Receptor activator of NF-κB (RANK) cytoplasmic motif, 369PFQEP373, plays a predominant role in osteoclast survival in part by activating Akt/PKB and its downstream effector AFX/FOXO4

Receptor activator of NF-kappa B ligand (RANKL) plays a crucial role in osteoclast differentiation, function, and survival. RANKL exerts its effect by activating its receptor RANK (receptor activator of NF-kappa B), which recruits various intracellular signaling molecules via specific motifs in its cytoplasmic tail. Previously, we identified three RANK cytoplasmic motifs (Motif 1, (369)PFQEP(373); Motif 2, (559)PVQEET(564); and Motif 3, (604)PVQEQG(609)) mediating osteoclast formation and function. Here, we investigated RANK cytoplasmic motifs involved in osteoclast survival. Motif 1, in contrast to its minimal role in osteoclast formation and function, plays a predominant role in promoting osteoclast survival. Moreover, whereas Motif 2 and Motif 3 are highly potent in osteoclast formation and function, they exert a moderate effect on osteoclast survival. We also investigated the role of these motifs in activating Akt/protein kinase B (PKB), which has been implicated in RANKL-induced osteoclast survival. Motif 1, but not Motif 2 or Motif 3, is able to stimulate Akt/PKB activation. Because Akt/PKB has been shown to utilize distinct downstream effectors (glycogen synthase kinase-3 beta, FKHR/FOXO1a, BAD, and AFX/FOXO4) to regulate cell survival, we next determined which downstream effector(s) is activated by Akt/PKB to promote osteoclast survival. Our data revealed that RANKL only stimulates AFX/FOXO4 phosphorylation, indicating that AFX/FOXO4 is a key downstream target activated by Akt/PKB to modulate osteoclast survival. Taken together, we conclude that Motif 1 plays a predominant role in mediating osteoclast survival in part by activating Akt/PKB and its downstream effector AFX/FOXO4.