Expression of somatostatin receptor subtypes in cultured astrocytes and gliomas

Expression of receptors for the neuropeptide somatostatin was investigated in vitro in rat and human astrocytes, glioma cell lines, and solid human glial tumors that were all immunopositive for the astrocytic marker glial fibrillary acidic protein. After affinity labelling with a peptide-gold conjugate of known biological activity, somatostatin-binding sites could be visualized at the light-and electron-microscopic level on the surface of glial cells. Glioma cells were generally labeled more strongly than were normal astrocytes and preferentially bound the ligand at their processes and not at their somata as were normal cells. Somatostatin transmembrane receptor (SSTR) subtype expression was probed by reverse transcription-polymerase chain reaction: In rat and human cortical astrocytes and in one glioma cell line (U 118), a pattern of three subtypes (SSTR-1, SSTR-2, and SSTR-4) was detected, whereas, in all other glioma cell lines and in six solid glial tumors investigated, the SSTR-2 subtype was relatively stronger, expressed either alone or in combination with SSTR-1; sometimes SSTR-3 or SSTR-4 was demonstrated in clearly reduced amounts. In astrocytes and gliomas, somatostatin reduced the levels of cyclic AMP elicited by the adenylate cyclase activator forskolin indicating that at least one of the receptor subtypes is negatively linked to adenylate cyclase. In contrast to other cell types, somatostatin did not inhibit the basal or the fetal calf serum-stimulated proliferation of astrocytes, glioma cell lines, or glial tumors in culture. Thus, strong SSTR-2 subtype expression characterizes glial tumors, but somatostatin is ineffective in inhibiting their growth.