Nuclear distribution of porphobilinogen deaminase (PBGD) in glioma cells: a regulatory role in cancer transformation?

Recently, considerable interest has been directed to red-fluorescence photodiagnosis of brain and other tumours during surgery using the protoporphyrin IX natural precursor, 5-aminolaevulinic acid. In the present study we focused on the role of the rate-limiting enzyme porphobilinogen deaminase in glioma C6 cell activity, differentiation and sub-cellular distribution. Over-expression of the human housekeeping porphobilinogen deaminase in the glioma cells, using the housekeeping-porphobilinogen deaminase plasmid, induced a G1 cell cycle attenuation accompanied by increases in enzyme activity and c6 differentiation toward astrocytes. Visualisation of subcellular localisation of the porphobilinogen deaminase using the independent techniques of fluorescence immuno-staining with specific anti-human porphobilinogen deaminase antibodies and cellular expression of porphobilinogen deaminase fused to green fluorescent protein, revealed (unexpectedly) a major fraction of porphobilinogen deaminase in the nucleus and only a minor fraction in the cytoplasm. Both C and N terminals of porphobilinogen deaminase fused to green fluorescent protein revealed a major fraction of the newly synthesized fused porphobilinogen deaminase in the nucleus. Furthermore, newborn rat brain cells grown in a primary culture showed the same localisation pattern of porphobilinogen deaminase in the nuclei. Stimulation of C6 glioma cell differentiation by butyrate induced a marked decrease in porphobilinogen deaminase both in the nucleus and in the cytoplasm as determined by Western blotting and fluorescence immuno-localisation. These findings suggest a possible dual role for housekeeping porphobilinogen deaminase in fast dividing glioma cells, one related to the porphyrin synthesis pathway and another coupled to nuclear function, which might be linked to tumorigenesis.