Abstract
The bone morphogenetic protein/Signaling mothers against decapentaplegic (BMP/Smad) and the WNT signaling pathways regulate the commitment of mesenchymal cells to the osteoblastic lineage. Nemo-like kinase (Nlk) is an evolutionary conserved kinase that suppresses Smad transactivation and WNT canonical signaling. However, it is not clear whether these effects of Nlk have any consequence on the differentiation of mammalian cells. To study the function of Nlk during the commitment of ST-2 bone marrow stromal cells to the osteoblastic fate, Nlk was downregulated by RNA interference (RNAi), following transfection of a specific small interfering (si)RNA. Nlk downregulation increased alkaline phosphatase and osteocalcin expression and sensitized ST-2 cells to the effects of BMP2 and WNT3 on alkaline phosphatase mRNA expression and activity. Accordingly, Nlk downregulation enhanced the effect of BMP2 on the transactivation of the BMP/Smad reporter construct 12xSBE-Oc-pGL3, and on the levels of phosphorylated Smad1/5/8, whereas it did not affect the transactivation of the transforming growth factor-β/Smad reporter pSBE-Luc. Nlk downregulation sensitized ST-2 cells to the effects of WNT3 on the transactivation of the WNT/T-cell factor (Tcf) reporter construct 16xTCF-Luc, whereas it did not affect cytosolic β-catenin levels. To understand the function of Nlk in cells committed to the osteoblastic lineage, Nlk was suppressed by RNAi in primary calvarial osteoblasts. Downregulation of Nlk increased alkaline phosphatase and osteocalcin transcripts and sensitized osteoblasts to the effects of BMP2 on alkaline phosphatase activity and Smad1/5/8 transactivation and phosphorylation. In conclusion, Nlk suppresses osteoblastogenesis by opposing BMP/Smad and WNT canonical signaling.