Department of Orthopedic Surgery, Taoyuan General Hospital, Ministry of Health and Welfare, Taoyuan, Taiwan.
Department of Orthopedics, Tri-Service General Hospital, National Defense Medical Center, Taipei City, Taiwan.
J Cell Biochem. 2022 Feb;123(2):275-288. doi: 10.1002/jcb.30165. Epub 2021 Oct 19.
Systemic and intracellular metabolic states are critical factors affecting immune cell functions. The metabolic regulator AMP-activated protein kinase (AMPK) senses AMP levels and mediates cellular responses to energy-restrained conditions. The ubiquitously expressed AMPK participates in various biological functions in numerous cell types, including innate immune cell macrophages and osteoclasts, which are their specialized derivatives in bone tissues. Previous studies have demonstrated that the activation of AMPK promotes macrophage polarization toward anti-inflammatory M2 status. Additionally, AMPK acts as a negative regulator of osteoclastogenesis, and upregulation of AMPK disrupts the differentiation of osteoclasts. However, the regulation and roles of AMPK in differentiated osteoclasts have not been characterized. Here, we report that inflammatory stimuli-regulated-AMPK activation of differentiated and undifferentiated osteoclasts in opposite ways. Lipopolysaccharide (LPS) inhibited the phosphorylation of AMPK in macrophages and undifferentiated osteoclasts, but it activated AMPK in differentiated osteoclasts. Inactivating AMPK decreased cellular responses against the activation of toll-like receptor signaling, including the transcriptional activation of proinflammatory cytokines and the bone resorption genes TRAP, and MMP9. The elevation of bone resorption by LPS stimulation was disrupted by AMPK inhibitor, indicating the pivotal roles of AMPK in inflammation-induced activities in differentiated osteoclasts. The AMPK activator metformin did not increase proinflammatory responses, possibly because other factors are also required for this regulation. Notably, changing the activation status of AMPK did not alter the expression levels of bone resorption genes in unstimulated osteoclasts, indicating the essential roles of AMPK in cellular responses to inflammatory stimuli but not in the maintenance of basal levels. Unlike its M2-polarizing roles in macrophages, AMPK was not responsive to the M2 stimulus of interleukin-4. Our observations revealed differences in the cellular properties of macrophages and osteoclasts as well as the complexity of regulatory mechanisms for osteoclast functions.
细胞内和全身的代谢状态是影响免疫细胞功能的关键因素。代谢调节剂 AMP 激活的蛋白激酶（AMPK）感应 AMP 水平，并介导细胞对能量受限条件的反应。广泛表达的 AMPK 参与多种细胞类型的各种生物学功能，包括先天免疫细胞巨噬细胞和破骨细胞，它们是骨组织中特化的衍生物。先前的研究表明，AMPK 的激活促进了巨噬细胞向抗炎 M2 状态的极化。此外，AMPK 作为破骨细胞生成的负调节剂，上调 AMPK 会破坏破骨细胞的分化。然而，AMPK 在分化的破骨细胞中的调节和作用尚未得到表征。在这里，我们报告炎症刺激以相反的方式调节分化和未分化的破骨细胞中的 AMPK 激活。脂多糖（LPS）抑制巨噬细胞和未分化破骨细胞中 AMPK 的磷酸化，但激活分化的破骨细胞中的 AMPK。失活 AMPK 会降低细胞对 Toll 样受体信号激活的反应，包括促炎细胞因子的转录激活和骨吸收基因 TRAP 和 MMP9。LPS 刺激引起的骨吸收增加被 AMPK 抑制剂破坏，表明 AMPK 在分化的破骨细胞中炎症诱导的活性中的关键作用。AMPK 激活剂二甲双胍不会增加促炎反应，可能是因为其他因素也需要这种调节。值得注意的是，改变 AMPK 的激活状态不会改变未刺激的破骨细胞中骨吸收基因的表达水平，这表明 AMPK 在细胞对炎症刺激的反应中起关键作用，但在维持基础水平方面不起作用。与它在巨噬细胞中的 M2 极化作用不同，AMPK 对白细胞介素-4 的 M2 刺激没有反应。我们的观察结果揭示了巨噬细胞和破骨细胞的细胞特性以及破骨细胞功能的调节机制的复杂性之间的差异。
