Macrophage Polarization and Osteoporosis: A Review

Abstract

Over 200 million people suffer from osteoporosis worldwide. Individuals with osteoporosis have increased rates of bone resorption while simultaneously having impaired osteogenesis. Most current treatments for osteoporosis focus on anti-resorptive methods to prevent further bone loss. However, it is important to identify safe and cost-efficient treatments that not only inhibit bone resorption, but also stimulate anabolic mechanisms to upregulate osteogenesis. Recent data suggest that macrophage polarization may contribute to osteoblast differentiation and increased osteogenesis as well as bone mineralization. Macrophages exist in two major polarization states, classically activated macrophages (M1) and alternatively activated macrophage (M2) macrophages. The polarization state of macrophages is dependent on molecules in the microenvironment including several cytokines and chemokines. Mechanistically, M2 macrophages secrete osteogenic factors that stimulate the differentiation and activation of pre-osteoblastic cells, such as mesenchymal stem cells (MSC's), and subsequently increase bone mineralization. In this review, we cover the mechanisms by which M2 macrophages contribute to osteogenesis and postulate the hypothesis that regulating macrophage polarization states may be a potential treatment for the treatment of osteoporosis.

Keywords: BMP-2; IL-4; M1 macrophage; M2 macrophage; TNF-α; macrophage polarization; macrophages; osteoporosis; osteoporosis treatment.

Figure 1

Schematic Diagram Showing the Relationship Between M2 Cytokine Profiles, Macrophage Phenotypes, and Osteoblast and Osteoclast Activity. 1. M2 cytokines, which include interleukin-13 (IL-13), bone morphogenetic protein 2 (BMP2), transforming growth factor beta (TGF-β), and interleukin-4 (IL-4), amongst others, participate in part of a feedback loop; They stimulate M2 macrophage polarization, which upregulates the production of M2 cytokines. 2. M2 cytokine signaling on osteoclasts through their respective receptors downregulates osteoclastic genes, including receptor activator of nuclear factor kappa-Β ligand (RANK) and tartrate resistant alkaline phosphatase (TRAP), which inhibits osteoclast differentiation and activation. 3. M2 cytokine signaling on osteoblasts through their respective receptors upregulates osteogenic genes including Runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), and type 1 collagen (COL 1), which increases osteoblast differentiation and activity. 4. The collective effects of M2 cytokines on different cell types may lead to increased calcium deposition and bone mineralization.