Abstract
The underlying mechanism regulating the expression of the cancer stem cell/tumor-initiating cell marker CD133/prominin-1 in cancer cells remains largely unclear, although knowledge of this mechanism would likely provide important biological information regarding cancer stem cells. Here, we found that the inhibition of mTOR signaling up-regulated CD133 expression at both the mRNA and protein levels in a CD133-overexpressing cancer cell line. This effect was canceled by a rapamycin-competitor, tacrolimus, and was not modified by conventional cytotoxic drugs. We hypothesized that hypoxia-inducible factor-1 alpha (HIF-1 alpha), a downstream molecule in the mTOR signaling pathway, might regulate CD133 expression; we therefore investigated the relation between CD133 and HIF-1 alpha. Hypoxic conditions up-regulated HIF-1 alpha expression and inversely down-regulated CD133 expression at both the mRNA and protein levels. Similarly, the HIF-1 alpha activator deferoxamine mesylate dose-dependently down-regulated CD133 expression, consistent with the effects of hypoxic conditions. Finally, the correlations between CD133 and the expressions of HIF-1 alpha and HIF-1 beta were examined using clinical gastric cancer samples. A strong inverse correlation (r = -0.68) was observed between CD133 and HIF-1 alpha, but not between CD133 and HIF-1 beta. In conclusion, these results indicate that HIF-1 alpha down-regulates CD133 expression and suggest that mTOR signaling is involved in the expression of CD133 in cancer cells. Our findings provide a novel insight into the regulatory mechanisms of CD133 expression via mTOR signaling and HIF-1 alpha in cancer cells and might lead to insights into the involvement of the mTOR signal and oxygen-sensitive intracellular pathways in the maintenance of stemness in cancer stem cells.
Highlights
The CD133/prominin-1 protein is a five-transmembrane molecule expressed on the cell surface that is widely regarded as a stem cell marker
The mTOR inhibitor rapamycin, but not cytotoxic drugs (5-fluorouracil, CPT11, and gemcitabine), increased the expression of CD133 in a dosedependent manner in CD133-overexpressing WiDr cells (Fig. 1C and D). These results indicate that mTOR signaling is involved in the expression of CD133 in cancer cells
The discrepancy might be explained by (a) a different cellular context in glioma from the others, because CD133 expressions of all cell lines including the WiDr, IM95, SNU16, OCUM1, 44As3, and DLD-1 cells were reproducibly downregulated by hypoxic condition (Supplemental Fig. S1; Fig. 3B), whereas the U251 cells failed to exhibit the down-regulation, and by (b) the different detection methods in our study (Western blot and quantitative real-time reverse transcription-PCR (RT-PCR)) from the previous report
Summary
The CD133/prominin-1 protein is a five-transmembrane molecule expressed on the cell surface that is widely regarded as a stem cell marker. Growing evidence indicates that CD133 can be used as a cell marker for cancer stem cells or tumor-initiating cells in colon. Some investigators have shown a relation between hypoxia and CD133 expression in brain tissue. With the exception of these studies in brain tissue, data on the expression of CD133 and the involvement of hypoxia and other signaling pathways in cancer cells remains limited. Several reports have indicated that mTOR is a positive regulator of hypoxia-inducible factor (HIF) expression and activity [7], and the inhibition of HIF-mediated gene expression is considered to be related to the antitumor activity of mTOR inhibitors in renal cell carcinoma [8]. We found that mTOR signaling was involved in CD133 expression in gastric and colorectal cancer cells. We investigated the regulatory mechanism of CD133 in cancer cells
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