cis-FFA do not alter membrane depolarization but block Ca2+ influx and GH secretion in KCl-stimulated somatotroph cells. Suggestion for a direct cis-FFA perturbation of the Ca2+ channel opening

Pathophysiology of GHRH-growth hormone-IGF1 axis in HIV/AIDS

Aberrations in GHRH-GH -IGF-I axis are common in the complex of HIV, HAART and AIDS. There are 2 distinct mechanisms at play in HIV and AIDS. One is primarly associated with development of lipodystrophy and results in complications such as chronic inflammation, insulin resistance, lipid and metabolic abnormalities. HIV lipodystrophy is found especially in those on highly active anti-retroviral therapy (HAART). The various processes involved in lipodystrophy result in the suppression of pituitary GH production. The mechanism of low GH levels relates to increased somatostatin tone, decreased Ghrelin, increased free fatty acids (FFA) and insulin resistance. On the other hand in AIDS wasting syndrome; elevated GH and low IGF-1 levels are seen suggesting GH resistance. The GHRH analog-Tesamorelin is the only treatment option, which is FDA approved for use in reduction of excess abdominal fat in patients with HIV-associated lipodystrophy. Although long-term clinical trials and experience is needed to further study the benefits and risks of Tesamorelin.

The intricate role of growth hormone in metabolism

Growth hormone (GH), a master regulator of somatic growth, also regulates carbohydrate and lipid metabolism via complex interactions with insulin and insulin-like growth factor-1 (IGF-1). Data from human and rodent studies reveal the importance of GH in insulin synthesis and secretion, lipid metabolism and body fat remodeling. In this review, we will summarize the tissue-specific metabolic effects of GH, with emphasis on recent targets identified to mediate these effects. Furthermore, we will discuss what role GH plays in obesity and present possible mechanisms by which this may occur.

GH/GHRH axis in HIV lipodystrophy

Approximately half of patients with HIV-infection develop abnormal body fat distribution, characterized by increased abdominal, breast, and dorsocervical adiposity and decreased fat in the limbs and face in association with antiretroviral therapy. Changes in fat distribution are associated with dyslipidemia, insulin resistance, and increased cardiovascular risk in patients with HIV lipodystrophy. Growth hormone secretion is reduced and responses to standardized stimulation testing altered, suggesting relative growth hormone deficiency in this population. Growth hormone secretion is characterized by normal pulse frequency, but decreased pulse amplitude, pulse width, and trough GH levels compared to weight matched, non-HIV-infected patients. Abnormalities in GH secretion are strongly associated with body composition and metabolic abnormalities in patients with HIV lipodystrophy, particularly with increased visceral fat and elevated free fatty acids. Increased somatostatin tone and decreased ghrelin concentrations may also contribute to reduced GH levels. Administration of exogenous GH or growth hormone releasing hormone (GHRH) to normalize growth hormone concentrations is effective to reduce visceral fat and improve lipid parameters in HIV-infected patients. Treatment with supraphysiologic GH is limited by side effects and exacerbation of insulin resistance, whereas administration of physiologic doses of GH demonstrates more modest treatment effects but fewer adverse effects. Initial studies of GHRH also show significant reductions in visceral adipose tissue (VAT) with potentially fewer adverse effects. GHRH may be particularly useful to normalize GH dynamics in patients with HIV lipodystrophy by increasing endogenous GH pulse height, GH pulse width, and trough GH levels, while preserving the negative feedback of IGF-I on pituitary GH secretion.

Effect of pioglitazone on glucose metabolism and luteinizing hormone secretion in women with polycystic ovary syndrome

OBJECTIVE

To thoroughly examine the mechanisms for insulin resistance in polycystic ovary syndrome (PCOS) and to evaluate the effects of pioglitazone treatment on insulin resistance, beta-cell function, LH secretion, and glucose metabolism.

DESIGN

Randomized, blinded, placebo-controlled study.

SETTING

Outpatient clinic, at a university hospital in Denmark.

PATIENT(S)

Thirty obese women with PCOS and 14 weight-matched healthy females.

INTERVENTION(S)

Sixteen weeks of blinded treatment with pioglitazone (30 mg/d) or placebo.

MAIN OUTCOME MEASURE(S)

Fasting blood samples, 24-hour 20-minute integrated blood sampling (LH, insulin, and C-peptide), euglycemic hyperinsulinemic clamps including 3-(3)H glucose, and indirect calorimetry were performed before and after the intervention period.

RESULT(S)

Patients with PCOS had significantly lower insulin sensitivity compared with controls, including significantly decreased insulin-stimulated oxidative and nonoxidative glucose metabolism. Pioglitazone treatment resulted in significantly lower levels of fasting insulin and significantly higher insulin sensitivity, increased insulin-stimulated glucose oxidation, and increased insulin-stimulated inhibition of lipid oxidation. During 24-hour blood sampling, significantly lower area under-the-curve insulin and lower median insulin levels were observed. Secretion profiles of LH and E(2) and T levels did not change significantly.

CONCLUSION(S)

Insulin resistance in PCOS was characterized by hyperinsulinemia, impaired insulin-stimulated oxidative and nonoxidative glucose metabolism, which was partly reversed by pioglitazone treatment.

Acipimox enhances spontaneous growth hormone secretion in obese women

We hypothesized that a high circulating free fatty acid (FFA) concentration is involved in the pathogenesis of hyposomatotropism associated with obesity. To evaluate this hypothesis, 10 healthy premenopausal women (body mass index 33.8 +/- 1.0 kg/m(2)) were studied in the follicular phase of their menstrual cycle at two occasions with a time interval of at least 8 wk, where body weight remained stable. Subjects were randomly assigned to treatment with either acipimox (an inhibitor of lipolysis, 250 mg orally 4 times daily) or placebo in a double-blind crossover design, starting 1 day before admission until the end of the blood sampling period. Blood samples were taken during 24 h with a sampling interval of 10 min for assessment of growth hormone (GH) concentrations, and GH secretion was estimated by deconvolution analysis. Identical methodology was used to study GH secretion in a historical control group of age-matched normal weight women. GH secretion was clearly blunted in obese women (total daily release 66 +/- 10 vs. lean controls: 201 +/- 23 mU x l(Vd)(-1) x 24 h(-1), P = 0.005, where l(Vd) is lite of distribution volume). Acipimox considerably enhanced total (113 +/- 50 vs. 66 +/- 10 mU x l(Vd)(-1) x 24 h(-1), P = 0.02) and pulsatile GH secretion (109 +/- 49 vs. 62 +/- 30 mU x l(Vd)(-1) x 24 h(-1), P = 0.02), but GH output remained lower compared with lean controls. Further analysis did not show any relationship between the effects of acipimox on GH secretion and regional body fat distribution. In conclusion, acipimox unleashes spontaneous GH secretion in obese women. It specifically enhances GH secretory burst mass. This might mean that lowering of systemic FFA concentrations by acipimox modulates neuroendocrine mechanisms that orchestrate the activity of the somatotropic ensemble.

Fatty acids inhibit growth-factor-induced diacylglycerol kinase alpha activation in vascular smooth-muscle cells

We have previously shown that unsaturated fatty acids amplify platelet-derived-growth-factor (PDGF)-induced protein kinase C (PKC) activation in vascular smooth-muscle cells (VSMCs). Diacylglycerol-induced PKC activation is normally terminated by diacylglycerol kinases (DGKs). We thus hypothesized that fatty acids act by inhibiting a DGK. Fractionation of VSMC extracts demonstrated that the DGK alpha isoform was the major DGK activity present. PDGF markedly increased the DGK activity of cultured cells. An inhibitor selective for the DGK alpha isoform, R59949 [3-[2-[4-(bis-(4-fluorophenyl)methylene]piperidin-1-yl)ethyl]-2,3-dihydro-2-thioxo-4(1H)-quinazolinone], abolished the growth-factor-induced increase in DGK activity, but had little effect on basal activity. PDGF thus selectively activates DGKalpha. Epidermal growth factor and alpha-thrombin stimulated total DGK activity similarly to PDGF. Activation by epidermal growth factor was sensitive to R59949, again suggesting involvement of DGKalpha. However, the alpha-thrombin-induced activity was unaffected by this agent. Unsaturated fatty acids inhibited growth-factor-induced DGKalpha activation, but had no effect on basal activity. Fatty acids also amplified the PDGF-induced increase in cell diacylglycerol content. These results indicate that inhibition of DGKalpha contributes to fatty-acid-induced amplification of PKC activation. Increased levels of fatty acids in diabetes may thus contribute to chronic PKC activation associated with this disorder.

Effects of free fatty acids and acipimox, a lipolysis inhibitor, on the somatotroph responsiveness to GHRH in anorexia nervosa

OBJECTIVE

Anorexia nervosa is characterized by low IGF-1 and high GH and free fatty acid (FFA) levels. As FFA exerts an inhibitory feedback action on GH secretion in physiological conditions, we hypothesized that somatotroph cells could be less sensitive to the negative feedback action of FFA in anorexia nervosa.

PATIENTS

Fifteen patients with anorexia nervosa (AN, age: mean +/- SEM: 20.8 +/- 1.2 years, BMI: 15.9 +/- 0.3 kg/m2) and 12 normal female controls (NW, age 27.2 +/- 2.1 years, BMI 21.2 +/- 2.2 kg/m2).

MEASUREMENTS

We studied the effects of lipid-heparin emulsion (Li-He, Intralipid 10% 250 ml + heparin 2500 U iv from -60 to + 90 minutes in seven AN and six NW) or acipimox (ACI, 250 mg p.o. at -60 minutes in eight AN and six NW), a lipolysis inhibitor, on the GH response to GHRH (1 microg/kg iv as a bolus at 0 minutes).

RESULTS

Basal IGF-1 levels were lower (P < 0.05) while GH levels were higher (P < 0.05) in AN than in NW. On the other hand, basal FFA levels in the two groups were not significantly different. In both groups Li-He increased FFA levels (P < 0.05), which became higher (P < 0. 02) in AN than in NW. Li-He infusion inhibited (P < 0.05) basal GH levels in AN to levels overlapping those in NW. The GH response to GHRH in the whole AN group was higher than in NW (P < 0.03). Li-He inhibited the somatotroph responsiveness to GHRH in AN (P < 0.03) as well as in NW (P < 0.03) and during Li-He the GH response to GHRH in AN became similar to that in NW. Whilst ACI pretreatment enhanced the GH response to GHRH in AN (P < 0.02), it did not significantly increase that in NW. Interestingly, after ACI administration, FFA levels were inhibited in both groups (P < 0.05) persisting higher in AN than in NW (P < 0.05).

CONCLUSION

Though GH hypersecretion in anorexia nervosa occurs in presence of enhanced lipolysis, our present findings indicate that the sensitivity of somatotroph cells to the inhibitory feedback action of free fatty acid is preserved.

Cis-unsaturated free fatty acids block VIP-mediated GH and PRL secretion by perturbing the cAMP/protein kinase A pathway

Cis-unsaturated free fatty acids (FFA) like oleic acid are strong blockers of both basal and stimulated GH secretion in vivo by acting directly on the somatotroph cell. Several lines of evidence suggest that this inhibitory action is the result of a perturbation of the function of several plasma membrane integral proteins. It has been reported recently that cis-FFA are able to block several steps in the inositolphosphates/phospholipase C/Ca2+ (InsPs/PLC/Ca2+) signal transduction pathway triggered by the activation of the TRH receptor. In this paper we present evidence showing that the inhibition of growth hormone (GH) and prolactin (PRL) secretion by cis-FFA in vitro is also exerted at several different levels on the cAMP-protein kinase A (cAMP/PKA) pathway triggered by the stimulation of the vasoactive intestinal peptide (VIP) receptor in pituitary clonal cells. By means of a sequential analysis of signal transduction events, we observed that cis-unsaturated FFA; (1) reduce the activity of adenylate cyclase; (2) perturb the activity of protein kinase A; (3) suppress the VIP-triggered Ca2+ influx, and (4) do not perturb VIP binding or the homologous desensitization of the VIP receptor.