6th ESACP Congress, Heidelberg, April 7-11, 1999

A030
Inhibition of Tyrosine Kinase and Protein Kinase C Sensitize Tumor Cells Overexpressing MRP But Not PGP
Benderra Z, Morjani H, Manfait M

UFR de Pharmacie, IFR 53, Unité Médian, Reims CEDEX, France

Multidrug-resistance (MDR) is a major limitation to chemotherapeutic treatment of cancer. MDR, conferred by overexpression of P-glycoprotein (Pgp) and/or the multidrug-resistance associated protein (MRP), is characterised by a decreased cellular drug accumulation due to an enhanced drug efflux. In the present study we have examined phosphorylation properties of Pgp and MRP. Both proteins are probably phosphorylated by protein kinase C (PKC) and/or tyrosine kinase (Tyr K). The efficacy of genestein (GNST), CGP57148B (CGP) (inhibitors of Tyr K) and GF109203X (GFX) (inhibitor of PKC) on the sensitivity and nuclear accumulation of daunorubicine (DNR) was examined using K562 cells lines overexpressing MRP (KH30) and Pgp (KH300). Nuclear accumulation of DNR was carried out using microspectrofluorometry confocal laser. We show that nuclear accumulation and cytotoxicity of DNR were increased in KH30 cells in presence of GNST, CGP and GFX. CGP and GFX have a moderate effect on modulation of MDR in KH300 cells. GNST was unable to increase the DNR nuclear accumulation and sensitivity of these cells. These findings suggested that MRP protein is highly phosphorylated by tyr K and PKC. In KH300 cells, CGP and GFX probably interact with Pgp directly. This result- does not support the concept of a major contribution of tyr K and PKC to a Pgp-associated MDR. Finally, GNST, GFX and CGP might be a useful for discrimination between Pgp and MRP phenotypes.