Regulation of somatotroph cell function by the adipose tissue - Nature

International Journal of Obesity (2000) 24, Suppl 2, S100±S103 ß 2000 Macmillan Publishers Ltd All rights reserved 0307±0565/00 $15.00 www.nature.com/ijo

Regulation of somatotroph cell function by the adipose tissue C Dieguez1*, E Carro1, LM Seoane1, M Garcia1, JP Camina2, R Senaris1, V Popovic3 and FF Casanueva2 1

Department of Physiology, Faculty of Medicine, University of Santiago, Santiago de Compostela, Spain; 2Department of Medicine, Faculty of Medicine, University of Santiago, Santiago de Compostela, Spain; and 3Institute of Endocrinology, Belgrade, Yugoslavia

Obese subjects exhibit a marked decrease in plasma growth hormone (GH) levels. However, the mechanisms by which increased adiposity leads to an impairment of GH secretion are poorly understood. Recent evidence suggests that the adipose tissue can markedly in¯uence GH secretion via two different signals, namely free fatty acids (FFA) and leptin. FFA appear to inhibit GH secretion mainly by acting directly at pituitary level. Interestingly, reduction in circulating FFA levels in obese subjects led to a marked increase in GH responses to different GH secretagogues. This indicates that FFA exert a tonic inhibitory effect that contributes to blunted GH secretion in obese subjects. Recent data have shown that leptin is a metabolic signal that regulates GH secretion, since the administration of leptin antiserum to adult rats led to a marked decrease in spontaneous GH secretion. However, leptin prevents,the inhibitory effect exerted by fasting on plasma GH levels. The effect of leptin in adult rats appears to be exerted at hypothalamic level by regulating growth hormone releasing hormone (GHRH), somatostatin and neuropeptide Y (NPY)-producing neurones. In addition, during fetal life or following the development of pituitary tumors, leptin can also act directly at the anterior pituitary. International Journal of Obesity (2000) 24, Suppl 2, S100±S103 Keywords: free fatty acids; leptin; growth hormone

Introduction Unlike other pituitary hormones, growth hormone (GH) exerts its biological actions not just on one target organ, but on almost every cell of the organism.1 Therefore, it is not at all surprising that GH plays an important role in the control of metabolic processes. Several studies have shown that GH de®ciency is associated with abnormalities in body composition, metabolic derangements and suboptimal physical performance. These impairments improve with GH replacement therapy.2 However, in obesity there is a marked decrease in GH secretion. For both children and adults, the greater the body mass index, the lower the GH response to tests. Although enhanced GH clearance rate may be a contributing factor, the main altered mechanism of GH secretion in obesity appears to be decreased GH secretion by the somatotroph cell.1 The primary cause of this alteration could be an altered hypothalamic function, abnormal pituitary function or a perturbation of peripheral signals acting at either the pituitary or hypothalamic level. In this review we will assess the role exerted by two different signals originating in the adipose tissue, namely free fatty acids (FFA) and leptin, in the control of GH secretion and their possible relevance

*Correspondence: C Dieguez, PO Box 563, 15700 Santiago de Compostela, Spain. E-mail: [email protected]

to the alterations in GH secretion present in obese subjects.

Free fatty acids and GH secretion A classic feedback relationship has been postulated between GH and FFA. GH has a direct lipolytic effect on adipose tissue, leading to the release of glycerol, FFA and ketones bodies.1 Pharmacological reductions of FFA are associated with GH release, whereas an increase in FFA reduces both basal and stimulated GH secretion. FFA elevations reduce or block GH secretion stimulated by a variety of stimuli or conditions, such as FFA depression, hypoglycemia, physical exercise or administration of GH releasing hormone (GHRH) or GHRP-6.3,4 There is now compelling evidence indicating that the FFA blockade of GH secretion is exerted at the pituitary level, probably by direct inhibition of somatotroph function.4,5 The effect of FFA on GH secretion is exerted at physiological levels, since reduction in circulating FFA levels following administration of acipimox, per se stimulates basal GH secretion as well as increasing GH responses to different GH secretagogues in normal subjects.6,7 Since obesity is associated with abnormal elevations in both fasting and postprandial FFA concentrations, it has been suggested that elevated FFA could be one of the mechanisms responsible for impaired

Adipose tissue and GH secretion C Dieguez et al

GH secretion in obese subjects. Direct support for this possibility has been reported recently.8 Following acipimox administration to obese subjects, which led to a marked decrease in circulating FFA levels, a marked increase in GH responses to different GH stimuli (pyridostigmine, GHRH or GHRH GHRP6) was observed, restoring the level of this secretion to 50 ± 70% of that observed in normal subjects.8 Thus this data indicate that abnormally high FFA levels may be a contributing factor to the disrupted GH secretory mechanism in obesity.

Leptin and GH secretion The possibility that leptin serves as a metabolic signal that acts on the hypothalamus in¯uencing GH secretion, has been recently studied both in vitro and in vivo. It is well established that the membrane spanning leptin receptor (OB-R), containing approximately 300 a.a. intracellular domain, is highly expressed in the hypothalamus. In addition, several shorter, and nonsignaling, OB-R isoforms with trun-

Figure 1 Representative plasma GH pro®les in individual male rats after i.c.v. injection of normal rabbit serum (NRS; control (fed)), or speci®c antiserum against leptin (Leptin-Ab); vehicle or leptin (i.c.v.) in fed rats (middle panel) or fasted rats (lower panel). All the substances or vehicle were administered at 10.15 h.

cated cytoplasmatic regions resulting from alternative splicing are expressed to different degrees in many tissues. Using RT-PCR, mRNA expression of the OBR intact long isoform as well as the in vitro effects of leptin on the pituitary have been reported.9 While the shorter isoforms of the OB-R are expressed in both fetal and adult human pituitary, the intact OB-R was only present in fetal pituitaries, thus indicating that leptin may well be involved in the regulation of normal pituitary development.9 Interestingly, while the long isoform could not be detected in normal anterior pituitary glands from adult subjects, it was highly expressed in 90% of GH, prolactin and nonfunctioning pituitary tumors. Furthermore, it was found that leptin stimulates in vitro GH secretion from tumoral GH secreting cells, while it does not modify in vitro GH secretion by anterior pituitary cells from adult rats.9,10 The role played by leptin in in vivo GH secretion has been studied by assessing the effects of immunoneutralization of endogenous leptin, after administration of leptin antibodies, as well as the action of exogenous leptin on spontaneous GH secretion in freely moving rats.10 Administration of leptin antiserum to normal fed rats led to a clear-cut decrease in plasma GH levels) indicating that physiological leptin levels are needed in order to ensure normal spontaneous GH secretion (Figure 1). As expected, following food deprivation for 48 h, there was a marked suppression on spontaneous GH secretion. Since fasting is associated with a marked decrease in circulating leptin levels,11 it was possible that spontaneous decrease in GH secretion in this animal model could be leptin mediated. In support of this hypothesis, it was found that following leptin administration there was a reversal of the inhibitory effect exerted by food deprivation on in vivo GH secretion (Figure 1). On the other hand, the lack of effect of exogenous leptin administration in normal fed animals suggests that normal circulating leptin levels exert a maximal effect on GH secretion (Figure 1). Although the mechanisms by which leptin exerts it effects are far from being completely understood, it has been postulated that the leptin-induced decrease in appetite as well as the stimulation of peripheral energy expenditure is mediated by a decrease in hypothalamic neuropeptide Y (NPY) gene expression.11,12 However, the fact that leptin is still able to exert its effects in NPY knock-out mice indicates the existence of additional hypothalamic efferent signaling pathways.13 Taking into account the fact that NPY appears to play an important physiologically inhibitory action on GH secretion, it was of great interest to assess the interaction of leptin and NPY on GH secretion.14 Central administration of NPY completely blunted leptin-induced GH secretion in fasted freely moving rats.15 Furthermore, the inhibitory effect exerted by leptin antiserum on spontaneous GH secretion in fed rats was prevented by the prior administration (120 min earlier) of anti-NPY serum. Thus it is

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Figure 2 In adult rats leptin appears to act mainly at hypothalamic level regulating GHRH and somatostatinergic tone, either directly or by in¯uencing NPY-gene expression. During fetal development or in pituitary tumors the effect of leptin can be exerted directly at pituitary level.

possible that NPY mediates the effects of leptin on GH secretion, acting on somatostatin and GHRH producing neurones. This hypothesis is supported by previous data showing that leptin receptors are present in NPY neurones located in the arcuate nucleus of the hypothalamus, as well as immunohistochemical data showing synaptic connections between NPY-connecting neurones and periventricular neurones of the hypothalamus.16 Further studies looking at the effects of leptin on GH secretion in NPY knock-out mice should help to clarify this possibility. Since we have previously found that leptin inhibits in vitro somatostatin secretion and somatostatin mRNA levels by rat fetal neurones in monolayer culture,17 it was possible that the stimulatory effect of leptin on GH secretion could be mediated by regulating hypothalamic somatostatin and=or GHRH tone. The administration of anti-GHRH antiserum completely blocked leptininduced GH release in fasted rats. In contrast, the treatment with anti-SS (anti-somatostatin) serum signi®cantly increased leptin-induced GH release. Furthermore, leptin administration to intact fasting animals reversed the inhibitory effect exerted by fasting on GHRH mRNA levels in the arcuate nucleus of the hypothalamus, and increased SS mRNA levels in the periventricular nucleus. Finally, leptin administration to hypophysectomized fasting rats increased GHRH mRNA levels and decreased SS mRNA content, indicating a direct effect of leptin on hypothalamic GHRH and an action on somatostatinergic neurons in¯uenced by the pituitary status.18 Taken together, these data suggest that in the adult male rat leptin plays a stimulatory role on GH secretion by acting at hypothalamic level on both GHRH and somatostatin-producing neurones, and that altered GH secretion in fasted rats may occur as a result of dysregulation of a central-peripheral loop system involving hypothalamic NPY and leptin synthesized by the adipose tissue. By contrast, during fetal life or in pituitary tumors, the expression of the long-form of International Journal of Obesity

the leptin receptor in somatotroph cells allows a direct stimulatory effect of leptin on GH secretion (Figure 2). Due to the existence of important interspecies differences on the effects of fasting on GH secretion, the translation of these data to human physiology is unclear at present. Thus, while fasting suppresses GH secretion in the rat, it markedly increases spontaneous GH secretion in humans.1 Also, in obese human subjects, GH secretion is impaired but leptin levels are elevated.19 Whether this later ®nding is related to the existence of a leptin-resistant state or to leptin playing an inhibitory role on human GH secretion is unknown at present, since data regarding the effects of leptin on GH secretion in normal human subjects are still lacking at present.

Acknowledgements

Supported by grants from the Fondo de InvestigacioÁn Sanitaria, Spanish Ministry of Health, and the Xunta de Galicia. References

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