Transcriptional program of osteoblast differentiation : coordinated activation of Notch, Wnt, and TGF-[beta] signaling pathways in MC3T3 cell line

In order to examine the early events in osteoblast differentiation, three different model systems for osteoblast differentiation were compared: mouse pre-osteoblastic cell line MC3T3; mouse myoblastic cell line C2C12 and primary mouse calvarial osteoblasts. Quantitative RT-PCR conditions were set for analysis of 8 markers of the osteoblast differentiation process: alkaline phosphatase, Msx2, Cbfa1, parathyroide hormone receptor, osteocalcin, osteopontin, osteonectin and collagen I α1.
Expression of these genes was analyzed in all three systems upon treatment with oesteogenic supplement, at days 1 and 3, by comparison with a non-stimulated time-matched control. In addition, classical cytochemical tests for following the osteoblast differentiation process, ALP and mineralization staining, were used. MC3T3 cells were shown to be the best model for examining osteoblast differentiation on the cytochemical, as well as on the transcriptional level, with most marker genes upregulated. Then, Affymetrix GeneCHIP analysis was used to probe the changes induced by differentiation stimuli in MC3T3 cells. These cells were stimulated for 1 and 3 days with an osteogenic stimulus containing BMP-2. Total RNA was extracted and analyzed with Affymetrix GeneCHIP oligonucleotide arrays.
A regulated expression of 394 known genes and 295 ESTs was tedected. The sensitivity and realiability of detection by microarrays was shown by confirming the expression pattern for 20 genes by radioactive quantitative RT-PCR. Extensive functional classification of regulated genes was performed. The most interesting finding was concomitant activation of TGF-β, Wnt and Notch signaling pathways, confirmed by strong upregulation of their target genes by PCR. The TGF-β pathway is activated by stimulated production of the growth factor itself, while the exact mechanism of Wnt and Notch activation remains elusive. We showed BMP-2 stimulated expression of Hey1, a direct Notch target gene, in mouse MC3T3 and C2C12 cells, in human mesenchymal cells and in mouse calvaria. Small interfering RNA-mediated inhibition of Hey1 induction led to an increase in osteoblast matrix mineralization, suggesting that Hey1 is a negative regulator of osteoblast maturation. This negative regulation is apparently achieved via interaction with Runx2, as Hey1 completely abrogated Runx2 transcriptional activity. These findings identify the Notch-Hey1 pathway as a negative regulator of osteoblast differentiation / maturation, which is a completely novel aspect of osteogenesis and could point to possible new targets for bone anabolic agents.