Mediation of the hepatic effects of growth hormone by its lipolytic activity

Towards Understanding the Direct and Indirect Actions of Growth Hormone in Controlling Hepatocyte Carbohydrate and Lipid Metabolism

Growth hormone (GH) is critical for achieving normal structural growth. In addition, GH plays an important role in regulating metabolic function. GH acts through its GH receptor (GHR) to modulate the production and function of insulin-like growth factor 1 (IGF1) and insulin. GH, IGF1, and insulin act on multiple tissues to coordinate metabolic control in a context-specific manner. This review will specifically focus on our current understanding of the direct and indirect actions of GH to control liver (hepatocyte) carbohydrate and lipid metabolism in the context of normal fasting (sleep) and feeding (wake) cycles and in response to prolonged nutrient deprivation and excess. Caveats and challenges related to the model systems used and areas that require further investigation towards a clearer understanding of the role GH plays in metabolic health and disease are discussed.

Effect of growth hormone on insulin signaling

Growth hormone (GH) is a pleiotropic hormone that coordinates an array of physiological processes, including effects on bone, muscle, and fat, ultimately resulting in growth. Metabolically, GH promotes anabolic action in most tissues except adipose, where its catabolic action causes the breakdown of stored triglycerides into free fatty acids (FFA). GH antagonizes insulin action via various molecular pathways. Chronic GH secretion suppresses the anti-lipolytic action of insulin and increases FFA flux into the systemic circulation; thus, promoting lipotoxicity, which causes pathophysiological problems, including insulin resistance. In this review, we will provide an update on GH-stimulated adipose lipolysis and its consequences on insulin signaling in liver, skeletal muscle, and adipose tissue. Furthermore, we will discuss the mechanisms that contribute to the diabetogenic action of GH.

Effects of growth hormone on hepatic insulin sensitivity and glucose effectiveness in healthy older adults

PURPOSE

Growth hormone (GH) replacement decreases insulin sensitivity in healthy individuals. However, the effects of GH on organ-specific insulin sensitivity and glucose effectiveness are not well characterized. The purpose of this study was to evaluate the effects of GH administration for 26 weeks on muscle and hepatic insulin sensitivity and glucose effectiveness in healthy older individuals.

METHODS

This report is from a 26-week randomized, double-blind, placebo-controlled parallel-group trial in healthy, ambulatory, community-dwelling older women and men. We compared surrogate indices of insulin sensitivity [quantitative insulin-sensitivity check index (QUICKI), muscle insulin sensitivity index (MISI), hepatic insulin resistance index (HIRI)] and glucose effectiveness [oral glucose effectiveness index (oGE)] derived from oral glucose tolerance tests (OGTTs) in subjects before and after 26 weeks of administration of GH (n = 17) or placebo (n = 15) as an exploratory outcome.

RESULTS

GH administration for 26 weeks significantly increased fasting insulin concentrations and HIRI but did not significantly change MISI or oGE compared to placebo.

CONCLUSIONS

GH administration for 26 weeks in healthy older subjects impairs insulin sensitivity in the liver but not skeletal muscle and does not alter glucose effectiveness.

Diabetes mellitus induced by somatostatin analogue therapy is not permanent in acromegalic patients

CONTEXT

Therapy with somatostatin analogues (SSAs) may have deleterious effects on glucose metabolism in patients with acromegaly, often leading to the development of diabetes mellitus (DM).

AIM

The aim of the study was to evaluate whether DM, developed during therapy with SSAs, may revert after drug withdrawal and cure of acromegaly with pituitary adenomectomy.

DESIGN

Retrospective cohort study, in a tertiary referral centre.

PATIENTS

Eighteen acromegalic patients without DM at the diagnosis of acromegaly treated with SSAs as a primary therapy, and then cured by pituitary adenomectomy.

METHODS

Endocrine status and glucose homeostasis were evaluated at diagnosis of acromegaly and at least every 6 months during SSA therapy. At each visit, patients were classified into one of the following classes: normal glucose tolerance, prediabetes, overt diabetes.

RESULTS

Median follow-up after starting SSAs therapy was 69 months (IQR 54.75-132.25). During SSA therapy, all patients had controlled acromegaly defined by normal serum IGF1 concentrations for the age. Of the 13 euglycaemic patients at diagnosis, three developed prediabetes and three diabetes, whereas, of the five prediabetic patients at diagnosis, two worsened to overt diabetes and three remained in the prediabetic range (P = 0.04). After curing acromegaly with pituitary adenomectomy and subsequent SSA withdrawal, prediabetes reverted in five of six patients, and diabetes in all five patients (three reverted to euglycaemia, while two reverted to prediabetes) (P = 0.01).

CONCLUSIONS

In acromegalic patients with controlled disease, changes in glycaemic status induced by SSAs are not permanent.

Islet insulin content and release are increased in male mice with elevated endogenous GH and IGF-I, without evidence of systemic insulin resistance or alterations in β-cell mass

It is clear that elevations in circulating GH can lead to an increase in insulin levels. This increase in insulin may be due to GH-mediated insulin resistance and enhanced lipolysis. However, there is also in vitro and in vivo evidence that GH acts directly to increase β-cell proliferation and insulin production. Our laboratory recently developed an animal model with elevated endogenous GH levels associated with a small (25%), but significant, increase in IGF-I (HiGH mice). As expected, insulin levels were elevated in HiGH mice; however, whole body insulin sensitivity was not altered and glucose tolerance was improved. This metabolic phenotype suggests that modest elevations in circulating GH and IGF-I may enhance β-cell mass and/or function, in the absence of systemic insulin resistance, thus improving glucose homeostasis.

OBJECTIVE

To determine if β-cell mass and/or function is altered in HiGH mice.

DESIGN

Male HiGH mice and their littermate controls were fed a low-fat or high-fat diet. Body composition and circulating metabolic endpoints were monitored overtime. The pancreas was recovered and processed for assessment of β-cell mass or in vitro basal and glucose-stimulated insulin secretion.

RESULTS

HiGH mice showed elevated circulating insulin and normal glucose levels, while non-esterified FFA levels and triglycerides were reduced or normal, depending on diet and age. β-cell mass did not differ between HiGH and control mice, within diet. However, islets from HiGH mice contained and released more insulin under basal conditions, as compared to control islets, while the relative glucose-stimulated insulin release did not differ.

CONCLUSIONS

Taken together, these results suggest moderate elevations in circulating GH and IGF-I can directly increase basal insulin secretion without impacting β-cell mass, independent of changes in whole body insulin sensitivity and hyperlipidemia.

Impact of growth hormone receptor blockade on substrate metabolism during fasting in healthy subjects

CONTEXT

Experimental studies in GH-deficient patients and in healthy subjects receiving somatostatin-infusion suggest that GH is an important regulator of substrate metabolism during fasting. These models may not adequately reflect the selective effects of GH, and GH receptor (GHR) blockade offers a new model to define the metabolic role of GH.

OBJECTIVE

The aim of this study was to investigate the impact of GHR blockade on substrate metabolism and insulin sensitivity during fasting.

DESIGN

We conducted a randomized, placebo-controlled, crossover study in 10 healthy young men.

INTERVENTION

After 36 h of fasting with saline or pegvisomant (GHR blockade), the subjects were studied during a 4-h basal period and 2.5-h hyperinsulinemic euglycemic clamp.

MAIN OUTCOME

We measured whole-body and forearm glucose, lipid, and protein metabolism, peripheral insulin sensitivity, and acyl and desacyl ghrelin.

RESULTS

GHR blockade significantly suppressed circulating free fatty acids (1226 +/- 83 vs. 1074 +/- 65 micromol/liter; P = 0.03) and ketone bodies (3080 +/- 271 vs. 2015 +/- 235 micromol/liter; P <or= 0.01), as well as forearm uptake of free fatty acids (0.341 +/- 0.150 vs. 0.004 +/- 0.119 micromol/100 ml x min; P < 0.01) and lipid oxidation (1.3 +/- 0.1 vs. 1.2 +/- 0.1 mg/kg x min; P = 0.03) in the basal period. By contrast, IGF-I levels in either serum or peripheral tissues were not impacted by GHR blockade, and protein metabolism was also unaffected. Basal glucose levels were elevated by GHR blockade, but insulin sensitivity was similar; this was associated with an increased acyl/desacyl ghrelin ratio.

CONCLUSION

GHR blockade, without changes in circulating or tissue IGF-I levels, selectively suppresses lipid mobilization and oxidation after short-term fasting. This supports the notion that stimulation of lipolysis is a primary and important effect of GH.

Effects of growth hormone and free fatty acids on insulin sensitivity in patients with type 1 diabetes

CONTEXT

Because GH stimulates lipolysis, an increase in circulating free fatty acid levels, as opposed to a direct effect of high GH levels, could underlie the development of insulin resistance in type 1 diabetes (T1D). Our aim was to explore the relative contributions of GH and free fatty acids to the development of insulin resistance in patients with T1D.

PATIENTS

Seven (four females, three males) nonobese patients with T1D aged 21-30 yr were studied on four occasions in random order. On each visit, overnight endogenous GH production was suppressed by octreotide. Three 1-h pulses of recombinant human GH (rhGH) or placebo were administered on two visits each. Acipimox, an antilipolytic drug, or a placebo were ingested every 4 h on two visits each. Stable glucose and glycerol isotopes were used to assess glucose and glycerol turnover. The overnight protocol was concluded by a two-step hyperinsulinemic euglycemic clamp on each visit.

MAIN OUTCOME

rhGH administration led to increases in the insulin infusion rate required to maintain euglycemia overnight (P = 0.008), elevated basal endogenous glucose production (P = 0.007), decreased basal peripheral glucose uptake (P = 0.03), and reduced glucose uptake during step 1 of the clamp (P < 0.0001). Coadministration of rhGH and acipimox reversed these effects and suppression of lipolysis in the absence of GH replacement led to further increases in insulin sensitivity.

RESULTS

GH pulses were associated with an increase in endogenous glucose production and decreased rates of peripheral glucose uptake, which was entirely reversed by acipimox. Therefore, GH-driven decreases in insulin sensitivity are mainly determined by the effect of GH on lipolysis.

Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects

In evolutionary terms, GH and intracellular STAT 5 signaling is a very old regulatory system. Whereas insulin dominates periprandially, GH may be viewed as the primary anabolic hormone during stress and fasting. GH exerts anabolic effects directly and through stimulation of IGF-I, insulin, and free fatty acids (FFA). When subjects are well nourished, the GH-induced stimulation of IGF-I and insulin is important for anabolic storage and growth of lean body mass (LBM), adipose tissue, and glycogen reserves. During fasting and other catabolic states, GH predominantly stimulates the release and oxidation of FFA, which leads to decreased glucose and protein oxidation and preservation of LBM and glycogen stores. The most prominent metabolic effect of GH is a marked increase in lipolysis and FFA levels. In the basal state, the effects of GH on protein metabolism are modest and include increased protein synthesis and decreased breakdown at the whole body level and in muscle together with decreased amino acid degradation/oxidation and decreased hepatic urea formation. During fasting and stress, the effects of GH on protein metabolism become more pronounced; lack of GH during fasting increases protein loss and urea production rates by approximately 50%, with a similar increase in muscle protein breakdown. GH is a counterregulatory hormone that antagonizes the hepatic and peripheral effects of insulin on glucose metabolism via mechanisms involving the concomitant increase in FFA flux and uptake. This ability of GH to induce insulin resistance is significant for the defense against hypoglycemia, for the development of "stress" diabetes during fasting and inflammatory illness, and perhaps for the "Dawn" phenomenon (the increase in insulin requirements in the early morning hours). Adult patients with GH deficiency are insulin resistant-probably related to increased adiposity, reduced LBM, and impaired physical performance-which temporarily worsens when GH treatment is initiated. Conversely, despite increased LBM and decreased fat mass, patients with acromegaly are consistently insulin resistant and become more sensitive after appropriate treatment.

The exon 3-deleted/full-length growth hormone receptor polymorphism does not influence the effect of puberty or growth hormone therapy on glucose homeostasis in short non-growth hormone-deficient small-for-gestational-age children: results from a two-year controlled prospective study

CONTEXT

The exon 3-deleted/full-length (d3/fl) GH receptor polymorphism (d3/fl-GHR) has been associated with responsiveness to GH therapy in short small-for-gestational-age (SGA) patients, although consensus is lacking. However, its influence on glucose homeostasis, at baseline or under GH therapy, has not been investigated.

OBJECTIVE

Our objective was to evaluate whether the d3/fl-GHR genotypes influence insulin sensitivity in short SGA children before or after puberty onset or during GH therapy.

DESIGN

We conducted a 2-yr prospective, controlled, randomized trial.

SETTING

Thirty Spanish hospitals participated. Auxological, GH secretion, and glucose homeostasis evaluation was hospital based, whereas molecular analyses and data computation were centralized.

PATIENTS

Patients included 219 short SGA children [body mass index sd score (SDS) < or = 2.0]; 159 were prepubertal (group 1), and 60 had entered puberty (group 2).

INTERVENTION

Seventy-eight patients from group 1 were treated with GH (66 microg/kg.d) for 2 yr (group 3).

MAIN OUTCOME MEASURES

Previous and 2-yr follow-up auxological and biochemical data were recorded, d3/fl-GHR genotypes determined, and data analyzed.

RESULTS

In groups 1 and 2, fasting glucose, insulin, homeostasis model assessment (HOMA), and quantitative insulin sensitivity check index (QUICKI) were similar in each d3/fl-GHR genotype. Group 2 glucose, insulin, and HOMA were significantly higher and QUICKI lower than in group 1. In group 3 GH-treated patients, height SDS, growth velocity SDS, fasting glucose, insulin, and HOMA significantly increased as did body mass index SDS at the end of the second year, and QUICKI decreased during the first and second years, with no differences among the d3/fl-GHR genotypes.

CONCLUSION

In short SGA patients, the d3/fl-GHR genotypes do not seem to influence prepubertal or pubertal insulin sensitivity indexes or their changes over 2 yr of GH therapy (66 mug/kg.d).

Maximizing acute fat utilization: effects of exercise, food, and individual characteristics

In discussion of the physiological mechanisms that regulate fat metabolism, and with consideration of the metabolic stimuli that modulate substrate metabolism, the issue of how an acute state of negative lipid balance can be maximized is addressed. The regulation of lipolysis by catecholamines and insulin is reviewed, and the mechanisms of fatty acid mobilization and uptake by muscle are also briefly discussed. The implications of substrate availability and the hormonal response during physiological states such as fasting, exercise, and after food intake are also addressed, with particular regard to the influences on fatty acid mobilization and/or oxidation from eliciting these stimuli conjointly. Finally, a brief discussion is given of both the nature of exercise and the exercising individual, and how these factors influence fat metabolism during exercise. It is also a primary thrust of this paper to underline gaps in the existing literature with regard to exercise timing concerning food ingestion for maximizing acute lipid utilization.

Somatropin and glucose homeostasis: considerations for patient management

More than 60 years ago it was shown, in dogs, that anterior pituitary extracts may cause glucose intolerance and that hypophysectomy was associated with increased insulin sensitivity. Accordingly, active acromegaly is characterized by insulin resistance at the hepatic and muscular level, whereas children with growth hormone (GH) deficiency are insulin hypersensitive and prone to developing fasting hypoglycemia. Somewhat unexpectedly, hypopituitary adults with untreated GH deficiency tend to be insulin resistant, which may be aggravated by somatropin (GH) therapy. The explanation for this apparent paradox has not been fully established. It is, however, likely that high circulating levels of free fatty acids (FFA) are responsible for insulin resistance, both before and after somatropin therapy. In the untreated state, patients have abdominal obesity, which increases circulating FFA levels. Since GH has potent lipolytic effects, somatropin therapy will further increase FFA levels. Theoretically, this GH replacement effect will eventually be compensated for by favorable alterations in body composition, including a reduction of fat mass. Subcutaneous somatropin therapy, however, will cause some degree of hypersomatropinemia in the prandial phase, which will inevitably antagonize the physiologic effects of insulin. At present, the best way to circumvent this inherent problem is to employ evening injections of somatropin and to ensure that the dosage is not too high. In the latter regard, it is important to realize that dosage requirements are lower in adults compared with children, and that the dosage will probably need to be reduced with age in the individual patient.

Glucose homeostasis in abdominal obesity: hepatic hyperresponsiveness to growth hormone action

It has been suggested that (abdominally) obese individuals are hypersensitive to growth hormone (GH) action. Because GH affects glucose metabolism, this may impact glucose homeostasis in abdominal obesity. Therefore, we studied the effect of GH on glucose metabolism in abdominally obese (OB) and normal-weight (NW) premenopausal women. A 1-h intravenous infusion of GH or placebo was randomly administered to six NW [body mass index (BMI) 21.1 +/- 1.9 kg/m(2)] and six OB (BMI 35.5 +/- 1.5 kg/m(2)) women in a crossover design. Insulin, glucagon, and GH secretion were suppressed by concomitant infusion of somatostatin. Glucose kinetics were measured using a 10-h infusion of [6,6-(2)H(2)]glucose. In both groups, similar physiological GH peaks were reached by infusion of GH. GH strongly stimulated endogenous glucose production (EGP) in both groups. The percent increase was significantly greater in OB than in NW women (29.8 +/- 11.3 vs. 13.3 +/- 7.4%, P = 0.014). Accordingly, GH responsiveness, defined as the maximum response of EGP per unit GH, was increased in OB vs. NW subjects (6.0 +/- 2.1 vs. 2.2 +/- 1.5 micromol.min(-1).mU(-1).l(-1), P = 0.006). These results suggest that the liver is hyperresponsive to GH action in abdominally obese women. The role of the somatotropic ensemble in the control of glucose homeostasis in abdominal obesity is discussed.

Effects of GH on urea, glucose and lipid metabolism, and insulin sensitivity during fasting in GH-deficient patients

Fasting-related states of distress pose major health problems, and growth hormone (GH) plays a key role in this context. The present study was designed to assess the effects of GH on substrate metabolism and insulin sensitivity during short-term fasting. Six GH-deficient adults underwent 42.5 h of fasting on two occasions, with and without concomitant GH replacement. Palmitate and urea fluxes were measured with the steady-state isotope dilution technique after infusion of [9,10-3H]palmitate and [13C]urea. During fasting with GH replacement, palmitate concentrations and fluxes increased by 50% [palmitate: 378 +/- 42 (GH) vs. 244 +/- 12 micromol/l, P < 0.05; palmitate: 412 +/- 58 (GH) vs. 276 +/- 42 microM, P = 0.05], and urea turnover and excretion decreased by 30-35% [urea rate of appearance: 336 +/- 22 (GH) vs. 439 +/- 43 micromol. kg-1. h-1, P < 0.01; urea excretion: 445 +/- 43 (GH) vs. 602 +/- 74 mmol/24 h, P < 0.05]. Insulin sensitivity (determined by a euglycemic hyperinsulinemic clamp) was significantly decreased [M value: 1.26 +/- 0.06 (GH) vs. 2.07 +/- 0.22 mg. kg-1. min-1, P < 0.01] during fasting with GH replacement. In conclusion, continued GH replacement during fasting in GH-deficient adults decreases insulin sensitivity, increases lipid utilization, and conserves protein.

Pharmacological antilipolysis restores insulin sensitivity during growth hormone exposure

Stimulation of lipolysis and the induction of resistance to insulin's actions on glucose metabolism are well-recognized effects of growth hormone (GH). To evaluate whether these two features are causally linked, we studied the impact of pharmacologically induced antilipolysis in seven GH-deficient patients (mean [+/- SE] age 37 +/- 4 years). Each subject was studied under four different conditions: during continuation of GH replacement alone (A), after discontinuation of GH replacement for 2 days (B), after GH replacement and short-term coadministration of acipimox (250 mg, p.o., b.i.d., for 2 days) (C), and after administration of acipimox alone (D). At the end of each study, total and regional substrate metabolisms were assessed in the basal state and after a 3-h hyperinsulinemic/euglycemic clamp. Serum levels of free fatty acids (FFAs) were elevated with GH alone (A) and suppressed with acipimox (C and D). Basal rates of lipid oxidation were highest with GH alone (A), and suppressed by 50% with acipimox (B versus D, P < 0.01; A versus C, P < 0.05). Basal glucose oxidation rates were lowest with GH alone (A) and highest with acipimox (C and D) (P = 0.01). Insulin-stimulated rates of total glucose turnover were significantly lower with GH alone as compared with all other conditions (P = 0.004). Insulin sensitivity as assessed by the M value (rate of glucose infusion) was reduced with GH alone as compared with all other conditions (M value in mg. kg(-1). min(-1): GH alone [A], 2.55 +/- 0.64; discontinuation of GH [B], 4.01 +/- 0.70; GH plus acipimox [C], 3.96 +/- 1.34; acipimox alone [D], 4.96 +/- 0.91; P < 0.01). During pharmacological antilipolysis, GH did not significantly influence insulin sensitivity (C versus D; P = 0.19). From our results, we reached the following conclusions: 1) Our data strongly suggest that the insulin antagonistic actions of GH on glucose metabolism are causally linked to the concomitant activation of lipolysis. 2) In addition, GH may induce residual insulin resistance through non-FFA-dependent mechanisms. 3) The cellular and molecular mechanisms subserving the insulin antagonistic effects of GH remain to be elucidated.

Increased insulin sensitivity in young, growth hormone deficient children

OBJECTIVE

Although growth hormone (GH) has well documented insulin antagonistic effects, GH deficient adults often demonstrate insulin resistance. In young GH deficient children, increased susceptibility to hypoglycaemia might indicate increased insulin sensitivity; however, this has not been documented. We therefore determined insulin sensitivity in GH deficient and GH sufficient children.

DESIGN AND PATIENTS

Prospective study of children undergoing insulin tolerance tests for clinical investigation of GH or cortisol secretion at a regional Paediatric Endocrine/Growth Clinic between October 1986 and December 1997. Ninety-one tests were performed in children with GH deficiency and 142 tests in children with normal GH response to insulin (peak GH > or = 20 IU/l).

MEASUREMENTS

The standard insulin tolerance test was modified to permit frequent measurements of glucose (0, 5, 10, 15, 20, 30, 45, 60 and 90 minutes). Rate of log glucose disappearance in the first 15 minutes was calculated as a direct measure of insulin sensitivity.

RESULTS

GH deficient children were more insulin sensitive than GH sufficient children (P = 0.004) and had lower glucose nadirs post-insulin (P = 0.005). Subgroup analysis revealed that these differences were greater in younger (< 12 years old) or pre/early pubertal children. In 14 prepubertal children, exogenous sex steroid priming resulted in lower insulin sensitivity (P < 0.05) compared to nonprimed tests.

CONCLUSIONS

Young GH deficient children were more insulin sensitive than children with normal GH secretion. This difference attenuated with age and puberty, possibly secondary to pubertal sex steroids; however, insulin resistance as reported in GH deficient adults, was not observed in adolescents.

Safety issues in children and adolescents during growth hormone therapy--a review

The action of growth hormone (GH) via its receptor involves many organ systems and metabolic pathways. These diverse actions are reviewed in this paper in the context that they may represent unwanted side-effects of GH therapy for growth promotion. The monitoring of GH therapy in large multicentre international databases has demonstrated a low frequency of adverse events. Tumour recurrence or new malignancy are not increased. Headaches, especially in the first few months of therapy, require close evaluation as benign intracranial hypertension is found infrequently, especially in children with GH deficiency and chronic renal failure (CRF). Children at risk for slipped capital femoral epiphysis and scoliosis require close monitoring during therapy. Decreased insulin sensitivity that is dose-dependent is observed during GH therapy. Glucose homeostasis, however, is not affected, but a recent report of increased incidence of Type 2 diabetes mellitus in children undergoing GH therapy requires prospective surveillance.