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.