PDK1 is important lipid kinase for RANKL-induced osteoclast formation and function via the regulation of the Akt-GSK3ß-NFATc1 signaling cascade.

Perturbations in the balanced process of osteoblast-mediated bone formation and osteoclast-mediated bone resorption leading to excessive osteoclast formation and/or activity is the cause of many pathological bone conditions such as osteoporosis. The osteoclast is the only cell in the body capable of resorbing and degrading the mineralized bone matrix. Osteoclast formation from monocytic precursors is governed by the actions of two key cytokines macrophage-colony-stimulating factor and receptor activator of nuclear factor-κB ligand (RANKL). Binding of RANKL binding to receptor RANK initiates a series of downstream signaling responses leading to monocytic cell differentiation and fusion, and subsequent mature osteoclast bone resorption and survival. The phosphoinositide-3-kinase (PI3K)-protein kinase B (Akt) signaling cascade is one such pathway activated in response to RANKL. The 3-phosphoinositide-dependent protein kinase 1 (PDK1), is considered the master upstream lipid kinase of the PI3K-Akt cascade. PDK1 functions to phosphorylate and partially activate Akt, triggering the activation of downstream effectors. However, the role of PDK1 in osteoclasts has yet to be clearly defined. In this study, we specifically deleted the PDK1 gene in osteoclasts using the cathepsin-K promoter driven Cre-LoxP system. We found that the specific genetic ablation of PDK1 in osteoclasts leads to an osteoclast-poor osteopetrotic phenotype in mice. In vitro cellular assays further confirmed the impairment of osteoclast formation in response to RANKL by PDK1-deficient bone marrow macrophage (BMM) precursor cells. PDK1-deficient BMMs exhibited reduced ability to reorganize actin cytoskeleton to form a podosomal actin belt as a result of diminished capacity to fuse into giant multinucleated osteoclasts. Notably, biochemical analyses showed that PDK1 deficiency attenuated the phosphorylation of Akt and downstream effector GSK3ß, and reduced induction of NFATc1. GSK3ß is a reported negative regulator of NFATc1. GSK3ß activity is inhibited by Akt-dependent phosphorylation. Thus, our data provide clear genetic and mechanistic insights into the important role for PDK1 in osteoclasts.

The Role and Mechanism of miRNA-1224 in the Polygonatum sibiricum Polysaccharide Regulation of Bone Marrow-Derived Macrophages to Osteoclast Differentiation.

MicroRNAs (miRNAs) are endogenous noncoding small molecule RNAs that regulate cell proliferation, differentiation, fat metabolism, and hormone secretion. Studies have shown that miRNAs regulate the processes related to osteoporosis, including the differentiation of osteoblasts, osteoclasts, and chondrocytes, and are one of the important regulatory factors of some bone metabolic diseases. In our previous study, it has been revealed that natural compound Polygonatum sibiricum polysaccharide (PSP) can promote osteoblast formation and block osteoclastogenesis through Wnt/ß-catenin signaling pathway. This study was designed to investigate whether PSP can inhibit expression of osteoclast-related genes by Hippo signaling pathway, which was prevented by effectively blocking the expression of miR-1224. This study showed that there were 27 differentially expressed miRNAs when PSP inhibits osteoclastogenesis, the most notable of which was miR-1224. Furthermore, the study showed that PSP increased the level of Limd1, which was the target gene of miR-1224. In conclusion, these findings demonstrate that PSP suppressed osteoclastogenesis in vitro through the Hippo signaling pathway based on miR-1224. This study may aid in the development of a therapeutic approach utilizing PSP for the enhancement of bone health and prevention of osteoporosis.

miR-136-5p Regulates the Inflammatory Response by Targeting the IKKß/NF-κB/A20 Pathway After Spinal Cord Injury.

BACKGROUND/AIMS: miR-136-5p participates in recovery after spinal cord injury (SCI) via an unknown mechanism. We investigated the mechanism underlying the involvement of miR-136-5p in the inflammatory response in a rat model of SCI. METHODS: Sprague-Dawley rat astrocytes were cultured in vitro to construct a reporter plasmid. Luciferase assays were used to detect the ability of miR-136-5p to target the IKKß and A20 genes. Next, recombinant lentiviral vectors were constructed, which either overexpressed miR-136-5p or inhibited its expression. The influence of miR-136-5p overexpression and miR-136-5p silencing on inflammation was observed in vivo in an SCI rat model. The expression of IL-1ß, IL-6, TNF-α, IFN-α, and related proteins (A20, IKKß, and NF-κB) was detected. RESULTS: In vitro studies showed that luciferase activity was significantly activated in the presence of the 3' untranslated region (UTR) region of the IKKß gene after stimulation of cells with miR-136-5p. However, luciferase activity was significantly inhibited in the presence of the 3'UTR region of the A20 gene. Thus, miR-136-5p may act directly on the 3'UTR regions of the IKKß and A20 genes to regulate their expression. miR-136-5p overexpression promoted the production of related cytokines and NF-κB in SCI rats and inhibited the expression of A20 protein. CONCLUSION: Overexpression of miR-136-5p promotes the generation of IL-1ß, IL-6, TNF-α, IFN-α, IKKß, and NF-κB in SCI rats but inhibits the expression of A20. Under these conditions, inflammatory cell infiltration into the rat spinal cord increases and injury is significantly aggravated. Silencing of miR-136-5p significantly reduces the protein expression results described after miR-136-5p overexpression and ameliorates the inflammatory cell infiltration and damage to the spinal cord. Therefore, miR-136-5p might be a new target for the treatment of SCI.