Keratin expression in astrocytomas: an immunofluorescent and biochemical reassessment

Several studies have shown that immunoenzymatic staining of formalin-fixed, paraffin-embedded astrocytomas with keratin antibodies frequently yields positive labelling, but no biochemical evidence of keratin expression in astrocytomas has been reported. We have investigated the presence of keratin in astrocytoma and normal brain tissues both by immunofluorescence on frozen sections and by 1D and 2D immunoblotting using seven monoclonal antibodies that, collectively, recognize most keratin polypeptides. Four of these antibodies did not stain neural tissues by immunofluorescence and were also negative by immunoblotting. The remaining three keratin antibodies stained normal brain and/or a high proportion of astrocytomas. Two of these three antibodies only stained glial fibrillary acidic protein (GFAP)-positive cells, while the third only stained GFAP-negative cells. 1D and 2D immunoblotting analysis showed that positive immunofluorescence staining of normal brain and/or astrocytomas seen with these three keratin antibodies was due to cross-reactivity with non-keratin proteins, such as GFAP. These results demonstrate that, contrary to earlier suggestions, keratin polypeptides are not frequently expressed in astrocytomas. Our studies also emphasize that keratin antibodies should be used cautiously for the differential diagnosis of undifferentiated gliomas from tumours of non-glial origin.