Lipocalin 2: A New Mechanoresponding Gene Regulating Bone Homeostasis

Abstract

Mechanical loading represents a crucial factor in the regulation of skeletal homeostasis. Its reduction causes loss of bone mass, eventually leading to osteoporosis. In a previous global transcriptome analysis performed in mouse calvarial osteoblasts subjected to simulated microgravity, the most upregulated gene compared to unit gravity condition was Lcn2, encoding the adipokine Lipocalin 2 (LCN2), whose function in bone metabolism is poorly known. To investigate the mechanoresponding properties of LCN2, we evaluated LCN2 levels in sera of healthy volunteers subjected to bed rest, and found a significant time-dependent increase of this adipokine compared to time 0. We then evaluated the in vivo LCN2 regulation in mice subjected to experimentally-induced mechanical unloading by (1) tail suspension, (2) muscle paralysis by botulin toxin A (Botox), or (3) genetically-induced muscular dystrophy (MDX mice), and observed that Lcn2 expression was upregulated in the long bones of all of them, whereas physical exercise counteracted this increase. Mechanistically, in primary osteoblasts transfected with LCN2-expression-vector (OBs-Lcn2) we observed that Runx2 and its downstream genes, Osterix and Alp, were transcriptionally downregulated, and alkaline phosphatase (ALP) activity was less prominent versus empty-vector transduced osteoblasts (OBs-empty). OBs-Lcn2 also exhibited an increase of the Rankl/Opg ratio and IL-6 mRNA, suggesting that LCN2 could link poor differentiation of osteoblasts to enhanced osteoclast stimulation. In fact, incubation of purified mouse bone marrow mononuclear cells with conditioned media from OBs-Lcn2 cultures, or their coculture with OBs-Lcn2, improved osteoclastogenesis compared to OBs-empty, whereas treatment with recombinant LCN2 had no effect. In conclusion, our data indicate that LCN2 is a novel osteoblast mechanoresponding gene and that its regulation could be central to the pathological response of the bone tissue to low mechanical forces.

Microgravity-Induced Transcriptome Adaptation in Mouse Paraspinal longissimus dorsi Muscle Highlights Insulin Resistance-Linked Genes

“…On the other hand, Guo and co-workers showed opposite results, that is Lcn2 deficiency potentiated diet-induced insulin resistance in mice (Guo et al, 2010). Interestingly, Lcn2 protein level was found to be increased in sera of individuals following bed rest (Rucci et al, 2015), therefore based on our results we may speculate that Lcn2 protein could be at least in part secreted in a soluble form in blood by skeletal muscle tissue similar to the known myokines.…”

Specific Commensal Bacterium Critically Regulates Gut Microbiota Osteoimmunomodulatory Actions During Normal Postpubertal Skeletal Growth and Maturation

“…F ). Notably, LCN2 is an acute‐phase reactant and antimicrobial peptide that has been shown to play roles in bone development and turnover . To corroborate that elevated LCN2 levels in the serum of MPF mice (Fig.…”

Lipocalin-2 inhibits osteoclast formation by suppressing the proliferation and differentiation of osteoclast lineage cells

“…Using transgenic mice that specifically overexpress LCN2 in osteoblasts, Costa et al reported that LCN2 negatively regulates bone development and formation by affecting growth plate development and attenuating osteoblast differentiation [31]. More recently, Rucci et al identified LCN2 as a novel osteoblast mechanoresponding gene that modulates bone homeostasis [39]. They further demonstrated that in vitro overexpression of LCN2 in primary osteoblasts resulted in reduced expression of Runx2, a master regulator of osteoblasts.…”