New mechanism of transforming growth factor-β signaling in hepatoma: Dramatic up-regulation of tumor initiating cells and epidermal growth factor receptor expression

Aim: Transforming growth factor-β (TGF-β) has dual activity in tumor cells. We studied the effect of TGF-β on tumor-initiating cells (TICs), which are similar in self-renewal and differentiation features to normal adult stem cells.: Methods: We used side population (SP) cells that exclude DNA binding dye Hoechst 33342 to obtain TICs, studied the differences in the kinetics of the SP cell response to TGF-β treatment between hepatic tumor cell lines, and performed gene analysis. Results: SP cells from all cell lines have higher proliferative ability compared to non-SP cells and they are drug resistant. TGF-β treatment increased the percentage of SP cells (%SP) and the survival rate; chemotherapeutic drug resistance developed only in K-251 SP cells. Gene analysis showed that TGF-β up-regulated epidermal growth factor receptor (EGFR) only in K-251 cells. There were no EGFR mutations in K-251, which had been reported in lung cancer. Knockdown of Smad4 using the small-interfering RNA technique in K-251 cells inhibited EGFR overexpression and significantly decreased the %SP. In contrast, the JNK inhibitor had little effect on EGFR expression or the %SP. Conclusion: TGF-β treatment of K-251 cells causes tumor progression and the anti-cancer drug resistant phenotype by increasing SP.