Growth hormone secretion pattern is an independent regulator of growth hormone actions in humans

Mechanisms of altered hepatic drug disposition during pregnancy: small molecules

INTRODUCTION

Pregnancy alters the systemic exposure and clearance of many hepatically cleared drugs that are commonly used by obstetric patients. Understanding the molecular mechanisms underlying the changes in factors that affect hepatic drug clearance (blood flow, protein binding, and intrinsic clearance) is essential to more precisely predict systemic drug exposure and dose requirements in obstetric patients.

AREAS COVERED

This review (1) summarizes the anatomic, physiologic, and biochemical changes in maternal hepatic, cardiovascular, endocrine, and renal systems relevant to hepatic drug clearance and (2) reviews the molecular mechanisms underlying the altered hepatic metabolism and intrinsic clearance of drugs during pregnancy via a comprehensive PubMed search. It also identifies knowledge gaps in the molecular mechanisms and factors that modulate hepatic drug clearance during pregnancy.

EXPERT OPINION

Pharmacokinetic studies have shown that pregnancy alters systemic exposure, protein binding, and clearance of many drugs during gestation in part due to pregnancy-associated decreases in plasma albumin, increases in organ blood flow, and changes in the activity of drug-metabolizing enzymes (DMEs) and transporters. The changes in the activity of certain DMEs and transporters during pregnancy are likely driven by hormonal-changes that inhibit their activity or alter the expression of these proteins through activation of transcription factors.

Impact of sex and pregnancy on hepatic CYP3A4 expression and activity in a humanized mouse model

Cytochrome P450 (CYP) 3A4 is an essential drug-metabolizing enzyme in humans, which shows substantial interindividual variation in response to various intrinsic and extrinsic factors such as sex and pregnancy. In humans, higher CYP3A4 metabolism has been observed in females compared with that in males and in pregnant compared with that in nonpregnant individuals, which has been linked to increased CYP3A4 expression in liver. However, sex differences and pregnancy-mediated changes in hepatic CYP3A4 expression and activity in vivo are not fully understood. In this study, we investigated the utility of a genetically engineered humanized mouse model that carries human CYP3A4/7, pregnane X receptor (PXR) and constitutive androstane receptor (CAR) (huPXR/CAR/CYP3A4/7) to recapitulate sex-associated and pregnancy-associated differences in the hepatic CYP3A4 expression and metabolism observed in humans. We found that female huPXR/CAR/CYP3A4/7 mice exhibited higher basal CYP3A4 mRNA levels and CYP3A4 absolute protein concentrations in liver, and higher 1-hydroxymidazolam formation in liver microsomes, compared with male humanized mice. In contrast, pregnant huPXR/CAR/CYP3A4/7 mice exhibited lower CYP3A4 mRNA levels, CYP3A4 absolute protein concentrations, and 1-hydroxymidazolam formation compared with nonpregnant and postpartum humanized mice. Expression of CAR and Cyp2b10 (a CAR responsive gene) were also higher in females and decreased during pregnancy and were positively correlated with hepatic CYP3A4 mRNA levels. Overall, the huPXR/CAR/CYP3A4/7 mouse model demonstrated utility to study higher basal hepatic CYP3A4 metabolism in females compared with that in males in vivo; however, this humanized mouse model did not demonstrate utility to study pregnancy-mediated increases in CYP3A4 drug substrate metabolism and clearance observed in humans. SIGNIFICANCE STATEMENT: This study assessed the impact of sex and pregnancy on hepatic CYP3A4 protein concentrations and metabolism in humanized PXR/CAR/CYP3A4 mice. Consistent with humans, female mice demonstrated higher hepatic CYP3A4 expression and activity than male mice. In contrast, pregnant mice showed decreased CYP3A4 expression and metabolism compared with nonpregnant mice. The humanized mouse model appeared useful to evaluate sex differences in basal hepatic CYP3A4 metabolism in vivo, but not to study the pregnancy-mediated increase in CYP3A4 metabolism observed during human pregnancy.

Update on regulation of GHRH and its actions on GH secretion in health and disease

This review focuses on our current understanding of how growth hormone releasing hormone (GHRH): 1) stimulates GH release and synthesis from pituitary growth hormone (GH)-producing cells (somatotropes), 2) drives somatotrope proliferation, 3) is negatively regulated by somatostatin (SST), GH and IGF1, 4) is altered throughout lifespan and in response to metabolic challenges, and 5) analogues can be used clinically to treat conditions of GH excess or deficiency. Although a large body of early work provides an underpinning for our current understanding of GHRH, this review specifically highlights more recent work that was made possible by state-of-the-art analytical tools, receptor-specific agonists and antagonists, high-resolution in vivo and ex vivo imaging and the development of tissue (cell) -specific ablation mouse models, to paint a more detailed picture of the regulation and actions of GHRH.

Maximizing plyometric training for adolescents: a meta-analysis of ground contact frequency and overall intervention time on jumping ability: a systematic review and meta-analysis

Plyometric training boosts adolescents' jumping ability, crucial for athletic success and health. However, the best total ground contact frequency (TGCF) and overall intervention time (OIT) for these exercises remain unclear. This meta-analysis aims to identify optimal TGCF and OIT in plyometric training for adolescents, focusing on countermovement jump (CMJ) and squat jump (SJ) outcomes. This systematic review encompassed five databases and included 38 studies with 50 randomized controlled experiments and 3347 participants. We used the Cochrane risk assessment tool for study quality and Review Manager 5.4 for data analysis. The current meta-analysis incorporated a total of 38 studies, comprising 50 sets of randomized controlled trials, to investigate the influence of different TGCFs and OITs on plyometric training. The Cochrane risk assessment tool indicated that all the included studies were classified as low risk. Various TGCFs in plyometric training positively affected CMJ and SJ heights in adolescents. The TGCF of less than 900 was ideal for enhancing CMJ, whereas more than 1400 was effective for SJ. The optimal OIT was 400-600 min, specifically, 500-600 min for CMJ and 400-500 min for SJ. Plyometric training improves jumping ability in adolescents. Lower ground contact frequency (< 900 contacts) enhances CMJ, while higher ground contact frequency (> 1400 contacts) is more effective for SJ. Optimal intervention time ranges from 400 to 600 min, with 500 to 600 min benefiting CMJ and 400 to 500 min improving SJ.

Impact of long-acting growth hormone replacement therapy in adult growth hormone deficiency: Comparison between adolescent, adult, and elderly patients

The predominant features of the adult growth hormone deficiency (GHD) syndrome may vary between patients of different age and age of onset of GHD. Evidence from clinical trials and long-term observational studies has informed our ability to understand the unique considerations regarding risks and benefits of daily growth hormone replacement therapy (GHRT) and specific dosing and monitoring strategies for these patient subgroups. High rates of nonadherence with daily GHRT presents a challenge to achieving optimal treatment outcomes and long-acting growth hormone (LAGH) formulations have been developed with the promise of improving treatment adherence resulting in improved therapeutic outcomes. While existing data from short-term studies have demonstrated noninferiority of efficacy and safety of LAGH compared to daily GHRT, long-term studies are needed to assess the full spectrum of outcomes of interest and long-term safety considerations specific to patients in adolescence, adulthood and the elderly GHD population. Since each LAGH formulation has a unique pharmacodynamic and pharmacokinetic profile optimal dosing and monitoring strategies will need to be developed to allow for the provision of individualized patient treatment.

MRI Findings of Pituitary Gland in Growth Hormone-Deficient Children and Their Correlation with Growth Hormone Peak during Growth Hormone Stimulation Tests

This study aims to explore the magnetic resonance imaging (MRI) findings of the pituitary gland (PG) in children with growth hormone deficiency (GHD) and their correlation with the growth hormone (GH) peak during clinical GH stimulation tests. Sixty-one children with GHD diagnosed and treated between December 2018 and December 2021 were retrospectively analyzed in terms of clinical and pituitary morphological MRI data. MRI measurements of various diameters of the adenohypophysis (AH) were obtained to analyze the differences of the measured values in different genders and age groups, as well as their relationship with the GH peak in GH stimulation tests. Among the 61 children with GHD, the superior PG margin was protuberant in 2 cases, flat in 13 cases, and concave in 46 cases. The three age groups showed similar pituitary morphology and stalk (P > 0.05). On T1-weighted images, the proportion of isointensity was lower while the proportion of slightly-low signal intensity was higher in the anterior pituitary gland (APG) of children aged >10 compared with those aged 7–10. The comparison of AH linear parameters and GH peak values of male patients among different age groups showed that the anteroposterior (sagittal) diameter of AH and GH peak were the highest in the >10-year-old group and the lowest in the ≤6-year-old group, with those of the 7–10-year-old group in between (P < 0.05). In females, the anteroposterior (sagittal) diameter and GH peak were higher in the 7–10-year-old group and >10-year-old group compared with the ≤6-year-old group (P < 0.05). The MRI coronal and sagittal heights of PG in children with GHD were positively correlated with the GH peak value. In conclusion, in GHD patients, the coronal and sagittal heights as well as the coronal width of AH do not change with sex or age, but the coronal and sagittal heights of PG are positively correlated with the GH peak of GH stimulation tests, which has high application value in the diagnosis of children with GHD.

Role of pulsatile growth hormone (GH) secretion in the regulation of lipolysis in fasting humans

Background

The increase in growth hormone (GH) secretion during a prolonged fast stimulates lipolytic rate, thereby augmenting the mobilization of endogenous energy at a time when fuel availability is very low.

Study aim

To identify the specific component of GH secretory pattern responsible for the stimulation of lipolytic rate during fasting in humans.

Study protocol

We measured lipolytic rate (using stable isotope dilution technique) after an overnight fast in 15 young, healthy, non-obese subjects (11 men and 4 women), and again on four separate occasions after a 59 h fast. These four prolonged fasting trials differed only by the contents of an infusion solution provided throughout the 59 h fasting period. Subjects were infused either with normal saline (“Control”; n = 15) or with graded doses of a GH Releasing Hormone Receptor Antagonist (GHRHa):10 μg/kg/h (“High”; n = 15), 1 μg /kg/h (“Medium”; n = 8), or 0.5 μg /kg/h (“Low”; n = 6).

Results

As expected, the 59 h fast completely suppressed plasma insulin levels and markedly increased endogenous GH concentrations (12 h vs 59 h Fast; p = 0.0044). Administration of GHRHa induced dose-dependent reduction in GH concentrations in response to the 59 h fast (p < 0.05). We found a strong correlation between the rate of lipolysis and GH mean peak amplitude (R = 0.471; p = 0.0019), and total GH pulse area under the curve (AUC) (R = 0.49; p = 0.0015), but not the GH peak frequency (R = 0.044; p = 0.8) or interpulse GH concentrations (R = 0.25; p = 0.115).

Conclusion

During prolonged fasting (i.e., 2–3 days), when insulin secretion is abolished, the pulsatile component of GH secretion becomes a key metabolic regulator of the increase in lipolytic rate.

Variation in Expression of Cytochrome P450 3A Isoforms and Toxicological Effects: Endo- and Exogenous Substances as Regulatory Factors and Substrates

The CYP3A subfamily, which includes isoforms CYP3A4, CYP3A5, and CYP3A7 in humans, plays important roles in the metabolism of various endogenous and exogenous substances. Gene and protein expression of CYP3A4, CYP3A5, and CYP3A7 show large inter-individual differences, which are caused by many endogenous and exogenous factors. Inter-individual differences can cause negative outcomes, such as adverse drug events and disease development. Therefore, it is important to understand the variations in CYP3A expression caused by endo- and exogenous factors, as well as the variation in the metabolism and kinetics of endo- and exogenous substrates. In this review, we summarize the factors regulating CYP3A expression, such as bile acids, hormones, microRNA, inflammatory cytokines, drugs, environmental chemicals, and dietary factors. In addition, variations in CYP3A expression under pathological conditions, such as coronavirus disease 2019 and liver diseases, are described as examples of the physiological effects of endogenous factors. We also summarize endogenous and exogenous substrates metabolized by CYP3A isoforms, such as cholesterol, bile acids, hormones, arachidonic acid, vitamin D, and drugs. The relationship between the changes in the kinetics of these substrates and the toxicological effects in our bodies are discussed. The usefulness of these substrates and metabolites as endogenous biomarkers for CYP3A activity is also discussed. Notably, we focused on discrimination between CYP3A4, CYP3A5, and CYP3A7 to understand inter-individual differences in CYP3A expression and function.

Harnessing natural variation to identify cis regulators of sex-biased gene expression in a multi-strain mouse liver model

Sex differences in gene expression are widespread in the liver, where many autosomal factors act in tandem with growth hormone signaling to regulate individual variability of sex differences in liver metabolism and disease. Here, we compare hepatic transcriptomic and epigenetic profiles of mouse strains C57BL/6J and CAST/EiJ, representing two subspecies separated by 0.5-1 million years of evolution, to elucidate the actions of genetic factors regulating liver sex differences. We identify 144 protein coding genes and 78 lncRNAs showing strain-conserved sex bias; many have gene ontologies relevant to liver function, are more highly liver-specific and show greater sex bias, and are more proximally regulated than genes whose sex bias is strain-dependent. The strain-conserved genes include key growth hormone-dependent transcriptional regulators of liver sex bias; however, three other transcription factors, Trim24, Tox, and Zfp809, lose their sex-biased expression in CAST/EiJ mouse liver. To elucidate the observed strain specificities in expression, we characterized the strain-dependence of sex-biased chromatin opening and enhancer marks at cis regulatory elements (CREs) within expression quantitative trait loci (eQTL) regulating liver sex-biased genes. Strikingly, 208 of 286 eQTLs with strain-specific, sex-differential effects on expression were associated with a complete gain, loss, or reversal of the sex differences in expression between strains. Moreover, 166 of the 286 eQTLs were linked to the strain-dependent gain or loss of localized sex-biased CREs. Remarkably, a subset of these CREs apparently lacked strain-specific genetic variants yet showed coordinated, strain-dependent sex-biased epigenetic regulation. Thus, we directly link hundreds of strain-specific genetic variants to the high variability in CRE activity and expression of sex-biased genes and uncover underlying genetically-determined epigenetic states controlling liver sex bias in genetically diverse mouse populations.

GHR signalling: Receptor activation and degradation mechanisms

Growth hormone (GH) actions via initiating cell signalling through the GH receptor (GHR) are important for many physiological processes, in addition to its well-known role in regulating growth. The activation of JAK-STAT signalling by GH is well characterized, however knowledge on GH activation of SRC family kinases (SFKs) is still limited. In this review we summarise the collective knowledge on the activation, regulation, and downstream signalling of GHR. We highlight studies on GH activation of SFKs and the important outcome of this signalling pathway with a focus on the different degradation mechanisms that can regulate GHR availability since this is an area that warrants further study considering its role in tumour progression.

Long-Acting Growth Hormone Preparations - Current Status and Future Considerations

CONTEXT

Long-acting GH (LAGH) preparations are currently being developed in an attempt to improve adherence. The profile of GH action following administration of LAGH raises practical questions about clinical monitoring and long-term safety and efficacy of these new therapeutic agents.

METHODS

Recent literature and meeting proceedings regarding LAGH preparations are reviewed.

RESULTS

Multiple LAGH preparations are currently at various stages of development, allowing for decreased GH injection frequency from daily to weekly, biweekly, or monthly. Following administration of LAGH, the serum peak and trough GH and IGF-I levels vary depending upon the mechanism used to prolong GH action. Randomized, controlled clinical trials of some LAGH preparations have reported non-inferiority compared with daily recombinant human GH (rhGH) for improved growth velocity and body composition in children and adults with GH deficiency (GHD), respectively. No significant LAGH-related adverse events have been reported during short-term therapy.

CONCLUSION

Multiple LAGH preparations are proceeding through clinical development with some showing promising evidence of short-term clinical efficacy and safety in children and adults with GHD. The relationship of transient elevations of GH and IGF-I following administration of LAGH to efficacy and safety remain to be elucidated. For LAGH to replace daily rhGH in the treatment of individuals with GHD, a number of practical questions need to be addressed including methods of dose adjustment, timing of monitoring of IGF-I, safety, efficacy, and cost-effectiveness. Long-term surveillance of efficacy and safety of LAGH preparations will be needed to answer these clinically relevant questions.

Rhythmic growth hormone secretion in physiological and pathological conditions: Lessons from rodent studies

Evolutionally conserved in all mammalians, the release of GH occurs in a rhythmic pattern, characterized by several dominant surges (pulsatile GH) with tonic low inter-pulse levels (tonic GH). Such pulsatile secretion pattern is essential for many physiological actions of GH on different tissues with defined gender dimorphism. Rhythmic release of pulsatile GH is tightly controlled by hypothalamic neurons as well as peripheral metabolic factors. Changes of GH pattern occur within a range of sophisticated physiological and pathological settings and significantly contribute to growth, ageing, survival and disease predispositions. Precise analysis of GH secretion pattern is vitally important for a comprehensive understanding of the function of GH and the components that regulate GH secretion pattern.

Brain Control of Sexually Dimorphic Liver Function and Disease: The Endocrine Connection

A multistep signaling cascade originates in brain centers that regulate hypothalamic growth hormone-releasing hormone (Ghrh) and somatostatin expression levels and release to control the pattern of GH secretion. This process is sexually fine-tuned, and relays important information to the liver where GH receptors can be found. The temporal pattern of pituitary GH secretion, which is sex-specific in many species (episodic in males and more stable in females), represents a major component in establishing and maintaining the sexual dimorphism of hepatic gene transcription. The liver is sexually dimorphic exhibiting major differences in the profile of more than 1000 liver genes related to steroid, lipid, and foreign compound metabolism. Approximately, 90% of these sex-specific liver genes were shown to be primarily dependent on sexually dimorphic GH secretory patterns. This proposes an interesting scenario in which the central nervous system, indirectly setting GH profiles through GHRH and somatostatin control, regulates sexual dimorphism of liver activity in accordance with the need for sex-specific steroid metabolism and performance. We describe the influence of the loss of sexual dimorphism in liver gene expression due to altered brain function. Among other many factors, abnormal brain sexual differentiation, xenoestrogen exposure and D2R ablation from neurons dysregulate the GHRH-GH axis, and ultimately modify the liver capacity for adaptive mechanisms. We, therefore, propose that an inefficient brain control of the endocrine growth axis may underlie alterations in several metabolic processes through an indirect influence of sexual dimorphism of liver genes.

Hepatocyte-Specific Deletion of Mouse Lamin A/C Leads to Male-Selective Steatohepatitis

BACKGROUND & AIMS

Lamins are nuclear intermediate filament proteins that comprise the major components of the nuclear lamina. Mutations in LMNA, which encodes lamins A/C, cause laminopathies, including lipodystrophy, cardiomyopathy, and premature aging syndromes. However, the role of lamins in the liver is unknown, and it is unclear whether laminopathy-associated liver disease is caused by primary hepatocyte defects or systemic alterations.

METHODS

To address these questions, we generated mice carrying a hepatocyte-specific deletion of Lmna (knockout [KO] mice) and characterized the KO liver and primary hepatocyte phenotypes by immunoblotting, immunohistochemistry, microarray analysis, quantitative real-time polymerase chain reaction, and Oil Red O and Picrosirius red staining.

RESULTS

KO hepatocytes manifested abnormal nuclear morphology, and KO mice showed reduced body mass. KO mice developed spontaneous male-selective hepatosteatosis with increased susceptibility to high-fat diet-induced steatohepatitis and fibrosis. The hepatosteatosis was associated with up-regulated transcription of genes encoding lipid transporters, lipid biosynthetic enzymes, lipid droplet-associated proteins, and interferon-regulated genes. Hepatic Lmna deficiency led to enhanced signal transducer and activator of transcription 1 (Stat1) expression and blocked growth hormone-mediated Janus kinase 2 (Jak2), signal transducer and activator of transcription 5 (Stat5), and extracellular signal-regulated kinase (Erk) signaling.

CONCLUSIONS

Lamin A/C acts cell-autonomously to maintain hepatocyte homeostasis and nuclear shape and buffers against male-selective steatohepatitis by positively regulating growth hormone signaling and negatively regulating Stat1 expression. Lamins are potential genetic modifiers for predisposition to steatohepatitis and liver fibrosis. The microarray data can be found in the Gene Expression Omnibus repository (accession number: GSE93643).

Effects of sexually dimorphic growth hormone secretory patterns on arachidonic acid metabolizing enzymes in rodent heart

The arachidonic acid (AA) metabolizing enzymes are the potential therapeutic targets of cardiovascular diseases (CVDs). As sex differences have been shown in the risk and outcome of CVDs, we investigated the regulation of heart AA metabolizing enzymes (COXs, LOXs, and CYPs) by sex-dependent growth hormone (GH) secretory patterns. The pulsatile (masculine) GH secretion at a physiological concentration decreased CYP1A1 and CYP2J3 mRNA levels more efficiently in the H9c2 cells compared with the constant (feminine) GH secretion; however, CYP1B1 mRNA levels were higher following the pulsatile GH secretion. Sex differences in CYP1A1, CYP1B1, and CYP2J11 mRNA levels were observed in both the wild-type and GHR deficient mice. No sex differences in the mRNA levels of COXs, LOXs, or CYP2E1 were observed in the wild-type mice. The constant GH infusion induced heart CYP1A1 and CYP2J11, and decreased CYP1B1 in the male C57/B6 mice constantly infused with GH (0.4 μg/h, 7 days). The activity of rat Cyp2j3 promoter was inhibited by the STAT5B protein, but was activated by C/EBPα (CEBPA). Compared with the constant GH administration, the levels of the nuclear phosphorylated STAT5B protein and its binding to the rat Cyp2j3 promoter were higher following the pulsatile GH administration. The constant GH infusion decreased the binding of the nuclear phosphorylated STAT5B protein to the mouse Cyp2j11 promoter. The data suggest the sexually dimorphic transcription of heart AA metabolizing enzymes, which might alter the risk and outcome of CVDs. GHR-STAT5B signal transduction pathway may be involved in the sex difference in heart CYP2J levels.

Effects of growth hormone-releasing hormone on visceral fat, metabolic, and cardiovascular indices in human studies

Increased visceral adipose tissue (VAT) is associated with reductions in endogenous GH secretion, possibly as a result of hyperinsulinemia, increased circulating free fatty acid, increased somatostatin tone, and reduced ghrelin. Reduced GH may, in turn, further exacerbate visceral fat accumulation because of decreased hormone-sensitive lipolysis in this depot. Data from multiple populations demonstrate that both reduced GH and increased VAT appear to contribute independently to dyslipidemia, increased systemic inflammation, and increased cardiovascular risk. The reductions in GH in states of visceral adiposity are characterized by reduced basal and pulsatile GH secretion with intact pulse frequency. Treatment with GH-releasing hormone (GHRH) provides a means to reverse these abnormalities, increasing endogenous basal and pulsatile GH secretion without altering pulse frequency. This review describes data from HIV-infected individuals and individuals with general obesity showing that treatment with GHRH significantly reduces visceral fat, ameliorates dyslipidemia, and reduces markers of cardiovascular risk. Further research is needed regarding the long-term efficacy and safety of this treatment modality.

Pituitary and brain dopamine D2 receptors regulate liver gene sexual dimorphism

Liver sexual gene dimorphism, which depends mainly on specific patterns of GH secretion, may underlie differential susceptibility to some liver diseases. Because GH and prolactin secretion are regulated by dopaminergic pathways, we studied the participation of brain and lactotrope dopamine 2 receptors (D2Rs) on liver gene sexual dimorphism, to explore a link between the brain and liver gene expression. We used global D2R knockout mice (Drd2(-/-)) and conducted a functional dissection strategy based on cell-specific Drd2 inactivation in neurons (neuroDrd2KO) or pituitary lactotropes. Disruption of neuronal D2Rs (which impaired the GH axis) decreased most of male or female-predominant class I liver genes and increased female-predominant class II genes in males, consistent with the positive (class I) or negative (class II) regulation of these genes by GH. Notably, sexual dimorphism was lost for class I and II genes in neuroDrd2KO mice. Disruption of lactotrope D2Rs did not modify class I or II genes in either sex, because GH axis was preserved. But surprisingly, 1 class II gene (Prlr) and female-predominant class I genes were markedly up-regulated in lacDrd2KO females, pointing to direct or indirect effects of prolactin in the regulation of selected female-predominant liver genes. This suggestion was strengthened in the hyperprolactinemic Drd2(-/-) female mouse, in which increased expression of the same 4 liver genes was observed, despite a decreased GH axis. We hereby demonstrate endocrine-mediated D2R actions on sexual dimorphic liver gene expression, which may be relevant during chronic dopaminergic medications in psychiatric disease.

Sex-dependent regulation of hepatic CYP3A by growth hormone: Roles of HNF6, C/EBPα, and RXRα

Sex-based differences in the pharmacological profiles of many drugs are due in part to the female-predominant expression of CYP3A4, which is the most important CYP isoform responsible for drug metabolism. Transcription factors trigger the sexually dimorphic expression of drug-metabolizing enzymes in response to sex-dependent growth hormone (GH) secretion. We investigated the roles of HNF6, C/EBPα, and RXRα in the regulation of human female-predominant CYP3A4, mouse female-specific CYP3A41, and rat male-specific CYP3A2 expression by GH secretion patterns using HepG2 cells, growth hormone receptor (GHR) knockout mice as well as rat models of orchiectomy and hypophysectomy. The constitutive expression of HNF6 and RXRα was GH-dependent, and GHR deficiency decreased HNF6/C/EBPα complex levels and increased HNF6/RXRα complex levels. Feminine GH secretion induced the binding of HNF6 and C/EBPα to the CYP3A4 and Cyp3a41 promoters and HNF6/C/EBPα complex levels was more efficiently compared with masculine pattern. Additionally, a greater inhibition of the binding of RXRα to the CYP3A4 and Cyp3a41 promoters and HNF6/RXRα complex levels was observed by feminine GH secretion, but less inhibition was observed by masculine pattern. The binding of HNF6, C/EBPα, and RXRα to the CYP3A2 promoter was not directly regulated by androgens. RXRα completely abolished the synergistic activation of the CYP3A4, Cyp3a41, and CYP3A2 promoters by HNF6 and C/EBPα. The results demonstrate that sex-dependent GH secretion patterns affect the expressions and interactions of HNF6, C/EBPα, and RXRα as well as their binding to CYP3A genes. RXRα mediates the sex-dependent influence of GH on CYP3A expression as an important signalling molecule.

Different modes of GH administration influence gene expression in the male rat brain

The endogenous secretion pattern in males of GH is episodic in rats and in humans, whereas GH administration is usually even. Different types of GH administration have different effects on body mass, longitudinal bone growth, and liver metabolism in rodents, whereas possible effects on brain plasticity have not been investigated. In this study, GH was administered as a continuous infusion or as two daily injections in hypophysectomized male rats. Thirteen transcripts previously known to respond to GH in the hippocampus and parietal cortex (cortex) were assessed by RT-PCR. To investigate the effects of type of GH administration on several transcripts with different variations, and categories of transcripts (neuron-, glia-, and GH-related), a mixed model analysis was applied. Accordingly, GH injections increased overall transcript abundance more than GH infusions (21% in the hippocampus, P<0.001 and 10% in the cortex, P=0.09). Specifically, GH infusions and injections robustly increased neuronal hemoglobin beta (Hbb) expression significantly (1.8- to 3.6-fold), and GH injections were more effective than GH infusions in increasing Hbb in the cortex (41%, P=0.02), whereas a 23% difference in the hippocampus was not significant. Also cortical connexin 43 was higher in the group with GH injections than in those with GH infusions (26%, P<0.007). Also, there were differences between GH injections and infusions in GH-related transcripts of the cortex (23%, P=0.04) and glia-related transcripts of the hippocampus (15%, P=0.02). Thus, with the exception of Hbb there is a moderate difference in responsiveness to different modes of GH administration.

GH administration patterns differently regulate epidermal growth factor signaling

Current GH administration protocols imply frequent s.c. injections, resulting in suboptimal compliance. Therefore, there is interest in developing delivery systems for sustained release of the hormone. However, GH has different actions depending on its continuous or pulsatile plasma concentration pattern. GH levels and circulating concentration patterns could be involved in the regulation of epidermal growth factor receptor (EGFR) expression in liver. Aberrant expression of this receptor and/or its hyperactivation has been associated with the pathogenesis of different types of carcinoma. Considering that one of the adverse effects associated with GH overexpression and chronic use of GH is the increased incidence of malignancies, the aim of this study was to analyze the effects of GH plasma concentration patterns on EGFR expression and signaling in livers of mice. For this purpose, GH was administered by s.c. daily injections to produce an intermittent plasma pattern or by osmotic pumps to provoke a continuously elevated GH concentration. Intermittent injections of GH induced upregulation of liver EGFR content, augmented the response to EGF, and the induction of proteins involved in promotion of cell proliferation in female mice. In contrast, continuous GH delivery in male mice was associated with diminished EGFR in liver and decreased EGF-induced signaling and expression of early genes. The results indicate that sustained delivery systems that allow continuous GH plasma patterns would be beneficial in terms of treatment safety with regard to the actions of GH on EGFR signaling and its promitogenic activity.

The G⁵¹⁶T CYP2B6 germline polymorphism affects the risk of acute myeloid leukemia and is associated with specific chromosomal abnormalities

The etiology of acute myeloid leukemia (AML) underlies the influence of genetic variants in candidate genes. The CYP2B6 enzyme detoxifies many genotoxic xenobiotics, protecting cells from oxidative damage. The CYP2B6 gene is subjected to a single-nucleotide polymorphism (G⁵¹⁶T) with heterozygotes (GT) and homozygotes (TT) presenting decreased enzymatic activity. This case-control study aimed to investigate the association of CYP2B6 G⁵¹⁶T polymorphism with the susceptibility of AML and its cytogenetic and clinical characteristics. Genotyping was performed on 619 AML patients and 430 healthy individuals using RCR-RFLP and a novel LightSNip assay. The major finding was a statistically higher frequency of the variant genotypes (GT and TT) in patients compared to the controls (GT:38.8% vs 29.8% and TT:9.3% vs 5.3% respectively) (p<0.001). More specifically, a significantly higher frequency of GT+TT genotypes in de novo AML patients (46.6%) and an immensely high frequency of TT in secondary AML (s-AML) (20.5%) were observed. The statistical analysis showed that the variant T allele was approximately 1.5-fold and 2.4-fold higher in de novo and s-AML respectively than controls. Concerning FAB subtypes, the T allele presented an almost 2-fold increased in AML-M2. Interestingly, a higher incidence of the TT genotype was observed in patients with abnormal karyotypes. In particular, positive correlations of the mutant allele were found in patients carrying specific chromosomal aberrations [-7/del(7q), -5/del(5q), +8, +21 or t(8;21)], complex or monosomal karyotypes. Finally, a strikingly higher frequency of TT genotype was also observed in patients stratified to the poor risk group. In conclusion, our results provide evidence for the involvement of the CYP2B6 polymorphism in AML susceptibility and suggest a possible role of the CYP2B6 genetic background on the development of specific chromosomal aberrations.

Complex rhythmicity and age dependence of growth hormone secretion are preserved in patients with acromegaly: further evidence for a present hypothalamic control of pituitary somatotropinomas

CONTEXT

Traditionally, acromegaly is viewed as a disease resulting from GH hypersecretion from an autonomous pituitary somatotropinoma.

OBJECTIVE

To test the hypothesis that GH secretion in acromegaly is still subjected to normal hypothalamic control, we studied the daily rhythmicity of GH secretion in normal controls and patients with newly diagnosed, untreated acromegaly.

DESIGN AND SETTING

This was an observational inpatient study in the General Clinical Research Center at the University of Michigan.

PATIENTS OR OTHER PARTICIPANTS

One hundred four normal controls and 67 acromegalic patients were included in the study.

INTERVENTION

The intervention consisted of frequent blood sampling over 24 hours.

MAIN OUTCOME MEASURE(S)

We hypothesized that acromegalic patients would show rhythmicity, sexual dimorphism, and age-related decline of GH secretion similar to normal controls.

RESULTS

Both normal controls and the patients exhibited 3 major GH waves with the highest values at 12:00 pm, 5:00 pm, and 1:00 am (P < .001 for all). Both controls and patients exhibited a clear appearance of the nocturnal GH waves, irrespective of the gender (P < .001 for all). The amplitude of the maximal (nocturnal) GH secretory wave (1:00 am) as compared with the nadir GH secretion (9:00 am) was clearly different between the 2 groups, with a significantly smaller magnitude in acromegaly (P < .001). A subsequent subanalysis of both groups was performed separately for both genders. Similar to the entire groups, both controls and patients exhibited a clear appearance of the nocturnal GH waves, irrespective of the gender (P < .001 for all). Patients with clearly elevated GH values have shown an age-related decline of GH secretion (r = -0.35, P < .001), similar to controls.

CONCLUSIONS

The analysis of GH profiles in multiple patients with untreated acromegaly discloses the persistence of the hallmarks of the central control of GH regulation, ie, nictohemeral rhythmicity, sexual dimorphism, and an age-related decline of GH output.

Induction of hepatic CYP3A enzymes by pregnancy-related hormones: studies in human hepatocytes and hepatic cell lines

CYP3A activity is induced by approximately 2-fold during the third trimester of human pregnancy. Placental growth hormone (PGH), estrogens (primarily 17β-estradiol), cortisol, and progesterone have the potential to modulate CYP3A activity. Therefore, we determined whether the elevated plasma concentrations of these hormones during pregnancy induce hepatic CYP3A expression. We incubated sandwich-cultured human hepatocytes (SCHH) from premenopausal female donors (n = 2) with the physiologic (unbound, 1× total) and the 10× total third trimester hormone plasma concentrations (individually and in combination) and determined their effect on CYP3A activity and the transcripts of CYP3A4, CYP3A5, and the respective hormone receptors (growth hormone receptor, glucocorticoid receptor, and estrogen receptor alpha). Of all the hormones, cortisol was the most potent inducer of CYP3A activity and CYP3A4, CYP3A5 mRNA expression. The combination of PGH/growth hormone and cortisol induced CYP3A activity and expression significantly more than did cortisol alone. When incubated with the unbound or total plasma concentration of all the hormones, CYP3A activity in SCHH was induced to an extent comparable to that observed in vivo during the third trimester. These hormones had only a modest effect on the mRNA expression of the hormone receptors. The pattern of induction observed in SCHH was reproduced in HepaRG cells but not in HuH7/HepG2 cells. SCHH or HepaRG cells could be used to determine the mechanistic basis of CYP3A induction during pregnancy and to predict the magnitude of induction likely to be observed during the first and second trimesters, when phenotyping studies to measure in vivo CYP3A activity are logistically difficult to perform.

Inherent sex-dependent regulation of human hepatic CYP3A5

BACKGROUND AND PURPOSE

Expression of hepatic cytochromes P450 (CYP) in all species examined, including humans, is generally sexually dimorphic. We examined the sex-dependent expression of CYP3A5 and the hormone-regulated molecular mechanism(s) responsible for any dimorphism.

EXPERIMENTAL APPROACH

CYP3A5 levels as well as nuclear translocation and promoter binding of transcription factors regulating CYP3A5 expression were measured in primary hepatocyte cultures derived from men and women exposed to physiological-like levels of growth hormone alone, dexamethasone alone and the combined regimen.

KEY RESULTS

We observed a dramatic inherent CYP3A5 sexual dimorphism (women > men) with all treatments as a result of a ~2-fold greater level of hormone-induced activation and nuclear accumulation of hepatocyte nuclear factor-4α (HNF-4α), pregnane X receptor (PXR) and retinoic X receptorα (RXRα) in female hepatocytes. Furthermore, PXR : RXRα exhibited significantly higher DNA binding levels to its specific binding motif on the CYP3A5 promoter in female hepatocytes, inferring a possible explanation for the elevated expression of the isoform in women. Results from experiments using HepG2 cells treated with siRNA-induced knockdown of HNF-4α and/or transfected with luciferase reporter constructs containing the CYP3A5 promoter were in agreement with the basic mechanism observed in primary hepatocytes of both sexes.

CONCLUSIONS AND IMPLICATIONS

Female-predominant expression of human CYP3A5 is due to an inherent, sex-dependent suboptimal activation of the transcription networks responsible for hormone-induced expression of the isoform in men. Accordingly, in conjunction with previous studies of other human CYPs, men and women are intrinsically unlikely to handle many drugs in the same way; thus, sex should be a requisite component factored into the design of personalized drug therapies.

Suppression in growth hormone during overeating ameliorates the increase in insulin resistance and cardiovascular disease risk

Previously, we reported that overeating for only a few days markedly suppressed the secretion of growth hormone (GH). The purpose of the present study was to determine the role of this reduction in GH concentration on key metabolic adaptations that occur during 2 wk of overeating. Nine nonobese, healthy adults were admitted to the hospital for 2 wk, during which time they ate ∼4,000 kcal/day (70 kcal·kg fat-free mass(-1)·day(-1); 50% carbohydrate, 35% fat, and 15% protein), and their plasma GH concentration was allowed to decline naturally (control). An additional eight subjects underwent the same overeating intervention and received exogenous GH treatment (GHT) administered in four daily injections to mimic physiological GH secretion throughout the 2-wk overeating period. We measured plasma insulin and glucose concentrations in the fasting and postprandial state as well as fasting lipolytic rate, proteolytic rate, and fractional synthetic rate (FSR) using stable-isotope tracer methods. GHT prevented the fall in plasma GH concentration, maintaining plasma GH concentration at baseline levels (1.2 ± 0.2 ng/ml), which increased fasting and postprandial assessments of insulin resistance (P < 0.05) and increased fasting lipidemia (all P < 0.05 vs. control). In addition, preventing the suppression in GH with overeating also blunted the increase in systemic proteolysis (P < 0.05 GHT vs. control). However, GHT did not alter lipolysis or FSR in response to overeating. In conclusion, our main findings suggest that the suppression in GH secretion that naturally occurs during the early stages of overeating may help attenuate the insulin resistance and hyperlipidemia that typically accompany overeating.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Intrinsic sexually dimorphic expression of the principal human CYP3A4 correlated with suboptimal activation of GH/glucocorticoid-dependent transcriptional pathways in men

Cytochrome P450 (CYP)3A4 is the principal and most abundant human isoform of CYP responsible for the metabolism of more than 50% of all consumed drugs and innumerable endogenous compounds. Expression of CYP3A4 is sexually dimorphic and regulated by the combined actions of GH and glucocorticoids. In the case of the rat, nearly all of the CYPs are "intrinsically" or "inherently" sexually dimorphic, meaning that the expressed sex differences are permanent and irreversible. Using primary hepatocyte cultures derived from men and women exposed to physiologic-like levels of continuous GH (the feminine circulating profile) alone, dexamethasone alone, and the combined regimen, we observed a dramatic inherent CYP3A4 sexual dimorphism (women more than men) with all treatments. The molecular basis for this intrinsic sexually dimorphic expression of CYP3A4 appears to be due, at least in part, to a greater level of hormone-dependent activation and nuclear translocation of both hepatocyte nuclear factor-4α (HNF-4α) and pregnane X receptor in female hepatocytes. Furthermore, these transcription factors exhibited significantly higher DNA binding levels to their specific motifs on the CYP3A4 promoter in female hepatocytes, inferring a possible explanation for the elevated expression of CYP3A4 in women. Accordingly, experiments using HepG2 cells treated with small inhibitory RNA-induced knockdown of HNF-4α and/or transfected with luciferase reporter constructs containing a CYP3A4 promoter lacking HNF-4α-binding motifs demonstrated that GH, to a greater extent dexamethasone, and to the greatest extent the combine hormone regimen, stimulated HNF-4α and pregnane X receptor promoter transactivation, signifying enhanced transcription of CYP3A4 and, thus, identifying a molecular mechanism contributing to the intrinsic sexual dimorphic expression of human CYP3A4.

Intrasplenic transplantation of isolated adult rat hepatocytes: sex-reversal and/or suppression of the major constituent isoforms of cytochrome P450

Adult male and female rat hepatocytes were individually transplanted into the spleens of adult male and female rats. The recipients were euthanized at either eight, sixteen, thirty, or forty-five weeks following transplantation, at which time hepatic and splenic levels of liver-specific rat albumin mRNA as well as sex-dependent transcript levels of CYP2C11, -2C12, -2C7, -2A1, and -3A2-which accounts for > 60% of the total concentration of hepatic constituent cytochrome P450-were determined. Whereas the pre-infused hepatocytes expressed their expected cytochrome P450 sexual dimorphisms (female-specific CYP2C12, male-specific CYP3A2, and female-predominant CYP2A1), their post-transplantational competence now reflected the sexual dimorphisms of the recipient (as observed in the host's liver), which supports the concept that the sex-dependent growth hormone circulating profiles are the determinants regulating the expression levels of hepatic cytochrome P450. Also expressed at normal concentrations in the pre-infused hepatocytes, male-specific CYP2C11 and female-predominant CYP2C7 were inexplicably undetectable in the spleens of both recipient males and females, regardless of the sex of the donor hepatocytes, almost one year after transplantation.

Effect of estradiol on gene expression profile in cynomolgus macaque liver: implications for drug-metabolizing enzymes

Estrogen regulation of gene expression is essential for physiological function of estrogen-responsive tissues, such as mammary glands, ovaries, and the uterus. In the liver, estrogen is responsible for sex-dependent gene expression of drug-metabolizing enzymes in rodents. However, the influence of estrogen on hepatic gene expression has not been fully investigated in primates, including human. Macaque, including cynomolgus macaque, is an important species for comparative studies aimed at understanding human physiology due to its evolutionary closeness to human. To identify estrogen-responsive genes in primate liver, therefore, hepatic gene expression was compared, by microarray analysis, in ovariectomized cynomolgus macaques treated with estradiol or solvent (control). The analysis identified 98 estradiol-responsive genes; 47 and 51 were up- and down-regulated by estradiol, respectively (≥2.0-fold, P < 0.05). Expression of drug-metabolizing enzyme genes was also influenced by estradiol treatment; estradiol enhanced expression of GSTM5 (3.8-fold, P < 0.05) and CYP3A8(4) (2.7-fold, P < 0.01), but lowered expression of CYP4F12 (2.2-fold, P < 0.01), as verified by quantitative polymerase chain reaction. In particular, CYP3A8(4), orthologous to human CYP3A4, is an essential drug-metabolizing enzyme in cynomolgus macaque liver. These results suggest that expression of hepatic genes, including drug-metabolizing enzyme genes, is at least partly regulated by estradiol in cynomolgus macaque.

Menstrual cycle and oral contraceptives' effects on growth hormone response to sprinting

The present study examined the impact of the menstrual cycle and oral contraceptive (OC) use on the growth hormone response to non-motorized treadmill sprinting. Nine monophasic OC users (21.5 ± 4.7 years old), and 8 normally menstruating women (NM; 21.4 ± 2.9 years old) participated in the study. Each participant completed 2 main trials, each consisting of an all-out 30-s treadmill sprint. The NM group performed one trial in the midfollicular phase (NM follicular) and one in the midluteal phase (NM luteal); the OC group's trials occurred one week into the start of the pill-taking cycle and once during the week in which pills were not taken.Venous blood samples were analyzed for growth hormone, pH, lactate, glucose, and progesterone concentrations. Peak and mean power output did not differ between the groups or with menstrual phase, or between the OC-free and OC trials. Integrated growth hormone was greater in the OC group than in the NM group (p = 0.04) with no phase difference (p = 0.80, mean (SD); NM follicular: 421 (335) and NM luteal: 345 (304) vs. OC free: 737 (471) and OC: 758 (389) µg·L(-1)·90 min(-1)). Blood lactate was higher in the OC group than in the NM group (p = 0.007) and, conversely, pH was lower in the OC group (p = 0.01). These results demonstrate that OC users who take high-androgenicity pills have a higher growth hormone response to sprint running than do normally menstruating women.

Transcriptional profiling of human liver identifies sex-biased genes associated with polygenic dyslipidemia and coronary artery disease

Sex-differences in human liver gene expression were characterized on a genome-wide scale using a large liver sample collection, allowing for detection of small expression differences with high statistical power. 1,249 sex-biased genes were identified, 70% showing higher expression in females. Chromosomal bias was apparent, with female-biased genes enriched on chrX and male-biased genes enriched on chrY and chr19, where 11 male-biased zinc-finger KRAB-repressor domain genes are distributed in six clusters. Top biological functions and diseases significantly enriched in sex-biased genes include transcription, chromatin organization and modification, sexual reproduction, lipid metabolism and cardiovascular disease. Notably, sex-biased genes are enriched at loci associated with polygenic dyslipidemia and coronary artery disease in genome-wide association studies. Moreover, of the 8 sex-biased genes at these loci, 4 have been directly linked to monogenic disorders of lipid metabolism and show an expression profile in females (elevated expression of ABCA1, APOA5 and LDLR; reduced expression of LIPC) that is consistent with the lower female risk of coronary artery disease. Female-biased expression was also observed for CYP7A1, which is activated by drugs used to treat hypercholesterolemia. Several sex-biased drug-metabolizing enzyme genes were identified, including members of the CYP, UGT, GPX and ALDH families. Half of 879 mouse orthologs, including many genes of lipid metabolism and homeostasis, show growth hormone-regulated sex-biased expression in mouse liver, suggesting growth hormone might play a similar regulatory role in human liver. Finally, the evolutionary rate of protein coding regions for human-mouse orthologs, revealed by dN/dS ratio, is significantly higher for genes showing the same sex-bias in both species than for non-sex-biased genes. These findings establish that human hepatic sex differences are widespread and affect diverse cell metabolic processes, and may help explain sex differences in lipid profiles associated with sex differential risk of coronary artery disease.

Novel ontogenetic patterns of sexual differentiation in arcuate nucleus GHRH neurons revealed in GHRH-enhanced green fluorescent protein transgenic mice

GH secretion and growth rates are developmentally regulated and sexually dimorphic, but the neuroregulatory mechanisms between birth and puberty are unclear. Using the GHRH-enhanced green fluorescent protein (eGFP) transgenic mouse, in which eGFP provides a strong surrogate signal for identifying GHRH neurons, we showed that numbers in the male arcuate nucleus were double those seen in females at x postnatal day (P)1 and P10, during which time numbers increased 2- to 3-fold. Thereafter (P20, P30, P60, P365) there was a significant trend for numbers to decrease in males and increase in females, such that sex differences were, surprisingly, absent in young and late adulthood. Conversely, we identified the emergence of male-dominant sex differences in the number of processes extended per GHRH perikarya across puberty. Intriguingly, prepubertal gonadectomy (P28), unlike adult gonadectomy, caused a dramatic 40% loss of GHRH cells in both sexes in adulthood and a significant (30%) increase in processes emanating from cell bodies only in females. These findings establish a novel ontogenetic profile for GHRH neurons and suggest previously undiscovered roles for peripubertal gonadal factors in establishing population size in both sexes. They also provide the first demonstration of emergent sex-specific GHRH architecture, which may signal the onset of sex-dependent regulation of activity reported for adult GHRH-eGFP neurons, and its differential regulation by gonadal factors in males and females. This information adds to our knowledge of processes that underpin the emergence of sex-specific GH secretory dynamics and hence biological activity of this pleiotropic hormone.

Effects of a growth hormone-releasing hormone analog on endogenous GH pulsatility and insulin sensitivity in healthy men

CONTEXT AND OBJECTIVE

Strategies to augment pulsatile GH may be beneficial in patients with excess visceral adiposity, in whom GH secretion is reduced. The objective of this study was to determine the effects of a novel GHRH (GHRH(1-44)) analog, tesamorelin, on endogenous GH pulsatility and insulin sensitivity in healthy men.

DESIGN, PARTICIPANTS, AND INTERVENTION

Thirteen males (mean age 45 ± 3 yr and body mass index 27.3 ± 1.2 kg/m(2)) received tesamorelin 2 mg sc once daily for 2 wk, with assessment made at baseline, after 2 wk of treatment, and after 2 wk of withdrawal.

OUTCOME MEASURES

The primary end point was change in mean overnight GH as determined by overnight frequent sampling. Secondary end points included insulin-stimulated glucose uptake as measured by euglycemic hyperinsulinemic clamp; IGF-I; and GH secretion parameters, including pulse area, pulse frequency, and basal secretion.

RESULTS

Tesamorelin treatment increased mean overnight GH (change +0.5 ± 0.1 μg/liter, P = 0.004), average log(10) GH peak area (change +0.4 ± 0.1 log(10) μg/liter, P = 0.001), and basal GH secretion (change +0.008 ± 0.003 μg/liter · min, P = 0.008). IGF-I increased by 181 ± 22 μg/liter (P < 0.0001). Neither fasting glucose (P = 0.93) nor insulin-stimulated glucose uptake (P = 0.61) was significantly affected by tesamorelin.

CONCLUSIONS

Once-daily short-term treatment with a GHRH(1-44) analog, tesamorelin, augments basal and pulsatile GH secretion. Moreover, although tesamorelin significantly increases IGF-I, peripheral insulin-stimulated glucose uptake appears to be preserved.

Gender, age, body mass index, and IGF-I individually and jointly determine distinct GH dynamics: analyses in one hundred healthy adults

BACKGROUND

GH secretion is quantifiable as mean, peak, and nadir GH concentrations; degree of irregularity (approximate entropy); and spikiness (brief staccato-like fluctuations).

HYPOTHESIS

Distinct GH dynamics reflect relatively distinct (combinations of) subject variables, such as gender, age, body mass index (BMI), and IGF-I concentrations.

LOCATION

The study took place at a clinical translational research unit.

SUBJECTS

Subjects included 100 healthy adults ages 20-77 yr (59 women and 41 men), BMI 18-42 kg/m(2), and IGF-I 9.2-38 nmol/liter.

MEASURES

Immunofluorometric GH assay was done on 10-min samples collected for 24 h.

RESULTS

Stepwise forward-selection multivariate regression analysis revealed that mean GH concentrations were simultaneously determined (overall r = 0.36; P < 0.001) by gender (higher in women, P < 0.001), BMI (negatively, P < 0.001), and IGF-I (positively, P < 0.001). Peak GH levels were influenced (r = 0.28) by both BMI (P < 0.001) and IGF-I (P = 0.001). Nadir GH values were jointly affected by gender (higher in women, P = 0.005) and BMI (negatively, P = 0.001). GH approximate entropy was triply defined (r = 0.29) by gender (greater irregularity in women, P < 0.001), age (P = 0.022), and BMI (P = 0.008) and dually (r = 0.25) by gender (P = 0.0001) and BMI (P = 0.017) if sex steroids were included. GH spikiness was determined (r = 0.29) by gender (higher in women, P = 0.0016) and BMI (positively, P = 0.0002).

CONCLUSION

In healthy adults, combinations of gender, age, BMI, and IGF-I specify distinct GH dynamics, thus requiring balanced representation of these variables in comparative GH studies.

Altered drug metabolism during pregnancy: hormonal regulation of drug-metabolizing enzymes

IMPORTANCE OF THE FIELD

Medication use during pregnancy is prevalent, but pharmacokinetic information of most drugs used during pregnancy is lacking in spite of known effects of pregnancy on drug disposition. Accurate pharmacokinetic information is essential for optimal drug therapy in mother and fetus. Thus, understanding how pregnancy influences drug disposition is important for better prediction of pharmacokinetic changes of drugs in pregnant women.

AREAS COVERED IN THIS REVIEW

Pregnancy is known to affect hepatic drug metabolism, but the underlying mechanisms remain unknown. Physiological changes accompanying pregnancy are probably responsible for the reported alteration in drug metabolism during pregnancy. These include elevated concentrations of various hormones such as estrogen, progesterone, placental growth hormones and prolactin. This review covers how these hormones influence expression of drug-metabolizing enzymes (DMEs), thus potentially responsible for altered drug metabolism during pregnancy.

WHAT THE READER WILL GAIN

The reader will gain a greater understanding of the altered drug metabolism in pregnant women and the regulatory effects of pregnancy hormones on expression of DMEs.

TAKE HOME MESSAGE

In-depth studies in hormonal regulatory mechanisms as well as confirmatory studies in pregnant women are warranted for systematic understanding and prediction of the changes in hepatic drug metabolism during pregnancy.

Basal, but not pulsatile, growth hormone secretion determines the ambient circulating levels of insulin-like growth factor-I

CONTEXT

Previous studies have shown that mean 24-h GH concentrations determine plasma IGF-I levels in patients with acromegaly. However, we have recently shown that continuous GH infusion, mimicking the interpulse GH levels, was significantly more effective than the pulsatile GH administration at increasing IGF-I concentrations.

OBJECTIVE

The aim of the study was to ascertain relative roles of total GH output (24-h mean), GH pulses, and interpulse GH level in determining plasma IGF-I concentrations.

DESIGN AND SETTING

We conducted a point-in-time observational inpatient study in the General Clinical Research Center at the University of Michigan.

PATIENTS OR OTHER PARTICIPANTS

Eighteen patients with acromegaly and 19 healthy control subjects participated in the study.

INTERVENTION(S)

We performed frequent (every 10 or 20 min) blood sampling over 24 h.

MAIN OUTCOME MEASURE(S)

Before data collection, we hypothesized that interpulse nadir levels of GH would correlate with IGF-I levels in normal and acromegalic subjects.

RESULTS

Mean and valley levels of GH correlated with serum IGF-I levels (r(2) = 0.44 and 0.48, respectively) in normal and acromegalic patients in a log-linear fashion. The strongest correlation, however, was observed between the log of nadir GH and IGF-I concentrations (r(2) = 0.77). GH pulse mass did not significantly correlate with IGF-I (r(2) = 0.001).

CONCLUSIONS

Plasma IGF-I concentrations correlated with mean 24-h GH concentrations. This relationship is dependent exclusively on the basal GH levels. GH pulses do not determine plasma IGF-I concentrations.

Study of sensory-motor and somatic development of the offspring of rats (Wistar) treated with caffeine

The influence of caffeine, administered to rats, on the somatic and sensory-motor development of the offspring was investigated. Female Wistar rats were divided into a control group and a treated group and received drinking water and a 0.1% solution of caffeine orally, respectively. The offspring, also divided into a control group and a treated group, received daily monitoring until the 20th day of life to verify alterations in somatic neural development. The offspring of the treated group had reduced weight on the day of birth and on the 1st, 5th, 15th and 20th days of life; shorter snout-anus length (evaluation done daily); shorter snout-tail length on the day of birth and on the 1st, 5th and 10th days of life, and signs of retardation of somatic and sensory-motor maturation. These results allowed the conclusion that administration of caffeine to rats affects somatic and sensory-motor development of offspring.

The pattern of growth hormone delivery to peripheral tissues determines insulin-like growth factor-1 and lipolytic responses in obese subjects

CONTEXT

It is unclear whether the pattern of GH delivery to peripheral tissues has important effects.

OBJECTIVE

The aim of the study was to compare the effects of pulsatile vs. continuous administration of GH upon metabolic and IGF-I parameters in obese subjects.

SETTING

The study was conducted at the General Clinical Research Center at the University of Michigan Medical Center.

PARTICIPANTS

Four men and five women with abdominal obesity (body mass index, 33 +/- 3 kg/m(2); body fat, 40 +/- 3%) participated in the study.

INTERVENTION

GH (0.5 mg/m(2) . d) was given iv for 3 d as: 1) continuous infusion (C); and 2) pulsatile boluses (P) (15% of the dose at 0700, 1300, and 1800 h and 55% at 2400 h). These trials were preceded by a basal period (B) when subjects received normal saline.

MAIN OUTCOME MEASURES

Rate of lipolysis and hepatic glucose production were evaluated using stable isotope tracer techniques. The composite index of insulin sensitivity (Matsuda index) was assessed using oral glucose tolerance test.

RESULTS

The increase in plasma IGF-I concentrations was greater (P < 0.05) with continuous GH infusion (211 +/- 31, 423 +/- 38, and 309 +/- 34 microg/liter for B, C, and P, respectively). Muscle IGF-I mRNA was significantly increased (P < 0.05) only after the continuous GH infusion (1.2 +/- 0.4, 4.4 +/- 1.3, and 2.3 +/- 0.6 arbitrary units, for B, C, and P, respectively). Only pulsatile GH augmented the rate of lipolysis (4.1 +/- 0.3, 4.8 +/- 0.7, and 7.1 +/- 1.1 mumol/kg . min for B, C, and P, respectively). GH had no effect on hepatic glucose production, but both modes of GH administration were equally effective in impairing insulin sensitivity.

CONCLUSION

These findings indicate that, in obese subjects, discrete components of GH secretory pattern may differentially affect IGF-I generation and lipolytic responses.

Sex differences in the expression of hepatic drug metabolizing enzymes

Sex differences in pharmacokinetics and pharmacodynamics characterize many drugs and contribute to individual differences in drug efficacy and toxicity. Sex-based differences in drug metabolism are the primary cause of sex-dependent pharmacokinetics and reflect underlying sex differences in the expression of hepatic enzymes active in the metabolism of drugs, steroids, fatty acids and environmental chemicals, including cytochromes P450 (P450s), sulfotransferases, glutathione transferases, and UDP-glucuronosyltransferases. Studies in the rat and mouse liver models have identified more than 1000 genes whose expression is sex-dependent; together, these genes impart substantial sexual dimorphism to liver metabolic function and pathophysiology. Sex differences in drug metabolism and pharmacokinetics also occur in humans and are due in part to the female-predominant expression of CYP3A4, the most important P450 catalyst of drug metabolism in human liver. The sexually dimorphic expression of P450s and other liver-expressed genes is regulated by the temporal pattern of plasma growth hormone (GH) release by the pituitary gland, which shows significant sex differences. These differences are most pronounced in rats and mice, where plasma GH profiles are highly pulsatile (intermittent) in male animals versus more frequent (nearly continuous) in female animals. This review discusses key features of the cell signaling and molecular regulatory mechanisms by which these sex-dependent plasma GH patterns impart sex specificity to the liver. Moreover, the essential role proposed for the GH-activated transcription factor signal transducer and activator of transcription (STAT) 5b, and for hepatic nuclear factor (HNF) 4alpha, as mediators of the sex-dependent effects of GH on the liver, is evaluated. Together, these studies of the cellular, molecular, and gene regulatory mechanisms that underlie sex-based differences in liver gene expression have provided novel insights into the physiological regulation of both xenobiotic and endobiotic metabolism.

Mechanism of gender-divergent UDP-glucuronosyltransferase mRNA expression in mouse liver and kidney

UDP-glucuronosyltransferases (UGTs) catalyze the addition of glucuronic acid to endo- and xenobiotics, increasing hydrophilicity and enhancing elimination. Gender-divergent glucuronidation rates are observed in humans and rats, and gender differences in UGT mRNA levels have been observed in rodents. The purpose of this study was to establish the hormonal regulation of gender-dependent Ugt mRNA expression in mouse liver and kidney. Therefore, three mouse models were used to characterize the involvement of sex hormones and gender-specific growth hormone (GH) secretion patterns, including 1) hypophysectomized mice treated with male- or female-pattern GH, testosterone, or 17beta-estradiol; 2) GH releasing hormone receptor-deficient little (lit/lit) mice treated with male- or female-pattern GH; and 3) gonadectomized mice treated with testosterone or 17beta-estradiol. Messenger RNA expression of mouse Ugt isozymes was determined by the branched DNA assay. In C57BL/6 mice, male-predominant expression of Ugt2b1 and Ugt2b38 was observed in liver and kidney, respectively. Female-predominant expression was observed for Ugt1a1 and Ugt1a5 in liver and Ugt1a2 in kidney. In liver, regulation of Ugt1a1 and Ugt1a5 expression was attributed to repression of Ugt mRNA by male-pattern GH secretion. Conversely, regulation of Ugt2b1 expression in liver was attributed to male-pattern GH secretion. In kidney, regulation of Ugt2b38 expression was attributed to inductive effects by testosterone. Conversely, Ugt1a2 expression in kidney was negatively regulated by testosterone. In conclusion, gender differences in mouse Ugt mRNA expression were influenced by male-pattern GH secretion in liver, whereas gender differences were regulated by the effects of androgens in kidney.

Both central command and exercise pressor reflex activate cardiac sympathetic nerve activity in decerebrate cats

Both static and dynamic exercise are known to increase cardiac pump function as well as arterial blood pressure. Feedforward control by central command and feedback control by the exercise pressor reflex are thought to be the neural mechanisms causing these effects during exercise. It remains unknown as to how each mechanism activates cardiac sympathetic nerve activity (CSNA) during exercise, especially at its onset. Thus we examined the response of CSNA to stimulation of the mesencephalic locomotor region (MLR, i.e., central command) and to static muscle contraction of the triceps surae muscles or stretch of the calcaneal tendon in decerebrate cats. We found that MLR stimulation immediately increased CSNA, which was followed by a gradual increase in heart rate, mean arterial pressure, and ventral root activity in a stimulus intensity-dependent manner. The latency of the increase in CSNA from the onset of MLR stimulation ranged from 67 to 387 ms. Both static contraction and tendon stretch also rapidly increased CSNA. Their latency from the development of tension in response to ventral root stimulation ranged from 78 to 670 ms. These findings suggest that both central command and the muscle mechanoreflex play a role in controlling cardiac sympathetic outflow at the onset of exercise.

Effects of growth hormone overexpression vs. growth hormone receptor gene disruption on mouse hindlimb muscle fiber type composition

OBJECTIVE

The present study characterizes the fiber type composition of selected hindlimb muscles from two transgenic mouse lines specifically engineered to alter the amounts of circulating growth hormone (GH) and insulin-like growth factor-1 (IFG-1).

DESIGN

The triceps surae muscle group (soleus m., gastrocnemius m., and plantaris m.) was harvested en masse from mice that were: (1) giant due to the expression of a bovine GH transgene (bGH), (2) dwarf due to the disruption of the GH receptor/binding protein gene (GHR-/-), and (3) normal-sized controls [non-transgenic (NT)]. Histochemical and immunohistochemical methods were utilized on serial cross sections to delineate eight fiber types (I, IC, IIC, IIA, IIAD, IID, IIDB, and IIB). Cross-sectional areas were subsequently determined on approximately 50 fibers/type.

RESULTS

Compared to NT littermates, muscles from bGH mice demonstrated a significant (p<0.05) fast-to-slow shift in fiber phenotype, as well as significantly larger fibers for most types. In contrast, significantly smaller fibers were found for all fiber types in the GHR-/- mice, with no significant differences in fiber type percentages compared to NT. Regardless of mouse genotype, the hierarchy of fiber size was maintained in each muscle with type I the largest in the soleus m. and type IIB the largest in the predominantly fast muscles (plantaris, superficial and deep gastrocnemius muscles).

CONCLUSION

In conclusion, the genetic manipulation of GH expression (bGH) and its receptor binding (GHR-/-) had profound and divergent effects on muscle phenotype. It is hoped that continued research in this area will help elucidate the direct (independent of IGF-1) vs. indirect (via IGF-1 mediating mechanisms) effects of GH.

Sex and the Liver – A Journey Through Five Decades

Metabolism of steroids and drugs in rodents is sexually differentiated. The reason for this turned out to be the sexually differentiated growth hormone (GH) secretory pattern regulating the expression of a number of hepatic cytochrome P-450 genes. Although not fully resolved, it is clear that several signaling pathways and transcription factors are involved in mediating the effects of GH. It may be argued that such a well-controlled physiological system should have an important biological role and we speculate that the demands of a robust hepatic steroid metabolism during pregnancy has led to the development of this sexually differentiated hypothalamo-pituitary-liver axis.

Role of growth hormone in regulating lipolysis, proteolysis, and hepatic glucose production during fasting

CONTEXT

Fasting is associated with suppressed insulin and augmented GH secretion. The involvement of each mechanism in the regulation of fuel mobilization during fasting is unknown.

OBJECTIVE

To ascertain the role of GH in the regulation of the rates of lipolysis, proteolysis, and hepatic glucose production (HGP) during the physiological daily feed/fast cycle and after 2 d of complete fasting, we used a model of selective GH suppression by the administration of GHRH receptor antagonist (GHRH-A).

DESIGN AND SETTING

We conducted an open label in-patient study in the General Clinical Research Center at the University of Michigan.

PARTICIPANTS

Six healthy, nonobese volunteers participated.

MAIN OUTCOME MEASURES

We assessed 24-h plasma GH concentration and rates of lipolysis, proteolysis, and HGP using stable isotope techniques after an overnight fast and after 2 d of fasting.

RESULTS

GHRH-A suppressed plasma GH by about 65% during the fed state (P = 0.015) but did not alter the rates of lipolysis, proteolysis, or HGP. Fasting for 2 d suppressed plasma insulin concentration by about 80% and elevated plasma GH about 4-fold (both P < 0.01). This was accompanied by a doubling in the rate of lipolysis, an approximately 40% increase in proteolysis, and an approximately 30% decline in HGP (all P < 0.05). Preventing the fasting-induced increase in GH with GHRH-A largely abolished the increase in the rate of lipolysis. GHRH-A also augmented the fasting-induced reduction in HGP but did not alter proteolysis.

CONCLUSIONS

Endogenous GH plays a very limited metabolic role during the daily feed/fast cycle but is essential for the increased lipolytic rate found with more prolonged fasting.