Mdr1 mRNA expression differs between grade III astrocytomas and glioblastomas.
104 N2-frozen samples from 33 astrocytic tumors previously untreated with cytotoxic drugs have been analyzed for the expression of p-glycoprotein transcripts (mdrl) by RT-PCR using beta2-microglobulin as an internal control. A remarkable variation was observed even within a single tumor in 50% of the cases. Nevertheless, a difference became visible between the groups of anaplastic astrocytomas and glioblastomas. While 78% of the grade 3 astrocytomas contained at least a minimum of 1 sample with a very low mdr1 expression, this was the case only in 23% of the glioblastomas. This supports an earlier observation revealing a positive correlation between tumor grading and the tumor cell fraction stained with the monoclonal antibody JSB1. On the other hand, no major differences were found between the histological groups when the samples with the highest mdr1 expression were selected to represent the individual tumors. Those samples are less informative. They might be derived from tumor regions in which capillaries deliver a larger fraction of the total mRNA pool. No induction of mdr1 was observed in some early astrocytoma or glioblastoma cell cultures even after administration of high concentrations of the drugs ACNU and VM26, often used in glioma therapy.