IL-1 alpha is myristoylated for monocyte surface translocation and promotes atherosclerosclerosis in mice
Authors: Maeder C , Speer T , Wirth A , Boeckel J N , Gadi I , Shahzad K , Isermann B , Freichel M , Laufs U , Gaul S 
Published: November 2023

Abstract

Background
The role of Interleukin-1 alpha (IL-1α) and beta (IL-1β) in the pathogenesis of atherosclerosis in vivo has been controversial for years. In contrast to IL-1β, which is dependent on the NLR family pyrin domain containing 3 (NLRP3) inflammasome, IL-1α can be tethered to the plasma membrane (csIL-1α) where it exerts pro-inflammatory effects. We characterised the role of IL-1α and the Nlrp3 inflammasome in a nongenetic hyperlipidemic mouse model and examined the function of csIL-1α in human monocytes.

Methods
Atherosclerosis was induced by a single injection of mutant PCSK9-AAV8 virus in wild-type (WT, referred to as PCSK9-AAV), Il1a-/-, Nlrp3-/-, and Il1b-/- mice, in combination with high-fat western diet (21% fat, 0.2% cholesterol) for 12 weeks. Plaque size and lipid content, were histologically analysed. The serum proteome was studied using a targeted multiplex PEA (Olink Proteomics). Human primary monocytes and human umbilical vein endothelial cells (HUVECs) were used to examine csIL-1α-IL1R interaction by analyzing VCAM1 expression and monocyte adhesion. LPS was used to induce IL-1α protein synthesis and membrane translocation in vitro. Myristoylation was measured via flow cytometry.

Results
Il1a-/- animals showed significantly reduced atherosclerotic lesion size (-62% vs. PCSK9-AAV) and fat accumulation within the plaque (-64% vs. PCSK9-AAV). Knockout of Nlrp3 and IL-1β did not reduce plaque size or lipid content, indicating an Nlrp3-independent mechanism of the protective effect of Il1a-/-. Circulating pro-inflammatory cytokines such as IL-1α, IL-1β, and IL-6 were downregulated in serum of Il1a-/-, Nlrp3-/-, and Il1b-/- mice compared to PCSK9-AAV mice suggesting that the development of atherosclerotic plaques is independent of circulating inflammatory cytokines at an early stage. Therefore, we investigated the role of csIL-1α on human monocytes. Stimulation with LPS showed enrichment of csIL-1α by 13.2% (p&lt;0.05) on monocytes without releasing IL-1α. Treatment of HUVECs with LPS-stimulated monocytes resulted in increased VCAM1 expression (+22.2%, p&lt;0.05) and monocyte adhesion (1.9 fold, p&lt;0.05) through a csIL-1α-IL1R interaction which was abolished by the addition of neutralising antibodies for IL1R1 and IL-1α (p&lt;0.05, vs. LPS-treated monocytes). To investigate whether the IL-1α translocation is mediated by protein myristoylation, human monocytes were pre-incubated with the n-myristoyl-transferase inhibitor IMP-1088 before LPS stimulation, resulting in a reduction of csIL-1α expression (p&lt;0.05, vs. LPS-treated monocytes).

Conclusion
This study demonstrated a protective effect of Il1a deficiency on the development of atherosclerosis. CsIL-1α mediates monocyte-to-endothelial adhesion by increasing VCAM1 expression through endothelial IL1R1 signaling. The data underscore the importance of the juxtacrine signaling of IL-1α and its role in the development of atherosclerosis independent of circulating cytokine levels.