Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 18 Pituitary Function
sequence may occur. Whereas the structures of TRH, GnRH,
and somatostatin (SS) (3, 10, and 14 amino acids, respectively)
are identical for mammals, those of GHRH and CRH
(44 and 41 amino acids) exhibit species specificity. The one
nonpeptide hypophysiotropic hormone is dopamine. In addition
to its major role as a neurotransmitter, dopamine is the
most important inhibitor of PRL release.
VP CRH GnRH GHRH
ACTH
Hypothalamus
Ghrelin
Adenohypophysis
SS TRH PRF PIF (DA)
LH FSH GH TSH PRL
FIGURE 18-3 Hypophysiotropic regulation of the secretion of pituitary
hormones. Solid lines denote hormones whose structures have been determined.
Dashed lines indicate a factor whose identity is still unknown.
2 . Regulation of Gene Expression
The developments in recombinant DNA technology have
enabled an increased knowledge on the genes encoding
pituitary hormones and the regulatory elements involved
in gene transcription. In short, the main elements regulating
the gene transcription will be mentioned ( Fig. 18-4 ).
Eukaryotic genes encoding peptide hormones consist of a
promoter (regulatory) unit and a transcription unit encoding
the primary transcript that after appropriate processing
will form the messenger RNA (mRNA).
The promoter unit is the upstream part (5 ) of the gene.
The promoter has specific DNA sequences (response elements)
permitting the binding of transcription factors that
enhance or inhibit gene expression by changes in the stability
of the RNA polymerase-TATA box complex at the
constitutive promoter. Also, with a higher distance to the
gene-specific enhancer elements, or locus control regions
as seen with GH expression ( Ho et al. , 2006 ), may regulate
gene transcription. Tissue-specific silencers and enhancers
that are mandatory for gene expression regulate the highly
FIGURE 18-4 Schematic representation
of potential control points
in pituitary hormone synthesis and
secretion.
5 RE RE
TATA
Exon 1 Intron
Exon 2
3
pol II
Transcription
hnRNA
mRNA
Post-transcriptional
modification
S
SR
Translation
S
Second messenger (s)
(cAMP, cGMP, RAS,
Calcium, Diacylglycerol)
Preprohormone
Prohormone
Hormone
Post-translational
modification
Secretion
S
P
Hormone