Comparison of somatostatin receptor expression in human gliomas and medulloblastomas

The expression of the five somatostatin receptor subtypes, sst(1-5) was compared on tissue containing glial tumours (glioblastomas or oligodendrogliomas), medulloblastomas, and on normal human cortex. By semiquantitative reverse transcription coupled to polymerase chain reaction, the receptor expression profiles were high in cortex and in tissue containing oligodendrogliomas. It was moderate in medulloblastomas. Tissue containing glioblastomas displayed lower expression of somatostatin receptor subtypes, sst(1) and sst(3) being mostly expressed. By I-125-Tyr(0)DTrp(8) somatostatin-14 or I-125-Leu(8)DTrp(22)Tyr(25) somatostatin-28 autoradiography combined with synaptophysin immunohistochemistry, it was possible to differentiate between isolated tumoral cell component infiltrating the cerebral parenchyma (cortex or white matter) and tumoral tissue (without residual parenchyma) in glioblastomas or oligodendrogliomas. Glial tumoral tissue per se presented few somatostatin receptors. By contrast, medulloblastoma tumoral cells exhibited numerous octreotide sensitive somatostatin receptors. sst(2) immunocytochemistry demonstrated immunostaining of neuronal cells and neuropile; sst(2) and sst(3) immunostaining was identified on glioblastoma proliferating vessels endothelial cells and on medulloblastomas tumoral cells. Faint sst(2) immunostaining among glial tumoral cells was due to microglia, while glioma cells did not significantly stain. In summary, medulloblastoma tumoral cells express sst(2) /sst(3) receptors at a high level while glioma cells do not. In gliomas, sst expression is restricted to endothelial cells on proliferating vessels (displaying both sst(2) and sst(3) receptors), including parenchyma and reactive microglia (only sst(2)). The differential expression of sst(2)/sst(3) receptors on gliomas and medulloblastomas has implications for the therapy of these tumours.