Disparity in expression of protein kinase C alpha in human glioma versus glioma-derived primary cell lines: therapeutic implications.

Intracellular signal transduction by the protein kinase C (PKC) family of enzymes plays a critical role in carcinogenesis and cellular growth regulation. Recent studies have suggested that the PKC isoform alpha may be a critical target for antiglioma therapy in humans (G. H. Baltuch et al., Can. J. Neurol. Sci., 22: 264-271, 1995). We studied the expression and subcellular distribution of the PKC alpha isoform in human high- and low-grade gliomas and also in glioma-derived cell lines with immunoblot analyses. Cell lines derived from high-grade gliomas expressed higher levels of PKC alpha than did cell lines derived from low-grade gliomas. In glioblastoma-derived cell lines, PKC alpha was mainly expressed in the soluble (cytosolic) fraction, indicating an inactive state of the enzyme. When analyzed in freshly frozen samples from human gliomas, the expression of PKC alpha was at similar levels in high- and low-grade tumors and was also similar to the levels in normal brain tissue controls. The PKC partial antagonist bryostatin 1, currently undergoing Phase II testing in patients with malignant gliomas, was capable of specifically down-regulating PKC alpha in vitro in glioblastoma-derived cell lines. However, this was not associated with significant growth inhibition. We conclude that the observed overexpression of PKC alpha in glioblastoma-derived cell lines may be an artifact of in vitro growth. Furthermore, we conclude that expression of PKC alpha in glioma-derived cell lines is not essential for cellular growth in vitro because down-regulation of PKC alpha following treatment with bryostatin 1 was not associated with growth inhibition.