Keratin intermediate filament expression in astrocytic neoplasms: analysis by immunocytochemistry, western blot, and northern hybridization.

In this study, 29 formalin-fixed, paraffin-embedded astrocytic tumors were analyzed immunocytochemically with the antikeratin monoclonal antibodies Mak-6 and Cam 5.2 and a polyclonal antibody against glial fibrillary acidic protein (GFAP). Immunoreactivity for Mak-6 was present in 29 cases (100%) including six well-differentiated astrocytomas, nine anaplastic astrocytomas, and 14 glioblastomas multiforme. Cam 5.2 immunoreactivity was focally present in one case of GBM (4%) but was absent in the remaining 28 cases. All cases were immunoreactive with an antibody against GFAP. Cytokeratin (CK) expression was examined in extracts of four separate well-characterized astrocytoma cell lines by Western blotting with the monoclonal antibodies Mak-6, Cam 5.2, and anti-CK 18 and by Northern analysis using a cDNA probe for the human CK 18 gene. The Western blots revealed the presence of immunoreactive bands corresponding to CK numbers 14/15, 16, and 18 in extracts from all four cell lines and additional bands corresponding to CK 8 in 3/4 lines and CK 19 in 1/4 lines. Northern analysis detected CK 18 mRNA in extracts from 2/4 astrocytic cell lines. These findings demonstrate that CK immunoreactivity is frequent in astrocytic tumors and confirm through the molecular and biochemical analysis of CK 18 gene expression and of keratin intermediate filament proteins that the basis for CK immunoreactivity in astrocytic tumors is bona fide CK expression, not cross-reactivity with other antigens or artifact. The demonstration of CK expression by astrocytic neoplasms has important implications for pathologists involved in the diagnosis of poorly differentiated tumors.