Expression of chorionic gonadotropin-beta-like messenger ribonucleic acid in an alpha-subunit-secreting pituitary adenoma.
The mRNA and protein products produced by an alpha-subunit-secreting pituitary tumor were studied to further characterize the early steps in glycoprotein hormone biosynthesis. alpha-Subunit mRNA was readily detected by RNA blot hybridization to alpha-subunit cDNA and was qualitatively normal, with a length of about 800 bases. Although only excess free alpha-subunit was secreted by the tumor, abundant beta-subunit mRNA hybridized to a cDNA which recognized both LH beta and CG beta sequences. Surprisingly, the beta-subunit mRNA in the pituitary tumor was characteristic of CG beta mRNA. The beta-subunit mRNA migrated in parallel with placental CG beta mRNA (approximately 1000 bases) and was easily resolved from LH beta mRNA (approximately 700 bases). Using site-specific hybridization and oligonucleotide-primed extension, the beta-subunit mRNA in pituitary tumor was found to have an extended 5'-untranslated tract (366 bases) characteristic of CG beta gene transcripts rather than LH beta gene transcripts, in which the 5'-untranslated tract is only 9 bases. Activation of the CG beta-like promoter rather than the LH beta promoter indicates that the cellular mechanisms regulating tissue-specific expression of the beta-subunit gene were altered by the neoplastic changes in the pituitary tumor. Protein analysis of tumor homogenate demonstrated large amounts of alpha-subunit in the form of intact FSH and small amounts of intact LH and free alpha-subunit. Despite the presence of abundant CG beta-like mRNA in the tumor, intact hCG or hCG beta was not detected. The uncoupling of beta-subunit gene expression and protein biosynthesis in the setting of ongoing alpha-subunit biosynthesis provides a potential mechanism for unbalanced synthesis and secretion of free alpha-subunit.
J L
JL
Jameson
C M
CM
Lindell
D W
DW
Hsu
J F
JF
Habener
E C
EC
Ridgway
D000236:Adenoma / Q000378:metabolism*; D000368:Aged; D000818:Animals; D006063:Chorionic Gonadotropin / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D018997:Chorionic Gonadotropin, beta Subunit, Human; D002850:Chromatography, Gel; D004247:DNA; D015292:Glycoprotein Hormones, alpha Subunit; D006651:Histocytochemistry; D006801:Humans; D007124:Immunoenzyme Techniques; D008297:Male; D009693:Nucleic Acid Hybridization; D010446:Peptide Fragments / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D010911:Pituitary Neoplasms / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism*; D011863:Radioimmunoassay; D051381:Rats; D014158:Transcription, Genetic
D006063:Chorionic Gonadotropin; D018997:Chorionic Gonadotropin, beta Subunit, Human; D015292:Glycoprotein Hormones, alpha Subunit; D010446:Peptide Fragments; D012333:RNA, Messenger; D004247:DNA