Thyroid glands: novel sites of growth hormone action.

Thyroid hormones inhibit the synthesis and release of GH in avian species. This may represent a feedback mechanism, since GH enhances the peripheral production of tri-iodothyronine (T3). The possibility that GH may also have direct effects on thyroidal function was therefore investigated. The basal and thyrotrophin-induced release of thyroxine (T4) from incubated chicken thyroid glands was not enhanced, however, in the presence of chicken GH. Contrarily, GH impaired T4 release in a dose-related way. These actions were probably mediated by specific receptors, since binding sites for radiolabelled GH were demonstrated on the plasma membranes of chicken thyroid glands. Expression of the GH receptor gene in these tissues was also demonstrated using a cRNA probe for the rabbit liver GH receptor, which specifically hybridized with RNA moieties of 4.4 kb, 2.7 kb and 1.0 kb. Moreover, reverse transcription of thyroidal RNA and its amplification in the presence of 3'- and 5'-oligonucleotide primers coding for the extracellular or intracellular domains of the GH receptor generated electrophoretically separable fragments of 500 bp and 800 bp respectively, as would be expected from analysis of the hepatic GH receptor cDNA sequence. Digestion of the 500 bp fragment with NcoI or EcoRI also produced moieties of expected size (350 bp and 150 bp or 325 bp and 175 bp respectively), as did BamHI or HaeIII digestion of the 800 bp fragment (yielding fragments of 550 bp and 275 bp or 469 bp and 337 bp respectively). Translation of the GH receptor mRNA was also indicated by the immunocytochemical demonstration of GH receptors in thyroid follicular and parafollicular cells, using a specific polyclonal antibody raised against the chicken GH-binding protein. These results therefore provide evidence, for the first time, of GH receptor gene expression in thyroid tissue and the translation of functional GH receptors in thyroid glands. These results also demonstrate differential effects of GH on the extracellular concentrations of T3 and T4, which may permit subtle regulation within the somatotroph-thyroid axis.

Growth hormone (GH)-binding proteins in GH-resistant guinea pigs.

The presence of growth hormone (GH) receptor (GHR) gene transcripts and GH-binding sites in guinea pig liver suggests normal expression and translation of a GHR gene in these animals. Guinea pigs are, however, resistant to GH action and appear to lack the circulating GH-binding proteins (GHBPs) that result from alternate splicing of the GHR message or from cleavage of the extracellular binding domain of membrane GHRs. The paradoxical absence of circulating GHBPs in guinea pigs was therefore examined. The presence of GHR/GHBP mRNA in guinea pig liver was confirmed by Northern blotting. In addition to a 4.4 kb transcript that probably encodes a full-length receptor, an additional 1.9 kb transcript was detected that may encode a binding protein, although this transcript is larger than rat GHBP mRNA. The possibility that these transcripts may be translated into GHBPs was assessed immunologically. A 46 kDa protein, identical in size to rat GHBP, was specifically detected in guinea pig liver by a monoclonal antibody (MAb 4.3) raised against the hydrophilic tail of rat GHBP. A single protein of approximately 48 kDa was also detected by MAb 4.3 in proteins precipitated from guinea pig serum by a polyclonal antibody raised against the rat GHBP. This protein was slightly larger than the two proteins (46 kDa and 40 kDa) in rat serum labelled by the same method. The presence of a putative GHBP in guinea pig serum was also supported by the cross-reactivity of guinea pig serum with a monoclonal antibody (MAb 263) raised against rat GHBP. The binding of radioiodinated hGHBP to this antibody was inhibited, in a dose-related way and parallel to that of the standard, by serial dilutions of guinea pig serum, indicating immunoreactive GHBP concentrations > 500 ng/ml. Immunoreactive GHBP concentrations in other mammalian serum (from rats, rabbits, pigs, cattle, horses, goats, dogs and humans) were, in contrast, < 30 ng/ml. Guinea pig sera similarly cross-reacted, but to a lesser degree, in other radioimmunoassays for rGHBP, in which p(Ab)1 or MAb 4.3 were used as the primary antibodies. Nevertheless, despite these immunological findings, hGH binding activity could not be detected in guinea pig serum using a number of different radioligand binding assays. These results suggest the novel presence of abundant, but possibly defective, GHBP-like proteins in guinea pig serum. The immunological detection of the hydrophilic sequence of rat GHBP in guinea pig hepatic and serum proteins also suggests that GHBPs in this species arise from the truncated GHR gene transcript identified in guinea pig liver.