A novel heterozygous IGF-1 receptor mutation associated with hypoglycemia.

Mutation in the insulin-like growth factor-1 receptor (IGF1R) gene is a rare cause for intrauterine and postnatal growth disorders. Patients identified with IGF1R mutations present with either normal or impaired glucose tolerance. None of the cases described so far showed hypoglycemia. We aimed to identify the genetic basis for small for gestational age, short stature and hypoglycemia over three generations in one family. The proband, a 9-year-old male, presented in infancy with recurrent hypoglycemic episodes, symmetric intrauterine growth retardation and postnatal growth retardation. Blood DNA samples from the patient, his parents, a maternal sister and maternal grandmother underwent Sanger sequencing of the IGF1R gene. Primary skin fibroblast cultures of the patient, his mother and age- and sex-matched control donors were used for gene expression and receptor functional analyses. We found a novel heterozygous mutation (c.94 + 1g > a, D1105E) affecting the splicing site of the IGF1R mRNA in the patient, his mother and his grandmother. Primary fibroblast cultures derived from the patient and his mother showed reduced proliferation and impaired activation of the IGF1R, evident by reduced IGF1R and AKT phosphorylation upon ligand binding. In conclusion, the newly identified heterozygous missense mutation in exon 1 of IGF1R (D1105E) results in impaired IGF1R function and is associated with small for gestational age, microcephaly and abnormal glucose metabolism. Further studies are required to understand the mechanisms by which this mutation leads to hypoglycemia.

IGF-1 Mediates EphrinB1 Activation in Regulating Tertiary Dentin Formation.

Eph receptors belong to a subfamily of receptor tyrosine kinases that are activated by membrane-spanning ligands called ephrins. Previously, we demonstrated that the ephrinB1-EphB2 interaction regulates odontogenic/osteogenic differentiation from dental pulp cells (DPCs) in vitro. The goal of this study was to identify the molecular mechanisms regulated by the EphB2/ephrinB1 system that govern tertiary dentin formation in vitro and in vivo. During tooth development, ephrinB1, and EphB2 were expressed in preodontoblast and odontoblasts at postnatal day 4. EphrinB1 was continuously expressed in odontoblasts and odontoblastic processes until the completion of tooth eruption. In addition, ephrinB1 was expressed in odontoblastic processes 2 wk following tooth injury without pulp exposure, whereas EphB2 was expressed in the center of pulp niches but not odontoblasts. In a model of tooth injury with pulp exposure, ephrinB1 was strongly expressed in odontoblasts 4 wk postinjury. In vitro studies with human and mouse DPCs treated with calcium hydroxide (CH) or mineral trioxide aggregate (MTA) showed an increased expression of insulin-like growth factor 1 (IGF-1). Experiments using several inhibitors of IGF-1 receptor signaling revealed that inhibiting the Ras/Raf-1/MAPK pathway inhibited EphB2 expression, and inhibiting the PI3K/Akt/mTOR pathway specifically inhibited ephrinB1 gene expression. Tooth injury in mice with odontoblast-specific IGF-1 receptor ablation exhibited a reduced tertiary dentin volume, mineral density, and ephrinB1 expression 4 wk following injury. We conclude that the IGF-1/ephrinB1 axis plays significant roles in the early stages of tooth injury. Further research is needed to fully understand the potential of targeting ephrinB1 as a regenerative pulp therapy.

Mammary tumor growth and pulmonary metastasis are enhanced in a hyperlipidemic mouse model.

Dyslipidemia has been associated with an increased risk for developing cancer. However, the implicated mechanisms are largely unknown. To explore the role of dyslipidemia in breast cancer growth and metastasis, we used the apolipoprotein E (ApoE) knockout mice (ApoE(-/-)), which exhibit marked dyslipidemia, with elevated circulating cholesterol and triglyceride levels in the setting of normal glucose homeostasis and insulin sensitivity. Non-metastatic Met-1 and metastatic Mvt-1 mammary cancer cells derived from MMTV-PyVmT/FVB-N transgenic mice and c-Myc/vegf tumor explants respectively, were injected into the mammary fat pad of ApoE(-/-) and wild-type (WT) females consuming a high-fat/high-cholesterol diet and tumor growth was evaluated. ApoE(-/-) mice exhibited increased tumor growth and displayed a greater number of spontaneous metastases to the lungs. Furthermore, intravenous injection of Mvt-1 cells resulted in a greater number of pulmonary metastases in the lungs of ApoE(-/-) mice compared with WT controls. To unravel the molecular mechanism involved in enhanced tumor growth in ApoE(-/-) mice, we studied the response of Mvt-1 cells to cholesterol in vitro. We found that cholesterol increased Akt(S473) phosphorylation in Mvt-1 cells as well as cellular proliferation, whereas cholesterol depletion in the cell membrane abrogated Akt(S473) phosphorylation induced by exogenously added cholesterol. Furthermore, in vivo administration of BKM120, a small-molecule inhibitor of phosphatidylinositol 3-kinase (PI3K), alleviated dyslipidemia-induced tumor growth and metastasis in Mvt-1 model with a concomitant decrease in PI3K/Akt signaling. Collectively, we suggest that the hypercholesterolemic milieu in the ApoE(-/-) mice is a favorable setting for mammary tumor growth and metastasis.

Inhibiting PI3K reduces mammary tumor growth and induces hyperglycemia in a mouse model of insulin resistance and hyperinsulinemia.

Women with type 2 diabetes mellitus (T2DM) are at a greater risk of developing and dying from breast cancer than women without T2DM. Insulin resistance and hyperinsulinemia underlie the pathogenesis of T2DM. In the MKR mouse model of insulin resistance, we have previously shown increased activation of the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR pathway in association with accelerated mammary tumor growth. In this study, we demonstrate that inhibiting PI3K with the oral pan-class I PI3K inhibitor, NVP-BKM120 reduced the growth of Met-1 and MCNeuA mammary tumor orthografts in the MKR mouse. NVP-BKM120 treatment decreased phosphorylation of Akt and S6 ribosomal protein (S6rp); no change in Erk1/2 phosphorylation was seen. Hyperglycemia, hypertriglyceridemia and greater hyperinsulinemia developed in the MKR mice treated with NVP-BKM120. We previously reported reduced tumor growth using intraperitoneal rapamycin in the MKR mouse, with the development of hyperglycemia and hypertriglyceridemia. Therefore, we examined whether the oral PI3K/mTOR inhibitor NVP-BEZ235 augmented the tumor suppressing effects of PI3K inhibition. We also investigated the effect of targeted PI3K/mTOR inhibition on PI3K/Akt/mTOR and Erk1/2 signaling, and the potential effects on glycemia. NVP-BEZ235 suppressed the growth of Met-1 and MCNeuA tumor orthografts, and decreased Akt and S6rp phosphorylation, despite increased Erk1/2 phosphorylation in Met-1 orthografts of MKR mice. Less marked hyperglycemia and hyperinsulinemia developed with NVP-BEZ235 than NVP-BKM120. Overall, the results of this study demonstrated that inhibiting PI3K/Akt/mTOR signaling with the oral agents NVP-BKM120 and NVP-BEZ235 decreased mammary tumor growth in the hyperinsulinemic MKR mouse. Inhibiting PI3K alone led to more severe metabolic derangement than inhibiting both PI3K and mTOR. Therefore, PI3K may be an important target for the treatment of breast cancer in women with insulin resistance. Monitoring for hyperglycemia and dyslipidemia should be considered when using these agents in humans, given the metabolic changes detected in this study.

Apolipoprotein E deficiency abrogates insulin resistance in a mouse model of type 2 diabetes mellitus.

AIMS/HYPOTHESIS: Although it is known that lipid metabolism plays a role in insulin resistance in type 2 diabetes and in obesity, the mechanism is still largely unknown. Apolipoprotein E (ApoE) regulates plasma lipid levels and also plays a role in the uptake of lipids into various tissues. To investigate whether the suppression of whole-particle lipoprotein uptake into tissues affects insulin responsiveness and the diabetic condition, we examined the effect of an ApoE (also known as Apoe) gene deletion in MKR mice, a mouse model of type 2 diabetes. METHODS: ApoE ( -/- ), MKR, ApoE ( -/- )/MKR and control mice were placed on a high-fat, high-cholesterol diet for 16 weeks. Glucose tolerance, serum insulin, blood glucose, insulin tolerance, tissue triacylglycerol content and atherosclerotic lesions were assessed. RESULTS: ApoE ( -/- )/MKR and ApoE ( -/- ) mice showed significantly improved blood glucose, glucose tolerance and insulin sensitivity. Reduced triacylglycerol content in liver and reduced fat accumulation in liver and adipose tissue were found in ApoE ( -/- )/MKR and ApoE ( -/- ) mice compared with control and MKR mice. ApoE ( -/- ) and ApoE ( -/- )/MKR mice demonstrated similarly large atherosclerotic lesions, whereas MKR and control mice had small atherosclerotic lesions. CONCLUSIONS/INTERPRETATION: We demonstrated that ApoE deficiency abrogates insulin resistance in a mouse model of type 2 diabetes, suggesting that lipid accumulation in tissue is a major cause of insulin resistance in this mouse model.

Diet-induced adiposity alters the serum profile of inflammation in C57BL/6N mice as measured by antibody array.

Morbid obesity is considered a systemic inflammatory state. The objective of this project was to characterize the adipokine, cytokine and chemokine protein profile in serum from control, lean and obese mice. We hypothesized that chemokines and cytokines are altered by caloric restriction and diet-induced obesity as a function of changes in body composition. Six-week-old female C57BL/6N mice (n = 12 per group) were randomized to one of three diets: control (fed ad libitum); lean (30% calorie-restricted regimen relative to control) and diet-induced obese (DIO; high calorie diet, fed ad libitum). Body weight, body composition and food intake were monitored throughout the study. After 10 weeks on the diets, blood samples were collected, and adipokine/cytokine/chemokine serum profiles were measured by antibody array. Lean mice, relative to the control group, displayed increased concentrations of insulin-like growth factor (IGF) binding protein-3, -5 and -6 and adiponectin and decreased IGF-1. These mice also showed increased concentrations of interleukin (IL)-10, IL-12 p40/p70, eotaxin, monocyte chemoattractant protein-5 and SDF-1. In contrast, DIO mice displayed increased leptin, IL-6 and LPS-induced chemokine and decreased concentrations of all chemokines/cytokines measured relative to control mice. As such, these data indicate that DIO may lead to an inflammatory state characterized as a shift towards a T helper lymphocyte type 1-skewed responsiveness. The demonstration of differential adipokine, cytokine and chemokine protein profile in control, lean and DIO mice may have implications for immune responsiveness and risk of disease.

Obesity and type 2 diabetes are associated with an increased risk of developing cancer and a worse prognosis; epidemiological and mechanistic evidence.

Both obesity and Type 2 diabetes are independently associated with an increased risk of developing cancer and an increased mortality. The etiology is yet to be determined but insulin resistance and hyperinsulinemia maybe important factors. Hyperglycemia, hyperlipidemia and inflammatory cytokines in addition to the insulin-like growth factors are also possible factors involved in the process.

Differential expression of IGF-I and insulin receptor isoforms in HPV positive and negative human cervical cancer cell lines.

Human papillomavirus (HPV) is the main risk factor for cervical cancer; however, some carcinomas occur in the absence of the virus. IGF-IR and an isoform of the insulin receptor, IR-A, play important roles in cancer. In this study we assessed the role of the IGF/insulin receptors in cervical cancer cell lines with different HPV status, SiHa (HPV positive), and C33a (HPV negative). Different patterns of receptor expression were found; while SiHa expressed IGF-IR, IR-A and IR-B, and IR/IGF-IR hybrid receptors, C33a cells expressed the IR-A only. Tyrosine phosphorylation of these receptors in response to their corresponding ligands correlated with the expression level of these receptors in the cell lines. Activation of PI3-K and MAPK pathways was revealed in both cell lines, however, no effects on proliferation, migration, or invasion were observed. Here we show that cervical cancer cell lines--positive and negative for HPV--differ in the type of insulin and IGF-1 receptors expressed. Additional studies are needed for characterization of the role of IR-A in cervical carcinogenesis.

Insulin-like growth factor-I protects cells from ER stress-induced apoptosis via enhancement of the adaptive capacity of endoplasmic reticulum.

Disruption of endoplasmic reticulum (ER) homeostasis causes accumulation of unfolded and misfolded proteins in the ER, triggering the ER stress response, which can eventually lead to apoptosis when ER dysfunction is severe or prolonged. Here we demonstrate that human MCF-7 breast cancer cells, as well as murine NIH/3T3 fibroblasts, are rescued from ER stress-initiated apoptosis by insulin-like growth factor-I (IGF-I). IGF-I significantly augments the adaptive capacity of the ER by enhancing compensatory mechanisms such as the IRE1 alpha-, PERK- and ATF6-mediated arms of ER stress signalling. During ER stress, IGF-I stimulates translational recovery and induces expression of the key molecular chaperone protein Grp78/BiP, thereby enhancing the folding capacity of the ER and promoting recovery from ER stress. We also demonstrate that the antiapoptotic activity of IGF-I during ER stress may be mediated by a novel, as yet unidentified, signalling pathway(s). Application of signal transduction inhibitors of MEK (U1026), PI3K (LY294002 and wortmannin), JNK (SP600125), p38 (SB203580), protein kinases A and C (H-89 and staurosporine) and STAT3 (Stattic) does not prevent IGF-I-mediated protection from ER stress-induced apoptosis. Taken together, these data demonstrate that IGF-I protects against ER stress-induced apoptosis by increasing adaptive mechanisms through enhancement of ER stress-signalling pathways, thereby restoring ER homeostasis and preventing apoptosis.

Studies involving the GH-IGF axis: Lessons from IGF-I and IGF-I receptor gene targeting mouse models.

The IGFs are ubiquitous and have pleoitropic effects. They are critical for normal growth and development, and for normal functioning of adult tissues. A liver-specific gene-deletion knockout of the IGF-I gene resulted in a mouse model with reduced circulating IGF-I levels, that led to insulin resistance due to the secondary elevation of circulating GH levels. The reduction in circulating IGF-I levels was also associated with a reduction in cancer growth and metastases in three cancer models, one for colon cancer and two for breast cancer. A second mouse model, using the transgenic approach, inhibited the IGF-I and insulin receptor function in skeletal muscle, and resulted in severe insulin resistance in muscle followed by insulin resistance in fat and liver and, eventually, beta-cell dysfunction and development of Type 2 diabetes. This progression from insulin resistance to Type 2 diabetes was most likely due to lipotoxicity with elevated serum and tissue triglyceride levels. Evidence supporting the hypothesis came from the use of fibrates and leptin injections, each of which enhanced fatty acid (FA) oxidation in liver and muscle and was associated with a reversal of the insulin resistance and diabetes.

Thiazolidinediones improve insulin sensitivity in adipose tissue and reduce the hyperlipidaemia without affecting the hyperglycaemia in a transgenic model of type 2 diabetes.

AIM/HYPOTHESIS: The aim of this study was to examine the effects of thiazolidinediones on the MKR mouse model of type 2 diabetes. METHODS: Six-week-old wild-type (WT) and MKR mice were fed with or without rosiglitazone or pioglitazone for 3 weeks. Blood was collected from the tail vein for serum biochemistry analysis. Hyperinsulinaemic-euglycaemic clamp analysis was performed to study effects of thiazolidinediones on insulin sensitivity of tissues in MKR mice. Northern blot analysis was performed to measure levels of target genes of PPAR gamma agonists in white adipose tissue and hepatic gluconeogenic genes. RESULTS: Thiazolidinedione treatment of MKR mice significantly lowered serum lipid levels and increased serum adiponectin levels but did not affect levels of blood glucose and serum insulin. Hyperinsulinaemic-euglycaemic clamp showed that whole-body insulin sensitivity and glucose homeostasis failed to improve in MKR mice after rosiglitazone treatment. Insulin suppression of hepatic endogenous glucose production failed to improve in MKR mice following rosiglitazone treatment. This lack of change in hepatic insulin insensitivity was associated with no change in the ratio of HMW : total adiponectin, hepatic triglyceride content, and sustained hepatic expression of PPAR gamma and stearoyl-CoA desaturase 1 mRNA. Interestingly, rosiglitazone markedly enhanced glucose uptake by white adipose tissue with a parallel increase in CD36, aP2 and GLUT4 gene expression. CONCLUSIONS/INTERPRETATION: These data suggest that potentiation of insulin action on tissues other than adipose tissue is required to mediate the antidiabetic effects of thiazolidinediones in our MKR diabetic mice.

Functional inactivation of the IGF-I and insulin receptors in skeletal muscle causes type 2 diabetes.

Peripheral insulin resistance and impaired insulin action are the primary characteristics of type 2 diabetes. The first observable defect in this major disorder occurs in muscle, where glucose disposal in response to insulin is impaired. We have developed a transgenic mouse with a dominant-negative insulin-like growth factor-I receptor (KR-IGF-IR) specifically targeted to the skeletal muscle. Expression of KR-IGF-IR resulted in the formation of hybrid receptors between the mutant and the endogenous IGF-I and insulin receptors, thereby abrogating the normal function of these receptors and leading to insulin resistance. Pancreatic beta-cell dysfunction developed at a relative early age, resulting in diabetes. These mice provide an excellent model to study the molecular mechanisms underlying the development of human type 2 diabetes.

Liver-specific igf-1 gene deletion leads to muscle insulin insensitivity.

Insulin and insulin-like growth factors (IGFs) mediate a variety of signals involved in mammalian development and metabolism. To study the metabolic consequences of IGF-I deficiency, we used the liver IGF-I-deficient (LID) mouse model. The LID mice show a marked reduction (approximately 75%) in circulating IGF-I and elevated growth hormone (GH) levels. Interestingly, LID mice show a fourfold increase in serum insulin levels (2.2 vs. 0.6 ng/ml in control mice) and abnormal glucose clearance after insulin injection. Fasting blood glucose levels and those after a glucose tolerance test were similar between the LID mice and their control littermates. Thus, the high levels of circulating insulin enable the LID mice to maintain normoglycemia in the presence of apparent insulin insensitivity. Insulin-induced autophosphorylation of the insulin receptor and tyrosine phosphorylation of insulin receptor substrate (IRS)-1 were absent in muscle, but were normal in liver and white adipose tissue of the LID mice. In contrast, IGF-I-induced autophosphorylation of its cognate receptor and phosphorylation of IRS-1 were normal in muscle of LID mice. Thus, the insulin insensitivity seen in the LID mice is muscle specific. Recombinant human IGF-I treatment of the LID mice caused a reduction in insulin levels and an increase in insulin sensitivity. Treatment of the LID mice with GH-releasing hormone antagonist, which reduces GH levels, also increased insulin sensitivity. These data provide evidence of the role of circulating IGF-I as an important component of overall insulin action in peripheral tissues.

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.

TCR stimulation protects CD8+ T cells from CD95 mediated apoptosis.

Activation of T cells through the T-cell receptor (TCR) induces the expression of Fas Ligand (CD95L). In turn, CD95L binds to the Fas receptor (CD95) and rapidly induces apoptosis in cycling cells. This interaction is involved in the elimination of reactive lymphocytes during an immune response. However, TCR activation cannot always trigger apoptosis because an effective immune response would then be compromised. Here we show that a short (2 to 3 h) activation of T cells through the TCR simultaneously induces an increase in CD95L mRNA and a dramatic decrease in caspase-8 mRNA levels and proteolytic activity in human CD8(+) T cells. In addition, there is a small reduction in CD95 mRNA and CD95 levels on the cell surface. We found that preactivation of T cells protected them from apoptosis induced by either religation of the TCR or direct exposure to CD95L. These results suggest a mechanism by which cycling CD95-sensitive peripheral T cells, become protected from CD95 mediated deletion when actively engaged in the specific recognition of target cells.

Mice deficient in liver production of insulin-like growth factor I display sexual dimorphism in growth hormone-stimulated postnatal growth.

Insulin-like growth factor I (IGF-I) is essential for normal intrauterine and postnatal growth and development. Using the Cre/loxP-induced conditional knockout system, we have established a liver-specific IGF-I-deficient (LID) mouse model. Circulating IGF-I levels were decreased by approximately 75% without any apparent effect on their growth and development. To determine the role of extra-hepatic IGF-I in GH-induced postnatal growth, we tested the effects of GH on growth rates in these mice. Female, but not male, LID mice displayed significantly accelerated growth rates in response to daily injections of GH for 5 weeks. The GH-induced peripubertal growth in female LID mice was not affected by ovariectomy, nor did castration change the growth pattern in male LID mice. Thus, factors other than gonadal steroids mediate this sexual dimorphism. We postulate that the sexual dimorphic response to GH observed in LID mice may be related to genetically programmed differences in GH secretion patterns.

The growth hormone/insulin-like growth factor-I system: implications for organ growth and development.

Growth hormone (GH) and insulin-like growth factors (IGFs) are essential for normal growth and development during embryonic stages as well as postnatally. While GH has little effect on these processes prenatally, the IGFs are important during these stages. On the other hand the GH-IGF-I axis is important for pubertal growth. To determine whether postnatal growth and development are dependent on circulating or locally produced IGF-I, we deleted the IGF-I gene in the liver using the cre/LoxP system used for tissue-specific gene deletion. These animals demonstrated approximately 75%-80% reduction in circulating IGF-I and an approximate fourfold increase in circulating GH. Despite the marked reductions in circulating IGF-I, growth and development was apparently normal. Thus the original somatomedin hypothesis needs to be re-evaluated in the light of these new findings.

CrkII participation in the cellular effects of growth hormone and insulin-like growth factor-1. Phosphatidylinositol-3 kinase dependent and independent effects.

We have examined the role of CrkII in the cellular response to both human growth hormone (hGH) and human insulin-like growth factor-1 (hIGF-1). We have demonstrated that overexpression of the adaptor molecule enhances both basal phosphatidylinositol 3-kinase (PI 3-kinase) activity and also dramatically enhances the ability of both hormones to stimulate PI 3-kinase activity in the cell. Many of the effects of CrkII overexpression on hGH- and hIGF-1-stimulated cellular function can then be attributed to CrkII enhancement of PI 3-kinase stimulation by these hormones. Thus, CrkII-enhanced PI 3-kinase activity is used to enhance actin filament reorganization in response to both hGH and hIGF-1, to enhance stress activated protein kinase (SAPK) activity in response to hGH, and to diminish STAT5-mediated transcription in response to hGH. It is apparent, however, that CrkII also regulates cellular function independent of its ability to stimulate PI 3-kinase activity. This is evidenced by the ability of CrkII, in a PI 3-kinase-independent manner, to diminish the activation of p44/42 mitogen-activated protein kinase in response to both hGH and hIGF-1 and to inhibit the activation of SAPK by hIGF-1. Therefore, despite the common use of CrkII to activate PI 3-kinase, CrkII also allows hGH or hIGF-1 to selectively switch the activation of SAPK. Thus, common utilization of CrkII by hGH and hIGF-1 allows the execution of common cellular effects of these hormones, concomitant with the retention of hormonal specificity.

Conditional knockout of mouse insulin-like growth factor-1 gene using the Cre/loxP system.

Insulin-like growth factor-1 (IGF-1) is an essential growth factor for normal intrauterine development and postnatal growth. Mice with a complete deficiency of IGF-1 (IGF-1-null mice), created by homologous recombination, were found to exhibit postnatal lethality, growth retardation, infertility, and profound defects in the development of major organ systems. Furthermore, IGF-1-null mice were resistant to growth hormone (GH) treatment in peri-pubertal somatic growth. Using the Cre/loxP-induced conditional knockout system, we generated a mouse that lacks IGF-1 specifically in the liver, the primary site of IGF-1 production. Interestingly, although circulating and serum levels of IGF-1 were decreased by approximately 75% in these mice, they exhibited no defect in growth or development. When administered exogenously, GH stimulated IGF-1 production in several extra-hepatic tissues as well as body growth. The "Somatomedin hypothesis" originally proposed that circulating IGF-1 acting in various tissues mediate the effects of GH. These striking in vivo results, obtained using homologous recombination technology, call for a major modification of the Somatomedin hypothesis. These gene targeting studies confirm that IGF-1 is essential for GH-stimulated postnatal body growth. However, liver-derived (endocrine) IGF-1 is not essential for normal postnatal growth, though it does exert a negative feedback on GH secretion. Instead, local production of IGF-1, acting in a paracrine/autocrine fashion, appears to mediate GH-induced somatic growth. This review will discuss the effects of tissue-specific IGF-1 gene deficiency created by the Cre/loxP system versus the conventional IGF-1 knockout.

Normal growth and development in the absence of hepatic insulin-like growth factor I.

The somatomedin hypothesis proposed that insulin-like growth factor I (IGF-I) was a hepatically derived circulating mediator of growth hormone and is a crucial factor for postnatal growth and development. To reassess this hypothesis, we have used the Cre/loxP recombination system to delete the igf1 gene exclusively in the liver. igf1 gene deletion in the liver abrogated expression of igf1 mRNA and caused a dramatic reduction in circulating IGF-I levels. However, growth as determined by body weight, body length, and femoral length did not differ from wild-type littermates. Although our model proves that hepatic IGF-I is indeed the major contributor to circulating IGF-I levels in mice it challenges the concept that circulating IGF-I is crucial for normal postnatal growth. Rather, our model provides direct evidence for the importance of the autocrine/paracrine role of IGF-I.