CHARACTERISTIC CHANGES IN PROTEIN-KINASE-C ACTIVITY AND ISOFORMS IN AH66 CELLS DURING THE ACQUISITION OF MULTIDRUG-RESISTANT PHENOTYPE

Changes in enzyme activity and the isoform pattern of protein kinase C (PKC) during the process of acquiring of drug resistance to doxorubic in (DXR) was examined in rat ascites hepatoma AH66 parental cells (AH6 6P) and several established clones of DXR-resistant AH66DR cells. In t he weakly resistant cell line, AH66DR-0.3, which was resistant to DXR at the concentration of 0.3 mu M, reduced PKC activity and significant ly decreased expression of its isoforms (alpha, delta and zeta) were o bserved. By contrast, AH66DR-30, the clone with the highest resistance , showed increased PKC activity, which was mainly either Ca2+-independ ent and phospholipid-dependent or Ca2+-independent and phospholipid-in dependent. PKC-isoform analysis of the AH66DR-30 cells disclosed high levels of PKC-delta and -zeta and a quite low level of PKC-alpha relat ive to that of the AH66P cells. In both cell lines, phosphorylation of P-glycoprotein (Pgp), which is known as a drug efflux pump, was induc ed by PKC not only with Ca2+-dependent, but also with Ca2+-independent reactions. These results indicate that the expression of one or more specific isoform(s) of PKC, particularly the Ca2+-independent type, ma y be necessary for expression as well as activation of the function of Pgp.