A long-acting GH receptor antagonist through fusion to GH binding protein

Characterization of dominant-negative growth hormone receptor variants reveals a potential therapeutic target for short stature

OBJECTIVE

Growth hormone insensitivity (GHI) encompasses growth restriction, normal/elevated growth hormone (GH), and low insulin-like growth factor I (IGF1). "Nonclassical" GHI is poorly characterized and is rarely caused by heterozygous dominant-negative (DN) variants located in the intracellular or transmembrane domains of the GH receptor (GHR). We sought to determine the molecular mechanisms underpinning the growth restriction in 2 GHI cases.

METHODS AND DESIGN

A custom-made genetic investigative pipeline was exploited to identify the genetic cause of growth restriction in patients with GHI. Nanoluc binary technology (NanoBiT), in vitro splicing assays, western blotting, and flow cytometry, characterized the novel GHR variants.

RESULTS

Novel heterozygous GHR variants were identified in 2 unrelated patients with GHI. In vitro splicing assays indicated both variants activated the same alternative splice acceptor site resulting in aberrant splicing and exclusion of 26 base pairs of GHR exon 9. The GHR variants produced truncated receptors and impaired GH-induced GHR signaling. NanoBiT complementation and flow cytometry showed increased cell surface expression of variant GHR homo/heterodimers compared to wild-type (WT) homodimers and increased recombinant human GH binding to variant GHR homo/heterodimers and GH binding protein (GHBP) cleaved from the variant GHRs. The findings demonstrated increased variant GHR dimers and GHBP with resultant GH sequestration.

CONCLUSION

We identified and characterized 2 novel, naturally occurring truncated GHR gene variants. Intriguingly, these DN GHR variants act via the same cryptic splice acceptor site, highlighting impairing GH binding to excess GHBP as a potential therapeutic approach.

Development of a Hypoparathyroid Male Rodent Model for Testing Delayed-Clearance PTH Molecules

CONTEXT

Parathyroid hormone (PTH) replacement is a promising approach in the management of hypoparathyroidism but long-acting analogues need to be developed. To date, animal models for testing PTH required parathyroidectomy by surgery. We have developed a nonsurgical rodent hypoparathyroid model and tested a delayed-clearance PTH molecule (DC-PTH).

OBJECTIVE

The aim of this study was to use cinacalcet to suppress calcium levels in normal rats and to reverse these effects with the administration of PTH or PTH analogues.

METHODS

Male Wistar rats were gavaged with either 30 mg/kg cinacalcet-HCl (cinacalcet) or vehicle only. Animals were then dosed with either single or repeated subcutaneous doses of PTH 1-34 or a DC-PTH at 20 nmol/kg. Control animals received vehicle only. Serum samples were analyzed for ionized calcium (iCa), phosphate, PTH, and DC-PTH. A pharmacokinetic-pharmacodynamic (PK-PD) model was built for cinacalcet, PTH 1-34, and DC-PTH using Phoenix64.

RESULTS

Cinacalcet reduced iCa levels between 2 and 24 hours, returning to baseline by 72 hours post dose with nadir at 8 hours (analysis of variance P < .001), associated with a fall in rat PTH. For phosphate there was a variable biphasic response. Single-dose PTH abrogated the cinacalcet-induced fall in iCa for up to 2 hours. DC-PTH prevented the fall in iCa from 4 hours post dose and gave a prolonged response, with iCa levels quicker to return to baseline than controls. DC-PTH has a half-life of 11.5 hours, approximately 44 times longer than human PTH 1-34. The PK-PD models defined the reproducible effect of cinacalcet on iCa and that DC-PTH had prolonged biological activity.

CONCLUSION

The administration of cinacalcet provides a robust and reproducible nonsurgical animal model of hypoparathyroidism. DC-PTH holds promise for the treatment of hypoparathyroidism in the future.

Growth hormone and chronic kidney disease

PURPOSE OF REVIEW

Elevated circulating levels of growth hormone (GH) and/or increased expression of the GH receptor in the kidney are associated with the development of nephropathy in type1 diabetes and acromegaly. Conditions of GH excess are characterized by hyperfiltration, glomerular hypertrophy, glomerulosclerosis and albuminuria, whereas states of decreased GH secretion or action are protected against glomerulopathy. The direct role of GH's action on glomerular cells, particularly podocytes, has been the focus of recent studies. In this review, the emerging role of GH on the biological function of podocytes and its implications in the pathogenesis of diabetic and chronic kidney disease will be discussed.

RECENT FINDINGS

Elevated GH levels impair glomerular permselectivity by altering the expression of podocyte slit-diaphragm proteins. GH stimulates the epithelial-mesenchymal transition of podocytes and decreases podocyte count. GH also induces the expression of prosclerotic molecules transforming growth factor beta, and TGFBIp.

SUMMARY

Our understanding of the cellular and molecular effects of GH in the pathogenesis of renal complications of diabetes and acromegaly has significantly progressed in recent years. These observations open up new possibilities in the prevention and treatment of diabetic nephropathy.

Targeting growth hormone function: strategies and therapeutic applications

Human growth hormone (GH) is a classical pituitary endocrine hormone that is essential for normal postnatal growth and has pleiotropic effects across multiple physiological systems. GH is also expressed in extrapituitary tissues and has localized autocrine/paracrine effects at these sites. In adults, hypersecretion of GH causes acromegaly, and strategies that block the release of GH or that inhibit GH receptor (GHR) activation are the primary forms of medical therapy for this disease. Overproduction of GH has also been linked to cancer and the microvascular complications that are associated with diabetes. However, studies to investigate the therapeutic potential of GHR antagonism in these diseases have been limited, most likely due to difficulty in accessing therapeutic tools to study the pharmacology of the receptor in vivo. This review will discuss current and emerging strategies for antagonizing GH function and the potential disease indications.

Emerging therapies are offering an expanded toolkit for combatting the effects of human growth hormone overproduction. Human growth hormone (GH) is a major driver of postnatal growth; however, systemic or localized overproduction is implicated in the aberrant growth disease acromegaly, cancer, and diabetes. In this review, researchers led by Jo Perry, from the University of Auckland, New Zealand, discuss strategies that either inhibit GH production, block its systemic receptor, or interrupt its downstream signaling pathways. The only licensed GH receptor blocker is pegvisomant, but therapies are in development that include long-acting protein and antibody-based blockers, and nucleotide complexes that degrade GHR production have also shown promise. Studies investigating GHR antagonism are limited, partly due to difficulty in accessing therapeutic tools which block GHR function, but overcoming these obstacles may yield advances in alleviating chronic disease.

Growth hormone - past, present and future

Growth hormone (GH) research and its clinical application for the treatment of growth disorders span more than a century. During the first half of the 20th century, clinical observations and anatomical and biochemical studies formed the basis of the understanding of the structure of GH and its various metabolic effects in animals. The following period (1958-1985), during which pituitary-derived human GH was used, generated a wealth of information on the regulation and physiological role of GH - in conjunction with insulin-like growth factors (IGFs) - and its use in children with GH deficiency (GHD). The following era (1985 to present) of molecular genetics, recombinant technology and the generation of genetically modified biological systems has expanded our understanding of the regulation and role of the GH-IGF axis. Today, recombinant human GH is used for the treatment of GHD and various conditions of non-GHD short stature and catabolic states; however, safety concerns still accompany this therapeutic approach. In the future, new therapeutics based on various components of the GH-IGF axis might be developed to further improve the treatment of such disorders. In this Review, we describe the history of GH research and clinical use with a particular focus on disorders in childhood.

Challenges and future for the delivery of growth hormone therapy

Growth hormone (GH) has multiple roles in sustaining human development and homeostasis. Its pulsatile secretion stimulates growth and contributes to an equilibrium in a process tightly regulated and coordinated by many organs. GH deficiency is a medical condition affecting all ages, with not only significant consequences in the health of the patient but also impact on the quality of life. This review gathers the different strategies used today with a glance at future technologies to treat GH deficiency. We present key aspects for consideration when developing new methods to deliver GH, mimicking or replacing its pulsatile activity. Today and in the future, the fusion of biochemistry, biology and nanotechnology will provide hybrid devices using microfluidic systems. But, until new technologies for GH delivery will become available, current methods must be reinforced in conjunction with the development of better communication strategies between the health system and patients. Treating GH deficiency represents a multidisciplinary effort for which this review provides a glance at potential future directions for this therapy.

Tumour-Derived Human Growth Hormone As a Therapeutic Target in Oncology

The growth hormone (GH) and insulin-like growth factor-1 (IGF1) axis is the key regulator of longitudinal growth, promoting postnatal bone and muscle growth. The available data suggest that GH expression by tumour cells is associated with the aetiology and progression of various cancers such as endometrial, breast, liver, prostate, and colon cancer. Accordingly there has been increased interest in targeting GH-mediated signal transduction in a therapeutic setting. Because GH has endocrine, autocrine, and paracrine actions, therapeutic strategies will need to take into account systemic and local functions. Activation of related hormone receptors and crosstalk with other signalling pathways are also key considerations.