212
Prince et al. for the first time accomplished the isolation of CSCs in HNSCC in
- They segregated a pool of cells in HNSCC with a high CD44 expression that
exhibited stem cell-like characteristics like self-renewal, generation of differenti-
ated progeny, lack of differentiation markers, and expression of immature cell mark-
ers. These CD44+ cells were shown to have the exclusive tumorigenic capacity
when introduced in immunosuppressed mice (Prince et al. 2007 ). Unlike to the bulk
tumor cells, CSCs are akin to normal stem cells having the unique ability of unlim-
ited self-renewal and hierarchical differentiation. Like normal stem cells, CSCs are
also prophesied to have the properties of enhanced resistance to DNA damage and
evasion of apoptosis. In addition to this, CSCs show some exceptional features; like
epithelial to mesenchymal transition (EMT) enhanced invasive capacity and meta-
static proficiency that helps in tumorigenesis, sustained growth and therapeutic
resistance (Visvader and Lindeman 2008 ). Most of the contemporay therapy mod-
ules target only the bulk tumor population escaping the assassin CSCs that serve as
a reservoir for post treatment tumor repopulation.
Oral CSCs may arise from normal adult epithelial stem cells which maintain the
self-renewal machinery of a pre-existing normal stem cell rather than developing
new self- renewal pathways like Notch, Hedgehog and Wnt signaling (Reya and
Clevers 2005 ). Additional oncogenic mutations that drive the de-differentiation pro-
cess followed by self-renewal in progenitors or partly differentiated cells would also
allow the re-acquisition of the stem-like properties (Zhou et al. 2009 ). Moreover,
de-differenciation of terminally differentiated adult oral epithelial cells via the
acquisition of stem-like mutations can also result in the origin and development of
CSCs (Zhang et al. 2013 ). Several factors are reported to be involved in the genera-
tion of CSC phenotypes in oral cancer. Nutrient starvation, reduced blood supply,
hypoxia, mild therapeutic stress and challenged microenvironment are documented
to reprogram the genetic and epigenetic landscapes that induce the acquisition of an
adapted inheritable state such as drug-resistant state or/and stem-like state (Pisco
and Huang 2015 ). One of the studies showed that long-term exposure with nicotine
elevates the ALDH1 population and enhanced the stemness gene expression, upreg-
ulated EMT mediators and increased the self-renewal and sphere-forming primary
oral cancer epithelial cells (Sinha et al. 2013 ). Moreover, smoking induced drug
resistance was reversed by inhibiting nicotinic acetylcholine receptors suggesting
nicotine as a potential inducer in oral CSC generation (An et al. 2012 ). In addition,
recent report indicates that chronic arecoline exposure to oral epithelial cells
enhances stem population by overexpressing, stemness-related transcription factors
Oct4, Nanog and Sox2 (Wang et al. 2016 ). Human Papilloma Virus (HPV) proteins
E6 and E7 activate Wnt signaling pathway in HPV16-positive oropharyngeal SCC
that may cause the de-differentiation of oral cancer cells to CSCs (Rampias et al.
2010 ). The isolation and characterization of CSCs in oral cancer has been achieved
successfully via the use of different techniques which mostly involve cell lines,
primary tumor specimens and xenograft models. CSC isolation and characterization
can be performed using flow cytometry and FACS based on the expression of spe-
cific cell surface markers, such as CD133, CD44 and ALDH1. Moreover, sorting the
side populations (SP) of tumor cells via intracellular Hoechst 33,342 exclusion has
P.P. Naik et al.