Growth hormone treatment in adults with GH deficiency: effects on new biochemical markers of bone and collagen turnover

Growth hormone replacement may influence the biological action of thyroid hormone on liver and bone tissue

OBJECTIVE

Growth hormone (GH) replacement alters the peripheral interconversion of thyroxine (T4) and triiodothyronine (T3). However, little is known about the clinical impact of these alterations. We aimed to compare changes observed in the serum T3:T4 ratio with known biological markers of thyroid hormone action derived from different peripheral tissues.

DESIGN

We prospectively studied twenty GH deficient men before and after GH replacement in a tertiary referral endocrine center. Serum biochemical measurements included insulin like growth factor-1 (IGF-1), thyroid hormones (free & total T3, free & total T4 and reverse T3) and TSH. Changes in thyroid hormone concentration were compared to alterations in hepatic and bone biomarkers of thyroid hormone action.

RESULTS

GH replacement provoked a decline in serum free T4 concentration (-1.09 ± 1.99 pmol/L; p = 0.02) and an increase in free T3 (+0.34 ± 0.15 pmol/L; p = 0.03); therefore, the free T3:free T4 ratio increased from 0.40 ± 0.02 to 0.47 ± 0.02 (p = 0.002). Sex hormone binding globulin (SHBG) level was unchanged. However, a decline in serum ferritin (-26.6 ± 8.5 ng/mL; p = 0.005) correlated with a fall in freeT4. Alterations in lipid profile, including a rise in large HDL sub-fractions and Lp (a) (+2.1 ± 21.1 nmol/L; p = 0.002) did not correlate with thyroid hormone levels. Significant increases were recorded in serum bone turnover markers - procollagen type 1 amino-terminal propeptide +57.4%; p = 0.0009, osteocalcin +48.6%; p = 0.0007; c-terminal telopeptides of type 1 collagen +73.7%; p = 0.002. Changes in bone formation markers occurred in parallel with fluctuations in thyroid hormone.

CONCLUSION

GH-induced alterations in the thyroid axis are associated with complex, tissue specific effects on thyroid hormone action. Modulation of bone turnover markers suggests that GH may improve the biological action of thyroid hormone on bone.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Focus on growth hormone deficiency and bone in adults

Growth hormone (GH) exerts several effects on the skeleton, mediated either directly or indirectly, leading to increased bone formation and resorption rates. Patients with growth hormone deficiency (GHD) of adult onset have decreased bone mineral density (BMD) and increased fracture risk. Some, but not all, studies have found that adults with childhood onset GHD also have lower BMD than healthy controls. Adults with GHD of childhood onset have smaller bone dimensions, leading to possible underestimation of areal BMD (measured by dual energy X-ray absorptiometry), thus potentially confounding the interpretation of densitometric data. Available data suggest that patients with childhood onset GHD are at increased fracture risk. Prospective studies and some clinical trials found that GH replacement for at least 18-24 months leads to increased BMD. Retrospective and prospective data suggest that GH replacement is associated with decreased fracture risk in adults. However, data from randomized clinical trials are lacking.

Effects of Growth Hormone on Bone

PURPOSE

Describe the effects of growth hormone (GH) and insulin-like growth factor 1 (IGF-1) on the skeleton.

FINDINGS

The GH and IGF-1 axis has pleiotropic effects on the skeleton throughout the lifespan by influencing bone formation and resorption. GH deficiency leads to decreased bone turnover, delayed statural growth in children, low bone mass, and increased fracture risk in adults. GH replacement improves adult stature in GH deficient children, increases bone mineral density (BMD) in adults, and helps to optimize peak bone acquisition in patients, during the transition from adolescence to adulthood, who have persistent GH deficiency. Observational studies suggest that GH replacement may mitigate the excessive fracture risk associated with GH deficiency. Acromegaly, a state of GH and IGF-1 excess, is associated with increased bone turnover and decreased BMD in the lumbar spine observed in some studies, particularly in patients with hypogonadism. In addition, patients with acromegaly appear to be at an increased risk of morphometric-vertebral fractures, especially in the presence of active disease or concurrent hypogonadism. GH therapy also has beneficial effects on statural growth in several conditions characterized by GH insensitivity, including chronic renal failure, Turner syndrome, Prader-Willi syndrome, postnatal growth delay in patients with intrauterine growth retardation who do not demonstrate catchup growth, idiopathic short stature, short stature homeobox-containing (SHOX) gene mutations, and Noonan syndrome.

SUMMARY

GH and IGF-1 have important roles in skeletal physiology, and GH has an important therapeutic role in both GH deficiency and insensitivity states.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.

Effect of long-term growth hormone treatment on bone mass and bone metabolism in growth hormone-deficient men

Long-term GH treatment in GH-deficient men resulted in a continuous increase in bone turnover as shown by histomorphometry. BMD continuously increased in all regions of interest, but more in the regions with predominantly cortical bone.

INTRODUCTION

Adults with growth hormone (GH) deficiency have reduced rates of bone turnover and subnormal BMD. GH treatment is effective in enhancing bone turnover as shown by biochemical markers and bone histomorphometric studies. However, it is uncertain whether long-term treatment will result in higher bone mass. In this study, we present BMD and histomorphometric data on 5 years of GH treatment in GH-deficient men.

MATERIALS AND METHODS

Thirty-eight adult men with childhood onset GH deficiency (20-35 years) were included in the study. Twenty-six of these had multiple pituitary hormone deficiencies and were on stable conventional hormone replacement. BMC (total body) and BMD (lumbar spine and hip) were measured before and after 1, 2, 3, 4, and 5 years of treatment. BMD in various regions of the total body was calculated by computer software (head, trunk, arms, and legs). Transiliac bone biopsies were obtained before and after 1 and 5 years of GH treatment.

RESULTS

Total body BMC increased 18% after 5 years of treatment. This increase was observed in all regions of interest: head, 13.7%; trunk, 27.8%; arms, 24.4%; legs, 13.8%. BMD also increased in all separately measured regions: lumbar spine, 9%; femoral neck, 11%; femoral trochanter, 16%. Lumbar spine area significantly increased (p=0.0002). Histomorphometric data showed increased osteoid surface (p<0.02), osteoid volume (p<0.01), and activation frequency (p<0.006), but trabecular bone volume did not increase significantly. Qualitative assessment of the cortical bone showed endosteal and periosteal bone formation.

CONCLUSIONS

In conclusion, GH considerably increases BMC after long-term treatment. The combination of BMD and histomorphometric data suggests that GH has a greater effect on cortical than on trabecular bone.

Effects of growth hormone replacement on parathyroid hormone sensitivity and bone mineral metabolism

Adult GH deficiency (AGHD) is associated with reduced bone mineral density, and decreased end-organ sensitivity to the effects of PTH has been suggested as a possible underlying mechanism. We investigated the effects of GH replacement (GHR) on PTH circulating activity and its association with phosphocalcium metabolism and bone turnover in 16 (8 men and 8 women) AGHD patients. Half-hourly blood and 3 hourly urine sampling was performed on each patient over a 24-h period before GHR and then after 1, 3, 6, and 12 months of GHR. GH was commenced at a dose of 0.5 IU/d and was titrated to achieve and maintain an IGF-I SD score within 2 SD of the age-related reference range. The target IGF-I SD score was achieved within 3 months and was maintained at 12 months after GHR in all patients. Our results demonstrated a significant decrease in serum PTH at all visits after GHR compared with baseline values (P < 0.001), with a concomitant increase in nephrogenous cAMP excretion at 1 (P < 0.001) and 3 (P < 0.05) months and increases in serum calcium (P < 0.001), serum phosphate (P < 0.001), 1,25-dihydroxyvitamin D(3) (P < 0.001), type I collagen C-telopeptide (a bone resorption marker; P < 0.001), and procollagen type I amino-terminal propeptide (a bone formation marker; P < 0.001). Simultaneously, we observed a significant decrease in urinary calcium excretion (P < 0.001) and an increase in maximum tubular phosphate reabsorption (P < 0.001). Together these results suggest increased end-organ responsiveness to the effects of circulating PTH resulting in increased bone turnover and reduced calcium excretion. Significant circadian rhythms were observed for serum PTH, phosphate, type I collagen C-telopeptide, and procollagen type I amino-terminal propeptide before and after GHR. However, sustained PTH secretion was observed between 1400-2200 h, with a reduced nocturnal rise in untreated AGHD patients, whereas PTH secretion decreased significantly between 1400-2200 h (P < 0.001), with a significant increase in nocturnal PTH secretion (P < 0.001) after 12 months of GHR. Our results demonstrate that GH may have a regulatory role in bone mineral metabolism, and our data provide a possible underlying mechanism for the development of osteoporosis in AGHD patients. The changes observed after GHR may further explain the beneficial effects of GHR on bone mineral density that have consistently been reported.

Parathyroid hormone secretory pattern, circulating activity, and effect on bone turnover in adult growth hormone deficiency

Adult growth hormone deficiency (AGHD) is associated with osteoporosis. Reports have associated parathyroid hormone (PTH) circadian rhythm abnormalities with osteoporosis. Furthermore, there is evidence of relative PTH insensitivity in AGHD patients. Factors regulating PTH circadian rhythm are not fully understood. There is evidence that serum phosphate is a likely determinant of PTH rhythm. The aim of this study was to investigate PTH circadian rhythm and its circulating activity and association with bone turnover in untreated AGHD patients compared to healthy individuals. We sampled peripheral venous blood at 30-min and urine at 3-h intervals during the day over a 24-h period from 1400 h in 14 untreated AGHD patients (7 M, 7 W; mean age, 49.5 +/- 10.7 years) and 14 age (48.6 +/- 11.4 years; P = NS) and gender-matched controls. Cosinor analysis was performed to analyze rhythm parameters. Cross-correlational analysis was used to determine the relationship between variables. Serum PTH (1-84), phosphate, total calcium, urea, creatinine, albumin, type I collagen C-telopeptides (CT(x)), a bone resorption marker, and procollagen type I amino-terminal propeptide (PINP), a bone formation marker, were measured on all samples. Nephrogenous cyclic adenosine monophosphate (NcAMP), which reflects the renal activity of PTH, was calculated from plasma and urinary cAMP. Urinary calcium and phosphate were measured on all urine samples. Significant circadian rhythms were observed for serum PTH, phosphate, CT(x), and PINP in AGHD and healthy subjects (P < 0.001). No significant rhythm was observed for serum-adjusted calcium. PTH MESOR (rhythm-adjusted mean) was significantly higher (P < 0.05), whereas the MESOR values for phosphate, CT(x) (P < 0.05), and PINP (P < 0.001) were lower in AGHD patients than in controls. AGHD patients had significantly lower 24-h NcAMP (P < 0.001) and higher urinary calcium excretion (P < 0.05). Maximum cross-correlation between PTH and phosphate (r = 0.75) was observed when PTH was lagged by 1.5 h in healthy individuals, suggesting that changes in phosphate precede changes in PTH concentration. PTH/CT(x) and PTH/PINP showed maximum correlation when CT(x) (r = 0.68) and PINP (r = 0.71) were lagged by 3 h. In AGHD patients, compared to controls the maximum correlation between PTH/phosphate (r = 0.88, P = 0.007), PTH/CTx (r = 0.61, P = 0.027), and PTH/PINP (r = 0.65, P = 0.028) was observed when the lag time was reduced by 1.5 h in all variables, with changes in PTH and phosphate occurring at concurrent time points. Our data suggest decreased end-organ sensitivity to the effects of PTH in AGHD patients, resulting in a significantly lower NcAMP, low bone turnover, and higher calcium excretion in the presence of significantly higher PTH concentrations. We have also demonstrated that changes in serum phosphate precede those of PTH, which in turn precede changes in bone resorption and formation in healthy individuals. This relationship was altered in AGHD patients. These results suggest a possible role for GH in regulating PTH secretion and the bone remodeling process.

Effects of 12 months of GH treatment on cortical and trabecular bone content of IGFs and OPG in adults with acquired GH deficiency: a double-blind, randomized, placebo-controlled study

To investigate the effects of 12 months of GH treatment on cortical and trabecular bone content of IGFs, iliac crest bone biopsies were obtained from 25 patients with GH deficiency (9 women and 16 men; ages, 21-61 yr; mean, 46 yr) who were randomized to sc injections with GH (2 IU/m(2).d) or placebo for 12 months. Levels of IGF-I, IGF-II, IGF binding protein (IGFBP)-3, IGFBP-5, osteocalcin, OPG, RANKL, and total protein were determined in extracts obtained after EDTA and guanidine hydrochloride extraction. Calcium was determined after HCl hydrolysis. Comparing changes during GH or placebo treatment, significant increases were observed during GH substitution for cortical and trabecular bone content of IGF-I [mean difference vs. placebo (mean +/- SEM), 97 +/- 30 and 72 +/- 38%] and OPG (mean difference vs. placebo, 109 +/- 59 and 51 +/- 19%). Also, a significant decline was found for cortical osteocalcin (mean difference vs. placebo, -49 +/- 22%) during GH treatment. In conclusion, our results indicate that long-term GH treatment increases the accumulation of IGF-I and OPG in cortical and trabecular bone in patients with GH deficiency, and this may in turn lead to an increase in bone mass and improved skeletal biomechanical competence.

Recombinant species-specific growth hormone increases hard callus formation in distraction osteogenesis

The effect of growth hormone (GH) on secondary fracture healing and callus formation has been demonstrated in several previously investigated animal models. The aim of this study was to investigate and quantify the effects of GH on bone regenerates in a distraction osteogenesis model. In 20 mature female Yucatan micropigs, the tibia and fibula were osteotomized, stabilized with an external fixator, and distracted at 2 mm/day for 10 days after a 4 day latency period. The regenerates were allowed to consolidate for 10 days. Micropigs in the study group (ten animals) received a daily injection of 100 microg per kilogram body weight of recombinant porcine growth hormone (r-pGH). Micropigs in the control group (ten animals) received sodium chloride as placebo. After killing on day 25, a quantitative histomorphometrical analysis of the formed callus and the adjacent cortical bone was performed and the results of polychrome in vivo labeling were assessed. The regenerates of the r-pGH-treated animals showed a significantly larger callus area but no change in callus structure. We found islands of cartilage tissue in the regenerates of both groups; the calli from the control group exhibited a higher fraction of cartilage compared with the r-pGH group, but this was not significant. Quantification of the fluorescent in vivo labeling revealed that the distraction gap in GH-treated group showed significant ossification even during distraction. These results demonstrate that growth hormone can accelerate the maturation of the regenerate in distraction osteogenesis without changing the callus microstructure. This may prove to be a useful clinical tool for shortening the healing time in limb lengthening and bone segment transport.

The effect of growth hormone on insulin-like growth factor I and bone metabolism in distraction osteogenesis

Limb lengthening in the left tibia of 30 mature female Yucatan micropigs was performed using distraction osteogenesis. A treatment group of 15 animals received recombinant porcine growth hormone (r-pGH) (100 microg/kg/day) while the others served as controls. Serial serum measurements of total insulin-like growth factor I (IGF-I), free IGF-I, IGF binding proteins -1, -2, -3 and -4 (IGFBP-1 to -4) were performed. Bone-specific alkaline phosphatase (bone-ALP) and the serum carboxyl-terminal telopeptide of type I collagen (ICTP) were measured as bone turnover markers. The GH-treated animals showed a significant increase in total IGF-I, free IGF-I and IGFBP-3 after surgery (P<0.001). Similarly, the treated animals showed a significantly higher level of bone-ALP (P<0.001) throughout the experiment compared to the controls. There was a significant correlation between bone-ALP and total IGF-I (r=0.76) in the GH-treated group and an even higher correlation for free IGF-I (r=0.90). There was no difference in the ICTP serum levels between the two groups. These data indicate that the application of species-specific growth hormone results in a stimulation of bone formation in distraction osteogenesis which may be mediated by IGF-I. The stronger correlation between free IGF-I and bone-ALP indicates that the anabolic effect of IGF-I may be regulated through the IGFBPs by binding and inactivating IGF-I.

Effects of 12-month GH treatment on bone metabolism and bone mineral density in adults with adult-onset GH deficiency

Serum bone-Gla protein (BGP), bone alkaline phosphatase (B-AP), and C-terminal cross-linked telopeptide of type I collagen (ICTP) levels were evaluated in 18 adults with acquired GH deficiency (GHD, 14 males and 4 females, age range: 25-59 yr) before, at 3, 6, 9 and 12 months of rec-GH treatment (0.125 IU/kg/week for the first month, followed by 0.25 IU/kg/week for 11 months) and 6 months after the withdrawal of therapy. Total body bone mineral density (BMD, g/cm2) was measured with dual energy X-ray absorptiometry (Hologic QDR 1000/W) before, at 12 months of GH treatment and 6 months after its withdrawal. Before treatment, BGP (mean+/-SE: 5.1+/-0.4 ng/ml), B-AP (59.4+/-6.5 IU/l), ICTP (3.1+/-0.3 ng/ml) levels of patients were similar to in healthy controls (BGP: 5.4+/-0.1 ng/ml; B-AP: 58.2+/-2.0 IU/l; ICTP: 4.1+/-0.3 ng/ml). GH treatment caused a significant increase of BGP, B-AP, ICTP levels, the maximal stimulation of bone resorption, occurring after 3 months of GH treatment, while the maximal effect on bone formation being evident later (at 6th month). A slight decline in BGP, B-AP, T-AP and ICTP levels occurred at 9-12 months of therapy, although the values remained significantly higher than in basal conditions and with respect to healthy controls. Before treatment, mean total body BMD of patients (1.110+/-0.027 g/cm2, range: 0.944-1.350 g/cm2) was not significantly different (z-score: +0.47+/-0.31, NS) from that observed in healthy controls (1.065+/-0.008 g/cm2, range: 1.008-1.121 g/cm2). GH therapy was associated with a significant reduction of mean total body BMD values (6th month: -1.8+/-0.5%, p<0.01; 12th month: -2.1+/-1.0%, p<0.05 vs baseline), particularly evident in the first six months of treatment. Six months after the withdrawal of GH therapy, BGP (5.9+/-0.5 ng/ml), B-AP (57.3+/-7.0 IU/l) and ICTP (3.2+/-0.1 ng/ml) levels returned similar to those recorded before treatment, while total BMD increased (+1.5+/-0.7, p<0.05), remaining however slightly lower than in basal conditions (-0.6+/-1.2, NS). In conclusion, our study shows that: a) acquired GHD in adulthood is associated with both normal bone formation/resorption indexes and normal total body BMD; b) GH therapy causes a significant rise of bone formation/resorption markers (earlier and greater for bone resorption); c) one-year GH therapy is associated with a reduction of total body BMD values, particularly evident in the first 6 months of treatment; d) the effects of GH therapy on bone turnover are transient, being completely reverted six months after the withdrawal of GH therapy; e) the increase of total body BMD (up to baseline values) after GH withdrawal might be explained as consequence of persisting effects of previous GH stimulation on bone remodeling.

Withdrawal of long-term physiological growth hormone (GH) administration: differential effects on bone density and body composition in men with adult-onset GH deficiency

Adults with acquired GH deficiency (GHD) have been shown to have osteopenia associated with a 3-fold increase in fracture risk and exhibit increased body fat and decreased lean mass. Replacement of GH results in decreased fat mass, increased lean mass, and increased bone mineral density (BMD). The possible differential effect of withdrawal of GH replacement on body composition compartments and regional bone mass is not known. We performed a randomized, single blind, placebo-controlled 36-month cross-over study of GH vs. placebo (PL) in adults with GHD and now report the effect of withdrawal of GH on percent body fat, lean mass, and bone density, as measured by dual energy x-ray absorptiometry. Forty men (median age, 51 yr; range, 24-64 yr) with pituitary disease and peak serum GH levels under 5 microg/L in response to two pharmacological stimuli were randomized to GH therapy (starting dose, 10 microg/kg x day, final dose 4 microg/kg x day) vs. PL for 18 months. Replacement was provided in a physiological range by adjusting GH doses according to serum insulin-like growth factor I levels. After discontinuation of GH, body fat increased significantly (mean +/- SEM, 3.18 +/- 0.44%; P = 0.0001) and returned to baseline. Lean mass decreased significantly (mean loss, 2133 +/- 539 g; P = 0.0016), but remained slightly higher (1276 +/- 502 g above baseline; P = 0.0258) than at study initiation. In contrast to the effect on body composition, BMD did not reverse toward pretreatment baseline after discontinuation of GH. Bone density at the hip continued to rise during PL administration, showing a significant increase (0.0014 +/- 0.00042, g/cm2 x month; P = 0.005) between months 18-36. Every bone site except two (radial BMD and total bone mineral content), including those without a significant increase in BMD during the 18 months of GH administration, showed a net increase over the entire 36 months. Therefore, there is a critical differential response of the duration of GH action on different body composition compartments. Physiological GH administration has a persistent effect on bone mass 18 months after discontinuation of GH.

Long-term monitoring of rec-GH treatment by serial determination of serum aminoterminal propeptide of type III procollagen in children and adults with GH deficiency

Serum aminoterminal propeptide of type III procollagen (PIIINP) levels, a reliable marker of collagen formation, were evaluated in children (C=7) and adults with childhood-onset (CO=10) and acquired (A=18) GH deficiency (GHD) before, during and after withdrawal of rec-GH therapy (C=0.6 IU/kg/week, CO=0.5 IU/kg/week, A=0.25 IU/kg/week). The duration of treatment was 12 months for C and A and 6 months for CO; investigations were carried out before and at 3, 6, 9 and 12 months (for C and A) and at 3 and 6 months (for CO) of GH treatment and 6 months after the withdrawal of therapy (for A and CO). Data obtained from patients were compared with those recorded in two age- and sex-matched control groups. Before treatment, serum PIIINP levels were significantly lower (p<0.001) in C with GHD (mean+/-SE: 2.9+/-0.4 ng/ml) than in controls (6.1+/-0.4 ng/ml), while no significant differences were recorded between adults with CO/A-GHD (3.7+/-0.5 ng/ml and 3.4+/-0.2 ng/ml) and controls (3.2+/-0.2 ng/ml). GH treatment caused a significant increase (p<0.0001) of PIIINP levels both in C (3rd month: 4.4+/-0.2 ng/ml, 6th month: 5.1+/-0.4 ng/ml, 12th month: 5.1+/-0.5 ng/ml), CO-GHD (3rd month: 12.7+/-1.2 ng/ml; 6th month: 10.2+/-0.6 ng/ml) and A-GHD (3rd month: 10.0+/-1.0 ng/ml; 6th month: 8.4+/-0.6 ng/ml; 12th month: 7.0+/-0.7 ng/ml), the increase being dose-dependent (more marked and sustained in adults with CO-GHD). The maximal stimulation of collagen synthesis occurred after 3 months of GH treatment in adults with GHD, while a more gradual and less relevant increase was observed in C with GHD. Six months after the withdrawal of GH therapy, serum PIIINP levels of adults with CO-GHD (3.6+/-0.3 ng/ml) were similar to those recorded before treatment, while in adults with A-GHD serum PIIINP levels (2.6+/-0.2 ng/ml) were significantly lower (p<0.01) than in basal condition. In conclusion, our study shows that: a) GHD is associated with a reduction of soft tissue formation in children, while it seems to exert no relevant effects in adults with GHD; b) GH therapy causes a rapid stimulation of collagen turnover, which shows a different pattern in children and adults; c) the GH-induced stimulation of collagen synthesis is rapidly removed after the withdrawal of GH treatment. For these reasons, the determination of peripheral markers of GH effects appears useful for the monitoring of GH therapy and can contribute to assess the "tailored" substitutive dose for the individual patient.

Growth hormone and bone

It is well known that GH is important in the regulation of longitudinal bone growth. Its role in the regulation of bone metabolism in man has not been understood until recently. Several in vivo and in vitro studies have demonstrated that GH is important in the regulation of both bone formation and bone resorption. In Figure 9 a simplified model for the cellular effects of GH in the regulation of bone remodeling is presented (Fig. 9). GH increases bone formation in two ways: via a direct interaction with GHRs on osteoblasts and via an induction of endocrine and autocrine/paracrine IGF-I. It is difficult to say how much of the GH effect is mediated by IGFs and how much is IGF-independent. GH treatment also results in increased bone resorption. It is still unknown whether osteoclasts express functional GHRs, but recent in vitro studies indicate that GH regulates osteoclast formation in bone marrow cultures. Possible modulations of the GH/IGF axis by glucocorticoids and estrogens are also included in Fig. 9. GH deficiency results in a decreased bone mass in both man and experimental animals. Long-term treatment (> 18 months) of GHD patients with GH results in an increased bone mass. GH treatment also increases bone mass and the total mechanical strength of bones in rats with a normal GH secretion. Recent clinical studies demonstrate that GH treatment of patients with normal GH secretion increases biochemical markers for both bone formation and bone resorption. Because of the short duration of GH treatment in man with normal GH secretion, the effect on bone mass is still inconclusive. Interestingly, GH treatment to GHD adults initially results in increased bone resorption with an increased number of bone-remodeling units and more newly produced unmineralized bone, resulting in an apparent low or unchanged bone mass. However, GH treatment for more than 18 months gives increased bone formation and bone mineralization of newly produced bone and a concomitant increase in bone mass as determined with DEXA. Thus, the action of GH on bone metabolism in GHD adults is 2-fold: it stimulates both bone resorption and bone formation. We therefore propose "the biphasic model" of GH action in bone remodeling (Fig. 10). According to this model, GH initially increases bone resorption with a concomitant bone loss that is followed by a phase of increased bone formation. After the moment when bone formation is stimulated more than bone resorption (transition point), bone mass is increased. However, a net gain of bone mass caused by GH may take some time as the initial decrease in bone mass must first be replaced (Fig. 10). When all clinical studies of GH treatment of GHD adults are taken into account, it appears that the "transition point" occurs after approximately 6 months and that a net increase of bone mass will be seen after 12-18 months of GH treatment. It should be emphasized that the biphasic model of GH action in bone remodeling is based on findings in GHD adults. It remains to be clarified whether or not it is valid for subjects with normal GH secretion. A treatment intended to increase the effects of GH/IGF-I axis on bone metabolism might include: 1) GH, 2) IGF, 3) other hormones/factors increasing the local IGF-I production in bone, and 4) GH-releasing factors. Other hormones/growth factors increasing local IGF may be important but are not discussed in this article. IGF-I has been shown to increase bone mass in animal models and biochemical markers in humans. However, no effect on bone mass has yet been presented in humans. Because the financial cost for GH treatment is high it has been suggested that GH-releasing factors might be used to stimulate the GH/IGF-I axis. The advantage of GH-releasing factors over GH is that some of them can be administered orally and that they may induce a more physiological GH secretion. (ABSTRACT TRUNCATED)

The effect of growth hormone (GH) on histomorphometric indices of bone structure and bone turnover in GH-deficient men

We investigated the effects of GH on bone structure and turnover by histomorphometry in GH-deficient adults. Therefore, transiliac bone biopsies were obtained before and after 1 yr of treatment in 36 GH-deficient men (mean age, 28 +/- 4 yr). Thirteen patients had isolated GH deficiency and 23 patients had multiple pituitary hormone deficiencies. Patients were randomly assigned to four treatment groups. Groups 1, 2, and 3 received 1, 2, and 3 IU/m2/day (0.35, 0.69, and 1.3 mg/m2/day) [corrected] GH, respectively, and the fourth group received placebo for the first 6 months and 2 IU/m2/day (5.8 mg/m2/day) GH for the subsequent 6 months. GH treatment resulted in an increase of cortical thickness from 0.98 +/- 0.27 to 1.20 +/- 0.35 mm (P = 0.005), but trabecular bone volume did not change. Bone formation variables increased significantly: osteoid surface increased from 8.5 +/- 5.3 to 15.5 +/- 6.1% (P = 0.0002), mineralizing surface increased from 6.7 +/- 2.5 to 10.8 +/- 4.4% (P = 0.0002), and bone formation rate increased from 0.04 +/- 0.02 to 0.08 +/- 0.04 mm3/mm2/day (P = 0.0001). Eroded surface did not change, but osteoclast number increased from 0.6 +/- 0.5 to 1.25 +/- 0.5 Oc/mm2 (P = 0.0001). The relative formation period increased significantly (P = 0.001), whereas the resorption period, including reversal phase, decreased from 65 to 40 days (P = 0.02). Activation frequency increased from 0.39 +/- 0.17 to 0.74 +/- 0.34 y(-1) (P = 0.0001). These data indicate a stimulated bone turnover as a result of GH treatment and a shorter resorption and reversal time. The increased turnover did not result in an increased trabecular bone volume, but the cortical thickness increased significantly.

Adults with childhood-onset growth hormone deficiency: effects of growth hormone treatment on cardiac structure

OBJECTIVES

To evaluate the effects of growth hormone deficiency (GHD) and of growth hormone (GH) therapy on cardiac structure in adults with childhood-onset GHD.

SETTING

Out-patient clinic in the Italian Institute for Auxology, Milan.

SUBJECTS

Eight adults with childhood-onset GHD and eight healthy controls, matched for sex, age, exercise and body mass index.

INTERVENTIONS

Recombinant GH (Saizen Serono, Italy), administered in a conventional dose of 0.5 IU kg-1 week-1 for 6 months.

MAIN OUTCOME MEASURES

Cardiac structure parameters, evaluated by two-dimensional, M-mode and Doppler echocardiograms, and stress test, by means of a modified Bruce protocol with a bicycle ergometer, were determined before and after 6 months GH therapy.

RESULTS

Before treatment, mean (+/- SE) intraventricular septal thickness (IVST: 7.1 +/- 0.2 mm), LV posterior wall thickness (LVPT: 5.2 +/- 0.1 mm), LV mass (LVM: 94.6 +/- 5.0 g), LV mass index (LVM/body surface area, LVMI: 65.1 +/- 3.0 g m-2) and left ventricular end-diastolic diameter (LVED: 41.4 +/- 0.6 mm) of patients were significantly lower (P < 0.01) than in controls, whilst LV end-systolic diameter (LVES) of patients (25.5 +/- 0.7 mm) was similar to controls (27.5 +/- 0.7). GH treatment significantly (P < 0.01) increased LVPT (6.8 +/- 0.2 mm), LVM (111.6 +/- 4.6 g) and LVMI (80.5 +/- 3.5 g m-2); no significant changes were observed in LVED, LVES and IVST values. The stress test showed a significant improvement of cardiac performance, as demonstrated by the reduction of blood pressure x heart rate product at the same workload (basal: 32,722.5 +/- 897.4 vs. after: 25,574.6 +/- 439.7).

CONCLUSIONS

GH plays a role in the maintenance of a normal cardiac structure in adulthood. The present study suggests that GH treatment might be able to improve the cardiac structure of patients with childhood-onset GHD.

Collagen formation and degradation increase during growth hormone therapy in children

A comprehensive set of serum markers of collagen turnover and growth was investigated in a longitudinal study of short children during growth induced by growth hormone (hGH) treatment. The study comprised 18 prepubertal children with short stature who had no other current illness or continuous medication. The growth rates and endogenous GH secretions covered a continuum from subnormal to normal. Before treatment, the concentrations of carboxyterminal propeptide of type I procollagen (PICP), reflecting type I collagen formation, of carboxyterminal telopeptide of type I collagen (ICTP), a degradation product of type I collagen, of amino-terminal propeptide of type III procollagen (PIIINP), a marker for type III collagen formation, of alkaline phosphatase (AP), and of insulin-like growth factor binding protein-3 (IGFBP-3) were within the lower limits of normal. The median IGF-I concentration was lower than the reference. One week after the start of treatment, the serum concentrations of ICTP, PIIINP, and osteocalcin (OC), and the increments in ICTP, PIIINP, and IGF binding protein-3 (IGFBP-3) correlated with the subsequent height velocity. During the 12-month treatment, all markers were higher than those of age-matched references, but only the three collagen markers paralleled the changes in height velocity. In molar concentrations, ICTP increased less than PICP. Throughout the study period, the serum level of ICTP correlated with that of PIIINP, but not with that of PICP. The findings suggest that during hGH treatment, linear body growth is closely associated with collagen formation and degradation.

Different responses of bone alkaline phosphatase isoforms during recombinant insulin-like growth factor-I (IGF-I) and during growth hormone therapy in adults with growth hormone deficiency

We studied serum bone alkaline phosphatase (ALP) isoforms and other markers of bone turnover in growth hormone-deficient (GHD) adults (n = 22). The patients were followed during 1 week of insulin-like growth factor-I (IGF-I) administration, 40 micrograms/kg of body weight/day (n = 6), and during 24 months of growth hormone (GH) therapy, 0.125 IU/kg of body weight/week for the first month, and then 0.250 IU/kg of body weight/week (n = 20). Six ALP isoforms were separated and quantified by high-performance liquid chromatography: one bone/intestinal, two bone (B1, B22), and three liver ALP isoforms. At baseline, the mean levels of B1, B22, and osteocalcin were higher in GHD adults than in healthy adults. After 2 week of IGF-I administration and 1 month of GH therapy, only B1 was decreased. We suggest that the initial decrease of B1 during GH therapy could be an effect of endocrine IGF-I action mediated by GH. After 3 months of GH therapy, both B1 and B2 increased as compared with placebo. Osteocalcin, carboxy-terminal propeptide of type I procollagen (PICP), cross-linked carboxy-terminal telopeptide of type I collagen (ICTP), and urinary pyridinoline cross-links/creatinine ratio increased during GH therapy. PICP increased significantly before bone ALP and osteocalcin, indicating early stimulation of type I collagen synthesis as previously demonstrated by in vitro models. Different responses of the bone ALP isoforms during IGF-I and during GH therapy suggest different regulations in vivo.

Effects of recombinant growth hormone (GH) treatment on bone mineral density and body composition in adults with childhood onset growth hormone deficiency

Lumbar spine, whole proximal femur and total body bone mineral density (BMD, g/cm2) and the regional soft tissue composition were measured with dual energy X-ray absorptiometry (Hologic QDR 1000/W) in eight adults with childhood onset GHD, before and after 6 months of recombinant GH treatment (0.5 IU/kg/week). Data obtained from patients were compared with those recorded in an age and sex matched control group. Before treatment, lumbar (L2-L4) spine BMD (mean +/- SD: 0.811 +/- 0.159 g/cm2), whole proximal femur BMD (0.739 +/- 0.094 g/cm2) and total body BMD (0.946 +/- 0.087 g/cm2) of patients were significantly (p < 0.001, 0.01 and 0.001, respectively) lower than those recorded in an age- and sex-matched control group (1.077 +/- 0.155 g/cm2, 0.968 +/- 0.166 g/cm2 and 1.168 +/- 0.058 g/cm2, respectively), although three patients showed BMD values at the lower limit of the normal range. Mean lumbar spine BMD, whole proximal femur BMD and total body BMD did not significantly change alter 6 months' GH treatment (-1.4 +/- 3.7%, +2.7 +/- 3.7% and -1.1 +/- 5.0% vs basal values, respectively). On the other hand, trochanteric subregion showed a significant 4.8 +/- 5.3% increase (vs basal, p < 0.05), while other hip subregions did not show significant changes. GH therapy caused marked effects on body composition; in fact, a significant decrease (p < 0.01) of trunk fat (-25.2 +/- 15.0%) and a marked increase (p < 0.01) of limbs lean mass (+10.0 +/- 5.3%), resulting in a significant (p < 0.02) reduction (-16.5 +/- 13.5%) of the axial to peripheral fat ratio (APFR), were clearly evident after six months of therapy. In conclusion, our study shows that six months of GH treatment do not exert relevant effects on the BMD of adults with childhood onset GHD. On the contrary, the effects of GH therapy on body composition are more marked, being clearly appreciable after six months of treatment.

Histomorphometric analysis of bone mass and bone metabolism in growth hormone deficient adult men

Transiliac bone biopsies were obtained from 36 growth hormone (GH) deficient men (mean age +/- SD, 28 +/- 4 years), of which 13 had an isolated GH deficiency and 23 had partial or complete hypopituitarism. The latter group was adequately substituted for the pituitary hormone deficiencies other than GH. Static histomorphometry was compared with eight controls, and dynamic histomorphometry was compared with six healthy men matched for age. Mean trabecular bone volume was not decreased and bone volume was high (> 30%) in ten patients. Osteoid thickness and mineralization lag time were slightly although not significantly higher than in controls. Osteoid surface, mineralizing surface and bone formation rate tended to be lower than in the controls. The eroded surface was significantly higher (p < 0.002) in the GH deficient patients. The results demonstrate that GH deficient patients do not show trabecular osteoporosis. The increased eroded surface together with normal to increased bone volume and bone surface suggests a prolonged reversal phase or a less sufficient coupling phenomenon.

Markers of type I collagen degradation and synthesis in the monitoring of treatment response in bone metastases from breast carcinoma

Thirty-six patients with bone metastases included in a trial of supportive calcitonin on the treatment response to systemic therapy were monitored by conventional radiography, conventional indicators of bone metabolism [alkaline phosphatase (AP), osteocalcin (gla), urinary hydroxyproline excretion (OHP), urinary calcium (uCa), serum calcium (sCa)] and collagen metabolites (ICTP, the pyridinoline cross-linked carboxy-terminal telopeptide of type I collagen; PICP, the carboxy-terminal propeptide of type I procollagen; and PIIINP the amino-terminal propeptide of type III procollagen). All patients had been on the same systemic treatment for at least 3 months at the start of the trial. There was a positive correlation between the concentrations of ICTP and PICP at baseline (Spearman's rank-order correlation coefficient rs = 0.62). Both ICTP and PICP showed statistically significant correlations to the other markers of bone metabolism (except sCa and uCa) as well as to the number of bone metastases on bone scans. Reduction in ICTP correlated significantly with the treatment response at three months (rs = - 0.57). while PICP showed a borderline negative correlation to therapy response (rs = - 0.37). Of all the biochemical parameters studied the changes in ICTP showed the best correlation with the treatment response. PICP and ICTP changes in patients with progressive disease differed significantly from those in patients with responding and stable metastases, whereas no difference was found between responders and stable patients.

Effects of rhIGF-I administration on bone turnover during short-term fasting

Insulin-like growth factor-I (IGF-I) is a nutritionally dependent bone trophic hormone which stimulates osteoblast function and collagen synthesis in vivo and in vitro. We hypothesized that in the fasting state, IGF-I levels would decline significantly and would establish a model in which we could investigate the effects of IGF-I administration on bone turnover. We therefore studied 14 normal women ages 19-33 (mean, 24 +/- 4 [SD] years) during a complete 10-d fast. After 4 d of fasting, subjects were randomized to receive rhIGF-I or placebo subcutaneously twice a day for 6 d. Bone turnover was assessed using specific markers of formation (osteocalcin and type I procollagen carboxyl-terminal propeptide [PICP]) and resorption (pyridinoline, deoxypyridinoline, type I collagen crosslinked N-telopeptide [N-telopeptide] and hydroxyproline). Serum levels of PICP and osteocalcin decreased from 143 +/- 52 to 60 +/- 28 ng/ml (P = 0.001) and from 7.6 +/- 5.4 to 4.2 +/- 3.1 ng/ml (P = 0.001) respectively with 4 d of fasting. Urinary excretion of pyridinoline and deoxypyridinoline decreased from 96 +/- 63 to 47 +/- 38 nmol/mmol creatinine (P < 0.05) and from 28 +/- 17 to 14 +/- 11 nmol/mmol creatinine (P < 0.05) respectively. Mean IGF-I levels decreased from 310 +/- 81 to 186 +/- 78 ng/ml (P = 0.001). In the second part of the experimental protocol, serum osteocalcin and PICP levels increased 5- and 3-fold, respectively with rhIGF-I administration and were significantly elevated compared with the placebo group at the end of treatment (20.9 +/- 17.3 vs. 5.9 +/- 6.4 ng/ml for osteocalcin [P < 0.05] and 188 +/- 45 vs. 110 +/- 37 ng/ml for PICP [P < 0.05]). In contrast, all four markers of bone resorption, including urinary pyridinoline, deoxypyridinoline, N-telopeptide and hydroxyproline were unchanged with rhIGF-I administration. This report is the first to demonstrate that bone turnover falls rapidly with acute caloric deprivation in normal women. RhIGF-I administration uncouples bone formation in this setting by significantly increasing bone formation, but not resorption. These data suggest a novel use of rhIGF-I to selectively stimulate bone formation in states of undernutrition and low bone turnover.

The effects of prolonged growth hormone replacement on bone metabolism and bone mineral density in hypopituitary adults

OBJECTIVE

Short-term GH replacement in hypopituitary adults increases bone turnover; data on the consequences of longer-term GH treatment are limited. We report on the effects of 12-18 months of GH replacement treatment with biosynthetic human GH on bone metabolism and bone mass in hypopituitary adults.

DESIGN

Patients were studied before and after GH treatment for 12 months (n = 11) and 18 months (n = 27) respectively in an open trial. GH dose was 0.04 +/- 0.01 IU/kg daily.

MEASUREMENTS

Plasma calcium, phosphate and intact PTH concentrations, 24-hour urinary calcium excretion, 3 markers of bone formation (total alkaline phosphatase, osteocalcin and procollagen 1 carboxy terminal peptide (P1CP)) and serum concentration of carboxyterminal cross-linked telopeptide of type 1 collagen (ICTP), as a marker of bone resorption, were measured at 6-month intervals. Lumbar spine and total body bone mineral mass was measured by dual-energy X-ray absorptiometry.

RESULTS

Small increases were observed in plasma calcium and phosphate concentrations at 12 months of GH therapy but the differences at 18 months were not statistically significant. Serum intact PTH concentration did not change. Plasma total alkaline phosphatase increased significantly on GH from 75 +/- 26 to 92 +/- 30 (P < 0.01) and 85 +/- 31 U/I (NS) at 12 and 18 months respectively. Serum osteocalcin increased from 6.5 +/- 3.7 to 15.7 +/- 6.2 (P < 0.0001) and 16.6 +/- 5.7 micrograms/I (P < 0.001) at 12 and 18 months respectively and P1CP increased significantly from 106.0 +/- 47.3 micrograms/I to 165.5 +/- 95.3 (P < 0.0001) and 177.2 +/- 72.2 micrograms/I (P < 0.01) at 12 and 18 months respectively. Plasma ICTP concentration increased also from 3.4 +/- 1.8 to 7.3 +/- 3.4 (P < 0.0001) and 7.0 +/- 2.7 micrograms/I (P < 0.003) at 12 and 18 months of GH therapy respectively. No significant change was observed in total body or lumbar spine bone mass, over the 18 months of GH treatment

CONCLUSIONS

Replacement therapy with GH in hypopituitary adults for 6-18 months produced a sustained increase in bone turnover (both formation and resorption). Bone mass was maintained but did not increase over the study period.

Growth hormone (GH) treatment in GH-deficient adults: effects on muscle size, strength and neural activation

The effects of 6 months of recombinant growth hormone (GH) treatment (0.5 IU kg-1 per week) on muscle size, strength and neural activation (EMG) was studied in eight adults with childhood onset GH deficiency (GHD). Before treatment, height, body mass (BM) and lean body mass (LBM) of the GHD subjects were significantly lower (P < 0.01) from those recorded in eight healthy controls, while no significant differences were found between the body mass index (BMI) of the two populations. Thigh muscle + bone cross-sectional area (CSAM+B) and lower limb muscle plus bone volume (LLVM+B) of the GHD patients were 66.1 +/- 13.7% and 47.6 +/- 6.8% of those recorded in the controls (P < 0.01), whereas no difference in CSA/height2 was found between the two groups. By contrast, LLVM+B/height3 was 82.0 +/- 19.0% that of the controls (P < 0.05). Similarly, quadriceps muscle strength (MVC) of the GHD patients was 63.2 +/- 12.4% that of controls (P < 0.01), while no significant differences in the force per unit area (F/CSA) and per body mass (F/BM) were found. After 6 months of GH treatment LBM increased by 6.0 +/- 4.2% (P < 0.02), CSAM+B by 14.5 +/- 12.7% (P < 0.01) and LLVM+B by 10.1 +/- 7.3% (P < 0.01), absolute differences from the normals still persisting. However, the LLVM+B/height3 of the GHD patients after treatment was no longer significantly different from that of the controls. Quadriceps MVC increased by 9.8 +/- 12.0% (P < 0.02), differences from the controls being still significant, whereas the F/CSA and F/BM did not change. A right shift of the integrated EMG/Force relation, with no change in the maximal integrated EMG (iEMG) activity, was observed in the patients after treatment. In conclusion, the current study shows that adults with childhood onset GHD have a reduced skeletal muscle mass and strength which seem to be positively influenced by 6 months of GH treatment.