A Cell Line That Produces the Glycoprotein Hormone α-Subunit Contains Specific Nuclear Factors Similar to Those Present in Thyrotropes

Abstract

A unique characteristic of thyrotrope-specific gene expression is the coordinated expression and regulation of the α- and β-subunits of TSH. A cell line (αTSH) derived from the transplantable mouse thyrotropic tumor MGH101A, which no longer expresses the TSHβ-subunit gene but continues to secrete large amounts of α-subunit, was used as a model to study α-subunit gene expression independent from the TSHβ-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 α-subunit gene promoter in both αTSH 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 α-subunit 5′ flanking DNA interactions with αTSH 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 α-subunit expression in thyrotropes are retained in α TSH cells. Moreover, since αTSH cells do not express the TSHβ-subunit gene, the factors that determine the expression of the α-subunit may not be sufficient for TSHβ-subunit gene expression.