Abstract:
Oral squamous cell carcinoma (OSCC) is the sixth common cancer worldwide. Despite the amount of research and the development of diagnosis and treatment methods, there is no significant improvement in the patient-survival rates. The treatment outcome prognostic and predictive biomarkers in OSCC still lack the clinical routine compared with other tumers in other regions of the human body, where such biomarkers have been more recognized as causative factors.
In recent studies, it is becoming more evident that most, perhaps all, malignancies are developing from the so-called cancer stem cell (CSC) compartment. Also from clinical observation, theories suggested the existence of residual stem cells after therapy that may be responsible for tumor regeneration, which demand developing new treatment modalities that can target the critical CSCs regenerating and self-renewal properties. In this manner, CD44, which is a cell-surface extracellular matrix receptor, is the most recognized and studied CSCs related marker.
Vitamin D (VD) and its metabolites, mainly 1,25-(OH)2D3 are known to regulate cell growth maintenance, cellular proliferation, and differentiation induction. Accordingly, Vitamin D receptor (VDR) expression is the mediator of anticancer VD activity and its analogues in some cancers. In head and neck squamous cell carcinoma and OSCC, VDR mediates the inhibition of neoplastic activities and cellular growth.
This is the first study to assess the link between putative CSC compartment and VDR expression in CD44+ tumor cells. This study was designed to analyze the expression of VDR in OSCC and normal mucosal tissue and to identify the relation between VDR expression and tumor relapse.
A retrospective analysis demonstrated VDR expression in OSCC and normal tissue samples by immunohistochemistry. Immunohistochemical and immunofluorescent double labeling experiments for VDR+, with CD44+ and Ki-67+ were carried out. VDR expression was analyzed in cells from the OSCC BICR3 and BICR56 cell lines in cytospins as positive control of VDR expression by cancer cells. VDR gene expression was measured by RT-PCR. Western blot analysis confirmed VDR antibody specificity.
This data confirmed that VDR expression is present in both cancer and non-cancer tissue samples. There was no association between VDR expression and clinicopathological characteristics although univariate analysis and multivariate analysis showed worse survival rates in OSCC patients with low VDR expression. Immunohistochemical double labeling experiments in OSCC tissue serial sections and OSCC cell lines confirmed that VDR expression is associated with CD44+ tumor cells. Our analysis of immunohistochemical double staining experiment data (CD44+/VDR+) showed increased co-expression of VDR+ with CD44+ cancer cells. Western blot expression in OSCC cell lines confirmed increased VDR expression in OSCC cell lines compared to normal tissue. In both tissue samples, VDR expression was mainly restricted to the basal cell layer. Correlation analysis from quantified immunohistochemical single staining revealed that Ki-67 expression in OSCC is inversely correlated to VDR expression.
Our finding supports the hypothesis that increased VDR expression may be associated with increased tumor-specific survival amongst patients with high VDR expression compared to patients with low VDR. From our results, we speculate that the use of 1,25-(OH)2D3 or its non-hypercalcemic analogues as adjuvant chemopreventive therapy could be useful.
In conclusion, this study provides the first evidence that decreased VDR expression in OSCC might be associated with tumor relapse. However, VDR expression found in a putative CD44+ CSC compartment may represent an entrance gate for other adjuvant treatment modalities in OSCC. Chemopreventive therapy using 1,25-(OH)2D3 or its analogues to target adjuvant residual tumor cells could be successful, which in turn may help enhancing tumor supportive treatments in the clinic. However, further in vivo and in vitro investigations are required to support this hypothesis.
