Abstract:
Sarcomas are highly aggressive cancers of mesenchymal origin whose clinical
management is highly complex. This is partly due to a lack of understanding of the
molecular mechanisms underpinning the transformation of mesenchymal stromal/stem
cells (MSCs) which are presumed to be the sarcoma-initiating cells. c-Myc is amplified/
overexpressed in a range of sarcomas where it has an established oncogenic role and
there is evidence that it contributes to the malignant transformation of MSCs. T-box
transcription factor 3 (TBX3) is upregulated by c-Myc in a host of sarcoma subtypes where
it promotes proliferation, tumor formation, migration, and invasion. This study investigated
whether TBX3 is a c-Myc target in human MSCs (hMSCs) and whether overexpressing
TBX3 in hMSCs can phenocopy c-Myc overexpression to promote malignant
transformation. Using siRNA, qRT-PCR, luciferase reporter and chromatinimmunoprecipitation assays, we show that c-Myc binds and directly activates TBX3
transcription in hMSCs at a conserved E-box motif. When hMSCs were engineered to
stably overexpress TBX3 using lentiviral gene transfer and the resulting cells characterised
in 2D and 3D, the overexpression of TBX3 was shown to promote self-renewal, bypass
senescence, and enhance proliferation which corresponded with increased levels of cell
cycle progression markers (cyclin A, cyclin B1, CDK2) and downregulation of the p14ARF/
MDM2/p53 tumor suppressor pathway. Furthermore, TBX3 promoted the migratory and
invasive ability of hMSCs which associated with increased levels of markers of migration
(Vimentin, SLUG, SNAIL, TWIST1) and invasion (MMP2, MMP9). Transcriptomic analysis
revealed that genes upregulated upon TBX3 overexpression overlapped with c-myc
targets, were involved in cell cycle progression, and were associated with
sarcomagenesis. Together, the data described indicate that the c-Myc/TBX3
oncogenic molecular pathway may be a key mechanism that transforms hMSCs
into sarcomas.
