L’efferocitosi difettiva dei fagociti presenti nelle lesioni aterosclerotiche avanzate dipende dal clivaggio del recettore Mertk da parte della metalloproteinasi ADAM17

Atherosclerosis is a chronic inflammatory disease characterized by a progressive accumulation of low density lipoprotein (LDL) into subendothelial space and it involves numerous activated cells (smooth muscle cells, macrophages and T lymphocytes). It’s responsible for life-threatening clinical events such as acute coronary syndrome and stroke, which occur when there is a plaque rupture. In the atherosclerotic plaque there is a significant degree of apoptosis of muscle cells and macrophages especially evident in the sites of breakage. Apoptosis is programmed cell death orderly and genetically controlled and apoptotic cells exhibit recognition signals (such as phosphatidylserine) for their removal and disposal. Under physiological conditions, they are quickly recognized and internalized by specialized macrophages (phagocytes or efferocytes) through a process called phagocytosis or efferocytosis. A rapid clearance leads to a suppression of proinflammatory response and so to reduced cellular component also in atherosclerotic plaques in the initial phase. Three cell receptors are involved in the recognition of apoptotic cells and they belong to the family TAM (Tyro3, Axl and Mertk) and the bridge molecule Gas6 (growth arrest-specific gene 6) is also involved. A defect at the level of these molecules may lead to defective efferocytosis, so to the lack of recognition of apoptotic cells which become secondarily necrotic and then in atherosclerotic plaques lead to expansion of the necrotic core. Studies in animals models have shown that the receptor Mertk, the TAM receptor more expressed in macrophages, in its extracellular portion is cleaved by a specific protease, ADAM17. This will form the soluble protein s-Mer that could probably compete for binding with the bridge protein Gas6 leading to efferocytosis defective. In our study, we investigate whether the efferocytosis in the lipid core of human carotid plaques may be due in part to the cleavage of the extracellular portion of the receptor Mertk by ADAM17. In fact, in ex vivo study, comparing the area surrounding the lipid core (LC) and periphery (P) of human carotid plaques only Mertk receptor and ADAM17 were significantly expressed (p<0.01). Moreover, through immunohistochemical analysis we have shown an opposite gradient of the two proteins expression from the outer edge of the fibrous cap to lipid core. In fact, closer to the lipid core positivity Mertk is decreasing, contrary ADAM17. Assuming that the area around the lipid core may be a strong inducer of ADAM17, we performed in vitro studies in macrophage-like THP-1 cells that were stimulated with plaques extract (PE). Our results show an increase of gene expression both Mertk that ADAM17, but not protein Mertk. We hypothesized that this could be due to the cleavage of the receptor by ADAM17, with the formation of soluble protein s-Mer, whose concentration is increased in the culture medium of THP-1 cells stimulated with PE (p<0.01). The addition of recombinant sMer in the culture medium lead to the formation of the complex sMer-GAS6 and so the soluble protein may inhibit the function of bridge molecule with apoptotic cells. This is supported by the dimostration that by silencing ADAM17 there is a significantly reduction of sMer in the culture medium (p<0.01). These results suggest that ADAM17, by cleaving the extracellular domain of Mertk may be one of the determinants of reduction of GAS6 availability and so lead to defective efferocytosis in the atherosclerotic human plaques.