Insulin-like growth factor-1 and growth hormone (GH) have distinct and overlapping anabolic effects in GH-deficient rats

Growth Hormone Upregulates Mediators of Melanoma Drug Efflux and Epithelial-to-Mesenchymal Transition In Vitro and In Vivo

Simple Summary

Growth hormone (GH) action is strongly implicated in the progression and therapy resistance in several types of solid tumors which overexpress the GH receptor (GHR). The aim of our study was to characterize the effects of GH and its downstream effector insulin-like growth factor 1 (IGF-1) on melanoma using in vitro and in vivo models. We confirmed an IGF-1-independent role of elevated circulating GH in upregulating key mechanisms of therapy resistance and malignancy with analyses conducted at the molecular and cellular level. We identified that GH upregulates key mechanisms of therapy resistance and metastases in melanoma tumors in an IGF-1 dependent and independent manner by upregulating multidrug efflux pumps and EMT transcription factors. Our study reveals that GH action renders an intrinsic drug resistance phenotype to the melanoma tumors—a clinically crucial property of GH verifiable in other human cancers with GHR expression.

Abstract

Growth hormone (GH) and the GH receptor (GHR) are expressed in a wide range of malignant tumors including melanoma. However, the effect of GH/insulin-like growth factor (IGF) on melanoma in vivo has not yet been elucidated. Here we assessed the physical and molecular effects of GH on mouse melanoma B16-F10 and human melanoma SK-MEL-30 cells in vitro. We then corroborated these observations with syngeneic B16-F10 tumors in two mouse lines with different levels of GH/IGF: bovine GH transgenic mice (bGH; high GH, high IGF-1) and GHR gene-disrupted or knockout mice (GHRKO; high GH, low IGF-1). In vitro, GH treatment enhanced mouse and human melanoma cell growth, drug retention and cell invasion. While the in vivo tumor size was unaffected in both bGH and GHRKO mouse lines, multiple drug-efflux pumps were up regulated. This intrinsic capacity of therapy resistance appears to be GH dependent. Additionally, epithelial-to-mesenchymal transition (EMT) gene transcription markers were significantly upregulated in vivo supporting our current and recent in vitro observations. These syngeneic mouse melanoma models of differential GH/IGF action can be valuable tools in screening for therapeutic options where lowering GH/IGF-1 action is important.

A rationale for the treatment of short stature in children with the combination of recombinant human growth hormone (rhGH) and recombinant human insulin-like growth factor-I (rhIGF-I)

Both rhGH and rhIGF-I are signaling molecules with the capacity to restore the rate of growth in certain subsets of slowly growing children. In some instances, heights attained at or near the time of cessation of linear growth are indistinguishable from the height distribution of the community as a whole or from the height distribution expected based on the heights of biological parents. The GH: IGF-I signaling system is sequential, forming a continuous loop wherein GH will stimulate production of IGF-I and IGF-I will inhibit production of GH. This feature suggests that a deficiency of GH will be accompanied by a deficiency of IGF-I and that treatment of GH deficiency with rhGH will restore IGF-I and the subnormal growth of combined GH: IGF-I deficiency. Although logical, this proposition is not always true. rhGH and rhIGF-I are distinct polypeptides, with distinct cell surface receptors and distinct intracellular signaling pathways both capable of amplifying distinct, yet overlapping, patterns of gene replication, protein synthesis and metabolic activities. These features suggest that neither treatment with rhGH nor rhIGF-I alone will invariably recapitulate the combined activities of the GH: IGF-I system, At the present time, this proposition appears both logical and true. The possibility that combined rhGH and rhIGF-I treatment can accomplish that which neither monotherapy can has been examined in gene knock-out experiments in animals and direct comparisons of GH, IGF-I and combined GH: IGF- treatments in animals and in children with short stature, normal GH and low IGF-I (primary IGF-I deficiency). In these experimental models, the growth rates with combined rhGH and rhIGF-I treatment exceed those of either monotherapy. The extent to which this proposition can be generalized to various short stature populations remains to be determined.

GH directly stimulates UCP3 expression

OBJECTIVE

We evaluated the direct action of GH signaling in energy homeostasis in myocytes.

DESIGN

We investigated the GH-induced expression of UCP3 in human embryonic kidney 293 cells, human H-EMC-SS chondrosarcoma cells, murine C2C12 skeletal muscle myoblasts, and rat L6 skeletal muscle cells, as well as its direct effect on the GHR/JAK/STAT5 pathway using a combination of a reporter assay, real-time quantitative polymerase chain reaction, and western blotting.

RESULTS

We demonstrated that the regulation of energy metabolism by GH involves UCP3 via activated STAT5, a signal transducer downstream of GH. UCP3 expression increased with STAT5 in a dose-dependent manner and was higher than that of UCP2. We confirmed the functional STAT5 binding site consensus sequences at -861 and -507 bp in the UCP3 promoter region.

CONCLUSION

The results suggest that GH stimulates UCP3 directly and that UCP2 and that UCP3 participate in the signal transduction pathway that functions downstream of the GHR/JAK/STAT.

Recombinant IGF-I: Past, present and future

Normal linear growth in humans requires GH and IGF-I. Diminished GH action resulting in reduced availability of IGF-I and IGF-binding proteins is the hallmarks of GH Insensitivity Syndromes (GHIS). The deficiencies are the perceived mechanisms for the growth failure of affected patients and the therapeutic targets for the restoration of normal growth. Early treatment attempts with pituitary-derived GH had limited effects in GHIS patients. Recombinant human insulin-like growth factor-I (rhIGF-I) treatment initially provides accelerated growth to GHIS children and provides substantial benefit. But, in general, catch up growth is less substantial with rhIGF-I treatment of GHIS than with rhGH treatment of GH Deficiency. Few classic GHIS patients have reached heights in the normal range (height SD score between -2.0 SD and +2.0 SD) with rhIGF-I monotherapy. A potential explanation is that while rhIGF-I treatment increases circulating concentrations of IGF-1 and IGFBP-3, such treatment reduces endogenous GH levels by negative feedback inhibition of pituitary GH release. In as much as both GH and IGF-I are required for good catch up growth, the loss of any residual GH signaling during IGF-I monotherapy in GHIS patients may attenuate possible catch up growth. Consistent with this explanation is the finding that, as predicted by the preclinical studies by Ross Clark, combination of rhGH & rhIGF-1 provides better growth responses than rhIGF-1 monotherapy in prepubertal children with short stature and low IGF-I levels despite normal stimulated GH responses. In the future, rhGH and rhIGF-I combination therapy can potentially improve growth outcomes over that seen with rhIGF-I monotherapy in all GHIS patients except in those with a total lack of functional GH signaling. Future alternative treatments for GHIS subjects may also include the use of post-growth hormone receptor signaling agonists which restore both GH signaling and IGF-I exposures or the addition of long-acting rhGH species to rhIGF-I. Additional etiologic factors for the growth failure in GHIS should be considered if the growth deficits of GHIS do not resolve with treatment.

Recombinant human growth hormone plus recombinant human insulin-like growth factor-1 coadministration therapy in short children with low insulin-like growth factor-1 and growth hormone sufficiency: results from a randomized, multicenter, open-label, parallel-group, active treatment-controlled trial

BACKGROUND/AIMS

Growth hormone (GH) and insulin-like growth factor-1 (IGF-1) both contribute to growth. To determine if recombinant human (rh)GH + rhIGF-1 therapy is more effective than rhGH alone to treat short stature, we assessed the efficacy and safety of coadministered rhGH + rhIGF-1 in short children with GH sufficiency and low IGF-1.

METHODS

In a 3-year, randomized, multicenter, open-label trial, patients with height SD score ≤-2.0 and IGF-1 SD score ≤-1.0 for age and sex, and with stimulated GH ≥10 ng/ml for age and sex, were randomized to receive (all doses in µg/kg/day): 45 rhGH alone (group A), 45 rhGH + 50 rhIGF-1 (group B), 45 rhGH + 100 rhIGF-1 (group C) or 45 rhGH + 150 rhIGF-1 (group D). Height velocity (HV) and Δ height SD score were measured.

RESULTS

The first-year HV (modified intention-to-treat population) was 9.3 ± 1.7 cm/year (group A), 10.1 ± 1.3 cm/year (group B), 9.7 ± 2.5 cm/year (group C) and 11.2 ± 2.1 cm/year (group D) (p = 0.001 for groups A vs. D). This effect was sustained, resulting in a height SD score improvement during the second and third years. Most treatment-emergent adverse events were mild and transient.

CONCLUSION

In children with short stature, GH sufficiency and low IGF-1, coadministration of rhGH/rhIGF-1 (45/150 µg/kg) significantly accelerated linear growth compared with rhGH alone, with a safety profile similar to the individual monotherapies.

Insulin-Like Growth Factor-Independent Effects of Growth Hormone on Growth Plate Chondrogenesis and Longitudinal Bone Growth

GH stimulates growth plate chondrogenesis and longitudinal bone growth directly at the growth plate. However, it is not clear yet whether these effects are entirely mediated by the local expression and action of IGF-1 and IGF-2. To determine whether GH has any IGF-independent growth-promoting effects, we generated (TamCart)Igf1r(flox/flox) mice. The systemic injection of tamoxifen in these mice postnatally resulted in the excision of the IGF-1 receptor (Igf1r) gene exclusively in the growth plate. (TamCart)Igf1r(flox/flox) tamoxifen-treated mice [knockout (KO) mice] and their Igf1r(flox/flox) control littermates (C mice) were injected for 4 weeks with GH. At the end of the 4-week period, the tibial growth and growth plate height of GH-treated KO mice were greater than those of untreated C or untreated KO mice. The systemic injection of GH increased the phosphorylation of Janus kinase 2 and signal transducer and activator of transcription 5B in the tibial growth plate of the C and KO mice. In addition, GH increased the mRNA expression of bone morphogenetic protein-2 and the mRNA expression and protein phosphorylation of nuclear factor-κB p65 in both C and KO mice. In cultured chondrocytes transfected with Igf1r small interfering RNA, the addition of GH in the culture medium significantly induced thymidine incorporation and collagen X mRNA expression. In conclusion, our findings demonstrate that GH can promote growth plate chondrogenesis and longitudinal bone growth directly at the growth plate, even when the local effects of IGF-1 and IGF-2 are prevented. Further studies are warranted to elucidate the intracellular molecular mechanisms mediating the IGF-independent, growth-promoting GH effects.

Genetic determination of the cellular basis of the ghrelin-dependent bone remodeling

OBJECTIVE

Bone mass is maintained through a balance of bone formation and resorption. This homeostatic balance is regulated by various systems involving humoral and local factors. The discovery that the anorexigenic hormone leptin regulates bone mass via neuronal pathways revealed that neurons and neuropeptides are intimately involved in bone homeostasis. Ghrelin is a stomach-derived orexigenic hormone that counteracts leptin's action. However, the physiological role of ghrelin in bone homeostasis remains unknown. In this study, through the global knockout of ghrelin receptor (Ghsr) followed by tissue-specific re-expression, we addressed the molecular basis of the action of ghrelin in bone remodeling in vivo.

METHODS

We performed molecular, genetic and cell biological analyses of Ghsr-null mice and Ghsr-null mice with tissue specific Ghsr restoration. Furthermore, we evaluated the molecular mechanism of ghrelin by molecular and cell-based assays.

RESULTS

Ghsr-null mice showed a low bone mass phenotype with poor bone formation. Restoring the expression of Ghsr specifically in osteoblasts, and not in osteoclasts or the central nervous system, ameliorated bone abnormalities in Ghsr-null mice. Cell-based assays revealed ghrelin induced the phosphorylation of CREB and the expression of Runx2, which in turn accelerated osteoblast differentiation.

CONCLUSIONS

Our data show that ghrelin regulates bone remodeling through Ghsr in osteoblasts by modulating the CREB and Runx2 pathways.

Growth Hormone and Insulin-like Growth Factor 1: New Endocrine Therapies in Cardiology?

The hormones growth hormone (GH) and insulin-like growth factor 1 (IGF-1) play a dominant role in whole body growth and metabolism. This is reflected in the use of human GH (hGH) in GH-deficient children to stimulate growth and in GH-deficient adults to reduce visceral fat mass. Recent data suggest that hGH may improve cardiac function in patients with heart failure, so there is current interest in methods to raise GH-IGF levels, including the testing of agents that release GH from the pituitary, administering IGF-1, and most recently, long-acting formulations of hGH. It is hoped that this ongoing integration of cardiology and endocrinology will uncover the pathophysiology of some cardiovascular diseases and yield new treatments based on the hormones of the GH axis. (Trends Cardiovasc Med 1997;7:264-268). © 1997, Elsevier Science Inc.

Recombinant insulin-like growth factor-1 as a therapy for IGF-1 deficiency in renal failure

Renal disease in children disrupts the growth hormone (GH) and insulin-like growth factor (IGF) axis and causes growth failure. Although GH therapy stimulates growth in these children, their short stature is likely due to a form of IGF-1 deficiency (IGFD) rather than GH deficiency. Recent experimental data have caused us to reconsider the importance of IGF-1 and IGFD to human growth. Pharmacology studies in rodents, as well as studies in patients with no functional GH receptors and primary IGFD, have shown that IGF-1 is an effective growth-promoting therapy. Gene knockout studies in mice have shown that IGF-1, rather than GH, is the major hormone controlling growth. In addition, both pharmacological and genetic studies have shown that there are effects of GH and IGF-1 that require their combined presence. In children with primary IGFD, where there is no GH signaling, recombinant human (rh)IGF-1 produces a large growth response, while in children who are GH and IGF-1 deficient, treatment with rhGH is the most-appropriate therapy. Children with short stature due to renal failure are GH sufficient and have some GH receptor signaling capacity, so that rhIGF-1, or rhIGF-1 plus rhGH, are logical therapeutic options and merit clinical testing.

Control of growth by the somatropic axis: growth hormone and the insulin-like growth factors have related and independent roles

The traditionally accepted theory has been that most of the biological effects of growth hormone (GH) are mediated by circulating (endocrine) insulin-like growth factor-I (IGF-I). This dogma was modified when it was discovered that most tissues express IGF-I that can act via an autocrine/paracrine fashion. In addition, both GH and IGF-I had independent effects on various target tissues. Using tissue-specific gene deletion of IGF-I in the liver, it has been shown that circulating IGF-I is predominantly liver-derived but is not essential for normal postnatal growth. Therefore, it is proposed that non-hepatic tissue-derived IGF-I may be sufficient for growth and development. Thus the original somatomedin hypothesis has undergone further modifications.

The somatomedin hypothesis: 2001

Since the original somatomedin hypothesis was conceived, a number of important discoveries have allowed investigators to modify the concept. Originally somatic growth was thought to be controlled by pituitary GH and mediated by circulating insulin-like growth factor-I (IGF-I, somatomedin C) expressed exclusively by the liver. With the discovery that IGF-I is produced by most, if not all, tissues, the role of autocrine/paracrine IGF-I vs. the circulating form has been hotly debated. Recent experiments using transgenic and gene-deletion technologies have attempted to answer these questions. In the liverspecific igf-1 gene-deleted mouse model, postnatal growth and development are normal despite the marked reduction in circulating IGF-I and IGF-binding protein levels; free IGF-I levels are normal. Thus, the normal postnatal growth and development in these animals may be due to normal free IGF-I levels (from as yet unidentified sources), although the role of autocrine/paracrine IGF-I has yet to be determined.

Comparison of methods of analysis of body composition in hypophysectomized rats treated with rat growth hormone

The present study compared estimates of body composition derived from dual-emission X-ray absorptiometry (DEXA) and from chemical analyses. The primary aim was to compare the two methods because growth hormone (GH) may cause fluid retention, and DEXA does not distinguish water from lean mass. Hypophysectomized rats were fed ad libitum and were treated with continuous infusions of rat GH in doses of 0, 10, 30, and 100 microg/day for 14 days. By chemical analysis, a decrease in percentage fat from 12.9% in the control group to 11.3%, 11.0%, and 10.2% in the low, medium, and high dose groups was observed (P < 0.0001). The fat percentages were about 3-4% higher by DEXA, but showed the same decline (P < 0.03). Lean mass increased from 74.4% in the control group to 75.8%, 78.0%, and 78.6% in the treatment groups (P < 0.001). A significant increase in the wet weight of the quadriceps muscle, but no difference in dry weight was observed in all four treatment groups, indicating that the increase in muscle weight was exclusively caused by water. This accumulation of water was reflected in the total water content of the carcasses, which increased from 62.0% in the control group to 64.9%, 66.1%, and 66.8% in the GH groups (P < 0.0001). The protein content decreased from 19.8% in the control group to 19.4%, 19.1%, and 18.9% in the GH groups (P < 0.001). Regardless of the decrease in protein, the GH treated groups contained more water in relation to protein as the g water/g protein ratio was increased by 13% from 3.14 in the control group to 3.55 in the group treated with the highest GH dose (P < 0.0001). Also, a close relationship between feed intake and body weight were found, together with increases in epiphyseal growth plate width, insulin-like growth factor I (IGF-I), and insulin-like growth factor binding protein 3 (IGFBP-3). In conclusion, the study shows that estimation of lean mass by DEXA should be carefully evaluated when used in connection with treatment of drugs that cause water retention.

Inactivation of the acid labile subunit gene in mice results in mild retardation of postnatal growth despite profound disruptions in the circulating insulin-like growth factor system

Insulin-like growth factors (IGFs) I and II are important regulators of cell proliferation and differentiation. After birth, plasma IGFs, representing mostly liver-derived IGFs, circulate in ternary complexes of 150 kDa consisting of one molecule each of IGF, IGF-binding protein (IGFBP) 3, and an acid labile subunit (ALS). Onset of ALS synthesis after birth is the primary factor driving the formation of ternary complexes. Capture of IGFs by ALS is thought to allow the development of a plasma reservoir without negative effects such as hypoglycemia and cell proliferation. To evaluate the importance of ALS and ternary complexes, we have created mice in which the ALS gene has been inactivated. The mutation was inherited in a Mendelian manner, without any effects on survival rates and birth weights. A growth deficit was observed in null mice after 3 weeks of life and reached 13% by 10 weeks. This modest phenotype was observed despite reductions of 62 and 88% in the concentrations of plasma IGF-I and IGFBP-3, respectively. Increased turnover accounted for these reductions because indices of synthesis in liver and kidney were not decreased. Surprisingly, absence of ALS did not affect glucose and insulin homeostasis. Therefore, ALS is required for postnatal accumulation of IGF-I and IGFBP-3 but, consistent with findings supporting a predominant role for locally produced IGF-I, is not critical for growth. This model should be useful to determine whether presence of ALS is needed for other actions of liver-derived IGF-I and for maintenance of homeostasis in presence of high circulating levels of IGF-II.

Long-term growth after hypophyseal stalk transection and hypophysectomy of beef calves

Hypothalamic hormones regulate episodic and basal secretion of hormones from the anterior pituitary gland that affect metabolism and growth in cattle. This study focused on long-term growth in young calves subjected to hypophysectomy (HYPOX), hypophyseal stalk transection (HST), and sham operation control (SOC). Cross-bred (Hereford x Aberdeen Angus) and Hereford, and Aberdeen Angus calves were HYPOX (n = 5), HST (n = 5), or SOC (n = 8) at 146 +/- 2 days of age, whereas another group was HST (n = 5) or SOC (n = 7) at 273 +/- 5 days of age. Body weight was determined every 21 days from birth to 1008 days of age. Anterior vena cava blood was withdrawn at 4-day intervals from day 64-360 for RIA of GH, TSH, T4, T3, and LH, and at 20-min intervals for 480 min to determine episodic hormone secretion. Daily feed intake was determined in HST and SOC calves during an 80-day period. Birth weight averaged 35 +/- 1 kg (+/- SE) and was 142 +/- 4 kg at 126 days and 208 +/- 8 kg at 252 days before surgery. From day 146-1008, growth was arrested (P < 0.001) in HYPOX (0.06 +/- 0.01 kg/day) compared with SOC (0.50 +/- 0.04 kg/day) calves. Growth continued but at a significantly lower rate (P < 0.05) in calves HST at 146 days (0.32 +/- 0.07 kg/day) and 273 days (0.32 +/- 0.06 kg/day) compared with SOC (0.50 +/- 0.09 kg/day). Growth continued to be impaired to 1008 days, but more so in those HST at 146 days (432 +/- 43 kg BW) than 273 days (472 +/- 5 kg BW) and less (P < 0.05) than SOC (586 +/- 37 kg BW). Daily feed intake was consistently less (P < 0.05) in HST compared with SOC calves. Although episodic GH secretion was abolished and peripheral serum GH concentration remained consistently lower in HST (2.4 ng/ml) than SOC (5.5 ng/ml; P < 0.01), the calves continued to grow throughout 1008 days. Peripheral serum TSH concentration was less (P < 0.05) HST compared with SOC calves. There was an abrupt decrease (P < 0.001) in serum T4 (4-fold) and T3 (3-fold) concentration after surgery that remained to 360 days in HST compared with SOC calves. At the time calves were killed, pituitary gland weight was markedly reduced (P < 0.001) in HST (0.18 +/- 0.01 g/100 kg BW) compared with SOC (0.54 +/- 0.03 g/100 kg BW). Histological examination of pituitary glands from HST calves indicated the persistence of secretory GH and TSH cells in the same areas of the adenohypophysis as SOC calves. Coronal sections of the gland stained with performic acid-Alcian blue-periodic acid-Schiff-orange G, revealed GH and TSH secreting cells in HST calves similar to controls. These results indicate that long-term growth continues, but at a slower rate, after hypophyseal stalk transection of immature calves in spite of complete abolition of episodic GH secretion and consistently decreased basal secretion of GH, TSH, T4, and T3 compared with sham-operated animals. Growth was abolished after hypophysectomy of immature calves in which circulating GH and TSH was undetectable.

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 role of hypertrophy and growth factors in heart failure

The physiologic trophic factors growth hormone (GH) and insulin-like growth factor 1 (IGF-1) generally increase body weight and cardiac mass proportionately, and several studies suggest that both growth factors cause vasodilation and increased myocardial contractility. Established clinical benefits of ACE inhibitors can be explained, at least in part, by inhibition of cell hypertrophy, lowered systemic vascular resistance (SVR) and afterload, leading to reduction of progressive left-ventricular (LV) enlargement. An alternative approach would be to administer IGF-1 or GH to stimulate compensatory hypertrophy and reduce afterload by their vasodilator action, as well as through potential favorable effects on myocardial contractility. In our initial study in the rat myocardial infarction (MI) model, when IGF-1 was administered early (at 2 days) post-MI and continued for 2 weeks, body weight (BW) increased and LV weight/BW remained unchanged, the LV end-diastolic volume (EDV) and stroke volume increased (but not when normalized to BW), and the LV ejection fraction increased in rats with large infarctions. These findings suggested a beneficial rather than detrimental effect of such treatment, and we then studied the action of combined IGF-1 and GH starting after infarct healing at 4-weeks' post-MI. BW increased substantially and LVEDV/BW was lower in treated rats than in control rats, suggesting relatively less LV dilation with little remodeling in this setting; IGF-1/GH increased the cardiac output by 46%, systemic vascular resistance (SVR) fell and the cardiac index (CI) was significantly elevated in treated rats with a large MI. Recently, others have used the rat MI model to study the effects of 2-weeks' of GH started at 4-weeks' post-MI, as well as IGF/GH for 2-weeks in rats treated with an ACE inhibitor for 3-month's post-MI. In both studies, in conscious treated rats the BW increased, LV/BW was not different compared to the control rats, but the CI increased, SVR fell, and estimated LV dP/dtmax was significantly augmented. Preliminary data in our laboratory suggest that beneficial effects may also occur with GH administration in the setting of chronic angiotensin II receptor blockade (losartan) after MI in the rat. Thus, growth factor therapy appears to have favorable effects in heart failure early and late after MI in the rat. Additional cardiac hypertrophy occurs early after MI, but the later beneficial effects appear to relate primarily to systemic vasodilation, improved cardiac output, and enhanced myocardial contractility.

Cardiovascular effects of insulin-like growth factor-1 and growth hormone in chronic left ventricular failure in the rat

BACKGROUND

Insulin-like growth factor-1 (IGF-1) appears to have favorable cardiac effects associated with left ventricular remodeling early after myocardial infarction in the rat. The present study was designed to determine whether IGF-1 combined with growth hormone would be beneficial later as well, when infarct healing and cardiac remodeling have occurred.

METHODS AND RESULTS

Four weeks after coronary occlusion, 36 rats were randomized to IGF-1 (3 mg.kg-1.d-1) plus growth hormone (0.1 mg BID) or to placebo for 4 weeks. Treated rats had significant increases in body weight (22%), while the ratio of heart weight to body weight was unchanged. Under anesthesia, cardiac output (fluorescent microspheres) increased 46%, and systemic vascular resistance decreased by 21% (P < .001) in the treated group; a significant (22%) increase of the cardiac index was limited to treated rats with large myocardial infarctions. Small increases in the reduced left ventricular ejection fractions and left ventricular dP/dt(max) values with treatment were not significant. Treated rats showed a borderline (16%) increase in left ventricular end-diastolic volume (angiography), whereas the ratio of left ventricular end-diastolic volume to body weight was reduced in the treated group.

CONCLUSIONS

IGF-1 plus growth hormone administered to rats with left ventricular failure starting 1 month after MI was associated with substantial body growth, decreased systemic vascular resistance, and increased cardiac output. The failing heart also underwent treatment-induced increases in left and right ventricular weights in proportion to body growth, but left ventricular remodeling was minor, and a decrease in the ratio of left ventricular end-diastolic volume to body weight reflected relatively less chamber dilation compared with controls. A significant interaction between size of the myocardial infarction and treatment was observed for several variables, and IGF-1 and growth hormone increased the cardia index (P < .035) in rats with a large myocardial infarction.