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
Contextual cues present in the tumour microenvironment are key inducers of EMT in epithelial cancer cells. In addition to the presence of numerous cell types (e.g. CAFs, vascular cells, immune cells such as TAMs), ECM and soluble mediators, the tumour microenvironment is also characterized by a modified metabolism (switch to glycolysis) and a disorganized vasculature compared to normal tissue, resulting in regions of acidic pHe, low nutrient levels and hypoxia (Hanahan and Coussens, 2012; Marie-Egyptienne et al., 2013; Gatenby and Gillies, 2004). Hypoxia is a prominent element of the microenvironment of many solid tumours, including cancers of the breast, lung, colon and pancreas, and is well recognized as a critical mediator of metastasis and drug resistance. Tumour cells adapt to hypoxia by activating hypoxia-inducible factor 1 (HIF-1a), the master regulator of hypoxia-induced gene expression, and modifying critical molecular pathways that allow for survival, proliferation and invasion (Lendahl et al., 2009). Hypoxia gradients in tumours are very dynamic (fluctuating levels of hypoxia due to changes in blood flow) and result in the activation of adaptive mechanisms that promote stemness, metastasis and treatment resistance, including the induction of EMT (Marie-Egyptienne et al., 2013).
Hypoxia has been shown to regulate many of the core components of the EMT programme. Through the stabilization and activation of HIF-1a, it is a potent inducer of EMT regulators, including Snail, Slug and Twist (Hill et al., 2009; Marie-Egyptienne et al., 2013). It also results in increased expression of TGF-β, a potent inducer of EMT, and activates several intracellular signalling pathways of importance to the activation of EMT, including TGF-β, Wnt-β-catenin, Notch, NF-kB and Hedgehog (Hh) (Bao et al., 2012). Hypoxia also induces the downregulation of E-cadherin and concomitant expression of N-cadherin in epithelial cells, and stimulates directed MMP production, resulting in the activation of cell motility and invasion (Philip et al., 2013). The level of chronicity of hypoxia is reported to have an effect on the reversibility of EMT, with the presence of acute hypoxia resulting in Twist-mediated, reversible EMT, and long-term, chronic hypoxia mediating a less transient ZEB2-dependent EMT (Philip et al., 2013). Therefore, hypoxia can serve to drive the EMT programme in cancer cells.
In addition to its direct impact on the regulation of the EMT programme, hypoxia results in the induction of a glycolytic switch by tumour cells, leading to increased production of acidic metabolites, including lactic acid, protons and CO2 (Parks et al., 2011). In order to avoid prolonged intracellular acidosis, tumour cells activate a network of proteins and buffer systems that function to maintain pH homeostasis (Gatenby and Gillies, 2008; Neri and Supuran, 2011). The membrane-bound, exofacial carbonic anhydrases, especially HIF-1a-inducible, tumour-assoicated carbonic anhydrase IX and XII (CAIX and CAXII), are critical components of this pH regulatory system (McDonald et al., 2012). By catalysing the reversible hydration of carbon dioxide (CO2 ) to bicarbonate (HCO3 ) and protons (H+ ), CAIX enables the maintenance of an intracellular pH favourable to cancer cell survival and growth, and simultaneously participates in extracellular acidification, facilitating tumour cell migration, invasion and metastasis (McDonald et al., 2012). In the context of EMT and metastasis, recent studies have demonstrated the importance of CAIX expression and activity for the growth and metastasis of hypoxic tumours (Chiche et al., 2009; Lou et al., 2011; McIntyre et al., 2012). Furthermore, inhibition of CAIX results in depletion of CSCs, inhibition of EMT and inhibition of tumour cell invasion in hypoxia in models of breast cancer (Lock et al., 2013), indicating the importance of hypoxia and of CAIX, EMT and metastasis.
Tumour hypoxia may also support metastasis of cancer cells that have undergone EMT, particularly those with CSC properties, by acting as a component of the microenvironment or niche favourable for CSC survival (Marie-Egyptienne et al., 2013). CSCs are thought to exist in microenvironments or niches that make them more resistant to chemo and radiotherapy, and hypoxia may represent a major part of such environments (Marie-Egyptienne et al., 2013). Indeed, hypoxia may provide tumour cells with cues for the maintenance of a stem-like state.