Intracellular changes in enzyme activity and the isoform pattern of protein kinase C (PKC) during treatment with doxorubicin (DXR) were examined in a rat ascites hepatoma AH66 parental cell line (AH66P), which shows slight expression of P-glycoprotein (Pgp) on the cell membrane and is classified as an intrinsic multidrug-resistant cell. AH66P cells, treated for 10 days with DXR at the concentrations of 0.3 or 5.0 muM, exhibited moderate to severe inhibition of PKC activity after transient increase of the activity, which mainly was either Ca2+-independent and phospholipid-dependent or Ca2+-independent and phospholipid-independent. PKC isoform analysis of the treated cells also indicated the transient 1.5- and 1.8-fold increase of PKC-delta and PKC-zeta as well as slight increase of PKC-alpha in treatment with 0.3 muM DXR and 1.9- and 1.6-fold increase of PKC-alpha and PKC-zeta with slight increase of PKC-delta in treatment with 5.0 muM, respectively. A small, but interesting discrepancy between the markedly elevated enzyme activity, which was composed particularly of both cofactor-independent forms, and the relatively low levels of isoform expression, was found when the cells were treated with 5.0 muM DXR. It is of great interest that DXR-treatment, while only slight increasing P-glycoprotein (Pgp) phosphorylation, enhanced the transient expression of Pgp on the cell membrane with good correlation with the fluctuating change in PKC activity. These results indicate that the possible expression of some or novel specific isoform(s) of PKC may modulate and be associated with expression of the Pgp.

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