Expression pattern of alpha-protein kinase C in human astrocytomas indicates a role in malignant progression

Abstract

Protein kinase C (PKC) is a family of isoenzymes which play an important role in regulating cell proliferation and differentiation. Constitutive activation of PKC, either by phorbol esters or overexpression of specific isoenzymes, leads to growth abnormalities in vitro and tumor promotion in vivo. Since stimulation of PKC in cultured astrocytes results in biochemical and morphological alterations associated with the transformed phenotype, we wanted to determine whether abnormal expression of specific isoenzymes of PKC was important in development of human astrocytomas in vivo. We have detected a specific pattern of alpha-PKC expression in human astrocytomas which is noteworthy because the highest transcript levels were detected in well-differentiated (Grade 1) tumors, with intermediate expression in anaplastic (Grade 2) astrocytomas and low or nondetectable levels in glioblastomas (Grade 3 astrocytomas) and normal controls. In comparison, the beta-PKC transcript was not detected in any of the tumors, while the gamma-PKC transcript was present in only one Grade 2 tumor. Immunohistochemistry, using a monoclonal antibody to alpha-PKC, revealed diffuse, positive cytoplasmic signals in most cells of the Grade 1 tumors. Grade 2 tumors exhibited heterogeneity of alpha-PKC expression, although a significant percentage of cells showed positivity. In contrast, only a small number of differentiated cells within Grade 3 tumors were positive for alpha-PKC expression, with the more malignant, dedifferentiated cells uniformly negative. Throughout all tumor grades, the staining pattern of alpha-PKC closely paralleled that of glial fibrillary acidic protein. Taken in conjunction with the established role of PKC in tumor promotion, these results suggest that the alpha-PKC isoenzyme plays a specific role in facilitating clonal expansion of transformed astrocytes in low-grade astrocytomas. Analysis of alpha-PKC may therefore serve as a direct biological marker of malignancy which may serve to enhance the current histopathological grading system.