Head and neck squamous cell carcinomas (HNSCCs) develop in the mucosal linings of the upper aerodigestive tract and are the sixth leading cause of cancer worldwide. They are initiated by tobacco and alcohol consumption, and by infection with high-risk types of human papillomavirus (HPV). The neoplastic process begins with the normal epithelium progressing through hyperplasia to dysplasia to carcinoma in situ and invasive carcinoma.

Retinoic acid (RA) and its derivatives (retinoids), metabolites of vitamin A, have been recognized as a group of cancer chemopreventive and therapeutic agents, because of their ability to induce differentiation of various types of stem cells and arrest of cellular proliferation. Recent studies show that RA sensitizes colorectal cancer cells to apoptosis by inducing the proapoptotic ligand, TNF-related apoptosis inducing ligand (TRAIL). RA increases the expression of TRAIL receptors, Death Receptors 4 & 5 (DR4/5), while suppressing TRAIL nonfunctional receptors, Decoy Receptors 1 & 2 (DcR1/2) in colorectal cancer cells. In this study, we analyzed the synergistic effects of RA and TRAIL on the induction of apoptosis in human oral squamous cell carcinoma derived cell-lines.

The human HNSCC cell lines, SCC-15, SCC-25, and OKF6/hTERT-1 (an immortalized oral mucosa cell line), were treated either with RA, TRAIL, or RA and TRAIL for 7 days. These treated cells were then analyzed for changes in cell proliferation (by growth curves), gene expression (by semi-quantitative PCR), and markers of apoptosis and differentiation (by Western blot). It was expected that cellular proliferation and expression of DcR1/2 would be reduced in cells treated with RA and TRAIL, while expression of DR4/5 and markers of apoptosis (PARP cleavage) would be higher in RA and TRAIL-treated cells. We show that RA and TRAIL decreased cellular proliferation in the OKF6/hTERT-1 cell-line; however, RA in combination with TRAIL did not result in a significant change in SCC-15 or SCC-25 cell-lines. In addition, Western blot results show that RA and TRAIL increased the cleavage of PARP in the SCC-25 cell-line. Finally, as expected, RA decreased the expression of Oct4, a stem cell marker, and increased the expression of E-cadherin, an epithelial cell marker.


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