK1 activation.
• The inhibitory effects of apoE were dependent on LRP1 expression and were abolished upon LRP1 silencing.
• ApoE stimulated PKA activity, and PKA inhibition restored IL-1beta signaling in apoE-treated VSMCs.
• ApoE also inhibited VSMC activation by IL-18 but not by tumor necrosis factor-alpha or polyinosinic:polycytidylic acid.

Conclusions:

• Apolipoprotein E (apoE) exerts anti-inflammatory effects in vascular smooth muscle cells by targeting the activation of IRAK-1.
• This mechanism effectively interrupts IL-1beta and IL-18 signaling pathways, representing a novel antiatherogenic activity of apoE.
• The findings highlight LRP1 as a key receptor mediating apoE's inhibitory effects on IL-1beta signaling.