Acta Padiatr Scand [Suppl] 367: 14-19, 1990
REVIEW PAPER Effects of Growth Hormone on Body Composition and Metabolism S. EDEN, B.A. BENGTSSON and J. OSCARSSON From the Department of Physiology and the Department of Internal Medicine II. University of Gothenburg, Gothenburg, Sweden
Growth hormone (GH) has profound effects not only on growth but also on body composition and metabolism. Previous studies have clearly shown that GH affects protein metabolism, carbohydrate metabolism and fat metabolism. This review will focus on the recent developments in our understanding of the role of GH apart from its effects on cell multiplication and growth. GH SECRETION GH is secreted throughout life from the late fetal period onwards. The role of GH during the late fetal and early neonatal period is poorly understood, since growth occurs without the presence of GH during this period (1). GH is also secreted in adult life, after the closure of the growth plates, when it clearly can have no importance for longitudinal bone growth. The secretion of GH is both age- and sex-dependent. Levels are high during the late fetal period and decrease during the neonatal period and childhood. At puberty, levels increase to a maximum. In the rat, GH secretion is markedly sex-dependent. Male rats have GH peaks at regular 3-4-hourly intervals with low trough levels, whereas females have more frequent episodes of GH secretion with higher trough levels (2-7). It has been shown that these differences in GH secretion are dependent on the levels of gonadal steroids during neonatal and adult life (3). After puberty, GH secretion declines but persists throughout life in both experimental animals (8) and man (9). EFFECTS OF GH ON BODY COMPOSITION It is well known that GH has a lipolytic effect (10). Studies in the 1930s demonstrated that the injection of rats with extracts from the anterior pituitary can reduce body fat. Subsequent studies using purified GH preparations demonstrated that GH was responsible for this effect. The increase in body fat observed in children with GH deficiency (GHD) is reduced by GH treatment (1 1, 12), and it has been noted that acromegalic patients have markedly less body fat than normal individuals (13, 14). By contrast with its effect on body fat, GH also has several anabolic actions. It causes an increase in body cell mass and nitrogen retention (1, 13-15), which is accompanied by a marked increase in extracellular water (13, 14). To date, very few studies have been performed to clarify the role of GH during adult life. In a short-term study of adults with GHD treated with GH for 4 months, Jorgensen ef al. (16) observed a significant increase in muscle volume and a significant decrease in body fat. In a study of adults with GHD of varying aetiology, 6 months’ treatment with GH produced an increase in lean body mass and a reduction in body fat towards the normal values (17). It may therefore be important for adults with GHD to receive GH in order to maintain a normal body composition. GH therapy may also be appropriate during the ageing process since GH treatment in elderly rats restores the rate of protein synthesis in skeletal muscle to that of younger animals (19). It has also been shown that GH therapy in older men induces increased levels of insulin-like growth factor I (IGF-I) (18). The relationship between the different effects of GH on body composition is unclear. Since GH increases the levels of IGF-I and insulin while also inducing insulin resistance, it is possible that all its effects are interdependent. GH has been shown to have effects in a variety of tissues in vitro (20), and the recent cloning of the GH receptor (21) and the finding that this receptor is expressed in a variety of tissues (22) raises the possibility that the effects of
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GH on metabolic events may vary according to the target tissue. Circumstantial evidence for this theory has been obtained from clinical and experimental observations. The relationship between GH levels and body composition in acromegalics before and after treatment has been studied. Lean body mass increased independently of the GH levels, but a marked inverse relationship between GH levels and body fat was observed, indicating differences in doseresponse relationships of the different GH effects (14). Gluckman et al. have recently made similar observations in cows, which were treated with different GH doses while on a controlled diet. In this study the effects of GH on body water and lean body mass were maximal even at the lowest dose of GH used, whereas higher doses of GH were required to cause any effects on body fat (see article by Gluckman, page 105). It is therefore possible that the effects of GH on adipose tissue, whether direct or indirect, are mediated by receptors other than those which mediate its effects on protein metabolism and body water. EFFECTS OF GH ON LIPIDS AND LIPOPROTEINS Acromegaly is a disease associated with increased mortality, which is mainly due to cardiovascular disease (23). In a study of a limited number of adults with GHD there were signs that the incidence of arteriosclerotic disease was reduced (24). The role of GH in the regulation of lipoproteins and lipid metabolism may be an important factor in this reduction, since lipoproteins are thought to be important risk factors for cardiovascular disease. GHD has been shown to be associated with hypercholesterolaemia (25-27), which can be reversed by treatment with GH (26-28). Other studies, however, have not been able to demonstrate that GH has any consistent effect on blood lipids (25, 29, 30). In both normal and hypercholesterolaemic subjects, GH treatment lowered cholesterol levels and increased triglyceride levels (3 1). In acromegaly, triglyceride levels seem to be increased, whereas the findings with respect to cholesterol levels are conflicting. Treatment with GH lowers the triglyceride levels (32-34). These studies clearly indicate that GH has a role in the regulation of serum lipid levels. However, the inconsistent results may reflect GH-dependent changes in insulin, IGF-I and carbohydrate metabolism. By contrast with humans, where most cholesterol is found in the low density lipoprotein (LDL) fraction, the main lipoprotein fraction in the rat is high density lipoprotein (HDL). Hypophysectomized rats show a decrease in HDL-cholesterol and an increase in LDLcholesterol. Replacement therapy with thyroid hormones partially reverses this effect on LDL-cholesterol, but has no effect on HDL-cholesterol (35). Gonadal steroids have been shown to influence lipoprotein levels in both humans and rats (36, 37). We have previously shown that the plasma concentration of phospholipids is decreased in hypophysectomized rats given replacement therapy with cortisone and thyroxine, and that gonadal steroids have no effect on plasma phospholipids in these rats. This evidence strongly indicates that the pituitary has an influence on the regulation of lipoprotein metabolism (38). Subsequent studies have shown that serum levels of cholesterol, apoprotein A-I (apo A-I) and apoprotein E (apo E) are decreased in hypophysectomized rats after replacement therapy with cortisone and thyroxine. Similar changes were observed in HDL. By contrast, apoprotein B (apo B), cholesterol and triglycerides were increased in LDL. GH treatment reduced the levels of apo B and triglycerides in both serum and LDL. In addition, serum and HDL levels of cholesterol and apo E were restored by GH treatment, but this effect was dependent upon the mode of administration of GH (39), indicating that the effects of GH on lipoproteins may be mediated by different metabolic pathways. This may be the reason for the inconsistent findings with respect to changes in plasma lipids and lipoproteins in GHD and acromegaly, and the effects of GH treatment in humans. Apo B is secreted from both the liver and the gut as triglyceride-rich particles (very low density lipoproteins (VLDL) and chylomicrons, respectively) (40). Apo BlOO binds to the LDL receptor, but the function of apo B48 is unclear. Apo B48 particles also contain apo E and are taken up by the liver through binding of apo E to the LDL receptor (apo B/E receptor). Stimulated triglyceride secretion from the liver is known to be accompanied by a relative increase in the secretion of apo B48 (41). The synthesis of triglycerides in isolated rat hepatocytes has been shown to increase after GH treatment in hypophysectomized rats (42). Recently,, the molecular basis for the synthesis of apo B48/100 in the liver has been characterized. The apo B48 mRNA is formed by co- or post-transcriptional modification of
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the apo BlOO mRNA which may be hormonally regulated (43). Primary cultures of hepatocytes have been used to demonstrate that in vivo GH treatment in hypophysectomized rats increases the ratio of apo B48/100to normal levels (44).This finding indicates that GH may have effects on the co- or post-transcriptional modification involved in the synthesis of apo B. Using such techniques, it might be possible to elucidate the molecular basis of the effects of GH on lipoprotein metabolism. Our group in Sweden has measured cholesterol, triglyceride, overall and individual phospholipid levels in acromegalic patients in a follow-up study of approximately 100 patients of various ages. No marked deviation from normal was found in the cholesterol, triglyceride or phospholipid levels. There was, however, a correlation between individual phospholipids (e.g. lecithin was found to have a positive correlation and lysolecithin a negative correlation) and the concentration of GH. A preliminary study by EdCn et al. showed that successful treatment of acromegalic patients resulted in a decrease in lecithin and an increase in lysolecithin, possibly reflecting changes in lipoprotein composition (unpublished observations). Since acromegaly is also associated with marked changes in glucose tolerance and insulin secretion, these effects may be indirect and not a true reflection of the direct effects of GH on lipid metabolism. However, there were no changes in the total cholesterol and triglyceride levels in these patients after successful GH treatment.
EFFECTS OF GH SECRETORY PATTERN ON METABOLISM Many liver functions are known to be sexually differentiated (e.g. enzymes involved in the metabolism of exogenous steroids and other enzymes in the P-450family, hormone receptors, plasma proteins and lipids (4)). Several studies in the rat have indicated that the sexually dimorphic pattern of GH secretion is important for these sexually differentiated functions. In the rat, high baseline levels of GH in plasma seem to ‘feminize’ liver functions, whereas intermittent secretion of GH with low baseline levels has a ‘masculinizing’ influence. Whether the plasma pattern of GH has similar effects on other tissues and in other species is not yet known. Marked changes in the plasma pattern of GH are, however, observed in ‘physiological’ states and also during human pregnancy. It appears that in pregnant women a variant of the GH gene is expressed in the placenta (45,46).This expression results in high and continuous levels of GH during late pregnancy (47).It is well recognized that pregnancy is associated with nitrogen retention, fluid retention, insulin resistance and changes in plasma proteins and lipids. These effects may be attributable to GH (48). Taking these considerations into account, it appears that the metabolic effects of GH treatment depend upon the relationship between the dose and the administration frequency.
MECHANISM OF ACTION OF GH Over the last few years, the mechanisms by which GH exerts its effects on cell proliferation and growth have become clearer. In initial studies, direct injections of GH into the growth plate of hypophysectomized rats revealed that GH can directly stimulate growth of the epiphyseal cartilage and subsequent bone growth. Later studies demonstrated that GH induces the production of IGF-I in its target cells (49).An important observation was the finding that GH induced differentiation of cultured pre-adipocytes into adipocytes (50). It has been hypothesized that GH promotes the differentiation of precursor cells by inducing the production of local growth factors, such as IGF-I, which are important for the resulting clonal expansion (49,50). However, fully differentiated cells also bind and respond to GH (51). Little is known about how GH exerts its effects in differentiated cells. The recent cloning and characterization of the GH receptor (21) will probably be a major step towards understanding the mechanism of action of GH. Previous studies using classic binding techniques have identified GH receptors in a variety of tissues (e.g. liver, adipose tissue, cartilage, lymphocytes and other blood cells), and binding assays in adipocytes and liver have shown that the half-life of the GH receptor is short (51-53). The receptor appears to be hormonally regulated, hence the long-term effect of GH is probably to induce the production of the receptor (54,5 3 , possibly depending upon the mode of GH secretion (55).
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GH binding has also been shown to be dependent upon other hormones such as insulin and thyroxine (56), and GH binding in the liver is increased during pregnancy (57). Results of binding studies agree with the findings that the message for the cloned GH receptor can be detected in a variety of tissues (22). There are previous indications of the presence of different GH receptors. GH derivatives have been shown to have different biological profiles when tested for insulin-like, diabetogenic and growth-promoting activities (58). A specific GH-dependent tyrosine phosphorylation site has been demonstrated in the GH receptor partially purified and characterized in 3T3 adipocytes (59), which is not found in the cloned receptor (21). The circulating GH binding protein was found to be the extracellular component of the cloned receptor (21). Different forms of the GH receptor may be produced by post-transcriptional modifications. This hypothesis is supported by a preliminary report of two different detectable mRNAs which seem to be regulated differently in liver and adipocytes (60). In conclusion, the diverse effects of GH on growth and metabolism may be mediated through different or modified GH receptors. The presence of a serum binding protein and the rapid turnover of the receptor further complicates the dynamics of the hormone-receptor interaction. Furthermore, differences in the pattern of secretion of GH may be of importance for the expression of various effects of the hormone. The mechanism by which the binding of GH to its receptor is translated into its various effects remains unknown.
REFERENCES 1. Cheek DB, Hill DE. Effect of growth hormone on cell and somatic growth. In: Knobil E, Sawyer WH, eds. Handbook of Physiology. Vol. 4, Part 2. Washington DC: Am Physiol Soc 1974; 159-85. 2. EdCn S. Age- and sex-related differences in episodic growth hormone secretion in the rat. Endocrinology 1979; 105: 555-60. 3. Jansson JO, Eden S, Isaksson 0. Sexual dimorphism in the control of growth hormone secretion. Endocrine Rev 1985; 6: 128-50. 4. EdCn S, Jansson, J-0, Oscarsson, J. Sexual dimorphism of growth hormone secretion. In: Isaksson 0, 5. 6. 7. 8. 9.
Binder C , Hall K, Hokfelt B, eds. Growth hormone - basic and clinical aspects. Amsterdam: Elsevier Science Publishers BV, 1987; 129-51. Clark RG, Chambers G, Lewin J, Robinson IF. Automated repetitive microsampling of blood: growth hormone profiles in conscious male rats. J Endocrinol 1986; 111: 27-35. Clark RG, Carlsson LMS, Robinson IF. Growth hormone secretory profiles in conscious female rats. J Endocrinol 1987; 114: 399-407. Clark RG, Carlsson LMS, Robinson IF. Growth hormone (GH) secretion in the conscious rat: negative feedback of GH on its own release. J Endocrinol 1988; 119: 201-9. Sonntag WE, Steger RW, Forman LJ,Meites J. Decreased pulsatile release of growth hormone in old male rats. Endocrinology 1980; 107: 1875-9. Zadik Z, Chalew SA, McCarter RJ, Meistas M, Kowarski AA. The influence of age on the 24-hour integrated concentration of growth homone in normal individuals. J Clin Endocrinol Metab 1985; 60:
513-16. 10. Goodman HM, Schwartz J . Growth hormone and lipid metabolism. In: Knobil E, Sawyer WH, eds. Handbook of Physiology. Vol. 4, Part 2. Washington DC: Am Physiol Soc 1974; 211-32. 1 I . Tanner JM, Whitehouse RH. The effect of human growth hormone on subcutaneous fat thickness in hyposomatotrophic and panhypopituitary dwarfs. J Endocrinol 1967; 39: 263-75. 12. Parra A, Argote RM, Garcia G, Cervantes C , Alatorre S, P6rez-Paten E. Body composition in hypopituitary dwarfs before and during human growth hormone therapy. Metabolism 1979; 28: 851-7. 13. Ikkos D, Luft R, Sjogren B. Body water and sodium in patients with acromegaly. J Clin Invest 1954; 33: 989-94. 14. Bengtsson B-A, Brummer R-J, EdCn S, Bosaeus I. Body composition in acromegaly. Clin Endocrinol 1989; 30: 121-30. 15. Kostyo JL, Nutting DF. Growth hormone and protein metabolism. In: Knobil E, Sawyer WH, eds. Handbook of Physiology. Vol. 4, Part 2. Washington DC: Am Physiol Soc 1974; 187-210.
16. Jorgensen JO, Pedersen SA, Thuesen L. Beneficial effects of growth hormone treatment in GH-deficient adults. Lancet 1989; i: 1221-5. 17. Salomon R, Cuneo RC, Hesp R , Sonksen PH. Growth hormone deficiency in adults. Acta Paediatr Scand [Suppl] 1989; 356: 69. 18. Johanson AJ, Blizzard RM. Low somatomedin-C levels in older men rise in response to growth hormone administration. Johns Hopkins Med J 1981; 149: 115-17. 19. Sonntag WE, Hylka VW, Meites J. Growth hormone restores protein synthesis in skeletal muscle of old male rats. J Gerontol 1985: 40:689-94.
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20. Kostyo JL, Isaksson 0. Growth hormone in regulation of somatic growth. In: Greep RO, ed. International review of physiology: Reproductive Physiology 11. Vol. 13. Baltimore: University Park Press 1977; 255-74. 21. Leung DW, Spencer SA, Cachianes G er al. Growth hormone receptor and serum binding protein: purification, cloning and expression. Nature 1987; 330-43. 22. Matthews LS. Engberg B, Norstedt G. Regulation of rat growth hormone receptor gene expression. J Biol Chem 1989; 2.64: 9905-10. 23. Bengtsson B-A, Edtn S, Ernest I, Odtn A, Sjogren B. Epidemiology and long-term survival in acromegaly. Acta Med Scand 1988; 223: 327-35. 24. Merimee TJ, Hollander W, Fineberg SE. Studies of hyperlipidemia in the HGH-deficient state. Metabolism 1972; 21: 1053-61. 25. Merimee TJ, Pulkkinen A. Familial combined hyperlipoproteinemia: evidence for a role of growth hormone deficiency in effecting its manifestation. J Clin Invest 1980; 65: 829-35. 26. Blackett PR, Weech PK, McConathy WJ, Fesmire JD. Growth hormone in the regulation of hyperlipidemia. Metabolism 1982; 31: 117-20. 27. Asayama K, Amemiya S, Kusano S, Kato K. Growth hormone-induced changes in postheparin plasma lipoprotein lipase and hepatic triglyceride lipase activities. Metabolism 1984; 33: 129-31. 28. Gacs G, Romics L. Effect of growth hormone on serum lipoproteins in growth hormone deficiency. Exp Clin Endocrinol 1987; 90: 227-31. 29. Winter RJ, Thompson RG, Green OC. Serum cholesterol and triglycerides in children with growth homone deficiency. Metabolism 1979; 28: 1244-9. 30. Aloia JF, Zanzi I, Cohn SH. Absence of an effect of chronic administration of growth hormone on serum lipids. Metabolism 1975; 24: 795-8. 31. Friedman M, Byers SO, Rosenman RH, Li CH, Neuman R. Effect of subacute administration of human growth hormone on various serum lipid and hormone levels of hypercholesterolemic and normocholesterolemic subjects. Metabolism 1974; 23: 905-12. 32. Takeda R, Tatami R, Ueda K, Sagara H, Nakabayachi H, Mabuchi H. The incidence and pathogenesis of hyperlipidaemia in 16 consecutive acromegalic patients. Acta Endocrinol 1982; 100: 358-62. 33. Nikkila EA, Pelkonen R. Serum lipids in acromegaly. Metabolism 1975; 7: 829-38. 34. Murase T, Yamada N, Ohsawa N, Kosaka K, Morita S, Yoshida S. Decline of post-heparin plasma lipoprotein lipase in acromegalic patients. Metabolism 1980; 29: 666-72. 35. Mwalkonen M, Manninen V. Failure of thyroid hormones to maintain the normal lipoprotein pattern after removal of the pituitary gland. Atherosclerosis 1981; 38: 121-8. 36. Patsch W, Kim K, Wiest W, Schonfeld G. Effects of sex hormones on rat lipoproteins. Endocrinology 1980; 107: 1085-94. 37. Furman RH, Alaupovic P, Howard RP. Effects of androgens and estrogens on serum lipids and the composition and concentration of serum lipoproteins in normolipidemic and hyperlipidemic states. Progr Biochem Pharmacol 1967; 2 : 215-49. 38. Edtn S, Oscarsson J, Jansson J-0, Svanborg A. The influence of gonadal steroids and the pituitary on the levels and composition of plasma phospholipids in the rat. Metabolism 1987; 36: 527-32. 39. Oscarsson J, Olofsson S-0, Bondjers G, Edtn S. Differential effects of continuous versus intermittent administration of growth hormone to hypophysectomized female rats on serum lipoproteins and their apoproteins. Endocrinology 1989; 125: 1638-49. 40. Elovson J, Huang YO, Baker N, Kannan R. Apolipoprotein B is structurally and metabolically heterogeneous in the rat. Proc Natl Acad Sci USA 1981; 78: 157-61. 41. Windmueller HG, Spaeth AE. Regulated biosynthesis and divergent metabolism of three forms of hepatic apolipoprotein B in the rat. J Lipid Res 1985; 26: 70-81. 42. Elam MB, Simkevich CP, Solomon SS, Wilcox HG, Heimberg M. Stimulation of in virro triglyceride synthesis in the rat hepatocyte by growth hormone treatment in vivo. Endocrinology 1988; 122: 1397-1402. 43. Davidson NO, Powell LM, Wallis SC, Scott J. Thyroid hormone modulates the introduction of a stop codon in rat liver apolipoprotein B messenger RNA. J Biol Chem 1988; 263: 13482-5. 44. Oscarsson J, Sjoberg A, EdCn S, Bondjers G, Olofsson S-0. Growth hormone influences serum apo B48/100 and has different effects on apo B48 and BlOO in primary culture of rat hepatocytes. American Heart Association 62nd Scientific Sessions 1989 (Abstr). 45. Frankenne F, Rentier-Delrue F, Scippo M, Martial J, Hennen G. Expression of the growth hormone variant gene in human placenta. J Clin Endocrinol Metab 1987; 64:635-7. 46. Frankenne F, Closset J, Gomez F, Scippo ML, Smal J, Hennen G. The physiology of growth hormones (GHs) in pregnant women and partial characterizationof the placental GH variant. J Clin Endocrinol Metab 1988; 66: 1171-80. 47. Eriksson L, Edtn S, Frohlander N, Bengtsson B-A, von Schoultz B. Continuous 24-hour secretion of growth hormone during late pregnancy. Acta Obstet Gynecol Scand 1988; 67: 543-7. 48. Eriksson L. Growth hormone in human pregnancy: maternal 24-hour serum profiles and experimental effects of continuous GH secretion. Acta Obstet Gynecol Scand [Suppl 1471, 1989; PhD thesis. 49. Isaksson OGP, Lindahl A, Nilsson A, Isgaard J. Mechanism of the stimulatory effect of growth hormone on longitudinal bone growth. Endocr Rev 1987; 8: 426-38.
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50. Green H, Morikawa M, Nixon T. A dual effector theory of growth-hormone action. Differentiation 1985;
29: 195-8. 51 Edtn S, Schwartz J, Kostyo JL. Effects of preincubation on the ability of rat adipocytes to bind and respond to growth hormone. Endocrinology 1982; 111: 1505-12. 52 Baxter RC. Measurement of GH and prolactin receptor turnover in rat liver. Endocrinology 1985; 117: 650-5. 53 Gorin E, Goodman HM. Turnover of GH receptors in rat adipocytes. Endocrinology 1985; 116: 1796-1805. 54. Baxter RC, Zaltsman Z. Induction of hepatic receptors for GH and prolactin infusion is sex dependent. Endocrinology 1984; 115: 2009-14. 5 5 , Gause I, EdBn S. Induction of growth hormone (GH) receptors in adipocytes of hypophysectomized rats by GH. Endocrinology 1986; 118: 119-24. 56. Gause I, Edtn S. Hormonal regulation of growth hormone binding and responsiveness in adipose tissue and adipocytes of hypophysectomized rats. J EndocrifoI 1985; 105: 331-7. 57. Husman B, Andersson G, Norstedt G, Gustavsson J-A. Characterization and subcellular distribution of the somatogenic receptor in rat liver. Endocrinology 1985; 116: 2605-1 1. 58. Reagan CR, Kostyo JL, Mills JB, Gennick S , Messina JL, Wagner SA, Wilhelmi AE. Recombination of fragments of human growth hormone: Altered activity profile of the recombinant molecule. Endocrinology 1981; 109: 1663-71. 59. Foster CM, Shafer JA, Rozsa FW, Lewis SD, Renken DA, Natale JE, Schwartz J, Carter-Su C. Growth hormone promoted tyrosyl phosphorylation of growth hormone receptors in murine 3T3-F442A fibroblasts and adipocytes. Biochemistry 1988; 27: 326-34. 60. Frick GP, Goodman HM. Hypophysectomy decreases mRNA for the growth hormone (GH) receptor in rat adipose tissue and increases it in liver. 71th Annual Meeting of the Endocrine Society 1989 (Abstr 434). (S.E.) Department of Physiology University of Gothenburg P.O.B. 33031 S-40033 Gothenburg Sweden
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REVIEW PAPER Effects of Growth Hormone on Body Composition and Metabolism S. EDEN, B.A. BENGTSSON and J. OSCARSSON From the Department of Physiology and the Department of Internal Medicine II. University of Gothenburg, Gothenburg, Sweden
Growth hormone (GH) has profound effects not only on growth but also on body composition and metabolism. Previous studies have clearly shown that GH affects protein metabolism, carbohydrate metabolism and fat metabolism. This review will focus on the recent developments in our understanding of the role of GH apart from its effects on cell multiplication and growth. GH SECRETION GH is secreted throughout life from the late fetal period onwards. The role of GH during the late fetal and early neonatal period is poorly understood, since growth occurs without the presence of GH during this period (1). GH is also secreted in adult life, after the closure of the growth plates, when it clearly can have no importance for longitudinal bone growth. The secretion of GH is both age- and sex-dependent. Levels are high during the late fetal period and decrease during the neonatal period and childhood. At puberty, levels increase to a maximum. In the rat, GH secretion is markedly sex-dependent. Male rats have GH peaks at regular 3-4-hourly intervals with low trough levels, whereas females have more frequent episodes of GH secretion with higher trough levels (2-7). It has been shown that these differences in GH secretion are dependent on the levels of gonadal steroids during neonatal and adult life (3). After puberty, GH secretion declines but persists throughout life in both experimental animals (8) and man (9). EFFECTS OF GH ON BODY COMPOSITION It is well known that GH has a lipolytic effect (10). Studies in the 1930s demonstrated that the injection of rats with extracts from the anterior pituitary can reduce body fat. Subsequent studies using purified GH preparations demonstrated that GH was responsible for this effect. The increase in body fat observed in children with GH deficiency (GHD) is reduced by GH treatment (1 1, 12), and it has been noted that acromegalic patients have markedly less body fat than normal individuals (13, 14). By contrast with its effect on body fat, GH also has several anabolic actions. It causes an increase in body cell mass and nitrogen retention (1, 13-15), which is accompanied by a marked increase in extracellular water (13, 14). To date, very few studies have been performed to clarify the role of GH during adult life. In a short-term study of adults with GHD treated with GH for 4 months, Jorgensen ef al. (16) observed a significant increase in muscle volume and a significant decrease in body fat. In a study of adults with GHD of varying aetiology, 6 months’ treatment with GH produced an increase in lean body mass and a reduction in body fat towards the normal values (17). It may therefore be important for adults with GHD to receive GH in order to maintain a normal body composition. GH therapy may also be appropriate during the ageing process since GH treatment in elderly rats restores the rate of protein synthesis in skeletal muscle to that of younger animals (19). It has also been shown that GH therapy in older men induces increased levels of insulin-like growth factor I (IGF-I) (18). The relationship between the different effects of GH on body composition is unclear. Since GH increases the levels of IGF-I and insulin while also inducing insulin resistance, it is possible that all its effects are interdependent. GH has been shown to have effects in a variety of tissues in vitro (20), and the recent cloning of the GH receptor (21) and the finding that this receptor is expressed in a variety of tissues (22) raises the possibility that the effects of
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GH on metabolic events may vary according to the target tissue. Circumstantial evidence for this theory has been obtained from clinical and experimental observations. The relationship between GH levels and body composition in acromegalics before and after treatment has been studied. Lean body mass increased independently of the GH levels, but a marked inverse relationship between GH levels and body fat was observed, indicating differences in doseresponse relationships of the different GH effects (14). Gluckman et al. have recently made similar observations in cows, which were treated with different GH doses while on a controlled diet. In this study the effects of GH on body water and lean body mass were maximal even at the lowest dose of GH used, whereas higher doses of GH were required to cause any effects on body fat (see article by Gluckman, page 105). It is therefore possible that the effects of GH on adipose tissue, whether direct or indirect, are mediated by receptors other than those which mediate its effects on protein metabolism and body water. EFFECTS OF GH ON LIPIDS AND LIPOPROTEINS Acromegaly is a disease associated with increased mortality, which is mainly due to cardiovascular disease (23). In a study of a limited number of adults with GHD there were signs that the incidence of arteriosclerotic disease was reduced (24). The role of GH in the regulation of lipoproteins and lipid metabolism may be an important factor in this reduction, since lipoproteins are thought to be important risk factors for cardiovascular disease. GHD has been shown to be associated with hypercholesterolaemia (25-27), which can be reversed by treatment with GH (26-28). Other studies, however, have not been able to demonstrate that GH has any consistent effect on blood lipids (25, 29, 30). In both normal and hypercholesterolaemic subjects, GH treatment lowered cholesterol levels and increased triglyceride levels (3 1). In acromegaly, triglyceride levels seem to be increased, whereas the findings with respect to cholesterol levels are conflicting. Treatment with GH lowers the triglyceride levels (32-34). These studies clearly indicate that GH has a role in the regulation of serum lipid levels. However, the inconsistent results may reflect GH-dependent changes in insulin, IGF-I and carbohydrate metabolism. By contrast with humans, where most cholesterol is found in the low density lipoprotein (LDL) fraction, the main lipoprotein fraction in the rat is high density lipoprotein (HDL). Hypophysectomized rats show a decrease in HDL-cholesterol and an increase in LDLcholesterol. Replacement therapy with thyroid hormones partially reverses this effect on LDL-cholesterol, but has no effect on HDL-cholesterol (35). Gonadal steroids have been shown to influence lipoprotein levels in both humans and rats (36, 37). We have previously shown that the plasma concentration of phospholipids is decreased in hypophysectomized rats given replacement therapy with cortisone and thyroxine, and that gonadal steroids have no effect on plasma phospholipids in these rats. This evidence strongly indicates that the pituitary has an influence on the regulation of lipoprotein metabolism (38). Subsequent studies have shown that serum levels of cholesterol, apoprotein A-I (apo A-I) and apoprotein E (apo E) are decreased in hypophysectomized rats after replacement therapy with cortisone and thyroxine. Similar changes were observed in HDL. By contrast, apoprotein B (apo B), cholesterol and triglycerides were increased in LDL. GH treatment reduced the levels of apo B and triglycerides in both serum and LDL. In addition, serum and HDL levels of cholesterol and apo E were restored by GH treatment, but this effect was dependent upon the mode of administration of GH (39), indicating that the effects of GH on lipoproteins may be mediated by different metabolic pathways. This may be the reason for the inconsistent findings with respect to changes in plasma lipids and lipoproteins in GHD and acromegaly, and the effects of GH treatment in humans. Apo B is secreted from both the liver and the gut as triglyceride-rich particles (very low density lipoproteins (VLDL) and chylomicrons, respectively) (40). Apo BlOO binds to the LDL receptor, but the function of apo B48 is unclear. Apo B48 particles also contain apo E and are taken up by the liver through binding of apo E to the LDL receptor (apo B/E receptor). Stimulated triglyceride secretion from the liver is known to be accompanied by a relative increase in the secretion of apo B48 (41). The synthesis of triglycerides in isolated rat hepatocytes has been shown to increase after GH treatment in hypophysectomized rats (42). Recently,, the molecular basis for the synthesis of apo B48/100 in the liver has been characterized. The apo B48 mRNA is formed by co- or post-transcriptional modification of
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the apo BlOO mRNA which may be hormonally regulated (43). Primary cultures of hepatocytes have been used to demonstrate that in vivo GH treatment in hypophysectomized rats increases the ratio of apo B48/100to normal levels (44).This finding indicates that GH may have effects on the co- or post-transcriptional modification involved in the synthesis of apo B. Using such techniques, it might be possible to elucidate the molecular basis of the effects of GH on lipoprotein metabolism. Our group in Sweden has measured cholesterol, triglyceride, overall and individual phospholipid levels in acromegalic patients in a follow-up study of approximately 100 patients of various ages. No marked deviation from normal was found in the cholesterol, triglyceride or phospholipid levels. There was, however, a correlation between individual phospholipids (e.g. lecithin was found to have a positive correlation and lysolecithin a negative correlation) and the concentration of GH. A preliminary study by EdCn et al. showed that successful treatment of acromegalic patients resulted in a decrease in lecithin and an increase in lysolecithin, possibly reflecting changes in lipoprotein composition (unpublished observations). Since acromegaly is also associated with marked changes in glucose tolerance and insulin secretion, these effects may be indirect and not a true reflection of the direct effects of GH on lipid metabolism. However, there were no changes in the total cholesterol and triglyceride levels in these patients after successful GH treatment.
EFFECTS OF GH SECRETORY PATTERN ON METABOLISM Many liver functions are known to be sexually differentiated (e.g. enzymes involved in the metabolism of exogenous steroids and other enzymes in the P-450family, hormone receptors, plasma proteins and lipids (4)). Several studies in the rat have indicated that the sexually dimorphic pattern of GH secretion is important for these sexually differentiated functions. In the rat, high baseline levels of GH in plasma seem to ‘feminize’ liver functions, whereas intermittent secretion of GH with low baseline levels has a ‘masculinizing’ influence. Whether the plasma pattern of GH has similar effects on other tissues and in other species is not yet known. Marked changes in the plasma pattern of GH are, however, observed in ‘physiological’ states and also during human pregnancy. It appears that in pregnant women a variant of the GH gene is expressed in the placenta (45,46).This expression results in high and continuous levels of GH during late pregnancy (47).It is well recognized that pregnancy is associated with nitrogen retention, fluid retention, insulin resistance and changes in plasma proteins and lipids. These effects may be attributable to GH (48). Taking these considerations into account, it appears that the metabolic effects of GH treatment depend upon the relationship between the dose and the administration frequency.
MECHANISM OF ACTION OF GH Over the last few years, the mechanisms by which GH exerts its effects on cell proliferation and growth have become clearer. In initial studies, direct injections of GH into the growth plate of hypophysectomized rats revealed that GH can directly stimulate growth of the epiphyseal cartilage and subsequent bone growth. Later studies demonstrated that GH induces the production of IGF-I in its target cells (49).An important observation was the finding that GH induced differentiation of cultured pre-adipocytes into adipocytes (50). It has been hypothesized that GH promotes the differentiation of precursor cells by inducing the production of local growth factors, such as IGF-I, which are important for the resulting clonal expansion (49,50). However, fully differentiated cells also bind and respond to GH (51). Little is known about how GH exerts its effects in differentiated cells. The recent cloning and characterization of the GH receptor (21) will probably be a major step towards understanding the mechanism of action of GH. Previous studies using classic binding techniques have identified GH receptors in a variety of tissues (e.g. liver, adipose tissue, cartilage, lymphocytes and other blood cells), and binding assays in adipocytes and liver have shown that the half-life of the GH receptor is short (51-53). The receptor appears to be hormonally regulated, hence the long-term effect of GH is probably to induce the production of the receptor (54,5 3 , possibly depending upon the mode of GH secretion (55).
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GH binding has also been shown to be dependent upon other hormones such as insulin and thyroxine (56), and GH binding in the liver is increased during pregnancy (57). Results of binding studies agree with the findings that the message for the cloned GH receptor can be detected in a variety of tissues (22). There are previous indications of the presence of different GH receptors. GH derivatives have been shown to have different biological profiles when tested for insulin-like, diabetogenic and growth-promoting activities (58). A specific GH-dependent tyrosine phosphorylation site has been demonstrated in the GH receptor partially purified and characterized in 3T3 adipocytes (59), which is not found in the cloned receptor (21). The circulating GH binding protein was found to be the extracellular component of the cloned receptor (21). Different forms of the GH receptor may be produced by post-transcriptional modifications. This hypothesis is supported by a preliminary report of two different detectable mRNAs which seem to be regulated differently in liver and adipocytes (60). In conclusion, the diverse effects of GH on growth and metabolism may be mediated through different or modified GH receptors. The presence of a serum binding protein and the rapid turnover of the receptor further complicates the dynamics of the hormone-receptor interaction. Furthermore, differences in the pattern of secretion of GH may be of importance for the expression of various effects of the hormone. The mechanism by which the binding of GH to its receptor is translated into its various effects remains unknown.
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