Résumé
BACKGROUND: Cyclic AMP stimulates the expression of the somatostatin (SRIF) receptor (sst1-5) and human growth hormone (GH)-secreting pituitary tumors with the gsp oncogene which increases intracellular cAMP levels, and shows a good inhibitory response of the GH to SRIF. Taken together, we hypothesized that the gsp oncogene may increase the SRIF receptor expression or and factors related to the postreceptor signal transduction of the SRIF, in order to enhance its responsiveness to SRIF. To test this hypothesis, we investigated if the gsp oncogene could increase the sst1, sst2, Gi2 alpha, and pit-1 alpha gene expression in GH3 cells. METHODS: GH3 cells were permanently transfected with the plasmid expressing Gs alpha gene, where the arginine of codon 201 was replaced with histidine. Intracellular cAMP levels and GH concentrations were measured by radioimmunoassays. Gene expressions of the sst1, sst2, Gi2 alpha, and pit-1 alpha were determined by RT-PCR. RESULTS: Intracellular cAMP levels and medium GH release were increased by 1.7 and 2.7-fold in GH3 cells expressing the gsp oncogene, respectively. In GH3 cells expressing the gsp oncogene, the sst1 mRNA levels were decreased, whereas those of the sst2, Gi2 alpha and pit-1 alpha mRNA were increased. A 4-h forskolin (10 M) stimulation remarkably increased the sst1 and sst2 mRNA levels in GH3 cells expressing wild and mutant Gs alpha . However, forskolin did not affect the Gi2 alpha and pit-1 alpha mRNA levels. In contrast, SRIF (1 M, 2 h) decreased the sst2 mRNA levels only in GH3 cells expressing the gsp oncogene. CONCLUSION: These results suggest that higher expressions of sst2, Gi2 alpha, and pit-1 alpha, induced by the gsp oncogene may be a mechanism by which gsp-positive pituitary tumors show a greater response to SRIF. The discrepancy between these and in vivo results should be explored further.