Lnc-PPP2R1B Mediates the Alternative Splicing of PPP2R1B by Interacting and Stabilizing HNRNPLL and Promotes Osteogenesis of MSCs

Osteogeinc differentiation from mesenchymal stem cells (MSCs) into osteoblasts is a key step for bone tissue engineering in regenerative medicine. The insight into regulatory mechanism of osteogenesis of MSCs facilitates achieving better recovery effect. Long non-coding RNAs are regarded as a family of important moderators in osteogenesis. In this study, we found a novel lncRNA, lnc-PPP2R1B was up-regulated during osteogenesis of MSCs by Illumina HiSeq transcritome sequencing. We demonstrated lnc-PPP2R1B overexpression promoted osteogenesis and knockdown of lnc-PPP2R1B inhibited osteogenesis of MSCs. Mechanically, it physically interacted with and up-regulated heterogeneous nuclear ribonucleoprotein L Like (HNRNPLL), which is a master regulator of activation-induced alternative splicing in T cells. We found lnc-PPP2R1B knockdown or HNRNPLL knockdown decreased transcript-201 of Protein Phosphatase 2A, Regulatory Subunit A, Beta Isoform (PPP2R1B) while increased transcript-203 of PPP2R1B, and did not affect transcript-202/204/206. PPP2R1B is a constant regulatory subunit of protein phosphatase 2 (PP2A), which activates Wnt/β-catenin pathway by removing phosphorylation and stabilization of β-catenin and translocation into nucleus. The transcript-201 retained exon 2 and 3, compared to transcript-203. And it was reported the exon 2 and 3 of PPP2R1B were one part of B subunit binding domain on A subunit in PP2A trimer, and therefore retaining exon 2 and 3 promised formation and enzyme function of PP2A. Finally, lnc-PPP2R1B promoted ectopic osteogenesis in vivo. Conclusively, lnc-PPP2R1B mediated alternative splicing of PPP2R1B through retaining exon 2 and 3 by interacting with HNRNPLL and then promoted osteogenesis, which may facilitate an in-depth understanding of function and mechanism of lncRNAs in osteogenesis. Lnc-PPP2R1B interacted with HNRNPLL, and regulated alternative splicing of PPP2R1B through retaining exon 2 and 3, which preserved enzyme function of PP2A and enhanced dephosphorylation and nuclear translocation of β-catenin, thereby promoting Runx2 and OSX expression and then osteogenesis. And it provided experimental data and potential target for promoting bone formation and bone regeneration.

hnRNP L enhances sensitivity of the cells to KW-2189

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are involved in several RNA-related biological processes. We demonstrated hnRNP L as a candidate protein of DARP (duocarmycin-DNA adduct recognizing protein) by gel shift assay and amino acid sequencing. Stable transfectants of hnRNP L showed high sensitivity of the cells to the growth inhibitory effect of KW-2189, a duocarmycin derivative in vitro. Immunostaining of hnRNP L demonstrated differential intracellular localization of hnRNP L among human lung cancer cell lines. A transfection study using a series of deletion mutants of hnRNP L fused to indicated that the N-terminal portions of RRM(RNA recognition motif)1, RRM3 and RRM2 are involved in localization of hnRNP L. We identified sequences in these portions that have high homology with the sequences of known NLS (nuclear localization signal) and NES (nuclear export signal). hnRNP L is a factor that determines the sensitivities of cancer cells to the minor groove binder, and overexpression and differential intracellular localization of hnRNP L are involved in its function in lung cancer.