Among the members of tumour necrosis factor family Fas ligand on binding to its receptor strongly induces apoptosis of tumour-infiltrating lymphocytes (TIL). Thus, FasL acts as an inhibitor of anti-tumour immune response. The present study demonstrates that retinoic acid morpholine amide (RAMA) significantly suppresses FasL expression in colon cancer cells in a dose- and time-dependent manner. The suppression of FasL mRNA and proteins was significant at a concentration of 30 μM after 48 h in CLT85 and HT26 colon cancer cells. There was around 2.6- and 3.2-fold decrease in FasL mRNA after incubation with 30 μM of RAMA in CLT85 cells and HT26 cells, respectively. The results from Western blot showed a decrease in FasL mRNA and protein expression in both CLT85 and HT26 cells after suppression of cyclooxygenase (COX)-2 and COX-1 by RNAi. However, when COX-2-specific silencer RNA (siCOX-2)- and siCOX-1-treated CLT85 and HT26 cells were exposed to RAMA, inhibition of FasL expression was further suppressed. The siCOX-2-treated CLT85 and HT26 cells on exposure to RAMA showed ∼87 and ∼54 % reduction in FasL mRNA, respectively. Co-culture of Jurkat T cells with RAMA-treated HT26 and CLT85 cells decreased the viability of Jurkat T cells by only 2 and 4.3 %, respectively, compared to 19.5 and 37.3 % in control HT26 and CLT85 cells. The results from real-time reverse transcription polymerase chain reaction (RT-PCR) and immunoblotting showed that suppression of EP1 prevented RAMA-induced FasL suppression in CLT85 cells at both the mRNA and protein levels. Thus, RAMA can be a potent therapeutic agent for the treatment of colon tumours.
Keywords: Cytotoxicity; Fas-sensitive; Immune response; Tumour necrosis.