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Head and neck squamous cell carcinomas (HNSCCs) are the most frequent malignancies of the upper
aerodigestive tract. The cancer stem cell (CSC) hypothesis concludes that CSCs constitute the dangerous
tumor cell population due to their ability of self-renewal and being associated with relapse of tumor disease,
invasiveness and resistance to chemo(radio)therapy. The aim of this study was to look for CSC candidates
and expression of MMP-9 that previously was implicated in HNSCC invasiveness.
Immunohistochemical, immunofluorescence and Western blot analysis were performed on HNSCC
tumor specimens using antibodies specific for MMP-9, CD44, ALDH1 and CK14. Gelatinolytic activity
was assessed by zymography. Pearson correlation analysis was used for statistical comparison.
Immunohistochemical analysis found CD44 and MMP-9 to co-localize in tumor cells at the invasive
front. Western blot analysis demonstrated a significant correlation (p = 0.0047) between CD44 and
MMP-9 in the tested tissues. In addition gelatinolytic activity of HNSCC tissues was found to significantly
correlate (p = 0.0010) with MMP-9 expression. The CD44+ invasive front of the tumor was also positive for
ALDH1 and CK14, all of them being typically expressed by cells in the basal cell layer of normal stratified
squamous epithelia that also harbors the epithelial stem cells.
The observations point to a role of a MMP-9 positive basal-cell-like cell layer in the process of HNSCC
invasiveness. This compartment likely contains CSCs since it is expressing the putative CSC markers
CD44, ALDH1 and CK14. This cell layer therefore should be considered a major therapeutic target in
the treatment of head and neck cancer.