Abstract
Botulinum neurotoxin is a potent inhibitor of acetylcholine secretion and acts by cleaving members of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor family, which are critical to exocytotic vesicular secretion. However, the potential of botulinum neurotoxin for treating secretory disease is limited both by its neural selectivity and the necessity for direct injection into the relevant target tissue. To circumvent these limitations, a technology platform called targeted secretion inhibitors (TSIs) is being developed. TSIs are derived from botulinum neurotoxin but are retargeted to specific cell types to inhibit aberrant secretion. A TSI called qGHRH-LHN/D, with a GHRH receptor targeting domain and designed to specifically inhibit pituitary somatotroph GH release through cleavage of the N-ethylmaleimide-sensitive factor-attachment protein receptor protein, vesicle-associated membrane protein (VAMP), has recently been described. Here we show this TSI activates GHRH receptors in primary cultured rat pituicytes is internalized into these cells, depletes VAMP-3, and inhibits phorbol-12-myristate-13-acetate-induced GH secretion. In vivo studies show that this TSI, but not one with an inactive catalytic unit, produces a dose-dependent inhibition of pulsatile GH secretion, thus confirming its mechanism of action through VAMP cleavage. Selectivity of action has been shown by the lack of effect of this TSI in vivo on secretion from thyrotrophs, corticotrophs, and gonadotrophs. In the absence of suitable in vivo models, these data provide proof of concept for the use of somatotroph-targeted TSIs in the treatment of acromegaly and moreover raise the potential that TSIs could be used to target other diseases characterized by hypersecretion.
MeSH terms
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Animals
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Biological Transport / drug effects
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Botulinum Toxins / chemistry
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Botulinum Toxins / genetics
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Botulinum Toxins / metabolism
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Botulinum Toxins / pharmacology*
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Cells, Cultured
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Dose-Response Relationship, Drug
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Down-Regulation / drug effects*
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Growth Hormone / blood
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Growth Hormone / metabolism*
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Growth Hormone-Releasing Hormone / chemistry
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Growth Hormone-Releasing Hormone / genetics
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Growth Hormone-Releasing Hormone / metabolism
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Growth Hormone-Releasing Hormone / pharmacology*
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Male
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Organ Specificity
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Peptide Fragments / chemistry
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Peptide Fragments / genetics
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Peptide Fragments / metabolism
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Peptide Fragments / pharmacology
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Periodicity
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Pituitary Gland, Anterior / cytology
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Pituitary Gland, Anterior / drug effects*
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Pituitary Gland, Anterior / metabolism
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Protein Interaction Domains and Motifs
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Proteolysis / drug effects
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Rats
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Rats, Sprague-Dawley
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Receptors, Neuropeptide / agonists*
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Receptors, Neuropeptide / chemistry
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Receptors, Neuropeptide / metabolism
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Receptors, Pituitary Hormone-Regulating Hormone / agonists*
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Receptors, Pituitary Hormone-Regulating Hormone / chemistry
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Receptors, Pituitary Hormone-Regulating Hormone / metabolism
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Recombinant Fusion Proteins / administration & dosage
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Recombinant Fusion Proteins / chemistry
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Recombinant Fusion Proteins / metabolism
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Recombinant Fusion Proteins / pharmacology*
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Somatotrophs / cytology
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Somatotrophs / drug effects
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Somatotrophs / metabolism
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Vesicle-Associated Membrane Protein 3 / antagonists & inhibitors
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Vesicle-Associated Membrane Protein 3 / metabolism
Substances
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Peptide Fragments
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Receptors, Neuropeptide
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Receptors, Pituitary Hormone-Regulating Hormone
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Recombinant Fusion Proteins
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SXN101742 protein
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Vesicle-Associated Membrane Protein 3
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vesicle-associated membrane protein 3, rat
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Growth Hormone
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Growth Hormone-Releasing Hormone
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Botulinum Toxins
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somatotropin releasing hormone receptor