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

Adipocyte Subpopulations Mediate Growth Hormone-induced Lipolysis and Glucose Tolerance in Male Mice

In adipose tissue, growth hormone (GH) stimulates lipolysis, leading to an increase in plasma free fatty acid levels and a reduction in insulin sensitivity. In our previous studies, we have found that GH increases lipolysis by reducing peroxisome proliferator-activated receptor γ (PPARγ) transcription activity, leading to a reduction of tat-specific protein 27 (FSP27, also known as CIDEC) expression. In previous studies, our laboratory uncovered 3 developmentally distinct subpopulations of white adipocytes. In this manuscript, we show that one of the subpopulations, termed type 2 adipocytes, has increased GH-induced signaling and lipolysis compared to other adipocyte subtypes. To assess the physiological role of GH-mediated lipolysis mediated by this adipocyte subpopulation, we specifically expressed human FSP27 (hFSP27) transgene in type 2 adipocytes (type2Ad-hFSP27tg mice). Systemically, male type2Ad-hFSP27tg mice displayed reduced serum glycerol release and nonesterified fatty acids levels after acute GH treatment, and improvement in acute, but not chronic, GH-induced glucose intolerance. Furthermore, we demonstrate that type2Ad-hFSP27tg mice displayed improved hepatic insulin signaling. Taken together, these results indicate that this adipocyte subpopulation is a critical regulator of the GH-mediated lipolytic and metabolic response. Thus, further investigation of adipocyte subpopulations may provide novel treatment strategies to regulate GH-induced glucose intolerance in patients with growth and metabolic disorders.

Metabolic surgery-induced changes of the growth hormone system relate to improved adipose tissue function

AIMS

Body weight loss improves insulin resistance and growth hormone secretion in obesity, which may be regulated by leptin according to preclinical studies. How changes in leptin, lipids and insulin sensitivity after bariatric (metabolic) surgery affect the human growth hormone system is yet unclear.

PARTICIPANTS AND METHODS

People with obesity (OBE, n = 79, BMI 50.8 ± 6.3 kg/m²) were studied before, 2, 12, 24 and 52 weeks after metabolic surgery and compared to lean healthy humans (control; CON, n = 24, BMI 24.3 ± 3.1 kg/m²). Tissue-specific insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamps with D-[6,6-²H₂]glucose. Fasting leptin, growth hormone (GH), insulin-like growth factor 1 (IGF-1) and IGF-binding proteins (IGFBP1, IGFBP3) were measured using ELISA.

RESULTS

At baseline, OBE exhibited higher glycemia and leptinemia as well as pronounced peripheral, adipose tissue and hepatic insulin resistance compared to CON. GH and IGFBP1 were lower, while IGF1 was comparable between groups. At 52 weeks, OBE had lost 33% body weight and doubled their peripheral insulin sensitivity, which was paralleled by continuous increases in GH, IGF-1 and IGFBP1 as well as decrease in leptin. The rise in GH correlated with reductions in free fatty acids, adipose tissue insulin resistance and insulinemia, but not with changes in body weight, peripheral insulin sensitivity, glycemia or leptinemia. The rise in IGF-1 correlated with reduction in high-sensitive C-reactive protein.

CONCLUSION

Reversal of alterations of the GH-IGF-1 axis after surgically-induced weight loss is unlikely related to improved leptin secretion and/or insulin sensitivity, but is rather associated with restored adipose tissue function and reduced low-grade inflammation.

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.

Insulin resistance induced by growth hormone is linked to lipolysis and associated with suppressed pyruvate dehydrogenase activity in skeletal muscle: a 2 × 2 factorial, randomised, crossover study in human individuals

AIMS/HYPOTHESIS

Growth hormone (GH) causes insulin resistance that is linked to lipolysis, but the underlying mechanisms are unclear. We investigated if GH-induced insulin resistance in skeletal muscle involves accumulation of diacylglycerol (DAG) and ceramide as well as impaired insulin signalling, or substrate competition between fatty acids and glucose.

METHODS

Nine GH-deficient male participants were randomised and examined in a 2 × 2 factorial design with and without administration of GH and acipimox (an anti-lipolytic compound). As-treated analyses were performed, wherefore data from three visits from two patients were excluded due to incorrect GH administration. The primary outcome was insulin sensitivity, expressed as the AUC of the glucose infusion rate (GIR_AUC), and furthermore, the levels of DAGs and ceramides, insulin signalling and the activity of the active form of pyruvate dehydrogenase (PDHa) were assessed in skeletal muscle biopsies obtained in the basal state and during a hyperinsulinaemic-euglycaemic clamp (HEC).

RESULTS

Co-administration of acipimox completely suppressed the GH-induced elevation in serum levels of NEFA (GH versus GH+acipimox, p < 0.0001) and abrogated GH-induced insulin resistance (mean GIR_AUC [95% CI] [mg min^-1 kg^-1] during the HEC: control, 595 [493, 718]; GH, 468 [382, 573]; GH+acipimox, 654 [539, 794]; acipimox, 754 [618, 921]; GH vs GH+acipimox: p = 0.004). GH did not significantly change either the accumulation of DAGs and ceramides or insulin signalling in skeletal muscle, but GH antagonised the insulin-stimulated increase in PDHa activity (mean ± SEM [% from the basal state to the HEC]: control, 47 ± 19; GH, -15 ± 21; GH+acipimox, 3 ± 21; acipimox, 57 ± 22; main effect: p = 0.02).

CONCLUSIONS/INTERPRETATION

GH-induced insulin resistance in skeletal muscle is: (1) causally linked to lipolysis; (2) not associated with either accumulation of DAGs and ceramides or impaired insulin signalling; (3) likely to involve substrate competition between glucose and lipid intermediates.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02782208 FUNDING: The work was supported by the Grant for Growth Innovation (GGI), which was funded by Merck KGaA, Darmstadt, Germany. Graphical abstract.

Reducing Type 1 Diabetes Mortality: Role for Adjunctive Therapies?

Individuals with type 1 diabetes (T1D) frequently fail to achieve glycemic goals and have excess cardiovascular risk and premature death. Adjunctive agents may play a role in reducing morbidity, mortality, and the adverse sequelae of insulin treatment. A surge in type 2 diabetes drug development has revealed agents with benefits beyond glucose lowering, including cardiovascular risk reduction. Could these benefits translate to T1D? Specific trials for T1D demonstrate substantial hemoglobin (Hb)A1c reductions with sodium glucose cotransporter inhibitors (SGLTis) and glucagon-like peptide (GLP)1 agonists, and modest improvements with metformin, dipeptidyl peptidase-4 inhibitor (DPP4i), and pramlintide. Studies exploring cardiovascular risk reduction are warranted. This review synthesizes the emerging literature for researchers and clinicians treating people with T1D. Challenges in T1D research are discussed.

Growth hormone acts along the PPARγ-FSP27 axis to stimulate lipolysis in human adipocytes

The lipolytic effects of growth hormone (GH) have been known for half a century and play an important physiological role for substrate metabolism during fasting. In addition, sustained GH-induced lipolysis is causally linked to insulin resistance. However, the underlying molecular mechanisms remain elusive. In the present study, we obtained experimental data in human subjects and used human adipose-derived stromal vascular cells (hADSCs) as a model system to elucidate GH-triggered molecular signaling that stimulates adipose tissue lipolysis and insulin resistance in human adipocytes. We discovered that GH downregulates the expression of fat-specific protein (FSP27), a negative regulator of lipolysis, by impairing the transcriptional ability of the master transcriptional regulator, peroxisome proliferator-activated receptor-γ (PPARγ) via MEK/ERK activation. Ultimately, GH treatment promotes phosphorylation of PPARγ at Ser273 and causes its translocation from nucleus to the cytosol. Surprisingly, FSP27 overexpression inhibited PPARγ Ser273 phosphorylation and promoted its nuclear retention. GH antagonist treatment had similar effects. Our study identifies a novel signaling mechanism by which GH transcriptionally induces lipolysis via the MEK/ERK pathway that acts along PPARγ-FSP27 in human adipose tissue.

Growth hormone controls lipolysis by regulation of FSP27 expression

Growth hormone (GH) has long been known to stimulate lipolysis and insulin resistance; however, the molecular mechanisms underlying these effects are unknown. In the present study, we demonstrate that GH acutely induces lipolysis in cultured adipocytes. This effect is secondary to the reduced expression of a negative regulator of lipolysis, fat-specific protein 27 (FSP27; aka Cidec) at both the mRNA and protein levels. These effects are mimicked in vivo as transgenic overexpression of GH leads to a reduction of FSP27 expression. Mechanistically, we show GH modulation of FSP27 expression is mediated through activation of both MEK/ERK- and STAT5-dependent intracellular signaling. These two molecular pathways interact to differentially manipulate peroxisome proliferator-activated receptor gamma activity (PPARγ) on the FSP27 promoter. Furthermore, overexpression of FSP27 is sufficient to fully suppress GH-induced lipolysis and insulin resistance in cultured adipocytes. Taken together, these data decipher a molecular mechanism by which GH acutely regulates lipolysis and insulin resistance in adipocytes.

Metabolic differences between short children with GH peak levels in the lower normal range and healthy children of normal height

OBJECTIVE

Severe growth hormone deficiency (GHD) leads to several metabolic effects in the body ranging from abnormal body composition to biochemical disturbances. However, less is known regarding these parameters in short children with GH peak levels in the lower normal range during provocation tests. Our aim was to study the metabolic profile of this group and compare it with that of healthy children of normal height.

DESIGN

Thirty-five pre-pubertal short children (<-2.5 SDS) aged between 7 and 10years, with peak levels of GH between 7 and 14μg/L in an arginine insulin tolerance test (AITT), were compared with twelve age- and sex-matched children of normal height. The metabolic profile of the subjects was analysed by blood samples, DEXA, frequently sampled intravenous glucose tolerance test, microdialysis and stable isotope examinations of rates of glucose production and lipolysis.

RESULTS

There were no overall significant metabolic differences between the groups. However, in the subgroup analysis, the short children with GH peaks <10μg/L had significantly lower fasting insulin levels which also correlated to other metabolic parameters.

CONCLUSION

The short pre-pubertal children with GH peak levels between 7 and 14μg/L did not differ significantly from healthy children of normal height but subpopulations within this group show significant metabolic differences.

Mixed model of dietary fat effect on postprandial glucose-insulin metabolism from carbohydrates in type 1 diabetes

In this study we introduce an extension of a previously developed model of glucose-insulin metabolism in type 1 diabetes (T1D) from carbohydrates that includes the effect of dietary fat on postprandial glycemia. We include two compartments that represent plasma triglyceride and nonesterified fatty acid (NEFA) concentration, in addition to a mathematical representation of delayed gastric emptying and insulin resistance, which are the most well-known effects of dietary fat metabolism. Simulation results show that postprandial glucose as well as lipid levels in our model approximates clinical data from T1D patients.

Low circulating levels of IGF-1 in healthy adults are associated with reduced β-cell function, increased intramyocellular lipid, and enhanced fat utilization during fasting

CONTEXT

Low serum IGF-1 levels have been linked to increased risk for development of type 2 diabetes. However, the physiological role of IGF-1 in glucose metabolism is not well characterized.

OBJECTIVE

Our objective was to explore glucose and lipid metabolism associated with variations in serum IGF-1 levels.

DESIGN, SETTING AND PARTICIPANTS

IGF-1 levels were measured in healthy, nonobese male volunteers aged 18 to 50 years from a biobank (n = 275) to select 24 subjects (age 34.8 ± 8.9 years), 12 each in the lowest (low-IGF) and highest (high-IGF) quartiles of age-specific IGF-1 SD scores. Evaluations were undertaken after a 24-hour fast and included glucose and glycerol turnover rates using tracers, iv glucose tolerance test to estimate peripheral insulin sensitivity (IS) and acute insulin and C-peptide responses (indices of insulin secretion), magnetic resonance spectroscopy to measure intramyocellular lipids (IMCLs), calorimetry, and gene expression studies in a muscle biopsy.

MAIN OUTCOME MEASURES

Acute insulin and C-peptide responses, IS, and glucose and glycerol rate of appearance (Ra) were evaluated.

RESULTS

Fasting insulin and C-peptide levels and glucose Ra were reduced (all P < .05) in low-IGF compared with high-IGF subjects, indicating increased hepatic IS. Acute insulin and C-peptide responses were lower (both P < .05), but similar peripheral IS resulted in reduced insulin secretion adjusted for IS in low-IGF subjects (P = 0.044). Low-IGF subjects had higher overnight levels of free fatty acids (P = .028) and β-hydroxybutyrate (P = .014), increased accumulation of IMCLs in tibialis anterior muscle (P = .008), and a tendency for elevated fat oxidation rates (P = .058); however, glycerol Ra values were similar. Gene expression of the fatty acid metabolism pathway (P = .0014) was upregulated, whereas the GLUT1 gene was downregulated (P = .005) in the skeletal muscle in low-IGF subjects.

CONCLUSIONS

These data suggest that serum IGF-1 levels could be an important marker of β-cell function and glucose as well as lipid metabolic responses during fasting.

Short-term administration of pegvisomant improves hepatic insulin sensitivity and reduces soleus muscle intramyocellular lipid content in young adults with type 1 diabetes

CONTEXT

Data on the metabolic effects of GH derived from studies using GH suppression by pharmacological agents may not reflect selective actions.

OBJECTIVE

The purpose of this study was to evaluate the effects of GH antagonism on glucose and lipid metabolism using pegvisomant, a selective GH receptor antagonist in patients with type 1 diabetes (T1D).

DESIGN AND PARTICIPANTS

In a randomized, placebo-controlled, crossover study, 10 young adults with T1D were evaluated at baseline and after 4 weeks of treatment with either 10 mg of pegvisomant or placebo. The assessments included an overnight euglycemic steady state followed by a hyperinsulinemic euglycemic clamp and used glucose and glycerol cold stable isotopes.

OUTCOME MEASURES

Hepatic and peripheral insulin sensitivity (IS), lipid turnover, and intramyocellular lipid (IMCL) were measured.

RESULTS

Compared with placebo, pegvisomant treatment resulted in lower IGF-I levels (P < .001). During the overnight steady state, insulin requirements for euglycemia (P = .019), insulin levels (P = .008), and glucose production rates (Ra) (P = .033) were reduced. During the clamp study, glucose infusion rates (P = .031) increased and glucose Ra (P = .015) decreased whereas glucose disposal rates were unchanged. Free fatty acid levels were similar during the steady state but were lower during the clamp (P = .040) after pegvisomant. Soleus muscle IMCL decreased after treatment (P = .024); however, no change in tibialis anterior muscle was observed.

CONCLUSIONS

The study demonstrates that GH antagonism in T1D results in improved hepatic insulin sensitivity. Lack of consistent changes in free fatty acid levels may suggest a direct effect of GH on IS. Unchanged peripheral IS despite reductions in IMCL indicate that GH-induced alterations in IMCL may not be causally linked to glucose metabolism.

Effect of Low Calorie Diet and Controlled Fasting on Insulin Sensitivity and Glucose Metabolism in Obese Patients With Type 1 Diabetes Mellitus

Obesity in T1DM patients is associated with the components of metabolic syndrome. The influence of controlled fasting and low calorie diet (LCD) on insulin sensitivity and glucose metabolism was studied in 14 obese patients with type 1 diabetes mellitus (T1DM) (42.6±9.4 years, BMI 32.4±2.1 kg m−2). Insulin sensitivity in obese T1DM patients was measured using a hyperinsulinemic-euglycemic clamp before fasting, immediately after 7 days of fasting, and after 21 days of LCD. Glucose oxidation and non-oxidative glucose disposal were measured before and during the clamp by indirect calorimetry. In the control group of 13 of non-obese T1DM patients (36.9±13.9 years, BMI 22.6±2.1 kg m−2), only one hyperinsulinemic-euglycemic clamp was performed. Obese T1DM patients lost 6.1±1.1 kg after fasting and maintained reduction in body weight after 21 days of LCD. Fasting transiently reduced insulin-mediated glucose disposal in the clamp (from 9.69±1.48 to 6.78±1.21 mg min−1 kg−1, P<0.001). This was caused by reduced glucose oxidation after the fasting period (from 2.81±0.52 to 0.88±0.98 mg min−1 kg−1, P<0.001). We conclude that one week of fasting transiently decreased insulin-mediated glucose disposal in T1DM patients. This was caused by reduced glucose oxidation.

Fagopyrum tataricum (buckwheat) improved high-glucose-induced insulin resistance in mouse hepatocytes and diabetes in fructose-rich diet-induced mice

Fagopyrum tataricum (buckwheat) is used for the treatment of type 2 diabetes mellitus in Taiwan. This study was to evaluate the antihyperglycemic and anti-insulin resistance effects of 75% ethanol extracts of buckwheat (EEB) in FL83B hepatocytes by high-glucose (33 mM) induction and in C57BL/6 mice by fructose-rich diet (FRD; 60%) induction. The active compounds of EEB (100 μg/mL; 50 mg/kg bw), quercetin (6 μg/mL; 3 mg/kg bw), and rutin (23 μg/mL; 11.5 mg/kg bw) were also employed to treat FL83B hepatocytes and animal. Results indicated that EEB, rutin, and quercetin + rutin significantly improved 2-NBDG uptake via promoting Akt phosphorylation and preventing PPARγ degradation caused by high-glucose induction for 48 h in FL83B hepatocytes. We also found that EEB could elevate hepatic antioxidant enzymes activities to attenuate insulin resistance as well as its antioxidation caused by rutin and quercetin. Finally, EEB also inhibited increases in blood glucose and insulin levels of C57BL/6 mice induced by FRD.

Antihypertriglyceridemia and anti-inflammatory activities of monascus-fermented dioscorea in streptozotocin-induced diabetic rats

The rice fermented by Monascus, called red mold rice (RMR), and has a long tradition in East Asia as a dietary staple. Monascus-fermented dioscorea called red mold dioscorea (RMD) contains various metabolites to perform the ability of reducing oxidative stress and anti-inflammatory response. We used Wistar rats and induced diabetes by injecting streptozotocin (STZ, 65 mg/kg i.p.). RMD was administered daily starting six weeks after disease onset. Throughout the experimental period, significantly (P < .05) lowered plasma glucose, triglyceride, cholesterol, free fatty acid and low density lipoprotein levels were observed in the RMD-treated groups. The RMD-treated diabetic rats showed higher activities of glutathione disulfide reductase, glutathione reductase, catalase and superoxide dismutase (P < .05) in the pancreas compared with the diabetic control rats. RMD also inhibited diabetes-induced elevation in the levels of interleukin (IL)-1β, IL-6, interferon-γ and tumor necrosis factor-α. Pancreatic β-cells damaged by STZ in the RMD supplemented groups were ameliorated. The results of this study clearly demonstrated that RMD possesses several treatment-oriented properties, including the control of hyperglycemia, antioxidant effects, pancreatic β-cell protection and anti-inflammatory effects. Considering these observations, it appears that RMD may be a useful supplement to delay the development of diabetes and its complications.