[Free alpha-subunit glycoprotein hormones: physiological and pathological data]

Unconventional Actions of Glycoprotein Hormone Subunits: A Comprehensive Review

The glycoprotein hormones (GPH) are heterodimers composed of a common α subunit and a specific β subunit. They act by activating specific leucine-rich repeat G protein-coupled receptors. However, individual subunits have been shown to elicit responses in cells devoid of the receptor for the dimeric hormones. The α subunit is involved in prolactin production from different tissues. The human chorionic gonadotropin β subunit (βhCG) plays determinant roles in placentation and in cancer development and metastasis. A truncated form of the thyrotropin (TSH) β subunit is also reported to have biological effects. The GPH α- and β subunits are derived from precursor genes (gpa and gpb, respectively), which are expressed in most invertebrate species and are still represented in vertebrates as GPH subunit paralogs (gpa2 and gpb5, respectively). No specific receptor has been found for the vertebrate GPA2 and GPB5 even if their heterodimeric form is able to activate the TSH receptor in mammals. Interestingly, GPA and GPB are phylogenetically and structurally related to cysteine-knot growth factors (CKGF) and particularly to a group of antagonists that act independently on any receptor. This review article summarizes the observed actions of individual GPH subunits and presents the current hypotheses of how these actions might be induced. New approaches are also proposed in light of the evolutionary relatedness with antagonists of the CKGF family of proteins.

[Pharmacokinetics and pharmacodynamics of triptorelin]

GnRH agonists are derived from the native molecule by substitution of a D-amino acid in position 6 which increases their resistance to enzymatic breakdown and their affinity for LH-RH receptors in comparison with the native hormone. Because of this improved resistance which increases their half-life they have a super-agonistic effect. In 1973, two years only after he characterized LH-RH, A.V. Schally synthesized several GnRH analogs, including D-TRP6-LHRH obtained by substituting the glycine-6 with a D-tryptophan. The biological half life of this agonist injected by the subcutaneous route is 10 times greater than that observed after intravenous injection because of the progressive release of the peptide from the injection site. Pharmaceutical research has led to the development of delayed-release formulations allowing doses to be spaced by intervals of several weeks, or even three months when needed in some indications (Decapeptyl slow release). Triptorelin, as the other GnRH agonists, strongly reduces LH secretion, by preventing the production of the LH-beta subunit. On the opposite, the production of LH-alpha subunit is markedly increased and remains responsive to exogenous GnRH injection, demonstrating that the agonist does not induce actual pituitary desensitization. Compared with LH-RH antagonists which inhibit both LH-alpha and LH-beta subunit secretion, agonists offer the advantage of a sustained efficacy even after one or two days of withdrawal, while the effect of the agonist disappeared as soon as the administration is stopped. On the other hand, GnRH antagonists do not induce the initial hyperstimulation of the gonadotrophs, the so-called flare up, characteristic of the superagonistic effect.

Pharmacokinetics and pharmacodynamics of GnRH agonists: clinical implications in pediatrics

Since 1981, GnRH agonist administration has been the treatment of choice for central precocious puberty. Continuous administration of the agonist, instead of permanently stimulating gonadotropin secretion, deeply suppresses LH and FSH levels and induces a marked inhibition of gonadal activity and regression of clinical symptoms. This inhibitory effect is due both to specific kinetic parameters relative to natural GnRH, and to marked alterations of the biosynthetic pathways of gonadotropin subunits. The half disappearance time of infused agonists is 3-10 fold that of natural GnRH. This means that the residence time of GnRH agonists is significantly longer than that of GnRH. The resistance of agonist to enzymatic degradation, mainly due to the substitution of a hydrophobic D-amino acid for glycine 6, is one of the factors involved in the increased availability of GnRH superagonists. The paradoxical effects of GnRH superagonists are still incompletely understood. In children long-term treated with depot formulations of triptorelin or leuprorelin, alpha-subunit secretion is markedly increased, and remains sensitive to exogenous GnRH, which demonstrates that the gonadotrophs are not totally desensitized. Despite the sustained stimulation of a-subunit secretion, no deleterious side effects, either during therapy or during post-therapy follow-up, have been reported in children treated with GnRH agonists. It should be noted that alpha-subunit responsiveness to exogenous GnRH decreases progressively after several years of treatment, although it is never completely abolished. On the other hand, LH beta-subunit secretion is suppressed as evidenced by radioimmunoassay of LH beta-subunit in serum chromatographic fractions from children treated with triptorelin. This differential pattern of secretion parallels that of mRNA levels in rat pituitary after in vivo exposure to triptorelin. Both pharmacodynamic and pharmacokinetic data can help diagnose the situations of resistance or escape. The lack of clinical effect of GnRH in the treatment of precocious puberty can be due to true resistance, or to an inappropriate injection schedule, or to abnormal metabolism. Measurement of serum alpha-subunit level, and, if needed, of serum agonist level, generally provides the answer.

Glycoprotein hormone alpha-subunit secretion in prolactinomas and in non-functioning adenomas: relation with the tumour size

OBJECTIVE

Free glycoprotein hormone alpha-subunit plasma levels have been reported to be increased in glycoprotein hormone-secreting adenomas and in acromegaly, but rarely in prolactinomas and in only two cases of Cushing's disease. The prevalence of elevated plasma alpha-subunit levels in patients with non-functioning adenomas is still unclear. In addition, no previous work has described plasma alpha-subunit levels in a comprehensive series of adenomas characterized by in-vivo secretion and/or immunocytochemistry.

PATIENTS

Thirty-seven patients with definite prolactinomas and 48 with non-functioning tumours characterized by immunocytochemistry were studied, from a series of 145 consecutive patients including 33 acromegalics, 18 patients with glycoprotein hormone-secreting adenomas and 9 with Cushing's disease.

MEASUREMENTS

Plasma free alpha-subunit was measured by radioimmunoassay in all patients and in a large sample of normal subjects to establish normal ranges according to sex, age and menstrual status. Tumour volume index was the product in cm3 of length, width and height of the adenoma as assessed by computerized tomography or magnetic resonance imaging.

RESULTS

Twelve of the 37 (32%) patients with prolactinomas had increased plasma alpha-subunit levels; their tumours were significantly larger with significantly higher plasma PRL levels than those of patients without increased plasma alpha-subunit levels (P < 0.02). All prolactinomas above 50 cm3 were associated with alpha-subunit secretion, whereas only 6 of 29 smaller tumours were similarly associated. Twelve of the 48 'non-functioning' adenomas actually secreted alpha-subunit in vivo: 8 gonadotrophin-secreting, 2 'pure' alpha-secreting, one with negative immunocytochemistry and one necrotic adenoma. Their volumes were significantly higher than those of adenomas without increased plasma alpha-subunit levels (P < 0.04). Plasma alpha-subunit levels were increased in the 6 patients with TSH-secreting adenomas, 8 of 12 with FSH-secreting adenomas, 11 of 33 acromegalics and none of those with Cushing's disease.

CONCLUSION

Plasma free alpha-subunit levels were increased in 49 of 145 patients (34%). For prolactinomas and 'non-functioning' adenomas, alpha-subunit hypersecretion was seen more often with larger tumours. Half of the cases with increased free alpha-subunit in this series were patients harbouring an adenoma which did not stain for an intact glycoprotein hormone.