Proteolytic cleavage of human growth hormone (hGH) by rat tissues in vitro: influence on the kinetics of exogenously administered hGH

Structural and functional characterization of recombinant human growth hormone isolated from transgenic pig milk

This study aimed to establish and reproduce transgenic pigs expressing human growth hormone (hGH) in their milk. We also aimed to purify hGH from the milk, to characterize the purified protein, and to assess the potential of our model for mass production of therapeutic proteins using transgenic techniques. Using ~15.5 L transgenic pig milk, we obtained proteins with ≥ 99% purity after three pre-treatments and five column chromatography steps. To confirm the biosimilarity of our milk-derived purified recombinant hGH (CGH942) with commercially available somatropin (Genotropin), we performed spectroscopy, structural, and biological analyses. We observed no difference between the purified protein and Genotropin samples. Furthermore, rat models were used to assess growth promotion potential. Our results indicate that CGH942 promotes growth, by increasing bone development and body weight. Toxicity assessments revealed no abnormal findings after 4 weeks of continuous administration and 2 weeks of recovery. The no-observed-adverse-effect level for both males and females was determined to be 0.6 mg/kg/day. Thus, no toxicological differences were observed between commercially available somatropin and CGH942 obtained from transgenic pig milk. In conclusion, we describe a transgenic technique using pigs, providing a new platform to produce human therapeutic proteins.

Bioanalytical approaches to assess the proteolytic stability of therapeutic fusion proteins

Therapeutic fusion proteins (TFPs) are designed to improve the therapeutic profile of an endogenous protein or protein fragment with a limited dose frequency providing the desired pharmacological activity in vivo. Fusion of a therapeutic protein to a half-life extension or targeting domain can improve the disposition of the molecule or introduce a novel mechanism of action. Prolonged exposure and altered biodistribution of an endogenous protein through fusion technology increases the potential for local protein unfolding during circulation increasing the chance for partial proteolysis of the therapeutic domain. Characterizing the proteolytic liabilities of a TFP can guide engineering efforts to inhibit or hinder partial proteolysis. This review focuses on considerations and techniques for evaluating the stability of a TFP both in vivo and in vitro.

CHO-recombinant human growth hormone as a protease sensitive reporter protein

Protein-free media are gaining more and more interest in mammalian cell culture technology. However, the range of commercially available protein-free media is wide, but lack of serum causes the lack of various substances (Keenan et al. in Cytotechnology, 50(1-3):49-56, 2006) which must be substituted case by case. Details on the composition of protein-free media are often unavailable or inaccessible in some cases, and as a consequence, there is an obvious need for testing procedures in order to evaluate the various commercialised products for their performance. Additionally, negative effects of tryptic meat digests on product quality have been reported in the literature (Gu et al. in Biotech Bioeng 56 (4):353-341, 1997). In the present studies of comparing various protein-free media for their suitability in propagation of recombinant CHO cells expressing human growth hormone (hGH), we have found somatotropin to be an excellent candidate for detection of protease activity. Somatotropin contains protease recognition sites for numerous proteases located around amino-acid residues 134-150. In this study, we demonstrate highly specific cleavage of recombinant hGH during batch cultivation. Analysis of the digested molecule was then performed by convergent methods like SDS-PAGE, HPLC and mass spectroscopy, and the results indicate hGH to be an ideal candidate for media and component screening in mammalian cell culture.

Proteolytic processing of human growth hormone by multiple tissue kallikreins and regulation by the serine protease inhibitor Kazal-Type5 (SPINK5) protein

BACKGROUND

Human growth hormone (hGH) is naturally present in numerous isoforms, some of which arise from proteolytic processing in both the pituitary and periphery. The nature of the enzymes that proteolytically cleave hGH and the regulation of this process are not fully understood. Our objective is to examine if members of a newly discovered human tissue kallikrein family (KLKs) are expressed in the pituitary and if these enzymes can cleave hGH in-vitro.

METHODS

Expression of 12 of the KLKs (KLKs 4-15) and serine protease inhibitor Kazal-type 5 (SPINK5) genes and their proteins in the pituitary was examined by RT-PCR and immunohistochemistry. Recombinant hGH was digested by various recombinant KLKs and fragments were characterized by N-terminal sequencing. SPINK5 recombinant fragments were used for inhibition of KLK activities.

RESULTS

We here describe for the first time expression of numerous KLKs (KLKs 5-8, 10-14) and SPINK5 in the pituitary. KLK6 and SPINK5 appeared to be localized to hGH-producing cells. KLKs 4-6, 8, 13 and 14 were able to cleave hGH, yielding various isoforms, in vitro. Inhibitor SPINK5 fragments were able to suppress activity of KLKs 4, 5 and 14 in vitro. Based on these data, we propose a model for the proteolytic processing of hGH in the pituitary and the regulation of this system by SPINK5 inhibitory domains. We speculate that loss of SPINK5 inhibitory domains, as in the case of Netherton syndrome, may lead to proteolytic over-processing of hGH and to growth retardation.

CONCLUSION

We conclude that many KLKs and SPINK5 are expressed in the pituitary. This serine protease-inhibitor system is likely to participate in the regulated proteolytic processing of hGH in the pituitary, leading to generation of hGH fragments. Our data suggest that KLKs 5, 6 and 14 might be involved in this process.

Methods for Predicting Human Drug Metabolism

Drug metabolism information is a necessary component of drug discovery and development. The key issues in drug metabolism include identifying: the enzyme(s) involved, the site(s) of metabolism, the resulting metabolite(s), and the rate of metabolism. Methods for predicting human drug metabolism from in vitro and computational methodologies and determining relationships between the structure and metabolic activity of molecules are also critically important for understanding potential drug interactions and toxicity. There are numerous experimental and computational approaches that have been developed in order to predict human metabolism which have their own limitations. It is apparent that few of the computational tools for metabolism prediction alone provide the major integrated functions needed to assist in drug discovery. Similarly the different in vitro methods for human drug metabolism themselves have implicit limitations. The utilization of these methods for pharmaceutical and other applications as well as their integration is discussed as it is likely that hybrid methods will provide the most success.

Host limits to accurate human growth hormone production in multiple plant systems

Human growth hormone (hGH) is not only a valuable recombinant therapeutic protein for hormone deficiency indications, but is also an extensively characterized molecule both from recombinant bacterial systems and as circulating in humans. We describe the characterization of hGH produced in three different plant systems: tobacco cell culture, soy seed, and maize seed. The data indicate highest production in the maize seed system, with continued productivity over multiple generations, and when bred to a new host genotype for improved productivity. Purification indicated significant material of the correct structure from both plant cell culture and maize seed, with maize seed also showing correct activity relative to that produced by Escherichia coli. However, all systems showed some proteolyzed hGH, with data from gel electrophoresis, mass spectrometry, and peptide mapping localizing to a region of the protein also prone to cleavage in some other systems. Together, the data indicate the dependence of recombinant protein accumulation on posttranslational processes in different host systems.

Synthesis and chromatographic purification of recombinant human pituitary hormones

Recombinant DNA-derived proteins and, in particular, human pituitary hormones, are increasingly used for research, diagnostic and therapeutic purposes. This trend has demanded new synthetic approaches and improved purification techniques. The type and sequence of the purification steps have to be selected in accordance with the cloning and protein expression strategy, the host organism and cellular localization of the protein of interest, with a view to producing the desired product at a required purity, biological activity and acceptable cost. This review article describes and analyzes the main synthetic and purification strategies that have been used for the production of recombinant human growth hormone, prolactin, thyrotropin, luteinizing hormone and follicle-stimulating hormone, giving special consideration to the few published downstream processes utilized by the biotechnology industry. Practically all types of prokaryotic and eukaryotic organisms utilized for this purpose are also reviewed.

Pharmacokinetics, metabolic stability, and subcutaneous bioavailability of a genetically engineered analog of DcR3, FLINT [DcR3(R218Q)], in cynomolgus monkeys and mice

Decoy receptor 3 (DcR3) is a novel member of the tumor necrosis factor receptor superfamily, which binds to and blocks the activities of the ligands, FasL and LIGHT (a cellular ligand for herpes virus entry mediator and lymphotoxin receptor), that play an important role in regulating apoptosis in normal physiology. DcR3 was rapidly degraded to a major circulating metabolic fragment, DcR3(1-218), after subcutaneous administration in primates and mice. DcR3 was molecularly engineered by changing the arginine residue at position 218 to glutamine to generate a potentially stable analog, DcR3(R218Q), which we termed FasLigand inhibitor protein [FLINT (LY498919)]. The influence of this modification on the kinetics and bioavailability of DcR3 was evaluated in primates and mice. After i.v. administration of FLINT and DcR3, both compounds were cleared from the plasma in a bi-phasic manner, with the terminal phase half-life being somewhat longer for FLINT than for DcR3. After s.c. administration, the exposure to the full-length form of FLINT was 5.7- to 6-fold greater than for DcR3. In both primates and mice, greater than 90% of circulating immunoreactivity after s.c. administration of FLINT was associated with intact molecule, whereas only 17 to 37% was associated with intact molecule after administration of DcR3. The absolute s.c. bioavailability of intact FLINT was approximately 4- to 6-fold higher than for DcR3. The improved s.c. bioavailability of FLINT is related to the increased metabolic stability afforded to the molecule as a result of the amino acid mutation at position 218 of the primary sequence of DcR3 and may translate to the need for lower therapeutic doses in a number of disease indications.

Decoy receptor 3 (DcR3) is proteolytically processed to a metabolic fragment having differential activities against Fas ligand and LIGHT

Fas ligand (FasL) and Fas receptor are members of the tumor necrosis factor (TNF) receptor and ligand family that play an important role in regulating apoptosis in normal physiology. Decoy receptor 3 (DcR3) is a novel member of the TNF receptor superfamily, which binds to and blocks the activities of the ligands FasL and LIGHT. We have demonstrated that DcR3 was degraded rapidly to a major circulating metabolic fragment after subcutaneous administration in primates and mice. This fragment was also generated in subcutaneous tissue homogenate in vitro. Mass spectrometry and N-terminal sequencing indicated that DcR3 was proteolytically cleaved between R218 and A219 in the primary sequence to yield the fragment DcR3(1-218). While retaining its ability to bind LIGHT and inhibit LIGHT-mediated activities, DcR3(1-218) no longer bound FasL and did not inhibit FasL-mediated apoptosis in vitro. The primary sequence of DcR3 was molecularly engineered, changing the arginine residue at position 218 to glutamine to generate an analog, DcR3(R218Q), which we termed FLINT (LY498919). We demonstrated that FLINT was more stable to proteolytic degradation in vitro and in vivo and maintained its activity against both soluble FasL and soluble LIGHT in vitro. As a result, the modification in the sequence of DcR3 to produce FLINT (LY498919) should result in a pharmacologically superior molecule in the therapeutic intervention of diseases in which the pathogenesis is linked to FasL-mediated apoptotic or inflammatory events.

Induction of growth hormone by the roots of Astragalus membranaceus in pituitary cell culture

The traditional Asian medicinal herb, roots of Astragalus (A.) membranaceus (Leguminosae), is used for many purposes, some of which are purported to stimulate the release of growth hormone in vivo. Extracts of A. membranaceus were tested to determine whether they stimulate the release of growth hormone in rat pituitary cell culture. A. membranaceus was extracted sequentially with 80% ethanol (fraction A), n-hexane (fraction B); the test compound from the herbal extraction was isolated using silica gel column chromatography and was identified with spectral data. Test compound was also extracted by traditional boiling water methods. Induction of growth hormone in pituitary cell culture was conducted with isolated compounds and extracted fractions of A. Radix (dried roots of A. membranaceus). The fraction A was not active in the rat pituitary cell culture, but the fraction B derived from the ethanol fraction stimulated the release of growth hormone in culture. Six compounds from fraction B (1-6) were isolated and identified previously. The compounds 1,2-benzendicarboxylic acid diisononylester (1), beta-sitosterol (2), and 3-O-beta-D-galactopyranosyl-beta-sitosterol (5) did not induce growth hormone release in the culture. Formononetin (3), 9Z,12Z-octadecadienoic acid (4), stigmast-4-en-6beta-ol-3-one (6) and 98-E, a mixture of 1'-9,12-octadecadienoic acid (Z,Z)-2',3'-dihydroxy-propylester (7) and 1'-hexadecanoic acid-2',3'-dihydroxy-propylester (8) stimulated the release of growth hormone in the rat pituitary cell culture significantly compared to the control. In conclusions, four compounds isolated from extracts of A. Radix induced growth hormone release in the rat pituitary cell culture. The 98-E isolate was the most active inducer of growth hormone release.

Proteolytic processing of human growth hormone (GH) by rat tissues in vitro: influence of sex and age

Although a wealth of evidence exists indicating that proteolytic cleavage can enhance the biological activity of the growth hormone (GH) molecule, the mechanisms responsible for the generation of GH fragments are not completely understood. In the present work we investigated the ability of different rat tissues to cleave 22 kDa GH, as well as the influence of sex and age, the two major physiological regulators of GH secretion on this process. Our results show that tissue homogenates obtained from rat liver, skeletal muscle or adipose tissue (three well-documented target organs for the hormone) are able to cleave 22K-GH, while the hormone is resistant to cleavage by rat brain homogenates. This process is rather selective for 22K-GH, since the 20 kDa GH variant exhibits stability to degradation by all tissue homogenates investigated. Moreover, only a minor fraction of 22 kDa GH is cleaved under our experimental conditions, suggesting that GH microheterogeneity within the 22 kDa range may also determine hormone susceptibility. Finally, we also found that 22K-GH processing shows important age-related changes (the greatest intensity observed in 4-day-old pups), while no gender-related differences exist in any of the tissues investigated.

Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism

A small synthetic peptide sequence of human growth hormone (hGH), AOD-9401, has lipolytic and antilipogenic activity similar to that of the intact hormone. Here we report its effect on lipid metabolism in rodent models of obesity and in human adipose tissue to assess its potential as a pharmacological agent for the treatment of human obesity. C57BL/6J (ob/ob) mice were orally treated with either saline (n = 8) or AOD-9401 (n = 10) for 30 days. From day 16 onward, body weight gain in AOD-9401-treated animals was significantly lower than that of saline-treated controls. Food consumption did not differ between the two groups. Analyses of adipose tissue ex vivo revealed that AOD-9401 significantly reduced lipogenic activity and increased lipolytic activity in this tissue. Increased catabolism was also reflected in an acute increase in energy expenditure and glucose and fat oxidation in ob/ob mice treated with AOD-9401. In addition, AOD-9401 increased in vitro lipolytic activity and decreased lipogenic activity in isolated adipose tissue from obese rodents and humans. Together, these findings indicate that oral administration of AOD-9401 alters lipid metabolism in adipose tissue, resulting in a reduction of weight gain in obese animals. The marked lipolytic and antilipogenic actions of AOD-9401 in human adipose tissues suggest that this small synthetic hGH peptide has potential in the treatment of human obesity.

Synthesis and purification of a deleted human growth hormone, hGH delta 135-146: sensitivity to plasmin cleavage and in vitro and in vivo bioactivities

Proteolytically cleaved human 22 kDa growth hormone (22K hGH) between the amino acid residues 134 and 150 by plasmin or other proteases in vitro has been reported to be most active in growth promoting activity. In this study a deleted mutant hGH lacking amino acid residues from 135 to 146 and having more sensitivity to plasmin digestion was produced using the inverse polymerase chain reaction method and the Escherichia coli expression system. The mutant, hGH delta 135-146, was folded and purified effectively and found to be more sensitive to plasmin cleavage to form the two-chain form in vitro. The biological activities of this plasmin sensitive hGH delta 135-146 were tested by in vitro cell proliferation assays and in vivo growth promoting assay. In Ba/F3-hGHR cells, which express receptors for hGH, hGH delta 135-146 showed 10-20% less growth promoting activity than 22K hGH, but expressed comparable quantities of IGF-I mRNA to that of 22K hGH. In Nb2 rat lymphoma cells, which proliferate in response to hGH via the lactogenic receptors, hGH delta 135-146 showed equivalent activities to those of 22K hGH at lower concentrations. By the body weight gain test using hypophysectomized rats, a lower dose (2.5 nmol kg-1) of hGH delta 135-146 exhibited an equivalent activity to that of wild type 22K hGH, but a higher dose (25 nmol kg-1) of the mutant showed less growth promoting activity than 22K hGH. These results indicated that the plasmin sensitive recombinant hGH delta 135-146 failed to show higher biological activity than the 22K hGH in vivo, suggesting the unsuccessful formation of the active two-chain form in vivo.

Expression and purification of a mutant human growth hormone that is resistant to proteolytic cleavage by thrombin, plasmin and human plasma in vitro

The region having a sequence from amino acid 134 to 150 in human growth hormone (hGH) is known to be cleaved by proteases in human plasma, plasmin and thrombin. In this study, oligonucleotide primer-directed mutagenesis was used to produce recombinant mutant hGHs resistant to limited proteolysis by these proteases. Substitution of Arg134 and Thr135 of hGH with Asp134 and Pro135 yielded a thrombin-resistant hGH mutant, and substitution of Arg134, Thr135 and Lys140 with Asp134, Pro135 and Ala140 yielded a plasmin-resistant hGH mutant. The latter mutant hGH was also insensitive to in vitro proteolysis by human plasma incubated for 7 days. These alterations in amino acid residues of hGH did not disrupt its biological conformation and retained full growth promoting activities on rat Nb2 cells and human T-47D breast cancer cells.

Proteolysis of human growth hormone by rat thyroid gland in vitro: application of electrospray mass spectrometry and N-terminal sequencing to elucidate a metabolic pathway

The present studies were designed to provide structural characterization of peptide metabolites of biosynthetic human growth hormone (hGH) formed by rat thyroid gland proteases in vitro. Electrospray ionization mass/spectrometry (ESI-MS) and N-terminal sequencing were used to characterize the peptide metabolites. The predominant enzyme in the thyroid gland preparations was a chymotrypsin-like serine protease which was biochemically similar to rat mast cell protease-I. Metabolic intermediates were formed by cleavage of hGH exclusively at Tyr/Phe/Leu-Xaa bonds. After a 5- or 45-min incubation of hGH with thyroid gland S9 pellet fraction, the majority of metabolites formed were two-chain variants of hGH having masses ranging from 16,002 to 22,143 Da. These metabolites were formed as a result of proteolysis in the large disulfide loop region of hGH in combination with processing at Tyr42-Ser43. Based upon the time-related appearance and structural characterization of these intermediates, a sequence of metabolic events is proposed. The initial event appears to be cleavage by the chymotrypsin-like protease between Tyr143-Ser144 to form a two-chain hGH. This intermediate is then cleaved between Tyr42-Ser43, liberating the N-terminal peptide, Phe1-Tyr42. Two other metabolites were generated as a result of the deletion of the peptides Lys140-Tyr143 and Ser144-Phe146 from the large loop region. The identification of similar metabolites truncated by a single amino acid at the carboxyl terminus indicated the action of a carboxypeptidase on these metabolic products. After a 4.5-hr incubation, the protease isolated from the S9 pellet fraction degraded hGH to > 20 small peptides, having masses < or = 2300 Da.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of recombinant human growth hormone in patients with severe sepsis

The objective of this study was to evaluate the safety and the effect of recombinant exogenous growth hormone (GH) on nitrogen production in patients with severe sepsis. It was designed as a prospective, randomized, placebo-controlled trial, and performed in the medical intensive care unit of a university hospital. Twenty patients admitted with septic shock and receiving standard parenteral nutrition served as subjects. Treatment consisted of GH 0.1 mg/kg/day or placebo administered as continuous intravenous infusion on the second, third, and fourth days after admission. The study period was eight days. During GH administration, nitrogen production decreased significantly in the GH group and increased in controls (p < 0.01). Nitrogen balance became slightly positive in the GH group during treatment: 1.2 +/- 6.4 versus controls -3.7 +/- 3.8 g/day (day 3) (p < 0.05). Within 24 hours after cessation of treatment, differences between GH and controls disappeared. 3-Methylhistidine excretion as a measure of absolute muscle breakdown declined during the study period, but did not differ between groups. The levels of insulin, insulinlike growth factor 1, glycerol, free fatty acids, and beta-hydroxybutyrate increased during treatment. Despite continuous intravenous administration, GH levels gradually declined during the 3 treatment days, indicating increased metabolic clearance. Side effects other than insulin resistance were not observed. Growth hormone administration reduces nitrogen production and improves nitrogen balance in patients with severe sepsis. These effects are not sustained after cessation of treatment.