Micro-RNAs and macrophage diversity in atherosclerosis: new players, new challenges…new opportunities for therapeutic intervention?

Evidence for constitutive bone morphogenetic protein-2 secretion by M1 macrophages: Constitutive auto/paracrine osteogenic signaling by BMP-2 in M1 macrophages

Mechanisms mediating vascular calcification recapitulate osteogenic processes encompassing bone formation and imply participation of bone related proteins such as bone morphogenetic protein-2 (BMP-2). Macrophages are amongst the cells that contribute to vascular ossification by releasing cytokines that induce an osteogenic program in vascular smooth muscle cells, and also by becoming themselves osteoclast-like cells. In inflammatory vascular disease, the macrophage population in the vascular wall is diverse, with the M1 or inflammatory, and the M2 or anti-inflammatory macrophage types being dominant. Yet, the osteogenic potential of M1 and M2 macrophages remains unknown. Prompted by recent studies from our laboratory showing that in macrophages the Transient Receptor Potential Canonical 3 (TRPC3) channel contributes to endoplasmic reticulum (ER) stress-induced apoptosis in M1, but not in M2 macrophages, and given the strong relationship between ER stress and vascular calcification, we wished to examine whether TRPC3 would play a role in the osteogenic signaling of polarized macrophages. The findings reported here indicate that a constitutive BMP-2-dependent signaling operates in M1 macrophages, which is not affected by deletion of Trpc3 and is not subject to regulation by ER stress. Our studies suggest operation of an auto/paracrine mechanism by which BMP-2 secreted by M1 macrophages maintains constitutive activation of a BMP-2 receptor/SMAD1/5 signaling axis.

Reduced Necrosis and Content of Apoptotic M1 Macrophages in Advanced Atherosclerotic Plaques of Mice With Macrophage-Specific Loss of Trpc3

In previous work we reported that ApoeKO mice transplanted with bone marrow cells deficient in the Transient Receptor Potential Canonical 3 (TRPC3) channel have reduced necrosis and number of apoptotic macrophages in advanced atherosclerotic plaques. Also, in vitro studies with polarized macrophages derived from mice with macrophage-specific loss of TRPC3 showed that M1, but not M2 macrophages, deficient in Trpc3 are less susceptible to ER stress-induced apoptosis than Trpc3 expressing cells. The questions remained (a) whether the plaque phenotype in transplanted mice resulted from a genuine effect of Trpc3 on macrophages, and (b) whether the reduced necrosis and macrophage apoptosis in plaques of these mice was a manifestation of the selective effect of TRPC3 on apoptosis of M1 macrophages previously observed in vitro. Here, we addressed these questions using Ldlr knockout (Ldlr^−/−) mice with macrophage-specific loss of Trpc3 (MacTrpc3^−/−/Ldlr^−/− → Ldlr^−/−). Compared to controls, we observed decreased plaque necrosis and number of apoptotic macrophages in MacTrpc3^−/−/Ldlr^−/− → Ldlr^−/− mice. Immunohistochemical analysis revealed a reduction in apoptotic M1, but not apoptotic M2 macrophages. These findings confirm an effect of TRPC3 on plaque necrosis and support the notion that this is likely a reflection of the reduced susceptibility of Trpc3-deficient M1 macrophages to apoptosis.