Modulation of Osteoblastic Cell Efferocytosis by Bone Marrow Macrophages

• Published in: Journal of cellular biochemistry 2016-12, Vol.117 (12), p.2697-2706
• Main Authors: Michalski, Megan N., Koh, Amy J., Weidner, Savannah, Roca, Hernan, McCauley, Laurie K.
• Format: Article
• Language: English
• Subjects: Animals; APOPTOSIS; BONE BIOLOGY; Bone Marrow - metabolism; Cell Proliferation; Cells, Cultured; Chemokine CCL2 - genetics; Chemokine CCL2 - metabolism; CYTOKINES; EFFEROCYTOSIS; Humans; Interleukin-10 - genetics; Interleukin-10 - metabolism; MACROPHAGES; Macrophages - cytology; Macrophages - metabolism; Mice; Mice, Inbred C57BL; OSTEOBLASTS; Osteoblasts - metabolism; Osteoblasts - pathology; Phagocytosis - physiology; Phosphorylation; Real-Time Polymerase Chain Reaction; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger - genetics; STAT3 Transcription Factor - genetics; STAT3 Transcription Factor - metabolism; Transforming Growth Factor beta - genetics; Transforming Growth Factor beta - metabolism
• ISSN: 0730-2312; 1097-4644
• DOI: 10.1002/jcb.25567

ABSTRACT Apoptosis occurs at an extraordinary rate in the human body and the effective clearance of dead cells (efferocytosis) is necessary to maintain homeostasis and promote healing, yet the contribution and impact of this process in bone is unclear. Bone formation requires that bone marrow stromal cells (BMSCs) differentiate into osteoblasts which direct matrix formation and either become osteocytes, bone lining cells, or undergo apoptosis. A series of experiments were performed to identify the regulators and consequences of macrophage efferocytosis of apoptotic BMSCs (apBMSCs). Bone marrow derived macrophages treated with the anti‐inflammatory cytokine interleukin‐10 (IL‐10) exhibited increased efferocytosis of apBMSCs compared to vehicle treated macrophages. Additionally, IL‐10 increased anti‐inflammatory M2‐like macrophages (CD206+), and further enhanced efferocytosis within the CD206+ population. Stattic, an inhibitor of STAT3 phosphorylation, reduced the IL‐10‐mediated shift in M2 macrophage polarization and diminished IL‐10‐directed efferocytosis of apBMSCs by macrophages implicating the STAT3 signaling pathway. Cell culture supernatants and RNA from macrophages co‐cultured with apoptotic bone cells showed increased secretion of monocyte chemotactic protein 1/chemokine (C‐C motif) ligand 2 (MCP‐1/CCL2) and transforming growth factor beta 1 (TGF‐β1) and increased ccl2 gene expression. In conclusion, IL‐10 increases M2 macrophage polarization and enhances macrophage‐mediated engulfment of apBMSCs in a STAT3 phosphorylation‐dependent manner. After engulfment of apoptotic bone cells, macrophages secrete TGF‐β1 and MCP‐1/CCL2, factors which fuel the remodeling process. A better understanding of the role of macrophage efferocytosis as it relates to normal and abnormal bone turnover will provide vital information for future therapeutic approaches to treat bone related diseases. J. Cell. Biochem. 117: 2697–2706, 2016. © 2016 Wiley Periodicals, Inc. The process of efferocytosis (clearance of apoptotic cells) has been characterized in various tissues but the role of efferocytosis in the bone microenvironment is unclear. Bone marrow macrophage efferocytosis of apoptotic osteoblastic cells was enhanced by interleukin‐10 in a STAT‐3 dependent manner and resulted in increased production of TGF‐β1 and CCL‐2. The process of efferocytosis is likely important in bone remodeling and osseous wound healing.