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Original Studies |
Department of Endocrinology and Diabetology, Karolinska Hospital (R.F., A.H., M.T.), SE-171 76 Stockholm, Sweden; Department of Endocrinology, University Hospital MAS (M.B.), SE-205 02 Malmo, Sweden; Department of Internal Medicine, Norrlands University Hospital (E.H.), SE-901 85 Umea, Sweden; Department of Mineral Metabolism, Jerry L. Pettis Veterans Administration Medical Center (D.J.B., S.M.), Loma Linda, California 92357
Address all correspondence and requests for reprints to: Dr. Marja Thorén, Department of Endocrinology and Diabetology, Karolinska Hospital, S-171 76 Stockholm, Sweden. E-mail: marja{at}divmed.ks.se
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Abstract |
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There were no changes in any of the measured variables during placebo treatment. GH treatment normalized serum IGF-I in a majority of the patients and increased IGFBP-3 and -5 as well as IGFBP-4 and IGF-II to values within normal range. Lean body mass was increased, and the increase at 6 and 12 months correlated with the increase in IGF-I (r = 0.46; P = 0.010 and r = 0.54, respectively; P = 0.003). GH treatment caused a modest, but highly significant, reduction of total body fat. Mean bone mineral density was not different from that in healthy subjects of the same age and did not change during the observation period. Markers for bone formation (bone-specific alkaline phosphatase activity, osteocalcin, and procollagen I carboxyl-terminal peptide in serum) increased within the normal range, and levels were sustained throughout the study. The bone resorption marker (pyridinoline in urine) was significantly elevated for 12 months. Side-effects were mild, mostly attributed to fluid retention. In two patients with normal glucose tolerance at the start of the study, pathological glucose tolerance occurred in one patient and was impaired in one.
In conclusion, elderly patients with GHD respond to replacement therapy in a similar manner as younger subjects, with an improvement in body composition and an increase in markers for bone metabolism. Side-effects are few, and elderly GHD patients can be offered treatment. As long-term risks are unknown, GH doses should be titrated to keep IGF-I within the age-related physiological range.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Despite the numerous publications on the effects of GH replacement therapy in adults in recent years, there is a paucity of information on the characteristics and the response to replacement in elderly GHD patients. The GH production rate declines by age and from 40 yr of age decreases about 14%/decade, primarily due to a decrease in the amplitude of GH pulses (12, 13). Despite the low GH secretion in old age, there was a significant reduction of spontaneous GH secretion and the GH response to arginine when patients with pituitary disease were compared with age-matched healthy elderly subjects (14). This indicates that older patients with GHD may also benefit from GH therapy.
Previous studies in younger adults have shown a wide interindividual variation in the GH-induced changes in body composition and BMD, and there is a need for serum markers that can predict these effects. As many of anabolic GH effects are mediated via stimulation of insulin-like growth factor I (IGF-I), circulating IGF-I levels could be a putative marker. However, we have found only weak correlations between the levels of or changes in lean body mass (LBM) and IGF-I levels during GH replacement therapy (10). Moreover, at least in the short term (1 week), a low GH dose (2 µg/kg·day) was sufficient to normalize serum IGF-I without affecting whole body protein and lipid kinetics (15).
The majority of IGFs exists as complexes bound to the six known fully characterized IGF-binding proteins (IGFBPs), which modulate IGF actions in both a positive and a negative manner (16, 17). IGFBP-1, IGFBP-2, and IGFBP-4 have been found to inhibit IGF actions, whereas the GH-dependent binding proteins, IGFBP-3 and IGFBP-5, can enhance the effects of IGF. IGF-independent effects of IGFBPs have also been described (18, 19). BMD has been found to have a positive relationship to serum levels of IGFBP-5 during GH replacement therapy (20). The potential efficacy of IGFBP-5 and -4 as markers for the changes in LBM has not been evaluated.
The aim of the present work was to study baseline characteristics and the effects of GH replacement therapy on body composition, markers for bone metabolism, and BMD as well as on levels of IGFs and IGFBP-1 through -5 in patients with adult-onset GHD, aged 60–79 yr. In addition we wanted to evaluate whether the IGFs and IGFBP-1 through -5 could serve as markers for potential changes in body composition and BMD.
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Subjects and Methods |
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Thirty-one patients, aged 60–79 yr (6 women and 25 men), with
adult-onset pituitary disease with a known duration of 0.5–40 yr
(mean, 11 yr) participated (Table 1
).
They were treated at 3 centers in Sweden; Department of Endocrinology
and Diabetology, Karolinska Hospital, Stockholm; Department of
Endocrinology, University Hospital Malmo, Malmo; and Department of
Medicine, Umea University Hospital, Umea. Written informed consent was
obtained from each patient, and the study was approved by the regional
ethics committees and the Swedish Medical Product Agency.
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Study design and protocol
During the first 6 months of the study the patients were randomized, in a double blinded and parallel fashion, to inject either biosynthetic human GH (Humatrope, Eli Lilly & Co., Stockholm, Sweden) or placebo as a single sc injection once daily at bedtime. The study was then continued unblinded for another 12 months with GH treatment to the whole group. The starting dose was 0.05 IU/kg·week the first month of treatment and then increased to 0.1 IU/kg·week for another 5 months. Thereafter, i.e. 6 months after the start of the study, all patients were again treated with 0.05 IU/kg·week for 4 weeks to avoid breaking the blind protocol and then with 0.1 IU/kg·week for 11 months. The total daily GH dose ranged from 0.75–1.25 IU (mean, 0.92 ± 0.2). The doses of T4, cortisone acetate, and gonadal steroids were kept constant during the study.
The study was performed on an out-patient basis, and blood samples were drawn in the morning after an overnight fast. At the start of the study and after 1, 2, 3, 6, 7, 8, 9, 12, 15, and 18 months, a physical examination, including height, weight, blood pressure, heart rate, and side-effects were registered, and blood samples were collected for measurement of IGF-I, IGF-II, and IGFBP-1, -2, -3, -4, and -5 as well as routine chemistry tests. Markers for bone metabolism in serum and urine were analyzed in samples obtained before and after 3, 6, 9, 12, 15, and 18 months of treatment.
LBM, total body fat (TBF), and BMD of total body, lumbar spine, and proximal femur (femoral neck and trochanter region) were assessed before and after 6, 12, and 18 months of treatment. In 26 patients bone quality was also assessed by ultrasound of the calcaneus at the same points in time.
An oral glucose tolerance test (OGTT) was performed before the start of treatment and then at 6-month intervals. Blood glucose was assessed 2 h after ingestion of 75 g glucose. A 2-h blood glucose value below 7.8 mmol/L was considered as normal, 7.8–11.1 mmol/L was defined as impaired glucose tolerance, and values above 11.1 mmol/L were considered pathological.
Assays
IGF-I was determined in serum by RIA after separation of IGFs from IGFBPs by acid-ethanol extraction and cryoprecipitation and with des(1, 2, 3)IGF-I as the radioligand (22) to minimize interference of remaining IGFBPs in the extract. The detection limit was 8 µg/L. Including the extraction step, the intra- and interassay coefficients of variation (CVs) were 4% and 11%, respectively. The normal range of IGF-I, which declines by age, was established in 448 healthy subjects, aged 20–96 yr (23). The geometrical mean concentration was 227 µg/L at 20 yr of age (range, 159–481 µg/L), 135 µg/L at 65 yr of age (78–235 µg/L), and 115 µg/L at 75 yr of age (range, 66–200 µg/L). The IGF-I values were also expressed as SD scores calculated from the regression line of values in these subjects.
Levels of IGF-II in serum were determined by a specific RIA as described by Blum and co-workers (24).
IGFBP-1 was analyzed by the method described by Povoa and co-workers (25). The sensitivity of the RIA was 3 µg/L, and the intra- and interassay CVs were 3% and 10%, respectively. The reference range was 7–100 µg/L.
IGFBP-2 was measured by the RIA described by Blum and co-workers (26).
IGFBP-3 was measured by RIA using a commercially available RIA kit with a slight modification (DSL 6700, Diagostics Systems Laboratories, Inc., Webster, TX). The intra- and interassay CVs were described as 5% and 7%, respectively. Cross-reactivity with IGFBP-1, -2 and -4 was less than 0.3%. The mean ± SD was 2966 ± 439 µg/L in individuals 50–70 yr of age.
IGFBP-4 in serum was measured by a RIA described by Honda and co-workers (27). Purified recombinant IGFBP-4 expressed in Escherichia coli resin was used as antigen, tracer, and standard. Intra- and interassay variations were less than 5% and 8%, respectively. There was no cross-reactivity with IGFBP-1, -2, -3, -5, or -6. The mean ± SD serum IGFBP-4 values in healthy men and women in the age group 61–87 yr was 546 ± 135 µg/L.
IGFBP-5 in serum was measured by a specific RIA described by Mohan and co-workers, using recombinant human IGFBP-5 as antigen, tracer, and standard (28). Intra- and interassay CVs were less than 4% and 8%, respectively. In 30 healthy women, aged 23–85 yr, the mean ± SD was 417 ± 194 µg/L.
Insulin in serum was measured by a conventional RIA technique.
Osteocalcin was determined by a commercial RIA kit (Cis-Bio osteocalcin ELSA), from CIS (Gif-sur-Yvette Cedex, France), with normal values for men of less than 30 µg/L and for postmenopausal women of less than 50 µg/L. Carboxyl-terminal propeptide of type I procollagen (PICP) was determined by RIA using a RIA kit from Orion Diagnostica with reference intervals (mean ± 2 SD) of 40–200 µg/L for men and 50–170 µg/L for women. Bone-specific alkaline phosphatase (ALP) activity was assessed by an immunoradiometric assay (Tandem-R Ostase, Hybritech, Fullerton, CA), with a normal value (mean ± SD) for men of 12.3 ± 4.3 µg/L and for women of 11.5 ± 4.3 µg/L. Urinary pyridinoline was determined by high pressure liquid chromatography (29, 30) in morning urine samples.
Body composition
LBM and TBF were determined by dual energy x-ray absorptiometry (DXA; DPX-L, Lunar Corp., Madison, WI) according to a standard procedure described previously (31). The same type of calibration phantom (Lunar Corp.) was used at all participating centers. The coefficient of variation was 1.6% for LBM and 4.0% for TBF.
BMD
BMD was measured by DXA (DPX-L, Lunar Corp.) of the total body, the lumbar spine (L2–L4), and the proximal femur (femoral neck and trochanter region). The measurements were made according to the standard procedure previously described (31). The precision of the method was 0.4% for total body BMD, 0.5% for spine BMD (L2–L4), and 1.6% for hip BMD (femoral neck). The BMD values in the patients were compared with data from reference material provided by the manufacturers. The BMD values were expressed as areal BMD (grams per cm2) or as SD scores from the mean of age-matched reference material and SD scores from the mean of young adults (T scores).
Ultrasound measurements
Ultrasound measurements were made at the right and left os calcaneus using the Lunar Corp. Achilles ultrasound bone densitometer (32). The subject’s heel was placed in a water bath at 37 C between two ultrasonic transducers. An acoustic wave with broad frequency band (100–600 kHz; center frequency, 500 kHz) was applied. Broad band ultrasound attenuation in decibels per MHz and speed of sound (SOS) in meter per second were calculated from the characteristics of the received signals. Broad band ultrasound attenuation and SOS were combined by the analysis software into a composite stiffness index, which was expressed as a percentage of age-matched and of young normal values. Calculations were based on the mean of the right and left heels.
Statistics
Results are presented as the mean ± SEM if not otherwise stated. To determine treatment effects of GH compared with baseline, normally and nonnormally distributed data were analyzed by one-way repeated measures ANOVA or Friedman’s repeated measures ANOVA on ranks, respectively, both followed by Dunnett’s test. The results from the first 6-month placebo-controlled period are presented separately; the differences between baseline values and the values at 6 months within the group were assessed by paired t test. In the comparison between the GH group and the placebo group regarding changes in parameters, the unpaired t test or Mann-Whitney rank sum test was used. Correlations between normally distributed variables were assessed using least square linear regression analysis. Relationships between variables with nonnormal distribution were analyzed by Spearman rank order correlation test. Statistical significance was set at P < 0.05. Statistical analyses were performed using SigmaStat for Windows (Jandel Scientific GmbH, Erkrath, Germany).
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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). At inclusion there was no difference between the
groups in age (mean, 67.8 and 68.6 yr) and body mass index (25.3 and
25.7 kg/m2). There was no difference between the
groups regarding the etiology of pituitary insufficiency with 13
patients with pituitary adenomas in the GH group and 12 in the placebo
group. Panhypopituitarism was found in 12 and 11 patients,
respectively. Twenty-eight patients completed the whole study period.
One patient who had history of intermittent claudication stopped
treatment after 10 months due to vascular surgery of arterial
insufficiency in a leg that subsequently led to an amputation. One
patient withdrew without stating the reason after 15 months, and 1
patient did not take her GH injections the last month; all 3 of these
patients received placebo the first 6 months and had active treatment
for 4, 9, and 11 months, respectively. Thus, for data analysis, all
patients were included during first 6-month placebo controlled period
(15 patients receiving GH treatment and 16 patients receiving placebo).
The effect of GH was then evaluated in the combined group of 28
patients receiving GH treatment for at least 12 months, and 15 patients
comprised the group with active GH treatment for 18 months. In the
placebo group, the visit at 6 months was considered comparable to the
starting visit in the GH group. Serum IGF, IGFBP, and insulin before and during treatment
At baseline, all patients had IGF-I levels below the normal mean
for age, and two thirds had IGF-I levels below –2 SD (Fig. 1). IGF-II was below the normal mean in
16 individuals and 5 of them had values below -2 SD. Of
the GH-dependent IGFBPs, IGFBP-3 concentrations were below -2
SD in 26, and IGFBP-5 levels were subnormal in 11 of the of
the 31 patients. In addition, 16 of the patients had subnormal IGFBP-4
levels, whereas IGFBP-1, IGFBP-2, and insulin levels were within normal
range. The levels of IGFBP-1 and insulin were negatively correlated
(r = -0.51; P < 0.01), whereas the expected
positive correlations were found between the GH-dependent peptides,
i.e. IGF-I and IGFBP-3 (r = 0.84; P <
0.001), IGF-I and IGFBP-5 (r = 0.61; P < 0.001),
and IGFBP-5 and IGFBP-3 (r = 0.62; P < 0.001). In
addition, there was a positive correlation between IGFBP-4 and IGFBP-5
(r = 0.53; P < 0.01).
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). With
the two groups combined, IGF-I increased during 12 months of treatment
from a mean of 65.1 ± 5.5 µg/L to levels normal for age in the
majority of the patients (mean, 145 ± 12 µg/L).
Subnormal IGF-I levels at 12 months were found in three patients, of
whom one was female (Fig. 1). The increase in IGF-I concentrations was
sustained throughout the study (Fig. 2).
IGF-II increased 21 ± 3.4% to values normal for age at 12 months
and did not change thereafter. The mean levels were 489 ± 27,
598 ± 31, and 571 ± 39 µ/L at baseline, 12 months, and 18
months, respectively. IGFBP-3 had increased by 60 ± 7.4% after
12 months of treatment and did not change thereafter (Fig. 2). The mean
molar ratio of IGF-I/IGFBP-3 increased from 0.269 at baseline to 0.389
at 12 months and 0.415 at 18 months (P < 0.001).
IGFBP-5, increased 30 ± 7.4% over 12 months, and the levels were
similar at 18 months (Fig. 2). IGFBP-4 increased 9.3 ± 5.0% over
12 months and was unaltered thereafter (Fig. 2). At the start of
treatment IGFBP-1 was 36.4 ± 5.5 µg/L, and IGFBP-2 was 525
± 47 µg/L. Both were roughly within the normal range, and neither
changed significantly seen over the 12/18-month period (Fig. 2). The
fasting insulin level at baseline was 55.2 ± 5.7 pmol/L and rose
to 68.9 ± 6.5 pmol/L after 6 months of GH treatment. No further
rise was observed. The changes from baseline in IGF-I; IGF-II; IGFBP-3,
-5, and –4; and insulin were significant at all times.
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During the first 6 months of the study there was no change in LBM in the placebo group; the mean values were 46.6 ± 2.7 and 46.9 ± 2.8 kg at baseline and 6 months, respectively. In the GH-treated group, LBM increased from 52.1 ± 1.6 to 54.5 ± 2.1 kg; the increase was significant (P < 0.05) compared to the change in the placebo group.
In the combined group (n = 28) significant elevations in LBM were
found at all time points. At 6 months the mean increase was 1.9 ±
0.4 kg, at 12 months it was 1.8 ± 0.4 kg, and at 18 months it was
2.0 ± 0.6 kg, all compared with baseline. However, when an
increase in LBM of 0.5 kg was arbitrarily defined as a significant
response with consideration taken to practical clinical purposes and
reproducibility, 8 of the 28 patients did not respond with an increase
in LBM after 12 months of treatment. IGF-I levels at the start of the
study were lower in the nonresponding group (51 ± 11
vs. 70 ± 6 µg/L) and did not increase as much as in
the responding group over 12 months (32 ± 8 vs.
100 ± 9 µg/L). LBM changes were independent of the age of the
patient, and responders and nonresponders had similar ages (68 ±
1.2 vs. 69 ± 2.2 yr). When the results from males and
females were analyzed separately, it was found that mean LBM did not
change in the females. Only 2 of the 5 females responded with an
increase in LBM after 12 months (0.8 and 1.1 kg, respectively), whereas
18 of the 23 males during the same time showed an increase, ranging
from 0.5–6.3 kg. The mean LBM increase in the males were 2.3 ±
0.5, 2.2 ± 0.4, and 2.1 ± 0.6 kg at 6, 12, and 18 months,
respectively (Fig. 3).
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). LBM and its changes did not
correlate to any of the IGFBPs.
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). TBF and its changes had no
relationship to serum IGFs and binding proteins. Body mass index and
waist/hip ratio remained unchanged. BMD and markers for bone metabolism
At baseline, mean BMD measured with DXA was not different from that found in healthy age- and sex-matched subjects. Mean levels, expressed as the SD score of age-matched reference material, were -0.02 ± 0.27 in the lumbar spine, -0.17 ± 0.18 in the femoral neck, 0.16 ± 0.22 in the trochanter, and -0.33 ± 0.21 for the total body BMD. In the six female patients mean BMD values at the various sites were similar to those in the males. There was no significant relationship between BMD and the known duration of pituitary disease. When the BMD values in the patients were compared with those in young adults, 15 of 31 patients (48%) fulfilled the WHO definition for osteoporosis, i.e. a BMD below -2.5 SD score from the mean of young adults at any measured site. The range was -4.3–1.79 SD score for total body measurements, -4.4–0.43 at the femoral neck, -3.47–1.27 SD score at the trochanter, and -3.75–1.92 SD score at L2–L4. BMD did not change at any site measured during treatment. At baseline, trochanter BMD showed a correlation to IGF-I (r = 0.37; P < 0.05) and IGFBP-5 (r = 0.37; P < 0.05), whereas femoral neck BMD correlated to IGF-I only (r = 0.38; P < 0.05).
In the heel ultrasound measurements at baseline, the mean speed of sound was 1511 ± 6 m/s, and the broad band ultrasound attenuation was 111 ± 2 decibels/MHz. The stiffness index was -0.73 ± 0.23 SD score compared with age-matched controls, with a range between -3.14 and 1.26. Twelve of 25 patients (48%) had a stiffness index below -2.5 SD score from the mean of young adults. Individual stiffness indexes correlated to total BMD (r = 0.64; P < 0.001), lumbar spine BMD (r = 0.39; P < 0.05), femoral neck BMD (r = 0.58; P < 0.01), and trochanter BMD (r = 0.72; P < 0.001).
None of the ultrasound measurements changed significantly during treatment (not shown). The SOS correlated to IGF-I in serum (r = 0.39; P < 0.05) at baseline.
Normal values for all of the markers of bone metabolism were found at
baseline, and no patient reached supranormal values during treatment.
None of the markers changed in the placebo group during the 6-month
placebo period, whereas a significant increase was found in the
GH-treated patients (Table 2).
In the combined group (n = 28), mean values of the formation
markers osteocalcin, PICP, and bone-specific ALP were significantly
increased at 6, 12, and 18 months compared with baseline values (Fig. 4). The resorption marker urinary pyridinoline was significantly
increased from baseline at 6 and 12 months, but not at 18 months (Fig. 5). The changes were similar in males and females.
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One patient had impaired glucose tolerance at the start of the study and at 12 months, but not at 6 and 18 months. In another patient glucose tolerance was impaired at study start and at 6 months, and the subject was diabetogenic at 12 months (2 h blood glucose, 11.2 mmol/L), which was this patient’s last visit. Of the patients with normal OGTT at the start of the study, one patient showed impaired glucose tolerance at 18 month only, and one patient had pathological blood glucose at 12 months (2 h blood glucose, 11.5 mmol/L).
Mild to moderate side-effects, probably due to fluid retention, were observed in eight patients during GH treatment and in one patient during placebo therapy, including peripheral edema, joint stiffness, and muscle pain, which all subsided spontaneously or after a minor dose reduction.
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Discussion |
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The mean changes in body composition seen in our patients, in particular concerning fat mass with unchanged waist/hip ratio, are smaller than those found in previous studies in younger subjects. In general, a mean increase in LBM of 2–5.5 kg and a mean decrease in fat mass of approximately 4–6 kg were seen (1). However, the doses employed were often more than twice as high per kg BW as those used in our study, leading to unphysiologically high IGF-I serum concentrations (7, 8). In one study using dose titration to keep IGF-I concentrations within the normal range, the effect on body cell mass was similar to that found in the present study (34). Furthermore, it has been reported that LBM is not reduced in elderly GHD patients compared with healthy age-matched controls (35). Possibly factors other than GH may be relatively more important for the maintenance of LBM in the elderly GHD patients. They may also be more resistant to the lipolytic effect of GH, in view of the modest effect of GH therapy on adipose tissue in the present study.
When women and men in the present study were analyzed separately, it was found that only 2 of the 5 females had increased LBM at 12 months compared with 18 of the 23 males. The same dose per kg BW was given to men and women. It has previously been shown that females are less sensitive to GH replacement therapy than males, and the time until the effect occurs is longer (36, 37, 38). Regrettably, the number of females in the present study was too small to assess differences in response between gender statistically.
The limitations in measuring LBM by DXA should also be recognized. It is built on the assumption of a constant hydration of lean tissue, and fluid retention may lead to overestimation of LBM (39, 40). However, in the present study using low GH doses, the problems with fluid retention were mild or absent, and body weight did not change.
BMD values varied widely, but the mean levels were not different from those found in healthy age- and sex-matched individuals. This is in accordance with the findings of Toogood et al. (41). Previous studies have shown that BMD is preserved to a greater extent in adult-onset GHD compared with childhood-onset GHD (1).
BMD did not change during 12–18 months of treatment in our elderly GHD patients. In younger patients, a number of investigations found reduced BMD after 6–12 months, but an increase of 4–10% after more than 12 months has been demonstrated (1). Thus, a longer observation time is needed for a definite assessment of the effect on BMD. In addition, it has been shown that the gain in BMD is higher when the initial BMD is low (11). In our patients with normal BMD from the start of the study, only a modest increase in BMD can be anticipated even in the long-term perspective.
In the ultrasound measurements of calcaneus, the stiffness index was reduced below -2.5 SD from the mean of young adults in the same percentage of patients as with the DXA measurements, and there were significant positive correlations between stiffness index and BMD at all sites. Stiffness index was positively related to IGF-I levels. Ultrasound measures clinically relevant properties of bone strength distinct from bone density. In vitro studies have shown that broad band ultrasound attenuation values correlate with histomorphometric parameters of trabecular structure. In contrast, speed of sound measurements reflect elastic properties of bone (42). These variables were not altered by GH treatment in our patients. In one study phalangeal ultrasound transmission velocity showed an initial decrease followed by a return to baseline (43).
In contrast to the study by Toogood et al. (41), who found decreased levels of osteocalcin and deoxypyridinoline, indicating reduced bone turnover in elderly GHD patients, baseline values of markers for bone metabolism were normal in our patients. We do not know the reason for this. It may be due to differences in methodology and reference material.
We found that the markers for bone metabolism were significantly increased within 3 months of therapy, indicating activation of bone remodeling. The increase in bone formation markers was sustained throughout 18 months, whereas the bone resorption marker urinary pyridinoline was significantly elevated until 12 months. Increase in markers of bone metabolism has been consistently reported from trials in younger patients (1). We found in younger patients that the levels peaked at 6–12 months and had returned to baseline at 36 months (44). The magnitude of the increase in our elderly patients appears similar to that in younger subjects.
In our patients, in addition to normalized IGF-I, there was a normalization of IGF-II and the GH-dependent IGFBPs, IGFBP-3 and IGFBP-5, during GH replacement therapy.
An age-related decline in circulating IGFBP-5 concentrations has been described in healthy subjects, possibly attributed to the declining GH secretion (13, 45). In contrast, IGFBP-4 concentrations tend to increase with age (27). In our GHD patients, IGFBP-4 levels also appeared higher in our older than in our previously studied younger subjects, with a mean of 418 ± 19 vs. 223 ± 23 µg/L (20). This indicates that an age-related rise in IGFBP-4 levels may also be present in GHD patients. Thus, a negative relationship between the two binding proteins is expected. We found, however, a positive correlation between IGFBP-4 and IGFBP-5 levels in our GHD patients. Possibly, the age-related pattern of IGFBP-5 is different when GH secretion is deficient. In addition, IGFBP-4 increased during GH therapy and that is in accordance with our findings in younger GHD patients. The physiological significance of the elevation of this binding protein, which is known to inhibit IGF actions, is unclear. It is established that IGFBP proteases play an important role in modulating IGFBP action (16). The antibodies used in the present study to measure IGFBP-4 detect both intact and fragment forms, and it is not known which forms are increased by GH treatment. Thus, the rise in IGFBP-4 can represent both synthesis and degradation. However, it has recently been found that IGFBP-4 given systemically in mice stimulates bone formation markers (46). A possible mechanism may be that more IGF-I can bind in a binary complex with IGFBP-4 and constitute a more readily available pool of IGF-I than the ternary complex with IGFBP-3 and the acid-labile subunit, which is confined to the intravascular space (16).
A decrease in IGFBP-1 was seen during the first 6 months. This probably reflected the increase in insulin, as IGFBP-1 is known to be negatively regulated by insulin (18, 19, 47). Over the whole treatment period, no significant alterations in IGFBP-1 were found. A similar pattern was seen with IGFBP-2. Although within the normal range in the present study, IGFBP-2 has been shown to be increased in GHD and decreased with GH replacement therapy (19).
As IGF-I mediates many of the anabolic effects of GH, and IGFBPs can modulate these effects, we investigated the relationships between LBM and serum IGFs and binding proteins. A positive correlation between LBM and IGF-I was found at baseline, 6 months, and 12 months. The increase in LBM was also correlated with the increase in IGF-I. IGF-I could thus be considered a marker for LBM changes during GH replacement. However, as some patients failed to respond with increase in LBM despite showing an increase in IGF-I, IGF-I does not seem to be an appropriate marker in the clinical setting. Relationships between LBM and serum IGFBP-1, -2, -3, -4, and -5 were not found. Thus, other markers for the changes in body composition should be sought.
IGFBP-5 is abundantly found in human bone and has been shown to stimulate IGF actions (18, 19). We have previously found positive correlations between BMD in femur (shaft and trochanter) and serum IGFBP-5 in younger GHD subjects (20). After long-term GH therapy when BMD had normalized, these associations were no longer found. In our older GHD patients with normal mean BMD, an association between trochanter BMD and IGFBP-5 was only found at baseline.
With the GH doses used (0.75–1.25 IU daily), serum IGF-I concentrations obtained were largely within the age-related normal range. It seems appropriate to administer a GH dose leading to IGF-I levels that do not exceed the upper physiological age-related range. It is possible that an IGF-I elevation above the normal range may have a greater impact in older than in younger patients due to their higher background risk of malignancy. In addition, population studies have indicated an association between serum IGF-I levels and prostate cancer risk, which has recently been reviewed (48). However, a causal relationship is not proven. IGFBP-3 is a strong inhibitor of IGF action in the prostate and even induces apoptosis (48, 49). With GH therapy, both IGF-I and IGFBP-3 increase, although the ratio of IGF-I/IGFBP-3 also increased in our patients. Patients with acromegaly have no increase in prostate cancer (48). As GHD patients often have low IGF-I, which normalizes during treatment, it does not seem likely at present that the cancer risk will increase above that in the normal population. Side-effects were few, mainly attributed to fluid retention, and subsided spontaneously or with minor dose reduction. Two patients who were normal at the start of the study had a deterioration of glucose tolerance.
In conclusion, elderly GHD patients respond to GH replacement therapy in a similar manner as younger patients with beneficial effects on body composition and increased bone metabolism. Although the increase in LBM correlated with the increase in IGF-I, the wide variation precluded IGF-I as a marker for change in LBM. There was no association between IGFBPs and LBM. With GH doses leading to IGF-I levels mostly within the age-related physiological range, side-effects were few, and elderly GHD patients can be considered for replacement therapy.
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Acknowledgments |
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Footnotes |
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Received January 5, 2000.
Revised July 17, 2000.
Accepted July 25, 2000.
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References |
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Male patients; •, female patients. The normal range for age (the
geometrical mean ± 2 SD) is indicated.
