Abstract: Objective To explore the role of lipocalin 2 (Lcn2) in bone development and senile osteoporosis. Methods Chromatin immunoprecipitation with H3K27AC and H3K9Me3 antibodies coupled with massively parallel sequencing (ChIP- Seq) was performed to analyze the changes in binding of modified histones at the Lcn2 gene locus in C3H10T1/2 mesenchymal stem cells (MSCs) induced with osteogenic induction medium for 3 days. ITRAQ-MS/MS and ELISA were used to compare Lcn2 protein expressions in the cancellous bone and the serum between 16- and 3-month-old male mice. The effect of treatment with recombinant Lcn2 protein on osteogenic differentiation of C3H10T1/2 cells was evaluated by detecting changes in ALP expression, and Western blotting was performed to examine the changes in OSX and OCN expression. Results The expression of Lcn2 protein in the cancellous bone and its serum levels were significantly higher in 16-month-old than in 3-month-old male mice (P<0.01). In C3H10T1/2 cells, ALP expression level decreased significantly after treatment with recombinant Lcn2 protein (P<0.05) accompanied by lowered protein expressions of OSX and OCN (P<0.05). Analysis with ROSE software revealed a super enhancer in the Lcn2 gene region of C3H10T1/2 cells after osteogenesis induction. RNA-seq results confirmed that Lcn2 was among the top 4 up- regulated genes in C3H10T1/2 cells following osteogenic induction, and a super enhancer was also detected in Zbtb16 gene, which showed the highest up- regulation after osteogenic induction. Real- time quantitative PCR further confirmed that the mRNA level of Lcn2 increased sharply in C3H10T1/2 cells after osteogenic induction (P<0.001). Conclusion Lcn2 is essential for normal osteogenic differentiation of MSCs, but its overexpression and excessive secretion in aging induce self-limited inhibition of osteogenic differentiation of MSCs.

Key words: lipocalin 2; senile osteoporosis; mesenchymal stem cells; osteogenic differentiation