Gonadotrophin Releasing Hormone: A
Peptide Hormone
Clinical background
The therapeutic use of gonadotrophin-releasing hormone (GnRH) and its
analogues is based on the discovery that pulsatile exposure of gonadotrophs to
GnRH is required to maintain normal anterior pituitary function, whereas
continuous GnRH secretion results in desensitization of the gonadotrophs and
suppression of LH and FSH release. Thus GnRH or its analogues can be used in a
pulsatile fashion to promote fertility in women with isolated GnRH deficiency
or given continuously to suppress sex hormone secretion in patients with
hormone-related cancers. Stable synthetic analogues of GnRH have been developed
since GnRH, although used therapeutically as gonadorelin to assess pituitary
function, is unstable and not satisfactory as a therapeutic agent. Stable
analogues, including buserelin, goserelin, leuprorede, deslorelin and
nafarelin, may be used to treat breast and prostate cancer, endometriosis,
uterine fibroids and infertility.
Introduction
Gonadotrophin-releasing hormone (GnRH) is an excellent example of a
peptide hormone for study, since so much is known about its chemistry,
production, release and actions. GnRH is an hypothalamic peptide which is
released in pulsatile fashion into the hypothalamo-hypophyseal portal blood
system which supplies the anterior pituitary gland. This pulsatile secretion
maintains the function of the anterior pituitary gonadotrophs in releasing the
gonadotrophins LH and FSH which are necessary for proper ovarian and testicular
function. This knowledge has led to the development of synthetic GnRH
analogues.
Synthesis and release of peptide hormones
Transcription. The first step in the synthesis of a peptide such
as GnRH is the transcription of the gene coding for the hormone mRNA (Fig. 6a).
An initiation site on the gene, upstream from the coding region, is activated
by a signal from the cytoplasm of the hypothalamic neurone in which it is
synthesized. In the case of GnRH, the signal originates from a
neurotransmitter, perhaps dopamine, which triggers an increase in cytoplasmic
cAMP, resulting in activation of the gene. Conversely, cAMP production may be
inhibited as a result of the action of an opioid neurotransmitter.
Preprohormone. The GnRH mRNA is called prepro-GnRH mRNA, since it
will be translated into a large precursor peptide called prepro-GnRH. The mRNA
moves out of the nucleus to the cytoplasm, where it is translated by ribosomes
on the endoplasmic reticulum into prepro-GnRH. This precursor peptide consists
of a signal sequence of 23 amino acids, followed by the sequence of GnRH itself and then by the 56 amino acids forming
the C-terminal portion of the peptide. This latter portion is termed GAP
(GnRH-associated peptide), which has been discovered to be an inhibitor of
prolactin secretion. This highlights the principle that more than one
physiologically active peptide can be generated from a single peptide
precursor. The signal peptide directs its transfer to the endoplasmic reticulum,
and during this processing it is cleaved to form a shorter prohormone.
Cleavage and packaging. From this point, the prohormone is
transferred to the Golgi apparatus, where it is cleaved further to form the
final hormone, in this case the decapeptide GnRH. The hormone is packaged into
storage vesicles and released on demand, in this case as a cellular response to
neurotransmitter activity.
Exocytosis. The hormone is released from the cell through the
process of exocytosis. On stimulation, intracellular free Ca2+ and cAMP rise,
causing contraction of myofilaments, and the vesicle is guided along
microtubules to the cell membrane. The vesicle fuses with the membrane through
a process which requires Ca2+. The membrane is lysed and the contents are
released into the extracellular space, and enter the bloodstream through
neighbouring capillaries. GnRH neurone terminals impinge on the portal vessels,
so that on release, a large proportion of exocytosed GnRH enters the portal
system.
Structure-function studies
Once the structure of the hormone is elucidated, attempts are made to
synthesize more stable analogues for therapeutic use, and, in the case of GnRH,
substitution with D-amino acids produces potent analogues resistant to enzyme
digestion (Fig. 6b; Table 6.1).
Receptor characterization. The stable analogues are radiolabelled
and used to study the localization and properties of the peptide receptor. In
the case of GnRH, these are situated on the plasma membrane of the anterior
pituitary gonadotroph, and stimulation of the GnRH receptor by GnRH causes a
rise in intracellular cAMP paralleled by the secretion of follicle-stimulating
hormone (FSH) and luteinizing hormone (LH).
Hormone measurement. Once the hormone has been identified and
synthesized in large quantities, antibodies can be raised against the hormone
and used to measure it under different physiological and pathological
conditions. In the case of GnRH, the availability of a radioimmunoassay enabled
the discovery that the hormone is released episodically, approximately every 90
minutes. This episodic release is necessary to maintain gonadotrophin release,
and thus fertility, in both the male and the female primate, including humans.
