✓ Levels of protein kinase C (PKC) isoforms in eight human glioblastoma cell lines and two normal human glial cell cultures were determined. Earlier studies identified PKC-α and PKC-γ in these cell lines but PKC-β was not present. In this study, PKC-ϵ and PKC-ζ are demonstrated immunologically in these cell lines and also in two normal human glial cell cultures. Protein kinase C-δ was not present. When levels of the four isoforms in the tumor cells were compared to levels in the normal cells, no increase was observed in PKC-α or PKC-γ, but PKC-ϵ was elevated three to 30 times in six of the eight tumors, and PKC-ζ was elevated approximately two times in all of the tumors. Incubation of cell line A172 with phorbol ester for 6 hours resulted in a 48-fold maximum increase in the nuclear PKC-ϵ and a sevenfold increase in the plasma membrane fraction with no change in the cytoplasmic fraction. A similar incubation for 4 hours produced a 0.5- to onefold increase of PKC-ζ in cytoplasmic, nuclear, and plasma membrane fractions. Other researchers have shown that overexpression of PKC-ϵ in fibroblasts results in tumorigenesis, and that blocking PKC-ζ function inhibits deoxyribonucleic acid synthesis. These data suggest that alteration in the expression of PKC-ϵ and PKC-ζ could be a factor in the conversion of normal glial cells to glioblastomas.
The protein kinase C-η isoform induces proliferation in glioblastoma cell lines through an ERK/Elk-1 pathway