Changes in enzyme activity and the isoform pattern of protein kinase C (PKC) during the process of acquiring of drug resistance to doxorubicin (DXR) was examined in rat ascites hepatoma AH66 parental cells (AH66P) and several established clones of DXR-resistant AH66DR cells. In the weakly resistant cell line, AH66DR-0.3, which was resistant to DXR at the concentration of 0.3 mu M, reduced PKC activity and significantly decreased expression of its isoforms (alpha, delta and zeta) were observed. By contrast, AH66DR-30, the clone with the highest resistance, showed increased PKC activity, which was mainly either Ca2+-independent and phospholipid-dependent or Ca2+-independent and phospholipid-independent. PKC-isoform analysis of the AH66DR-30 cells disclosed high levels of PKC-delta and -zeta and a quite low level of PKC-alpha relative to that of the AH66P cells. In both cell lines, phosphorylation of P-glycoprotein (Pgp), which is known as a drug efflux pump, was induced 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, may be necessary for expression as well as activation of the function of Pgp.

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