IL-6 regulates the bone metabolism and inflammatory microenvironment in aging mice by inhibiting Setd7
Graphical abstract
In the normal microenvironment of bone marrow, β-catenin accumulates in the cytoplasm of BMSCs. Then the β-catenin transfers into the nucleus and increases the expression of Setd7. Setd7 further promotes the osteogenic differentiation of BMSCs and inhibits the expression of IL-1α and TNF-α. However, the level of IL-6, which is considered to be a potent pro-inflammatory factor, increases in the aging microenvironment of bone marrow. With the stimulation of IL-6, the AKT pathway is activated, which further leads to the decrease of β-catenin in BMSCs. Therefore, the expression of Setd7 is reduced. On the one hand, the osteogenic differentiation of BMSCs is inhibit, on the other hand, the expression of IL-1α and TNF-α is increased, which further aggravated the inflammatory microenvironment of bone marrow.
Introduction
As an important worldwide problem, aging leads to the degeneration of multiple organs and tissues. Senescence not only causes metabolic disorders of the whole-body bone tissue (Morgan et al., 2018) but also inhibits bone tissue generation and reconstruction and accelerates bone absorption. Therefore, the need to prevent the adverse effects of aging on bone metabolism is urgent.
Dysregulation of the tissue microenvironment and the abnormal functioning of specific stem cells are significant changes in senescence (Oh et al., 2014). The generation of stress factors is increased and maintained at a high level in the aging body (Butcher and Lord, 2004). The autophagic scavenging ability of cells is decreased during aging, leading to an increase in reactive oxygen species and glycation end products (Roca et al., 2014; Salminen et al., 2012). At the same time, the aging microenvironment exhibits extensive expression of inflammatory factors and low-level, persistent, aseptic inflammation, also known as inflammatory senescence (Fougere et al., 2017). Establishment of the inflammatory microenvironment can lead to disordered self-renewal and differentiation of stem cells or even disease. IL-6 has been proved to be increased with age. Excessive production or reduced clearance of oxygen free radicals with aging stimulate IL-6 production (Milan-Mattos et al., 2019; Wei et al., 1992). IL-6 is involved in multiple age-related diseases (Bonda et al., 2019; Tyrrell et al., 2020; Wang and Shah, 2015) through P13 K/AKT, MAPK and JAK/STAT pathways (Maggio et al., 2006). Moreover, IL-6 is the primary inflammatory response factor associated with bone diseases (Singh and Newman, 2011). IL-6 at high levels can bind receptors on osteoclasts, promoting bone catabolic metabolism and increasing the bone absorption rate (Feng et al., 2017). Studies have shown a link between high level of IL-6 with hip fracture and poor outcomes after hip implantation (Saribal et al., 2019). Anti-inflammatory drugs and anti-IL-6 receptor antibodies can prevent the loss of bone structure and bone strength (Dubrovsky et al., 2018; Yoshida et al., 2018).
The inflammatory microenvironment decreased the osteogenic differentiation of bone marrow stem cells (BMSCs) in osteoporotic mice. IL-6 is one of the most important factors involved in the decreased ability of BMSCs to undergo osteogenic differentiation (Li et al., 2016). Furthermore, BMSCs also have immunomodulatory effects on the surrounding microenvironment. BMSCs have been found to produce proinflammatory cytokines through the TLR4-NF-κB pathway (Sun et al., 2018). Toll-like receptors (TLRs) play a key role in the inflammatory response, and various regulatory mechanisms control the degree and duration of TLR-induced inflammation (Foster et al., 2007). In addition, IL-6 can activate other intracellular pathways and kinases, such as the MAPK pathway and AKT pathway (Shen et al., 2017). When phosphorylated, these kinases activate multiple other proteins through a cascade of reactions.
Protein methylases have critical regulatory effects on the biological responses of cells in different microenvironments. We previously found that Setd7 mediated the boron-promoted osteogenic differentiation process in the BMSCs of osteoporotic rats (Yin et al., 2018). Setd7, a protein lysine methyltransferase in the SET family, is composed of pre-SET, SET, i-SET, post-SET and N-terminal domains (Nishioka et al., 2002). Setd7 has been found to regulate inflammation-induced cell proliferation, migration (Wu et al., 2019) and the oxidative stress response (Dang et al., 2018). However, whether Setd7 can regulate the immunomodulatory properties of BMSC remains unknown.
In this study, we demonstrated the decreased expression of Setd7 in the bone tissue of aging mice. Under IL-6 stimulation, the AKT pathway in BMSCs was activated, while β-catenin and Setd7 expression was inhibited. BMSCs in which Setd7 was inhibited showed decreased osteogenic gene expression and increased inflammatory gene expression.
Section snippets
Micro-CT analysis
The femurs of 8-week-old and 2-year-old were excised and fixed in 4 % (w/v) paraformaldehyde for 2 days. The Micro-CT system (SCANCO μCT 100, Scanco Medical AG, Switzerland) was used to evaluate the bone volume and trabecular number. The samples were scanned with an X-ray beam energy of 70 kV, beam intensity of 200 mA, and spatial resolution of 30 μm.
Histological analysis
Female C57BL/6 L mice of 8-week-old and 2-year-old were purchased from Sun Yat-sen University and used for this study with all procedures in
The expression of Setd7 was decreased in aging mice
The micro-CT results of 2-year-old mice revealed the typical osteoporosis characteristics. Less bone trabecular was observed in the images of the femur sections of 2-year-old mice compared to that of 8-week-old mice. The BV/TV and Tb.N decreased significantly in 2-year-old mice group (Fig. 1A). Immunohistochemical staining showed that Setd7 was expressed in osteocytes and BMSCs of 8-week-old mice. However, as the mice aged, the expression of Setd7 decreased significantly. Setd7-positive cells
Discussion
As the worldwide population ages, age-related osteoporosis has become a growing public health problem. In 2000, almost 9 million osteoporotic fractures (1.6 million hip fractures, 1.7 million forearm fractures, and 1.4 million clinical vertebral fractures) occurred. Morbidity is expected to increase exponentially in an aging demographic (Kiernan et al., 2017). Unlike postmenopausal osteoporosis, age-related osteoporosis damages bone tissue in both men and women. However, there are still no
Conclusion
In the present study, we discovered a novel cellular mechanism of the dual functions of Setd7 in regulating osteogenic differentiation and the immunomodulatory effects of BMSCs. The inhibition of Setd7 by IL-6 may be another target for the treatment of age-related osteoporosis.
Author contribution
Jia Xiaoshi: Conceptualization, Validation, Writing- Reviewing and Editing, Supervision and funding acquisition. Wang Jiwei: Methodology, Data curation, Writing- Original draft preparation. Chen Jianming: Validation and Software. Zhang Bin: Software. Revising manuscript content: all authors. Approving final version of manuscript: all authors.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was supported by funds from the National Natural Science Foundation of China (81901025 to Xiaoshi Jia), the China Postdoctoral Science Foundation (2019M653232 to Xiaoshi Jia), the Guangdong Basic and Applied Basic Research Foundation (2019A1515011326 to Xiaoshi Jia) and the Fundamental Research Funds for the Central Universities (19ykpy82 to Xiaoshi Jia).
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