Retinoid modulation of c-myc and max gene expression in human breast carcinoma.

Retinoic acid (RA) modulation of c-myc and max gene expression was investigated in human breast carcinoma (HBC) cell lines. Our results demonstrate that c-myc and max genes are differentially expressed in HBC cells which was accompanied by increases in [3H]-thymidine incorporation and percent of cells in the S phase of cell cycle. RA-mediated increase in c-myc mRNA levels were noticed as early as 30 min. and maximum levels were attained by 1 h. RA effect on max mRNA levels was slow and gradual with the maximum effect noticed by 48 h. Nuclear run-on assays demonstrated that RA mediated its effect by increasing the rate of transcription of both of these genes. We thus report for the first time that RA, during its growth inhibitory effects on MCF-7 HBC cells, positively regulates the gene expression of c-myc and max.