PBX3 is targeted by multiple miRNAs and is essential for liver tumour-initiating cells

Tumour-initiating cells (TICs) are advocated to constitute the sustaining force to maintain and renew fully established malignancy; however, the molecular mechanisms responsible for these properties are elusive. We previously demonstrated that voltage-gated calcium channel alpha 2 delta 1 subunit marks hepatocellular carcinoma (HCC) TICs. Here we confirm directly that alpha 2 delta 1 is a HCC TIC surface marker, and identify let-7c, miR-200b, miR-222 and miR-424 as suppressors of alpha 2 delta 1(+) HCC TICs. Interestingly, all the four miRNAs synergistically target PBX3, which is sufficient and necessary for the acquisition and maintenance of TIC properties. Moreover, PBX3 drives an essential transcriptional programme, activating the expression of genes critical for HCC TIC stemness including CACNA2D1, EpCAM, SOX2 and NOTCH3. In addition, the expression of CACNA2D1 and PBX3 mRNA is predictive of poor prognosis for HCC patients. Collectively, our study identifies an essential signalling pathway that controls the switch of HCC TIC phenotypes.