Pituitary tumors arising from glycoprotein hormone alpha-subunit-deficient mice contain transcription factors and receptors present in thyrotropes.

Glycoprotein-hormone alpha-subunit deficient (alphaSUnull) mice are hypothyroid and hypogonadal due to the absence of functional TSH, LH and FSH, despite normal production of the corresponding beta subunits. Pituitary tumors spontaneously developing in alphaSUnull mice were propagated in hypothyroid mice. The purpose of the current studies was to compare the gene expression profile of these alphaSUnull tumors with previously characterized TtT-97 thyrotropic tumors. A group of animals bearing each tumor type was treated with thyroid hormone (T4) prior to tumor removal. Both tumor types equally expressed TSHbeta mRNA, which significantly decreased when exposed to T4, whereas alpha-subunit mRNA was absent in alphaSUnull tumors. Northern blot analysis was performed using cDNA probes for the following transcription factors: Pit1, GATA2, pLIM, Msx1, Ptx1 and Ptx2. Both tumors were found to contain identical transcripts with similar responses to T4, with the exception of Pit1. In contrast to the signal pattern seen in TtT-97, only two bands were seen in alphaSUnull tumors, which were similar in size to those in alphaTSH cells, a thyrotropic cell line that lacks TSHbeta-subunit expression and Pit1 protein. However, western blot analysis revealed a protein band in the alphaSUnull tumors consistent with Pit1, while this signal was absent in alphaTSH cells. Northern blot analysis was also performed with specific cDNA probes for the following receptors: TRbeta1, TRbeta2, TRalpha1, non-T3 binding alpha2, RXRgamma and Sst5. Similarly-sized transcripts were found in both types of tumor, although the signal for Sst5 was seen in T4-treated alphaSUnull tumors only with a more sensitive RT-PCR analysis. The overall similarity between the two tumor types renders the alphaSUnull tumor as a suitable thyrotropic tumor model.

Targeted disruption of the pituitary glycoprotein hormone alpha-subunit produces hypogonadal and hypothyroid mice.

Pituitary thyrotropin (TSH) and gonadotropins (LH and FSH) are thought to be critical for thyroid and gonadal development and function. Each of these pituitary hormones is a heterodimer composed of a common alpha-subunit and unique beta-subunit, and heterodimerization is required for function. No mutations in the alpha-subunit or any of the beta-subunit genes have been reported in mice. To assess directly the functional role of TSH, LH, and FSH in thyroid and gonadal development, we created a disruption of the alpha-subunit gene by homologous recombination. The homozygous mutant animals were hypogonadal and exhibited profound hypothyroidism resulting in dwarfism. Thyroid development was arrested in late gestation, but GnRH neuron migration, development of secondary sex organs, and fetal and neonatal gonadal development were normal. This establishes the importance of thyrotropin in ontogeny and reveals that fetal pituitary gonadotropins are not required for sexual differentiation or genital development in male or female fetuses. The pituitary cells that produce TSH beta-subunit exhibited dramatic hypertrophy and hyperplasia as a result of the lack of thyroid function. This proliferation response occurred at the expense of somatotrope and lactotrope cells, consistent with a derivation of these three cell types from a common precursor.