Patho-physiological Analysis of Radio-resistant Cancer Stem Cells

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There is increasing evidence that cancers contain their own stem-like cells called cancer stem cells(CSCs). A small subset of cells, termed side population(SP), has been identified using flow cytometric analysis. The SP cells have the ability to exclude the DNA binding dye, Hcechst33342, and are highly enriched for stem cells in many kinds of normal tissues. Because CSCs are thought to be drug resistant, SP cells in cancers might contain CSCs. We initially examined the presence of SP cells in several human thyroid cancer cell lines. A small percentage of SP cells were found in ARO(0.25%), FRO(0.1%), NPA(0.06%), and WRO(0.02%) cells but not TPC1 cells. After sorting, the SP cells generated both SP and non-SP cells in culture. The clonogenic ability of SP cells was significantly higher than that of non-SP cells. Moreover, the SP prevalence was dependent on cell density in culture, suggesting that SP cells preferentially survived at lower cell density. Microarray experiment revealed diffe … More rential gene expression profile between SP and non-SP cells, and several genes related to sternness were up-regulated. However, non-SP population also contained cells that were tumorigenic in nude mice, and non-SP cells generated a small number of SP cells. These results suggest that cancer stem-like cells are partly, but not exclusively, enriched in SP population.Thyroid hormone receptors(THRs) widely govern cell growth, differentiation and metabolism acting in a ligand-and cofactordependent manner to modulate tissue-specific gene expression. Given a large variety of genes regulated by THRs and multiplicity of cellular processes potentially influenced by THRs, we addressed the role of THRB(thyroid hormone receptor beta) in cellular radiosensitivity. On the other hand, SP-rich cell lines showed low level expression of THRB. Wild-type and mutant THRB were overexpressed in several cell lines using an adenovirus-mediated gene delivery and their effects were examined after cell exposure to gamma-rays. Wild-type THRB decreased clonogenic survival of the cell lines with low levels of endogenous THRB, retarded their growth and synergized with radiation in decreasing proliferative potential and promoting cellular senescence. These changes were accompanied by the accumulation of p21(CDKNIA, CIP1, WAF1) and p16(CDKN2A, TNK4a)inhibitors of cyclin-dependent kinases and by the decrease of Rb(retinoblastoma protein)phosphorylation. Mutant THRB produced a radioprotective effect, attenuated radiation-induced growth inhibition and cellular senescence. The results suggest that THRB may modulate cellular radiosensitivity and stress-induced senescence.These data is suggesting that radio-resistant property of SP cell is involved in THRB expression and its function. Less