GLYCOPROTEIN HORMONE ALPHA-SUBUNIT PRODUCTION IN SOMATOTROPH ADENOMAS WITH AND WITHOUT GS-ALPHA MUTATIONS

Activating mutations of the Gs-alpha subunit have been identified in a subset of somatotroph adenomas. The mutant form of the Gs-alpha subunit causes persistent activation of adenylyl cyclase and consequently results in high intracellular levels of cAMP. Because cAMP is known to stimulate the synthesis of the glycoprotein hormone (GPH) alpha-subunit as well as GH, we examined somatotroph tumors with and without Gs-alpha mutations for GPH alpha-subunit production. GPH alpha-subunit production was assessed in vivo by measuring serum hormone levels and in vitro by analyzing hormone secretion by cultured pituitary tumor cells. DNA was extracted from the pituitary tumors of 26 acromegalic patients. The Gs-alpha gene was amplified by the polymerase chain reaction and screened for mutations at codons 201 and 227 using oligonucleotide specific hybridization. Nine of the 26 tumors (35%) had point mutations at Arg 201. Seven of these tumors contained a CGT to TGT mutation (Arg to Cys) and 2 contained a CGT to CAT mutation (Arg to His). No mutations were detected at codon 227. There were no significant differences in age, sex distribution, tumor size, or serum levels of GH or insulin-like growth factor-1 between the groups of patients with or without Gs-alpha mutations. The mean serum level of the free GPH alpha-subunit was 1.9-fold higher in the group with Gsa mutations (0.48 +/-0.37-mu-g/L) than in patients without mutations (0.25 +/- 0.17) (P < 0.05). In pituitary tumor cell culture, 75% of somatotroph tumors with Gs-alpha mutations secreted free GPH alpha-subunit into the media compared with 45% of tumors without Gs-alpha mutations. The amount of GPH alpha-subunit secretion was 12-fold greater in the group of tumors containing the Gs-alpha mutation (P < 0.05). Immunocytochemical detection of the free GPH alpha-subunit was similar in the two groups of patients with 75% positive for the GPH alpha-subunit in tumors with Gs-alpha mutations and 67% positive in tumors without mutations (P = 0.69). We conclude that GPH alpha-subunit production occurs in somatotroph tumors with and without Gs-alpha mutations. The increased levels of GPH alpha-subunit secretion in vivo and in vitro suggest that the Gs-alpha mutation may increase the amount of preexisting GPH alpha-subunit biosynthesis in the tumors, perhaps via activation of the cAMP pathway.