Abstract
BackgroundSubsets of cells with stem-like properties have been previously isolated from human epithelial cancers and their resistance to apoptosis-inducing stimuli has been related to carcinoma recurrence and treatment failure. The aim of this study was to investigate the mechanisms of resistance to apoptosis-inducing agents of cells with stem-like properties in both normal and malignant human epithelia.MethodsCells isolated from fresh human head and neck carcinomas (n = 11), cell lines derived from head and neck, prostate and breast human carcinomas (n = 7), and from normal human oral mucosa (n = 5), were exposed to various apoptosis-inducing stimuli (UV, Tumour Necrosis Factor, Cisplatin, Etoposide, and Neocarzinostatin). Flow cytometry for CD44 and epithelial-specific antigen (ESA) expression, colony morphology, tumour sphere formation and rapid adherence assays were used to identify the subset of cells with stem-like properties. Apoptosis, cell cycle and expression of various cell cycle checkpoint proteins were assessed (Western Blot, qPCR). The role of G2-checkpoint regulators Chk1 and Chk2 was investigated by use of debromohymenialdisine (DBH) and siRNA.ResultsIn both cancer biopsies and carcinoma cell lines a subset of CD44high cells showed increased clonogenicity, a significantly lower rate of apoptosis, and a significantly higher proportion of cells in the G2-phase of the cell cycle. An inverse correlation between the percentage of cells in G2-phase and the rate of apoptosis was found. Pulse-chase with iododeoxyuridine (IdU) demonstrated that CD44high carcinoma cells spent longer time in G2, even in un-treated controls. These cells expressed higher levels of G2 checkpoint proteins, and their release from G2 with BDH or Chk1 siRNA increased their rate of apoptosis. Low passage cultures of normal keratinocytes were also found to contain a subset of CD44high cells showing increased clonogenicity, and a similar pattern of G2-block associated with apoptotic resistance.ConclusionsThese data indicate that both normal and malignant human epithelial cells with stem-like properties show greater resistance to apoptosis associated with extended G2 cell cycle phase, and that this property is not a consequence of neoplastic transformation. Targeting G2 checkpoint proteins releases these cells from the G2-block and makes them more prone to apoptosis, implying an opportunity for improved therapeutic approaches.
Highlights
Subsets of cells with stem-like properties have been previously isolated from human epithelial cancers and their resistance to apoptosis-inducing stimuli has been related to carcinoma recurrence and treatment failure
Some tumour cells have the ability for indefinite self-renewal while generating cells that enter differentiation pathways, properties that correspond to the essential basic properties of normal adult somatic stem cells [10]. Further support for this idea has lately been generated by the ability to isolate and assess the tumour-initiating properties of various cell fractions isolated by fluorescence-activated cell sorting (FACS) based on certain cell surface markers such as CD34, CD44 or CD133 [7]
CD44 expression correlates with in vitro colony morphology, clonogenicity and predicts the ability to grow in suspension as "tumour spheres" Each of the seven human carcinoma cell lines investigated in this study (CA1, H357, 5PT, CaLH2, CaLH3, MCF7, and DU145) developed colonies with morphologies that corresponded, as previously described, to holoclone, meroclone and paraclone colonies (Fig. 1A, 1D)
Summary
Subsets of cells with stem-like properties have been previously isolated from human epithelial cancers and their resistance to apoptosis-inducing stimuli has been related to carcinoma recurrence and treatment failure. Some tumour cells have the ability for indefinite self-renewal while generating cells that enter differentiation pathways, properties that correspond to the essential basic properties of normal adult somatic stem cells [10]. Following the early identification of cells with stem-like properties in haematopoietic malignancies [11,12], prospective identification and isolation of such cell subpopulations has been achieved for an expanding range of solid human tumours, including head and neck, breast and prostate cancers [13,14,15,16,17,18]
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