Stability of mRNA influences osteoporotic bone mass via CNOT3.

Osteoclastogenesis is under the control of posttranscriptional and transcriptional events. However, posttranscriptional regulation of osteoclastogenesis is incompletely understood. CNOT3 is a component of the CCR4 family that regulates mRNA stability, but its function in bone is not known. Here, we show that Cnot3 deficiency by deletion of a single allele induces osteoporosis. Cnot3 deficiency causes an enhancement in bone resorption in association with an elevation in bone formation, resulting in high-turnover type bone loss. At the cellular level, Cnot3 deficiency enhances receptor activator of NF-κB ligand (RANKL) effects on osteoclastogenesis in a cell-autonomous manner. Conversely, Cnot3 deficiency does not affect osteoblasts directly. Cnot3 deficiency does not alter RANKL expression but enhances receptor activator of NF-κB (RANK) mRNA expression in bone in vivo. Cnot3 deficiency promotes RANK mRNA stability about twofold in bone marrow cells of mice. Cnot3 knockdown also increases RANK mRNA expression in the precursor cell line for osteoclasts. Anti-CNOT3 antibody immunoprecipitates RANK mRNA. Cnot3 deficiency stabilizes luciferase reporter expression linked to the 3'-UTR fragment of RANK mRNA. In contrast, Cnot3 overexpression destabilizes the luciferase reporter linked to RANK 3'-UTR. In aged mice that exhibit severe osteoporosis, Cnot3 expression levels in bone are reduced about threefold in vivo. Surprisingly, Cnot3 deficiency in these aged mice further exacerbates osteoporosis, which also occurs via enhancement of osteoclastic activity. Our results reveal that CNOT3 is a critical regulator of bone mass acting on bone resorption through posttranscriptional down-regulation of RANK mRNA stability, at least in part, even in aging-induced osteoporosis.