Production and release of thyrotropin and its subunits by monolayer cultures containing bovine anterior pituitary cells

The Human TSHβ Subunit Proteins and Their Binding Sites on the TSH Receptor Using Molecular Dynamics Simulation

To gain further insight into the binding of the normal and variant human TSHβ subunits (TSHβ and TSHβv), we modeled the 2 monomeric proteins and studied their interaction with the TSH receptor ectodomain (TSHR-ECD) using molecular dynamics simulation Furthermore, analyzed their bioactivity in vitro using recombinant proteins to confirm that such binding was physiologically relevant. Examining the interaction of TSHβ and TSHβv with the TSHR-ECD model using molecular dynamic simulation revealed strong binding of these proteins to the receptor ECD. The specificity of TSHβ and TSHβv binding to the TSHR-ECD was examined by analyzing the hydrogen-bonding residues of these subunits to the FSH receptor ECD, indicating the inability of these molecules to bind to the FSH receptors. Furthermore, the modelling suggests that TSHβ and TSHβv proteins clasped the concave surface of the leucine rich region of the TSHR ECD in a similar way to the native TSH using dynamic hydrogen bonding. These mutually exclusive stable interactions between the subunits and ECD residues included some high-affinity contact sites corresponding to binding models of native TSH. Furthermore, we cloned TSHβ and TSHβv proteins using the entire coding ORF and purified the flag-tagged proteins. The expressed TSHβ subunit proteins retained bioactivity both in a coculture system as well as with immune-purified proteins. In summary, we showed that such interactions can result in a functional outcome and may exert physiological or pathophysiological effects in immune cells.

LHRH-stimulated release of stored and newly synthesized gonadotrop polypeptide (GP-87) by cultured rat gonadotrophs

Enriched gonadotrophs from rat pituitaries were used to analyze the kinetics of release in vitro of the Gonadotrop Polypeptide (GP-87) under LHRH stimulation. Proteins in cultured cells were labeled with [35S]methionine. Labeling of the intracellular GP-87 pool was effected during 16 hours prior to LHRH (10(-7) M) stimulation. Proteins were analyzed either by one-dimensional SDS-PAGE (Medium content) or by two-dimensional PAGE (Cell content). An apparent half-life (intracellular catabolism + release) of 31 h for GP-87 was estimated from control cells; it dropped to 2.5 h in stimulated cells due to intense release (26% after 1 h and about 80% after 8 h of stimulation). When cells were simultaneously labeled and stimulated, the newly synthesized species appeared in the medium after a lag phase of 30 minutes, time required for synthesis and full subsequent processing. From both series of experiments, it is concluded that the hypothalamic decapeptide promotes exocytosis of the newly synthesized GP-87 well before the endogenous GP-87 pool is exhausted. Furthermore, the release of another discrete protein (B2, Mr: 81 kDa) is also stimulated by LHRH. These proteins being co-released with LH could be part of the sorting and/or routing process of hormones towards exocytosis.

Acute effect of triiodothyronine on the dynamics of thyrotropin release from superfused anterior pituitary cells

The effects of triiodothyronine (T3) on the dynamics of thyrotropin (TSH) release induced by TSH-releasing hormone (TRH) were examined in the presence or absence of a protein synthesis inhibitor, cycloheximide (CX), in a superfusion system using primarily cultured cells of the rat anterior pituitary gland on microcarrier beads. When the cells were continuously stimulated with TRH (10 nM, 180 min), TSH release occurred in a biphasic manner and the profile of TSH release was characterized by an initial sharp peak (phase I), followed by a lower plateau form phase (phase II). Both phase-I and phase-II releases were significantly suppressed in the presence of T3 (1 ng/ml), which was added to the superfusion medium 1 h before initiation of TRH stimulation. The biphasic nature of the release profile was maintained in the presence of T3, suggesting that the site of the T3 action may be common between phase-I and phase-II release. We have already suggested that phase-I release is protein synthesis-independent and phase-II release protein synthesis-dependent using CX in TRH-stimulated cells. In the presence of CX, phase-I release was not suppressed by T3, while phase-II release was still suppressed by T3. The inability of CX to reverse the T3-induced suppression of phase-II release may be masked by the direct CX effect on phase-II release of TSH. The present study indicates that each component (phase I and phase II) of the biphasic release of TSH induced by TRH stimulation was acutely suppressed by T3 and suggests that the T3 action is mediated through protein synthesis.

Regulation of hormone release by cultured cells from a thyrotropin-growth hormone-secreting pituitary tumor. Direct inhibiting effects of 3,5,3'-triiodothyronine and dexamethasone on thyrotropin secretion

The regulation of TSH and GH secretion was investigated in cultured tumor cells prepared from a mixed TSH/GH secreting pituitary tumor. The tumor tissue had been removed transsphenoidally from a patient with hyperthyroidism and inappropriately high serum TSH levels and acromegaly. TSH and GH secretion by cultured cells were stimulated in a parallel way by TRH (300 nM) and LHRH (50 nM), but were unaffected by bromocriptine (10 nM). Exposure of the tumor cells to dexamethasone (0.1 microM) or T3 (50 nM) had differential effects on hormone secretion. GH secretion was greatly stimulated by dexamethasone, but unaffected by T3. TSH secretion was inhibited both by T3 and by dexamethasone. So, T3 and glucocorticoids inhibit TSH release by the human pituitary tumor cells studied at least partly by means of a direct effect.

Quantitation of LH subunits released by rat anterior pituitary cells in primary culture

Immunoreactive rLH, rLH alpha, rLH beta and rFSH in the cell extracts and medium of rat anterior pituitary cell cultures were measured after gel filtration on Sephadex G-100 Superfine. Cell extracts contained uncombined rLH alpha and rLH beta present as approximately 40% and 4%, respectively, of native rLH on a molar basis. Unstimulated cultures appeared to release a large excess of rLH alpha (approximately 2560% of rLH on a molar basis) as well as a minimum of uncombined rLH beta. Stimulation of cultures with physiological (LHRH) or non-physiological agents (A23187, elevated K+) increased the absolute but not the relative quantities of uncombined subunits released relative to those present intracellularly. Thus, rat anterior pituitary cell cultures appear to produce and release an excess of free alpha subunit as well as a minimum of uncombined rLH beta. However, one cannot be certain whether the uncombined rLH beta represents a pool of 'free' rLH beta or simply results from the dissociation of rLH.

Dopaminergic modulation of TSH and its subunits: in vivo and in vitro studies

We have studied the effects of dopamine on the secretion of TSH and its subunits in vivo and in vitro. Four normal controls, seven patients with primary hypothyroidism, two patients with peripheral resistance to thyroid hormone (PRTH), and two patients with alpha-secreting pituitary tumours underwent a 3-h dopamine infusion (4 micrograms/kg/min). Serial blood samples were drawn for TSH, PRL, alpha, and TSH-beta subunit. In normal subjects, TSH fell from 2.1 +/- 0.9 (+/- SE) to 0.7 +/- 0.1 microU/ml (P less than 0.05), and alpha declined from 1.5 +/- 0.4 to 1.0 +/- 0.1 ng/ml (P less than 0.01). TSH-beta was at or slightly above the detection limits of the assay before and after dopamine. In hypothyroidism, basal serum TSH was 81 +/- 14 microU/ml. With dopamine, TSH fell to 35 +/- 8 microU/ml (P less than 0.001), while alpha decreased from 3.2 +/- 0.4 to 2.0 +/- 0.3 ng/ml (P less than 0.01). Serum TSH-beta also declined from 0.97 +/- 0.06 to 0.57 +/- 0.05 ng/ml (P less than 0.001). A similar fall in TSH and alpha was seen in the two patients with PRTH. In normals and hypothyroid patients, the percentage change in alpha concentration was significantly less than that observed for intact TSH. This is due presumably to the contribution of the gonadotrophs to the circulating alpha pool. TSH and TSH-beta were undetectable in the two pituitary tumour patients, and alpha declined only slightly in each patient after dopamine. The in vitro effects of dopamine were studied using cultured bovine anterior pituitary cells. Dopamine (10(-4)-10(-8) mol/l) did not change basal TSH, alpha, or TSH-beta release. However, dopamine at all doses significantly blunted TRH (10(-7) mol/l)-stimulated TSH and TSH-beta release, and blunted TRH-mediated alpha release at the two highest dopamine doses. These data suggest that dopamine modulates both TSH and TSH subunit secretion. These effects may be exerted directly at the level of the thyrotroph.

The effect of thyroid hormones on prolactin secretion by cultured bovine pituitary cells

The effect of thyroid hormones and thyrotropin releasing hormone (TRH) on prolactin (PRL) secretion has been studied using a primary calf anterior pituitary cell culture system. After mechanical and enzymatic dispersion, cultured pituitary cells were preincubated with T3 or T4 for 48 hr prior to a 24 hr experimental incubation. T3 stimulated the release of PRL into the medium in a dose-related fashion, with an ED50 of 3 nM; at 10 nM T3, a maximal 52 +/- 5% stimulation (p less than 0.001) was observed. T4 at 100 nM stimulated medium PRL 27 +/- 10% (p less than 0.05); the ED50 for T4 was 20 nM. Neither T3 nor T4 affected intracellular PRL content. The stimulation of medium PRL by T3 was observed in medium containing 10% euthyroid as well as 10% charcoal-stripped hypothyroid calf serum. The relative stimulation by TRH of PRL release into the medium was significantly diminished by 10 nM T3 in euthyroid and stripped hypothyroid serum medium, but only as a consequence of the stimulation of basal medium PRL by T3; there was no change in maximal TRH-stimulated PRL release. In medium supplemented with unstripped hypothyroid serum, however, T3 did decrease absolute TRH-stimulated PRL release.