PDK1 is important lipid kinase for RANKL‐induced osteoclast formation and function via the regulation of the Akt‐GSK3β‐NFATc1 signaling cascade

Abstract

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 GSK3β, and reduced induction of NFATc1. GSK3β is a reported negative regulator of NFATc1. GSK3β activity is inhibited by Akt‐dependent phosphorylation. Thus, our data provide clear genetic and mechanistic insights into the important role for PDK1 in osteoclasts.

Asiatic Acid Attenuates Osteoporotic Bone Loss in Ovariectomized Mice Through Inhibiting NF-kappaB/MAPK/ Protein Kinase B Signaling Pathway

“…In particular, it has been shown that GSK3β suppresses the capacity of NFATc1 to bind to DNA, which hinders NFATc1-dependent gene transcription ( Neal and Clipstone, 2001 ). Furthermore, the latest research shows that a wide range of cytokines, guanine nucleotide binding protein subunit α13 (G α 13), 3-phosphoinositide-dependent protein kinase 1 (PDK1), and microtubule actin crosslinking factor 1 (MACF1) can manage osteoclastogenesis by regulating the Akt-GSK3β-NFATc1 signaling axis ( Wu et al, 2017a ; Lin et al, 2019 ; Xiao et al, 2020 ). Our results showed that asiatic acid inhibited Akt phosphorylation and activation of the NFATc1 protein.…”

Sec-O-Glucosylhamaudol Inhibits RANKL-Induced Osteoclastogenesis by Repressing 5-LO and AKT/GSK3β Signaling

“…GSK3β-mediated phosphorylation of NFATc1 at serine residues promotes NFATc1 nuclear export and localization in the cytoplasm ( 33 , 34 ). RANKL stimulates the phosphorylation of GSK3β at serine 9, leading to its inactivation, the blockage of NFATc1 phosphorylation, and promotion of NFATc1 nuclear localization.…”

Sec-O-glucosylhamaudol Inhibits RANKL-induced Osteoclastogenesis Via Repressing 5-LO and AKT/GSK3β Signaling

“…GSK3β-mediated phosphorylation of NFATc1 at serine residues promotes NFATc1 nuclear export and localization in the cytoplasm 21,31 . RANKL stimulates phosphorylation of GSK3β at serine 9 and inactivates GSK3β.…”