c-myc gene inactivation during induction of nasopharyngeal carcinoma cells with retinoic acid

OBJECTIVE: To investigate the effects of retinoic acid (RA) on the growth, morphology, oncogene expression and regulation of nasopharyngeal carcinoma cells. METHODS: Nasopharyngeal carcinoma cell line (HNE1) was induced by RA. The RA-treated and control cells were established and cellular morphology and growth patterns were defined. Oncogene expression and regulation were detected by Northern hybridization and DNase-I hypersensitive site analysis. RESULTS: RA markedly inhibited cell growth. The growth of HNE1 cells was reduced to 50% of the control level on the 4th day of RA (10(-4) mol/L) treatment. After 4 days of treatment, the rapidly growing polygonal cells were reversed into a slow growing phenotype, with flattened morphology similar to fibroblast-like cells. Northern hybridization showed that c-myc and c-Ha-ras expression was high in HNE1 cells and undetectable in normal blood cells. c-myc was down-regulated at 48 h of RA treatment. In contrast, the c-Ha-ras was not affected. DNase I hypersensitive site analysis detected changes in the regulatory elements of c-myc and c-Ha-ras genes. 5 hypersensitive sites were found in the c-myc of HNE1 cells, while 3 hypersensitive sites disappeared upon HNE1 induction. However, only 1 hypersensitive site was found in c-Ha-ras of RA treated cells and controls. In normal peripheral white blood cells, no DNase I hypersensitive sites were found in the inactive c-myc and c-Ha-ras gene. CONCLUSION: RA can induce differentiation in a nasopharyngeal carcinoma cell line at high concentration of RA; HNE1 shows some similar patterns of DNase I hypersensitive sites with the common one in other types of cells expressing c-myc. The repression of c-myc expression with induction is accompanied by the loss of 3 DNase-I hypersensitive sites; c-myc has more than one inactive conformation.