As shown in Fig. 2, TGFβ1, and not starvation, significantly induced CD133 expression. In addition, we performed dose- and time-dependent experiments on the effect of TGFβ1 on CD133 expression. CD133− Huh7 cells were stimulated ICG-001 with up to 20 ng/mL TGFβ1 for 48 hours. As depicted in Fig. 3A, CD133 expression was induced by TGFβ1 in a dose-dependent manner up to 2.5
ng/mL, and dosages between 2.5 and 10 ng/mL had similar effects on CD133 expression induction. CD133− cells were then treated with 5 ng/mL TGFβ1 for up to 48 hours, followed by repeat treatment with 0 to 10 ng/mL TGFβ1 for an additional 24 hours. As shown in Fig. 3B, TGFβ1-induced CD133 expression was in a time-dependent fashion, and once CD133 expression was induced, the expression remained elevated even after TGFβ1 stimulation was removed. As CD133 is a CSC marker in Huh7 cells, we questioned if the TGFβ1-induced CD133+ Huh7 cells have the property of tumor initiation in vivo, comparable to native CD133+ Huh7 cells. Freshly isolated, untreated CD133+ and CD133− Huh7 cells were used as controls. Thirty
days after inoculation all of the mice transplanted with native CD133+ cells were sacrificed because the tumor size reached the endpoint according to our protocol (>3,500 mm3). As demonstrated in Fig. 4A, 6 and Protein Tyrosine Kinase inhibitor 12 hours of TGFβ1 stimulation increased CD133 expression in CD133− cells; 35 days after HSP90 inoculation in nude mice, TGFβ1-induced CD133+ cells were significantly more tumorigenic compared to native CD133− cells (Fig. 4B,C). Following activation of TGFβ receptors, Smad2 and Smad3 are phosphorylated and form a heterocomplex, Smad2/3/4, which
translocates to the nucleus to regulate responsive gene transcription.28 In order to test whether TGFβ induces CD133 expression through Smad-dependent pathways, we used inhibitory Smads, Smad6 and Smad7, which are able to block heterocomplex formation. Huh7 cells were transfected with Smad629 and Smad730 vectors, and 48 hours after transfection cells were stimulated with 5 ng/mL TGFβ1. In qPCR analysis, elevated CD133 mRNA induced by TGFβ1 was significantly attenuated by inhibitory Smads (Fig. 5A). This expression pattern was confirmed at the protein level (Fig. 5B). In colon cancer cells CD133 expression is regulated by promoter methylation. Compared with the parental HCT116 cell line, a double knockout line with disruption of DNA methyltransferases DNMT1 and DNMT3β demonstrates increased CD133 expression.8 To test if similar epigenetic regulation is involved in CD133 expression in liver cancer, a DNMT inhibitor (5-aza-2′-deoxycytidine, DAC) and a histone deacetylase inhibitor (trichostatin A, TSA) were introduced. As shown in Supporting Information Fig. 1, CD133 expression in CD133− Huh7 cells was up-regulated by DNMT inhibitor in a time- and dose-dependent manner.
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