EFFECT OF THE ACTIVATED RAF PROTEIN-KINASE ON THE HUMAN MULTIDRUG-RESISTANCE-1 (MDR1) GENE PROMOTER

Revealing the regulatory mechanism of the multidrug resistance 1 (MDR1 ) gene is important to gain understanding of MDR in tumor cells. Using MDR1 deletion constructs and the 22W mutant of c-Raf in which the NH2 -terminal half has been deleted, we examined the effect of the activat ed Raf on human MDR1 promoter activity in transient expression assay a nd stable transfectants of GHE-L cells. A DNA sequence exhibiting stro ng activation of MDR1 promoter by 22W was located between -197 and -13 6 containing the upstream heat shock element (HSE) motifs without othe r regulatory elements, whereas the MDR1 deletion construct containing downstream HSE motif showed a relatively weaker activation by 22W. We observed that the activated Raf significantly potentiated the inductio n of MDRCAT activity in GHE-L cells by sodium arsenite or heat shock, which stimulates heat shock factor (HSF) binding to HSE. In addition, protein kinase A inhibitor (H-87) blocked the activation of the MDR1 p romoter by 22W in GHE-L cells in a dose-dependent manner. From these r esults, we propose the possibility that Raf- and protein kinase A-depe ndent pathways control the transcription of MDR1 gene via a mechanism involving the modulation of HSF activity.