Principles of stem cell biology and cancer: future applications and therapeutics. Edited by T. Regad, T. J. Sayers and R. C. Rees. John Wiley & Sons (2015)

Part II. Cancer stem cells

Quiescent cells in tissues characterized by their propensity to retain DNA labels such as tritiated thymidine or BrdU over an extended period of time following labelling have classically been considered indicative of the presence of stem cells in the gut. Such cells tend to remain quiescent and may undergo asymmetrical cell divisions to produce progeny. They may also function as an ‘injury reserve’ that helps maintain homeostasis in the gut following conditions of stress. Subsequently, label-retaining cells (LRCs) may mark quiescent ISCs, which primarily expand the stem cell compartment when necessary. Indeed, small numbers of LRCs have been observed for around 4 weeks after established homeostasis and can re-enter the cell cycle following stress (Potten et al., 1978, 2002). More recent studies have employed transgenic mice expressing H2B-YFP under the Cyp1a1 promoter (Buczacki et al., 2013). This transgenic model has an advantage over nucleotide labelling, which marks noncycling quiescent cells incompletely. An expression pulse of the yellow fluorescent H2B-YFP fusion protein, which binds to DNA, is induced through a bolus injection of β-naphthoflavone (βNF), and nuclear fluorescence is lost with subsequent cell divisions. The Cyp1a1 promoter shows expression in all cells of the crypt – villus axis, with the exception of mature Paneth cells, and H2B-YFP expression is restricted to cells at the crypt base 7 days post-injection with βNF. At 3 weeks following βNF, approximately two YFP-LRCs are observed per crypt, predominantly at the +3 position (Buczacki et al., 2013). By contrast, no LRCs are observed in the colon. Expression profiling of flow-sorted YFP-LRCs shows that these cells are distinct from Paneth cells and from cycling cells from the lower crypt. However, transcripts enriched in Paneth cells, enteroendocrine cells and stem cell markers (Lgr5, Lrig1, CD133, CD44 and Peg3) are enriched in this cell population. Further experiments have concluded that the YFP-LRCs may be precursor cells that are committed to mature into differentiated secretory cells or into the Paneth and enteroendocrine lineages (Buczacki et al., 2013). Following intestinal injury, these cells appear capable of extensive proliferation and give rise to progeny from all epithelial lineages. Subsequently, it has been shown that YFP-LRCs can function as stem cells under conditions that are stressful to the small intestine, and function as a clonogenic reserve. These data demonstrate the plasticity of slowly cycling cell lineages in the GIT and may further aid in identifying CRC cells that cause treatment resistance.

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