A cell line that produces the glycoprotein hormone alpha-subunit contains specific nuclear factors similar to those present in thyrotropes.

Abstract

A unique characteristics of thyrotrope-specific gene expression is the coordinated expression and regulation of the alpha- and beta-subunits of TSH. A cell line (alpha TSH) derived from the transplantable mouse thyrotropic tumor MGH101A, which no longer expresses the TSH beta-subunit gene but continues to secrete large amounts of alpha-subunit, was used as a model to study alpha-subunit gene expression independent from the TSH beta-subunit gene and was compared with the expression in TSH-secreting TtT97 tumors. Transient transfection studies showed a striking similarity in the activity of 5' deletions of the mouse alpha-subunit gene promoter in both alpha TSH and TtT97 cells and localized two regions important for expression that spanned 100 base pairs, from -480 to -417 and from -417 to -381. These regions were found to have no activity in nonthyrotrope pituitary GH4 cells and L-cell fibroblasts. Analysis of the alpha-subunit 5' flanking DNA interactions with alpha TSH and TtT97 nuclear extracts showed two DNase I protected sequences, from -474 to -452 and from -447 to -400, both of which colocalized with the functionally important regions. Gel retardation analysis demonstrated the specificity of these interactions, and a similar migration of the DNA-protein complexes suggested that protein factors were similar in the two cell types. We conclude that the nuclear factors necessary for alpha-subunit expression in thyrotropes are retained in alpha TSH cells. Moreover, since alpha TSH cells do not express the TSH beta-subunit gene, the factors that determine the expression of the alpha-subunit may not be sufficient for TSH beta-subunit gene expression.