Effects of growth hormone on serum lipids and lipoproteins: possible significance of increased peripheral conversion of thyroxine to triiodothyronine

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.

Administration of recombinant human growth hormone normalizes GH-IGF1 axis and improves malnutrition-related disorders in patients with anorexia nervosa

High serum level of GH in the presence of low plasma level of insulin-like growth factor-I (IGF-I) is one of the endocrinological features of anorexia nervosa (AN). Whether the amount of endogenous GH is not enough to increase IGF-I is not certain. We studied the effect of recombinant human growth hormone (rhGH) on the GH-IGF-I axis and on malnutrition-related disorders in this syndrome. Twenty patients with AN were divided into two groups; one (N = 13) was given rhGH (0.33 mg/day), and the other (N = 7) was given placebo for 6 or 12 months, respectively. During each treatment, levels of serum GH, plasma IGF-I, serum thyroid hormones, serum cholesterol, fasting plasma glucose and cardiac function were monitored. Changes in body mass index (BMI) and calorie taken were also evaluated. Plasma IGF-I level increased from 74.4 +/- 41.9 to 269.0 +/- 31.2 microg/L (P<0.001) during administration of rhGH, which associated with a decrease in serum GH level from 17.0 +/- 15.0 to 1.6 +/- 0.8 microg/L (P<0.001). Administration of rhGH increased BMI, body temperature, fasting plasma glucose level, and food intake. Serum level of triiodothyronine, but not thyroxine, increased during treatment with rhGH. The treatment decreased serum levels of both total and HDL-cholesterol. Studies with echocardiography showed an increase in cardiac output during the treatment with rhGH. These improvements were not observed in patients treated with placebo. Administration of rhGH is recommended as one of the methods of managing the patients with AN.

Effects of food deprivation on expression of growth hormone receptor and proximate composition in liver of black seabream Acanthopagrus schlegeli

The effects of food deprivation on the hepatic level growth hormone receptor (GHR) were investigated in black seabream (Acanthopagrus schlegeli) both at the protein level (by radioreceptor assay) and at the mRNA level (by ribonuclease protection assay). Serum levels of growth hormone (GH) and triiodothyronine (T(3)) were also measured. Condition factor and hepatic proximate composition of the fish were also assessed. Significant decrease in hepatic GHR binding was recorded as early as on day 2 of starvation. On day 30 this decrease was even more pronounced, with the level in the starved fish reaching less than 20% the fed control level. A concomitant decrease in the hepatic GHR mRNA content was also noted during this period, with a progressive decrease from day 2 to day 30 of starvation. The extent of decrease in the mRNA content was less pronounced than the decrease in receptor binding, with the hepatic GHR mRNA content in the day 30 starved fish representing approximately 30% of the level in the fed control. In large contrast, serum GH level increased progressively during starvation. After 30 days of starvation, serum GH levels in the starved fish were more than three times the concentration found in the fed control. Serum T(3) levels, on the other hand, decreased during starvation, with the difference reaching significance on day 15 and day 30. After 30 days of starvation, serum T(3) levels in the starved fish were only approximately 40% the concentration found in the fed control. The hepatic lipid content exhibited an increasing trend during starvation. On day 30 the hepatic lipid content of the starved fish had doubled the level found in the fed control. However, the hepatic protein content did not exhibit much change during starvation. There was also a minor decrease in the moisture content of the liver during starvation, but the condition factor of the fish as a whole registered a gradual decrease during the course of food deprivation.

Lipoprotein(a): an emerging cardiovascular risk factor

Lipoprotein(a) is a cholesterol-enriched lipoprotein, consisting of a covalent linkage joining the unique and highly polymorphic apolipoprotein(a) to apolipoprotein B100, the main protein moiety of low-density lipoproteins. Although the concentration of lipoprotein(a) in humans is mostly genetically determined, acquired disorders might influence synthesis and catabolism of the particle. Raised concentration of lipoprotein(a) has been acknowledged as a leading inherited risk factor for both premature and advanced atherosclerosis at different vascular sites. The strong structural homologies with plasminogen and low-density lipoproteins suggest that lipoprotein(a) might represent the ideal bridge between the fields of atherosclerosis and thrombosis in the pathogenesis of vascular occlusive disorders. Unfortunately, the exact mechanisms by which lipoprotein(a) promotes, accelerates, and complicates atherosclerosis are only partially understood. In some clinical settings, such as in patients at exceptionally low risk for cardiovascular disease, the potential regenerative and antineoplastic properties of lipoprotein(a) might paradoxically counterbalance its athero-thrombogenicity, as attested by the compatibility between raised plasma lipoprotein(a) levels and longevity.

Presence of thyroxine deiodinases in mammary gland: possible modulation of the enzyme-deiodinating activity by somatotropin

Thyroid hormones (TH) and somatotropin (ST) play critical role in lactation. One explanation of their multiple physiological actions is based on the functional interrelationships among ST, TH, and thyroxin deiodinase (5'D). This enzyme is present in the mammary tissue, milk cellular components, and whole milk and is responsible for intramammary production of triiodothyronine (T3). In rats in which the 5'D isozymes in the mammary gland and in the liver are similarly of type I (5'D-I), an enhancement of mammary 5'D-I causes a reduction of hepatic 5'D-I activities. This opposite rearrangement in the mammary and hepatic deiodinating activities is thought to be a factor of a homeorhetic response characterized by an increased and compartmentalized energy expenditure of the mammary gland. In the cow, the mammary 5'D is the type II (5'D-II) deiodinase. The 5'D-II, owing to its high catalytic efficiency, secures T3 production, making tissues relatively independent from the circulatory levels of TH and from variations in the hepatic 5'D-I activity. No significant alterations of 5'D-II isozymes were found during a low T3 syndrome. Location of tissue deiodinases in the cow, the 5'D-II in the mammary gland, and the 5'D-I in the liver make it so that T3 production in these two tissues can be dissociated in time to secure better local requirement for T3 supporting lactation. To date, attempts to evidence that the alterations in iodothyronines blood levels and in tissues' 5'Ds activity during lactation are due to ST action have not received clear experimental support in either cows or rats.

Dyslipidaemia in Adult Growth Hormone (GH) Deficiency and the Effect of GH Replacement Therapy: A Review

Adult growth hormone (GH) deficiency is associated with a lipid profile known to be related to atherosclerosis. GH replacement therapy improves the lipid profile with the exception of lipoprotein (a) concentrations, which tend to increase after GH therapy. Plasma lipid concentrations depend on its plasma carriers, the lipoproteins. Possible mechanisms involved in the dyslipidaemia of GH-deficient patients and the effects of GH replacement therapy are discussed with a special focus on hepatic lipoprotein metabolism.

Basal growth hormone levels in women are positively correlated with high-density lipoprotein cholesterol and apolipoprotein A-I independently of insulin-like growth factor 1 or insulin

Previous studies in growth hormone (GH)-deficient or acromegalic patients yielded contradictory results on the effect of GH on lipoprotein metabolism. In a cross-sectional study, we analyzed the relationships between unstimulated GH, insulin-like growth factor 1 (IGF1), insulin, and lipoprotein metabolism in 44 non-obese young women. On univariate analysis, basal serum levels of GH correlated positively with triglycerides, high-density lipoprotein (HDL) cholesterol, apolipoprotein A-I (apoA-I) and apoA-II and negatively with lipoprotein lipase (LPL) activity. These associations remained significant on multivariate analyses that, in addition to GH, took into account the effects of insulin or C-peptide, as well as the effects of total, protein-bound, or free IGF1. In most cases, the relationships of these lipid parameters with insulin/C-peptide and IGF1 and its free or protein-bound subfractions were opposite of those with GH and not significant. Thus, GH appears to regulate the metabolism of HDL and triglycerides independently of IGF1 and insulin.

Basal growth hormone levels are positively correlated with high-density lipoprotein cholesterol levels in women

Previous studies in patients with either a deficiency or an excess of growth hormone (GH) yielded contradictory results on the regulation of lipoprotein metabolism by GH. In a cross-sectional study of 563 male and 126 female participants of the Prospective Cardiovascular Münster (PROCAM) Study, we determined biometric and demographic data, serum levels of total cholesterol, triglycerides, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein (apo)A-I, A-II, and B, and unstimulated GH levels. The median of basal GH levels was higher in women than in men. Moreover, 44.2% of men but only 8.7% of women had basal GH levels less than the detection limit of 0.05 microgram/L. The relationship between basal GH and lipoprotein metabolism was investigated by univariate and multivariate regression analysis of data from 315 men and 126 women with detectable basal GH levels. In men, GH correlated positively with HDL cholesterol and negatively with body mass index (BMI), age, and triglycerides. After multivariate analysis, the correlation with triglycerides remained independent of age and BMI. Among women, GH correlated positively with the use of hormonal contraception, HDL cholesterol, apoA-I, and apoA-II, and negatively with BMI, age, menopause, triglycerides, and apoB. With multivariate analysis, the positive correlations of GH with HDL cholesterol and apoA-I in women were independent of age. BMI, menopause, and oral contraception. We conclude that GH contributes to the regulation of HDL cholesterol levels. Moreover, in women the well-known effects of exogenous estrogen or estrogen loss on HDL metabolism may be partially mediated via GH.