Clinical Uses of Insulin-like Growth Factor I

Dr. Carolyn Bondy (Developmental Endocrinology Branch, National Institute of Child Health and Human Development): Insulin-like growth factor I (IGF-I; somatomedin-C) is an anabolic polypeptide that is structurally homologous to insulin [1]. Its actions are mediated primarily by the IGF-I receptor, which is structurally and functionally homologous to the insulin receptor. The ligand-binding domains of these receptors are sufficiently different that each binds its cognate hormone with about ten times more affinity than does the related ligand [2]. The signal-transducing, tyrosine kinase domains of the two receptors, however, are very similar [2] and activate common intracellular pathways [3]. Thus, it appears that the difference in physiologic effects of insulin and IGF-I are not due primarily to intrinsic differences in signaling capacities of their receptors [4]. Furthermore, with a few notable exceptions, both receptors are widely expressed, with some tissues apparently expressing hybrid receptors that combine insulin and IGF-I receptor subunits [5, 6]. Because insulin and IGF-I are subject to very different regulatory influences and have markedly different patterns of secretion and circulating profiles, hormone bioavailability is probably an important factor in determining the different roles served by IGF-I and insulin. Recombinant human IGF-I recently became available for clinical studies, allowing, for the first time, direct investigation of the metabolic and anabolic effects of IGF-I and its relations with insulin and growth hormone. Our view of the regulatory relations among IGF-I, growth hormone, and insulin is outlined in Figure 1. Growth hormone and insulin stimulate the constitutive secretion of IGF-I from the liver [7] and IGF-I, in turn, suppresses growth hormone and insulin secretion, even under euglycemic conditions [8-11]. In contrast to the highly regulated secretory patterns and fluctuating serum profiles of growth hormone and insulin, circulating IGF-I levels are relatively stable. This stability is due to its constitutive pattern of secretion and to the fact that most circulating IGF-I is bound to high-affinity IGF-binding proteins, which prolong the half-life and titrate the supply of this hormone to its receptors [12, 13]. Six IGF binding proteins have been identified, but clinical data are most abundant for IGF-binding protein-3. This IGF-binding protein binds IGF-I and another component, the acid-labile subunit, and forms a high molecular weight ternary complex, which constitutes the primary reservoir of circulating IGF-I. Circulating levels of this complex are positively regulated by growth hormone. Insulin-like growth factor-binding protein-1 binds a smaller fraction of the total circulating IGF-I, but this fraction may be disproportionately influential in terms of the effects of IGF-I on intermediary metabolism, because IGF-binding protein-1 levels are potently suppressed by insulin. Figure 1. Relations between insulin-like growth factor I (IGF-I) and IGF-binding proteins, growth hormone (GH), and insulin. Originally, the somatomedin hypothesis [1] suggested that circulating IGF-I mediates most of the effects of growth hormone on linear growth. Recently, however, growth hormone was found to stimulate the local production of IGF-I in several tissues in addition to the liver in rodents [1], and thus local autocrine or paracrine effects of IGF-I appeared to be important for normal growth. There is, however, little evidence for growth hormone-stimulated IGF-I synthesis in human tissues other than the liver, and the apparent success of systemic IGF-I treatment in producing linear growth in growth hormone-resistant children, discussed in the following section by Dr. Underwood, suggests that neither local IGF-I production nor direct anabolic effects of growth hormone are essential for statural growth in children. Local autocrine/paracrine growth processes in humans might be regulated by another member of the insulin family of peptides. Insulin-like growth factor II is structurally closely related to IGF-I [1] and binds the IGF-I receptor with high affinity, but unlike IGF-I it is not regulated by growth hormone. In rodents, IGF-II expression is abundant during embryonic development but is largely suppressed after birth. In humans, however, IGF-II levels are equal to or greater than IGF-I in the circulation and in many tissues during adulthood [1, 14-16]. Growth hormone and IGF-I have continuing roles in fuel metabolism and in the maintenance of musculoskeletal mass in adults. Many of the changes in body composition, such as increasing adiposity and decreasing muscle mass, that occur during aging correlate specifically with decreasing levels of these hormones [17]. Several clinical situations exist in which the anabolic or metabolic effects of growth hormone, IGF-I, or both may prove to have substantial therapeutic benefit. Starvation, cachexia, hyperalimentation, and insulin-dependent diabetes mellitus are all associated with a state of functional growth hormone resistance in which, despite normal or high growth hormone levels, circulating IGF-I levels are low and do not respond to growth hormone treatment. A common factor in these conditions is under-insulinization of the liver, which impairs normal IGF-I and IGF-binding protein synthesis. Recent clinical trials evaluated the short-term metabolic effects of IGF-I administration in calorically deprived adult volunteers, as described by Dr. Clemmons, and in insulin-dependent diabetic patients, as described by Dr. Bach. Another area in which IGF-I may have important therapeutic benefit is the hyperglycemic disorders characterized by insulin resistance. In the short term, recombinant IGF-I reduces blood glucose and triglyceride levels in obese patients with noninsulin-dependent diabetes mellitus [11]. These salutary effects have been attributed to improved insulin sensitivity due to suppression of growth hormone and insulin secretion by IGF-I and to the direct, insulin-like metabolic effects of IGF-I. A few studies have reported that recombinant IGF-I treatment improves the hyperglycemia of patients with extreme insulin resistance caused by genetic defects in the insulin receptor, thus suggesting that IGF-I may act through its own receptor to regulate blood glucose [18-21]. Not all insulin-resistant patients respond well to IGF-I treatment, however, as reported by Drs. Guler and Skarulis in a following section. Insulin-like Growth Factor I in Growth Hormone-Resistant Short Stature Dr. Louis Underwood (Department of Pediatrics, University of North Carolina, Chapel Hill, North Carolina): We are treating two kinds of patients with short stature secondary to growth hormone insensitivity: patients with Laron-type dwarfism, now called the Laron syndrome [22], and growth hormone-deficient patients in whom large amounts of growth-attenuating antibodies have developed after treatment with growth hormone. Normally, growth hormone binds to the growth hormone receptor to induce hepatic IGF-I production, which in turn stimulates growth and feeds back at the level of the pituitary and hypothalamus to suppress growth hormone secretion (Figure 2). Patients with the Laron syndrome lack functional growth hormone receptors and thus do not respond to growth hormone; their IGF-I levels are very low, growth is slow, and circulating growth hormone levels are high because of decreased feedback suppression of growth hormone by IGF-I (Figure 2). Patients with a deletion of the growth hormone gene may recognize growth hormone as a foreign protein, and large numbers of antibodies may develop that attenuate or obliterate their response to it. Figure 2. Diagram of the growth hormone (GH) and insulin-like growth factor I (IGF-I) growth axis in healthy persons (left), those with the Laron syndrome (middle), and those with a deletion of the gene-encoding growth hormone (right). We studied a boy with the Laron syndrome [23] who was very short (111 cm at 9 years) and had the physical appearance of a person with growth hormone deficiency. Basal serum levels of growth hormone were elevated (10 to 12 g/L) and increased to 40 to 60 g/L after pharmacologic stimulus. His serum IGF-I level was low (5 to 6 g/L; normal for age, 100 g/L), and he had no increase in serum IGF-I after injections of growth hormone. He received growth hormone therapy for 6 months without an increase in growth rate. We admitted him to our Clinical Research Center for 5 weeks and ensured a constant dietary intake. In the second week, he was given three injections of growth hormone at therapeutic doses, and in weeks 3 and 4 he received continuous infusion of recombinant IGF-I (Genentech, San Francisco, California). This treatment was followed by 1 week of postinfusion observation. He showed no metabolic responses to growth hormone, but he had a marked decrease in urinary excretion of urea and in serum urea nitrogen with IGF-I infusion. His urinary calcium level increased and his urinary phosphate and sodium excretion levels decreased [24]. These all are fairly typical growth hormone-like effects and are similar to those that would occur in patients with growth hormone deficiency who are sensitive to growth hormone. Because of the insulin-like effects of IGF-I, he tended to become hypoglycemic when he was infused overnight in a fasting state. However, in the postprandial state, his glucose increased to high levels and his insulin level was suppressed, the latter because of a direct effect of IGF-I on insulin secretion. He was treated with subcutaneous injections of recombinant IGF-I (120 g/kg every 12 hours). After IGF-I injection, serum IGF-I concentrations were in the normal range for at least 7 hours. In general, however, acute metabolic responses to subcutaneous injections are less pronounced than are those observed with intravenous infusion. He has been treated with IGF-I for nearly 2 years and has grown at