Retinoic acid and forskolin activate the human MDR1 gene promoter in differentiated neuroblasts.

To determine whether over-expression of the MDR1 gene may result from activation of its promoter by differentiating agents, the activity of the human MDR1 proximal promoter (MDR1 pp) transfected to 2 neuroblastoma lines treated with retinoic acid and forskolin was first measured using transient expression assays while the MDR1 mRNA levels were measured by Northern blots. The results indicate that retinoic acid and forskolin were able to activate the human MDR1 pp in a dose dependent manner after transfection of the MDR1pp- CAT constructs in the 2 cell models tested, i.e., SK-N-SH and IGR-N-91, a new human neuroblastoma cell line. A significant increase in MDR1 gene transcript levels was observed upon treatment with differentiation inducers in SK-N-SH but not in IGR-N-91 neuroblasts. These results suggest that the induction of the MDR1 gene promoter is necessary but not sufficient to lead to an increase in MDR1 gene transcript levels, according to the neuroblast cell line considered.