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

Cyclic 3',5'-adenosine monophosphate (cAMP) signaling in the anterior pituitary gland in health and disease

The cyclic 3',5'-adenosine monophosphate (cAMP) was the first among the so-called "second messengers" to be described. It is conserved in most organisms and functions as a signal transducer by mediating the intracellular effects of multiple hormones and neurotransmitters. In this review, we first delineate how different members of the cAMP pathway ensure its correct compartmentalization and activity, mediate the terminal intracellular effects, and allow the crosstalk with other signaling pathways. We then focus on the pituitary gland, where cAMP exerts a crucial function by controlling the responsiveness of the cells to hypothalamic hormones, neurotransmitters and peripheral factors. We discuss the most relevant physiological functions mediated by cAMP in the different pituitary cell types, and summarize the defects affecting this pathway that have been reported in the literature. We finally discuss how a deregulated cAMP pathway is involved in the pathogenesis of pituitary disorders and how it affects the response to therapy.

Rapid suppression of growth hormone concentration by overeating: potential mediation by hyperinsulinemia

CONTEXT

The very low GH concentration in obesity is commonly attributed to high body fat mass; however, the influence of overeating on GH secretion is not clear.

OBJECTIVE

The aim of the study was to examine the effects of 2 wk of overeating on changes in GH secretion.

SETTING

Subjects were admitted to the hospital and stayed within the Michigan Clinical Research Unit throughout the entire 2-wk overeating period.

PARTICIPANTS

We studied seven healthy, nonobese men (body mass index, 24 ± 1 kg/m(2); age, 25 ± 1 yr).

INTERVENTION

Subjects ate standardized meals containing 70 kcal/kg fat free mass/d (∼4000 kcal/d) for 2 wk.

MAIN OUTCOME MEASURES

Twenty-four-hour plasma concentrations of GH (every 20 min) and insulin (every 2 h) were measured before overeating (baseline), on d 3, and after 2 wk of overeating.

RESULTS

Compared with baseline, average 24-h plasma GH concentration declined nearly 80% by d 3 of overeating (1.30 ± 0.18 vs. 0.36 ± 0.09 ng/ml; P = 0.01). This marked suppression of GH secretion occurred in the absence of an increase in body weight (77.0 ± 2.2 vs. 76.4 ± 2.4 kg). At the same time, average 24-h insulin concentration doubled (16.6 ± 2.1 vs. 31.7 ± 5.8 μU/ml; P = 0.009). After 2 wk, body weight significantly increased (79.0 ± 2.1 kg; P < 0.001), and body fat increased by more than 10% (P = 0.002). However, this did not induce a further suppression in plasma GH concentration (0.33 ± 0.08 ng/ml).

CONCLUSION

Only a few days of overeating markedly suppressed GH secretion before any measurable weight gain and was accompanied by chronic hyperinsulinemia. Increased body weight and body fat by 2 wk of overeating did not further suppress GH secretion.

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.

Unsaturated fatty acids prevent desensitization of the human growth hormone secretagogue receptor by blocking its internalization

The composition of the plasma membrane affects the responsiveness of cells to metabolically important hormones such as insulin and vasoactive intestinal peptide. Ghrelin is a metabolically regulated hormone that activates the G protein-coupled receptor GH secretagogue receptor type 1a (GHSR) not only in the pituitary gland but also in peripheral tissues such as the pancreas, stomach, and T cells in the circulation. We have investigated the effects of lipids and altered plasma membrane composition on GHSR activation. Oligounsaturated fatty acids (OFAs) disrupt the structure of membranes and make them more fluid. Prolonged (96 h), but not acute, treatment of the GHSR cells with the 18C OFAs oleic and linoleic acid caused a significant increase in sensitivity of the receptor to ghrelin (EC(50) reduced by a factor of 2.4 and 2.9 at 60 and 120 microM OFAs, respectively). OFAs were found to block the inhibitory effects of ghrelin pretreatment on subsequent ghrelin responsiveness, suggesting that OFAs suppress desensitization of GHSR. Radioligand displacement studies did not show a significant shift in receptor binding after incubation with OFAs. However, it was found that OFA treatment suppressed GHSR internalization, likely explaining OFA-induced refractoriness to ligand-induced desensitization. The involvement of lipid rafts in this process was indicated by the altered responsiveness of GHSR under conditions that alter membrane cholesterol. In conclusion, our findings demonstrate the importance of membrane composition for GHSR activation and desensitization and indicate at least part of the mechanism through which OFAs and cholesterol could affect ghrelin's activity in vivo.

Interactions of VIP with rigid phospholipid bilayers: implications for vasoreactivity

The purpose of this study was to determine whether vasoactive intestinal peptide (VIP), a pleiotropic amphipathic peptide, interacts with rigid liposomes composed of gel phase phospholipids. We found that incubation of VIP with small unilamellar gel phase liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and egg phosphatidylglycerol (ePG) for 2h at room temperature had no significant effects on VIP secondary structure. Moreover, suffusion of VIP (0.01, 0.1 and 1.0nmol) incubated in saline or with DPPC/ePG liposomes (size, 30 and 100nm) for 2h at room temperature or 4 degrees C onto the intact hamster cheek pouch microcirculation elicited a similar concentration-dependent vasodilation except for 0.01nmol VIP (P<0.05). By contrast, incubation of VIP with gel phase liposomes overnight at 4 degrees C significantly potentiated vasodilation evoked by all three concentrations of the peptide in comparison to aqueous VIP (P<0.05). VIP-induced vasodilation was liposome size-independent. The ratio of VIP to phospholipids in DPPC/ePG liposomes was concentration-independent. Collectively, these data indicate that short-term interactions of VIP with rigid phospholipid bilayers are limited resulting in only modest effects on VIP vasoreactivity in vivo.

Free fatty acids in human cerebrospinal fluid following subarachnoid hemorrhage and their potential role in vasospasm: a preliminary observation

OBJECT

The mechanisms leading to vasospasm following subarachnoid hemorrhage (SAH) remain unclear. Accumulation in cerebrospinal fluid (CSF) of free fatty acids (FFAs) may play a role in the development of vasospasm; however, in no previous study have concentrations of FFAs in CSF been examined after SAH.

METHODS

We collected samples of CSF from 20 patients with SAH (18 cases of aneurysmal SAH and two cases of spontaneous cryptogenic SAH) and used a high-performance liquid chromatography assay to determine the FFA concentrations in these samples. We then compared these findings with FFA concentrations in the CSF of control patients. All FFA concentrations measured 24 hours after SAH were significantly greater than control concentrations (p < 0.01 for palmitic acid and < 0.001 for all other FFAs). All measured FFAs remained elevated for the first 48 hours after SAH (p < 0.05 for linoleic acid, p < 0.01 for palmitic acid, and p < 0.001 for the other FFAs). After 7 days, a second elevation in all FFAs was observed (p < 0.05 for linoleic acid, p < 0.01 for palmitic acid, and p < 0.001 for the other FFAs). Samples of CSF collected within 48 hours after SAH from patients in whom angiography and clinical examination confirmed the development of vasospasm after SAH were found to have significantly higher concentrations of arachidonic, linoleic, and palmitic acids than samples collected from patients in whom vasospasm did not develop (p < 0.05).

CONCLUSIONS

Following SAH, all FFAs are initially elevated. A secondary elevation occurs between 8 and 10 days after SAH. This study provides preliminary evidence of FFA elevation following SAH and of a potential role for FFAs in SAH-induced vasospasm. A prospective study is warranted to determine if CSF concentrations of FFAs are predictive of vasospasm.

Human lipocalin-1, a physiological scavenger of lipophilic compounds, is produced by corticotrophs of the pituitary gland

Lipocalin-1 (Lcn-1), a member of the lipocalin superfamily that binds a broad array of different chemical classes of lipophilic ligands, is believed to act as a physiological scavenger of potentially harmful lipophilic molecules. Thus far, it was thought to be produced exclusively by a number of exocrine glands and tissues, including lachrymal and lingual glands, prostate, secretory glands of the tracheobronchial tract, and sweat glands. Using Northern blotting analysis, we were able to demonstrate Lcn-1 expression by the human pituitary gland. Moreover, double immunolabeling with antibodies against Lcn-1 and pituitary gland hormones and detection with fluorophore-conjugated secondary antibodies revealed that Lcn-1 is specifically produced by corticotrophs, clearly indicating that its distribution is not restricted to exocrine tissues.

Interaction between body composition, leptin and growth hormone status

Administration of growth hormone (GH) induces changes in body composition, namely, increases in both bone and lean mass and a decrease in fatty tissue. However, the contrary issue, i.e. the way in which body composition affects the secretion of GH, is highly controversial. Disease states such as obesity and chronic hypercortisolism are associated with increased adiposity and/or the central distribution of fat. Ageing, characterized by excess adiposity, is also associated with impaired secretion of GH. In these states, both spontaneous and stimulated secretion of GH is severely impeded. At the other extreme, malnutrition and fasting are both associated with increased secretion of GH when confronted with most, if not all, stimuli. As the common factor in all of these situations is the increased or decreased adiposity, or the changes in energy homeostasis, it has been postulated that adipose tissue exerts a relevant role in the control of GH secretion in man. The link between adipose tissue and GH seems to be exerted through at least two signals produced by adipocytes: free fatty acids (FFA) and the recently cloned protein, leptin. An increase in FFA blocks secretion of GH, while a decrease in FFA enhances secretion. Leptin, a hormone whose main role is to regulate the intake of food and energy expenditure, seems to regulate GH secretion by acting at the hypothalamic level. In summary, body composition affects GH secretion by way of the degree of adiposity, and free fatty acids and leptin would appear to be the messages through which adipocytes participate in the regulation of GH secretion. This framework clarifies the metabolic control of GH, a hormone with profound metabolic activities.