Thyrotropin Regulates the Levels of Free Ubiquitin and Ubiquitin-Protein Conjugates in the Male Mouse Thyroid

The conjugation of ubiquitin to proteins can be a signal for their degradation, but can also be involved in regulatory processes not directly involved in protein degradation. Because thyrotropin (TSH) is the major physiological regulator of the thyroid, we have investigated whether changes in the circulating level of TSH influence the level of immunoreactive ubiquitin in the thyroid, as assessed by dot-blot assays. Putting male Balb/c mice on a low iodine diet with methimazole (MMI) in their drinking water for 14 days raised the level of ubiquitin by 425% (p < 0.001) per microgram nonthyroglobulin protein (the mean thyroglobulin level dropped by 35% (p < 0.05)). Western blots similarly indicated that immunoreactivity migrating in the region of monoubiquitin, ubiquitin oligomers, and ubiquitinated thyroid proteins increased on the low iodine/MMI diet. Injecting 1 μg triiodothyronine (T3) 6 hours prior to sacrifice appeared to reduce the ubiquitin levels by 31% when compared with mice only on the low iodine/MMI (p < 0.07), but injection of T3 had no effect on ubiquitin levels in mice on the control diet. Injecting male ICR mice with a large dose of TSH (200 mU) increased immunoreactive ubiquitin levels by 50% (p < 0.05) 2 hours later. After a second dose of TSH was injected 12 hours later, the level of immunoreactive thyroglobulin fell by 17% (p < 0.05). With further 12 hourly injections, thyroglobulin levels then began to reaccumulate, and they had returned to the level of saline-injected controls after 50 hours (five TSH injections), while ubiquitin levels fell, but remained significantly elevated above the saline-injected controls (36%, p < 0.01). When ICR mice were given perchlorate in their drinking water to block the iodide pump, thus preserving thyroidal responsiveness to repeated TSH injections, the responses to the initial two TSH injections were similar to mice who received ordinary tap water. However, with further TSH injections, the reaccumulation of thyroglobulin did not occur: at 50 hours, thyroglobulin levels remained suppressed by 28% (p < 0.05), and ubiquitin levels actually rose slightly, and were significantly higher (57%, p < 0.01) than the saline-injected controls. These in vivo responses of free ubiquitin, oligoubiquitin, and ubiquitinated protein to changes in the level of circulating TSH suggest that ubiquitin-mediated mechanisms are involved in some of the thyroid's metabolic responses.