Gene Expression of Somatostatin Receptor (Subtype 2 & 5), Gi2 alpha and Pit-1 in GH-secreting Pituitary Adenomas / 대한내분비학회지

BACKGROUND: Mutation of Gs protein subunit (gsp oncogene), detected in about 30~40% of growth hormone (GH)-secreting pituitary tumors, is associated with an increased long-acting somatostatin analog octreotide sensitivity. However, the mRNA expression of somatostatin receptor (sst) was not changed in the GH-secreting pituitary tumor, regardless of whether they were gsp oncogene positive or negative. This suggests that the expression of genes coding for Gi2 alpha , Pit-1 and the other factors involved in the regulation of secretory activity in somatotrophs is likely to be altered in gsp oncogene positive tumors. We observed the impact of the gsp oncogene on the expression of the genes coding for Gi2 alpha, Pit-1 and sst (2&5) in GH-secreting pituitary tumors. METHODS: The GH response to octreotide was examined in 13 acromegalic patients before transsphenoidal adenomectomy. Genomic DNA and RNA were extracted from fresh frozen tumor tissues. PCR was performed to amplify and sequence the region between codon 184 and 251 that includes exons 8 and 9 of the Gs gene. Sst2, sst5, Gi2 alpha and Pit-1 mRNA levels were measured by semi-quantitative RT-PCR. RESULTS: Sst2 and sst5 mRNA transcripts were detected in all tumors (7 gsp +, 6 gsp-). The amount of sst transcripts varied considerably varied between the tumors. There were no significant differences in sex, age, tumor size, grade or basal GH levels. Pit-1 and sst2 mRNA levels were not different. In contrast, Gi2 alpha mRNA levels were significantly higher in gsp (+) while sst5 mRNA levels were higher in gsp (-). CONCLUSION: These data suggests that gsp oncogene may increase Gi2 alpha levels but decrease sst5 mRNA levels. However, Pit-1 and sst2 mRNA expression may not be affected by gsp oncogene. The increased expression of the Gi2 alpha gene might be an inhibitory compensatory response to the action of gsp oncogene.

Thyrotropin-releasing Hormone(TRH) Receptor Gene Expression in GH3 Cells Permanently Transfected with a Mutant Gs alpha Gene / 대한내분비학회지

BACKGROUND: Gs alpha gene mutation, that constitutively increases intracellular cAMP, is found in some acromegalic patients. It was demonstrated that increased intracellular cAMP levels suppress the expression of rat TRH receptor (TRH-R) mRNA. We previously demonstrated that transient expression of a mutant Gs alpha gene suppress the rat TRH-R gene expression in the cultured rat growth hormone-secreting tumor cell line (GH3), whereas TRH-R gene expression in adenomas with Gs alpha gene mutation (gsp oncogene) did not differ from that in tumors without the mutation. The discrepancy suggests the possibilities that the effect of permanent expression of mutant Gs alpha gene on TRH-R gene expression is different from that of transient expression of the mutant gene and hypothalamic hormones including TRH regulate the gene expression. METHODS: We investigated whether permanent expression of the mutant-type Gs alpha does not suppress the TRH receptor gene expression in GH3 cells, and whether TRH suppresses the gene expression by using reverse transcription-polymerase chain reaction (RT-PCR) and in vitro transcription. RESULTS: Permanent expression of a mutant-type Gs alpha increased basal cAMP levels up to 1.7-fold relative to the controls, whereas the wild-type cell line did not show increased cAMP levels. Permanent expression of a mutant-type Gs alpha increased TRH receptor mRNA level up to 2.8 fold compared with the controls. Treatment of the permanently transfected GH3 cells with TRH suppressed TRH-R gene expression more prominently compared to the wild type GH3 cells. CONCLUSION: These results suggest that permanent expression of mutant Gs alpha enhances the expression of TRH-R in GH-secreting pituitary tumors with gsp oncogene, but the gene expression may also be regulated by other factors including TRH.

Characteristics of gsp-Positive Growth Hormone-secreting Pituitary Tumors

A subset of human growth hormone(GH)-secreting pituitary tumors contains the gsp oncogene that encodes an activation mutation of the alpha subunit of Gsalpha, a stimulatory GTP-binding protein. The purpose of this study was to investigate the frequency of the gsp oncogene in GH-secreting pituitary tumors in Korean acromegalic patients and to elucidate the clinical reaction of these patients to endocrine testing. Direct PCR sequencing revealed gsp oncogene mutation in nine of 21 tumors(43%) at amino acid 201 of the Gsalpha protein. A single nucleotide mutation in tumors carrying the gsp oncogene was observed in eight tumors, this replaced an arginine(CGT) in normal protein with cysteine(TGT), and in one, with serine(AGT). Patients in the in whom gsp oncogene mutation occurred were older(54 years vs. 41 years, p=0.01) than those in the normal group. Sex, tumor grade, basal GH and PRL levels, GH response to oral glucose loading, GH fluctuation, and paradoxical response to TRH or GnRH did not differ between the groups : gsp oncogene was found mostly in somatotroph adenomas. Octreotide-induced GH suppression was significantly higher in the mutation group than in the normal group(95% vs. 81%, p=0.03), but the GH response to bromocriptine did not differ between the groups. These results suggest that Gsalpha mutations of GH-secreting tumors occur in Korean acromegalic patients with a similar frequency to that found in Western countries. Patients with gsp oncogene are likely to be older than those without it, and show high levels of octreotide-induced GH suppression.