Functional pathways associated with human carotid atheroma: a proteomics analysis

The traditional view of atherosclerosis as a lipid storage disease crumbles in the face of extensive and growing evidence that inflammation participates centrally in all stages of this disease, from the initial lesion to the end-stage thrombotic complications. Investigators now appreciate that narrowing arteries do not necessarily presage myocardial infarction and that simply treating narrowed blood vessels does not prolong life. Although invasive approaches such as angioplasty and coronary artery bypass will remain necessary in some cases, we now understand that at least some of the cardiovascular benefits attributable to medical treatment and lifestyle modification (diet and physical activity) may result from reductions in inflammatory processes.

In response to microenvironmental signals, macrophages undergo different activation, including the "classic" proinflammatory phenotype (also called M1), the "alternative" activation induced by the IL-4/IL-13 trigger, and the related but distinct heterogeneous M2 polarization associated with the anti-inflammatory profile. The latter is induced by several stimuli, including IL-10 and TGF-β. Macrophage-polarized activation has profound effects on immune and inflammatory responses and in tumor biology, but information on the underlying molecular pathways is scarce. In the present study, we report that alternative polarization of macrophages requires the transcription factor c-MYC. In macrophages, IL-4 and different stimuli sustaining M2-like polarization induce c-MYC expression and its translocation to the nucleus. c-MYC controls the induction of a subset (45%) of genes associated with alternative activation. ChIP assays indicate that c-MYC directly regulates some genes associated with alternative activation, including SCARB1, ALOX15, and MRC1, whereas others, including CD209, are indirectly regulated by c-MYC. c-MYC up-regulates the IL-4 signaling mediators signal transducer and activator of transcription-6 and peroxisome proliferator-activated receptorγ, is also expressed in tumor-associated macrophages, and its inhibition blocks the expression of protumoral genes including VEGF, MMP9, HIF-1α, and TGF-β. We conclude that c-MYC is a key player in alternative macrophage activation, and is therefore a potential therapeutic target in pathologies related to these cells, including tumors.